TW202335384A - Simulation modeling method for an organic laser device, program for performing the simulation modeling method, and method for manufacturing an organic laser device - Google Patents

Abstract

Disclosed are a current excitation type organic semiconductor laser containing a pair of electrodes, an organic laser active layer and an optical resonator structure between the pair of electrodes and a laser having an organic layer on a distributed feedback grating structure. The lasers include a continuous-wave laser, a quasi-continuous-wave laser and an electrically driven semiconductor laser diode.

Description

Simulation modeling method of organic laser device, program for executing simulation modeling method, and method of manufacturing organic laser device

The invention relates to continuous wave organic thin film dispersed feedback laser and electrically driven organic semiconductor laser diodes.

This application discloses the following inventions: 1. A current-excited organic semiconductor laser, which contains two electrodes, an organic optical gain layer and an optical resonator structure. In the specification, the current-excited organic semiconductor laser may be called a current-injected organic semiconductor laser or an electrically driven organic semiconductor laser. 2. The laser of item 1, wherein the organic optical gain layer is composed of at least one charge transport layer and at least one light amplification layer caused by stimulated emission. 3. For the laser of Item 1 or 2, at least one of the two electrodes is transparent. 4. The laser of any one of items 1 to 3, wherein the optical resonator structure consists of a distributed feedback (DFB) structure. 5. The laser of any one of items 1 to 4, wherein the optical resonator structure is a one-dimensional resonator structure. 6. The laser of item 5, wherein the optical resonator structure consists of a second-order Bragg scattering region surrounded by a first-order Bragg scattering region. 7. The laser of any one of items 1 to 4, wherein the optical resonator structure is a two-dimensional resonator structure. 8. As for the laser of item 7, the optical resonator structure is a circular resonator structure. 9. The laser of any one of items 1 to 3, 7 and 8, wherein the optical resonator structure consists of a distributed Bragg reflector (DBR) structure. 10. A laser such as any one of items 1 to 9, which contains at least one hole transport or injection region and at least one electron transport or injection region. The hole transport region may be present in the organic optical gain layer or may be a layer formed on the organic optical gain layer. This layer may contain no organic gain medium and may consist solely of dopants. The regions present in the organic optical gain layer may contain dopants and organic gain media. The hole transport region in the organic optical gain layer can be formed by doping the organic optical gain layer with a dopant. Doping can be performed by doping a dopant from one surface of the organic optical gain layer. The electron transport region may be present in the organic optical gain layer or may be a layer formed on the organic optical gain layer. This layer may contain no organic gain medium and may consist solely of dopants. The regions present in the organic optical gain layer may contain dopants and organic gain media. The electron transport region in the organic optical gain layer can be formed by doping the organic optical gain layer with a dopant. Doping can be performed by doping a dopant from one surface of the organic optical gain layer. 11. For the laser of item 10, the hole transport or injection region contains receptors. 12. As in the laser of item 10, the hole transport or injection region is doped with acceptors. 13. The laser of any one of items 10 to 12, wherein the receptor is a metal oxide. The metal oxide may be MoO ₃ . 14. The laser according to any one of items 10 to 13, further comprising a receiving layer between the hole transport or injection region and the anode. The anode is one of the two electrodes. 15. A laser as in any one of items 10 to 14, wherein the electron transport or injection region contains a donor. The donor can be an alkali metal such as Cs. 16. As in the laser of item 15, the electron transport or injection region is doped with a donor. 17. The laser of any one of items 10 to 16, further comprising a donor layer between the electron transport or injection region and the cathode. The cathode is one of two electrodes. 18. A laser as in any one of items 1 to 17, which does not have an organic layer other than an organic optical gain layer. 19. The laser of any one of items 1 to 18, wherein the optical resonator structure is an external optical resonator structure. 20. The laser of any one of items 1 to 18, wherein the optical resonator structure is located between two electrodes. 21. The laser of any one of items 1 to 20, further comprising a current limiting structure between two electrodes. 22. The laser of any one of items 1 to 21, wherein the optical resonator structure is a whispering high-resonance gallery optical resonator structure. 23. A laser as in any one of items 1 to 22, wherein excitons are generated by current excitation and overlap with the optical resonance mode of the optical resonator structure. 24. For example, the laser of any one of items 1 to 23 does not exhibit substantial exciton mutual destruction at the laser wavelength. The loss caused by exciton mutual destruction is preferably less than 10%, more preferably less than 5%, further preferably less than 1%, further preferably less than 0.1%, still further preferably less than 0.01%, optimal is 0%. 25. For example, the laser of item 24 does not exhibit substantial singlet-singlet state and triplet-triplet mutual destruction. 26. A laser such as any one of items 1 to 25 that exhibits no substantial polaron absorption loss at the laser wavelength. There is no substantial overlap between the polaron absorption spectrum and emission spectrum of organic semiconductor lasers. The loss due to polaron absorption is preferably less than 10%, more preferably less than 5%, further preferably less than 1%, still further preferably less than 0.1%, still further preferably less than 0.01%, optimal is 0%. 27. The laser of any one of items 1 to 26, wherein the ratio of electron mobility to hole mobility in the organic optical gain layer is in the range of 1/10 to 10/1. The ratio is preferably 1/5 to 5/1, more preferably 1/3 to 3/1, and still more preferably 1/2 to 2/1. 28. A laser with an organic layer on a DFB grating structure, wherein the organic layer contains an organic gain medium. 29. As for the laser of item 28, the DFB grating structure is a mixed-order DFB grating structure in which the second-order Bragg scattering area is surrounded by the first-order scattering area. 30. As for the laser of item 28, the DFB grating structure is a mixed-order DFB grating structure in which a second-order Bragg scattering region and a first-order scattering region are formed. 31. For example, in the laser of item 28, the second-order Bragg scattering region and the first-order scattering region are alternately formed. 32. The laser of any one of items 28 to 31, wherein the DFB grating structure has a circular structure. 33. A laser as in any one of items 28 to 32, wherein there is no substantial spectral overlap between excited state absorption and laser emission. 34. For example, the laser of any one of items 28 to 33, wherein the stimulated emission cross section σ _em is 100 times or more than the triplet excitation state cross section σ _TT , preferably 400 times or more than 400 times, and more The good land is 700 times or more than 700 times larger. 35. The laser of any one of items 28 to 34, wherein the organic layer contains a host material and at least one dopant. 36. A laser as in any one of items 28 to 35, wherein the organic layer is directly or indirectly covered with sapphire. 37. The laser of any one of items 28 to 36, wherein the organic layer is directly or indirectly covered with a fluoropolymer. 38. For the laser of item 37, the thickness of the fluoropolymer is less than 3 μm. 39. The laser of item 38, wherein the organic layer covered with fluoropolymer is further covered with sapphire. 40. If the laser is in any of items 29 to 39, it is a continuous wave laser. 41. If the laser is in any of items 29 to 39, it is a quasi-continuous wave laser. 42. The laser of any one of items 1 to 41, wherein the organic layer contains an organic compound having at least one stilbene unit. 43. The laser according to any one of items 1 to 41, wherein the organic layer contains 4,4'-bis[( N - carbazole)styryl]biphenyl (BSBCz). 44. Such as the laser of item 43, in which the organic layer contains 4,4'-bis[( N - carbazole)styryl]biphenyl (BSBCz) and 4,4'-bis( N - carbazolyl)- 1,1'-Biphenyl (CBP). 45. The laser of any one of items 1 to 41, wherein the organic layer contains a compound having at least one fluorine unit. The compound having a fluorine unit may be a compound having at least two fluorine structures, such as seven fluorine, eight fluorine, and two fluorine centrum dendrons. 46. For example, the laser of any one of items 1 to 45, wherein the thickness of the organic layer is 80 to 350 nm, preferably 100 to 300 nm, more preferably 150 to 250 nm. 47. The laser of any one of items 1 to 46, wherein the depth of the grating structure is less than 75 nm, preferably 20 to 70 nm. 48. The laser of any one of items 1 to 47, wherein the grating structure is made of _SiO2 . 49. For example, the laser of any one of items 1 to 48 does not contain a triplet quencher.

[ 1 ] Continuous wave organic thin film dispersion feedback laserSince the discovery of organic solid-state lasers, ^{[ 1 − 6 ]}Significant efforts have been devoted to the development of continuous wave (cw) lasers for organic materials, including small molecules, oligomers and polymers. ^{[ 7 − 10 ]}However, operating organic solid-state lasers at extremely high repetition rates (quasi-CW excitation) under optical CW excitation or pulse excitation is extremely challenging. When organic films are optically pumped under these conditions, accumulation of long-lived triplet excitons and charge carriers usually occurs. ^{[ 11 − 14 ]}Resulting in increased absorption losses by triplet exciton formation and singlet exciton quenching by triplet excitons (ie, singlet-triplet mutual destruction). ^{[ 11 − 16 ]}These absorption losses and emission quenching are significant issues that must be addressed to achieve CW and quasi-CW operation, as they cause the laser threshold to greatly increase and, in the worst case, completely stop the laser. ^{[ 17 − 19 ]}To curb absorption losses and emission quenching, it has been proposed to incorporate triplet quenchers, such as oxygen, into organic films. ^{[ 15 , 16 ]}Cycloctatetraene, ^{[ 20 ]}or anthracene derivatives ^{[ 19 ]}. However, as suggested by Schols et al. ^{[ 20 ]}The requirements for triplet quenchers are low triplet energy, short triplet lifetime, and the large difference between the energies of singlet and triplet states, making it difficult to find a suitable triplet quencher that satisfies these conditions without hindering the laser. agent. Rabe et al. demonstrated the use of 6,6'-(2,2'-octyloxy-1,1'-binaphthyl)binaphthyl spacers containing 12% (BN-PFO) without a triplet quencher. Quasi-cw operation at a repetition rate of 5 MHz in radical poly(9,9-dioctyl fluoride) derivatives. ^{[ 9 ]}This high repetition rate is achieved due to less spectral overlap between emission and triplet absorption in BN-PFO. ^{[ 10 ]}Therefore, the development of organic laser dyes with less spectral overlap between excited state absorption and emission is crucial to realizing cw and quasi-cw lasers with low thresholds. In the group, we have continuously studied the optical and amplified spontaneous emission (ASE) properties of many organic materials with the aim of realizing electrically pumped organic laser diodes. ^{[ twenty one − 27 ]}Among them, 4,4'-double[( N -Carbazole) styryl]biphenyl (BSBCz) is one of the most promising candidates because it is blended with 6 wt% BSBCz as the host material 4,4'-bis( NVacuum-deposited thin films of -carbazolyl)-1,1'-biphenyl (CBP), whose chemical structure is shown in Figure 1a, have excellent optical and ASE properties, such as a high photoluminescence quantum yield close to 100% ( Φ _PL) and a short PL lifetime (τ _PL), yielding approximately 10 ⁹s ^{− 1}The huge radiative decay constant (k _r) and about 0.3 μJ cm ^{− 2}The low ASE threshold energy. ^[23,26]In this paper, we report quasi-cw surface-emitting laser in a distributed feedback (DFB) device based on this BSBCz:CBP blended film. In this laser device, we obtained the highest repetition rate (up to 8 MHz) and the lowest threshold (about 0.25 μJ cm) ever reported for a quasi-CW laser based on an organic thin film system. ^{− 2}). The incorporation of triplet quenchers is not necessary in our blended films due to their high Φ _PLAnd there is no significant spectral overlap between the emission and triplet absorption of BSBCz. ^{[ twenty four ]}In the DFB structure, laser oscillation occurs when the following Bragg condition is met: mλ _Bragg= 2 n _eff Λ,in mis the diffraction order, λ _Braggis the Bragg wavelength, n _effis the effective refractive index of the gain medium and Λis the period of the grating. ^{[ 28 , 29 ]}When considering second-order modes ( m= 2), use the one reported for BSBCZ n _effand λ _BraggCalculate the grating period as Λ= 280 nm. ^{[ twenty one , twenty two ]}have Λ =The 280 nm grating provides surface emitting laser light in a direction perpendicular to the plane of the substrate as shown in Figure 1b. Although second-order gratings generally produce higher laser thresholds than first-order gratings, surface-emitting lasers using second-order gratings are suitable for fabricating electrically pumped organic lasers with organic light-emitting diode structures that exhibit the same surface emission. diode. ^{[ 30 , 31 ]}Using electron beam lithography and reactive ion etching, these gratings were engraved directly to 5 × 5 mm ²area on the silicon dioxide surface (Fig. 1c). Figures 1d and 1e show SEM images of representative gratings fabricated in this study. We obtained from SEM images Λ= 280±2 nm and d= 70±5 nm grating depth, which meets our specifications perfectly. A 6 wt% BSBCz:CBP blended film or a pure BSBCz film with a thickness of 200 nm was prepared on the grating by vacuum deposition to fabricate the laser device. First, we examined the surface-emitting laser characteristics of our DFB system under 0.8 ns wide pulse excitation at 20 Hz from a nitrogen laser. This excitation light with a wavelength of 337 nm is mainly absorbed by CBP in the blended film. However, the large spectral overlap between CBP emission and BSBCz absorption ensures efficient Förster-type energy transfer between the two molecules (Fig. 1f). ^{[ 26 ]}Therefore, we did not observe any emission from CBP even under high excitation. Figures 2a and 2b show the emission spectra measured from the laser device ((a) BSBCz:CBP film and (b) pure BSBCz film) with different excitation intensities. Both devices exhibit laser emission with extremely narrow peaks relative to certain excitation light intensities. We confirm that there is no surface emitted laser from the non-grating areas on the same substrate. As a result of stimulated emission, ^{[ 32 − 35 ]}In our laser device, we found τ _PLand full width at half maximum (FWHM) in E _thThe results are significantly reduced at high excitation energies (Fig. 2a and Fig. 2b), indicating that our optical grid is suitable for extracting light from waveguide films as surface emission. We observed a Bragg dip at about 478 nm for the blended film and a Bragg dip at 474 nm for the pure film in the emission spectra measured at low excitation intensity (inset of Figure 2a and Figure 2b). The Bragg dip is caused by the grating inhibiting the propagation of waveguide light and can be imagined as a photonic stopband for waveguide modes. ^{[ 36 ]}Lasing occurs at the short wavelength edge of the Bragg dip (477 nm for blended films and 473 nm for pure films). The difference in the position of the Bragg dip is most likely due to the different refractive indices used for the blended and pure films. As the excitation intensity increases, the emission intensity increases linearly and then begins to amplify for the laser as the FWHM decreases to <0.30 nm for the blended film and <0.40 nm for the pure film (see Figure 2c and Figure 2d). Laser threshold energy measured from the intersection of two straight lines fitted to emission intensity ( E _th) for the blended film is E _th= 0.22 μJcm ^{− 2}And for pure films, it is E _th= 0.66 μJcm ^{− 2}, which corresponds to 275 W cm ^{− 2}and 825 W cm ^{− 2}The power density. Due to the excellent quality of our gratings, this value is lower than its 375 W cm without grating. ^{− 2}and 1625 W cm ^{− 2}The ASE critical power density. ^{[ twenty three , 26 ]}obtained E _thThe value is the lowest value ever reported among all quasi-cw organic thin film lasers. Due to the suppressed concentration quenching, the concentration in the blended film is lower than that in the pure film. E _thThis is attributed to the fact that the blended film (98%) has a higher Φ than the pure film (76%). _PL. ^{[ 36 ]}Generally speaking, E _thAnd laser gain and Φ _PLInversely proportional. ^{[ 37 , 38 ]}Our device was operated in quasi-cw mode using optical pulses from a Ti-sapphire laser with a wavelength of 365 nm and a width of 10 ps. Figure 3 shows the frame camera image of laser oscillation and the corresponding time variation of the laser intensity in the BSBCz:CBP blended film at the laser wavelength. The excitation light intensity is fixed at approximately 0.44 μJ cm ^{− 2}, its ratio E _thAbout twice as high. At a repetition rate of 0.01 MHz, laser oscillations were observed at intervals of 100 μs. Reduce the time interval between laser oscillations at higher repetition rates. Neighboring laser oscillations occur continuously at 8 MHz over a broad time scale of 500 μs (Figures 3a and 3b); however, even at 8 MHz, 125 Individual laser oscillations at ns intervals (Figure 3c). We confirm that similar quasi-cw operations are possible for BSBCz pure films. The emission intensity of both laser devices with blended films and pure films remains almost constant up to 8 MHz, as shown in Figure 3. This maximum repetition rate is the highest ever reported and is attributed to small absorption losses and emission quenching caused by triplet exciton formation. BSBCz Φ _PLExtremely high, thereby minimizing the generation of triplet excitons via intersystem crossing, especially for blended films. Furthermore, the spectral overlap between emission and triplet absorption is negligible, reducing the possibility of collisions between singlet and triplet excitons. When operating a laser device at 80 MHz (the highest frequency possible with our equipment), the emission intensity decreases rapidly, and it is probably impossible to estimate a clear laser threshold due to rapid material degradation. Furthermore, the FWHM of the emission peaks observed at 80 MHz is approximately twice that of the emission peaks at lower frequencies. At this stage, we're not sure if it's lasering. Figure 4a shows a graph of the laser threshold as a function of repetition rate for blended films and pure films. Of concern, the laser threshold is almost independent of the repetition rate of the blended film due to negligible absorption losses and emission quenching. However, for pure films, a gradually increasing threshold is observed with increasing repetition rate. We do not know the exact reason for the gradual increase in the threshold, and therefore further research is needed to elucidate this observation. We studied the operational stability of laser oscillation when operating the device continuously at 8 MHz (Figure 4b). The emission intensity gradually decreases with time. The change is irreversible, indicating photodegradation of the material. The lifetime until the emission intensity decreases to 90% of the initial value is 900 s for the blended film, which is longer than 480 s for the pure film. Due to the higher threshold, stronger excitation light is required to achieve lasing in pure films compared to blended films. Therefore, photodegradation can be expected to be faster in pure films. Lowering the threshold is critical to curbing photodegradation. In summary, a DFB laser device combining a BSBCz:CBP blended film and a second-order grating as a gain medium was fabricated and evaluated. We obtain excellent surface-emitted lasers from the device under self-calibrated CW operation, where the emission intensity and laser threshold are independent of the repetition rate. For our laser device, the maximum repetition rate is 8 MHz, which is the highest repetition rate ever reported, and the laser threshold is approximately 0.25 μJ cm ^{− 2}, which is the lowest laser threshold ever reported. Due to the negligible accumulation of triplet excitons and the small spectral overlap between emission and triplet absorption, the triplet quenchers typically used to fabricate organic thin film lasers are not necessary in our device. We therefore consider BSBCz to be the most promising candidate for the first realization of electrically pumped organic laser diodes in terms of optical properties. However, electrical properties such as charge carrier mobility, charge carrier capture cross-section, etc. are also extremely important and will need to be further studied and enhanced for the realization of electrically pumped organic lasers. Experimental partNeutral detergent, pure water, acetone and isopropyl alcohol were used to clean the silicon substrate covered with a thermally grown silicon dioxide layer of 1 μm thickness by ultrasonic treatment, followed by UV ozone treatment. The silica surface was treated with hexamethyldisilazane (HMDS) by spin coating at 4000 rpm for 15 s. A resist layer with a thickness of approximately 70 nm was applied to the substrate from a ZEP520A-7 solution (ZEON Co.) by spinning at 4000 rpm for 30 s and baked at 180°C for 240 s. Use a 0.1 nC cm ^{− 2}A JBX-5500SC system (JEOL) with optimized dosage was used for electron beam lithography to draw the grating pattern on the resist layer. After electron beam irradiation, the pattern was developed in a developer solution (ZED-N50, ZEON Co.) at room temperature. The patterned resist layer was used as an etch mask, while an EIS-200ERT etch system (ELIONIX) was used with CHF ₃Plasma etching the substrate. To completely remove the resist layer from the substrate, use a FA-1EA etching system (SAMCO) with O ₂Plasma etching the substrate. The grating formed on the silicon dioxide surface was observed using scanning electron microscopy (SU8000, Hitachi). To complete the laser device, by ^{− 4}Thermal evaporation under a pressure of Pa is 0.1 nm s ^{− 1}to 0.2 nm s ^{− 1}The total evaporation rate was used to prepare 200 nm thick 6 wt% BSBCz:CBP blended films and BSBCz pure films on the grating. For laser operation, the pulsed excitation light from a nitrogen laser (USHO, KEN-2020) is concentrated on a 6×10 area of the device through a lens and a slit. ^{− 3}cm ²area. The excitation wavelength is 337 nm, the pulse width is 0.8 ns, and the repetition rate is 20 Hz. The excitation light is incident on the device at an angle of approximately 20° relative to the normal line of the device plane. The emitted light perpendicular to the device surface was collected using an optical fiber connected to a multichannel spectrometer (PMA-50, Hamamatsu Photonics) placed 3 cm away from the device. Use a set of neutral density filters to control excitation intensity. For quasi-cw operation, a mode-locked frequency doubled Ti-sapphire laser (Millennia Prime, Spectra physics) was used to generate excitation with an excitation wavelength of 365 nm, a pulse width of 10 ps, and a repetition rate ranging from 0.01 MHz to 8 MHz. Light. The excitation light is concentrated on 1.9×10 of the device through lenses and slits. ^{− 4}cm ²area, and the emitted light was collected using a streak scope (C10627, Hamamatsu Photonics) with a temporal resolution of 15 ps connected to a digital camera (C9300, Hamamatsu Photonics). As described previously, the same illumination and detection angles were used for this measurement. 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Phillips, A. M. Sergent, K. W. Wecht, Appl. Phys. Lett. 1994, 64, 2634. 36. G. A. Turnbull, P. Andrew, M. J. Jory,W. L. Barnes, I. D.W. Samuel, Phys. Rev. B 2001, 64, 125122. 37. B. Zhen, S.-L. Chua, J. Lee, A. W. Rodriguez, X. Liang, S. G. Johnson, J. D. Joannopoulos, M. Soljacic, O. Shapira, Proc. Natl. Acad. Sci. USA 2013, 110, 13711. 38. J. C. Ribierre, G. Tsiminis, S. Richardson, G. A. Turnbull, H. S. Barcena, P. L. Burn, I. D. W. Samuel, Appl. Phys. Lett. 2007, 91, 081108. [ 2 ] Using oxygen as a triplet quencher to improve quasi-continuous wave laser properties in organic semiconductor lasersWe demonstrate the quasi-continuous wave laser in a solvent-free liquid organic semiconductor dispersion feedback laser based on a blend of a liquid 9-(2-ethylhexyl)carbazole host doped with a blue-emitting heptamine derivative. . The liquid gain medium is bubbled with oxygen or nitrogen to study the effect of triplet quenchers such as molecular oxygen on the quasi-continuous wave laser properties of organic semiconductor lasers. The oxidized laser device exhibits 2 μJ cm ^{- 2}A low threshold value that is lower than the threshold value measured in the nitriding device and is independent of the repetition rate in the range between 0.01 MHz and 4 MHz. Since the first optically pumped organic solid-state semiconductor laser was demonstrated in 1996, ^{1 , 2}Organic lasers have been the subject of intensive research mainly due to several attractive characteristics of organic semiconducting materials, such as their broad absorption and emission spectra, and their high optical gain coefficients. ^{3 , 4}The performance of organic solid-state lasers has greatly improved over the past two decades, and applications are now emerging including the development of integrated light sources for spectral analysis and vapor chemical sensors. ⁵Although pulsed inorganic light-emitting diodes can now be used to optically pump organic solid-state lasers, ⁶However, further breakthroughs are still needed to demonstrate optically pumped organic semiconductor lasers operating in the continuous wave (cw) state and ultimately to realize optically pumped organic laser diodes. It has been established that the generation of long-lived triplet excitons via intersystem crossing can lead to high photon and singlet losses that prevent lasers in the cw optical pump regime. ^{7 - 12}To address this critical issue, the incorporation of triplet quenchers into organic semiconductor gain media has been proposed. Zhang et al. (8-hydrixyquinoline) aluminum doped with 4-(dicyanomethylene)-2-methyl-6-juraleridinyl-9-enyl-4H-pyran (DCM2) (Alq ₃) uses anthracene derivatives as triplet quenchers and can extend the laser duration of its distributed feedback (DFB) organic device to nearly 100 μs. ⁸Meanwhile, some other studies have demonstrated that triplet loss in optically pumped organic semiconductor lasers can be reduced by using oxygen or cyclooctatetraene (COT) as a triplet quencher. ^{9 - 11}Although the use of triplet quenchers to develop true cw organic solid-state laser technology is highly promising, it should be mentioned that other methods have been proposed to achieve this goal. Recently, 4,4'-bis(N-carbazolyl)-1,1'-biphenyl doped with 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) has been studied. A quasi-cw laser with a repetition rate of up to 8 MHz was demonstrated in the organic DFB laser of benzene (CBP) host. ¹³This achievement is explained by the negligible overlap between the laser emission and triplet absorption spectra of BSBCz and the photoluminescence quantum yield of the material close to 100%, which results in triplet under optical pumping. The state is extremely weak. Another approach to achieving high-power cw organic solid-state dye lasers is based on extremely fast rotation of the device during its operation, but the long-term power output stability of these devices appears to be limited for practical applications. ¹⁴In this study, we report on the use of solvent-free liquid organic semiconductor materials as laser gain media to fabricate organic semiconductor DFB lasers operating in a quasi-cw state. ^{15 - twenty three}This laser material consists of a host of 9-(2-ethylhexyl)carbazole (EHCz) doped with hexafluoride derivatives. ¹⁷composition. ^{twenty four}The chemical structure of these molecules is shown in Figure 5a. Through the blend's photoluminescence quantum yield (PLQY) of 85% and its 0.4 μJ cm under pulsed optical pumping ^{- 2}The low amplified spontaneous emission (ASE) threshold drives the selection of this blend. ^{twenty two}In this case, we examine here the effect of oxidation on the quasi-cw DFB laser properties of EHCz:QiFu blends. The results provide clear evidence that the use of triplet quenchers such as molecular oxygen is very promising for the future realization of optically pumped cw organic semiconductor lasers. According to previously published in the literature ²⁵Seven-year-old derivatives were synthesized using the method in , and liquid carbazole and EHCz (Sigma-Aldrich) were purchased and used without further purification. EHCz, which is liquid at room temperature and exhibits a glass transition temperature well below 0°C, ¹⁷Mix it with Qifu in chloroform solution. Then the EHCz:Qifu (90:10 wt.%) blended solution is bubbled with oxygen or nitrogen for about 20 minutes. The gas was incorporated into the solution by using a needle with an inner diameter of 0.7 mm and a pressure of approximately 0.02 MPa. After complete evaporation of the solvent, the blend is used as a gain medium in a laser device. The device structure of the liquid DFB laser is schematically shown in Figure 5b. To fabricate these devices, ultraviolet (UV) curable polyurethane acrylate (PUA) mixtures were synthesized according to previously reported methods. ²⁶Corrugated polymeric DFB patterns are easily fabricated on polyethylene terephthalate (PET) substrates by replicating a silicon grating master with a PUA mixture. ²⁷Grating period for desired laser wavelength λ ΛPrague conditions must be met Λ=mλ/(2n _eff), where m is the number of orders and n _effis the effective refractive index of the guided mode. To achieve low-threshold laser operation, a first-order feedback corresponding to m = 1 is chosen, which results from the laser emission at the edge of the device. It is worth noting that the refractive index of the PUA film and the EHCz blend are approximately 1.54 and 1.7 respectively, which means a relative refractive index difference of 0.16. ^{twenty two}As shown in Figure 5c, the patterned corrugated structure on the PUA layer consists of a 1D grating with a period of 140 nm and a height of 100 nm. This grating period was chosen for first-order DFB laser operation with an emission wavelength of about 450 nm based on Bragg's formula and the emission spectrum of the blue-emitting hexafluoride derivative. The corrugated PUA layer was then covered with a molten silica substrate, and silica particles with a diameter of 1 μm were used to fix the gap distance between the PUA replica and the covering. The empty interstitial space is then filled with liquid gain medium via capillary action. To study its quasi-cw laser properties, a Ti-sapphire laser system (Millennia Prime, Spectra Physics) delivering optical pulses at 365 nm with a pulse width of 10 ps was used to optically nitride and oxidize EHCz: Qifu DFB laser. The repetition rate of optical excitation varies in the range of 0.01 MHz to 4 MHz. The spot area of the laser pump beam concentrated on the device is 1.9×10 ^{- 4}cm ². Emissions are detected from the edge of the device using a Hamamatsu frame eye (C10627) connected to a Hamamatsu digital camera (C9300). Qifu derivatives were previously used in solution-processed fluorescent organic light-emitting diodes (OLEDs) with external quantum efficiencies as high as 5.3%. ²⁸Such good electroluminescent performance can be achieved due to the horizontal orientation of the seven emitters in the 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) host. In another study, heptafluorine molecules were also incorporated into the EHCz host to demonstrate solvent-free liquid organic second-order DFB lasers operating in the blue region of the visible spectrum. ^{twenty two}For this purpose, a pulsed nitrogen laser (λ = 337 nm, pulse duration 800 ps and repetition rate 8 Hz) was used to optically pump the device and detect the laser output emission in the direction normal to the surface. Here, we use the same liquid composite material to create an edge-emitting first-order DFB laser. As shown in Figure 5d and Figure 5e, the blue laser emission detected from the edge of the nitrided and oxidized liquid DFB laser has peak wavelengths of 450 nm and 449 nm respectively. The minimal difference between the laser wavelengths of the two devices is presumably due to small changes in the thickness of the organic liquid layer. ²⁹Figures 6a and 6b show frame camera images of laser emission for two nitrided and oxidized solvent-free liquid organic DFB lasers at several repetition rates. For these measurements, the excitation intensity was held constant at 2.5 μJ cm ^{- 2}value. While the laser pulse emitted from the DFB laser can be clearly observed in the 100 μs time scale window, the time interval between pulses gradually decreases as the repetition rate increases. For the highest repetition rates of 1 MHz and 4 MHz, the DFB laser output emissions in Figures 6c and 6d appear to emit continuously during this time range, providing evidence that both nitrided and oxidized devices are in a quasi-cw state Operate appropriately. It is noteworthy, however, that the output intensity of oxidized devices in the quasi-cw state (especially at 4 MHz) is consistently found to be significantly higher than that of nitrided devices. ³⁰The laser output intensity and the full width at half maximum (FWHM) of the emission spectrum were plotted against the excitation intensity at different repetition rates in nitrided and oxidized solvent-free liquid organic DFB lasers (Figures 7 and 8). ³⁰The FWHM of the emission peak in both samples was found to decrease to 1.8 nm at high excitation densities, which was attributed to amplification by stimulated emission. This linewidth is higher than the resolution of a spectrometer of 0.7 nm. Looking at the curve showing output intensity versus excitation intensity, sudden changes in slope efficiency are directly related to the laser threshold. ^{29 , 31 - 34}Using this data, the laser threshold is then determined based on the repetition rate in both devices. The results in Figure 9a demonstrate that the laser threshold is lower and almost the same as with 2 μJ cm ^{- 2}The values are independent of the repeatability in the oxidized sample. Of concern, it was found that the laser threshold in the nitrided sample gradually increased from 2.8 μJ cm as the repetition rate of optical picosecond pulse excitation increased from 0.01 MHz to 4 MHz. ^{- 2}increased to 4.4 μJ cm ^{- 2}. A non-negligible overlap was observed between the triplet-triplet absorption spectrum of the hexafluoride molecule in chloroform solution and the representative laser spectrum of the gain material (Figure 10). ³⁰In fact, previous work reported that the stimulated emission cross section was seven times larger than the triplet absorption cross section at ASE/laser wavelengths. ¹⁰It is noteworthy that triplet-triplet absorption completely disappears in oxidized solutions due to the presence of molecular oxygen, which acts as triplet quencher. To provide additional evidence that molecular oxygen can efficiently quench triplet states in heptafluoride-based laser gain media, we next examined the mutual destruction of singlet-triplet excitons in liquid blended materials bubbled with oxygen or nitrogen. (STA) Quenching of singlet excitons. For this purpose, nitrided and oxidized gain material is sandwiched between two flat fused silica substrates. By 325 nm light pulses (with pulse duration varying from 50 μs to 800 μs) at 0.5 kW cm ^{- 2}The sample was irradiated with an excitation density of 100% and we monitored the temporal evolution of the photoluminescence intensity. ^{8 - 10}The transient curve in the nitrided sample shows that after the onset of optical pumping, the emission intensity significantly decreases by almost 60% after 300 μs before reaching its steady state (Figure 11). ³⁰These data demonstrate that singlet excitons are quenched by STA in nitrided liquid materials. ^{8 - 10}In contrast, oxidized liquid gain media did not exhibit such quenching and, in addition, did not show any signs of degradation under high-intensity cw irradiation of 800 μs. This and the previous research ¹⁰The results reported in are consistent and provide clear evidence that molecular oxygen can in fact be used to quench the triplet state without affecting the singlet state in materials based on hexafluoride. Suppression of singlet quenching by STA in oxidized samples is also consistent with the fact that the intensity of DFB laser emission appears to be stronger in oxidized devices than in nitrided devices. For these reasons, the highest threshold in nitrided DFB laser devices and the repetition rate dependence of this threshold can be attributed to the generation and accumulation of long-lived triplet excitons in the gain medium, which Resulting in additional losses associated with triplet absorption and singlet-triplet exciton mutual destruction. It is important to note that the liquid blend exhibits a high PLQY of 85% and a low ASE/laser threshold. In addition, the inter-system crossing yield is usually small (about 3%) in oligomeric and polyfluorine derivatives. ³⁵In this case, it is highly reasonable that the concentration of triplet states generated via intersystem crossing under optical pumping is kept low enough in the nitrided 7-F based gain material to be observed for repetition rates up to 4 MHz. Laser in accurate cw state. Importantly, the fact that the laser threshold in oxidized DFB devices becomes lower and independent of the repetition rate can be directly explained by the presence of molecular oxygen acting as a triplet quencher. The quasi-cw laser emission was also evaluated by monitoring the time evolution of the output intensity from the edge of the liquid layer for two nitrided and oxidized DFB lasers at repetition rates above 1 MHz. Photostability. The characteristic photostability time constant was estimated by measuring the duration associated with a 10% decrease in output intensity from its initial value. As shown in Figure 9b, the time constants for the nitrided and oxidized devices were found to be 4 minutes and 5 minutes respectively. This decrease in output laser intensity with time is probably attributed to the bleaching of the heptafluoride molecules. Of course, this photodegradation problem can be solved by using microfluidic circuits used to achieve true quasi-cw solvent-free liquid organic semiconductor laser technology. ^{twenty two}Of concern, despite the formation of a highly chemically reactive oxygen singlet state upon quenching of triplet excitons, the presence of oxygen does not lead to faster photodegradation of liquid devices. ³⁶It was shown that the photoluminescence intensity from the oxidized sample was at 0.5 kW cm ^{- 2}This is well supported by the nearly constant results after 800 μs under cw optical pumping with high excitation density. ³⁰In conclusion, we demonstrate that the use of oxygen as a triplet quencher is a promising approach to the development of continuous wave organic semiconductor laser technology. The gain medium used in our first-order organic DFB laser is based on a solvent-free liquid carbazole host doped with a blue fluorescent heptafluoride derivative. 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Here, we report a low-threshold surface-emitting organic dispersion feedback laser operating in a quasi-continuous wave regime at 80 MHz and under 30 ms of continuous wave optical excitation. This outstanding performance is achieved using an organic semiconductor film that combines with a mixed-order dispersed feedback grating to achieve a low laser threshold. The organic semiconductor film has high optical gain, high photoluminescence quantum yield and is insensitive at laser wavelengths. Triplet absorption losses. The simple encapsulation technology greatly reduces laser-induced thermal degradation and inhibits the erosion of the gain medium that otherwise occurs under severe continuous wave optical excitation. In summary, this study provides evidence that the development of true continuous wave organic semiconductor laser technology is possible through engineering modifications of the gain medium and device architecture. introductionOrganic semiconductor materials are generally considered to be well suited for photonics applications due to their ability to emit, modulate, and detect light ( 1). In particular, considerable research has been conducted over the past two decades due to its excellent characteristics in terms of low-cost fabrication, ease of processing, chemical versatility, mechanical flexibility, and wavelength tunability across the entire visible range. Work to use such organic semiconductor materials in optically pumped solid-state laser sources ( 2- 6). Since the first demonstration of optically pumped organic semiconductor lasers (OSL) ( 2), whose performance has been greatly improved due to significant advances in both high-gain organic semiconductor materials and device design ( 7 - 15). Recent developments in low-threshold distributed feedback (DFB) OSL demonstrate direct optical pumping of inorganic light-emitting diodes via electrically driven nanosecond pulses, providing a path towards new compact and low-cost visible laser technologies. path( 12 , 13). Applications based on these OSLs are currently emerging, including the development of spectroscopic tools, data communication devices, medical diagnostic equipment, and chemical sensors ( 16 , 20). Nonetheless, OSLs are optically pumped by pulsed optical excitation (with pulse width typically varying in the range of 100 fs to 10 ns) and driven at repetition rates (f) in the range of 10 Hz to 10 kHz. Under this situation, further breakthroughs are still needed to demonstrate optically pumped OSLs operating in the continuous wave (CW) regime and ultimately realize electrically pumped organic laser diodes ( twenty one , twenty two). Operating OSL in CW regime has proven challenging ( twenty three , twenty four). Thermal degradation of organic gain media under severe long-pulse optical pumping presents a serious problem for long-term laser operation ( 25). Another important issue to overcome concerns losses due to long-lived triplet excitons generated via intersystem crossings ( 26 - 29). When organic films are optically pumped in the long pulse regime, an accumulation of triplet excitons often occurs, leading to increased absorption at the laser wavelength due to triplet absorption (TA) and due to singlet-triplet Quenching of singlet excitons by mutual exciton destruction (STA). To overcome these obstacles, it has been proposed to incorporate triplet quenchers, such as oxygen ( 30 , 31), cyclooctatetraene ( 32) and anthracene derivatives ( 33). Another way to significantly reduce triplet losses is based on the use of emitters that exhibit high photoluminescence quantum yield (PLQY) and no spectral overlap between the absorption band of the triplet excited state and the emission band of the singlet excited state ( 34 - 36). Two methods of suppressing triplet losses in OSL have been successfully used to improve device performance in quasi-CW (qCW) regime ( 31 , 35). At the same time, a CW laser duration of nearly 100 μs can be achieved in OSL containing anthracene derivatives as triplet quenchers ( 33). In this paper, we propose an improved DFB OSL architecture that enables quasi-CW (qCW) lasers (at extremely high repetition rates of 80 MHz) and CW surface-emitting lasers with outstanding and unprecedented performance. These results represent a major development in the field of organic photonics and open new prospects towards the development of reliable and cost-effective organic CW solid-state laser technology. resultIn this study, the surface-emitting OSL was fabricated using the 4,4'-bis[( N -Carbazole) styryl]biphenyl (BSBCz) as emitter ( 34). Incorporation of triplet quenchers into BSBCz films is not necessary due to the extremely weak generation of triplet states via intersystem crossing and negligible triplet absorption at laser wavelengths in this material ( 35). The manufacturing method and structure of the organic semiconductor DFB laser produced in this study are schematically shown in Figure 12 and Figure 13A respectively. To achieve a low laser threshold with laser emission in the direction perpendicular to the substrate plane, we design a hybrid-order DFB grating architecture with a second-order Bragg scattering region surrounded by a first-order scattering region that induces strong feedback, thereby providing Efficient vertical extraction of laser radiation ( 8). In the DFB structure, laser oscillation occurs when the following Bragg conditions are met: mλ _Bragg=2 n _eff Λ( 5),in mis the diffraction order, λ _Braggis the Bragg wavelength, n _effis the effective refractive index of the gain medium and Λis the period of the grating. Use the ones reported for BSBCz n _effvalue and λ _Braggvalue( 37 - 39), mixed order ( m=1,2) The grating periods of the DFB laser device are calculated to be 140 nm and 280 nm respectively. Using electron beam lithography and reactive ion etching, these gratings were engraved directly to 5 × 5 mm ²area on the silicon dioxide surface. It should be noted that the optical simulations and experimental data reported in Figures 16 to 17 and Tables S1 to S3 (see Section A, Supplementary Material) are considered to select parameters for the resonator design. As shown by the scanning electron microscopy (SEM) images in Figures 13B to 13C, the DFB grating fabricated in this work has grating periods of 140±5 nm and 280±5 nm and a grating of approximately 65±5 nm. Depth, it meets our standards. The lengths of each first-order and second-order DFB grating are approximately 15.12 µm and 10.08 µm respectively. BSBCz pure films and BSBCz:CBP (6:94 wt.% and 20:80 wt.%) blended films with a thickness of 200 nm were prepared by vacuum deposition on top of the grating. As shown in Figures 13D to 13E, the surface morphology of the organic layer exhibits a grating structure with a surface modulation depth of 20 nm to 30 nm. To greatly improve the efficiency and stability of DFB lasers operating in qCW regime and long pulse regime, the device was then encapsulated in a nitrogen-filled glove box (40). For this purpose, 0.05 ml of CYTOP (a chemically stable, optically transparent fluoropolymer with a refractive index of approximately 1.35) was spin-coated directly on top of the organic layer, and the polymer film was then covered by a transparent sapphire cap. Sealed organic laser device, the sapphire cover was chosen because of its good thermal conductivity at the BSBCz laser wavelength (TC about 25 W m at 300 K ^{- 1}K ^{- 1}) and good transparency. CYTOP films typically have a thickness of about 2 µm and were found not to affect the photophysical properties of BSBCz films (Figure 18). The use of BSBCz pure films or BSBCz:CBP (6:94 wt.%) blends was first examined under pulsed optical pumping using a nitrogen laser delivering 800 ps pulses at a repetition rate of 20 Hz and a wavelength of 337 nm. Laser characteristics of encapsulated mixed-stage DFB devices with thin films as gain media (see Section B and Figure 19, Supplementary Material). In the case of CBP blended films, the excitation light is mainly absorbed by the CBP host, but the large spectral overlap between CBP emission and BSBCz absorption ensures efficient Förster-type energy transfer from host molecules to guest molecules ( 39). This was confirmed by the absence of CBP emission under 337 nm light excitation. Based on the results shown in Figure 19, it was found that the pure thin film device and the blended thin film device each exhibited 0.22 µJ cm in the 800 ps pulsed state. ^{− 2}and 0.09 µJ cm ^{− 2}The low laser threshold value. In both cases, these values are lower than previously reported for amplified spontaneous emission (ASE) in BSBCz:CBP blends (0.30 μJ cm ^{− 2})(39) and second-order DFB laser (0.22 μJ cm ^{− 2}) (35) reported threshold value, ( 35− 39) supports the possibility of hybrid-level gratings being used in high-efficiency organic solid-state lasers ( 8). Importantly, it was found that device encapsulation in this pulsed optical pump regime does not change the threshold value and laser wavelength of the mixed-order DFB laser. organic semiconductor DFB Laser hits accurately CW laserVarious BSBCz and BSBCz:CBP (6:94 wt.%) with different resonator structures were studied in the qCW regime for optical pumping using optical pulses with a wavelength of 365 nm and a width of 10 ps from a Ti-sapphire laser. Laser properties of DFB device. Figures 14A-14C show frame camera images of laser oscillations above threshold in a representative encapsulated blend mixing stage DFB device and the corresponding changes in emission intensity at different repetition rates. The excitation light intensity is fixed at approximately 0.5 µJ cm ^{− 2}. When the repetition rate of optical excitation is increased from 10 kHz to 80 MHz, the time interval between laser oscillations gradually decreases from 100 µs to 12.5 ns. For the highest repetition rates (>1 MHz), the DFB laser output emission appears continuous in the 500 µs window, indicating that the device is operating properly in the qCW state even at the highest repetition rate of 80 MHz. The possibility of operating a DFB device at these high repetition rates is clearly related to small TA losses and STA quenching originating from negligible triplet exciton formation in the BSBCz:CBP blend ( 35). Similar experiments were performed using non-encapsulated hybrid stage devices and second-stage DFB devices based on BSBCz pure films or blended films. For each device, the laser output intensity obtained at several repetition rates was measured based on the excitation intensity to determine the laser threshold, and for representative encapsulated blend mixing stages at repetition rates of 10 kHz and 80 MHz. The results of the DFB device are shown in Figure 20. The dependence of repetition rates on laser thresholds in different devices is summarized in Figure 14D. The laser threshold in 6 wt.% blended DFB lasers is essentially due to the close to 100% PLQY and the suppression of concentration quenching in this gain medium (e.g. compared to 76% PLQY in BSBCz pure films) value( E _th) is always lower ( 36). The results also show that the lowest threshold is obtained with a mixed-order DFB resonator structure. It is worth noting that when the repetition rate is increased from 10 kHz to 8 MHz, the laser threshold for all devices increases only very slightly. Due to the lack of significant triplet accumulation in the BSBCz system ( 35), we attribute the small increase in the threshold repetition rate to slight degradation of the device under high-intensity qCW irradiation (see Figure 21). Of concern, the encapsulated blend mixed-order DFB laser exhibits the lowest threshold (from 0.06 µJ cm at 10 kHz ²to 0.25 µJ cm at 80 MHz ²changes) and is the only device that operates properly at 80 MHz. When other devices are optically pumped at 80 MHz, the emission intensity decreases very rapidly and the FWHM value of the emission spectrum detected with a strip camera before rapid degradation of the organic film is usually larger, about 7 nm to 8 nm. (Figure 22). This situation indicates that encapsulation of DFB devices is necessary to significantly reduce degradation and that laser ablation of organic films presumably occurs under high-intensity 80 MHz light excitation. This reduction in device degradation due to encapsulation is presumably responsible for the decrease in laser threshold observed in Figure 14D. The operational stability of different blended DFB devices was studied under 8 MHz qCW optical pumping. Similar experiments were also conducted using an encapsulated mixed-order DFB laser at a repetition rate of 80 MHz. For each device, the time evolution of different DFB laser output intensities was monitored for 20 minutes using a pump intensity greater than 1.5 times the laser threshold (Figure 23). These results demonstrate that operational stability is improved when the laser threshold is reduced through grating structure and encapsulation choices. Higher pump intensities are required to achieve lasers in devices with higher thresholds, which results in faster laser-induced thermal degradation. More importantly, although none of the unencapsulated DFB devices operated well under qCW optical pumping at 80 MHz, the emission output intensity from the encapsulated organic laser decreased to only that of 96% of the initial value. This excellent operational stability emphasizes the critical role that encapsulation plays in the performance of organic semiconductor DFB lasers operating in the qCW regime. organic semiconductor DFB Reality in the laser CW laserThe variable stripe length method was used to study the amplified spontaneous emission (ASE) properties of 200 nm thick BSBCz:CBP (20:80wt.%) films to gain an understanding of the optical gain and loss coefficients under long pulse light irradiation. As shown in Figure 24 (see Table S4 and Section C in the Supplementary Material), the film using 50 μs long pulse optical pumping at 405 nm exhibits 1.5 kW cm ^{− 2}of pump intensity of 40 cm ^{− 1}high net gain coefficient and 3 cm ^{− 1}the loss coefficient. This clearly supports our idea that BSBCz is an excellent candidate for organic semiconductor lasers operating under long pulse light excitation. The laser characteristics of the DFB device in CW mode were then studied using an inorganic laser diode emitting at 405 nm. Since the absorption of CBP is negligible at this excitation wavelength ( 30), add the mixture to The concentration of BSBCz was increased to 20 wt.% to improve laser diode pump emission harvest. The PLQY of this 20 wt.% blend was measured to be approximately 86%. Figure 15A shows the frame camera integrated within 100 pulses of an encapsulated 20 wt.% blend hybrid stage DFB laser shot for CW excitation pulse widths of 800 µs and 30 ms respectively. at 200 W cm ^{− 2}and 2.0 kW cm ^{− 2}Measured at the pump intensity. The corresponding emission spectra in Figure 25 and the image in Figure 15B provide additional evidence that the encapsulated DFB laser operates properly in the long pulse regime, with laser durations that can be significantly extended to over 30 ms. Additional data in Figure 26 provide additional evidence of laser excitation with 30 ms long pulse light. As shown in Figure 27, when the number of consecutive 30 ms long excitation pulses is increased from 10 to 500, the DFB laser emission output intensity decreases, which is probably attributed to the thermal degradation of the gain medium under such severe irradiation. Although the encapsulation of the device between high thermal conductivity silicon and sapphire has significantly improved the performance and stability of OSL to unprecedented levels, this situation shows that the development of practical CW organic laser technology will still need to be completed in the future. Improved heat dissipation. Figure 27 also shows that quenching of singlet excitons by TA or STA does not occur in BSBCz (see Section D, Supplementary Material). The results confirm the negligible overlap between the emission of BSBCz and the triplet absorption of BSBCz and the absence of deleterious triplet losses in the gain medium even under severe CW light excitation ( 35). To identify the requirements for CW lasers, check the divergence of the emitted beam below and above the threshold and its polarization. The results shown in Figures 28 to 29 confirm that proper laser operation occurs in the BSBCz DFB device under long pulse light irradiation. The organic DFB laser output intensity and emission spectrum were measured based on the excitation intensity and various long pulse durations in the range of 0.1 µs to 1000 µs in devices with different structures. An example of data obtained from a representative encapsulated blend mixing stage device is shown in Figure 30. The sudden change in the slope efficiency of the laser output intensity is again used to determine the laser threshold. Figure 15C summarizes the pulse duration dependence of laser thresholds measured in different devices. Similar to the trend observed in the qCW regime, incorporating BSBCz into the CBP host, using a mixed-order DFB resonator structure and encapsulation device resulted in a substantial reduction in the laser threshold. Encapsulated blended mixed-stage organic DFB lasers exhibit the lowest laser threshold while encapsulated mixed-stage DFB devices based on BSBCz pure films can operate properly in long pulse regimes for durations longer than 100 µs value (at 5 W cm ^{− 2}to 75 W cm ^{− 2}range) and is the only device that can effectively generate laser radiation for a duration longer than 800 µs. To provide additional evidence of the critical role played by the choice of high TC sapphire as the encapsulation cap on the performance of organic semiconductor lasers in the long pulse regime, we compared the performance of mixed-level blended DFBs encapsulated with sapphire caps or glass caps. The dependence of the excitation duration on the laser threshold obtained in the device. Figure 31 clearly demonstrates that using a high TC cover made of sapphire results in lower thresholds and improved operational stability. The operational stability of encapsulated or unencapsulated mixed-stage DFB lasers in the long-pulse regime is characterized by monitoring in such devices greater than 200 W cm ^{− 2}The laser emission output intensity of the laser threshold value changes with the number of 100 µs excitation pulses of the pump intensity. As shown in Figure 15D, in all devices, the emission intensity gradually decreased over time, and these decreases were irreversible, indicating laser-induced thermal degradation of the organic gain medium. It is noteworthy that operational stability is greatly improved by encapsulation and is clearly optimal for encapsulated blending devices. In the latter case, after 500 pulses, the laser output intensity decreased by only 3%. Figure 32 shows the performance of an unencapsulated hybrid hybrid-stage DFB laser with a width of 1 ms and 200 W cm ^{− 2}Laser microscope images before and after 100 incident pulses of excitation intensity. Although no signs of laser-induced thermal degradation were observed in the encapsulated devices, laser ablation occurred in the unencapsulated devices with an ablation depth of approximately 125 nm. Significantly reducing the possibility of laser ablation through the proposed encapsulation technology is obviously key to the future development of CW organic semiconductor laser technology. To draw conclusions on how to limit existing devices in terms of practical CW operation, thermal simulations of heat dissipation in the device were performed and reported in Figures 38 to 42 (see Table S4 and Section E, Supplementary Material). These results demonstrate the effect of pump pulse width and the effect of encapsulation on the thermal properties of the device. Specifically, although encapsulation was considered an important element in this study, simulations suggest that CYTOP should be replaced by another material with better thermal conductivity in further studies. DiscussInorganic CW solid-state lasers were first demonstrated about 40 years ago (41) and their development has proven to be extremely successful, especially at wavelengths in the near-infrared and ultraviolet/blue regions of the electromagnetic spectrum ( 42 - 45). Although these devices typically require cutting-edge microfabrication techniques with high vacuum and temperature conditions, it has recently been demonstrated that solution-processed inorganic quantum wells can also be used to achieve CW laser ( 46). On the other hand, the performance of organic semiconductor lasers in qCW and long pulse conditions has so far remained much lower than that of inorganic semiconductors ( 33 , 35). Therefore, our demonstration of an organic semiconductor laser operating in the qCW state at 80 MHz and still operating in the long pulse state after 500 consecutive pulses of 30 ms represents an important step towards the development of practical CW organic solid-state laser technology. Progress. This study strongly supports the fact that organic laser materials with high PLQY, high optical gain, and no spectral overlap between the laser emission peak and the TA band are useful for containing triplet losses and achieving this when combined with mixed-order DFB gratings. Low threshold CW lasers are highly desirable. The results are also shown using thermal conductivity ( 47) The mutual thermal conductivity of silicon encapsulation and sapphire encapsulation, which is higher than that of conventional glass and molten silica, significantly improves the efficiency and stability of organic DFB lasers. However, under severe CW optical pumping, organic Laser-induced thermal degradation of gain media remains the most serious problem that will need to be overcome in the near future. Therefore, given the potential development of the aforementioned methods for improving thermal management in CW inorganic solid-state lasers, further research into greatly enhancing the operational stability of CW organic semiconductor lasers should now focus on having low CW laser thresholds and Research and development of organic semiconductor gain media with enhanced thermal stability and focus on integrating efficient heat dissipation systems into devices ( 48 , 49). Furthermore, in addition to the discovery of better and more efficient gain materials, further optimization of resonator geometries and laser structures should lead to lower laser thresholds and should still represent the development of CW organic laser technology and electrical pumping The realization of organic laser diodes is an important future direction. Materials and methods Device manufacturingThe silicon substrate covered with a thermally grown silicon dioxide layer of 1 μm thickness was cleaned by ultrasonic treatment using alkaline cleaner, pure water, acetone and isopropyl alcohol, followed by UV ozone treatment. The silica surface was treated with hexamethyldisilazane (HMDS) by spin coating at 4000 rpm for 15 s and annealed at 120 °C for 120 s. A resist layer with a thickness of approximately 70 nm was spin-coated on the substrate from a ZEP520A-7 solution (ZEON Co.) at 4000 rpm for 30 s and baked at 180°C for 240 s. Use a 0.1 nC cm ^{− 2}A JBX-5500SC system (JEOL) with optimized dosage was used for electron beam lithography to draw the grating pattern on the resist layer. After electron beam irradiation, the pattern was developed in a developer solution (ZED-N50, ZEON Co.) at room temperature. The patterned resist layer was used as an etch mask, while an EIS-200ERT etch system (ELIONIX) was used with CHF ₃Plasma etching the substrate. To completely remove the resist layer from the substrate, use a FA-1EA etching system (SAMCO) with O ₂Plasma etching the substrate. The grating formed on the silicon dioxide surface was observed using SEM (SU8000, Hitachi). To complete the laser device, by ^{− 4}Thermal evaporation under a pressure of Pa is 0.1 nm s ^{− 1}to 0.2 nm s ^{− 1}The total evaporation rate is used to prepare 200 nm thick 6 wt% or 20 wt% BSBCz:CBP blended films and BSBCz pure films on the grating. Finally, 0.05 ml of CYTOP (Asahi Glass Co., Ltd., Japan) was directly spin-coated onto the DFB laser device at 1000 rpm for 30 s, sandwiched with a sapphire cover to seal the top of the laser device, and dried in a vacuum overnight. Spectral measurementIn order to characterize the pulsed organic laser, the pulsed excitation light from the nitrogen laser (USHO, KEN-2020) was concentrated on 6×10 of the device through the lens and slit. ^{− 3}cm ²area. The excitation wavelength is 337 nm, the pulse width is 0.8 ns, and the repetition rate is 20 Hz. The excitation light is incident on the device at an angle of approximately 20° relative to the normal line of the device plane. The emitted light perpendicular to the device surface was collected using an optical fiber connected to a multichannel spectrometer (PMA-50, Hamamatsu Photonics) and placed 3 cm away from the device. Use a set of neutral density filters to control excitation intensity. For qCW operation, a mode-locked frequency doubled Ti-sapphire laser (Millennia Prime, Spectra physics) was used to generate excitation light with an excitation wavelength of 365 nm, a pulse width of 10 ps, and a repetition rate ranging from 0.01 MHz to 80 MHz. . The excitation light is concentrated on 1.9×10 of the device through lenses and slits. ^{− 4}cm ²area, and the emitted light was collected using a frame eye (C10627, Hamamatsu Photonics) with a time resolution of 15 ps connected to a digital camera (C9300, Hamamatsu Photonics). For real CW operation, a CW laser diode (NICHIYA, NDV7375E, maximum power 1400 mW) was used to generate excitation light with an excitation wavelength of 405 nm. In these measurements, an acousto-optic modulator (AOM, Gooch & Housego) triggered by a pulse generator (WF 1974, NF Co.) was used to deliver pulses. The excitation light is concentrated on 4.5×10 of the device through lenses and slits. ^{− 5}cm ²area, and the emitted light was collected using a frame eye (C7700, Hamamatsu Photonics) with a time resolution of 100 ps connected to a digital camera (C9300, Hamamatsu Photonics). A photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics) was used to record the emission intensity. 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Much effort has been focused on reducing the energy threshold of optically pumped organic lasers by enhancing the properties of the gain medium [2], [3] and optimizing the resonant cavity [4], [5], [6]. Given that achieving electrically pumped organic lasers is not yet achievable, more optimization is needed to further lower the energy threshold. Regarding resonant cavities, there are several types compatible with organic gain media, including distributed feedback (DFB) resonators [7], [8], distributed Bragg resonators (DBR) [9], microrings [10], Microdisk [11] and microsphere cavity [12]. The function of the resonator is to provide positive optical feedback in addition to the optical amplification provided by the gain medium. The laser architecture used in current state-of-the-art organic lasers is based on DFB resonators [5], [4], [13]. These resonators do not use conventional cavity mirrors, but instead use periodic nanostructures responsible for Bragg diffraction. DFB resonators are compact and can be easily integrated into planar organic films. In addition, it provides a higher degree of spectral selection. The laser structure studied in this work consists of an organic thin film deposited on a 2-order DFB grating. In this type of grating, light generated by the gain medium is waveguided along a high refractive index organic film and then scattered by periodic structuring. Optical feedback occurs due to the coupling between forward propagating waves and backward propagating waves [14]. This coupling is the maximum at a specific wavelength that satisfies the following Bragg condition: (1) in mis the diffraction angle, λ _Bragg is the resonance wavelength in the cavity, _neff is the effective refractive index of the uniform waveguide, and Λ is the grating period. In the second-order grating ( m=2), the first-order diffracted light is extracted vertically from the surface of the film, and the coplanar feedback is provided by the second-order diffraction. According to the coupling mode theory, wavelengths that satisfy the Bragg condition (1) are not allowed to propagate in the film [15]. This is due to periodic modulation of the refractive index, which results in the appearance of a photon stop band concentrated at the Bragg wavelength. Therefore, in λ _Bragg Below, a sudden drop in emission is observed and laser oscillation appears at a pair of wavelengths located on the edge of the stop band. In second-order gratings, the laser oscillations are only at one edge of the stop band (at the highest wavelength). At this wavelength, the threshold is lower due to lower radiation losses [16]. The resonant cavity affects laser performance through two parameters: limiting factor Γand quality factor Q. The exciton density at the laser threshold and Γand QThe two are inversely proportional [17]. Therefore, the optimization of the geometry of the DFB resonant cavity is crucial to reducing the loss, which can be achieved by Γand QQuantitative. The goal of this work is to study the effect of organic film thickness on laser performance (including energy threshold and laser wavelength). First, the laser is designed to be fixed. This step is accomplished by calculating the effective refractive index of the waveguide structure in order to deduce the grating period required to obtain laser light at the ASE wavelength. The thickness of the organic film was changed from 100 nm to 300 nm, and the effective refractive index at each thickness was calculated. Second, to gain a physical understanding of how the laser threshold energy changes with thickness, optical simulations are performed. The quality factor and limiting factor of the resonant cavity are calculated based on the film thickness and compared with the experimental energy threshold of the organic laser device. 2 . Device structure and simulation detailsThe geometry of the grating-coupled waveguide that constitutes the second-order DFB organic laser studied in this work is depicted in Figure 33. The waveguide structure consists of a gain medium (6%wt BSBCz:CBP), which is composed of SiO with a lower refractive index. ₂It is composed of a grating and a high refractive index layer surrounded by air. The gain medium consists of a 6wt% BSBCz:CBP blend film deposited in vacuum on a 2nd order DFB grating. Gratings are fabricated on SiO by electron beam lithography ₂on the substrate. The fabrication of DFB lasers is described elsewhere [4]. The input parameters used for the simulation are the layer thickness and refractive index. think air ( n _a =1) and SiO ₂substrate ( n _s =1.46) is a semi-infinite layer. Consider the refractive index of 6wt% BSBCz:CBP blend _f is equal to the refractive index of CBP reported in [18] ( _f About 1.8). The thickness of the organic film changes from 100 nm to 300 nm. The structure of the laser is designed so that the laser oscillates at the amplified spontaneous emission (ASE) wavelength of BSB-Cz (approximately 477 nm) [19], [20]. Simulation software :Use self-made python 3.5 software instruction code to perform effective refractive index calculation and Fano fitting The finite element method in the RF module of Comsol 5.2a software is used to extract the quality factor and limiting factor from the calculation of the eigenvalues of the resonant cavity mode. 3. Results and discussion 3.1 Waveguide characterization ( Effective refractive index calculation )In order to calculate the grating period using the Bragg condition (Equation 1), the effective refractive index of the uniform waveguide (without grating) is required _neff . In this model, the grating is ignored, so the waveguide thickness is the thickness of the organic film. The effective refractive index is calculated by solving the propagating wave equation [21] based on the thickness of the organic film at a wavelength of 477 nm. _neff value. In this calculation, we consider the asymmetric waveguide to have no grating (Fig. 34(a)). In the case of an asymmetric 3-layer thick block waveguide, the electric field in each region is given by: in: in k ₀ For the vacuum propagation constant mode ,and βis the propagation constant of the boot mode . The effective refractive index of the waveguide mode is calculated from the transcendental equation obtained after applying the following boundary conditions: for TE mode (6) for TM mode (7) Figure 34(b) presents the waveguide dispersion curve derived from Equations 6 and 7, which shows the effective refractive index as a function of organic film thickness at a laser wavelength of 477 nm. From these curves, one can infer the number of propagation modes at a given thickness and the cutoff thickness for a specific propagation mode. In this work, the thickness was chosen to change from 100 nm to 300 nm. For thicknesses below 280 nm, only the fundamental mode TE is allowed ₀oscillation. Increasing thickness above 280 nm results in higher order (TE ₁,TE ₂)exist. Once the effective refractive index is calculated, one can deduce the grating period at λ using the Bragg condition (Eq. 1) at different film thicknesses. _ASE= value at 477 nm. For a 200 nm film thickness, _neff =1.7. The value of the grating period Λ that satisfies the Bragg condition (Equation 1) is 280 nm. In the following, we fix the grating period to 280 nm and the grating depth to 70 nm thickness. The thickness of the organic film only changes from 100 nm to 300 nm. 3.2 DFB Resonance cavity optimizationThe resonant cavity is described by its photon lifetime and limiting factor. Photon lifetime τ represents the time a photon spends in the cavity (the rate at which photons are lost in the cavity). Photons can be lost by escaping the cavity or by being absorbed by the material. This photon lifetime τ is related to the quality factor Q of the cavity as follows: where ω ₀is the resonant angular frequency. The Q-factor of an optical cavity is calculated in two different ways. (1) Eigenmode calculationIn the first method, the finite element method in the RF module of the Comsol software is used to extract the quality factor from the calculation of the eigenvalues of the resonant cavity modes. The computational domain is limited to one period unit cell of the grating. Floquet periodic boundary conditions are applied to the lateral boundaries, and scattering boundary conditions are used in the top and bottom domains [22], [23]. The natural frequency solver is used to find the propagating eigenmodes of the resonant cavity. According to the real part and imaginary part of the eigenvalue, the Q factor is derived: in αfor damped decay . Additionally, the limiting factor of the eigenmode is calculated using the following expression: in E _norm is the normalized electric field intensity distribution of the eigenmode. (2) Fano fitting of reflectance spectrumThe second method for extracting the figure of merit consists of calculating the reflection spectrum using a scattering matrix implemented in Comsol software for normally incident TE polarized plane waves (whose electric field is parallel to the grating) [ref]. Subsequently, the Q factor (Eq. 8) is obtained by fitting the resonance linewidth present in the simulated reflection spectrum with the following Fano resonance equation [24]: in ω ₀ is the center frequency, τ is the resonance life, rand thas the same thickness and effective refractive index as the grating _neff,g The amplitude reflection and transmission coefficients of a uniform thick block. In the case of binary gratings, the effective refractive index can be described using the following effective medium theory [25]: in ffis defined as the raster width wRatio to period Λ. Figure 35 shows the calculated reflection spectra as a function of wavelength and film thickness and the corresponding fitted Fano resonance curve using Equation 11. For cavities with film thicknesses of 100, 150, 200, 250 and 300 nm, reflection peaks at wavelengths of 448, 462, 472, 478 and 483 nm were observed, respectively. At these wavelengths, resonance occurs due to phase matching between waves diffracted by grating and leaky waveguide modes [26], [27]. Therefore, multiple reflections occur in the waveguide and the wavelength of the incident light is selected by the resonance of the waveguide grating. As confirmed by the calculations presented in Section 3.1 and by previously reported work [28], _f The increase makes the modal _neff increases (Fig. 34(b)), which causes tuning of the laser wavelength. As we can see in Figure 36(a), _f The increase causes the spectral red shift of laser emission. The experimental laser wavelength is consistent with the Fano model and the model indicated in Chapter 3.1. _f + h _g A comparison of the calculated laser wavelengths for the model is presented in Figure 36(b), where h _g Refers to the depth of the grating. Both models provide approximately the same results, close to experimental values, but at smaller _f (<200 nm), the gap between the experimental wavelength and the calculated wavelength is still significant (Δλ>10 nm). It is reported that when the ratio h _g / _f When exceeding 0.3 [28], in _f At about 200 nm and below, exponential coupling is the dominant mechanism. When exponential coupling is more dominant than gain coupling, the laser will not λ _Bragg appear nearby. Therefore, the deviation between the experimental laser wavelength and the calculated laser wavelength can be determined by focusing on the wavelength below 200 nm. _f Explained by the dominance of exponential coupling. Figure 37(a) shows the calculated Qfactors and Γvalue. used for calculation QBoth methods of factoring give the same result. It can be seen that ΓWith _f increase, demonstrating good optical limits. This system is attributed to the fundamental harmonic mode TE ₀Of _neff increase. However, the resonance cavity Qfactor between 200 nm _f value becomes the highest. different _f The measured energy threshold value E _th Presented in Figure 37(a). we can observe Qfactor and E _th Inversely proportional. Furthermore, when _f When increasing from 100 nm to 200 nm, E _th decrease. This is attributed to Qfactors and ΓThe increase of both. Between 200 nm _f value, E _th show the minimum value, followed by _f Increase. Larger _f higher E _th This is due to the lower resonant cavity Qfactor. Finally, the full width at half maximum (FWHM) of the peak reflection extracted from the calculation and Fano fitting was compared with the FWHM of the experimental laser emission [Figure 37(b)]. Both experimental FWHM values and calculated FWHM values are shown and are for a wavelength equal to 200 nm. _f The minimum values obtained follow the same trend. 3.3 Use the encapsulation DFB Laser OptimizationIn this section, CYTOP is used to calculate the Γand Qfactor. The input parameters used for the optical simulation are the thickness of the organic film and the refractive index of the layer. Think CYTOP ( n _CYTOP=1.35) and SiO ₂substrate ( n _SiO2=1.46) is a semi-infinite layer. Consider 6wt% BSBCz: refractive index n of CBP film _fis equal to the reported refractive index of CBP ( _f =1.85) ( 1). BSBCz: Thickness of CBP film d ₀ From 100 nm to 300 nm. Due to the structured top surface, the thickness ( h _g - h _{g ( top )})/2=30 nm thin layer with depth h _{g ( top )}=5 nm thin grating. 3.3.1 Film thickness changesFirst, we calculate Γand Qfactor to make the raster depth h _gremain constant ( h _g=65 nm) while studying the film thickness d ₀ the effect of change. Table S1 shows the calculation results. surface S1 .Film thickness, resonance wavelength, quality factor and limiting factor. d ₀ (nm) λ ₀ (nm) Q factor Γ 100 465 717 0.34 200 481 5050 0.78 300 494 6674 0.88 When the thickness increases, Γand Qfactor increases, but due to the resonant wavelength λ ₀ASE wavelength shift of self-gaining materials, between 200 nm d ₀ Maintain optimal thickness for device operation. 3.3.2 Raster depth changesSecondly, we are calculating Γand Qfactor makes d ₀ remain constant ( d=200 nm) simultaneous study h _gThe effect of change. Table S2 below shows the calculation results. surface S2 .Grating depth, resonance wavelength, quality factor and limiting factor. h _g (nm) λ ₀ (nm) Q factor Γ 30 481 8026 0.79 65 481 5050 0.78 80 483 1915 0.74 By reducing the grating depth, Qfactor increases ΓStill pretty much the same. However, the fabrication of shallow gratings is challenging because small changes in depth will greatly affect the optical response of the grating. Although this aspect will certainly be improved upon in future work, a depth of 65 nm seemed to be the most appropriate for this study. 3.3.3 Comparison between encapsulated and non-encapsulated devicesCalculations were performed using the same geometry. In the encapsulated case, the top layer was CYTOP with a refractive index of 1.35. Without encapsulation, CYTOP is replaced by air ( n=1). In this case, Qfactors and Γincreased and the resonance wavelength was slightly blue-shifted, as shown in Table S3. surface S3 .Comparison of resonance wavelength, quality factor, and limiting factor between encapsulated and non-encapsulated devices. λ ₀ (nm) Q factor Γ encapsulated 481.2 5050 0.78 Unencapsulated 479 6455 0.82 However, based on experimental results, the encapsulated devices demonstrated better performance (FWHM) than the non-encapsulated devices. This could be attributed to changes in the top surface when we encapsulated the device or to protection from moisture. 3.3.4 2 Effect of size of step grating areaDifferent sizes of the 2nd-order region were used to experimentally determine the laser threshold for mixed-order DFB lasers of BSBCz:CBP (6:94wt.%) blends. The results are shown in Figure 17. It can be seen that the DFB architecture used for this study (which corresponds to a number of cycles equal to 36) is not fully optimized, indicating that further improvements in device performance should be possible by playing only on the resonator structure. References1. C. Ge, M. Lu, X. Jian, Y. Tan, and B. T. Cunningham, “Large-area organic distributed feedback laser fabricated by nanoreplica molding and horizontal dipping,” vol. 18, no. 12, pp. 12980 -12991, 2010. 2. H. Nakanotani, S. Akiyama, D. Ohnishi, M. Moriwake, M. Yahiro, T. Yoshihara, S. Tobita, and C. Adachi, “Extremely low-threshold amplified spontaneous emission of 9,9-disubstituted- spirobifluorene derivatives and electroluminescence from field-effect transistor structure," Adv. Funct. Mater., vol. 17, no. 14, pp. 2328-2335, 2007. 3. G. Tsiminis, Y. Wang, P. E. Shaw, A. L. Kanibolotsky, I. F. Perepichka, M. D. Dawson, P. J. Skabara, G. A. Turnbull, and I. D. W. Samuel, “Low-threshold organic laser based on an oligofluorene truxene with low optical losses,” pp . 3-5, 2009. 4. A. S. D. Sandanayaka, K. Yoshida, M. Inoue, C. Qin, K. Goushi, J.-C. Ribierre, T. Matsushima, and C. Adachi, “Quasi-Continuous-Wave Organic Thin-Film Distributed Feedback Laser ," Adv. Opt. Mater., vol. 4, no. 6, pp. 834-839, Jun. 2016. 5. E. R. Martins, Y. Wang, A. L. Kanibolotsky, P. J. Skabara, G. A. Turnbull, and I. D. W. Samuel, “Low-Threshold Nanoimprinted Lasers Using Substructured Gratings for Control of Distributed Feedback,” Adv. Opt. Mater., vol. 1, no. 8, pp. 563-566, 2013. 6. V. Qaradaghi, V. Ahmadi, and G. Abaeiani, “Design of organic vertical-cavity surface-emitting laser for electrical pumping,” IEEE Electron Device Lett., vol. 33, no. 11, pp. 1616-1618, 2012. 7. M. D. McGehee, M. A. Díaz-García, F. Hide, R. Gupta, E. K. Miller, D. Moses, and A. J. Heeger, “Semiconducting polymer distributed feedback lasers,” Appl. Phys. Lett., vol. 72, no. 13, pp. 1536-1538, 1998. 8. G. Heliotis, R. Xia, D. D. C. Bradley, G. A. Turnbull, I. D. W. Samuel, P. Andrew, and W. L. Barnes, “Blue, surface-emitting, distributed feedback polyfluorene lasers,” Appl. Phys. Lett., vol. 83, no. 11, pp. 2118-2120, 2003. 9. A. E. Vasdekis, G. Tsiminis, J.-C. Ribierre, L. O' Faolain, T. F. Krauss, G. A. Turnbull, and I. D. W. Samuel, “Diode pumped distributed Bragg reflector lasers based on a dye-to-polymer energy transfer blend .," Opt. Express, vol. 14, no. 20, pp. 9211-9216, 2006. 10. R. Osterbacka, M. Wohlgenannt, M. Shkunov, D. Chinn, and Z. V. Vardeny, “Excitons, polarons, and laser action in poly(p-phenylene vinylene) films,” J. Chem. Phys., vol. 118, no. 19, pp. 8905-8916, 2003. 11. C. X. Sheng, R. C. Polson, Z. V. Vardeny, and D. A. Chinn, “Studies of pi-conjugated polymer coupled microlasers,” Synth. Met., vol. 135-136, no. April, pp. 147-149, 2003. 12. M. Berggren, A. Dodabalapur, Z. N. Bao, and R. E. Slusher, “Solid-state droplet laser made from an organic blend with a conjugated polymer emitter,” Adv. Mater., vol. 9, no. 12, pp. 968-971, 1997. 13. G. Tsiminis, Y. Wang, A. L. Kanibolotsky, A. R. Inigo, P. J. Skabara, I. D. W. Samuel, and G. A. Turnbull, “Nanoimprinted organic semiconductor laser pumped by a light-emitting diode,” Adv. Mater., vol. 25, no. 20, pp. 2826-2830, 2013. 14. I. D. W. Samuel and G. a Turnbull, “Organic semiconductor lasers.,” Chem. Rev., vol. 107, no. 4, pp. 1272-1295, 2007. 15. H. Kogelnik and C. V. Shank, “Coupled-wave theory of distributed feedback lasers,” J. Appl. Phys., vol. 43, no. 5, pp. 2327-2335, 1972. 16. R. F. Kazarinov and C. H. Henry, “Second-Order Distributed Feedback Lasers with Mode Selection Provided by First-Order Radiation Losses,” IEEE J. Quantum Electron., vol. 21, no. 2, pp. 144-150, 1985. 17. S. Schols, Device Architecture and Materials for Organic Light-Emitting Devices. Springer, 2011. 18. D. Yokoyama, A. Sakaguchi, M. Suzuki, and C. Adachi, “Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films,” Org. Electron., vol. 10, no. 1, pp. 127-137, 2009. 19. J. Chang, Y. Huang, P. Chen, R. Kao, X. Lai, C. Chen, and C. Lee, “Reduced threshold of optically pumped amplified spontaneous emission and narrow line-width electroluminescence at cutoff wavelength from bilayer organic waveguide devices,” vol. 23, no. 11, pp. 67-74, 2015. 20. M. Inoue, T. Matsushima, and C. Adachi, “Low amplified spontaneous emission threshold and suppression of electroluminescence efficiency roll-off in layers doped with ter(9,9’-spirobifluorene),” Appl. Phys. Lett., vol. 108, no. 13, 2016. 21. A. K. Sheridan, G. A. Turnbull, A. N. Safonov, and I. D. W. Samuel, “Tuneability of amplified spontaneous emission through control of the waveguide-mode structure in conjugated polymer films,” vol. 62, no. 18, pp. 929-932, 2000 . 22. J.-H. Hu, Y.-Q. Huang, X.-M. Ren, X.-F. Duan, Y.-H. Li, Q. Wang, X. Zhang, and J. Wang, “Modeling of Fano Resonance in High-Contrast Resonant Grating Structures,” Chinese Phys. Lett., vol. 31, no. 6, p. 64205, Jun. 2014. 23. T. Zhai, X. Zhang, and Z. Pang, “Polymer laser based on active waveguide grating structures.,” Opt. Express, vol. 19, no. 7, pp. 6487-6492, 2011. 24. A. E. Miroshnichenko and Y. S. Kivshar, “Fano resonances in nanoscale structures,” vol. 82, no. September, pp. 2257-2298, 2010. 25. M. Foldyna, R. Ossikovski, A. De Martino, B. Drevillon, E. Polytechnique, and P. Cedex, “Effective medium approximation of anisotropic lamellar nanogratings based on Fourier factorization,” vol. 14, no. 8, pp. 3055-3067, 2006. 26. S. S. Wang and R. Magnusson, “Theory and applications of guided-mode resonance filters.,” Appl. Opt., vol. 32, no. 14, pp. 2606-2613, 1993. 27. T. Khaleque and R. Magnusson, “Light management through guided-mode resonances in thin-film silicon solar cells,” J. Nanophotonics, vol. 8, no. 1, p. 83995, 2014. 28. V. Navarro-Fuster, I. Vragovic, E. M. Calzado, P. G. Boj, J. A. Quintana, J. M. Villalvilla, A. Retolaza, A. Juarros, D. Otaduy, S. Merino and M. A. Díaz-García, “Film thickness and grating Depth variation in organic second-order distributed feedback lasers,” J. Appl. Phys. Vol.112,no.4, pp. 43104, 2012. Chapter B . Mixed order DFB Laser properties of the deviceThe use of BSBCz pure films or BSBCz:CBP (6:94 wt.%) blended films was examined under pulsed optical pumping with a nitrogen laser delivering 800 ps pulses at a repetition rate of 20 Hz and a wavelength of 337 nm. Laser properties of encapsulated mixed-stage DFB devices as gain media. In the case of CBP blended films, the excitation light is mainly absorbed by the CBP host. However, the large spectral overlap between CBP emission and BSBCz absorption ensures efficient Förster-type energy transfer from host molecules to guest molecules ( 2- 6). This was confirmed by the absence of CBP emission under 337 nm light excitation. Figures 19A to 19E show emission spectra collected perpendicular to the surface of the BSBCz film and the BSBCz:CBP (6:94 wt.%) film at different excitation intensities below and above the threshold. Under low excitation intensity, corresponding to the DFB grating ( 2) of the photoresist band. The small change in the Bragg dip position is probably due to the slightly different refractive index of the blended and pure films ( 2- 6). As the pump intensity increases above the critical threshold, a narrow emission peak appears in both pure and blended devices, indicating the onset of lasing. It can also be seen that the intensity of the laser peak increases faster than the photoluminescence background, providing evidence of nonlinearity associated with stimulated emission. The laser wavelength was found to be 484 nm for the blended film and 481 nm for the pure film. Figures 19C to 19D show the output emission intensity and full width at half maximum (FWHM) of the two DFB devices as a function of pump intensity. The FWHM was found to become lower than 0.2 nm at high excitation intensity. The laser threshold value of DFB laser is determined by the sudden change in output intensity. It was found that devices based on pure films and blended films exhibit 0.22 μJ cm respectively. ^{− 2}and 0.09 μJ cm ^{− 2}The laser threshold value. In both cases, these values are below previously reported thresholds for amplified spontaneous emission (ASE) and second-order DFB lasers in BSBCz:CBP blends ( 2- 6), supporting the possibility of using mixed-order gratings for high-performance organic solid-state lasers. Chapter C . optical gainBased on these experimental ASE data, the net gain and loss coefficients can be determined and their values are listed in Table S4. surface S4 .Pulse width, excitation power, net gain and loss coefficient. Pulse width(μs) Power (kW cm ^-2 ) Net gain (cm ^-1 ) Loss coefficient (cm ^-1 ) 0.5 8.1 0.1 1.0 11.3 1.5 1.5 19.8 0.5 13.9 10.0 1.0 17.0 2.2 1.5 32.6 0.5 25.1 50.0 1.0 30.8 3.4 1.5 40.1 These ASE results provide clear evidence that large net optical gains can be achieved in BSBCz-based films in the CW regime. Therefore, this clearly supports our statement that BSBCz is one of the best candidates for CW lasers and quasi-CW lasers. Chapter D . transient absorptionThe results in Figure 27A indicate that the PL intensity remains constant after several μs of irradiation. This implies that there is no quenching of singlet excitons by STA in the device. Figure 27C also shows that there is no significant spectral overlap between laser and triplet absorption spectra. These results provide clear evidence that there is no deleterious triplet loss in the gain medium used in this study. Based on this information, we also estimate the stimulated emission cross-section as previously reported σ _emand triplet excited state cross section σ _TT( 3 , 9). Below 480 nm σ _emis 2.2×10 ^{− 16}cm ², which is significantly larger than 3.0×10 ^{− 19}cm ²Of σ _TT, indicating that triplet absorption has little effect on the long pulse state. We separately estimate the triplet lifetime in solution ( τ _TT), triplet absorption cross section ( σ _TT) and inter-system crossover yield ( ϕ _ISC), τ _TT=5.7×10 ³s ^-1, σ _TT=3.89×10 ^-17cm ²(at 630 nm, Figure 27D) and ϕ _ISC=0.04. Estimated by comparing the excitation power dependence of the transient absorption with benzophenone as a reference (Figure 27E) ϕ _ISC( 9). However, it is important to note that using our transient absorption measurement system, we were unable to observe any triplet specific gravity in the film. For example, due to ϕ _PLThe value is close to 100%, and the crossover between systems in the blended film is negligible. Overall, it has been measured to be higher than E _thThe emission spectrum does not largely overlap with the triplet absorption spectrum, resulting in a larger net gain in light amplification in the long pulse state. Therefore, we are convinced that BSBCz is one of the best candidates for CW lasers and quasi-CW lasers. Chapter E . Thermal simulationIn order to detect the temperature distribution within the device, COMSOL 5.2a was used to perform a transient 2D heat transfer simulation. Figure 38 shows a schematic diagram of the geometric structure of the laser device. It should be noted that we ignored the grating in this simulation. The governing local difference equation of the temperature distribution is expressed as: in ρis the material density, C _pis the specific heat capacity, Tis the temperature, tfor time, kis the thermal conductivity and Qis the laser heat source item. The laser pump beam has a Gaussian shape. Due to the circular symmetry of the laser beam, the heat transfer equation is solved in cylindrical coordinates. For a pulsed Gaussian laser beam, write the heat source as follows ( 10): in αis the absorption coefficient, Ris the reflection of the pump beam at the bottom facet of the device, Pis the incident pump power reaching the gain region, rand zis the spatial coordinate, r ₀ is 1/ of the pump laser beam e ²radius, r=0 is the center of the laser beam, z _gis the z-coordinate of the interface between the gain region and the top layer (see Figure 38), H( t) is the pulse width τ _pThe rectangular pulse function, n _gis absorbed in the gain area in the absence of laser field ( 11) is the fraction of pump power converted into heat, which is given by: in ϕ _PL is the fluorescence quantum yield ( ϕ _PL (BSBCz:CBP)=86%), λ _{Pump Pu} is the wavelength of the pump laser, and λ _laser is the extracted laser wavelength. Regarding the boundary conditions in the radial direction, a symmetry boundary condition is used at the axis of rotation. Apply thermal insulation boundary conditions (ignoring air convection) at the bottom, top, and edge surfaces. The radius of the device was set to 2.5 mm. Power density is 2 kW/cm ². Table S5 presents the thermophysical parameters and geometric parameters used for the simulation obtained from the COMSOL database. For the BSBCz:CBP layer, we chose the same thermal parameters as in Ref (11) for the organic material. surface S5 .Thermophysical parameters and geometric parameters of materials. Layer name k (WK ^-1 m ^-1 ) C _p (J kg ^-1 K ^-1 ) ρ (kg m ^-3 ) α at 405 nm (m ^-1 ) D (μm) Glass 1.4 730 2210 0 717 Sapphire 27 900 3900 0 759 CYTOP 0.12 861 2200 0 2 BSBCz:CBP 0.2 1400 1200 1.55 x 10 ⁶ 0.2 SiO ₂ 1.38 703 2203 0 100 Si 130 700 2329 8.00 x 10 ⁶ 333 After absorbing the pump laser energy, the BSBCz layer acts as a heat source. The heat generated is transferred by conduction towards the top and bottom layers. 1.1 Pulse width changesFigure 39 and Figure 40 show the pulse width τ of 10, 30 and 40 ms each time respectively. _pMaximum temperature rise after pumping and temperature rise at the BSBCz/CYTOP layer interface. These simulation results demonstrate that the temperature rise caused by long-pulse pump irradiation increases with pulse duration, but this effect tends to saturate for pulses longer than 30 ms. It can also be seen from these calculations that the temperature rise is not expected to increase significantly with the number of incident pulses. 1.2 exist 10 ms Effect of encapsulation in the case of pulse widthThe simulation results in Figure 41 provide clear evidence of the importance of encapsulation used in our devices to improve thermal management in devices operating under long pulsed light irradiation. 1.3 CYTOP Thickness variationAs shown in Figure 42, due to the low thermal conductivity of CYTOP, increasing the thickness of CYTOP results in an increase in temperature in the gain region. Although encapsulation of DFB lasers by CYTOP was found to be crucial for improving device performance under long-pulse light excitation, the poor thermal conductivity of CYTOP is clearly a limiting factor, and this aspect should be explored in future research by selecting more appropriate encapsulation. sealing materials to demonstrate actual CW organic semiconductor technology. References1. D. Yokoyama, A. Sakaguchi, M. Suzuki, C. Adachi, Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films. Org. Electron. 10(1), 127-137 (2009). 2. A. S. D. Sandanayaka, K. Yoshida, M. Inoue, K. Goushi, J. C. Ribierre, T. Matsushima, C. Adachi, Quasi-continuous-wave organic thin film distributed feedback laser. Adv. Opt. Mater. 4, 834-839 (2016). 3. H. Nakanotani, C. Adachi, S. Watanabe, R. Katoh, Spectrally narrow emission from organic films under continuous-wave excitation. Appl. Phys. Lett. 90, 231109 (2007). 4. D. Yokoyama, M. Moriwake, C. Adachi, Spectrally narrow emissions at cutoff wavelength from edges of optically and electrically pumped anisotropic organic films. J. Appl. Phys. 103, 123104 (2008). 5. H. Yamamoto, T. Oyamada, H. Sasabe, C. Adachi, Amplified spontaneous emission under optical pumping from an organic semiconductor laser structure equipped with transparent carrier injection electrodes. Appl. Phys. Lett. 84, 1401 (2004). 6. T. Aimono, Y. Kawamura, K. Goushi, H. Yamamoto, H. Sasabe, C. Adachi, 100% fluorescence efficiency of 4,4'-bis[( N-carbazole)styryl]biphenyl in a solid film and the very low amplified spontaneous emission threshold. Appl. Phys. Lett. 86, 071110 (2005). 7. M. D. Mcgehee, A. J. Heeger, Semiconducting (conjugated) polymers as materials for solid-state lasers. Adv. Mater. 12, 1655-1668 (2000). 8. J. C. Ribierre, G. Tsiminis, S. Richardson, G. A. Turnbull, I. D. W. Samuel, H. S. Barcena, P. L. Burn, Amplified spontaneous emission and lasing properties of bisfluorene-cored dendrimers. Appl. Phys. Lett. 91, 081108 (2007). 9. S. Hirata, K Totani, T. Yamashita, C. Adachi, M. Vacha, Large reverse saturable absorption under weak continuous incoherent light. Nature Materials. 13, 938-946 (2014). 10. Z. Zhao, O. Mhibik, T. Leang, S. Forget, S. Chénais, Thermal effects in thin-film organic solid-state lasers. Opt. Express. twenty two, 30092-30107 (2014). 11. S. Chenais, F. Druon, S. Forget, F. Balembois, P. Georges, On thermal effects in solid-state lasers: The case of ytterbium doped materials, Prog. Quantum Electron. 30(4), 89-153 (2006). [ 5 ] Electrically driven organic semiconductor laser diodes OverviewDespite significant advances in the performance and applications of optically pumped organic semiconductor lasers, electrically driven organic laser diodes have not yet been achieved. Here, we report the first demonstration of an organic semiconductor laser diode. The reported device regenerates mixed-stage dispersed SiO ₂The grating is incorporated into the organic light-emitting diode structure. Can convert up to 3.30 kA cm ^{− 2}A current density of 0.54 kA cm was injected into the device and the blue laser was observed to be higher than about 0.54 kA cm ^{− 2}the critical value. The realization of organic semiconductor laser diodes is mainly due to the selection of high-gain organic semiconductors that do not exhibit triplet absorption losses at laser wavelengths and the suppression of electroluminescence efficiency roll-off at high current densities. This represents a significant advance in the field of organic electronic devices and the first step toward novel cost-effective organic laser diode technology that enables full integration of organic optoelectronic circuits. Detailed descriptionThe properties of optically pumped organic semiconductor lasers (OSLs) have been greatly improved over the past two decades due to significant advances in the development of high-gain organic semiconductor materials and the design of high-quality factor resonator structures. ^1-5. The advantages of organic semiconductors as gain media for lasers include their high photoluminescence quantum yield (PLQY) and large stimulated emission cross section, their chemical tunability, their broad emission spectrum across the visible region, and their ease of fabrication. . Recent advances in low-threshold distributed feedback (DFB) OSL demonstrate that optical pumping of inorganic light-emitting diodes via electrically driven nanosecond pulses provides a path towards new compact and low-cost visible laser technologies. way ⁶. This type of miniaturized organic laser is particularly promising in chip lab applications, chemical sensing and biological analysis. However, to achieve complete integration of organic photonic circuits and optoelectronic circuits, electrically driven organic semiconductor laser diodes (OSLDs) are required, which remains an unrealized scientific challenge so far. Problems preventing direct electrical pumping of organic semiconductor devices by lasers are primarily due to optical losses from the electrical contacts and the additional triplet and polaron losses that occur at high current densities. ^4,5,7-9. Different approaches to solving these problems have been proposed, involving, for example, the use of triplet quenchers ^{10 - 12}To curb triplet absorption loss and singlet quenching through singlet-triplet exciton mutual destruction, and reduce the device's active area ¹³To spatially separate the exciton formation and exciton radiation decay regions and minimize the polaron quenching process. Considering the current advanced technology of optically pumped organic semiconductor DFB laser ⁵With high efficiency, careful combination of these methods together with optimization of the device structure can lead to electrically driven laser emission from organic thin films. Previous studies have suggested that if the additional losses associated with electrical pumping were to be fully contained, higher than a few kA/cm would be required ²current density to achieve the laser from OSLD ¹⁴. Demonstrating amplified spontaneous emission (ASE) threshold below 0.5 μJ/cm ²Among different organic semiconductor films, ⁵One of the most promising molecules for observing laser emission under electrical pumping is 4,4'-bis[(N-carbazole)styryl]biphenyl (BSBCz) (see Figure 43 chemical structure) ¹⁵. The ASE threshold of BSBCz-based films has been reported to be as low as 0.30 μJ cm under 800 ps pulsed light excitation. ^{−2 16}. Meanwhile, another work demonstrated up to 2.8 kA cm under pulse operation using a pulse width of 5 μs. ^{− 2}The current density can be injected into BSBCz-based organic light-emitting diodes (OLEDs) ¹³. These devices exhibit maximum electroluminescence external quantum efficiency (EQE) values above 2%. In addition, efficiency roll-off due to singlet-thermal and singlet-polaron mutual destruction at high current densities is substantially reduced by reducing one dimension of the current injection/transport region to 50 nm. Recently, quasi-continuous wave laser at 80 MHz and true continuous wave laser lasting at least 30 ms were demonstrated in the optically pumped BSBCz-based organic DFB laser. ¹⁷. This unprecedented performance is achieved because the PLQY of BSBCz in the 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) blend is close to 100%, and because There is no significant triplet absorption loss at the laser wavelength of the BSBCz film. Here, we achieve this by combining an inverted OLED structure with a mixed-level DFB SiO integrated into the active area of the device. ₂The gratings were combined to demonstrate electrically driven laser emission from BSBCz films, thus providing the first clear evidence of electrically driven laser emission from organic semiconductors. The fabrication method and architecture of the OSLD developed in this study are schematically shown in Figures 43 to 45 (see the detailed description of the experimental procedures in the Materials and Methods section). First, a 100 nm thick dielectric SiO ₂layer was sputtered onto a pre-cleaned patterned indium tin oxide (ITO) glass substrate. We then designed a hybrid-order DFB grating in which the first-order Bragg scattering area is surrounded by the second-order Bragg scattering area. These areas generate strong optical feedback and provide efficient vertical extraction of laser emission respectively. ^17,18. In DFB laser, it is well known that when the Bragg condition is satisfied ^4,19(mλ _Bragg= 2n _effLaser oscillation occurs when Λ), where m is the diffraction order, λ _Braggis the Bragg wavelength, n _effis the effective refractive index of the gain medium, and Λ is the grating period. Use the ones reported for BSBCz _effvalue and λ _Braggvalue ^20,21, the grating periods of the mixed-order (m=1, 2) DFB laser devices are calculated to be 140 nm and 280 nm respectively. Using electron beam lithography and reactive ion etching, these mixed-order DFB gratings are engraved on SiO ₂Within the 140×200 μm area in the layer (Figure 46A). As shown by the scanning electron microscopy (SEM) image in Figure 46B, the DFB grating fabricated in this work has periods of 140±5 nm and 280±5 nm and a grating depth of approximately 65±5 nm, which is perfect The land complies with our specifications provided above. The lengths of each 1st-order and 2nd-order DFB grating are approximately 10 µm and 15.1 µm respectively. Energy dispersive X-ray spectroscopy (EDX) analysis is then performed to ensure that the ITO layer was not damaged during grating fabrication and that the SiO is completely removed in the etched areas ₂layer. The EDX results shown in Figures 46C and 46D provide evidence that charge injection from ITO to the organic semiconductor layer deposited on top of the DFB grating can occur in the etched areas where the ITO contacts are located. In addition, we propose that DFB resonators can also be fabricated using a low-cost and simple nanoimprint lithography process (Figure 45). As demonstrated by the schematic representation shown in Figure 47A, the OSLD fabricated in this work has the following simple inverted OLED structure vapor deposited on top of a DFB grating: ITO (100 nm)/20wt.%Cs: BSBCz (60 nm)/BSBCz (150 nm)/MoO ₃(10 nm)/Ag (10 nm)/Al (90 nm). In this type of reverse device structure, electron injection into the organic layer is improved by Cs doping of the BSBCz film in the region close to the ITO contact, while MoO ₃Used as hole injection layer (Figure 48 to Figure 49). As shown in Figure 50, the surface morphology of all layers exhibits a grating structure with a surface modulation depth of 20 nm to 30 nm. Although the most efficient OLEDs generally use multi-layer architectures to optimize charge balance ^22,23, but charge accumulation can occur at organic heterointerfaces under high current densities, which can be detrimental to device performance and stability. ^{twenty four}. The OSLD fabricated in this work contains only BSBCz as the organic semiconductor and is specifically designed to minimize the number of organic heterointerfaces. It should be noted that also manufactured without SiO ₂A device for DFB gratings and used as a reference to obtain additional information on the effect of gratings on electroluminescence properties. In addition, we are eager to find out more about SiO ₂Organic semiconductor laser diodes with different one-dimensional DFB resonator structures are made in , ITO and polymers or on top of the active layer (Figure 51A to Figure 51D). As shown in Figure 52, organic semiconductor laser diodes with two-dimensional DFB resonator structures are also promising for lower threshold 2D DFB lasers. Figures 47B and 53A-53D show optical microscopy images of an OSLD, and Figure 47C shows those optical microscopy images of a reference OLED without a grating, both operating at a direct current (DC) of 4.0 V. Electroluminescence is emitted uniformly from the active area of the reference OLED. In the case of OSLD, more intense emission is seen from the 2nd order DFB grating area of OSLD, which is specifically designed to facilitate vertical light extraction. Current-voltage (J-V) and EQE-J curves measured in representative devices with and without DFB gratings are shown in Figures 47D-47E. The device was characterized under both DC and pulsed (with voltage pulse width of 500 ns and repetition rate of 100 Hz) conditions. To estimate the active area of OSLD based on SEM and laser microscope images, it is necessary to calculate the current density injected into the device. The reference device under DC and pulsed operation each demonstrated 70 A cm before device failure. ^{− 2}and 850 A cm ^{− 2}The maximum current density (J _max). Due to the smaller effective installation area ^13,25The Joule heat is reduced, and the OSLD clearly exhibits 80 A cm under DC and pulse operation respectively. ^{− 2}and 3220 A cm ^{− 2}The higher J _max. All BSBCz devices were found to exhibit maximum EQE values higher than 2% at lower current densities. However, in DC operation above 15 A cm ^{− 2}Significant efficiency roll-off was observed in the OSLD and reference devices at 100% current density, which can be attributed to thermal degradation of the organic gain medium. Under pulsed operation, the reference device demonstrates performance above 410 A cm ^{− 2}The efficiency roll-off at current density is consistent with previous reports ¹³The result. More importantly, efficiency roll-off in OSLDs was curbed under pulsed operation and the EQE was even found to increase substantially to above 800 A cm ^{− 2}to achieve a maximum value of 3.3%. When the current density increases above 3200 A cm ^{− 2}At this time, the rapid decrease in EQE is presumably attributed to the thermal degradation of the organic semiconductor. As shown in Figure 54, the electroluminescence spectrum of the reference device is similar to the steady-state PL spectrum of the BSBCz pure film and does not change with changes in current density. Figure 53E, Figure 55A, Figure 55C and Figure 56A show the evolution of the electroluminescence spectra of several OSLDs under different current densities of pulse operation. These spectra were measured from the ITO side of the OSLD in a direction perpendicular to the plane of the substrate. It can be clearly seen that when J becomes higher than 800 A cm ^{− 2}It produces a strong spectral line narrowing effect at 456.8 nm. For further understanding, the output intensity and full width at half maximum (FWHM) as a function of current density are plotted in Figure 53F, Figure 55B, Figure 55D, and Figure 56B. The FWHM of the steady-state PL spectrum of the pure BSBCz film is about 35 nm, which decreases to less than 0.2 nm at the highest current density. At the same time, a sudden change in the slope efficiency of the output intensity was also observed, which is consistent with the state of the EQE-J curve and can be used to determine 960 A cm ^{− 2}the critical value. Similar to what is seen in the EQE-J curve, when J > 3.2 kA cm ^{− 2}When , the output intensity decreases with J, which is attributed to thermal degradation leading to device collapse. In this regime, however, it is noteworthy that the emission spectrum of the OSLD remains extremely steep. The observed conditions clearly indicate that optical amplification occurs at high current densities and that the OSLD exhibits laser emission above the laser threshold. The search for the first organic semiconductor laser diode, which has been linked to several controversial reports in the past, means that attention should be paid before arguing that the OSLD fabricated in this research provides electrically driven laser emission. ⁹. First, a few studies ^{20 , 26 , 27}It is shown that edge emission from the waveguide mode of organic light-emitting devices can lead to extremely strong line narrowing effects without laser amplification. In contrast to these previous works, emissions from our OSLD were detected in the direction perpendicular to the substrate plane and exhibited clear threshold conditions. It should also be emphasized that the ASE linewidth of organic films is usually in the range of several nm, while the FWHM of organic DFB lasers can be well below 1 nm. ⁵. At FWHM below 0.2 nm, the emission spectrum from our OSLD cannot be attributed solely to ASE and corresponds to elements commonly obtained in optically pumped organic DFB lasers. Second, previous reports demonstrated extremely narrow emission spectra by unintentionally stimulating transitions in ITO. ²⁸The atomic spectral lines of ITO include those at 410.3 nm, 451.3 nm and 468.5 nm. ²⁹The peak emission wavelength of the OSLD in Figure 55A is 456.8 nm, which cannot be attributed to emission from ITO. It should also be emphasized that the emission from an OSLD should be characteristic of the resonator mode, and therefore the output should be extremely sensitive to any modification of the laser cavity. A simple way to tune the emission wavelength in optically pumped organic DFB lasers is to change the grating period ^{4 , 5 , 30 , 31}. Figures 55C to 55D show the emission spectra at different current densities and the OSLD output intensity as a function of current density for a grating period of 300 nm (for 2nd order scattering) and 150 nm (for 1st order scattering) respectively. This device demonstrates FWHM as low as 0.16 nm and a threshold of 1.07 kA cm at 475.5 nm ^{− 2}The laser peak (Figure 57). In addition, we demonstrated organic DFB laser based on BSBCz thin film using modified resonator design (Figure 58). Since laser radiation extracted by second-order gratings represents loss channels, these lasers typically exhibit higher thresholds compared to their first-order counterparts. To study the trade-off between extraction and threshold, we fabricated gratings with different widths and with first- and second-order regions. The threshold value is deduced from the laser input-output curve and plotted against the width of the second-order region in Figure 59. It can be seen that the oscillation threshold increases linearly with the size of the second-order region. This can be understood as being proportional to the waveguide loss in terms of laser threshold, which increases linearly with increasing period. Thus, the threshold of a mixed-order resonator increases with the fraction of extracted light, but remains low even for strong extractions. By varying the grating parameters, organic solid-state lasers can therefore be tuned to have optimized properties (low threshold and high extraction). Figure 56B shows the emission spectra at different current densities and for grating periods with 300 nm (for 2nd order scattering) and 150 nm (for 1st order scattering) and 4 first-order periods and 12 second-order periods, respectively. OSLD output intensity changes with current density (Figure 60). This device demonstrates FWHM as low as 0.18 nm at 500.5 nm and a threshold of 540 A cm ^{− 2}The laser peak. This provides clear evidence that the laser emission from our OSLD is significantly affected by the DFB resonator structure, and that this can be used to tune the laser wavelength within a range of wavelengths. Laser emission from OSLD should also follow certain parameters regarding output beam polarization, output beam shape and time coherence. ⁹of principles. As shown in Figure 61, the output beam of the OSLD is largely linearly polarized along the grating pattern, providing clear evidence of true one-dimensional DFB laser action in an electrically driven device. Another important issue that needs clarification is to see how the laser threshold of electrically driven OSLDs compares with that obtained by optical pumping. Figure 62 shows the laser characteristics of the OSLD optical pumping through the ITO side by delivering a 500 ns pulse to the laser diode at an excitation wavelength of 405 nm. Laser emission occurs at 481 nm, which is consistent with the wavelength of electrically driven lasers. The laser threshold measured under optical pumping is approximately 450 W cm ^{− 2}, which is higher than the 36 W cm obtained in an optically pumped BSBCz-based DFB laser without two electrodes ^{− 2}value. It should be noted that the thickness of the different layers used in the OSLD has been optimized in this work to minimize optical losses due to the presence of these electrodes. Assuming no additional loss mechanisms in BSBCz OSLD operation at high current densities, threshold values measured in optically pumped devices indicate that electrically driven laser emission should achieve current densities higher than 1125 A cm ^{− 2}. Similar thresholds for optical and electrical pumping indicate that they are almost limited to high current densities ³²The following are additional losses that generally occur in organic electroluminescent devices (including exciton mutual destruction, triplet and polaron absorption, quenching by high electric fields, Joule heating). This is completely consistent with the fact that no electroluminescence efficiency roll-off was observed in OSLD under severe pulse electrical excitation. To explain this result, it should be remembered that the BSBCz film does not exhibit significant triplet absorption at laser/ASE wavelengths, and it exhibits extremely weak quenching of the singlet state through singlet-triplet mutual destruction. Importantly, previous work has shown that device active area reduction can be used to separate exciton formation from radiative decay of excitons and generally reduce polariton/thermal quenching processes. We have also fabricated a device with nine DFBs on one chip as shown in Figure 63, and this device provides effective output of laser emissions. For low-threshold organic semiconductor laser diodes, we have also successfully fabricated circular DFB resonators (Figure 64 to Figure 65). In conclusion, this study demonstrates the first realization of an electrically driven organic semiconductor laser diode that implements mixed-order distributed feedback SiO ₂resonator into the active area of the organic light emitting diode structure. Specifically, the device exhibits threshold current densities as low as 540 A cm ^{− 2}blue laser emission. Different criteria regarding emission linewidth, polarization and threshold values can be used to distinguish laser emission from other phenomena that have been carefully examined and fully support the claim that this is the first observation of electrically driven lasers in organic semiconductors. This report opens up new opportunities and perspectives in organic photonics and should clearly serve as a strong foundation for the future development of organic semiconductor laser diode technology that is simple to apply, cheap and tunable laser sources and its suitability for complete and direct integration into organic-based optoelectronic platforms. Materials and methods Device manufacturing and characteristicsIndium tin oxide (ITO)-coated glass substrates (100 nm ITO, Atsugi Micro Co.) were cleaned by ultrasonic treatment followed by UV ozone treatment using neutral detergent, pure water, acetone, and isopropyl alcohol. 100 nm thick SiO at room temperature ₂Sputter on 100 nm ITO-coated glass substrate to engrave DFB on the ITO substrate. The argon gas pressure during sputtering was 0.2 Pa. The RF power is set to 100 W (Figure 43 and Figure 44). The substrate was again cleaned by ultrasonic treatment followed by UV ozone treatment using isopropyl alcohol. The silica surface was treated with hexamethyldisilazane (HMDS) by spin coating at 4000 rpm for 15 s and annealed at 120 °C for 120 s. A resist layer with a thickness of approximately 70 nm was spin-coated on the substrate from ZEP520A-7 solution (ZEON Co.) at 4000 rpm for 30 s and baked at 180°C for 240 s. Use a 0.1 nC cm ^{− 2}A JBX-5500SC system (JEOL) with optimized dosage was used for electron beam lithography to draw the grating pattern on the resist layer. After electron beam irradiation, the pattern was developed in a developer solution (ZED-N50, ZEON Co.) at room temperature. The patterned resist layer was used as an etch mask, while an EIS-200ERT etch system (ELIONIX) was used with CHF ₃Plasma etching the substrate. To completely remove the resist layer from the substrate, use a FA-1EA etching system (SAMCO) with O ₂Plasma etching the substrate. Etch conditions optimized to completely remove SiO from DFB pitch modulation ₂Until ITO contact. The grating formed on the silicon dioxide surface was observed using SEM (SU8000, Hitachi) (Fig. 46B). Perform EDX (at 6.0 kV, SU8000, Hitachi) analysis to confirm complete removal of SiO from the DFB spacing ₂(Figure 46C and Figure 46D). The DFB substrate is cleaned by conventional ultrasonic treatment. Subsequently by using the 2.0 × 10 ^{− 4}Thermal evaporation under a pressure of Pa is 0.1 nm s ^{− 1}to 0.2 nm s ^{− 1}The total evaporation rate of the organic layer and metal electrode is placed in a vacuum with SiO ₂Fabricated with indium tin oxide (ITO) (100 nm)/20 wt% Cs on insulator DFB substrate: BSBCz (60 nm)/BSBCz (150 nm)/MoO ₃(10 nm)/Ag (10 nm)/Al (90 nm) structure i-OLED. SiO remaining on the ITO surface ₂The layer acts as an insulator. Therefore, the current area of OLED is limited to the DFB area where BSBCz is in direct contact with ITO. A reference OLED with an active area of 140 × 200 µm was also prepared using the same current region. The current density-voltage-EQE (J-V-EQE) characteristics (DC) of OLED were measured at room temperature using a integrating sphere system (A10094, Hamamatsu Photonics). Measurement under pulse driving using pulse generator (NF, WF1945), amplifier (NF, HSA4101) and photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics) J- V- Lcharacteristic. Both the PMT response and the driving square wave signal were monitored on a multi-channel oscilloscope (Agilent Technologies, MSO6104A). A rectangular pulse with a pulse width of 500 ns, a pulse period of 5 μs, and a repetition rate of 100 Hz was applied in a device with varying peak current. Spectral measurementThe emitted laser light perpendicular to the device surface was collected using an optical fiber connected to a multichannel spectrometer (PMA-50, Hamamatsu Photonics) and placed 3 cm away from the device. For CW operation, a CW laser diode (NICHIYA, NDV7375E, maximum power 1400 mW) was used to generate excitation light with an excitation wavelength of 405 nm. In these measurements, an acousto-optic modulator (AOM, Gooch & Housego) triggered by a pulse generator (WF 1974, NF Co.) was used to deliver pulses. The excitation light is concentrated on 4.5×10 of the device through lenses and slits. ^{− 5}cm ²area, and the emitted light was collected using a frame eye (C7700, Hamamatsu Photonics) with a time resolution of 100 ps connected to a digital camera (C9300, Hamamatsu Photonics). A photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics) was used to record the emission intensity. Both the PMT response and the driving square wave signal were monitored on a multi-channel oscilloscope (Agilent Technologies, MSO6104A). As described previously, the same illumination and detection angles were used for this measurement. 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Lett. 85, 1886 (2004). 32. Murawski, C., Leo, K. & Gather, M. C. Efficiency roll-off in organic light-emitting diodes. Adv. Mater. 25, 6801-6827 (2013). Electrical simulation of distributed feedback electrically driven organic laser 1. Device model and parametersIn this study, a so-called "first-generation model" was used to describe charge transport in organic light-emitting diodes (OLEDs). In this model, the electron density is solved self-consistently using a two-dimensional time-independent drift-diffusion model n, hole density pand electrostatic potential Ψ. The Poisson equation relates the electrostatic potential Ψ to the space charge density as follows: where F is the vector electric field, q is the basic charge, ε _ris the relative permittivity of the material and ε ₀is the vacuum permittivity, is the electron (hole) concentration, is the concentration of filled electron (hole) trap states. Assuming parabolic density of states (DOS) and Maxwell-Boltzmann statistics, the electron and hole concentrations are expressed as: in and is the energy state density of carriers in the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), and is the energy level of LUMO and HOMO, and is the quasi Fermi level of electrons and holes, is Boltzmann's constant and T is the device temperature. Charge carrier traps in organic semiconductors exist due to structural defects and/or impurities. The injected charge needs to first fill these traps before current can be established. This state is called trap limited current (TLC). ^1,2Exponential or Gaussian distributions are used to model trap distribution within organic semiconductors. ³In this work, the Gaussian distribution for hole trap states is used: ^{4 , 5} in is the total density of traps, is the energy trap depth above the HOMO level, and is the width of the distribution. The density of trapped holes is estimated by taking the integral of the Gaussian energy state density times the Fermi-Dirac distribution. Charge transport is controlled by drift in the electric field F and diffusion due to charge density gradients. The steady-state continuity equations for electrons and holes in the drift-diffusion approximation method are given by: in is the electron (hole) mobility, is the electron (hole) diffusion constant, and Ris the recombination rate. The charge carrier mobility is inferred to be field dependent and has the Pool-Frenkel form: ^6,7 in is zero field mobility, and It is the characteristic field of electrons (holes). High-energy chaos is not considered in this model, so we assume the validity of Einstein's relation to calculate the diffusion constant from charge mobility. By Langevin model ⁸gives the recombination rate R: When electrons recombine with holes, they form excitons. The resulting excitons can decay radiatively or nonradiatively with a characteristic diffusion constant D _s migration. The continuity equation for singlet excitons is given by: in Sis the exciton density. The first period is the singlet exciton production rate based on electron hole recombination, which is 1/4, the second period represents exciton diffusion, and the third period represents the decay constant with radiation. and nonradiative decay constant of exciton decay, and the last period is represented by having a field-dependent dissociation rate The dissociation of excitons in the electric field is given by the Onsager-Braun model: ^9,10 in is the exciton radius, is the exciton binding energy, is a first-order Bessel function, and are field-related parameters. In this model, the impact of electric field quenching (EFQ) depends on the exciton binding energy . 2. Simulation results and comparison with experiments 2.1. Unipolar and bipolar reference devicesBefore performing bipolar device simulations, pure hole and pure electron devices were considered to test the electrical model, simulation parameters, and charge carrier mobility. The purely electronic device consists of a 190 nm BSBCz layer sandwiched between Cs (10nm)/Al and 20 wt% Cs:BSBCz (10nm)/ITO electrodes. By converting 10 nm MoO ₃The layer is inserted between BSBCz (200 nm) and ITO and Al to obtain a pure hole device. The bipolar OLED device contains the following structure: ITO/20 wt% Cs: BSBCz (10 nm)/BSBCz (190 nm)/MoO ₃(10 nm)/Al. The work function of the cathode (ITO/20 wt% Cs: BSBCz) is 2.6 eV, and the anode (MoO ₃/Al) one of the work functions is 5.7 eV. A diagram of energy levels for these device structures is shown in Figure 48. Use the reported charge carrier mobility (measured by time of flight) for BSBCz [11]. Figure 49a shows the measured reported mobilities of electrons and holes for BSBCz and the corresponding fits to the Pool-Frenkel field correlation model. The values of the fitted mobility parameters and other values of the input parameters required for the electrical simulation are shown in the table below. The hole and electron mobilities of BSBCz are approximately the same order of magnitude, indicating that BSBCz can transport both types of charge carriers. Table. Electrical simulation parameters parameters BSBCz Cs:BSBCz unit ε _r 4 4 - E _HOMO 5.8 5.8 eV _ELUMO 3.1 2.6 eV N _HOMO 2× ^10-19 2× ^10-19 cm ^-3 _NLUMO 2× ^10-19 2× ^10-19 cm ^-3 _nnJC 2.8× ^10-17 - cm ^-3 E _t 0.375 - eV σtp _{_} 0.017 - eV μn0 _{_} 6.55× ^10-5 6.55× ^10-5 cm ² V ^-1 s ^-1 μ _p0 1.9× ^10-4 1.9× ^10-4 cm ² V ^-1 s ^-1 _f 175561 175561 V/cm F _p0 283024 283024 V/cm k _r 10 ⁺⁹ 10 ⁺⁹ s ^-1 _n 0.11 × 10 ⁺⁹ 0.11 × 10 ⁺⁹ s ^-1 φPL _{_} 0.9 0.4 - L _s 18× ^10-9 18× ^10-9 m Experiments and simulations of unipolar and bipolar devices J ( V )The curve is shown in Figure 49b. Experiments measured under direct current (DC) drive Jlower than 18V, and under pulse driving is higher than 18V. The current in a pure hole device is largely affected by V ＜ 20 VAt the trap limit. Obtained through optimization of simulated experimental data , and The values are given in the table above. The results demonstrate good agreement between experiments and simulations of monopolar devices. For bipolar devices, the small deviation between measurements and simulations at lower current densities is attributed to the presence of experimental leakage currents. The simulation model predicts that pure hole devices and pure electron devices will have similar current densities at high voltages, demonstrating good electron and hole transport balance. Bipolar devices exhibit current densities that are an order of magnitude higher than unipolar current densities. 2.2. bipolar DFB deviceThe use of DFB grating resonator not only amplifies light ^{12 - 14}Providing positive optical feedback affects the optical properties of organic lasers and also affects the electrical properties of organic lasers. The effect of nanostructured cathodes on the electrical properties of DFB OLEDs was calculated and compared with a reference OLED (without grating). The structure of DFB OLED is similar to that of bipolar OLED. The difference is that the nanostructured cathode is composed of periodic grating SiO deposited on ITO. ₂-Cs: BSBCz composition. The grating period is 280 nm and the grating depth is 60 nm, as represented in Figure 66a. In this structure, the thickness of BSBCz is 150 nm. For comparison, reference OLEDs with the same thickness (ITO/20 wt% Cs: BSBCz (60 nm)/BSBCz (150 nm)/MoO ₃(10 nm)/Al) and does not have a grating. All parameters and conditions for the bipolar device have been retained since the DFB and reference OLED have no additional fitting parameters. Experiments and simulations of DFB grating and reference OLED J ( V )The curve is shown in Figure 66b. Electrical simulation predicts that at DC ( V ＜ 18 V) and pulse operation ( V≥ 18V) in two cases J ( V )The curve is in good agreement with the experimental results. remove J ( V )In addition to curve predictions, electrical simulations can access physical parameters that are difficult to determine experimentally, such as the spatial distribution of charge carrier densities, electric fields, and the location of recombination regions. First, we consider referring to OLED. Figures 67a to 67b show the charge carrier distribution and electric field profile of the reference OLED at 10V and 70V. Free electrons are injected from the cathode of ITO/CS:BSBCz into BSBCZ (at x = 0 μmtime), and the free holes come from Al/MoO ₃Anode injection ( x = 0 . 215 μm). Due to carrier recombination, the carrier density decreases as it leaves the contact. when n = p, the electric field increases and reaches a maximum value at its center. At 10V, the electric field is screened by high charge carrier density close to the cathode and anode. At higher voltages (70 V), the electrons penetrate deeper and the electric field near the anode remains higher. In the case of DFB grating OLED, at 70 VExtract entity parameters below. Figures 68a-68b show the spatial distribution of charge carrier densities n and p. Since the periodic nanostructured electrons of the cathode are not injected uniformly, their spatial distribution follows the periodic injection, as can be clearly seen in Figure 68b and Figure 68c. The holes are injected from the uniform anode and extend relatively uniformly in the bulk (Figure 68a, Figure 68c). When the hole reaches the cathode, it decays for the reference OLED (Fig. 67(b)). However, since SiO ₂The existence of gratings and holes in SiO ₂The /BSBCz interface accumulates and exhibits high density (Fig. 68a). Figure 69a shows the periodic profile of the electric field, which is higher in the insulator and slightly modulated in the BSBCz layer for the reference OLED (approximately 3.5×10 ⁶ V / cm) remains at the same intensity. The current density profile shown in Figure 69b is modulated to a large extent and is shown in SiO ₂/Cs: The higher value near the BSBCz interface. To clarify SiO ₂/Cs: Reason for the high current density value near the BSBCz interface, recombination rate profile RThis is shown in Figure 70a. As we can see, RIt also shows the cyclic changes within the device. In the region bounded by the cathode/anode, the cross-section is the same as that of the reference OLED, which in the region bounded by SiO ₂/ decreases in the region bounded by the anode (see Figure 70c). In Cs: BSBCz/SiO ₂at the interface, RShowing the maximum value due to hole and electron accumulation, as demonstrated in Figure 70d. The electric field inside the device is approximately MV/cm ², as shown in Figure 69a. Therefore, the exciton dissociation caused by the electric field cannot be ignored and affects the device performance to a great extent. The singlet exciton binding energy of organic semiconductors ranges from 0.3 eV to 1.6 eV ^{15 - 18}within the range. Under low electric fields, the dominant deactivation processes are radiative decay and non-radiative decay. Under high electric fields, the probability of exciton dissociation is greatly increased and depends on the exciton binding energy. To account for electric field-induced exciton dissociation, the field-dependent dissociation rate given by Equation 10 is included in the singlet exciton continuity Equation 9. Figure 71a shows the calculated exciton density of the reference device S, including those with different exciton binding energies E _b ( 0 . 2 - 0 . 6 eV )EFQ. when E _b When reduced, EFQ becomes a serious loss mechanism. Using molecules with high exciton binding energies requires overcoming the EFQ. The exciton binding energy of BSBCz is estimated using PL quenching yield experiment and its lower limit is 0 . 6 eV. Figure 71b shows the results for the reference device and the DFB device with and without electric field-induced exciton dissociation. S ( J )characteristic. Without EFQ, S follows Jadded and displayed in J = 3KA / cm ² DFB device 9×10 ¹⁷ cm ^{- 3} relative to a reference device of 2×10 ¹⁷ cm ^{- 3} high value. SThis difference comes from different device architectures, which lead to differences within the device Rdistribution, as shown in Figures 70a, 70b. By considering the EFQ model and BSBCz E _b = 0 . 6 eV, one of the two devices Sboth increase, and until J = 0 . 5KA / cm ² Then it decreases due to the electric field dissociation of excitons. The EFQ in the DFB device is slightly lower than that in the reference device and may explain the experimentally lower EQE roll-off of the DFB device compared to the reference device shown in Figure 47E. To gain further physical understanding of the cause of the EQE enhancement in DFB devices, the one-dimensional exciton distribution inside a reference device with and without EFQ is shown in Figure 72a. In the case of the DFB device, the two-dimensional exciton distributions without and with EFQ are shown in Figure 72b and Figure 72c respectively. Comparison of the exciton density distribution (Fig. 73, bottom right) and the optical mode distribution in the device (Fig. 74, bottom) indicates that there is a large overlap at the 2nd grating region, contributing to light amplification. This significant overlap certainly contributes to the lower laser threshold. In the reference device, in the absence of EFQ, SDisperse evenly. In the presence of EFQ, due to the high electric field (which reaches 3.5 in the bulk MV / cm), among the blocks Sdecrease (see Figure 79b). Close to the Cs: BSBCz/ITO interface, the electric field is lower, which avoids the EFQ of excitons. In the case of the DFB device, excitons are generated from two recombination sites (site 1 and site 2), as shown in Figure 71a. Close to SiO ₂The accumulation of charge in the grating creates a high exciton density ( S=6×10 ¹⁷ cm ^{- 3} ), the recombination region is named site 1. Site 2 has the same characteristics as the reference device ( S=1×10 ¹⁷ cm ^{- 3} )identical S. Without EFQ, the maximum Sis provided by site 1 and explains the high value in the DFB device compared to the reference device at low electric fields (see Figure 71b). When considering EFQ, the excitons in site 1 are quenched by the high electric field in this site ( F = 3 . 5MV / cm), and the maximum value SProvided by site 2, where the electric field close to the interface is lower ( F = 1 . 2MV cm) and gradually increases relative to the reference device in the bulk region. Thus, some excitons close to the interface may survive if they are not quenched by another mechanism (which we did not include in this simulation). References1. G. Malliaras, J. Salem, P. Brock, and C. Scott, “Electrical characteristics and efficiency of single-layer organic light-emitting diodes,” Phys. Rev. B, vol. 58, no. 20, pp. R13411-R13414, 1998. 2. J. Staudigel, M. Stößel, F. Steuber, and J. Simmerer, “A quantitative numerical model of multilayer vapor-deposited organic light emitting diodes,” J. Appl. Phys., vol. 86, no. 7, p. 3895, 1999. 3. V. Kumar, S. C. Jain, A. K. Kapoor, J. Poortmans, and R. 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[ 6 ] Extremely low amplified spontaneous emission threshold and blue electroluminescence of spin-coated pure filmsOrganic semiconductor lasers have been the subject of intensive research during the past two decades, leading to significant developments regarding laser thresholds and operational device stability. ^{1 - 3}These devices are currently being considered for use in a variety of applications, including spectroscopic tools, data communications devices, medical diagnostics, and chemical sensors. ^{1 - 5}However, there is currently no demonstration of electrically driven organic laser diodes and breakthroughs are still needed to develop real continuous wave optically pumped organic semiconductor laser technology. ^1-3,6-8The challenges of realizing electrically pumped organic laser diodes are well recognized and involve (i) additional absorption losses at the laser wavelength due to polarons and long-lived triplet states, (ii) due to Singlet-triplet, singlet-polaron and singlet-thermal mutual destruction, quenching of singlet excitons, and (iii) stability of organic materials in electroluminescent device operation at high current densities. It should be noted that methods have been proposed to reduce triplet and polaron losses, including the use of triplet quenchers and reducing the active area of organic light-emitting diodes (OLEDs) to spatially separate exciton formation and exciton decay. area. ^9,10Although these problems need to be fully overcome and further research is still needed, it is also critical to substantially lower the threshold for amplified spontaneous emission (ASE) and laser in organic semiconductor films. ³To this end, there is a need for the development of novel high laser gain organic materials and improved resonator structures that can be incorporated into electrically pumped organic light-emitting devices. Radiative decay rate ( _k ) is directly related to the Einstein B coefficient, as expressed by the following equation: , where ν ₀ is the frequency of light, his Planck's constant, and cis the speed of light. The ASE threshold is inversely proportional to the B coefficient, which means larger k _RIt is usually better to achieve a low ASE threshold. ^{11 , 12}As outlined in a recently reviewed article on organic lasers, ³The lowest reported ASE threshold for organic thin films based on small molecules is 110 nJ/cm ², and obtained using 9,9'-snail derivatives. ¹³At approximately 300 to 400 nJ/cm ²Two other excellent organic semiconductor laser materials that exhibit low ASE threshold values in thin films are 4,4'-bis[( N -Carbazole) styryl] biphenyl (BSBCz) and heptafluoride derivatives. ^12,14Although the ASE threshold usually depends on the characteristics of the light source used for optical pumping, it is worth noting that the ASE threshold mentioned above was determined by using a similar nitrogen laser for optical excitation. It is considered that futon derivatives are very promising for achieving low ASE thresholds, and some of these futon derivatives exhibit higher than 1×10 ⁹s ^{- 1}The rate of radiative decay. ^{13 - 19}Significantly, previous work has specifically investigated the photophysical properties of triphenyl, penta, and heptafluoride derivatives functionalized with hexyl side chains. ¹⁸The results demonstrate that when increasing the length of oligomeric fluorine molecules, the radiative decay rate increases and the ASE threshold decreases. In this case, it is important to verify whether the ASE/laser properties can be further improved by increasing the oligomer length. Here, we report on Bafu derivatives, demonstrating no concentration quenching in spin-coated neat films with a PLQY of 87% and a fluorescence lifetime of approximately 600 ps. The chemical structure of this molecule is shown in Figure 75a. The large PLQY value and short PL lifetime of Bafu pure film are associated with about 90 nJ/cm ²The ASE threshold reaches an unprecedented level of ASE performance in organic non-polymer gain media. ³The performance of organic dispersive feedback (DFB) lasers and OLEDs based on pure Bafu thin films provides further evidence that this Fu derivative is very promising for further work devoted to organic semiconductor laser devices and their applications. The experimental procedures used in this work are described in the supplementary material. ¹⁹The absorption and steady-state PL spectra of a pure film spin-coated on a molten silica substrate are shown in Figure 75b. The film is almost transparent in the visible range of wavelengths and exhibits a major absorption band with a maximum absorption peak wavelength of 375 nm in the ultraviolet radiation region. This absorption peak has previously been attributed to exciton coupling between fluorine monomers. ¹⁸The optical energy gap from the long wavelength absorption edge is calculated to be approximately 2.9 eV. The PL spectrum and image shown in inset 75b indicate that the Bafu pure film fluoresces blue. The spectrum shows a clear vibronic structure with two peaks that can be assigned to the (0,0) and (0,1) transitions and a shoulder at the longer wavelength associated with the (0,2) transition. The maximum PL peak wavelength was found to be approximately 423 nm. containing dispersed to 4,4'-bis( N -The measured absorption and steady-state PL spectra of spin-coated blends of 10 wt.% and 20 wt.% of CBP in carbazolyl-1,10-biphenyl (CBP) host are shown in Figure 76 in (see Supplementary Materials). The CBP host was chosen in this work because efficient Förster-type energy transfer is known to occur from CBP to most oligomeric fluorine derivatives. ¹⁴Although the absorption spectrum of the blend is controlled by CBP absorption, it can be seen that its PL spectrum is not significantly different from that of the pure film. PLQY and PL lifetime were then measured in neat films and CBP blends. The 10 wt.% and 20 wt.% blends exhibited PLQY values of 88% and 87%, respectively, which are close to those found in neat films. The pure film and the 10 wt.% and 20 wt.% blends also exhibit similar single-exponential fluorescence decays, with characteristic PL lifetimes of 609, 570, and 611 ps, respectively (see Figure 77 in the supplemental material). This provides evidence that Bafu pure films do not exhibit any PL concentration quenching, unlike what has been reported for similar triple, Wu, and Qifu derivatives. ¹⁸Considering that in eight pure films, approximately 1.7 × 10 ⁹s ^{- 1}With a relatively large radiative decay rate, this oligomeric fluoride derivative can be expected to exhibit excellent ASE properties. ^{11 , 12}Variable-angle ellipsometry was used to measure the optical constants of the eight pure films and are shown in Figure 75c (the optical constants were calculated from ellipsometry data and can be found in Figure 78 in the supplementary material). The small optical anisotropy of the pure film indicates an almost random orientation of the octagonal molecules, which is consistent with the ellipsometry results reported previously in the octagonal pure film. ²⁰As schematically represented in Figure 79a, the ASE properties of eight pure films were characterized by optically pumping the sample at 337 nm with a nitrogen laser delivering 800 ps pulses at a repetition rate of 10 Hz. The excitation beam is concentrated into a strip with a size of 0.5 cm × 0.08 cm, and PL is collected from the edge of the organic film. Figure 79b shows the PL spectra measured from the edge of a 260 nm thick eight-foot pure film at various pump intensities. The spectral line narrowing effect is clearly visible at high excitation densities, with the full width at half maximum (FWHM) falling to 5 nm, providing evidence that ASE occurs in this sample. Light amplification occurs at approximately 450 nm due to spontaneously emitted photons that are waveguided in the organic film and amplified by stimulated emission. ^{twenty one}The ASE threshold is then determined based on the curve of the output intensity emitted from the edge of the film versus the excitation intensity. The abrupt change in ramp efficiency seen in Figure 79c results in approximately 90 nJ/cm ²The ASE threshold value. It should be noted that the ASE properties were measured in eight pure films with different film thicknesses ranging between 53 nm and 540 nm. The data presented in Figure 79d and Figure 80 (see Supplementary Information) indicate that films with a thickness of 260 nm have the lowest ASE threshold. Similar thickness dependence of the ASE threshold has been reported in poly(9,9-dioctylfluoride) films. ^{twenty two}This state is attributed to the interaction between the increase in mode confinement and the decrease in pump mode overlap when increasing thickness. Remarkably, the ASE threshold measured in a 260 nm thick 8-nm pure film is lower than the lowest value ever reported for an organic film based on small molecules. ³This good performance should also mean that the Bafu film exhibits extremely low loss coefficient values. For this purpose, the ASE intensity was measured based on the distance between the edge of the film and the pump strip. The results shown in Figure 81 (see Supplementary Information) lead to a 5.1 cm ^-1the loss coefficient. This low value is close to poly(9,9-dioctyl fluoride) film ^{twenty three}are low values reported in and provide evidence of the excellent optical waveguide properties of the films. It should be emphasized that, unlike most polyfluoroquinone systems, polyfluoroquinone, as well as most small molecules based on fluoroquinone, are ^{twenty four - 26}It does not exhibit any significant degradation of its photophysical properties under severe light irradiation. Furthermore, the results shown in Figure 82 (see Supplementary Information) demonstrate that the 8-pure film exhibits excellent photostability at high pump intensities above the ASE threshold in both ambient and nitrogen atmospheres. This may involve a high radiative decay rate of the film, which presumably leads to a reduction in photobleaching of the material under high intensity irradiation. Comparing the ASE threshold values measured in shorter oligofluorocarbons, ^14,18The results indicate that increasing oligomer length results in improved ASE performance. However, it should be noted that preliminary experiments performed on Shifu films demonstrated higher ASE thresholds than those obtained on Bafu films, indicating that Bafu derivatives are certainly organic semiconductor lasers in this series of oligomers. the most promising candidate. We then designed and fabricated a hybrid-order DFB grating structure consisting of a second-order Bragg scattering region surrounded by a first-order scattering region. ¹⁷This grating architecture is chosen to obtain a low laser threshold along with laser emission in a direction normal to the substrate. In DFB laser, the laser emits at Bragg wavelength ( λ _Bragg) occurs near, defined as: mλ _Bragg= 2 n _effΛ, where n _effis the effective refractive index of the laser gain medium, mis the Bragg order and Λ is the grating period. ^1-3Using the refractive index of eight pure films determined by ellipsometry (Figure 75c) and the ASE wavelength measured in this study, for m=1, 2, select the grating period as 260 nm and 130 nm respectively. Figure 83a and Figure 83b show this type of DFB SiO ₂Schematic representation and scanning electron microscope (SEM) image of a grating fabricated using electron beam lithography and reactive ion etching techniques. It should be noted that the depth of the DFB grating is approximately 70 nm. To complete the laser device, a 260-nm-thick 8-millimeter pure film was spin-coated on top of the DFB structure. Figure 83c shows the emission spectra detected normal to the substrate plane at several excitation densities below and above the laser threshold. Below the threshold, a Bragg dip due to the optical stopband of the DFB grating is observed. Above the laser threshold, a steep laser emission peak is clearly visible at the laser wavelength of approximately 452 nm. The output emission intensity and FWHM of this DFB laser as a function of excitation intensity are plotted in Figure 81d. The FWHM of the laser emission peak was found to be lower than 0.3 nm at high excitation density. At the same time, it was found that the laser threshold value determined based on the change in the slope of the output intensity curve is approximately 84 nJ/cm ², which is slightly lower than the previously reported ASE threshold. Overall, the extremely low ASE and laser threshold values measured in this work together with the excellent photostability of the films under high optical excitation intensities demonstrate that these eight derivatives are very promising for laser applications in organic semiconductors. gain medium material. To fully evaluate the potential of this octagonal derivative for use in organic laser diodes, it is also critical to study the electroluminescence (EL) properties of this compound in pure films and CBP blends using standard OLED structures. A schematic representation of the OLED fabricated in this study is provided in Figure 82a. The architecture of these devices is as follows: Indium Tin Oxide (ITO) (100 nm)/Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) (PEDOT: PSS) (45 nm)/ EML (approximately 40 nm)/2,8-bis(diphenylphosphonyl)dibenzo[b,d]thiophene (PPT) (10 nm)/2,2',2''-(1,3 ,5-Phenyltriyl)-Phenyl(1-phenyl-1-H-benzimidazole) (TPBi) (55 nm)/LiF (1 nm)/Al (100 nm), where the emissive layer (EML) corresponds In Bafu pure film or Bafu: CBP blend. In these devices, PEDOT:PSS functions as the hole injection layer, while PPT and TPBi serve as the hole blocking layer and electron transport layer respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy values of PEDOT:PSS, PPT and TPBi in Figure 84a are taken from the literature. ²⁰The ionization potential of the pure film measured by photoelectron spectroscopy in air was 5.9 eV (see Figure 85 in the supplemental material). Using the optical band gap value of 2.9 eV determined from the absorption spectrum of the pure film, the electron affinity of Bafu can be estimated to be approximately 3 eV. As shown in Figure 86a (see Supplementary Information), in these OLEDs at 10 mA/cm ²The EL spectra measured here are similar to the PL spectra measured in the Bafu pure film and in the CBP blend, indicating that the blue EL emitted from these devices comes only from the Bafu chromophore. Device current density-voltage-brightness ( J- V- L )The curve is shown in Figure 86b (see Supplementary Information). at 1 cd/m ²Here, OLEDs based on pure Bafu film, 10 wt.% CBP blend, and 20 wt.% CBP blend exhibit driving voltages of 5.0 V, 4.9 V, and 4.5 V respectively. The highest brightness value obtained in these OLEDs is 4580 cd/m for pure film ²(at 12.6 V), 8520 cd/m for 20 wt.% blend ²(at 10.4 V) and 8370 cd/m for 10 wt. % blend ²(at 11.2 V). External quantum efficiency of the device in terms of current density (η _ext) is plotted in Figure 84b. Its maximum value was found to be 3.9% for the neat film, 4.3% for the 20 wt.% blend, and 4.4% for the 10 wt.% blend. The difference in efficiency cannot be explained by the PLQY values of the three films, which are almost the same. In fact, current research devoted to the molecular orientation of oligomeric fluorine molecules in spin-coated films demonstrates that although octanoic acid molecules are randomly oriented in neat films, 20 wt.% and 10 wt.% octanoic acid:CBP blends Demonstrates relatively good horizontal orientation of the eight molecules. ²⁶These horizontal molecular orientations of the emitting dipole should lead to improvements in light extraction efficiency and thus explain the slightly higher η measured in OLEDs based on CBP blends. _extvalue. ^{20 , 26}In the case of organic laser diodes, the maximum η obtained in these OLEDs _extThe value is clearly promising. However, higher current densities should be injected into the device and above 100 mA/cm ²Efficiency roll-off occurs at such current densities, and this efficiency roll-off needs to be contained in additional work through improved device architecture before these eight derivatives can be carefully considered as candidates for electrically driven organic laser devices. In summary, this study demonstrates an unprecedented 90 nJ/cm in non-polymeric organic films ²The low ASE threshold value. Used in spin-coated neat films demonstrating 87% PLQY and 1.7 × 10 ⁹s ^{- 1}Eight derivatives with large radiative decay rates achieve this achievement. This blue-emitting material is then used in low-threshold organic semiconductor DFB lasers and has an external quantum efficiency as high as 4.4% and a maximum brightness value close to 10,000 cd/m ²in fluorescent OLEDs. 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Experimental procedures Photophysics and ASE MeasureThe octagonal derivative was synthesized following methods previously disclosed in the literature. ¹The fused silica substrate was cleaned by ultrasonic treatment using detergent, pure water, acetone and isopropyl alcohol followed by ozone treatment by ultraviolet radiation. Bafu pure film and CBP: Bafu blended film was deposited on a molten silica substrate by spin coating from a chloroform solution in a nitrogen-filled glove box. It should be noted that the concentration of the solution and the spin speed are changed to control the thickness of the pure film. UV-vis spectrophotometer (Perkin-Elmer Lambda 950-PKA) and spectrofluorometer (Jasco FP-6500) were used to measure the absorption and steady-state emission spectra respectively. The PLQY in the film was measured using a xenon lamp with an excitation wavelength of 340 nm and an integrating sphere (C11347-11 Quantaurus QY, Hamamatsu Photonics). PL decay was measured using a frame camera and a Ti-sapphire laser system (Millenia Prime, Spectra Physics) delivering optical pulses with a width of 10 ps and a wavelength of 365 nm. Variable-angle ellipsometry (VASE) (J.A. Wollam, M-2000U) was performed at various angles from 45° to 75° by steps 5° in 75 nm thick eight-foot pure films. Analysis software (J.A. Woollam, WVASE32) was then used to analyze the ellipsometry data to determine the anisotropic extinction coefficient and refractive index of the film. For the characterization of ASE properties, the sample was optically pumped with a pulsed nitrogen laser (KEN2020, Usho) emitting at 337 nm. This laser delivers pulses with a pulse duration of 800 ps at a repetition rate of 10 Hz. Vary the pump beam intensity using a set of neutral density filters. The pump beam is concentrated into a 0.5 cm × 0.08 cm strip. Emission spectra from the edge of the organic film were collected using an optical fiber connected to a charge-coupled device spectrometer (PMA-11, Hamamatsu Photonics). organic DFB Laser manufacturing and characterizationFollow the same cleaning procedure as above to clean thermally grown 1 μm thick SiO ₂layer of silicon substrate. Hexamethyldisilazane (HMDS) was then spin-coated on SiO ₂On top of the surface the sample was annealed at 120°C for 2 minutes. Thereafter, a 70 nm thick resist layer was spin-coated on the substrate from ZEP520A-7 solution (ZEON Co.) and annealed at 180°C for 4 minutes. Next, electron beam lithography using a JBX-5500SC system (JEOL) was used to pattern the DFB grating on the resist layer. After electron beam irradiation, the pattern was developed in a developer solution (ZED-N50, ZEON Co.). In the following steps, the patterned resist layer acts as an etch mask. Using EIS-200ERT etching system (ELIONIX) by CHF ₃Plasma etching the substrate. Finally, FA-1EA etching system (SAMCO) was used by O ₂Plasma - Etches the substrate to completely remove the resist layer. SEM (SU8000, Hitachi) is used to check the quality of DFB gratings. To complete the organic laser device, a 260 nm thick eight-foot pure film was finally spin-coated from the chloroform solution on the top of the DFB grating. For laser operation, pulsed excitation light from a nitrogen laser (SRS, NL-100) is concentrated on a 6 × 10 area of the device through lenses and slits. ^{− 3}cm ²on the area. The excitation wavelength is 337 nm, the pulse width is 3.5 ns, and the repetition rate is 20 Hz. The excitation light is incident on the device at an angle of approximately 20° relative to the normal line of the device plane. The emitted light perpendicular to the device surface was collected using an optical fiber connected to a multichannel spectrometer (PMA-50, Hamamatsu Photonics) placed 6 cm away from the device. Use a set of neutral density filters to control excitation intensity. OLED Manufacturing and characterizationOLEDs are fabricated by depositing organic layers and cathodes on pre-cleaned ITO glass substrates. The structure of the OLED fabricated in this study is as follows: ITO (100 nm)/PEDOT: PSS (45 nm)/EML (about 40 nm)/PPT (10 nm)/TPBi (55 nm)/LiF (1 nm)/ Al (100 nm), where the emissive layer (EML) corresponds to either a pure film or a CBP:CBP blend. PEDOT:PSS layer was spin-coated on ITO and annealed at 130°C for 30 minutes. Eight pure and blended films were spin-coated from a chloroform solution on top of a PEDOT:PSS layer in a glovebox environment. The thickness of the EML layer is typically about 40 nm. Next, a 10 nm thick PPT layer and a 40 nm thick TPBi layer were deposited by thermal evaporation. Finally, the cathode made of a thin LiF layer and a 100 nm thick Al layer was prepared by thermal evaporation through a mask. The active area of the device is 4 mm ². Prior to characterization, the device was encapsulated in a nitrogen atmosphere to prevent any degradative effects associated with oxygen and moisture. Under DC driving, use a power meter (Keithley 2400, Keithley Instruments Inc.) and an absolute external quantum efficiency measurement system (C9920-12, Hamamatsu Photonics) to measure current density-voltage-brightness ( J- V- L)characteristic. The EL spectrum was measured using an optical fiber connected to a spectrometer (PMA-12, Hamamatsu Photonics). References1. R. Anemian, J.C. Mulatier, C. Andraud, O. Stephan, J.C. Vial, Chem. Comm. 1608 (2002). [ 7 ] by CW amplified spontaneous emission ( ASE ) experimentCW amplified spontaneous emission (ASE) experiments were performed on double-core dendrites and eight-core spin-coated pure films. Thin films were deposited onto pre-cleaned flat fused silica substrates without encapsulation. The film thickness is approximately 250 nm. To study the properties of CW ASE, the film was optically pumped by a CW laser diode at 355 nm. An acousto-optic modulator (AOM, Gooch & Housego) triggered by a pulse generator (WF 1974, NF Co.) was used to deliver pulses with different widths. Emitted light was collected from the edge of the film using a frame eye (C7700, Hamamatsu Photonics) with a time resolution of 100 ps connected to a digital camera (C9300, Hamamatsu Photonics). A photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics) was used to record the emission intensity. Both the PMT response and the driving square wave signal were monitored on a multi-channel oscilloscope (Agilent Technologies, MSO6104A). In both materials, frame camera images and emission spectra at various pump strengths show clear line-narrowing effects above a threshold that are attributed to stimulated emission and can be assigned to the ASE. The ASE threshold is measured by comparing the output intensity of the excitation intensity curves for different pulse widths. The results show that the ASE threshold remains almost constant for varying pulse widths in the range of 100 μs to 5 ms. Furthermore, it can be noted that these ASE thresholds are consistent with those measured in these materials using a pulsed nitrogen laser (pulse width of 800 ps and repetition rate of 10 Hz). The possibility of achieving CW lasers in both eight-core and two-core dendrites implies negligible triplet loss. This is consistent with the fact that both materials exhibit extremely high photoluminescence quantum yields (PLQY) (92% PLQY in double-core dendrites and 82% PLQY in eight-fold pure films). In addition, transient absorption measurements were performed in Bafu and Shuangfuxin dendron solutions to examine the triplet-triplet absorption spectra. It can be seen that there is no overlap between the ASE and triplet absorption spectra, which provides clear evidence that triplet absorption does not play any harmful role in CW lasers in the two materials. [ 8 ] Current injection into organic semiconductor laser diodes OverviewThis laser diode is mainly based on inorganic semiconductors, but through a unique manufacturing route, organic matter can also be an excellent gain medium. However, despite advances in optically pumped organic semiconductor lasers, electrically driven organic semiconductor laser diodes have not yet been achieved. Here, we report the first demonstration of organic semiconductor laser diodes. Device incorporating mixed-order dispersed feedback SiO into organic light-emitting diode structure ₂grating and emits blue laser. These results prove that lasers that directly inject current into organic films can contain high current densities by selecting high-gain organic semiconductors that do not exhibit triplet and polaron absorption losses at the laser wavelength and designing appropriate feedback structures. losses to achieve. This represents a first step toward simple organic-based laser diodes that can cover the visible and near-infrared spectrum, and represents a major advance toward future organic optoelectronic integrated circuits. Detailed descriptionDue to the development of high-gain organic semiconductor materials and high-quality factor resonator structures ^{1 - 5}Significant advances in both design and design have greatly improved the properties of optically pumped organic semiconductor lasers (OSLs) over the past two decades. The advantages of organic semiconductors as gain media for lasers include their high photoluminescence (PL) quantum yield, large stimulated emission cross section, and broad emission spectrum across the visible region, as well as their chemical tunability and ease of processing. Recent advances in low-threshold distributed feedback (DFB) OSL demonstrate optical pumping of inorganic light-emitting diodes by electrically driving nanosecond pulses, providing a path towards new compact and low-cost visible laser technologies. ⁶route. This type of miniaturized organic laser is very promising for wafer laboratory applications. However, the ultimate goal is electrically driven organic semiconductor laser diodes (OSLDs). In addition to enabling the full integration of organic photonic and optoelectronic circuits, the implementation of OSLD will open novel applications in high-performance displays, medical sensing, and biocompatible devices. Problems with the realization of laser blocking by direct electrical pumping of organic semiconductors are mainly due to optical losses from the electrical contacts and occur at high current densities ^4,5,7-9Triplet and polaron losses below. Solutions to these fundamental loss problems have been proposed, including the use of triplet quenchers ^{10 - 12}The singlet-triplet exciton mutual destruction is used to curb triplet absorption loss and singlet quenching, and reduce the device's active area. ¹³It spatially separates exciton formation and exciton radiative decay and minimizes the polaron quenching process. However, even organic light-emitting diodes (OLEDs) and optically pumped organic semiconductor DFB lasers ⁵Progress has been made, but current injection into OSLDs has not yet been conclusively demonstrated. Previous research suggested that if the additional losses associated with electrical pumping were completely contained ¹⁴, then it needs to be higher than several kA/cm ²current density to achieve the laser from OSLD. One of the most promising molecules for realizing OSLD is 4,4'-bi[( N -Carbazole)styryl]biphenyl (BSBCz) (chemical structure in Figure 89a) ¹⁵, because of its excellent combination of optical and electrical properties (such as thin films (0.30 µJ cm under 800 ps pulsed light excitation) ^{− 2}) ¹⁶Low to medium amplified spontaneous emission (ASE) threshold) and tolerance greater than 2% at 5 µs pulse operation ¹³The maximum electroluminescence (EL) external quantum efficiency ( n _EQE) up to 2.8 kA cm in OLED ^{− 2}The ability of current density injection. In addition, recent optically pumped BSBCz-based DFB lasers ¹⁷It is demonstrated that the laser is possible under the repetition rate of 80 MHz and the long pulse light excitation of 30 ms and due to the extremely small triplet absorption loss at the laser wavelength of the BSBCz film. Here we demonstrate without a doubt the first example of laser from an organic semiconductor thin film based on mixed-order DFB SiO with an active area integrated into the device ₂The BSBCz film in the grating-inverted OLED structure was developed and completely characterized by electrical direct excitation through OSLD. The architecture and fabrication of the OSLD developed in this study are schematically shown in Figures 89a and 90 (see Materials and Methods for a detailed description of the experimental procedures). SiO on indium tin oxide (ITO) glass substrate ₂The sputtered layer was engraved by electron beam lithography and reactive ion etching to create a hybrid DFB grating with an area of 30 × 90 µm (Figure 89b), and the organic layer and metal cathode were vacuum deposited on the substrate to complete the device. We design a hybrid-order DFB grating with first-order and second-order Bragg scattering regions, which provide strong optical feedback and efficient vertical extraction of laser emission respectively. ^{17 , 18}. Based on Bragg conditions ^{4 , 19}, mλ _Bragg=2 n _effΛ _m , select the grating periods (Λ) of 140 nm and 280 nm for the first-order and second-order regions respectively. ₁and Λ ₂),in mis the diffraction order, λ _Braggis the Bragg wavelength, set to the reported maximum gain wavelength (477 nm) for BSBCz, and n _effis the effective refractive index of the gain medium, which is calculated for BSBCz ^{20 , twenty one}is 1.70. The lengths of individual first-order and second-order DFB grating regions in the characterized first set of devices, hereafter referred to as OSLDs, are 1.12 µm and 1.68 µm, respectively. Scanning electron microscopy (SEM) images in Figures 89c and 89d confirm that the fabricated DFB grating has periods of 140±5 nm and 280±5 nm, with a grating depth of approximately 65±5 nm. Complete removal of SiO in etched areas ₂Exposing the ITO layer is critical to making good electrical contact with the organic layer and was verified by energy dispersive X-ray spectroscopy (EDX) analysis (Figure 90c, Figure 90d). Cross-sectional SEM and EDX images of the complete OSLD are shown in Figure 89d and Figure 89e. The surface morphology of all layers exhibits a grating structure with a surface modulation depth of 50 nm to 60 nm. Although the interaction between the resonant laser mode and the electrode is expected to reduce the quality factor of the feedback structure, such a grating structure on the metal electrode should also reduce the device structure. ^{twenty two , twenty three}The pattern of inner guidance absorbs losses. The OSLD fabricated in this work has a simple reverse OLED structure ITO (100 nm)/20 wt.% Cs with energy levels: BSBCz (60 nm)/BSBCz (150 nm)/MoO ₃(10 nm)/Ag (10 nm)/Al (90 nm), as shown in Figure 91a. Doping the BSBCz film with Cs in the area close to the ITO contact improves the electrons injected into the organic layer, and MoO ₃Used as hole injection layer (Figure 92). Although the most efficient OLEDs generally use multi-layer architectures to balance charge ^24,25to achieve optimal performance, but at high current densities charges can accumulate at the organic heterointerface, which can affect device performance and stability. ²⁶harmful. The OSLD fabricated in this work contains only BSBCz as the organic semiconductor layer and is specifically designed to minimize the number of organic heterointerfaces. Also manufactured without SiO ₂A reference device of DFB grating (hereinafter referred to as OLED) was used to study the effect of grating on EL properties. Figure 91b shows optical microscopy images of OSLD and reference OLED operating at 3.0 V direct current (DC). In addition to the previously described DFB gratings, five other DFB grating geometries (Table 1) were optimized and characterized in OSLD. Although the EL emits uniformly from the active area of the reference OLED, more intense emission can be seen from the second-order DFB grating region in the OSLD, which is specifically designed to facilitate vertical light extraction (Figure 91b and Figure 93). Current density-voltage ( J- V)and n _EQE- JThe characteristics are shown in Figures 91c and 91d, and the characteristics obtained under DC conditions are shown in Figure 94. The active area used to calculate the current density of OSLD is estimated based on SEM and laser microscope images. Reference OLED's maximum current density before device failure is 6.6 A cm under DC operation ^{− 2}Increases to 5.7 kA cm under pulsed operation ^{− 2}, this is because by pulse operation ^13,27Joule plus after reduction. Under DC operation, the device exhibits greater than 2% maximum power at lower current densities. n _EQEand above 1 A cm ^{− 2}A strong efficiency roll-off appears at high current densities, which is presumably attributed to thermal degradation of the device. On the other hand, the efficiency roll-off in OLEDs under pulsed operation (Figure 91c, Figure 91d) is above 110 A cm ^{− 2}starting at the current density, which is consistent with previous reports ¹³consistent. Efficiency roll-off in OSLDs under pulsed operation was further curbed, and even found n _EQEsubstantially increases above 200 A cm ^{− 2}to achieve a maximum value of 2.9%. above 2.2 kA cm ^{− 2}under the current density n _EQEThe rapid decrease is most likely due to thermal degradation of the device. Although the EL spectrum of OLED is similar to the steady-state PL spectrum of pure BSBCz film (Figure 94c) and does not change with changes in current density, the EL spectrum from the glass surface of OSLD shows a spectrum with increasing current density under pulse operation. Line narrowing (Fig. 95a). at 478.0 nm for below 650 A cm ^{− 2}A Bragg dip in the current density corresponding to the stop band of the DFB grating was observed (Fig. 95b). When the current density increases above this value, a strong spectral line narrowing occurs at 480.3 nm, indicating the onset of laser emission. The intensity of the narrow emission peak was found to increase faster than the intensity of the EL emission background, which can be attributed to the nonlinearity associated with stimulated emission. The output intensity and full width at half maximum (FWHM) of the OSLD as a function of current are plotted in Figure 95c. Although the FWHM of the steady-state PL spectrum of the pure BSBCz film is about 35 nm, the FWHM of the OSLD decreases to values below 0.2 nm at high current densities, which is close to our spectrometer (0.17 nm for the 57 nm wavelength range) Spectral resolution limitations. The ramp efficiency of the output intensity changes suddenly with increasing current and can be used to determine 600 A cm ^{− 2}(8.1 mA) threshold value. Above 4.0 kA cm ^{− 2}In this case, the output intensity decreases with increasing current, presumably due to the onset of device collapse due to the strong increase in temperature, but the emission spectrum remains extremely steep. This increase and subsequent decrease are related to n _EQE- JThe curves match. The maximum output power measured by a power meter placed at a distance of 3 cm between the front of the OSLD and the ITO glass substrate (Figure 95d) is 3.3 kA cm ^{− 2}is 0.50 mW. These observed EL properties largely indicate that optical amplification occurs at high current densities and that electrically driven lasers are achieved above the current density threshold. The beam polarization and shape were characterized to provide further evidence that this is a laser ⁹. The output beam of the OSLD is largely linearly polarized along the grating pattern (Figure 96a), with the expected laser emission coming from a one-dimensional DFB. The spatial profile of the OSLD emission measured above the laser threshold at different current densities (Figure 96b) demonstrates the presence of well-defined Gaussian beams. Also, if this were a laser there should be the appearance of a spot pattern, providing preliminary evidence of spatial coherence. Before we can claim lasers, several phenomena that have been misinterpreted as lasers in the past must be excluded as observed states ⁹the cause. The emission of our OSLD is detected in the direction perpendicular to the substrate plane and exhibits a clear threshold state, so the line narrowing caused by the edge emission of the waveguide mode without laser amplification can not be considered ^20,28,29. ASE can occur in a similar manner to lasers, but the FWHM in our OSLDs (<0.2 nm) is much narrower than the typical ASE linewidths of organic films (a few nanometers) and is similar to that of optically pumped organic DFB lasers. Radiation (< 1 nm) ⁵consistent with typical FWHM. The extremely narrow emission spectrum obtained by unintentionally stimulating transformations in ITO has also been mistakenly attributed to the organic layer. ³⁰of launch. However, the emission peak wavelength of the OSLD in Figure 95a is 480.3 nm and cannot be attributed to emission from ITO, which has atomic spectral lines at 410.3 nm, 451.3 nm and 468.5 nm. ³¹If this is truly a laser from a DFB structure, the emission from the OSLD should be characteristic of the resonator mode, and the output should be extremely sensitive to any modification of the laser cavity. Therefore, OSLDs with different DFB geometries (labeled OSLD-1 to OSLD-5 (Table 1)) were fabricated and characterized (Figure 93) to confirm that the emission wavelength can be predictably tuned, which is consistent with optically pumped organic DFBs. laser ^4,5,32,33Very common in . The laser peaks of OSLD, OSLD-1, OSLD-2 and OSLD-3 are almost the same (480.3 nm, 479.6 nm, 480.5 nm and 478.5 nm respectively), which have the same DFB grating period. Additionally, OSLD-1, OSLD-2, and OSLD-3 owners have lower minimum FWHMs (0.20 nm, 0.20 nm, and 0.21 nm, respectively) and clear thresholds (1.2 kA cm, respectively) ^{− 2},0.8 kA cm ^{− 2}and 1.1 kA cm ^{− 2}). On the other hand, OSLD-4 and OSLD-5 with different DFB grating periods exhibit a FWHM of 0.25 nm and 1.2 kA cm at 459.0 nm. ^{− 2}threshold (OSLD-4), with a FWHM of 0.38 nm and 1.4 kA cm at 501.7 nm ^{− 2}The laser peak of the threshold value (OSLD-5). These results clearly demonstrate that the laser wavelength is controlled by the DFB geometry. To verify that the laser threshold of electrically driven OSLDs matches that obtained by optical pumping, N delivering a 3.0 ns pulse was used ₂Laser (excitation wavelength of 337 nm) measured the laser characteristics of the OSLD (OLSD-6) optically pumped through the ITO side (Figure 97). The laser peak of OLSD-6 under optical pumping (481 nm) is consistent with the laser peak of OSLD under electrical pumping (480.3 nm). The measured laser threshold under optical pumping is approximately 430 W cm ^{− 2}. Although this value is higher than without two electrodes ¹⁷30 W cm obtained in optically pumped BSBCz-based DFB laser ^{− 2}value, but the thickness of the layers in this OSLD is optimized to minimize optical losses caused by the presence of electrodes. Assuming no additional loss mechanisms in OSLD-6 at high current densities, 1.1 kA cm under electrical pumping ^{− 2}The laser threshold can be estimated based on the threshold under optical pumping. This value is consistent with the threshold value (0.6 to 0.8 kA cm) measured in smaller devices with the same grating period (OSLD and OSLD-2) under electrical pumping ^{− 2}) reasonably consistent. These results indicate that at high current densities ³⁴Additional losses that normally occur in OLEDs (including exciton mutual destruction, triplet and polaron absorption, quenching due to high electric fields, and Joule heating) have been almost suppressed in BSBCz OSLD. This is fully consistent with the fact that the EL efficiency roll-off is not observed in OSLD under severe pulse electrical excitation. Loss containment can be explained based on the properties of the BSBCz and the device. As mentioned previously, BSBCz films do not exhibit significant triplet loss ³⁵, and the reduction of the active area of the device leads to Joule heat-assisted exciton quenching ³⁶decrease. In addition, based on the respective measurements of composite films BSBCz:MoO ₃And BSBCz: The overlap between the polaron absorption and emission spectra of Cs for both radical cations and radical anions in BSBCz is negligible (Figure 98). Electrical and optical simulations of the device were performed to further confirm that current injection laser occurs in the OSLD (Figure 99). Simulations of devices with and without gratings using carrier mobilities extracted from fitting of experimental data for monopole devices (Fig. 99a, Fig. 99b) J- VThe curves are in excellent agreement with the experimental characteristics (Figure 99a, Figure 99c, Figure 99d), indicating sufficient etching for good electrical contact with devices with gratings. The recombination rate profiles (Figure 99e, Figure 99f) show periodic changes within the device as electrons pass from the ITO electrode through the insulating SiO ₂Periodic injection of grating. Similar to this reorganization, the exciton density (Fig. 100a) is distributed throughout the thickness of the organic layer, but is mainly concentrated in SiO ₂In an area that does not obstruct the path from cathode to anode. The average exciton density of OSLD and OLED (Figure 99g) is similar, indicating that it is close to SiO ₂The high accumulation compensation of excitons results in a low exciton density between gratings (non-injection regions) with similar exciton densities relative to the reference device. Calculated resonant wavelengths of light traps in OSLD for light extraction from the second-order grating and waveguide losses in the ITO layer λ ₀= Stimulated electric field distribution of light field at 483 nm E( x, y) is clearly visible (Fig. 100b). The DFB resonator cavity is limited by a 40% ΓAnd the quality factor representation of 255. The modal gain ( g _m) (which is an indicator of light amplification in laser mode), has a value for 2.8 10 ^{− 16}cm ²BSBCz ³⁵stimulated emission cross section σ _Stimulateand is shown in Figure 100c for the second-order region. Above 500 A cm ^{− 2}The high modal gain and increasing modal gain are consistent with laser observations. exist J=500 A cm ^{− 2}The area where there is strong spatial overlap between the exciton density and the optical mode (Fig. 100d) corresponds to the area where both the exciton density and the optical field (Fig. 100a, Fig. 100b) are high. Therefore, the DFB structure also helps enhance localization and optical mode coupling via high exciton densities in and above the valleys of the grating. In summary, this study demonstrates that through appropriate design and selection of resonators and organic semiconductors, lasers driven from current-driven organic semiconductors may contain losses and enhance coupling. The laser demonstration here has been reproduced in multiple devices and fully characterized to rule out other phenomena that could be mistaken for lasers. This result fully supports the claim that this is the first observation of electrically pumped laser in organic semiconductors. Low losses in BSBCz are essential to enable lasers, so the development of strategies to design new laser molecules with similar or improved properties is a critical next step. This report opens up new opportunities in organic photonics and serves as a basis for the future development of organic semiconductor laser diode technology that is simple, cheap, and tunable and enables complete and direct integration into organic-based optoelectronic platforms. Materials and methods Device manufacturingClean indium tin oxide (ITO)-coated glass substrate (100 nm thick ITO, Atsugi Micro Co.) using neutral detergent, pure water, acetone and isopropyl alcohol by ultrasonic treatment followed by UV ozone treatment. . SiO 100 nm thick ₂The layer (which will become the DFB grating) is sputtered onto an ITO coated glass substrate at 100°C. The argon gas pressure during sputtering was 0.66 Pa. RF power was set to 100 W. The substrate was again cleaned by ultrasonic treatment followed by UV ozone treatment using isopropyl alcohol. SiO was treated with hexamethyldisilazane (HMDS) by spin coating at 4,000 rpm for 15 s. ₂surface and annealed at 120°C for 120 s. A resist layer with a thickness of approximately 70 nm was spin-coated on the substrate from ZEP520A-7 solution (ZEON Co.) at 4,000 rpm for 30 s and baked at 180°C for 240 s. Use a 0.1 nC cm ^{− 2}A JBX-5500SC system (JEOL) with optimized dosage was used for electron beam lithography to draw the grating pattern on the resist layer. After electron beam irradiation, the pattern was developed in a developer solution (ZED-N50, ZEON Co.) at room temperature. The patterned resist layer was used as an etch mask, while an EIS-200ERT etch system (ELIONIX) was used with CHF ₃Plasma etching the substrate. To completely remove the resist layer from the substrate, use a FA-1EA etching system (SAMCO) with O ₂Plasma etching the substrate. Etch conditions optimized to completely remove SiO from grooves in DFB ₂Until ITO is exposed. Using SEM (SU8000, Hitachi) to observe the formation of SiO ₂Grating on the surface (Fig. 89c). Perform EDX (at 6.0 kV, SU8000, Hitachi) analysis to confirm complete removal of SiO from the DFB spacing ₂(Figure 90c and Figure 90d). Cross-section SEM and EDX were measured by Kobelco using cold field emission SEM (SU8200, Hitachi High-Technologies), energy dispersive X-ray spectrometry (XFlash FladQuad5060, Bruker) and focused ion beam system (FB-2100, Hitachi High-Technologies). (Fig. 89d, Fig. 89e). The DFB substrate is cleaned by conventional ultrasonic treatment. Then by using the 1.5 × 10 ^{− 4}Thermal evaporation under a pressure of Pa is 0.1 nm s ^{− 1}to 0.2 nm s ^{− 1}The total evaporation rate of the organic layer and metal electrode is placed on the substrate in vacuum to produce a structure with indium tin oxide (ITO) (100 nm)/20 wt% BSBCz: Cs (60 nm)/BSBCz (150 nm)/MoO ₃(10 nm)/Ag (10 nm)/Al (90 nm) structure OSLD. SiO on ITO surface ₂layer acts as an insulator in addition to the DFB grating. Therefore, the current area of OLED is limited to the DFB area where BSBCz is in direct contact with ITO. A reference OLED with an active area of 30 × 45 µm was also prepared using the same current area. Device characterizationAll devices were encapsulated in a nitrogen-filled glove box using glass covers and UV-cured epoxy to prevent any degradation caused by moisture and oxygen. The current density-voltage- of OSLD and OLED was measured at room temperature using a integrating sphere system (A10094, Hamamatsu Photonics). n _EQE(J-V- n _EQE) characteristics (DC). For pulse measurements, a rectangular pulse with a pulse width of 400 ns, a pulse period of 1 µs, a repetition frequency of 1 kHz and varying peak current was applied to the device using a pulse generator (NF, WF1945) at ambient temperature. Measured by amplifier (NF, HSA4101) and photomultiplier tube (PMT) (C9525-02, Hamamatsu Photonics) under pulse driving J- V-Brightness characteristics. Both the PMT response and the driving square wave signal were monitored on a multi-channel oscilloscope (Agilent Technologies, MSO6104A). Calculated by dividing the number of photons calculated from the PMT response EL intensity with a correction factor multiplied by the number of injected electrons calculated from the current n _EQE. The output power was measured using a laser power meter (OPHIR Optronics Solution Company, StarLite 7Z01565). To measure the spectrum, the emitted laser light for the optically and electrically pumped OSLD perpendicular to the device surface was collected using an optical fiber connected to a multichannel spectrometer (PMA-50, Hamamatsu Photonics) and placed 3 cm away from the device. . The beam profile of the OSLD is checked by using a CCD camera (beam profiler WimCamD-LCM, DataRay). For the characteristics of OSLD-6 under optical pumping, the nitrogen laser (NL100, N ₂laser, Stanford Research System), the pulsed excitation light is concentrated on 6 × 10 of the device ^{− 3}cm ²in the area. The excitation wavelength is 337 nm, the pulse width is 3 ns, and the repetition rate is 20 Hz. The excitation light is incident on the device at an angle of approximately 20° relative to the normal line of the device plane. Use a set of neutral density filters to control excitation intensity. Steady-state PL spectral analysis was monitored using the spectrofluorometer (FP-6500, JASCO) in Figure 98 and the spectrometer (PMA-50) in Figure 94. Device modeling and parametersThe optical simulation of the resonant DFB cavity was performed using Comsol Multiphysics 5.2a software. Use the finite element method (FEM) to solve the Helmholtz equation for each frequency in the RF module of Comsol software. Each layer is represented by its birefringence and thickness. The computational domain is limited by a supercell consisting of a second-order grating surrounded by a first-order grating. Froquat periodic boundary conditions are applied to the lateral boundaries, and scattering boundary conditions are used to the top and bottom domains. Only the TE mode is considered as it is suppressed since the TM mode experiences more losses (due to metal absorption) than the TE mode. Charge transport through OSLD is described using a two-dimensional time-independent drift diffusion equation coupled to Poisson's equation and a continuity equation for charge carriers using Silvaco's Technical Computer Aided Design (TCAD) software. Use parabolic density of states (DOS) and Maxwell-Boltzmann statistics to represent electron and hole concentrations. Gaussian distribution for modeling organic semiconductors ³⁷The distribution of traps within. Assume that the charge carrier mobility is field dependent and has the Pool-Frenkel form ^{38 , 39}. High-energy chaos is not considered in this model, so we assume the validity of Einstein's relation to calculate the charge carrier diffusion constant from charge mobility. By Langevin model ⁴⁰gives the recombination rate R. Solve the continuity equation for singlet excitons by considering exciton diffusion, radiative and non-radiative processes. Experimental data on pure holes and pure electrons , in Lis the cavity length (second-order grating area only) and dis the thickness of the film. Table 1 | Parameters of different OSLD geometric structures device w (µm) l (µm) Λ ₁ (nm) Λ ₂ (nm) w ₁ (µm) w ₂ (µm) A (µm ² ) OLED 30 45 - - - - 1,350 OSLD 30 90 140 280 1.68 1.12 1,350 OSLD-1 35 90 140 280 14.00 7.00 1,575 OSLD-2 90 30 140 280 1.68 1.12 1,350 OSLD-3 101 30 140 280 45.36 10.08 1,515 OSLD-4 30 90 134 268 1.608 1.072 1,350 OSLD-5 30 90 146 292 1.752 1.168 1,350 OSLD-6 560 800 140 280 1.68 1.12 224,000 Parameters for different grating geometries and the total exposed ITO area used to calculate current density are shown in Figure 90 Avalue. Table 2. Parameters of optical simulation and electrical simulation. parameters BSBCz BSBCz:Cs unit ε _r 4 4 - E _HOMO 5.8 5.8 eV ELUMO _{_} 3.1 3.1 eV N _HOMO 2 × 10 ⁻¹⁹ 2 × 10 ⁻¹⁹ cm ⁻³ NLUMO _{_} 2 × 10 ⁻¹⁹ 2 × 10 ⁻¹⁹ cm ⁻³ nnJC _{_} 2.8 × 10 ⁻¹⁷ - cm ⁻³ E _t 0.375 - eV σtp _{_} 0.017 - eV n0 _{_} 6.55 × 10 ⁻⁵ 6.55 × 10 ⁻⁵ cm ² V ⁻¹ s ⁻¹ µ _p0 1.9 × 10 ⁻⁴ 1.9 × 10 ⁻⁴ cm ² V ⁻¹ s ⁻¹ f _{_} 175,561 175,561 V cm ⁻¹ F _p0 283,024 283,024 V cm ⁻¹ k _r 10 ⁹ 10 ⁹ s ⁻¹ n _{_} 0.11 × 10 ⁹ 0.11 × 10 ⁹ s ⁻¹ φPL _{_} 0.76 0.4 - L _S 18 18 nm ε _ris the relative permittivity of the material. E _HOMOand E _LUMOThey are the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) respectively. N _HOMOand N _LUMOis the energy state density of HOMO level and LUMO level. N _tpis the total density of traps, E _tpis the energy depth of the trap above the HOMO level, and σ _tpis the width of the Gaussian distribution. µ _n0and µ _p0is zero field mobility. 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Figure 1. (a) Chemical structures of BSBCz and CBP, (b) Schematic diagram of BSBCz:CBP film embedded in a second-order DFB structure , (c) used to fabricate DFB resonator structures by electron beam lithography and dry etching. Schematic representation of the method, namely SEM images of the DFB structure at (d) 4500× magnification and (e) 120000× magnification, and (f) absorption and photoluminescence spectra of pure films of BSBCz and CBP. DFB structures are engraved directly onto the silicon dioxide surface. When the second-order mode ( m = 2) is used in the Bragg equation ( mλ = 2 n _eff Λ ), the diffracted beam is emitted in a direction perpendicular to the plane of the film as shown in (b). The large overlap of CBP emission and BSBCz absorption enables efficient Förster-type energy transfer from CBP to BSBCz. In the BSBCz:CBP blended film, CBP molecules are mainly excited by 337 nm light. Next, due to efficient energy transfer, emission comes from BSBCz, while no emission is observed from CBP. Figure 2. Emissions as a function of excitation intensity for (a, c) BSBCz:CBP blended films and (b, d) BSBCz pure films embedded in DFB structures, when excited with 0.8 ns wide pulses at 20 Hz. Emission spectra and graphs of intensity and FWHM excitation. The insets in (a, b) show measured emission spectra close to the laser threshold. Figure 3. (a) is the frame camera image of laser oscillation and (b, c) are the time changes of laser intensity in BSBCz:CBP blended film. The repetition rate changes from 0.01 MHz to 8 MHz. The excitation light intensity is fixed at approximately 0.44 μ J cm ⁻² , which is 1.8 times higher than E _th = 0.25 μ J cm ⁻² . The time scale of (a, b) is 500 μs and the time scale of (c) is 2 μs. Figure 4. (a): Curve plot of laser threshold value as a function of repetition rate and (b): Operation stability of laser oscillation in organic DFB laser based on BSBCz:CBP blended film and BSBCz pure film . To measure the stability, the laser device was operated continuously and accurately at 8 MHz. The excitation light intensity is 1.5 times higher than the laser threshold value. Figure 5. (a): Chemical structure of liquid carbazole (ECHz) main body and seven derivatives. (b): Schematic representation of liquid DFB laser prepared using first-order 1D grating patterns. (c): SEM image of a PUA pattern with a period of 140 nm and a height of 100 nm. Scale bar represents 200 nm. (d): The emission spectrum of a nitrided and oxidized liquid DFB laser measured above the threshold value at a repetition rate of 1 MHz. (e): Image of an oxidized blue liquid DFB laser operating in a quasi-cw state. Figure 6. (a, b) are frame camera images of laser emission and (c, d) are the time changes of laser intensity in nitrided and oxidized EHCz:7-DFB lasers. Repetition rate increased from 0.01 MHz to 4 MHz. The excitation light intensity was kept constant at 2.5 μJ cm ⁻² . Figure 7. Changes in the output emission intensity and full width at half maximum of the nitrided EHCz: Qifu DFB laser with the excitation intensity at different repetition rates. Figure 8. Changes in output emission intensity and full width at half maximum of oxidized EHCz: Qifu DFB laser with excitation intensity at different repetition rates. Figure 9. (a) Laser threshold value of nitrided and oxidized liquid DFB organic semiconductor laser as a function of repetition rate. (b) Photostability of an oxidized and nitrided liquid organic semiconductor DFB laser operating in a quasi-cw state at a repetition rate of 1 MHz. The excitation light intensity is 2.5 μJ cm ^{- 2} . Figure 10. (a) Triplet-triplet absorption spectrum of Qifu in oxidized and nitrided chloroform solution. The triplet-triplet absorption disappears in the oxidized solution due to quenching of the triplet by molecular oxygen. Also shown is a representative laser spectrum measured in an oxidized DFB device. (b) Transient decay of triplet-triplet absorption specific gravity with a time constant much higher than 1 ms. Figure 11. Kinetics of photoluminescence intensity measured at 0.5 kW cm ^-2 in nitrided and oxidized EHCz:Qifu blends for 4 different pulse durations ranging from 50 to 800 μs. For this experiment, the gain medium package was sandwiched between two identical flat fused silica substrates. The sample is then optically pumped by laser pulses from a cw HeCd laser emitting at 325 nm. The light spot area is 2.0 × 10 ^{- 5} cm ² . The sample was irradiated in a direction perpendicular to the substrate plane and the PL intensity was measured using a photomultiplier tube at an angle of 45°. These results clearly demonstrate that oxygenation induces suppression of STA loss due to quenching of the triplet state by molecular oxygen. Figure 12. Manufacturing method of organic semiconductor DFB laser. It is a schematic representation of the method used to make organic DFB lasers. Different sequential steps involve fabrication of the DFB resonator structure by electron beam lithography, thermal evaporation of the organic semiconductor film and spin-coating of the CYTOP polymer film, followed by sealing the device with a high thermal conductivity (TC) sapphire cap. Figure 13. Structure of mixed-order DFB resonator. ( A ) Schematic representation of the mixed-order DFB grating structure used in this study. SEM images with ( B ) 2500× magnification and ( C ) 100000× magnification after depositing a 200 nm thick BSBCz:CBP blended film, and ( D ) SEM image of the device and (E) cross-sectional SEM of the device image. Figure 14. Laser properties of organic DFB laser in quasi-CW state. Bar camera images showing laser oscillations from a representative BSBCz:CBP encapsulated hybrid-stage DFB device at repetition rates from 0.01 MHz to 80 MHz over a period of ( A ) 500 µs or ( B ) 200 ns (only 80 MHz). The excitation intensity is fixed at approximately 0.5 µJ cm ^{− 2} , which is above the laser threshold ( E _th ). ( C ) Time evolution of the laser output intensity at various repetition rates (f) in the encapsulated BSBCz:CBP hybrid-stage DFB laser. ( D ) Laser thresholds as a function of repetition rate in certain types of DFB devices. Lines guide the line of sight. Figure 15. Laser properties of organic DFB laser in long pulse state. ( A ) Frame camera image showing integration of 100 pulses from an encapsulated mixed-stage DFB device using BSBCz:CBP (20:80 wt.%) film as gain medium and by 30 ms and 2.0 kW Optically pumped laser emission with pulses of cm ^-2 (top) or 800 µs and 200 W cm ^-2 (bottom) ^. ( B ) Photograph of the DFB device operating in the long pulse regime (30 ms excitation). ( C ) Laser threshold ( E _th ) as a function of excitation duration in various DFB devices. The dotted line guides the line of sight. ( D ) Laser output intensity from organic DFB laser changes with the number of incident pulses (100 µs, 200 W cm ^{− 2} ). Figure 16. Schematic representation of the geometry used for optical simulations. Figure 17. Laser threshold values of mixed-order DFB laser based on BSBCz:CBP (6:94 wt.%) 200 nm thick film for second-order gratings of different sizes. The device was optically pumped by delivering an 800 ps pulse of nitrogen laser at a repetition rate of 20 Hz and a wavelength of 337 nm. Figure 18. (A) Absorption spectrum, (B) steady-state photoluminescence spectrum and (C) transient state of BSBCz:CBP (6:94 wt.% ) films encapsulated with or without CYTOP film Photoluminescence decay. Figure 19. Representative organic hybrid - stage DFB lasers based on (A) BSBCz:CBP (6:94 wt.%) and (B) BSBCz pure films at various pump intensities below and above the laser threshold. The emission spectrum below. The inset shows the emission spectrum near the laser threshold. Emission output intensity and FWHM change with pump intensity in (C) blended film and (D) pure film DFB laser. In these experiments, the organic film was covered by a spin-coated CYTOP film and a sapphire cap. The optical pump source is a nitrogen laser that emits a 0.8 ns wide pulse excitation at a repetition rate of 20 Hz and a wavelength of 337 nm. Emissions from the laser device are collected in a direction perpendicular to the plane of the substrate. Figure 20. Emission output intensity and FWHM as a function of pump intensity in a representative encapsulated blend mixed-stage DFB laser at repetition rates of (A ) 0.01 MHz and (B) 80 MHz. Figure 21. Laser threshold as a function of repetition rate measured in the forward direction (increasing repetition rate) and reverse direction (decreasing repetition rate). The irreversible change in the laser threshold is attributed to the degradation of the gain medium under strong light excitation. Figure 22. (A) Frame camera image and (B) corresponding emission spectra from an unencapsulated 2-stage DFB device, an unencapsulated mixed-stage DFB device , and an encapsulated mixed-stage DFB device. Emission at a repetition rate of 80 MHz and a pump intensity of 0.5 μJ cm ^{− 2} . Figure 23. (A) Operational stability of laser output oscillations from different organic DFB lasers based on BSBCz:CBP (6:94 wt . %) blend films. To measure the stability, operate the laser device continuously in quasi-CW at 8 MHz or 80 MHz for 20 minutes. For each device, the excitation light intensity is 1.5 times its laser threshold. (B) Image of an encapsulated blended DFB laser operating in quasi-CW regime at 80 MHz. Figure 24. (A) In a 200 nm thick BSBCz:CBP ( 20:80wt.%) film, the output intensity of ASE concentrated at 472 nm changes with pump energy and strip length. The dashed line was fitted to the data using the equation reported in references ( 7 , 8 ). The net gain of the waveguide is determined based on these fits. (B) ASE intensity emitted from the edge of the waveguide film versus the distance between the pump strip and the edge of the blended film. The ASE characteristics of the films were studied using inorganic laser diodes with different pulse widths (0.1, 10 and 50 μs) emitting at 405 nm. Figure 25. Encapsulated 20 wt.% blend mixed - order DFB mine measured at pump intensities of 200 W cm ^{− 2} and 2.0 kW cm ^{− 2} for long pulse durations of 800 µs and 30 ms, respectively. Radiation emission spectrum. Figure 26. Frame camera image showing laser emission integrated within 100 pulses from an encapsulated mixed-stage DFB device during a 30 ms long optical excitation with a pump power of 2.0 kW cm ^-2 . The gain medium is BSBCz:CBP (20:80 wt.%). The excitation wavelength is 405 nm. To observe the laser during 30 ms using our frame camera system, the duty cycle percentage is changed to 2% to observe the 30 ms pulse in a 1 ms frame (0.02 × 30 ms = 0.6 ms). Figure 27. (A) Time evolution of laser output intensity , (B) encapsulated laser measured after 10 and 500 average pulses with a width of 30 ms and a pump intensity of 2.0 kW cm ^{− 2} Emission spectrum of mixed-order DFB laser. (C) Stimulated emission and triplet absorption cross-section spectra of BSBCz. The emission spectrum of the DFB laser measured from the BSBCz pure film is higher than E _th . (D) Triplet absorption spectrum measured under Ar in a solution containing BSBCz. ( E ) Excitation power dependence of transient absorption spectra in solutions (dichloromethane containing BSBCz and benzene containing benzophenone). Figure 28. (A) Images showing the divergence of the emitted beam of a DFB laser near (B) and above (C) the threshold. The active gain medium is BSBCz:CBP (20:80 wt.%) film. The device was pumped in the long pulse regime (10 ms excitation). Figure 29. (A) Emission spectrum as a function of polarization angle and (B ) Emission intensity as a function of polarization angle. The device is based on a BSBCz:CBP (20:80 wt.%) blend and uses a mixed-order DFB grating. The pump intensity is 200 W cm ^{− 2} and the pump pulse duration is 800 µs. It should be noted that 0 ^° corresponds to the direction parallel to the groove of the DFB grating. Figure 30. Emission spectra and laser output intensity of the encapsulated mixed-stage doping device as a function of pump intensity for long pulse excitation widths of (A, B) 1 µs and (C, D) 800 µs . and FWHM. Figure 31. Frame camera images showing laser emissions from mixed-stage blended BSBCz:CBP (20:80 wt.%) DFB devices encapsulated with (A ) sapphire or (B) glass cover. (C) Emission spectra of the device encapsulated with a glass cover for various pulse widths with a pump intensity of 2.0 kW cm ⁻² . The emission spectrum becomes broader with longer pulse width, which can be explained by the significant increase in laser threshold with pulse width. For example, with pulse widths of 2 ms and 3 ms, the device operates below the laser threshold. (D) Laser threshold ( E _th ) as a function of excitation pulse width measured in DFB devices encapsulated with sapphire or glass lids. The dotted line guides the line of sight. Figure 32. Laser microscopy images of unencapsulated blend hybrid - stage DFB lasers before (A) and (B) after irradiation by 100 excitation pulses with a width of 1 ms. The thickness profile in inset (B) shows the ablation of an organic film during high-intensity CW irradiation. The pump intensity is 200 W cm ⁻² . The framed camera images in (C) and (D) show the results of encapsulated and non-encapsulated blends under the same irradiation conditions (1 ms pulse, 200 W cm ^{− 2} pump intensity and 100 pulses Integration of framed camera images). Figure 33. Scheme of DFB organic laser. Figure 34. (a) Scheme of 3 - layer thick block waveguide. (b) Effective refractive index ( n _eff ) of TE and TM modes as a function of film thickness d at laser wavelength 477 n. Figure 35. Reflection spectrum calculated numerically (solid line) and Fano - profile fitted reflection spectrum (dashed line) for different values of film thickness under normal incident TE polarization. Figure 36. (a) Experimental laser emission spectra of fabricated laser devices with different d _f . (b) Experimental laser wavelength and calculated resonance wavelength. Figure 37. (a) E _th (square), Q factor extracted from numerical calculation ( Q _{FEM calculation} (triangle) and Fano fit Q _{Fano fit} (star)) and Γ (circle) with d Graph of _f change, (b) graph of experimental FWHM of laser emission and FWHM calculated from Fano resonance. Figure 38. Schematic of the geometry used for thermal simulation. Figure 39. Maximum temperature at the end of each pulse. Figure 40. Temperature rise in the gain region with time and different pulse widths: (A) 1, (B) 10, (C) 30 and ( D ) 40 ms. Figure 41. Temperature rise over time for a pulse width of 10 ms in encapsulated and unencapsulated devices. Figure 42. Temperature rise in the gain region with time or number of pulses for τ _p =30 ms. Figure 43. Schematic representation of methods for fabricating DFB gratings and organic laser diodes for electrically driving organic semiconductor DFB laser diodes. Different sequential steps involve sputtering a 100 nm thick _SiO2 layer on top of the patterned ITO electrode, fabricating the DFB resonator structure in _SiO2 by electron beam lithography and dry etching, and thermal evaporation of the organic semiconductor film and top electrode. Figure 44. Schematic representation of the substrate after different steps of DFB fabrication on ITO. ( A ) Patterned ITO, ( B ) after sputtering _SiO2 onto ITO, ( C ) after fabricating DFB on ITO and ( D ) DFB structure. Figure 45. Schematic representation of the method used to fabricate nanoimprinted DFB gratings. Different sequential steps involve the preparation of a 70 nm thick polymer layer on top of the patterned ITO electrode, followed by the fabrication of the DFB resonator structure in the polymer using a simple, low-cost nanoimprint lithography process. Figure 46. Structural characterization of mixed - stage DFB resonators for organic laser diodes. ( A ) Laser microscopy and ( B ) SEM images (with 5000× and 200000× (in inset) magnification) of mixed-level DFB SiO _grating structures prepared on top of ITO patterned glass substrates. ( C , D ) EDX and SEM analysis of mixed-level DFB gratings prepared on top of ITO. These images confirm the exposure of ITO used in contact with the device. Figure 47. Structure and properties of electrically driven organic semiconductor DFB laser. ( A ) Schematic representation and energy level diagram of an organic semiconductor laser diode. ( B ) Micrographs of organic DFB laser diodes and ( C ) reference device (OLED without grating) with and without DC operation at 4 V. The device area is 140 × 200 μm. ( D ) Current density-voltage (JV) curves and ( E ) external quantum efficiency-current density (EQE-J) curves measured in a reference OLED device and organic DFB laser diode under DC and pulse operation. Figure 48. Energy level diagrams of (a) pure electronic devices (b) pure hole devices and (c) bipolar devices. Figure 49. (a) Reported (symbols) and fitted ( solid line) mobilities of the Pool-Frankel field correlation model (solid line) for holes (blue) and electrons (red) for BSBCz. (b) Experimental (symbols) and simulated (solid lines) J ( V ) curves of pure hole devices (blue), pure electronic devices (red), and bipolar devices (black). Figure 50. SEM ( A , B ) surface morphology images and ( C , D ) cross-sectional images of the laser diode structure after all layers have been deposited. Figure 51. Schematic representation of some possible configurations of organic semiconductor laser diodes. DFB resonator structures (second-order and mixed-order gratings) can be produced ( A ) by electron beam lithography and dry etching in _SiO2 , ( B ) by electron beam lithography and dry etching in ITO, ( C ) by electron beam lithography and dry etching in ITO Nanoimprint lithography in the polymer on top of a patterned ITO electrode, or ( D ) fabricated by nanoimprint lithography on top of the active layer. Figure 52. Schematic representation of organic semiconductor laser diodes with two -dimensional DFB resonator structures (second-order and mixed-order gratings) for 2D DFB laser. Figure 53. Micrographs (A to D ) of organic DFB laser diodes ( A to D ) with and without DC operation at 4 V. The device in A , B was constructed with 36 first-order periods surrounded by 324 The DFB structure of the second-order period was prepared, and the devices in D and C were prepared using a repeating structure with 4 second-order periods surrounded by 12 first-order periods). The device area is 30 × 101 μm. ( E ) Emission spectra of an electrically driven organic semiconductor DFB laser (the first-order and second-order grating periods of this device are 140 nm and 280 nm, respectively) collected perpendicular to the substrate plane for different injection current densities and ( F ) with current density Varying output intensity. Figure 54. Electroluminescence and PL spectra of BSBCz (reference device black PL spectrum, red EL spectrum, and blue EL spectrum with grating below laser threshold). Figure 55. Laser properties of electrically driven organic semiconductor DFB laser diodes. ( A ) The emission spectrum of an electrically driven organic semiconductor DFB laser (the first-order and second-order grating periods of this device are 140 nm and 280 nm, respectively) collected perpendicular to the substrate plane for different injection current densities and ( B ) its current dependence Output intensity of density changes. ( C ) Emission spectrum and ( D ) output intensity versus current density obtained in an organic DFB laser diode using a 1st-order grating period of 150 nm and a 2nd-order grating period of 300 nm, respectively. Figure 56. Laser properties of electrically driven organic semiconductor DFB laser diodes. ( A ) The emission spectrum of an electrically driven organic semiconductor DFB laser (the first-order and second-order grating periods of this device are 140 nm and 280 nm, respectively) collected perpendicular to the substrate plane for different injection current densities and ( B ) its current dependence Output intensity of density changes. Figure 57. ( A ) Current density-voltage (JV) curves with and without DFB. Devices with DFB have a first-order grating period of 150 nm and a second-order grating period of 300 nm. ( B ) External quantum efficiency versus current density in an electrically driven organic DFB solid-state laser without DFB at a 500 ns pulse. Figure 58. SEM images of mixed - order gratings with different numbers of periods in the first-order and second-order regions ( A , B , C , D , and E ) . ( F ) Table of mixed-order gratings with different numbers of periods in the first-order and second-order regions used to design DFB. Figure 59. ( A ) Laser spectra of mixed -order gratings with different numbers of periods. The number of cycles at the top of each graph corresponds to the number of 2nd order cycles and the number of 1st order cycles that can be seen in Figure 58 . The bottom graph shows the output characteristics of the device. ( B ) Threshold energy of mixed-order grating lasers in the second-order region for different numbers of periods. In the case of optical pumping, the lowest threshold is demonstrated using 4 and 12 cycles in the first-order and second-order regions respectively. The optical pump source is a nitrogen laser that emits a 0.8 ns wide pulse excitation at a repetition rate of 20 Hz and a wavelength of 337 nm. Emissions from the laser device are collected in a direction perpendicular to the plane of the substrate. Figure 60. ( A ) SEM image of a mixed - order grating with 12 and 4 periods in the first-order and second-order regions, respectively. ( B , C ) Organic DFB laser diode with or without DC operation at 5 V. The device area is 2.9 × 10 μm. ( D ) Reference OLED emission spectrum. Figure 61. ( A ) Schematic representation of the experimental setup for examining the polarization of lasers. The EL spectrum of the DFB laser diode ( B ) is below ^the laser threshold value (at 415 A cm ^{− 2} ) and ( C ) is above the laser threshold value (at 823 A cm ⁻² ) ionization correlation and ( D ) reference (at 800 A cm ^{− 2} , no DFB), ( E ) EL intensity as a function of polarization angle. EL is polarized in the plane of the device (TE mode). Figure 62. ( A ) Schematic diagram of OLED with DFB. ( B ) Microscope image of a DFB with a device area of 560 × 800 μm on an ITO substrate. ( C ) Optical pump output intensity as a function of excitation density for DFB laser; excited by CW laser at 405 nm. Figure 63. Micrographs of ( A , B ) organic DFB laser diodes and ( C , D ) reference devices (grating-less OLEDs) with and without DC operation at 4 V. Figure 64. Laser microscope image of a circular mixed-order DFB grating structure prepared using SiO ₂ on an ITO patterned substrate. Figure 65. Microscope images of organic circular DFB lasers with and without drive. Current density-voltage (JV) curves for devices with and without circular DFB. External quantum efficiency versus current density in OLEDs with or without DFB. Figure 66. (a) Schematic representation of DFB grating OLED and (b) experimental (symbol) and simulated (solid line) J ( V ) curves of DFB grating and reference OLED. Figure 67. Spatial distribution of charge carrier density ( n , p ) and electric field F at (a) 10 V and (b) 70V . Figure 68. (a) The hole density p , (b) the electron density n at 70 V , (c) the spatial distribution of the n and p tangent lines passing through the 2D part at y = 0.11 μ m . Figure 69. Drawing of cross-sections of (a) electric field F and (b) current density J at 70 V. Figure 70. Plot of profile recombination rate R for (a) DFB device, (b) R for reference device, (c ) R tangent through 2D section for DFB device at y = 0.10 μm , and (d) at 70 V. When the DFB device is at y = 0.164 μ m , the n and p tangents pass through the 2D part. Figure 71. (a) S ( J ) characteristics for different E _b (a ) without EFQ and after EFQ for the reference device and (b) for the DFB device and the reference device at E _b =0.6eV S ( J ) characteristics. Figure 72. At 70 V , (a) exciton distribution inside the reference device with and without EFQ, (b ) exciton distribution inside the DFB device without EFQ, (c) exciton distribution inside the EFQ device. Exciton distribution inside the DFB device. Figure 73. Exciton density distribution inside the DFB device without quenching (upper left), exciton density distribution inside the DFB device after EFB (lower left), exciton density distribution inside the DFB device after PQ ( Upper right), exciton density distribution inside the DFB device after PQ and EFQ (lower right). Figure 74. Optical density distribution inside the DFB device (upper left) Air/BSBCz/SiO ₂ and ( upper right) Air/BSBCz/SiO ₂ /ITO (bottom) actual device Al/Ag/MoO ₃ /BSBCz/SiO ₂ /ITO . Figure 75. (a) Chemical structure of Bafu derivatives. (b) Absorption and steady-state PL spectra measured in spin-coated Bafu pure films at room temperature. The excitation wavelength of the measured PL spectrum is 376 nm. An image of the eight pure films under UV illumination is shown in the inset. (c) Ordinary and extraordinary optical constants ( k and n ) of eight pure films measured by variable angle ellipsometry. The film thickness is approximately 75 nm. Figure 76. (a) Absorption and (b) steady - state PL spectra of blended films containing 10 and 20 wt.% of octanoic acid in CBP host. The excitation wavelength used for the emission spectra of both films was 424 nm. Figure 77. PL decay measured in Bafu pure films and in blended films containing 10 and 20 wt . % Bafu in CBP host. The excitation wavelength is 365 nm. Figure 78. Experimental and simulated ellipsometric data ψ and Δ measured at different incident angles in spin-coated Bafu pure films. Figure 79. (a) Schematic representation of an experimental configuration used to characterize the ASE properties of organic films. (b) Emission spectra of a 260 nm thick eight-foot pure film collected from the edge of the organic layer for different excitation intensities below and above the ASE threshold. Steady-state PL spectra are also shown with dashed lines. An image of a pure film under intense light illumination is shown in the inset. (c) Output intensity from the edge of a 260 nm thick film (integrated over all wavelengths) as a function of excitation density. (d) ASE threshold as a function of Bafu pure film thickness. The excitation wavelength is 337 nm. Figure 80. Output intensity from the edge of the organic layer (integrated over all wavelengths) as a function of excitation density in several pure films with various thicknesses in the range between 53 and 540 nm. This data was used to examine the thickness dependence of the ASE threshold shown in Figure 2d. Figure 81. ASE intensity plotted as a function of the distance between the pump strip and the edge of a 260 nm thick eight-foot pure film. The solid line corresponds to the fit obtained from the single exponential decay function to determine the loss coefficient. Figure 82. Time decay of emission intensity above the ASE threshold for 260 nm thick 8-nm pure films self-exposed in air or nitrogen atmospheres. The pump intensity is 873 μJ/cm ² and 10 Hz. Figure 83. (a) Schematic representation of the experimental configuration used to characterize the properties of the eight-foot DFB laser. (b) SEM image of the mixed-order DFB grating used in this work. (c) Emission spectra of a DFB laser based on a pure film collected perpendicular to the substrate plane for different excitation intensities below and above the laser threshold. (d) The output intensity of the DFB laser changes with the excitation density. Figure 84. (a) Schematic representation of the OLED structure used in this study. HOMO and LUMO of organic materials used in these devices are also provided. (b) External quantum efficiency versus current density in OLEDs based on Bafu pure film and CBP blended film. Figure 85. Photoelectron spectroscopy in air determines the ionization potential of a pure film in a thin film. Taking into account the optical band gap value determined from the absorption spectrum of the pure film, the electron affinity of the eight atoms in the pure film is then roughly estimated. However, it is important to note that Koopman's theorem, which states that the vertical ionization potential is equal to the absolute value of the calculated HOMO energy, is usually not satisfied due to relaxation processes during the ionization process and electron correlation. Although the optical band gap usually differs from the true electron gap, the electron affinity and LUMO can be roughly estimated from the difference between the ionization potential and the optical band gap value. Figure 86. (a) EL spectrum measured at a current density of 10 mA/cm 2 and (b) J - V - L curves in OLEDs based on Bafu pure film and CBP ^blended film. Figure 87. CW ASE in eight pure films. Figure 88. CW ASE in eight pure films. Figure 89. Organic semiconductor DFB laser diode structure . a , Schematic representation of organic laser diode. b , c , Laser microscope image ( b ) and SEM image ( c ) of the DFB SiO ₂ grating structure prepared on the top of ITO at 5,000× and 200,000× (inset) magnification. d , Cross-sectional SEM image of the complete OSLD. e , Cross-sectional EDX image of OSLD. To improve the visibility of low-concentration Cs and enhance contrast. Figure 90. Fabrication and structure of OSLD. a , Schematic diagram of the manufacturing steps of OSLD. b , Structure of the ITO-coated glass substrate used in this study and the general structure of the DFB grating. The specific values of different grating parameters can be seen in Table 1. c , d , EDX and SEM analysis of mixed-level DFB gratings prepared on top of ITO. These images confirm the possibility of electrical contact with the ITO. Figure 91. Electrical properties of electrically pumped organic semiconductor DFB laser. a , Energy level diagram of OSLD with highest occupied and lowest unoccupied molecular orbital levels indicated by work functions for organic and inorganic species. b , Micrograph of OSLD and reference OLED under DC operation of 3.0 V. The lengths of individual first-order grating areas and second-order grating areas are 1.68 µm and 1.12 µm. c , d , Current density-voltage ( J - V ) characteristics ( c ) and eta _EQE - J characteristics ( d ) in OLED and OSLD under pulse operation (pulse width of 400 ns and repetition rate of 1 kHz). Figure 92. Holes and electron transport in organic layers . a and b , the structures used to evaluate the transmission of pure hole devices ( a ) and pure electronic devices ( b ). c , Representative current density-voltage in a pure hole device (HOD) and a pure electron device (EOD) under DC operation (filled symbols) and pulse operation (empty symbols) on logarithmic and linear (inset) levels ( J - V )Characteristics. The device area is 200×200 µm. These J - V curves indicate good transport of holes and electrons in the high voltage region in the laser diodes fabricated in this study. Due to the trap limitation of hole current, the electron current at lower voltage is higher than the hole current. Figure 93. Properties of OSLD with different DFB geometries. a , Micrograph of OSLD operating at DC 3.0 V. b , c , d , current density-voltage ( J - V ) characteristics and eta _EQE - J characteristics of OSLD. e , electroluminescence intensity and FWHM changing with J. f , Emission spectrum as a function of J collected in the direction perpendicular to the substrate plane. Figure 94. DC characteristics and emission spectrum of the device. a , b , current density-voltage ( J - V ) curve ( a ) and eta _EQE - J curve ( b ) of OLED and OSLD measured under DC operation. c , PL spectrum of pure BSBCz film (black line) and EL spectrum of OLED (red line) and OSLD below the laser threshold (blue line). Figure 95. Laser properties of OSLD. a , Emission spectra of OSLD collected in the direction perpendicular to the substrate plane for different injection current densities. b , Emission spectrum close to the laser threshold. c , Output intensity and FWHM as the current changes . d , output power as a function of current . The illustration is a photo of an OSLD operating under 50V pulses. Figure 96. Characterization of emissions from OSLD. a , The emission spectrum and emission intensity of OSLD measured at different polarization angles (inset). Here, 0 ^° corresponds to the direction parallel to the groove of the DFB grating. b , CCD camera image showing the spatial Gaussian profiles of the emitted beam from the OSLD at different current densities. Figure 97. Characteristics of OSLD under optical pumping. a , Microscope image of the DFB grating used for optical pump measurement. As with other OSLDs (see Figure 89a), the same layer is deposited on the grating before measurement. b , Emission spectra of OSLD-6 collected in the direction perpendicular to the substrate plane under optical pumping with different optical excitation densities. The geometric structure of OSLD-6 is given in Table 1. c , The output intensity and FWHM of OSLD-6 as the optical excitation density changes. Excitation by _N2 laser at 337 nm was 3.0 ns with the device at ambient temperature. Figure 98. Absorption spectra of radical cations and anions of BSBCz. To study the spectral overlap between components, the absorption spectra of pure film BSBCz (50 nm) and composite films BSBCz: _MoO3 and BSBCz:Cs, (1:1 molar ratio, 50 nm) were measured. The differential absorption spectra of BSBCz cations and anions were calculated by subtracting the absorption spectra of pure films from the absorption spectra of _MoO3 -doped films and Cs-doped films, respectively. An absorption spectrometer (Lamda 950, PerkinElmer) was used to measure the UV/visible/near-infrared absorption spectra of pure films and composite films. The steady-state PL spectrum of a pure BSBCz film and a representative laser emission spectrum from a BSBCz DFB laser under optical pumping are also shown to demonstrate that polaron absorption in BSBCz OSLD should be negligible. Figure 99. Optical and electrical simulation. a , Experimental (symbols) and simulated (solid lines) J - V curves of pure hole devices (blue circles), pure electronic devices (red squares) and bipolar devices (black triangles). Model parameters were extracted from Figure 92 by fitting to a unipolar device, and they were used to simulate a bipolar device. b , Comparison of the calculated mobilities using parameters extracted from the monopolar device (solid line) with the reported ⁴¹ mobilities (symbols) of holes (blue) and electrons (red) in BSBCz. c , Experimental (symbol) and simulated (solid line) J - V curves of OSLD. d , Schematic diagram of the OSLD structure used for calculations. e , Spatial distribution of the recombination rate profile R of OSLD at J =500 mA cm ^{− 2} . f , cross section of the DFB device through (e) at y = 0.11 µm. g , the average exciton density of OSLD and OLED as the current density changes. Figure 100. Simulation of OSLD. a , Spatial distribution of exciton density S. b , Electric field distribution of the passive DFB resonant cavity expanded to include the structure of the first-order region at the resonance wavelength λ ₀ =483 nm. c , Modal gain as a function of current density. d , Spatial overlap between the exciton density S (x,y) and the optical mode | E (x,y)| ² for one period in the second-order region at J =500 A cm ^{− 2} . Layers other than the grating are modeled flat (see Figure 99d), and y = 0 corresponds to the BSBCz/ _MoO interface.

Claims (8)

A simulation modeling method of an organic laser device, which has at least one of the following steps: Calculate the charge carrier transport and charge carrier recombination rate in the device, and perform electrical simulation; Use the Helmholtz equation to obtain The light propagation mode in the device is used to perform optical simulation; Calculate the heat source and thermal conductivity in the device, conduct temperature simulation, and estimate the operational stability; Calculate the radiative decay constant k _r and non-radiative decay constant including gain, exciton The rate equation of k _nr estimates the laser properties. The simulation modeling method of claim 1 includes at least two of the above steps. The simulation modeling method of claim 1 has all the above steps. The simulation modeling method according to any one of claims 1 to 3, wherein the organic laser device is a current-excited organic semiconductor laser device. The simulation modeling method of any one of claims 1 to 3, wherein the organic laser device is a photoexcited organic laser device. A program for executing the simulation modeling method of any one of claims 1 to 5. Such as the program of claim 6, which has the step of outputting at least one of the following: current density-voltage curve, electric field distribution, charge carrier density, exciton density, eigenmode of the optical resonator structure, and Q value and limiting factor, as well as gain. A method of manufacturing an organic laser device, which includes the step of designing an organic laser device using the simulation modeling method of any one of claims 1 to 5.