Corrugated Organic Light Emitting Diodes Using Low Tg Electron

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Corrugated Organic Light Emitting Diodes Using Low Tg Electron Transporting Materials Cheng Peng, Shuyi Liu, Xiangyu Fu, Zhenxing Pan, Ying Chen, Franky So, and Kirk S. Schanze ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02669 • Publication Date (Web): 01 Jun 2016 Downloaded from http://pubs.acs.org on June 1, 2016

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ACS Applied Materials & Interfaces

Corrugated Organic Light Emitting Diodes Using Low Tg Electron Transporting Materials Cheng Peng†§, Shuyi Liu†, Xiangyu Fu†§, Zhenxing Pan‡, Ying Chen†, Franky So†* and Kirk S. Schanze‡* †Department

of Materials Science and Engineering, North Carolina State University,

Raleigh, North Carolina, 27695, United States of America §

Department of Materials Science and Engineering, University of Florida, Gainesville,

Florida, 32611, United States of America ‡

Department of Chemistry, University of Florida, Gainesville, Florida, 32611, United

States of America

KEYWORDS: OLED, light extraction, corrugation, low glass transition temperature material ABSTRACT: A corrugated organic light emitting diode (OLED) with enhanced light extraction is realized by incorporating a corrugated composite electron transport layer (ETL) consisting of two ETLs with different glass transition temperatures. The morphology of the corrugated structure is characterized with atomic force microscopy (AFM). The results show that the corrugation can be controlled by the layer thicknesses and annealing temperature. Compared with the 1 ACS Paragon Plus Environment


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control planar device, the corrugated OLED shows a more than 35% enhancement in current efficiency from 31 cd/A to 43 cd/A and a 20% enhancement in external quantum efficiency (EQE) from 10% to 12% at 100 cd/m2. In addition, the corrugated OLED also has a greatly improved operational stability. The LT90 lifetime of a device operated at 1000 cd/m2 is improved as much as 40 times in the corrugated OLED.

Introduction Since the first bilayer organic light emitting diode (OLED) was reported by Tang and van Slyke in 19871, OLEDs have been drawing increasing attention for display and lighting applications.

With the development of organometallic phosphorescent

chromophores and thermally activated delayed fluorescent emitting molecules, the internal quantum efficiency (IQE) of OLEDs have reached almost 100%.2-4 However, due to the higher refractive indices (~1.7-1.8) of organic semiconductor materials and indium tin oxide (ITO) compared with glass substrate (~1.5) and air (~1), most of the photons generated in an OLED are trapped within the device and the external quantum efficiency (EQE) of most high-efficiency bottom emitting OLED is typically less than 25%. In a conventional bottom emitting OLED, the organic semiconducting layers are sandwiched between the ITO and the metal electrodes, and as a result only a small fraction of photons can be extracted. Approximately 25% of the light is lost by coupling to the surface plasmon polariton (SPP) mode at the metal/organic interface, about 25% of the light is trapped in the waveguide mode due to the refractive index difference between ITO and the glass substrate (~1.5), and about 20% of the light is trapped in the substrate 2 ACS Paragon Plus Environment

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mode due to the refractive index difference between the glass substrate (~1.5) and air (~1).5, 6 Considerable effort has been made by the research community to enhance the light extraction from OLEDs. By using substrates with high refractive indices (~1.8) matching that of the ITO and organic layers, the waveguide mode can be significantly suppressed. In this case, most of the photons are trapped in the substrate mode which can be extracted by patterning the backside of the substrate or attaching external lenses.7 A corrugated device stack with in-plane quasi-period comparable to light wavelength was found to effectively extract both the waveguide and SPP modes. Corrugated device stack can be fabricated by thermal evaporation on corrugated substrates. Kim et al. demonstrated an easily fabricated metal oxide nanostructure on glass substrate improving white OLED EQE by 70%.8 Koo et al. developed spontaneously formed buckled structures using polydimethylsiloxane and aluminum and demonstrated a significant enhancement in device efficiency.9-11 Based on a similar concept, using a high refractive index corrugated substrate with a macro lens attached on the backside, Youn et al. have recently demonstrated a phosphorescent green OLED with an EQE as high as 63%.12 A corrugated device stack can also be fabricated on planar substrates by corrugating active layers such as ITO anode and solution processed injection polymer layer.13,14 Most of the corrugated structures are fabricated via a complicated processes, which can significantly increase the device fabrication costs. Herein we report a simple process to fabricate a corrugated OLED by incorporating a bilayer electron transport layer (ETL) consisting of two electron transport materials with distinctly different glass transition temperatures. Because of the large difference in

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thermal and mechanical properties in the two electron transport materials in this bilayer ETL, a buckled surface is formed by thermal annealing. With an appropriate material combination, the corrugated bilayer ETL can be finely tuned and it serves two functions: i) serves as a normal ETL; and ii) enhances the light out-coupling efficiency. With the internal bilayer corrugated electron transport layers, we demonstrate an OLED with a 35% enhancement in current efficiency compared to a control device with a planar ETL. Additionally, the corrugated bilayer ETL greatly improves the operational stability of the device. Specifically, the LT 90 lifetime at 1000 cd/m2 is improved by a factor of 100 compared to that of the reference device. Results and Discussion Bilayer Corrugated Structure Characterization. A corrugated structure fabricated by using a metal/elastomer bilayer thin film and thermal annealing has been investigated previously.15

The formation of the corrugated structure was attributed to a large

difference in thermal expansion coefficient between the metal and the polymer layers. When the metal/elastomer bilayer thin film is thermally annealed, the metal layer, which has a much smaller thermal expansion coefficient, induces compressive strain in the underlying elastomer layer. When this compressive strain exceeds a certain critical value at sufficiently high annealing temperatures, corrugation forms. Using a similar strategy, we fabricated corrugated structures using a bilayer approach with films of two different organic (or metal organic) small molecules. Since Tg reflects the temperature at which large amplitude molecular motion in an amorphous solid begins,16 it is strongly related to the magnitude of molecular interaction, as well as the thermal expansion coefficient. A survey of thermal properties of polymers 4 ACS Paragon Plus Environment

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reveals a general trend that polymers with higher Tg have relatively smaller thermal expansion coefficients.17 It is reasonable to expect a similar trend with small molecule materials. Therefore, based on the model discussed here, we propose to use two small molecule materials with distinctly different Tg to form the bilayer corrugated structure. Tris-(8-hydroxyquinoline)aluminum (Alq3) is a well-studied electron transport material with a high Tg (~172 oC)18. In order to form a bilayer corrugated structure which can be applied as a functional layer, i.e. an ETL in the OLED stack, we used 2-(4-biphenylyl)-5(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), which is a known low Tg electron transport material, along with Alq3 in the bilayer. In the PBD/Alq3 bilayer system, the Tg of Alq3 (~172 oC) is much higher than that of PBD (~60 °C)18. We expect that Alq3 has a smaller thermal expansion coefficient compared to PBD resulting in the formation of corrugation after thermal annealing, which was later confirmed by the experimental results outlined below. It should be noted that various bilayer systems consisting of a high Tg material atop a low Tg material give rise to a corrugated morphology after thermal annealing, in a similar fashion as reported here for PBD/Alq3. (AFM images for other bilayer material systems are shown in Figure S1 in Supporting Information). Fenter et al. measured the thermal expansion coefficients of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) and Alq3, and their results show that the thermal expansion coefficient of TPD is approximately 10 fold larger than that of Alq3.

Moreover, the glass transition

tempereature of TPD is approximately 60o C, which his considerably lower than that of Alq3.19 As shown in Supporting Information, the TPD/Alq3 bilayer also forms a similar corrugated structure as PBD/Alq3 after the bilayer is thermally annealed at 70° C. This



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finding supports the model described above about the mechanism of the corrugation formation. Nevertheless, since we did not measure the thermal expansion coefficients of the other materials investigated in bilayers with Alq3, and since there is a dearth of such information available in the literature concerning the thermal properties of organic glassy materials, the model described above for the formation of the corrugated structure in the bilayers remains a hypothesis. Based on the optical analysis of the conventional bottom emitting OLEDs,

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corrugation structure with in-plane quasi-period comparable to or below the considered light wavelength is necessary to provide sufficiently large additional in-plane wave number in order to effectively extract light trapped in the SPP mode and the waveguide mode. Light trapped in the glass substrate can be extracted by a corrugated structure with an in-plane quasi-period in the low µm range because only a small additional in-plane wave number is sufficient. The corrugated structure also needs a sufficient out-of-plane modulation depth to provide an effective scattering effect. In order to optimize the effect of the bilayer corrugation structure on light extraction in a working OLED, both the inplane corrugation length scale and the out-of-plane modulation depth need to be taken into consideration, and optimized for the OLED emission wavelength. In an initial series of experiments, we sought to explore the relationship between the corrugated structure (modulation depth and characteristic length scale) and the thickness of the ETL bilayers. In these exploratory studies, we fabricated material stacks of Alq3 on top of PBD on 40 nm 2, 2′, 2"-(1, 3, 5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) films. (TPBi was chosen as the bottom layer because in the OLED structure stack used in the device work the layer under the bilayer ETL is TPBi). In fact, it was found


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later that the underlying layer almost has no effect on the corrugation structure compared with the ETL bilayer, as long as the Tg of the underlying material is higher than the annealing temperature. The schematic of the structure under investigation is shown in Figure 1.

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Figure 1. a) Schematic of the material stack under morphology investigation substrate/underlying layers/PBD/Alq3 and the fabrication process. The molecular structures of the two materials in the bilayer ETL are shown in b) for PBD and c) for Alq3. The underlying layer is 40 nm TPBi in the morphology study described here. First, we investigated how the corrugation is affected by the PBD thickness in a series of samples with a fixed thickness of the Alq3 layer (20 nm) but varying PBD layer thicknesses (40 nm, 50 nm and 60 nm). After deposition, the samples were subjected to a thermal annealing step for 30 min at 70 oC. The atomic force microscopy (AFM) images showing the surface morphology of the resulting films are shown in Figure 2. Fast 7 ACS Paragon Plus Environment


Fourier transformation was done on the AFM images to analyze the in-plane periodicity. The results from the 40 nm PBD sample are also shown in Figure 2.

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Figure 2. AFM topographical images of substrate/TPBi/PBD/Alq3 (20 nm) samples (with different PBD thicknesses) annealed at 70 oC for 30 min. The thickness of PBD layer is: a) 40 nm, b) 50 nm and c) 60 nm. The AFM section profile image for the 40 nm PBD sample is shown in d) and the power spectral density from the fast Fourier transform of the 40 nm PBD sample AFM image is shown in e). The AFM images clearly show that corrugated structures formed on all three samples after annealing. With increasing PBD thickness, the root-mean-square (RMS) roughness increases from 31.9 nm to 39.3 nm. Large particles can be seen on the sample with 60 nm PBD and the corrugation is not as well defined as in the samples with thinner PBD layers. 8 ACS Paragon Plus Environment

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We attribute the particles to the crystallization of PBD film which lies below the top Alq3 layer. Crystallization of the layer may occur during annealing because the temperature used, 70 oC, is higher than the Tg of PBD (~60 oC).18 Between the two other samples, according to the Fourier analysis, the one with a thicker PBD layer (50 nm) has a larger in-plane peak period (corrugation length scale ~1.5 µm) compared with the other bilayer (40 nm PBD, ~1.3 µm). The peak-to-valley roughness (corrugation modulation depth) of the two samples with well-defined corrugation is approximately 100 nm. Figure 2d shows the AFM section profile image for the 40 nm PBD sample. The peak-to-valley roughness can be clearly seen. Figure 2e shows the power spectral density as a function of in-plane period, which results from the fast Fourier transform of the 40 nm PBD sample AFM image (Figure 2b). It can be seen that the corrugation structure has a broad distribution of periods with a maximum at 1.3 µm. The broad distribution of periods is also consistent with the structure formed by polydimethylsiloxane/Al in the literature.9

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Figure 3. AFM topographical images of substrate/TPBi/PBD (40 nm)/Alq3 samples with different Alq3 thicknesses annealed at 70 oC for 30 min. The thickness of Alq3 layer is a) 15 nm, b) 20 nm, and c) 25 nm. 9 ACS Paragon Plus Environment


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A second study shows that the corrugated structure also depends on the Alq3 layer thickness. A series of samples with fixed PBD thickness (40 nm) but varying Alq3 layer thickness (15 nm, 20 nm, 25 nm) were fabricated and characterized. The annealing condition was also 70 oC for 30 min and the AFM images of the resulting films are shown in Figure 3. Here it is seen that a rough texture develops on the surface of all three samples; however, a well-defined corrugated structure according to Fourier analysis only forms on the sample with 20 nm Alq3 layer. These results indicate that the corrugated structure is very sensitive to the thickness of the Alq3 layer. With the PBD layer thickness fixed at 40 nm, a well-defined corrugated structure only forms with a narrow range of Alq3 layer thickness, which appears to be optimal at 20 nm.

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Figure 4. AFM topographical images of substrate/TPBi /PBD (40 nm)/Alq3 (20 nm) samples annealed for 30 min at different temperatures. The annealing temperature was a) 50 oC, b) 60 oC and c) 70 oC. Another study reveals that the corrugated structure that forms in the PBD/Alq3 bilayers is also sensitive to annealing temperature. A series of samples with fixed thickness of PBD (40 nm) and Alq3 layers (20 nm) annealed at different temperatures (50 oC, 60 oC, 10 ACS Paragon Plus Environment

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70 oC) for 30 min were fabricated and characterized, and AFM images of the resulting corrugated structures are shown in Figure 4. The AFM images show that the surface roughness increases as the annealing temperature increases. The RMS roughness is 14.3 nm for 50 oC annealed sample, 24.2 nm for 60 oC annealed sample and 31.9 nm for 70 oC annealed sample. This is because a higher annealing temperature induces a larger strain between the two layers. Device Performance Characterization. OLED devices that incorporate the corrugated bilayer ETL were fabricated and compared to analogous device structures that lack the corrugation as a control. These OLEDs have the following structure: indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene

sulfonate (PEDOT:PSS)

nm)/4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]

(TAPC)

(~30 (35

nm)/2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) doped with 8 wt% iridium, tris[2-(2-pyridinyl-N)phenyl-C] (Ir(ppy)3) (30 nm)/TPBi (20 nm)/PBD (40 nm)/Alq3 (20 nm)/(thermal annealing)/Alq3 (6 nm)/LiF/Al. The OLED structure and energetics are shown in Figure 5.20-27

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b Figure 5. a) OLED material stack structure. b) Energy diagram of OLED relative to the vacuum level. The units of the energy values is eV. Corrugated devices were transferred out of the vacuum chamber after deposition of the Alq3 layer and thermally annealed in the glovebox under a nitrogen atmosphere. The annealing condition was 70 oC for 30 min. As shown in the previous bilayer corrugation characterization section, samples fabricated under this annealing condition have an inplane period peak ~1.3 µm and a peak-to-valley roughness ~100 nm. We expect this corrugated structure is able to enhance the light extraction efficiency when applied in OLED devices, mainly targeting the light trapped in the glass substrate. After the annealing process, the devices were transferred back to the vacuum chamber for deposition of the rest of the OLED stack. During the fabrication process, the devices were transferred and treated without any exposure to ambient atmosphere (e.g., air/humidity). Control devices were


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Figure 6. Control device and 70oC annealed device characteristics. Device structure shown in Figure 5. a) Luminance and current density plotted as a function of driving voltage. b) Calculated current efficiency plotted as a function of luminance. c) Calculated external quantum efficiency (EQE) as a function of luminance. Results shown represent the average of measurements of several pixels on a single substrate. Very similar results were found when the same experiments were carried out using devices fabricated in different runs. Variation of the device current efficiency and EQE is less than 5%. fabricated by following the exact same fabrication procedure, but they were not subjected to the thermal annealing step. Control device and corrugated device were fabricated in the same run of device fabrication to allow for direct comparison. After device fabrication, all the devices were encapsulated with cavity glass and UV-curable epoxy.



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Luminance-current-voltage (LJC) characteristics, and calculated efficiencies of the devices are shown in Figure 6. Compared with the control device, the corrugated device exhibits a higher current efficiency and external quantum efficiency (EQE). (The EQE of the devices was determined from the angular dependent electroluminescence intensity data and the EL spectra, see Figure S3). The enhancement in current efficiency and EQE are 35% and 20% respectively throughout the luminance values from 10 to 10000 cd/m2. This consistency in efficiency enhancement throughout all bias levels implies that the enhancement arises from an optical effect rather than an electrical effect, e.g., a change in charge balance. The discrepancy between the enhancement of current efficiency and EQE comes from the difference in the EL spectra and the angular dependence of EL intensity between the corrugated device and the control device. The change in the EL spectrum is expected because the corrugation significantly changes the optical structure of the device. For example, the thin film waveguide has uniform thickness across the pixel in the planar control device, while the thin film waveguide has different thickness at different locations in the corrugated device. The difference can be as large as 100 nm. Note that the total thin film waveguide thickness of the control device, combining the whole organic stack and the ITO electrode, is less than 300 nm. In order to confirm that the enhancement in efficiency arises from an optical rather than an electrical effect, the charge transport property of the corrugated PBD/Alq3 bilayer electron transport layer (ETL) must be characterized. Note that compared with the control device, the 70 oC annealed corrugated OLED exhibits a slightly lower current density and higher EL turn-on voltage (see Figure 6a). This behavior is distinct from the behavior of


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OLEDs fabricated on the corrugated substrate which have been explored previously.9, 12 It is understood that in devices fabricated on corrugated substrates, the effective thickness of the device stack is thinner compared to that of the corresponding planar device structure.

As a result, the current density in devices fabricated on the corrugated

substrates is higher than that in a control device with the same nominal stack thickness.9 In the corrugated device studied here, there are active regions with much smaller thickness than the nominal. These regions are expected to contribute significantly more to the current, which results in an overall higher current going through the corrugated device than the control device. In order to understand why the 70 oC annealed corrugated OLED structure studied here has a slightly lower current density than the control device, electron dominant devices based on PBD/Alq3 bilayer ETL were fabricated and characterized under different annealing conditions. Corrugated electron dominant device structures annealed at 60 oC or 70 oC for 30 min were fabricated for this purpose. The control device with the same bilayer structure was fabricated without annealing. The device structure and currentvoltage characterization data are shown in Figure 7.

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Figure 7. a) Electron dominant device structure and energy diagram. b) Device currentvoltage characteristics for control device and corrugated devices annealed at 60 oC and 70 o

C. The ITO was not UV-ozone treated, so the work function is lower than in Figure 5.

The thicknesses of TPBi, PBD and Alq3 layers are 40 nm, 40 nm and 20 nm, respectively. As shown in Figure 7b, the control electron dominant device features the highest current density at 10 V drive voltage. The device annealed at 60 oC, which has a less pronounced corrugation than the device annealed at 70oC, (see Figure 4), has the lowest current density. The turn-on voltage of the corrugated device annealed at 60 oC is greater than for the control device, indicating a higher injection barrier as a consequence of annealing. Even though the corrugation and a higher injection barrier are both consequences of annealing, they have opposite effects on the current density. We conclude that for the device annealed at 60 oC, the effective “increase” in current density due to corrugation is less than the “decrease” due to a higher injection barrier. This results in a lower overall current density compared with the control device. However, the device annealed at 70 oC has a higher current density than the device annealed at 60 oC, though still lower than the control device at high voltages (> 8 V), which is consistent with the OLED device JV data (see Figure 6a). This indicates that the increase in current density due to a more significant corrugation almost negates the decrease due to a higher injection barrier. In addition, at low drive voltage (< 5 V) the device annealed at 70 oC has the highest current density, with a turn on ~ 1 V. This could be due to the large peakto-valley roughness in the corrugated structure (~100 nm, see Figure 4c). In the OLED device stack, since the hole mobility of the TAPC HTL is almost 1000 times higher than the electron mobility of both materials in the bilayer PBD/Alq3 ETL,28,


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charge transport in the overall device is imbalanced and dominated by holes. Annealing

further reduces the electron transport property of the PBD/Alq3 ETL, which increases the charge imbalance. As a result, it is expected that thermal annealing would decrease the overall device efficiency. However, as shown above, the results reveal an enhancement in device efficiency as a result of annealing at 70 oC for 30 min. This strongly suggests that the enhancement is an optical effect caused by enhanced light extraction efficiency due to the corrugated structure that is created by the annealing step. One may argue about the overall relatively low EQE of the control device. One possible reason is the use of PBD. Devices with the same structure are made except for using 60 nm Alq3 to replace 40 nm PBD/20 nm Alq3 as the ETL. The Alq3 ETL control device peak current efficiency is 53 cd/A, higher than 35 cd/A of the PBD/Alq3 ETL control device. The performance of the Alq3 ETL devices is summarized in Table S1 in Supporting Information. In order to look into the effect of thermal annealing on device performance, Alq3 ETL devices were fabricated and different annealing conditions were applied. The annealing conditions and device performance are shown in Supporting Information. It can be seen that efficiency change by either annealing the HBL TPBi layer or annealing the ETL Alq3 layer is less than 10%, which confirms that the PBD layer and the corrugation morphology are important in the improvement of device efficiency. Finally, the operational stability of the corrugated and control devices was compared. Since PBD has comparatively poor morphological stability due to its low Tg, devices that incorporate PBD as electron transport material are expected to have a poor operational stability, which is expected to limit the practical use of the approach discussed here.30 We 17 ACS Paragon Plus Environment


replaced the hole transport material TAPC in the device structure with a more stable30 hole transport material N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) and tested the operational stability of the control device with that of a corresponding device structure that was annealed at 70 oC during fabrication to create the corrugated PBD/Alq3 bilayer ETL structure. Other than the HTL, the rest of the device stack for the stability test was the same as in Figure 5. The stability test was carried out with an initial luminance of ~1000 cd/m2 and a constant driving current density (~2 mA/cm2 for corrugated device and ~3 mA/cm2 for control device). The luminance was monitored and recorded as a function of time, as shown in Figure 8.

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Figure 8. Luminance at a constant driving current density (~2 mA/cm2 for corrugated device and ~3 mA/cm2 for control device) decays as a function of time. Device stack structure same as shown in Figure 5, except HTL material TAPC is replaced by NPD. Interestingly, the LT90 lifetime at 1000 cd/m2 is increased by more than a factor of 100 in the annealed corrugated device compared with the planar control device. One possible explanation for this large improvement is the greatly enhanced morphological stability of


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the device ETL. On one hand, we attribute the poor stability of the planar control device to the morphology change of the PBD layer during operation due to local Joule heating.30 The morphology change of the PBD layer might cause the delamination of the aluminum cathode, which may eventually lead to device failure.31 Indeed, fundamental change could be easily observed of the planar control device even by naked eyes after the device was stored in dry box with encapsulation for only one day. By contrast, for the corrugated device, during the annealing process before the deposition of aluminum cathode layer, the strain due to the thermal expansion coefficient difference was already relaxed, giving rise to a more stable morphology. Since the local temperature during normal operation can hardly exceed the annealing temperature used in this work, the morphology is less likely to change under operational conditions.32 These results show that this approach not only improves the device current efficiency, but also improves the operational lifetime so that even low Tg materials like PBD can be used to make stable devices. This lowers the difficulty in the design and synthesis of novel organic semiconductor materials. Summary and Conclusions In this work, corrugated OLEDs were fabricated by incorporating a bilayer ETL into the active layers. Corrugation is formed by thermal annealing because of the very different thermal properties used in the ETLs. The corrugated morphology of the bilayer structure can be controlled by tuning the ETL layer thicknesses and annealing temperature. With a corrugated PBD/Alq3 electron transporting layer, the resulting OLED showed a 35% enhancement in current efficiency and a 20% enhancement in EQE compared with the planar control device. This enhancement was attributed to an 19 ACS Paragon Plus Environment


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enhanced light out-coupling efficiency. The corrugated devices also showed a significantly enhanced operational stability. The LT90 lifetime starting from 1000 cd/m2 was extended more than 100 times compared with the planar control device. The effect of enhanced out-coupling efficiency and operational stability makes this bilayer corrugation a promising approach to make OLEDs with enhanced light extraction. Experimental Section Organic small molecule materials except PBD were purchased from Luminescence Technology Corp. PBD was purchased from Sigma-Aldrich. No additional purification was done. Glass substrates were purchased from Kintech and were coated with prepatterned ITO with a thickness ~100 nm. The substrates were cleaned with acetone and isopropanol and, for OLED device, were UV-Ozone treated to increase the ITO work function. Polymer layer PEDOT:PSS (Al 4083) was purchased from CleviosTM and was deposited by spin-coating. The PEDOT:PSS film was baked at 140 oC for 30 min. Organic small molecule films were fabricated by thermal evaporation in a vacuum chamber built by Infovion. The pressure in the vacuum chamber was kept lower than 1 x 10-6 Torr. The deposition rate and film thickness are monitored by quartz crystal microbalance pre-calibrated by Bruker DektakXT profilometer. The deposition rate was kept at around 2 Å/s for organic materials. Thermal annealing to form bilayer corrugation was done on a hot plate in a glove box with a nitrogen atmosphere. The hot plate temperature was stabilized at the annealing temperature before the sample was placed on it. After annealing, the sample was removed from the hot plate without changing the temperature and cooled down in nitrogen atmosphere. Bilayer corrugated film morphology was characterized by atomic force microscopy.


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OLED and electron dominant device JV characterization was measured with a Keithley 2400 source meter. The OLED EL luminance was measured with a silicon photodiode pre-calibrated with a luminance meter. The OLED EL spectrum was taken with an Ocean Optics HR4000 high-resolution spectrometer pre-calibrated by an Ocean Optics LS-1 tungsten halogen lamp. Angular dependent EL measurement was carried out with a Hamamatsu photo-multiplier tube detector. OLED devices were encapsulated with cover cavity glass and UV-curable resin in the glove box. Atomic force microscopy was carried out using a Bruker AUTHOR INFORMATION Corresponding Authors [email protected] [email protected] Author Contributions The manuscript was written through contributions of all authors. Funding Sources BOE Technology Group Co., Ltd., China Acknowledgments: The authors acknowledge the support of BOE for this work. Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.XXX.



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Atomic force microscopy images of corrugated bilayers of other materials after thermal annealing, electroluminescence spectra and angular dependence of electroluminescence for control and corrugated devices, X-ray scattering for Alq3/PBD bilayer, performance data for device constructed with Alq3 only ETL, comparison of power efficiencies for control and corrugated devices.

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Corrugated Organic Light Emitting Diodes Using Low Tg Electron

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