KR102227787B1 - High brightness and efficiency of polymer-blend based light-emitting layers without the assistance of the charge-trapping effect - Google Patents

Abstract

An object of the present invention is to find out whether the performance of the mixed polymer EML system is due to the charge-trapping effect or more dependent on the characteristics of the FRET process, and according to the result, a mixed polymer EML system that can increase brightness and efficiency without charge trapping effect. I want to provide.
According to the above object, the present invention,
Light emission characterized by mixing a polyspirobifluorene-based copolymer (SPB) host and a poly (para-phenylenevinylene) copolymer (SuperYellow, SY) guest It provides a light emitting layer for a device.

Description

High brightness and efficiency of polymer-blend based light-emitting layers without the assistance of the charge-trapping effect

The present invention relates to a light emitting layer and a light emitting device based on a high luminance high efficiency polymer blend that is not related to the charge trapping effect.

Polymer light emitting materials have recently been developed as an attractive solution process to replace the existing vacuum evaporated small molecules in organic/polymer light emitting diodes, but are still limited in terms of performance, especially in terms of low brightness and low efficiency. With the development of electronics over the past 20 years, solution-processed polymer semiconductor materials have been successfully applied to various devices such as polymer light-emitting devices (PLEDs), polymer photovoltaic cells, polymer thin film transistors (TFTs), sensors and electronic devices. I've been doing it. The unique features of these simple and lightweight devices include flexibility and cost effectiveness, and are mass-producible. PLEDs are even more special, providing a simple device architecture for investigating the basic luminescent properties associated with polymer semiconductor materials, as well as a bright and efficient flat panel display and a variety of electroluminescent (EL) such as solid state light sources and/or electrically driven organic lasers. It can also be used to develop optoelectronic devices. Several other polymer materials and device structures have also been developed to further improve performance. In Korean Patent Application Publication No. 10-2018-0028892, a new blue organic light-emitting device emission layer material is proposed. In addition, high-performance PLEDs have been adopted and developed for the design of multi-layer emitting material layers (EML) structures, high-density thick EMLs, and various efficient interfaces/electrodes. However, the imperfections and/or high driving voltages of the formed multilayer EMLs are still a problem, hindering various applications of PLEDs. The use of polymer mixed EML in PLEDs has been proposed as an alternative solution to improve efficiency and improve luminous performance (see S. Tasch et al., Appl. Phys. Lett. 71(20), 2883-2885 (1997)). .

For example, mixed polymer EMLs that utilize Forster resonance energy transfer (FRET) (Forster resonance energy transfer) between fluorophore pairs located close to each other (less than 10 nm) are known as energy transfer host polymer (donor) EMLs. It has been attempted to introduce an efficient light-emitting guest polymer (Exceptor) into the structure.

Recently, a polymer mixing system for EML has been reported, including a host poly (9,9-diocylfluorne) copolymer (known as F8) and a guest poly (para-phenylenevinylene) copolymer (known as SuperYellow, SY). (Reference 26: MU Hassan et al., Appl. Mater. Today 1(1), 45-51, 2015, Reference 27: MU Hassan et al., Nano Energy 21, 62-70, 2016) eV) and SY (approximately 5.2 eV), the large difference (approximately 0.6 eV) in the highest occupied molecular orbital (HOMO) level causes a large number of injection holes to be trapped at the SY guest site on host F8, which is approximately the same lowest unoccupied It is clearly arranged with the molecular orbital (LUMO) level (~2.7V). This hole trap effect improves the balance between electron and hole transmission in EML by reducing the hole transmission rate. This improved charge balance, combined with FRET between the F8 host and the SY guest polymer, greatly improves the radiative recombination of excitons, showing a maximum efficiency of 21-27 cd/A. These studies show that by means of efficient hole trapping and FRET, the use of single polymer EML can be a key factor in the development of a very bright and efficient next-generation PLED technology.

However, the hole trapping effect in polymer EML is still limited to a few materials because the difference between HOMO levels in light emitting polymers developed so far is generally less than 0.6 eV. Moreover, it has not yet been confirmed whether the hole trapping effect is essential to achieve the improved brightness and efficiency of the mixed polymer. Understanding the mechanism of action of mixed EML in high-performance PLEDs remains a significant challenge.

An object of the present invention is to find out whether the performance of the mixed polymer EML system is due to the charge-trapping effect or more dependent on the characteristics of the FRET process, and according to the result, a mixed polymer EML system that can increase brightness and efficiency without charge trapping effect. I want to provide.

The present invention will describe the use of a mixed polymer EML system to evaluate whether the performance of the mixed polymer EML system is due to the charge-trapping effect or more due to the nature of the FRET process.

The mixed polymer system was obtained by mixing a host polymer of a commercial blue light-emitting polyspirobifluorene-based copolymer (SPB-02T) and a guest polymer of yellow light-emitting PDY-132 (SY), and was mixed in a simple PLED structure. Used as EML. Each of these polymers has been studied extensively, but the present invention has found that mixing these polymers together can form a single EML without charge trapping. The present invention investigated three types of polymer EML based on single SPB host polymer, single SY guest polymer and mixed SPB:SY (see Fig. 1). The present inventors view the strong overlap between the emission spectrum of SPB and the absorption spectrum of SY as the core of an efficient FRET. Using these polymer EMLs, the impact of the FRET process on the performance of EMLs was identified. When SPB:SY = 95 wt%:5 wt% in mixed EML, the performance of the PLED is about 142,000 cd/m ² in luminance and the peak current efficiency is optimized to about 14 cd/A, much better than a single host or single guest device. It was high. This performance improvement is due to the efficient FRET between the SPB host and the SY guest, even when there is no general charge trapping effect. Although the device performance of the present invention was not as good as that of the previous F8:SY device, the luminance and efficiency were more than half of that of the device without charge trapping effect. Therefore, it can be confirmed that the FRET process plays an important role in improving the luminescence performance of the polymer mixed EML. Considering the difficulty of implementing efficient charge trapping, these results show an important contribution of FRET to mixed polymer EML, and thus can be applied to various PLED devices. Also, their excellent color stability also adds to its appeal.

That is, the present invention,

Light emission characterized by mixing a polyspirobifluorene-based copolymer (SPB) host and a poly (para-phenylenevinylene) copolymer (SuperYellow, SY) guest It provides a light emitting layer for a device.

In the above, the poly (para-phenylenevinylene) (poly (para-phenylenevinylene) copolymer (SuperYellow, SY) guest is provided with a light emitting layer for a light emitting device, characterized in that 1 to 9wt% is added.

The present invention,

Transparent substrate;

A transparent anode formed on the transparent substrate;

A hole injection layer formed on the transparent anode;

The light emitting layer of any one of claims 1 to 3 formed on the hole injection layer; And

It provides a polymer light emitting device comprising; a cathode including an electron transport layer formed on the light emitting layer.

In the above, the hole injection layer provides a polymer light emitting device comprising PEDOT:PSS.

The present invention,

Prepare a transparent substrate,

Forming a transparent anode on the transparent substrate,

Forming a hole injection layer on the transparent anode,

The light emitting layer of any one of claims 1 to 3 is formed on the hole injection layer, wherein the mixed solution of the host polymer and the guest polymer is prepared by dissolving the polymers in toluene, and is coated on the hole injection layer,

It provides a method for manufacturing a polymer light emitting device, characterized in that a cathode including an electron transport layer is formed on the light emitting layer.

According to the present invention, a mixed polymer EML device capable of displaying luminance and efficiency at a significant level without implementing charge trapping is provided, and various PLED devices can be implemented by applying this.

PLED using SPB:SY EML of the present invention exhibits much improved luminance-voltage (LV) performance than blue and yellow PLEDs, and exhibits excellent color stability. In other words, it is possible to efficiently emit green light without charge trapping effect by using the physically mixed polymer of the SPB host and the SY guest. Even in a simple PLED structure, low voltage operation and ^{improved luminance of up to 142,000 cd/m 2} can be achieved with CIE coordinates (0.35, 0.60).

_{In addition, the current efficiency η C} of the PLED using the SPB:SY EML of the present invention is at least 14 cd/A, which is significantly improved than the efficiency of blue and yellow PLEDs, which can obtain the same brightness even at low J, resulting in a long device life. Losing adds an important advantage.

The present invention provides a new platform for the development of advanced light emitting devices and/or solution process light emitting displays with FRET-based polymer EML that is very bright and efficient with a simple structure and an easy process and excellent color stability.

1 is a cross-sectional view of a PLED according to a widely known process for manufacturing a PLED.
2 is a UV-vis absorption spectrum and CV (Cyclic voltammogram) of an SPB and SY polymer film and an SPB:SY mixed polymer film of the present invention.
3 shows the surface morphology and surface potential of the fabricated polymer layer.
Figure 4 shows the applied voltage (V) for a sample PLED of mixed SPB:SY EMLs compared with two reference PLEDs fabricated using a single SPB EML (blue-PLED) and a single SY EML (yellow-PLED). This is a graph showing device characteristics in terms of current (J) and luminance (L) for each.
FIG. 5 shows the hall current density vs. applied voltage ( J _h -V ) characteristics of the HOD, and the energy level diagram of the HOD is shown inside the diagram.
6 is a silicified EL and PL emission spectrum focusing on the mixed SPB:SY EML emission mechanism.
7 is a light emission photograph and color index coordinates of the SPB:SY EML light emitting device of the present invention and comparative examples.
8 to 11 are tables summarizing characteristics of SPB, SY, etc., as well as SPB:SY EML of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1. Materials and properties of luminescent polymers and films.

All reagents were purchased from commercial sources and used without further purification. Poly(styrene sulfonic acid) doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) aqueous solution (Clevios TM PVP AI 4083, H.C. Stark) was purchased from Heraeus, Germany. Light-emitting polymer, blue light-emitting SPB-02T (SPB for blue PLED production), and yellow light-emitting PDY-132 (yellow-PLED SY) were purchased from Merck.

Cyclic voltammetry (CV) measurement of the light-emitting polymers used contained acetonitrile (99.8%, Aldrich) and tetrabutylammonium hexafluorophosphate (TBAPF ₆ , 100 mM, Aldrich) at room temperature with a scanning speed of 0.05 V/s. A potentiostat (DY2113, Digi-Ivy, Inc.) with DY2000 multi-channel potentiostat software was run on one non-aqueous electrolyte.

In the CV measurement, a platinum wire was used as the counter electrode with a glassy carbon working electrode coated with a thin SPB or SY polymer layer. An Ag/AgCl (3.5 M KCl) electrode was used as the reference electrode. To calibrate the electrode, ferrocene/ferrocinium ions (Fc/Fc+) (Sigma Aldrich) forming a redox pair were used as a redox probe. The HOMO level of the drop-cast polymer film in the carbon working electrode was calculated using the onset potential, which was determined at the intersection of the two tangents derived from the rising current obtained from the CV measurement and the background current.

To investigate the characteristics of the luminescent polymer film, three types of materials were tested. i) Single blue light-emitting SPB was used as a standard, ii) single yellow light-emitting SY was used as a comparison standard, and iii) SY (5%) and SPB were mixed as sample EML. The ultraviolet-visible absorption spectrum of the luminescent polymer was measured using an HP 845 spectrometer. The photoluminescent spectrum (PL) of the polymer film was recorded using a fluoroscopic spectrum analyzer (Cary Eclipse Fluorescence Spectrophotometer, Agilent technologies). The surface morphology image and surface potential of the prepared polymer film were obtained using a non-contact atomic force microscope (AFM) and a Kelvin probe force microscope (KPFM, FlexAFM, Nanosurf AG). It was obtained by applying to a tip (resonant frequency = 87 kHz, Force constant = 3.9 N m ^{-1, NanoWorld, Inc).} To correct the surface potential of the polymer film and monitor the integrity of the KPFM tip, high orientation pyrolytic graphite (HOPG, ZYB, Optigraph GmbH) was used as a reference surface.

2. PLED production

PLED fabrication followed a widely known process (see Fig. 1). The ITO layer (80 nm, surface resistance 30 ohm/square) on the glass substrate was used as a transparent anode, and was washed in an ultrasonic bath with detergent, deionized water, acetone, and isopropanol in that order. After washing, the ITO glass was dried and treated with ultraviolet (UV) ozone for 5 minutes. A 40 nm-thick PEDOT:PSS layer was directly spin-coated on the ITO anode to form a hole injection layer (HIL). PEDOT:PSS HIL was annealed at 120°C for 20 minutes. The thickness of the PEDOT:PSS layer was optimized at about 40 nm and measured using non-contact AFM. In the case of polymer EML, a mixed solution of SPB host polymer and SY guest polymer was prepared by dissolving the polymers in toluene, and spin-coated on PEDOT:PSS HIL. The EML thickness was optimized to about 85 nm. Then, the CsF electron injection layer (EIL) (2 nm) and the Al cathode (80 nm) were ^{thermally evaporated in a vacuum at a pressure near about 2.0 × 10 -4} Pa using a vacuum chamber, and placed in a glove box filled with an inert atmosphere. Was brought in. The manufactured PLED was encapsulated with a UV-curing epoxy resin to stably operate in the air. Current density-voltage-luminance (JVL) and CIE-coordinate measurements of the prepared PLED were performed at room temperature using a computer-controlled chromameter (CS-200, Minolta) equipped with a source meter (Keithley 2400). The EL spectrum of the device was also determined using a spectrometer (HR4000, Ocean Optics). To investigate the exciton lifetime, the time-analytical PL spectrum of EML was recorded using a time-correlated single photon counting system (TCSPC, Fluotime 300, Picoquant) with an excitation wavelength of 405.5 nm. The absolute PL quantum yields (PLQYs) of the film were measured using PLQY measurement (Hamamatsu C11347) with an excitation wavelength of 405.5 nm. The dielectric permittivity of EMLs was measured with a capacitance meter (VC6013, VICHY).

3. Results and discussion

As the first major point of view, we look at the energy band, that is, the UV-Vis absorption spectrum and the HOMO and LUMO levels of SPB and SY measured by CV. First, the UV-vis absorption spectra of the SPB and SY polymer films were recorded, and absorption peaks were observed at 390 nm and 440 nm at the absorption edges of 436 nm and 528 nm, respectively, as shown in FIG. 2(a). Therefore, the optical band-gap values of SPB and SY were estimated to be 2.84 eV and 2.35 eV, respectively.

Cyclic voltammograms (CVs) of SPB and SY are shown in FIG. /reduction)) means to indicate a process.

The HOMO level of these polymers is related to the vacuum level, and was calculated for the reduction potential of the ferrocene probe according to the following equation [-(Eox/onset-E1/2(Fc/Fc+))-4.8] eV, where 4.8 eV Is the absolute energy level (level) of the ferrocene/ferricium (Fc/Fc+) redox pair below the vacuum level.

Using an observation of 0.45 V for E 1/2 of ferrocene _(FC/FC+) , the HOMO level of SPB was calculated as -5.27 eV, which was estimated from the onset oxidation potential of the polymer (E _ox/onset = 0.92 V). .

The HOMO level of this SPB is almost equal to the value of SY (-5.24 eV) calculated as 0.89V E _ox/onset. The HOMO level (-5.27 eV) of SPB is much lower than that of the fluorescent analog F8 (~5.8 eV), which was used as a host polymer in previous work.

The low HOMO level of SPB can be caused by the presence of hole-transporting amine units in the polymer backbone of SPB, which can cause the SPB polymer to cause more electron donating.

Based on the broadband-gap values of SPB and SY and the HOMO level, the LUMO levels of the polymer can be estimated as -2.43 eV and -2.89 eV, respectively. The HOMO and LUMO values obtained here agree relatively well with the previously reported values (see Table 1 in FIG. 8). Therefore, the present invention assumes that the SY guest polymer does not trap the holes injected into the mixed SPB:SY EML. This is because the HOMO level of EML is almost the same as in F8:SY EML.

The basic physical and electrical properties of the three polymer films were investigated as follows. The surface morphology and surface potential of the prepared polymer layer were obtained using non-contact AFM and simultaneous KPFM measurements (Fig. 3). A topographic AFM image of SPB EML is displayed on the left panel of FIG. 3(a). This image looks quite smooth (RMS roughness about 0.34 nm). This AFM result was confirmed through the corresponding partial surface potential map (right panel in Fig. 3(a)) of the contact potential differences (V _{CPD s: contact potential differences) for SPB EML, which is relative to the HOPG reference surface in the EML plane.} Indicates a low surface potential (0.86 V). The estimated mean V _CPD of SPB EML in the KPFM map was about 0.17V. For comparison, the SY layer was examined in the same manner as in FIG. 3(b). The SY layer has an average V _CPD of 0.13 V similar to that of SPB EML (0.17 V) and AFM morphology with an RMS roughness of -0.47 nm is shown.

Likewise, the mixed SPB:SY layer (5wt%) showed a smooth AFM image with an RMS roughness of 0.36 nm and an average V _{CPD of} ~0.23 V (Fig. 3(c)). These results for the surface roughness and surface potential of the SPB:SYB layer are almost identical to the SPB reference EML, and in the mixed EML the SPB host polymer without noticeable aggregation and/or phase separation of the SY guests. The layer indicates that the SY guest is homogeneously dispersed. The present invention also notes that the surface potentials (V _CPD ) of the three EMLs studied are almost the same, which means that there is little difference in their electrical properties because of the similar HOMO levels with each other.

To investigate the EL performance of the three polymer EML described above, ITO anode, PEDOT:PSS hole injection layer (HIL, 40nm), polymer light emitting layer (EML, 85nm), CsF electron injection layer (EIL, 2nm), Al metal cathode ( 80m) in turn, including PLED (Fig. 1a) was produced.

Now, compared to two reference PLEDs fabricated using a single SPB EML (blue-PLED) and a single SY EML (yellow-PLED), for the applied voltage (V) for a sample PLED of mixed SPB:SY EMLs. The device characteristics in terms of current (J) and luminance (L) will be described (Fig. 4).

As shown in Fig. 4(a), the current density-voltage (JV) characteristic of the sample PLED was slightly higher than the current density through a single SPB EML in the current density through the mixed EML, because SY was added to the SPB EML. I can. However, the current density is still much lower than that of a single SY EML. These results indicate that mixing a small amount of SY guests into the SPB host causes only minor changes in the electrical properties of the mixed EML compared to single SPB EML.

Nevertheless, unlike the JV characteristics, the sample PLED using SPB:SY EML exhibits much improved luminance-voltage (LV) performance than the blue and yellow PLEDs (Fig. 4(b)). For example, the maximum luminance of the sample PLED is L ~ 142,000 cd/m ² at V = 16.5 V, and the threshold voltage ( V _onset ) is as low as ~ 2.7 V. This maximum luminance L is about 5.9 times higher than that of the blue PLED (L ~ 24,000 cd/m ² at V = 16.5 V, V _onset of ~ 3.2 V), and the yellow PLED's (V = 13.0V, ~ L ~ 51,000 cd/ m ² , V _onset ~ 2.2 V).

This improved result clearly indicates that the radiative recombination process of mixed SPB:SY EML is improved. Therefore, the current efficiency η _C of the specimen device was significantly improved than that of the blue and yellow PLEDs (inset in Fig. 4(b)). For example, the highest observed value η _C was about 14.3 cd/A at V = 4.5 V, much higher than the reference PLED (5.1 cd/A for yellow PLED and 3.6 cd/A for blue PLED). Even at a luminance of 500 cd/m ^2, the sample PLED has η _C of 13.7 cd/A, about 4.6 times that of blue-PLED ( η _C = 3.0 cd/A) and 2.9 times that of yellow PLED ( η _C = 4.7 cd/A) Emits EL light.

Also, even at the high luminance level of 5,000 cd/m ² , the sample PLED maintains an improved η _C of about 13.9 cd/A, which provides other important advantages. The use of this value, η _C , provides an important advantage of extending device life as the same brightness can be obtained for low J. Therefore, it is clear that the SPB:SY EML of the sample PLED provided improved device performance in both the luminance L and the current efficiency η _{C, even though the cathode was not fully optimized.}

In particular, in the case of mixed SPB:SY EMLs, when the doping is high, the aggregation and concentration quenching of guest light emission occurs. Therefore, when the SY concentration is 5 wt% or more, EL emission and efficiency are observed to decrease. Therefore, an appropriate SY concentration seems to be 1 to 9 wt%, preferably 3 to 6 wt%.

Details on device performance are summarized in Table 2 of Figure 9 and compared to other reported devices. The best device performance is that provided by the F8:SY EML. As shown in the table, the maximum brightness and efficiency of the sample device (5 wt%) was more than half of the previous F8:SY PL, even without charge trapping effects. In order to understand the charge-transfer dynamics of the mixed EML, single charge carrier devices, holes (holes)-only and electron-only devices were fabricated and investigated. In order to form hole-only devices (HODs), (ITO/PEDOT:PSS/EML/MoO ₃ /Ag) was fabricated _{by replacing the CsF/Al cathode of the PLED with a MoO 3 /Ag electrode.}

FIG. 5(a) shows the hall current density vs. applied voltage ( J _h -V ) characteristics of the HOD, and the energy level diagram of the HOD is displayed inside the diagram. In the J _h -V observation, the ITO electrode was biased with a positive voltage to observe the hole movement from the ITO anode in HOD.

The J _h -V characteristic curve shows that the hole current density of SPB:SY HOD is slightly lower than that of other reference HODs. The current density J _h of SPB:SY HOD decreases slightly to about 1/3 of SPB HOD after mixing a small amount (5wt%) of SY in SPB. These results show that the hole transmission through the SPB:SY active layer of SPB:SY HODs is not significantly different from the hole transmission through the SPB active layer of the reference SPB HOD. This contradicts the previous result that the conventional F8:SY HOD significantly reduced the size of J _h by 3 orders (~3 orders) due to the hole trapping effect. The small decrease in SPB:SY HOD J _h of the present invention may be a result of a small difference between the HOMO levels of SPB and SY, which may result in a sharp decrease in the hole trapping effect.

을 적용하여 SPB:SY EML의 홀 이동도와 홀 전도도를 평가하기 위해 정공 전송 곡선이 분석되었다. 도 5(a)의 J _h-V 데이터에 대한 곡선 맞춤은 SPB:SY 레이어에 대한 정공 μ₀의 영점 이동도(zero field mobility)에 대한 값(~1.26 × 10^-8 cm²/(V s))을 제공하며, SPB 레이어의 경우 SPB:SY 레이어에 대한 것(μ₀ ~ 4.75 × 10^-8 cm²/(V s))보다 약간 더 낮다(표 3 참조). Simplified space charge limited current (SCLC) model

The hole transport curve was analyzed to evaluate the hole mobility and hole conductivity of SPB:SY EML. The curve fit for the J _h - V data in FIG. 5(a) is the value for the zero field mobility _{of the hole μ 0} for the SPB:SY layer ^{(~1.26 × 10 -8} cm ² /(V s). )), and for the SPB layer it _{is slightly lower than that for the SPB:SY layer (μ 0} ~ 4.75 × 10 ^-8 cm ² /(V s)) (see Table 3).

This is in contrast to the fact that the size _{of μ 0} in the conventional F8:SY layer has been significantly reduced by a third order. This small reduction in J _h and μ ₀ of the SPB:SY layer according to the present invention indicates that the β phase formation of the SPB polymer can be affected to some extent by the introduction of the SY guest. Similar to the behavior of μ ₀ , the field-effect mobility coefficient β _F is ~1.41 × 10 ^-3 cm ^1/2 V ^-1/2 for the SPB:SY layer, so that the SPB layer (~1.10 × 10 ^-3 cm ^{1/ 2} V ^-1/2 ) shows a similar value (Table 3). Therefore, it is clear that introducing a small amount of SY guests to the SPB host does not seriously suppress the Hall current of SPB:SY EML.

Next, in order to manufacture an electronic device (EOD), the PLED's ITO/PEDO:PSS anode _{was replaced with an ITO/ZnO/Cs 2} CO ₃ electrode (ITO/ZnO/Cs ₂ CO ₃ /EML/CsF/Al ). 5(b) shows the electron current density versus applied voltage ( J _e -V ) characteristics of the fabricated EOD. In this observation, the Al electrode was negatively biased to study electron transfer from the Al cathode in the EOD of the present invention. The J _e -V curve of EOD shown in FIG. 5(b) did not fit well with the SCLC and Pool-Frankel models. However, the J _e -V curve of SPB:SY EOD shows slightly higher electron current density than SPB EOD, and a single SPB shows electron current density J _e ~ 4.63 × 10 ^-2 mA/cm ² at V = 6.0V, In the case of mixed SPB:SY, it slightly increased to J _e ~ 7.35 × 10 ^-2 mA/cm ^2. Through this small J _e increment, it can be seen that even if the SY guest is added to the SPB host, the electron current is not suppressed in the SPB:SY EML, even if the difference in the LUMO level between SPB and SY is large. This slight increase in the electron current may occur due to the decrease in the electron injection barrier of SPB:SY due to the introduction of the SY guest, and the LUMO level is between the LUMO level of the SPB host and the work function of the CsF/Al electrode. Therefore, the inventors are convinced that adding the SY guest to the SPB host does not cause any charge trapping effect in the SPB:SY layer.

Next, mixed SPB:SY EML. The investigation focused on the luminescence mechanism shows the normalized EL and PL emission spectra of three PLED EMLs (Fig. 6(a)). All figures show that the PL spectrum and the EL spectrum are similar, showing that the mechanism is similar. The EL(PL) spectrum of a single SPB EML is a well-known blue light emission with a strong electron 0-0 peak at about 458 nm, a strong first shoulder peak located at about 487 (484 nm), and a weak second shoulder peak at about 522 (521 nm). Showed. In the case of SY EML, the EL(PL) spectrum showed a strong electron 0-0 peak at 550 (552 nm) and yellow light emission with the first oscillation 0-1 peak at 590 (591) nm. In contrast to the EL(PL) spectra of the reference EMLs, the EL(PL) spectra of the mixed EML showed a strong green emission peak located about 525 (534) nm between the emission peaks of the single SPB and SY polymers as shown. Therefore, the green emission of SPB:SY EML can be understood in terms of the intermolecular interaction between SPB and SY.

In order to clarify the release mechanism of SPB:SY EML, different PL characteristics were measured as shown in FIGS. 6(b) and (c). 6(b) shows the PL spectra of SPB:SY (5 wt%) EML at various excitation wavelengths. As shown in Fig. 6, the PL spectra of SPB:SY EML shows almost the same spectral image, which is applied even at a long excitation wavelength of 500 nm to excite only the SY guest polymer. The PL emission showed an electronic weak 0-0 peak at 520 nm, a strong vibronic peak at 528 nm, and a distinct blueshifted spectra with a shoulder peak at 542 nm. These results indicate that the PL emission spectrum of the SPB:SY mixture can be explained by SY emission. The blue-shifted spectra is caused by diluted SY polymers, and the intermolecular interactions between SY polymers that are not aggregated in the host matrix are reduced. Therefore, note that when the mixed SPB:SY EML is approximately 520 nm, the strong blue-shifted emission is due to the increase of the diluted SY guest emission via efficient energy transfer (FRET) from the SPB host. Note that, as shown in Fig. 6(a), this emission by energy transfer cannot ideally compensate for the blue emission of the SPB polymer in the 458 nm region. More sophisticated photophysical properties are needed to quantitatively determine the exact contribution of emission from SPB:SY EML.

Next, time-resolved PL spectra was also measured to investigate the fluorescence lifetime of SPB, SY, and SPB:SY EMLs, and the curves can be seen in FIG. 6(c). It was observed that the fluorescence decay curves of the two reference polymers fit well with the double-exponential decay function (Table 4).

SPB and SY average fluorescence lifetime <τ> s is <τ _SPB > ~ 1.78 ns and <τ _SY > ~ 0.90 ns. Interestingly, the mixed SPB:SY EML had an average PL decay lifetime (< τ _SPB:SY >) of 0.98 ns, which is a significant reduction, and as shown in the figure, this is the double decay index of a single SY EML. (< τ _{SY > ~ 0.90 ns) The <τ SPB: SY} > of SPB:SY EML, which is reduced compared to SPB EML, verifies the FRET for SY guests. In addition, in the present invention, η _FRET , which is the FRET efficiency of SPB:SY EML, is converted to η _FRET = (1-τ _SPB:SY / τ _SPB ) was calculated using the formula. When calculated from τ ₁ , η _FRET = 22.6%, and _{when calculated from <τ SPB:SY} > η _FRET = 44.9%. This high FRET efficiency indicates that these two polymers are significantly mixed in SPB:SY EML. In addition, the present invention demonstrates an efficient FRET process between the SPB host and the SY guest in the mixed SPB:SY EML. The fluorescence lifetime results for this are summarized in Table 4.

Next, SPB, SY, and SPB:SY EMLs, PLQYs were also measured as shown in Table 4. The PLQYs of the standard SPB and SY EMLs were ~14.8% and ~13.7%, respectively. These values of SY EML are comparable to those reported for SY film (~17.0%). Conversely, the mixed SPB:SY EML showed a very increased PLQY value of 38.7%, which is 2.6 times larger than the value of SPB EML. This increased PLQY of SPB:SY EML shows that efficient FRET between SPB host and SY guest can greatly improve the light-emitting performance of mixed EML.

Although the device performance of the present invention using SPB:SY EML is not as high as that of the previous F8:SY EML, the luminous power and efficiency of the device of the present invention without charge-trapping effect is that of the conventional F8:SY device. It is clear that there is a significant improvement over half. These results show that an important factor contributing to the improvement of device performance for mixed EMLs contributes to the FRET process, and that FRET of mixed polymer EMLs is a very effective means of reaching high-performance PLEDs.

Finally, the green-emitting ability of the sample PLED was investigated together with the blue and yellow-emitting reference PLED (Fig. 7(a)). Here, PLEDs were manufactured using an active area of ^{3 × 2 mm 2.} Snapshots were taken during PLED operation at two applied voltages of 5.0 V and 12.0 V. Compared to the reference PLED, the light output of the sample PLED was clearly bright green. The 1931 Commission Internationale de l'Eclairage (CIE) chromaticity diagram relative to the EL emission of the sample PLED shows green in the CIE coordinates (0.35, 0.60) in the yellow-green area, compared to the single host SPB and single guest SY reference devices. For each, the color coordinates of blue (0.18, 0.31) and yellow (0.47, 0.52) are shown (CIE color coordinates can be seen in Fig. 7(b)).

The present invention is green (dx/d L ~ 2.9 × 10 ^-7 (cd/m ^-2 ) ^-1 , dy/d L ~ 0.7 × 10 ^-7 (cd/m ^-2 ) ^-1) ), despite the fact that the FRET emission cannot fully compensate for the blue emission (~458nm) of the SPB polymer host, the excellent mixture of SPB:SY EML as shown in Fig.6(a) Note that it shows color stability.

This high color stability of the sample PLED can be attributed to the molecularly agglomerated homogeneous dispersion of the SY guest polymer in the host SPB polymer, enabling efficient energy transfer between the SPB:SY FRET pairs in the mixed EML. The aforementioned results clearly demonstrate the remarkable light emission performance of the FRET donor SPB and acceptor SY pair in mixed EML. This is the first demonstration of a high-performance green PLED without charge-trapping effect, significantly improving brightness and current efficiency by exceeding ^{142,000 cd/m 2 and 14 cd/A, respectively.}

In addition, considering the difficulty of implementing an efficient charge-trapping effect, combining the FRET-based EML and the PLED charge-balancing architecture enables the development of low-cost, high-speed, large-area, color-stabilized, and high-performance light-emitting devices. Can be expected clearly.

In summary, the present invention explored a new type of EML single polymer layer, a copolymer mixed with a blue-emitting polyspirobifluorene-based host SPB and a yellow-emitting poly(p-phenylene vlinene) derivative guest SY polymer. And both have a HOMO level of about 5.2 eV. A physically mixed polymer of SPB host and SY guest enables efficient green light emission without charge trapping effect. Even in a simple PLED structure, ^{it was confirmed that low voltage operation and improved luminance of up to 142,000 cd/m 2} can be achieved with CIE coordinates (0.35, 0.60). In addition, the current efficiency of the sample PLED was at least 14 cd/A, much higher than that of the reference PLED (3.6 and 5.1 cd/A) using a single SPB EML and a single SY EML, respectively. This significant improvement in device performance is due to the efficient FRET of the SPB:SY mixed EML. FRET-based polymer EML, which is very bright, efficient and color-stabilized, along with its simple structure and easy processing, provides a new platform for the development of advanced light-emitting devices and/or solution-processed light-emitting displays.

On the other hand, in addition to the configuration examples of the PLED above, the transparent substrate may be various flexible substrates other than glass, and the transparent electrode may be IZO, AZO, etc. in addition to ITO, and the hole transport layer, electron transport layer, and cathode material may also be replaced by other widely known materials. I can.

The rights of the present invention are not limited to the embodiments described above, but are defined by what is described in the claims, and that a person having ordinary knowledge in the field of the present invention can make various modifications and adaptations within the scope of the rights described in the claims. It is self-explanatory.

Claims (5)

delete delete Transparent substrate;
A transparent anode formed on the transparent substrate;
A hole injection layer formed on the transparent anode;
A light emitting layer formed on the hole injection layer; And
Including; a cathode including an electron transport layer formed on the light emitting layer,
The light emitting layer,
Foster without charge trapping by mixing 1 to 9 wt% of a poly (para-phenylenevinylene-based copolymer) guest in a polyspirobifluorene-based copolymer host (Forster) A polymer light emitting device, characterized in that it is formed of a light emitting layer that emits light through resonance energy transfer. The polymer light emitting device of claim 3, wherein the hole injection layer comprises PEDOT:PSS. Prepare a transparent substrate,
Forming a transparent anode on the transparent substrate,
Forming a hole injection layer on the transparent anode,
To form a light emitting layer formed on the hole injection layer,
The light emitting layer,
Foster without charge trapping by mixing 1 to 9 wt% of a poly (para-phenylenevinylene-based copolymer) guest in a polyspirobifluorene-based copolymer host (Forster) is formed of a single light-emitting layer that emits light by resonant energy transfer,
A method for manufacturing a polymer light emitting device, comprising forming a cathode including an electron transport layer on the light emitting layer.

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