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Outcoupling-enhanced organic light-emitting diodes using simple phase-separated polymer films
Optik - International Journal for Light and Electron Optics 192 (2019) 162944
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Short note
Outcoupling-enhanced organic light-emitting diodes using simple phase-separated polymer films
T
Trang T.K. Tu1, Joo Won Han1, Dong Jin Lee, Dong Woo kim, Han Eun Park, ⁎ ⁎ Yong Hyun Kim , Kwon Taek Lim Department of Display Engineering, Pukyong National University, Busan 48513, Republic of Korea
A R T IC LE I N F O
ABS TRA CT
Keywords: Organic light-emitting diodes Light outcoupling Poly(amic acid) Polyimide Phase separation Microdent and nanobump structures
A high-performance external light outcoupling film for organic light-emitting diodes (OLEDs) is developed using a novel phase-separated polymer (PSP) which is spontaneously formed from a homogeneous solution of poly(amic acid) and polyimide in dimethylacetamide. The phase separation during the spin-coating process leads to complex morphologies of film with random microdent and nanobump patterns by a self-assembly of polymers. The PSP film is utilized on the backside of a glass substrate in green OLEDs as a light outcoupling film, leading to a considerable enhancement of light outcoupling efficiency. Both the current efficiency and power efficiency of OLEDs with the PSP film are improved by a factor of 1.45 and 1.53, respectively, in comparison with those of the reference device. The results demonstrate that the PSP film could serve as a promising outcoupling film for OLEDs.
1. Introduction Organic light-emitting diodes (OLEDs) have demonstrated their promising potential for use in lighting and display applications [1–3]. Since the efficiencies of OLEDs are still lagging behind that of inorganic LEDs, the development of the high-performance light outcoupling structures is of great importance for realizing the high efficiency of OLEDs. Typical OLEDs exhibit the limit of their light outcoupling characteristics where 70–80% of generated photons are confined at interfaces of the glass substrate/air (substrate mode) and the organic layer/glass substrate (waveguide mode), or bound at the metal electrode due to surface plasmon polaritons [4,5]. The total internal reflection phenomenon at the interfaces is caused by the mismatch of refractive indices between air (n˜1.0), glass substrates (n˜1.5), organic layers (n˜1.8) and indium tin oxide (ITO) transparent electrode (n˜1.7˜2.1), leading to the limited light extraction performance of conventional OLEDs. In this manner, extensive research has been performed to enhance the light outcoupling efficiency for OLEDs. The internal outcoupling structures, which have rough morphologies formed by scattering particles and/or structures, often cause damages on organic layers in devices [6–8]. To extract the substrate mode, microlens arrays [9–12], nanostructured substrates [13–15], corrugated substrates [16], nanoporous films [17,18], and polymer films with dispersed light scattering nanoparticles [19,20] have been widely investigated. However, many approaches still lead to rise in cost with complex fabrication processes for forming outcoupling patterns [5]. Here, spontaneously formed light outcoupling structures based on phase-separated polymers are employed into OLEDs as an external light outcoupling film with randomly dispersed microdent and nanobump patterns. They are formed by phase separation
⁎
Corresponding authors. E-mail addresses: [email protected] (Y.H. Kim), [email protected] (K.T. Lim). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ijleo.2019.162944 Received 26 March 2019; Accepted 12 June 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 192 (2019) 162944
T.T.K. Tu, et al.
Fig. 1. Schematic illustration for phase separated patterns: a) spin-coating of binary polymer mixture solution on the glass substrate, b) arrangement of two-phase during evaporation of the solvent, c, d) proposed mechanism of phase-separated structure model (nucleation, growth and spinodal decomposition processing), e) theoretical thin-film formation, and f) real thin-film formation.
Fig. 2. SEM images of micro/nanopatterns formed by phase separation with various magnifications X 5 m, b: 1 m X.
phenomena of binary immiscible polymer blends. The method could extract light confined at the substrate/air interface. In addition, the patterns can be easily built up only using a spin-coating technique and size and architecture can be controlled thanks to the spontaneous nature of the phase separation. The OLEDs with the PSP film show an enhancement of current efficiency as well as the power efficiency which are improved by a factor of 1.45 and 1.53, respectively, in comparison to those of a reference device. We anticipate that the simple and solution-processable light outcoupling film developed here has great potential for high-performance OLEDs. 2. Experimental 2.1. Materials 3,3′-Diaminodiphenyl sulfone (3DDS), 4,4′-(Hexafluoroisopropylidene)bisphthalic anhydride (6FDA), pyridine (≥ 99%), acetic anhydride (AA, ≥99%) and dimethyl acetamide (DMAC) were purchased from Sigma-Aldrich and used without purification. Other chemicals of analytical grade were used as received. 2.2. Preparation of PAA and PI Poly(amic acid) (PAA) and polyimide (PI) were prepared according to the previous report [21]. 13.33 g (0.03 mol) of 6FDA and 7.45 g (0.03 mol) of 3DDS were dissolved in 45 mL of DMAC in a round bottom flask equipped with a stirring bar. After 1 h of stirring 2
Optik - International Journal for Light and Electron Optics 192 (2019) 162944
T.T.K. Tu, et al.
Fig. 3. AFM images of micro/nanopatterns formed by phase separation (a: images of microdent (5 μm × 5 μm), b: images of nanobumps inside and outside of microdent in dimension (1 μm × 1 μm)).
at 0 °C under nitrogen, the reaction was continuously stirred at room temperature overnight. One half of the solution was precipitated into deionized water to isolate PAA. The filtered white solid was dried at 50 °C in vacuo overnight. Meanwhile, another half of the PAA solution was added to the mixture of 1.19 g (0.015 mol) of pyridine and 1.53 g (0.015 mol) of AA at 40 °C and then the solution was stirred overnight until the chemical imidization reaction was completed. The solution was precipitated in ethanol. The white PI solids were collected and washed many times with ethanol and dried in a vacuum-oven overnight at 50 °C. 2.3. Preparation of the PSP light extraction layer A polymer mixed solution was prepared by adding 0.5 g of PAA and 0.5 g of PI into 10 mL of DMAC. After complete dissolution, the homogeneous solution of the polymer mixture was coated on the backside of a glass substrate by using a spin-coating technique with a speed of 1200 rpm for 30 s in step 1 and 2500 rpm for 4 s in step 2. Afterwards, samples were dried at 80 °C by using a hot plate. The temperature was increased from 80 to 120 °C with 10 °C per 10 min. The field emission scanning electron microscopy (SEM, JEM-2100 F, JEOL) and atomic force microscopy (AFM, Icon-PT-PLUS, BRUKER) were utilized for investigating the morphologies of the polymer surface and size of patterns. 2.4. Fabrication of OLEDs OLEDs were fabricated in air by a solution process. PEDOT:PSS (AI4083, Heraeus), as a hole transport layer (HTL), was spun onto the ITO coated glass substrate with a spin speed of 1500 rpm for 30 s and subsequently annealed at 120 °C for 10 min. Poly(9vinylcarbazole) (PVK), 4,4′-N,N′-dicarbazole-biphenyl (CBP) and tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3) (PVK/CBP/Ir (mppy)3) were mixed and used as an emission layer (EML) with a weight ratio of 0.45:0.45:0.1. The mixture was dissolved in chlorobenzene at 10 mg/mL concentration and spun on the PEDOT:PSS layer with a spin-speed of 800 rpm, and subsequently dried at 100 °C. As an electron transport layer (ETL), 0.5 wt.% of 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was dissolved in methanol and spin-coated on the EML. The thickness of HTL, EML, and ETL were measured to be 35, 61, and 20 nm, respectively. Finally, a cathode which consists of LiF and Al (1 nm and 100 nm) was evaporated thermally on TPBi. The luminancecurrent density–voltage (L-J–V) characteristics were examined using a sourcemeter (Keithley 2401) and spectrophotometer (Minolta CS-2000). 3. Results and discussion Phase separation of two immiscible polymers in a solution enables forming randomly dispersed nanopore and nanopillar structures on substrates. PAA and PI (a ratio of 1:1) are dissolved in DMAC (10 wt.%). The homogeneous solution of the polymer mixture is spin-coated on glass substrates. During the spin-coating process, lateral nano-patterns are spontaneously built up as the solvent evaporates. This phenomenon obeys the incompatible free energy changes, and phase separation occurs only in two ways either the spinodal decomposition process or the nucleation and growth process [22,23]. Fig. 1 depicts the formation of pattern structures. The PAA-rich phase tends to approach the substrate due to the interaction between the surface polar groups of the glass and polar groups 3
Optik - International Journal for Light and Electron Optics 192 (2019) 162944
T.T.K. Tu, et al.
Fig. 4. (a) Device structures, (b) Current density-voltage curves, (c) luminance-voltage curves, (d) current efficiency, and (e) power efficiency for OLEDs. Table 1 Device performance of OLEDs. Device
Maximum current efficiency [cd/A]
Maximum power efficiency [lm/W]
Reference With PSP film
15.49 22.45
5.78 8.82
in polymer chains while the PI-rich phase moves to the opposite direction [24,25]. As a consequence, the PI-rich phase lies over the PAA phase. Subsequently, the fluctuation and instability at the interface of the two phases during the spin-coating process lead to the laterally phase-separated structures. The resulting phase-separated film shows an interesting topography containing microdents and nanobumps. Fig. 2 shows the SEM images of the PSP film with different magnifications. The PSP film shows randomly distributed microdents with a size of several micrometers (see Fig. 2a). Fig. 2b clearly shows the microdents at a high magnification. The nanobump structures with a size of several nanometers are well-distributed at the surface of microdents as can be seen in Fig. 2c. In Fig. 3, the AFM images also reveal the microdents with well-distributed nanobumps. It is considered that PAA has higher molar volume than PI so that it forms convex structures (nanobumps, brighter structures) while PI produce concave microdents (darker parts) after phase separation. The distinctive surface morphologies such as microdent and the nanobump structures of the PSP film are expected to effectively extract the substrate mode in OLEDS. The outside and inside of the microdents are covered by innumerable 4
Optik - International Journal for Light and Electron Optics 192 (2019) 162944
T.T.K. Tu, et al.
"lens arrays" of nanobumps, similar to a concept of "microlens arrays" which are widely used as an external outcoupling structure. The nanobump structures might contribute to the outcoupling enhanment of OLEDs shown in the following section. To identify the performance of the PSP light extraction film, we attempt to use the PSP film as an external light outcoupling film for solution-processed green OLEDs. Fig. 4a describes the structures of OLEDs with and without the PSP film. The characteristics of current density-voltage (J-V) and luminance-voltage (L-V) in fabricated green OLEDs are shown in Fig. 4b and c. Almost equal J–V curves of both devices indicate that the external light outcoupling film does not effect on the electrical properties of the device. The OLED with the PSP film exhibits a higher luminance than the OLED without the film at the given voltages. The current efficiencies and power efficiencies of OLEDs are shown in Fig. 4d and e, respectively, and the values are represented in Table 1. The OLEDs with the PSP film achieve a current efficiency of 22.45 cd/A and a power efficiency of 8.82 lm/W, which are enhanced by a factor of 1.45 and 1.53, respectively, in comparison to the reference device without the film. These results suggest that the PSP film effectively reduces the total internal reflection at the surface of glass substrate by modifying the critical angle and it can serve as an outstanding external light outcoupling film for high efficiency OLEDs. 4. Conclusion In summary, we have designed successfully the high-quality external light outcoupling film for OLEDs. The outcoupling film is prepared by using the mixed solution of PAA and PI. The spontaneous phase separation process during spin-coating renderes random microdent and nanobump patterns in the film. By applying the PSP film into OLEDs, the current efficiency of OLEDs is greatly enhanced by a factor of 1.45 compared with the reference one. Furthermore, the power efficiency is improved by a factor of 1.53. We expect that the light outcoupling film with spontaneously formed phase separation patterns can be a simple, solution-processable, and high-performance light outcoupling system. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant (NRF-2018R1D1A3B07041437) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016R1C1B2012490). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
S. Reineke, Complementary LED technologies, Nat. Mater. 14 (2015) 459–462. S. Reineke, M. Thomschke, B. Lüssem, K. Leo, White organic light-emitting diodes: status and perspective, Rev. Mod. Phys. 85 (2013) 1245–1293. M.C. Gather, A. Köhnen, K. Meerholz, White organic light-emitting diodes, Adv. Mater. 23 (2011) 233–248. M.C. Gather, S. Reineke, Recent advances in light outcoupling from white organic light-emitting diodes, J. Photonics Energy 5 (2015) 057607. J. Feng, Y.-F. Liu, Y.-G. Bi, H.-B. Sun, Light manipulation in organic light-emitting devices by integrating micro/nano patterns, Laser Photon. Rev. 11 (2017) 1600145. H.-W. Chang, J. Lee, T.-W. Koh, S. Hofmann, B. Lüssem, S. Yoo, C.-C. Wu, K. Leo, M.C. Gather, Bi-directional organic light-emitting diodes with nanoparticleenhanced light outcoupling, Laser Photon. Rev. 7 (2013) 1079–1087. H.-W. Chang, Y.H. Kim, J. Lee, S. Hofmann, B. Lüssem, L. Müller-Meskamp, M.C. Gather, K. Leo, C.-C. Wu, Color-stable, ITO-free white organic light-emitting diodes with enhanced efficiency using solution-processed transparent electrodes and optical outcoupling layers, Org. Electron. 15 (2014) 1028–1034. Y.H. Kim, J. Lee, W.M. Kim, C. Fuchs, S. Hofmann, H.-W. Chang, M.C. Gather, L. Müller-Meskamp, K. Leo, We want our photons back: simple nanostructures for white organic light-emitting diode outcoupling, Adv. Funct. Mater. 24 (2014) 2553–2559. J.-H. Han, J. Moon, D.-H. Cho, J.-W. Shin, H.Y. Chu, J.-I. Lee, N.S. Cho, J. Lee, Luminescence enhancement of OLED lighting panels using a microlens array film, J. Inf. Disp. 19 (2018) 179–184. J.W. Han, C.W. Joo, J. Lee, D.J. Lee, J. Kang, S. Yu, W.J. Sung, N.S. Cho, Y.H. Kim, Enhanced outcoupling in down-conversion white organic light-emitting diodes using imprinted microlens array films with breath figure patterns, Sci. Technol. Adv. Mater. 20 (2019) 35–41. Y. Sun, S.R. Forrest, Organic light emitting devices with enhanced outcoupling via microlenses fabricated by imprint lithography, J. Appl. Phys. 100 (2006) 1–7. J.Y. Oh, J.H. Kim, Y.K. Seo, C.W. Joo, J. Lee, J.-I. Lee, S. Yu, C. Yun, M.H. Kang, B.H. Choi, Y.H. Kim, Down-conversion light outcoupling films using imprinted microlens arrays for white organic light-emitting diodes, Dyes Pigm. 136 (2016) 92–96. C. Lee, J.-J. Kim, Enhanced light out-coupling of OLEDs with Low Haze by inserting randomly dispersed nanopillar arrays formed by lateral phase separation of polymer blends, Small 9 (2013) 3858–3863. C. Lee, K.-H. Han, K.-H. Kim, J.-J. Kim, Direct formation of nano-pillar arrays by phase separation of polymer blend for the enhanced out-coupling of organic light emitting diodes with low pixel blurring, Opt. Express 24 (2016) A488. J.H. Son, J.U. Kim, Y.H. Song, B.J. Kim, C.J. Ryu, J.-L. Lee, Design rule of nanostructures in light-emitting diodes for complete elimination of total internal reflection, Adv. Mater. (2012) n/a-n/a. W.H. Koo, S.M. Jeong, F. Araoka, K. Ishikawa, S. Nishimura, T. Toyooka, H. Takezoe, Light extraction from organic light-emitting diodes enhanced by spontaneously formed buckles, Nat. Photonics 4 (2010) 222–226. H. Go, T.-W. Koh, H. Jung, C.Y. Park, T.-W. Ha, E.M. Kim, M.H. Kang, Y.H. Kim, C. Yun, Enhanced light-outcoupling in organic light-emitting diodes through a coated scattering layer based on porous polymer films, Org. Electron. 47 (2017) 117–125. B. Pyo, C.W. Joo, H.S. Kim, B.-H. Kwon, J.-I. Lee, J. Lee, M.C. Suh, A nanoporous polymer film as a diffuser as well as a light extraction component for top emitting organic light emitting diodes with a strong microcavity structure, Nanoscale 8 (2016) 8575–8582. K.M. Lee, R. Fardel, L. Zhao, C.B. Arnold, B.P. Rand, Enhanced outcoupling in flexible organic light-emitting diodes on scattering polyimide substrates, Org. Electron. Physics, Mater. Appl. 51 (2017) 471–476. J.B. Preinfalk, T. Eiselt, T. Wehlus, V. Rohnacher, T. Hanemann, G. Gomard, U. Lemmer, Large-area screen-printed internal extraction layers for organic lightemitting diodes, ACS Photonics 4 (2017) 928–933. D.W. Kim, J.W. Han, K.T. Lim, Y.H. Kim, Highly enhanced light-outcoupling efficiency in ITO-Free organic light-emitting diodes using surface nanostructure embedded high-refractive index polymers, ACS Appl. Mater. Interfaces. 10 (2018) 985–991. E.P. Favvas, A.C. Mitropoulos, What is spinodal decomposition? J. Eng. Sci. Technol. Rev. 1 (2008) 25–27. W.J. MacKnight, F.E. Karasz, Polymer blends, Compr. Polym. Sci. Suppl. Elsevier, 1989, pp. 111–130. A. Baldan, Adhesion phenomena in bonded joints, Int. J. Adhes. Adhes. 38 (2012) 95–116. F. Awaja, M. Gilbert, G. Kelly, B. Fox, P.J. Pigram, Adhesion of polymers, Prog. Polym. Sci. 34 (2009) 948–968.
5
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Short note
Outcoupling-enhanced organic light-emitting diodes using simple phase-separated polymer films
T
Trang T.K. Tu1, Joo Won Han1, Dong Jin Lee, Dong Woo kim, Han Eun Park, ⁎ ⁎ Yong Hyun Kim , Kwon Taek Lim Department of Display Engineering, Pukyong National University, Busan 48513, Republic of Korea
A R T IC LE I N F O
ABS TRA CT
Keywords: Organic light-emitting diodes Light outcoupling Poly(amic acid) Polyimide Phase separation Microdent and nanobump structures
A high-performance external light outcoupling film for organic light-emitting diodes (OLEDs) is developed using a novel phase-separated polymer (PSP) which is spontaneously formed from a homogeneous solution of poly(amic acid) and polyimide in dimethylacetamide. The phase separation during the spin-coating process leads to complex morphologies of film with random microdent and nanobump patterns by a self-assembly of polymers. The PSP film is utilized on the backside of a glass substrate in green OLEDs as a light outcoupling film, leading to a considerable enhancement of light outcoupling efficiency. Both the current efficiency and power efficiency of OLEDs with the PSP film are improved by a factor of 1.45 and 1.53, respectively, in comparison with those of the reference device. The results demonstrate that the PSP film could serve as a promising outcoupling film for OLEDs.
1. Introduction Organic light-emitting diodes (OLEDs) have demonstrated their promising potential for use in lighting and display applications [1–3]. Since the efficiencies of OLEDs are still lagging behind that of inorganic LEDs, the development of the high-performance light outcoupling structures is of great importance for realizing the high efficiency of OLEDs. Typical OLEDs exhibit the limit of their light outcoupling characteristics where 70–80% of generated photons are confined at interfaces of the glass substrate/air (substrate mode) and the organic layer/glass substrate (waveguide mode), or bound at the metal electrode due to surface plasmon polaritons [4,5]. The total internal reflection phenomenon at the interfaces is caused by the mismatch of refractive indices between air (n˜1.0), glass substrates (n˜1.5), organic layers (n˜1.8) and indium tin oxide (ITO) transparent electrode (n˜1.7˜2.1), leading to the limited light extraction performance of conventional OLEDs. In this manner, extensive research has been performed to enhance the light outcoupling efficiency for OLEDs. The internal outcoupling structures, which have rough morphologies formed by scattering particles and/or structures, often cause damages on organic layers in devices [6–8]. To extract the substrate mode, microlens arrays [9–12], nanostructured substrates [13–15], corrugated substrates [16], nanoporous films [17,18], and polymer films with dispersed light scattering nanoparticles [19,20] have been widely investigated. However, many approaches still lead to rise in cost with complex fabrication processes for forming outcoupling patterns [5]. Here, spontaneously formed light outcoupling structures based on phase-separated polymers are employed into OLEDs as an external light outcoupling film with randomly dispersed microdent and nanobump patterns. They are formed by phase separation
⁎
Corresponding authors. E-mail addresses: [email protected] (Y.H. Kim), [email protected] (K.T. Lim). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ijleo.2019.162944 Received 26 March 2019; Accepted 12 June 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 192 (2019) 162944
T.T.K. Tu, et al.
Fig. 1. Schematic illustration for phase separated patterns: a) spin-coating of binary polymer mixture solution on the glass substrate, b) arrangement of two-phase during evaporation of the solvent, c, d) proposed mechanism of phase-separated structure model (nucleation, growth and spinodal decomposition processing), e) theoretical thin-film formation, and f) real thin-film formation.
Fig. 2. SEM images of micro/nanopatterns formed by phase separation with various magnifications X 5 m, b: 1 m X.
phenomena of binary immiscible polymer blends. The method could extract light confined at the substrate/air interface. In addition, the patterns can be easily built up only using a spin-coating technique and size and architecture can be controlled thanks to the spontaneous nature of the phase separation. The OLEDs with the PSP film show an enhancement of current efficiency as well as the power efficiency which are improved by a factor of 1.45 and 1.53, respectively, in comparison to those of a reference device. We anticipate that the simple and solution-processable light outcoupling film developed here has great potential for high-performance OLEDs. 2. Experimental 2.1. Materials 3,3′-Diaminodiphenyl sulfone (3DDS), 4,4′-(Hexafluoroisopropylidene)bisphthalic anhydride (6FDA), pyridine (≥ 99%), acetic anhydride (AA, ≥99%) and dimethyl acetamide (DMAC) were purchased from Sigma-Aldrich and used without purification. Other chemicals of analytical grade were used as received. 2.2. Preparation of PAA and PI Poly(amic acid) (PAA) and polyimide (PI) were prepared according to the previous report [21]. 13.33 g (0.03 mol) of 6FDA and 7.45 g (0.03 mol) of 3DDS were dissolved in 45 mL of DMAC in a round bottom flask equipped with a stirring bar. After 1 h of stirring 2
Optik - International Journal for Light and Electron Optics 192 (2019) 162944
T.T.K. Tu, et al.
Fig. 3. AFM images of micro/nanopatterns formed by phase separation (a: images of microdent (5 μm × 5 μm), b: images of nanobumps inside and outside of microdent in dimension (1 μm × 1 μm)).
at 0 °C under nitrogen, the reaction was continuously stirred at room temperature overnight. One half of the solution was precipitated into deionized water to isolate PAA. The filtered white solid was dried at 50 °C in vacuo overnight. Meanwhile, another half of the PAA solution was added to the mixture of 1.19 g (0.015 mol) of pyridine and 1.53 g (0.015 mol) of AA at 40 °C and then the solution was stirred overnight until the chemical imidization reaction was completed. The solution was precipitated in ethanol. The white PI solids were collected and washed many times with ethanol and dried in a vacuum-oven overnight at 50 °C. 2.3. Preparation of the PSP light extraction layer A polymer mixed solution was prepared by adding 0.5 g of PAA and 0.5 g of PI into 10 mL of DMAC. After complete dissolution, the homogeneous solution of the polymer mixture was coated on the backside of a glass substrate by using a spin-coating technique with a speed of 1200 rpm for 30 s in step 1 and 2500 rpm for 4 s in step 2. Afterwards, samples were dried at 80 °C by using a hot plate. The temperature was increased from 80 to 120 °C with 10 °C per 10 min. The field emission scanning electron microscopy (SEM, JEM-2100 F, JEOL) and atomic force microscopy (AFM, Icon-PT-PLUS, BRUKER) were utilized for investigating the morphologies of the polymer surface and size of patterns. 2.4. Fabrication of OLEDs OLEDs were fabricated in air by a solution process. PEDOT:PSS (AI4083, Heraeus), as a hole transport layer (HTL), was spun onto the ITO coated glass substrate with a spin speed of 1500 rpm for 30 s and subsequently annealed at 120 °C for 10 min. Poly(9vinylcarbazole) (PVK), 4,4′-N,N′-dicarbazole-biphenyl (CBP) and tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3) (PVK/CBP/Ir (mppy)3) were mixed and used as an emission layer (EML) with a weight ratio of 0.45:0.45:0.1. The mixture was dissolved in chlorobenzene at 10 mg/mL concentration and spun on the PEDOT:PSS layer with a spin-speed of 800 rpm, and subsequently dried at 100 °C. As an electron transport layer (ETL), 0.5 wt.% of 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was dissolved in methanol and spin-coated on the EML. The thickness of HTL, EML, and ETL were measured to be 35, 61, and 20 nm, respectively. Finally, a cathode which consists of LiF and Al (1 nm and 100 nm) was evaporated thermally on TPBi. The luminancecurrent density–voltage (L-J–V) characteristics were examined using a sourcemeter (Keithley 2401) and spectrophotometer (Minolta CS-2000). 3. Results and discussion Phase separation of two immiscible polymers in a solution enables forming randomly dispersed nanopore and nanopillar structures on substrates. PAA and PI (a ratio of 1:1) are dissolved in DMAC (10 wt.%). The homogeneous solution of the polymer mixture is spin-coated on glass substrates. During the spin-coating process, lateral nano-patterns are spontaneously built up as the solvent evaporates. This phenomenon obeys the incompatible free energy changes, and phase separation occurs only in two ways either the spinodal decomposition process or the nucleation and growth process [22,23]. Fig. 1 depicts the formation of pattern structures. The PAA-rich phase tends to approach the substrate due to the interaction between the surface polar groups of the glass and polar groups 3
Optik - International Journal for Light and Electron Optics 192 (2019) 162944
T.T.K. Tu, et al.
Fig. 4. (a) Device structures, (b) Current density-voltage curves, (c) luminance-voltage curves, (d) current efficiency, and (e) power efficiency for OLEDs. Table 1 Device performance of OLEDs. Device
Maximum current efficiency [cd/A]
Maximum power efficiency [lm/W]
Reference With PSP film
15.49 22.45
5.78 8.82
in polymer chains while the PI-rich phase moves to the opposite direction [24,25]. As a consequence, the PI-rich phase lies over the PAA phase. Subsequently, the fluctuation and instability at the interface of the two phases during the spin-coating process lead to the laterally phase-separated structures. The resulting phase-separated film shows an interesting topography containing microdents and nanobumps. Fig. 2 shows the SEM images of the PSP film with different magnifications. The PSP film shows randomly distributed microdents with a size of several micrometers (see Fig. 2a). Fig. 2b clearly shows the microdents at a high magnification. The nanobump structures with a size of several nanometers are well-distributed at the surface of microdents as can be seen in Fig. 2c. In Fig. 3, the AFM images also reveal the microdents with well-distributed nanobumps. It is considered that PAA has higher molar volume than PI so that it forms convex structures (nanobumps, brighter structures) while PI produce concave microdents (darker parts) after phase separation. The distinctive surface morphologies such as microdent and the nanobump structures of the PSP film are expected to effectively extract the substrate mode in OLEDS. The outside and inside of the microdents are covered by innumerable 4
Optik - International Journal for Light and Electron Optics 192 (2019) 162944
T.T.K. Tu, et al.
"lens arrays" of nanobumps, similar to a concept of "microlens arrays" which are widely used as an external outcoupling structure. The nanobump structures might contribute to the outcoupling enhanment of OLEDs shown in the following section. To identify the performance of the PSP light extraction film, we attempt to use the PSP film as an external light outcoupling film for solution-processed green OLEDs. Fig. 4a describes the structures of OLEDs with and without the PSP film. The characteristics of current density-voltage (J-V) and luminance-voltage (L-V) in fabricated green OLEDs are shown in Fig. 4b and c. Almost equal J–V curves of both devices indicate that the external light outcoupling film does not effect on the electrical properties of the device. The OLED with the PSP film exhibits a higher luminance than the OLED without the film at the given voltages. The current efficiencies and power efficiencies of OLEDs are shown in Fig. 4d and e, respectively, and the values are represented in Table 1. The OLEDs with the PSP film achieve a current efficiency of 22.45 cd/A and a power efficiency of 8.82 lm/W, which are enhanced by a factor of 1.45 and 1.53, respectively, in comparison to the reference device without the film. These results suggest that the PSP film effectively reduces the total internal reflection at the surface of glass substrate by modifying the critical angle and it can serve as an outstanding external light outcoupling film for high efficiency OLEDs. 4. Conclusion In summary, we have designed successfully the high-quality external light outcoupling film for OLEDs. The outcoupling film is prepared by using the mixed solution of PAA and PI. The spontaneous phase separation process during spin-coating renderes random microdent and nanobump patterns in the film. By applying the PSP film into OLEDs, the current efficiency of OLEDs is greatly enhanced by a factor of 1.45 compared with the reference one. Furthermore, the power efficiency is improved by a factor of 1.53. We expect that the light outcoupling film with spontaneously formed phase separation patterns can be a simple, solution-processable, and high-performance light outcoupling system. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant (NRF-2018R1D1A3B07041437) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016R1C1B2012490). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
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