KR20130089395A - Flexible organic light emitting device including buckle structure and manufacturing method thereof - Google Patents

Abstract

PURPOSE: A flexible organic light emitting device including a buckle structure and a manufacturing method thereof are provided to improve outcoupling efficiency by forming a spontaneous buckle structure in the flexible organic light emitting device including a PEDOT:PSS anode. CONSTITUTION: A flexible and transparent substrate having is prepared. A polymer anode is laminated on the flexible and transparent substrate. An electroluminescent layer is laminated on the polymer anode. A cathode is laminated on the electroluminescent layer. The electroluminescent layer and the cathode include a buckle structure. [Reference numerals] (AA) Heating process; (BB) Cooling process

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flexible organic light emitting device having a buckle structure,

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a structure and a manufacturing method of a flexible organic electroluminescence device, and more particularly, to a buckling structure formation technique for increasing light extraction efficiency in extracting light generated in a light emitting layer of a flexible organic electroluminescence device, .

Recent research on organic electroluminescent devices (hereinafter referred to as "OLEDs") has focused on the development of organic materials and device structures for OLED applications, in addition to light weight, flexibility, performance versus price ratio and sizing. To achieve the above object, the scientific development sector, which is a great concern of researchers, is to simplify the device fabrication process, improve the device performance, and enhance stability. OLED performance has improved significantly in recent years, and some OLEDs now have near 100% internal quantum efficiency of phosphorescent materials. However, most of the OLEDs are trapped in the glass substrate or ITO (indium tin oxide) electrode / organic layer (waveguide mode) and can not escape from the inside of the device. This is because ITO / organic layer (n _{ITO , In} the case of the glass substrate ( _{organic material} = 1.7 to 2) and the glass substrate (n _glass to 1.5), due to the total reflection phenomenon caused by the refractive index difference between these and the _air layer (n _air = 1.0) outside the device. Such a trap phenomenon seriously limits the out-coupling efficiency (outcoupling efficiency) of light emitted from the light emitting device to the outside of the device. In order to overcome this problem, some use Bragg diffraction gratings, some use a low refractive index grid for light extraction in the waveguide mode of ITO / organic layer, .

Recently, we have introduced a randomly oriented buckle structure made using printing technology to extract from trap mode. By using these techniques, the outcoupling efficiency can be improved. However, to date, there have been few studies on light extraction from the trap mode for flexible OLEDs (hereinafter referred to as FOLEDs).

On the other hand, ITO, which is a transparent electrode widely used in FOLED, can not provide a stable anode due to its weakness and incompatibility. Therefore, intensive studies on polymer-based electrodes to replace ITO have been made. Among them, poly (3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) is a colorless transparent, highly conductive, Ease of lamination and low surface roughness. However, the satisfactory electroluminescent performance of the FOLED, in particular, the efficiency of light exiting outside the device is not improved when the above polymer anode is used. Therefore, for the production of a stable ITO-free FOLED with improved outcoupling efficiency, a simple and efficient way to extract light over a broad wavelength band should be further sought.

It is therefore an object of the present invention to improve the outcoupling efficiency in the FOLED with respect to light emission of the light emitting layer in a simpler manner.

The present invention provides a self-assembled and self-assembled buckle structure with a PEDOT: PSS anode on a flexible PES (polyethersulfone) substrate, thereby improving the outcoupling efficiency and facilitating the manufacturing method of the FOLED.

That is, the present invention,

A transparent substrate having flexibility;

A polymer anode laminated on the transparent substrate;

An electroluminescent layer laminated on the polymer anode; And

And a cathode stacked on the electroluminescent layer,

The electroluminescent layer and the cathode may include a buckle structure.

Further, the present invention can provide an organic electroluminescent device characterized in that the electroluminescent layer and the cathode have different coefficients of thermal expansion.

Further, the present invention provides an organic electroluminescent device characterized in that the polymer anode and the transparent substrate have a thermal conductivity of 0.7 W / m.K or less.

Further, the buckle structure of the present invention is characterized in that the electroluminescent layer and the cathode have different thermal expansion coefficients and are formed spontaneously in a cooling process after heating.

Further, according to the present invention,

Stacking a polymer anode on a flexible transparent substrate;

Laminating an electroluminescent layer on the polymer anode layer;

Depositing a cathode on the electroluminescent layer by thermal evaporation to form an organic electroluminescent device; And

And cooling the organic electroluminescent device to form a buckle structure. The organic electroluminescent device of the present invention may be manufactured by a method comprising the steps of:

Further, the present invention can provide a method of manufacturing an organic electroluminescent device, wherein the electroluminescent layer and the cathode have different thermal expansion coefficients from each other.

Further, the present invention provides a method of manufacturing an organic electroluminescent device, wherein the polymer anode and the transparent substrate have a thermal conductivity of 0.7 W / mK or less.

The present invention also provides a method of manufacturing an organic electroluminescent device, wherein the radius or the curvature of the buckle structure is controlled by controlling a ratio of a guest molecule concentration to a host material of the electroluminescent layer composition.

The buckle structure can efficiently extract the trapped light inside the FOLED and is easy to manufacture due to the spontaneous formation method.

According to the buckle structure of the present invention, the outcoupling luminous efficiency is improved by about 3.1 times and the external quantum efficiency of a peak as high as 15% is enabled, and the highest light extraction efficiency of the ITO-free FOLED known to date is recorded .

1 is a cross-sectional view schematically showing a method of manufacturing a buckle structure FOLED having a PEDOT: PSS anode on a flexible PES substrate.
Fig. 2 is a graph and a photograph evaluating the optical characteristics of the PEDOT: PSS anode formed on the PES and the glass substrate.
3 is an SEM photograph, a cross-sectional photograph, and an OLED cross-sectional photograph of a glass substrate showing the buckle structure of the FOLED surface of the PES substrate.
4 (a) is a photograph of a FOLED prototype operating under a bias voltage of 10 V. FIG.
FIG. 4B is a graph showing current density-voltage (JV: red) characteristics and luminance-voltage (LV: blue) characteristics of a FOLED device having an EL layer on a PEDOT: PSS anode.
4C is a graph showing the efficiency of the FOLED in comparison with the reference OLED.
5 (a) is a graph showing the normalized angle dependence of the EL intensity for the reference OLED and the FOLED of this embodiment.
5 (b) is a graph showing the EL spectrum of the reference OLED and the FOLED of this embodiment.
FIG. 6 is a graph showing the external quantum efficiencies (EQEs)? _EXT of the device and the emission power ratio.
7 is a graph showing the current efficiency (FIG. 7 (a)) and the power efficiency (FIG. 7 (b)) of the FOLED and the reference OLED of this embodiment as a function of output luminance.

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

It is known that compressive stresses due to heat shrinkage of the polymer film underneath some film can cause buckle structure on the film surface above.

1 is a cross-sectional view schematically showing a method of manufacturing a buckle structure FOLED having a PEDOT: PSS anode on a flexible PES substrate.

A thin EL layer (EL layer) was coated on a PES substrate on which a thin layer of PEDOT: PSS (poly (3,4-ethylenedioxythiophene) + poly (styrenesulfonate)) was previously formed, . When the Al source metal is heated and deposited by a thermal evaporation boat in a vacuum atmosphere, the heat reaches the EL layer at the same time as the Al electrode is formed, and the EL layer is thermally expanded.

Thereafter, when the evaporation process is completed and the sample including the formed Al electrode layer is cooled, heat shrinkage occurs and the compressive stress is slowly released, thereby forming a wavy pattern or a buckle structure in a random orientation on the surface, a it is due to the difference between the thermal expansion coefficient (~ 10 ^-4 / K) and the Al electrode layer of thermal expansion coefficient (~ 10 ^-6 / K) of the. Therefore, the electrode material has a significant difference in thermal expansion coefficient from the EL layer, and can be replaced if it is suitable for the FOLED characteristic.

By using such a process, a buckle structure can be formed immediately in the process of forming an element without the inconvenience of forming a separate buckle structure by printing or the like.

The organic electroluminescent device having the above-described buckle structure can be manufactured as follows.

First, a pre-cleaned flexible PES film is prepared as a substrate. In this embodiment, the substrate is Glastic PES, I-components Co., thickness: 0.1 mm, and surface roughness: about 8 nm, but this is not a limitation.

PEDOT: PSS is formed as a polymer anode on the substrate. PEDOT: PSS can use CLEVIOSTM PH 500. PEDOT: PSS is preferably doped with DMSO (dimethyl sulfoxide) and D-sorbitol in order to improve conductivity. D-sorbitol is used at about 0.625 wt%, DMSO is used at 7.5 wt% Solution. The modified PEDOT: PSS has a thickness of about several tens to 100 nm, a surface roughness of about 5.5 nm, and a surface resistance of about 220 Ω / □, and is manufactured by a method such as spin coating can do.

Next, a green electroluminescent (EL) layer is formed on the PEDOT: PSS anode by spin coating or the like. The mixed solution for forming the EL layer is a mixture of N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'biphenyl-4,4'- N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'biphenyl-4,4'- diamine: TPD) (4-tert-butylphenyl) -1,3,4-oxadiazole: Bu-PBD) (Tris (2-phenylpyridinato) iridium: Ir (ppy) 3) as a green luminescent material and a poly (vinylcarbazole ): PVK) is contained in a solvent in which 1,2-dichloroethane and chloroform are mixed at a weight ratio of about 3: 1.

Next, an electron injection boundary layer was formed to a thickness of about 2 nm by CsF on the EL layer at a rate of about 0.2 nm / s by using thermal deposition at a base pressure of about 2 × 10 ^-6 Torr, and an Al cathode layer was formed at about 80 nm . After the Al electrode is formed, the chamber containing the sample is vented to the atmosphere and cooled to the atmosphere for 30 minutes or more. The cooling temperature can be advantageously adjusted to form the buckle structure, but in the present embodiment, the atmosphere is simply used without using a separate refrigerant (such as liquid nitrogen).

Thus, the compressive stress induced in the EL layer causes the buckle structure to occur on the FOLED.

After fabricating the device, post-processing is performed by applying field annealing to the device to improve the OLED efficiency with the PEDOT: PSS anode.

Cooling for forming the buckle structure of the device and post-treatment for improving the characteristics are carried out at room temperature under the atmosphere without the sealing process.

In order to investigate the surface morphologies of the prepared film layers, the degree of surface roughness variation of the film layers was observed with an atomic force microscope (AFM, Nanosurf easyscan2 FlexAFM, Nanosurf AG Switzerland Inc.).

During the measurement, the contact mode was used with a cantilever (CONTR-10 point probe-silicon, Nanoworld, Inc.).

EL characteristics were measured using a chroma meter (Chroma Meter CS-200 from Konica Minolta Sensing, IN), a spectrometer (Ocean's Optics) and a source meter (Keithley 2400). The light emission characteristics were measured with an LED measurement system (LCS-100, ShereOptics Inc.) in which the spheres were integrated.

First, the optical characteristics of the PEDOT: PSS anode formed on the PES and the glass substrate were evaluated. 2 (a) shows that the transmittance spectrum of a sample coated with a PEDOT: PSS anode on a flexible PES substrate is slightly higher than the transmittance spectrum of a sample coated with a PEDOT: PSS anode on a glass substrate in a visible region with a transmittance of about 80% .

FIG. 2 (b) is a photograph of a sample formed on a PES substrate and a glass substrate by spin casting of a PEDOT: PSS anode, which is placed on a letter It shows its transparency.

 Next, the morphology of both samples was observed, and the morphology of the PES sample was compared with the EL layer and the Al cathode after glass substrate was formed.

3, a FOLED SEM picture of the PES substrate is shown on the upper side. In the image of the Al layer, in terms of wavelength, the waveform buckle having a length of about 450 to 500 nm is periodically randomly and widely dispersed, and the buckle depth (amplitude) appears to be as small as 30 to 50 nm.

3 shows that the corrugated structure is formed on both the EL and Al layers, while the PEDOT: PSS layer remains flat and flat.

In contrast to the buckle structure of the FOLED, the topographic image of the reference OLED on which the PEDOT: PSS anode is formed on the glass substrate is very flat and has a root mean square of about 3 nm, Respectively.

 The flat surface formation of the reference OLED is presumed to be due to the fact that there was little thermal expansion of the EL layer during the Al layer deposition because the thermal conductivity of the glass substrate was about 0.96 W / m · K and the thermal conductivity of the PES substrate was 0.17 W / m · K . That is, the glass substrate functions as a heat radiator of good quality.

On the other hand, when the ITO anode is formed on the PES substrate, the buckle structure does not appear at all. This is because ITO has a thermal conductivity of about 5.9 W / m · K and a PEDOT: PSS thermal conductivity of about 0.17 W / K is very high. Therefore, the formation of the buckle structure of the present invention requires the adoption of a flexible and transparent material substrate having a low thermal conductivity, and can be replaced with a material having such characteristics in addition to PES. That is, it is preferable that the polymer anode and the transparent substrate have a thermal conductivity of 0.7 W / mK or less, and that the liquid state has a thermal conductivity of at most 0.6 W / mK and the thermal grease has a thermal conductivity of 0.7 W / mK, which is a reasonable choice.

The performance of the next OLED was investigated. 4 (a) is a photograph of a FOLED prototype operating under a bias voltage of 10 V. FIG. The photograph clearly shows that the EL emission from the active region of the FOLED of the buckle structure is very bright. As a result of measuring and analyzing the performance of the FOLED having the buckle structure surface, the brightness of the PES substrate depends on the thickness of the EL layer as in the case of using the glass substrate. In the case of using a glass substrate, the well-functioning EL layer thickness operated relatively well even when a PES substrate was used. Also, comparing the performance of the FOLED device for a given thickness, it is more consistent with the use of a PES substrate than with a glass substrate.

FIG. 4B shows current density-voltage (J-V: red) characteristics and luminance-voltage (L-V: blue) characteristics of an OLED device having a 160 nm EL layer on a 100 nm PEDOT: PSS anode.

As shown, the JV curve shows excellent diode-like characteristics in both the reference OLED (dotted line) without the buckle structure and the sample FOLED (solid line) with the buckle structure according to the present embodiment, Can be confirmed.

It should be noted that the FOLED of this embodiment exhibits a more pronounced diode slope than the reference OLED. In buckle-structured samples, the current density is higher due to the electric field that is stronger due to the thinner organic layer thickness at the midpoint between the peak and peak of the sinusoidal grating due to the buckle structure.

 The L-V characteristic indicates that the buckle-structured FOLED is turned on at a relatively low turn-on voltage (about 3 V) and exhibits high intensity EL emission; It exhibits a higher operating voltage of about 7.0 V and a higher luminance than an OLED using a glass substrate.

Next, efficiency of the OLED is inferred as shown in FIG. 4C. The present embodiment is FOLED, 100 cd / m ² was in the luminance of the month to the current efficiency _{(η C,) 50.5 cd /} A, η C up to the luminance of 1000 cd / m ² is 47.6 was maintained in cd / A, η _C up to the luminance of 14,900 cd / m ² was maintained at below 30.0 cd / A. The power efficiency η _P was also evaluated, which reached 32.0 lm / W at a luminance of 10 cd / m ² , and then gradually decreased with increasing bias voltage. On the contrary, the peak current and the peak power efficiency of the reference device without the buckle structure were as low as 20.8 cd / A and 10.9 lm / W, respectively. In order to understand the improved efficiency of the FOLED of this embodiment, the emission characteristics of the device were observed as shown in FIG.

 5 (a) shows the normalized angular dependence of the EL intensity on the reference OLED and the FOLED of this embodiment. Fig. 5 (a) clearly shows the Lambertian luminescence pattern showing the maximum value in the perpendicular direction with and without the buckle structure. Since the buckle structure exhibits a random orientation and a broad periodicity, the outcoupling luminescence is concentrated in the perpendicular direction, resulting in a Lambertian luminescence pattern.

 Next, the EL spectra were examined in a direction perpendicular to the surface of the device. FIG. 5 (b) shows EL spectra, and the reference OLED and the FOLED of this embodiment are very similar.

Next, the external quantum efficiencies (EQEs)? _{EXT of the device} are deduced, assuming a perfectly diffused EL light emitting surface, and are shown in FIG. 6 (a).

In Figure 6 (a), unlike the reference element without a buckle structure that decreases monotonically from 5.2% depending on the input power density increases, EQE of the present embodiment FOLED with a buckle structure showed an about 15% peak, η _EXT Was about 3.1 times higher than that of the reference device.

 In another comparison, the EQE (15%) of the FOLED of this example fabricated on a flexible PES substrate is higher than the EQE (8-12%) of a heterostructured fluorescent OLED fabricated with ITO optimized on a conventional glass substrate .

The FOLED of this embodiment can be realized by: i) efficient light extraction of the waveguide mode due to the buckle structure from the EL layer, ii) waveguide of the anode layer due to the low refractive index of PEDOT: PSS (refractive index n _{PEDOT : PSS} ~ 1.42 at 550 nm) Mode suppression, iii) additional light extraction of the substrate mode through multiple reflections by a random buckle structure cathode layer due to a PES substrate (0.1 mm) thinner than the glass substrate (0.7 mm) used in the reference OLED, EQE could be improved by superior outcoupling in the extraction process.

To confirm the improved outcoupling efficiency, the radiant power was measured using an integrating sphere at a fixed input power (222 mW / cm ² ) (FIG. 6 (b)). Fig. 6 (b) shows the peak ratio of the radiated power of about 3.4: 1 at about 548 nm in comparison with the reference device of the FOLED of this embodiment, and the average ratio of the radiated power in the entire EL spectrum range is almost 3.2: 1, and it was confirmed that the FOLED of the buckle structure was greatly improved in simultaneous light extraction. Such a light extraction enhancement is far more improved by a factor of three, while the previously reported maximum enhancement was about 2.2 times.

For a better comparison of the FOLED of this embodiment, we have inferred the improved ratio of current efficiency and power efficiency at various output brightnesses.

Fig. 7 shows the current efficiency (Fig. 7 (a)) and the power efficiency (Fig. 7 (b)) of the FOLED and the reference OLED of this embodiment as a function of output luminance. From FIG. 7 (a) and FIG. 7 (b), it is possible to infer the improvement ratio of current efficiency and power efficiency at various luminance. For example, improvement of the current efficiency ratio of approximately, the luminance 10 cd / in m ² 2.7, at 100 cd / m ² 3.3, 740 is in cd / m ² 3.5, improvement in the power efficiency ratio is approximately, the luminance 10 cd / m ² to 3.9, 100 cd / m ² to 5.8, and 740 cd / m ² to 7.1. From these results, it can be assured that the spontaneously formed FOLED buckle structure enables repeat regeneration, reliability and high performance FOLED production.

Finally, with reference to FOLED production, the inventors have found that the radius and / or curvature of the buckle structure can be controlled by varying the ratio of the guest molecule concentration to the host material of the EL composition layer. This finding means that the phase separation of the guest molecules from the PVK host matrix can be induced to the temperature during the thermal deposition of the Al electrode, which is due to the surface tension change of the EL layer. This phenomenon is fundamentally similar to the Bernard-Marangoni pattern formed on the film surface, and the change in surface tension is caused by the induction of a temperature gradient into the layer. Thus, precise control of the phase separation in the EL composition layer can be useful for further optimization of the buckle structure pattern.

Since the thermal deposition of the Al electrode proceeds by a conventionally used process, the temperature is not particularly controlled for the buckle structure of the present embodiment, but a suitable temperature range for forming the buckle structure can be estimated as follows.

The glass transition temperature (Tg) of the materials used in the device is Tg ~ 200 ºC for PVK host material, Tg ~ 65 ºC for guest molecule TPD, melting point ~ 179 ºC, Bu-PBD case Tg ~ 60 ºC, melting point ~ 137-139 ºC. The temperature range of buckling structure formation by thermal evaporation considering this

Buckling is most likely to occur at temperatures between 50ºC and 210ºC at the lowest Tg - 10 ºC (60 ºC - 10 ºC = 50 ºC) to the maximum Tg + 10 ºC (200 ºC + 10 ºC = 210 ºC). Below the above temperature range, it is difficult to expect thermal motion of the material, and it is difficult to maintain the formation of the thin film at the above temperatures.

In short, highly efficient light extraction is possible from the buckle structure of the ITO-free FOLED with the modified PEDOT: PSS anode. The buckle structure pattern of the ITO-free FOLED can be successfully formed by the thermal expansion technique and has a peak efficiency of 50.5 cd / A and a luminance of 14,900 cd / m ² . In addition, a buckled FOLED with a PEDOT: PSS anode increases light extraction by a factor of three or more without introducing significant spectral changes and directional changes.

This spontaneously formed buckle-structured FOLED manufacturing method can provide a solid foundation for cost reduction, large size, high performance flexible display and wide wavelength band adaptability.

The rights of the present invention are not limited to the embodiments described above, but are defined by the claims, and those skilled in the art can make various modifications and adaptations within the scope of the claims. It is self-evident.

No reference symbol.

Claims (8)

A transparent substrate having flexibility;
A polymer anode laminated on the transparent substrate;
An electroluminescent layer laminated on the polymer anode; And
And a cathode stacked on the electroluminescent layer,
The electroluminescent layer and the cathode comprises an buckle structure. The organic electroluminescent device according to claim 1, wherein the electroluminescent layer and the cathode are composed of different thermal expansion coefficients. The organic electroluminescent device according to claim 2, wherein the polymer anode and the transparent substrate have a thermal conductivity of 0.7 W / mK or less. The organic light emitting device of claim 1, wherein the buckle structure is formed spontaneously in a cooling process after heating because the thermal expansion coefficients of the electroluminescent layer and the cathode are different from each other. Stacking a polymer anode on a flexible transparent substrate;
Laminating an electroluminescent layer on the polymer anode layer;
Depositing a cathode on the electroluminescent layer by thermal evaporation to form an organic electroluminescent device; And
Cooling the organic electroluminescent device to form a buckle structure; manufacturing method of an organic electroluminescent device comprising a. The method of claim 5, wherein the electroluminescent layer and the cathode are configured to have different thermal expansion coefficients. The method of claim 5, wherein the polymer anode and the transparent substrate have a thermal conductivity of 0.7 W / m · K or less. The method of claim 5, wherein the radius or curvature of the buckle structure is controlled by controlling a guest molecular concentration ratio of the electroluminescent layer composition to a host material.

Images