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Optimization of polymer light emitting devices using TiOx electron transport layers and prism sheets
Organic Electronics 13 (2012) 2667–2670
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Optimization of polymer light emitting devices using TiOx electron transport layers and prism sheets Yu-Hsuan Ho a,b, Yung-Ting Chang c, Shun-Wei Liu d, Hsiao-Han Lai a, Chih-Wei Chu a, Chih-I Wu c, Wei-Cheng Tian b, Pei-Kuen Wei a,e,⇑ a
Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 106, Taiwan c Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 106, Taiwan d Department of Electronic Engineering, Mingchi University of Technology, New Taipei 24301, Taiwan e Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung, Taiwan b
a r t i c l e
i n f o
Article history: Received 19 April 2012 Received in revised form 13 July 2012 Accepted 15 July 2012 Available online 9 August 2012 Keywords: Polymer light emitting devices Current efficiency Light extraction
a b s t r a c t The internal and external efficiency of polymer light emitting devices were found can be simultaneously improved by insertion a high refractive index material, titanium oxide (TiOx), to the emission layer and a prism sheet attached to the substrate. The TiOx layer increased the internal efficiency due to a better electron injection and hole confinement. However, it led a wider angular emission profile with more photons trapped in the substrate. By using the prism sheet, those trapped light was efficiently coupled to the air. The extraction efficiency enhancement was increased from 33.1% to 54.4% and the overall current efficiency was improved up to 86%. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Due to the large refractive index mismatch between organic layers (n = 1.7–1.9), glass substrate (n = 1.52) and air, substantial photons trapped in the organic light emitting devices are usually critical problems to constrain the external quantum efficiencies. In general, lower than one third of the generated photons can escape from emitting devices. One third of these photons are guided in the glass substrate and the others are trapped in the organic layers. Many researches had been proposed to increase the coupling ratio of external mode (coupling to air) by extracting trapped photons from substrate mode and high-index guided mode [1–3]. The surface modifications at the air/ substrate interface with microlens arrays, micro-pyramid arrays, silica micro-spheres and photonic crystals all had been reported to efficiently extract the light confined in ⇑ Corresponding author at: Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan. E-mail address: [email protected] (P.-K. Wei). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.07.014
the substrate [4,5]. Other techniques, such as the fabrication of the micro/nanostructures into devices or insertion of low index material were also proposed to couple light out of the high-index guided mode [6–8]. In addition to the surface modification for extracting the trapped photons, the device efficiency can also be improved by using the micro-cavity effect in the organic layers [9]. However, the modification of the electrontransport or hole-transport layer also leads to the degradation of internal quantum efficiency and rapid increase of operation voltage. In this work, we combined both surface roughening and cavity-tuning methods to achieve an optimal increase of current efficiency. A solution based sol–gel and spin-coating process was applied to inset a titanium oxide (TiOx) on the emissive polymer. The HOMO and LUMO levels were measured to be 7.1 and 3.6 eV, respectively, which can assist to the electron injection, transport and the hole blocking [10]. The large refractive index (n = 2.41) of TiOx also provides an effective means to tune the optical cavity mode in the emitting layers [11]. By combination the TiOx layer with the prism sheets, the
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Y.-H. Ho et al. / Organic Electronics 13 (2012) 2667–2670
extraction efficiency enhancement was increased from 33.1% to 54.4% without any device electricity degeneration. In addition, 86.0% improvement of current efficiency was achieved.
2. Device fabrication The devices were attached with prism sheets by index matching oil as illustrated in Fig. 1(a). The prism sheet had an apex angle of 90° and the prism pitch was 50 lm. This prism sheet acts as the backlight enhancement film (BEF) which can efficiently extract the substrate guiding wave. For the emitting layers, the glass substrates was coated with 250 nm-thick indium tin oxide (ITO) and then soaked into concentrated HCl for 15 min to pattern the transparent ITO anode. After UV-O3 plasma treatment, hole injection layer (Poly (3,4-ethylenedioxythiophene) – poly (styrenesulfonate), PEDOT:PSS), hole transport layer (Poly (9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine, TFB) and emission layer (polyfluorene derivative Green B) were spin-coated onto this ITO anode sequentially. The thickness of the first three organic layers was set to be around 120 nm, which is thought to be the suitable thickness for green light emission [9]. We used a solution based sol–gel process to deposit TiOx on the emissive polymer by spin coating without high temperature sintering. The rotation speed varied from 2000 to 4000 rpm for 30 s, then the samples were baked at 100 °C for 30 min. The effect of titanium oxide was revealed by Li and Yang [10], where the synthesis of titanium oxide was followed by the method published by Takahashi and Matsuoka [11]. After organic layer deposition, the opaque cathode, Cesium Carbonate
(Cs2CO3) and aluminum (Al) were evaporated by a thermal evaporator. The energy band diagram is plotted in Fig. 1(b). The energy levels were measured by using a photoelectron spectrometer (AC-2, Riken Keiki) in ambient condition. The energy levels (work function, HOMO) were calculated by plotting the square root of the total electron counts versus photoelectron energy and extrapolating the linear fit of the counts to zero. The LUMO was calculated by the HOMO and Eg, where Eg was determined from the absorption onset of the solid film. With the treated ITO, the work function raised to 5.0 eV, which matched well with the HOMO level of PEDOT:PSS. The TFB layer reduced the hole barrier and form a ladder path from PEDOT:PSS to emission layer. The HOMO of TiOx layer is very deep, which can provide an effective hole and exciton blocking property.
3. Measurement results Fig. 2 showed the characteristics of these devices including electricity, current efficiency and emission spectra. As depicted in Fig. 2(a), the current density of the device increase significantly by the insertion of the TiOx layer (10 nm) compared with the device without the TiOx layer. Due to the better electron-injection, the devices with TiOx layer implementation showed better or comparable electrical characteristics to the control device (TiOx: 0 nm). With a better hole confinement and carrier balance, the devices with TiOx all had higher current efficiencies. They were 8.01 cd/A for control device, 10.02 cd/A (TiOx: 10 nm), 10.35 cd/A (TiOx: 20 nm) and 9.40 cd/A (TiOx: 30 nm). The thicker TiOx layers resulted in a longer cavity length and made the emission spectra broadening and red shift. As illustrated in Fig. 2(c), the peak wavelength shifted from 532 nm to 540 nm and the FWHM (Full width at half maximum) was increased from 68 nm to 78 nm. The optical effect could be explained by Fabry–Perot equation [12–13].
pffiffiffiffiffi T 2 ½1 þ R1 þ 2 R1 cosð4pk xÞ pffiffiffiffiffiffiffiffiffiffi jEout ðkÞj ¼ jEin ðkÞj 1 þ R1 R2 2 R1 R2 cosð4pk LÞ 2
Fig. 1. (a) Diagrams of device structure and SEM graph of prism sheet (b) Energy level diagram of the device.
2
where Eout ðkÞ is the output intensity, Ein ðkÞ is the free space luminous intensity, x is the effective distance of the dipole from the metal cathode, R1 is the reflectivity of organic layer to the opaque cathode, R2 and T 2 are the effective reflectivity and transmission of the organic layer to the transparent anode. L is the total optical length of the cavity (including anode, organic layers, and cathode). When the total thickness of the organic layers was increased, the resonant wavelength was also increased and resulted in the red shift of the emission spectra. The calculated optical response was shown in Fig. 3(a). The optical response is the experimental electroluminescence (EL) spectrum divided by the intrinsic spectrum of the emitter, which represents whether constructive or destructive interference in the device layered structure. Fig. 3(b) shows the calculated EL spectra of the OLEDs with different TiOx thickness. When the TiOx thickness increased, the EL spectrum red shift and broadening were also observed in the simulation result.
Y.-H. Ho et al. / Organic Electronics 13 (2012) 2667–2670
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Fig. 3. Calculated optical response (a) and electroluminescence spectra (b) of the devices with different TiOx thickness.
Fig. 2. Characteristics of the emitting devices with cavity tuning layers, TiOx, thickness of 0, 10, 20, and 30 nm. (a) Current density vs. bias voltage, (b) current efficiency vs. current density, and (c) EL spectra.
When we applied the TiOx layer into the devices, the emission pattern was broadened as shown in Fig. 4(a). The wider emission pattern implies more light impinged at the substrate/air surface with a larger incident angle and more photons were trapped in the device. If we plotted the relation of the FWHM of the emission profile versus the corresponding TiOx thickness, a positive correlation was observed as shown in Fig. 4(b). Before the insertion of the TiOx layer, the angular FWHM was 60.2° which was similar to a Lambertian light source. When 30 nm TiOx layer was inserted, the angular FWHM was increased to 64°. After the application of a prism sheet onto the device,
a higher luminous enhancement was obtained for larger angular FWMH. The higher luminance came from the increase of extracted photons at larger incident angles and the reduction of the total reflection of the normal incident light. The additional inclined surface could help to couple out the light with large incident angle, but the one in small incident angle would be trapped due to the total internal reflection occurred at the inclined surface. This kind of trade-off concern had been reported in some other works [14]. Using the cavity tuning layer to change the emission profile in cooperation with surface roughening techniques can maximize the out-coupling efficiency of these emission devices. As illustrated in Fig. 4(b), the luminous enhancement ratio increased from 33.1% to 54.4% after the TiOx layer was inserted. Although thicker TiOx layer had a higher extraction improvement, too thick TiOx layer also depressed the electricity and internal quantum efficiency. There is an optimal condition for the TiOx layer with prism sheet attachment. Fig. 5 shows the current efficiency and efficiency enhancement ratio for different thickness of TiOx layers with and without the prism sheet. As depicted in Fig. 5(a), the best efficiency occurred as the thickness of TiOx was 20 nm and the angular FWMH was 62.8°. In this case, the current efficiency increased from 8.0 cd/A to 15.0 cd/A, which indicates that the efficiency enhancement was up to 86.0%.
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Fig. 4. (a) Relative luminous intensity at different viewing angles with constant current injection, 3 mA/cm2 (normalized to the individual luminance in the normal direction) (b) Angular FWHM of emission profile and luminance enhancement ratio under different TiOx deposition thickness.
Fig. 5. (a) Current efficiency under different TiOx deposition thickness with and without the prism sheet. (b) The efficiency enhancement ratio for different configuration of the device.
References 4. Conclusion In summary, the extraction efficiency enhancement is not only influenced by the extraction ability of the microor nanostructure but also the emission profile. We used a high-index n-type material, TiOx to tune the emission profile in cooperation with the prism sheet to achieve the best extraction efficiency. The current efficiency improvement of individual techniques of TiOx layer implementation and prism sheet attachment was 33.1% and 29.3%, respectively. By combining both techniques, the overall current efficiency was increased up to 86.0% higher than the summation of the individual improvement. The optimal enhancement was attributed to the simultaneous improvement of both electron–hole balance and light extraction. Acknowledgement This work was supported by National Science Council, Taipei, Taiwan, under Contract No. NSC-100-2120-M-007006 and NSC-100-2221-E-001-010-MY3.
[1] Y.R. Do, Y.-C. Kim, Y.-W. Song, Y.-H. Lee, J. Appl. Phys. 96 (2004) 7629. [2] C.F. Madigan, M.-H. Lu, J.C. Sturm, Appl. Phys. Lett. 76 (2000) 1650. [3] J.-S. Kim, P.K.H. Ho, N.C. Greenham, R.H. Friend, J. Appl. Phys. 88 (2000) 1073. [4] S. Möller, S.R. Forrest, J. Appl. Phys. 88 (2000) 1073. [5] A. Chutinan, K. Ishihara, T. Asano, M. Fujita, S. Noda, Org. Electron. 6 (2005) 3. [6] S.-Y. Hsu, M.-C. Lee, K.-L. Lee, P.-K. Wei, Appl. Phys. Lett. 92 (2008) 013303. [7] D.K. Gifford, D.G. Hall, Appl. Phys. Lett. 81 (2002) 4315. [8] K. Ishihara, F. Masayuki, I. Matsubara, T. Asano, S. Noda, H. Ohata, A. Hirasawa, H. Nakada, N. Shimoji, Appl. Phys. Lett. 90 (2007) 111114. [9] T. Nakamura, N. Tsutsumi, N. Juni, H. Fujii, J. Appl. Phys. 96 (11) (2004) 6016. [10] J.-H. Li, Y. Yang, Solution processed organic light-emitting diodes with improved cathode interfacial structure, Proc. SPIE 7051 (2008) 705116. [11] Y. Takahashi, Y. Matsuoka, J. Mater. Sci. 23 (1988) 2259. [12] N. Takada, T. Tsutsui, S. Saito, Appl. Phys. Lett. 63 (1993) 2032. [13] K.-Y. Chen, J.-H. Lee, M.-K. Wei, Y.-T. Chang, Y.-H. Ho, J.-R. Lin, H.Y. Lin, J. Soc. Inf. Display 19 (2011) 21. [14] H.-Y. Lin, Y.-H. Ho, J.-H. Lee, K.-Y. Chen, J.-H. Fang, S.-C. Hsu, M.-K. Wei, H.-Y. Lin, J.-H. Tsai, T.-C. Wu, Opt. Express 16 (2008) 11044.
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Optimization of polymer light emitting devices using TiOx electron transport layers and prism sheets Yu-Hsuan Ho a,b, Yung-Ting Chang c, Shun-Wei Liu d, Hsiao-Han Lai a, Chih-Wei Chu a, Chih-I Wu c, Wei-Cheng Tian b, Pei-Kuen Wei a,e,⇑ a
Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 106, Taiwan c Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 106, Taiwan d Department of Electronic Engineering, Mingchi University of Technology, New Taipei 24301, Taiwan e Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung, Taiwan b
a r t i c l e
i n f o
Article history: Received 19 April 2012 Received in revised form 13 July 2012 Accepted 15 July 2012 Available online 9 August 2012 Keywords: Polymer light emitting devices Current efficiency Light extraction
a b s t r a c t The internal and external efficiency of polymer light emitting devices were found can be simultaneously improved by insertion a high refractive index material, titanium oxide (TiOx), to the emission layer and a prism sheet attached to the substrate. The TiOx layer increased the internal efficiency due to a better electron injection and hole confinement. However, it led a wider angular emission profile with more photons trapped in the substrate. By using the prism sheet, those trapped light was efficiently coupled to the air. The extraction efficiency enhancement was increased from 33.1% to 54.4% and the overall current efficiency was improved up to 86%. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Due to the large refractive index mismatch between organic layers (n = 1.7–1.9), glass substrate (n = 1.52) and air, substantial photons trapped in the organic light emitting devices are usually critical problems to constrain the external quantum efficiencies. In general, lower than one third of the generated photons can escape from emitting devices. One third of these photons are guided in the glass substrate and the others are trapped in the organic layers. Many researches had been proposed to increase the coupling ratio of external mode (coupling to air) by extracting trapped photons from substrate mode and high-index guided mode [1–3]. The surface modifications at the air/ substrate interface with microlens arrays, micro-pyramid arrays, silica micro-spheres and photonic crystals all had been reported to efficiently extract the light confined in ⇑ Corresponding author at: Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan. E-mail address: [email protected] (P.-K. Wei). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.07.014
the substrate [4,5]. Other techniques, such as the fabrication of the micro/nanostructures into devices or insertion of low index material were also proposed to couple light out of the high-index guided mode [6–8]. In addition to the surface modification for extracting the trapped photons, the device efficiency can also be improved by using the micro-cavity effect in the organic layers [9]. However, the modification of the electrontransport or hole-transport layer also leads to the degradation of internal quantum efficiency and rapid increase of operation voltage. In this work, we combined both surface roughening and cavity-tuning methods to achieve an optimal increase of current efficiency. A solution based sol–gel and spin-coating process was applied to inset a titanium oxide (TiOx) on the emissive polymer. The HOMO and LUMO levels were measured to be 7.1 and 3.6 eV, respectively, which can assist to the electron injection, transport and the hole blocking [10]. The large refractive index (n = 2.41) of TiOx also provides an effective means to tune the optical cavity mode in the emitting layers [11]. By combination the TiOx layer with the prism sheets, the
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Y.-H. Ho et al. / Organic Electronics 13 (2012) 2667–2670
extraction efficiency enhancement was increased from 33.1% to 54.4% without any device electricity degeneration. In addition, 86.0% improvement of current efficiency was achieved.
2. Device fabrication The devices were attached with prism sheets by index matching oil as illustrated in Fig. 1(a). The prism sheet had an apex angle of 90° and the prism pitch was 50 lm. This prism sheet acts as the backlight enhancement film (BEF) which can efficiently extract the substrate guiding wave. For the emitting layers, the glass substrates was coated with 250 nm-thick indium tin oxide (ITO) and then soaked into concentrated HCl for 15 min to pattern the transparent ITO anode. After UV-O3 plasma treatment, hole injection layer (Poly (3,4-ethylenedioxythiophene) – poly (styrenesulfonate), PEDOT:PSS), hole transport layer (Poly (9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine, TFB) and emission layer (polyfluorene derivative Green B) were spin-coated onto this ITO anode sequentially. The thickness of the first three organic layers was set to be around 120 nm, which is thought to be the suitable thickness for green light emission [9]. We used a solution based sol–gel process to deposit TiOx on the emissive polymer by spin coating without high temperature sintering. The rotation speed varied from 2000 to 4000 rpm for 30 s, then the samples were baked at 100 °C for 30 min. The effect of titanium oxide was revealed by Li and Yang [10], where the synthesis of titanium oxide was followed by the method published by Takahashi and Matsuoka [11]. After organic layer deposition, the opaque cathode, Cesium Carbonate
(Cs2CO3) and aluminum (Al) were evaporated by a thermal evaporator. The energy band diagram is plotted in Fig. 1(b). The energy levels were measured by using a photoelectron spectrometer (AC-2, Riken Keiki) in ambient condition. The energy levels (work function, HOMO) were calculated by plotting the square root of the total electron counts versus photoelectron energy and extrapolating the linear fit of the counts to zero. The LUMO was calculated by the HOMO and Eg, where Eg was determined from the absorption onset of the solid film. With the treated ITO, the work function raised to 5.0 eV, which matched well with the HOMO level of PEDOT:PSS. The TFB layer reduced the hole barrier and form a ladder path from PEDOT:PSS to emission layer. The HOMO of TiOx layer is very deep, which can provide an effective hole and exciton blocking property.
3. Measurement results Fig. 2 showed the characteristics of these devices including electricity, current efficiency and emission spectra. As depicted in Fig. 2(a), the current density of the device increase significantly by the insertion of the TiOx layer (10 nm) compared with the device without the TiOx layer. Due to the better electron-injection, the devices with TiOx layer implementation showed better or comparable electrical characteristics to the control device (TiOx: 0 nm). With a better hole confinement and carrier balance, the devices with TiOx all had higher current efficiencies. They were 8.01 cd/A for control device, 10.02 cd/A (TiOx: 10 nm), 10.35 cd/A (TiOx: 20 nm) and 9.40 cd/A (TiOx: 30 nm). The thicker TiOx layers resulted in a longer cavity length and made the emission spectra broadening and red shift. As illustrated in Fig. 2(c), the peak wavelength shifted from 532 nm to 540 nm and the FWHM (Full width at half maximum) was increased from 68 nm to 78 nm. The optical effect could be explained by Fabry–Perot equation [12–13].
pffiffiffiffiffi T 2 ½1 þ R1 þ 2 R1 cosð4pk xÞ pffiffiffiffiffiffiffiffiffiffi jEout ðkÞj ¼ jEin ðkÞj 1 þ R1 R2 2 R1 R2 cosð4pk LÞ 2
Fig. 1. (a) Diagrams of device structure and SEM graph of prism sheet (b) Energy level diagram of the device.
2
where Eout ðkÞ is the output intensity, Ein ðkÞ is the free space luminous intensity, x is the effective distance of the dipole from the metal cathode, R1 is the reflectivity of organic layer to the opaque cathode, R2 and T 2 are the effective reflectivity and transmission of the organic layer to the transparent anode. L is the total optical length of the cavity (including anode, organic layers, and cathode). When the total thickness of the organic layers was increased, the resonant wavelength was also increased and resulted in the red shift of the emission spectra. The calculated optical response was shown in Fig. 3(a). The optical response is the experimental electroluminescence (EL) spectrum divided by the intrinsic spectrum of the emitter, which represents whether constructive or destructive interference in the device layered structure. Fig. 3(b) shows the calculated EL spectra of the OLEDs with different TiOx thickness. When the TiOx thickness increased, the EL spectrum red shift and broadening were also observed in the simulation result.
Y.-H. Ho et al. / Organic Electronics 13 (2012) 2667–2670
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Fig. 3. Calculated optical response (a) and electroluminescence spectra (b) of the devices with different TiOx thickness.
Fig. 2. Characteristics of the emitting devices with cavity tuning layers, TiOx, thickness of 0, 10, 20, and 30 nm. (a) Current density vs. bias voltage, (b) current efficiency vs. current density, and (c) EL spectra.
When we applied the TiOx layer into the devices, the emission pattern was broadened as shown in Fig. 4(a). The wider emission pattern implies more light impinged at the substrate/air surface with a larger incident angle and more photons were trapped in the device. If we plotted the relation of the FWHM of the emission profile versus the corresponding TiOx thickness, a positive correlation was observed as shown in Fig. 4(b). Before the insertion of the TiOx layer, the angular FWHM was 60.2° which was similar to a Lambertian light source. When 30 nm TiOx layer was inserted, the angular FWHM was increased to 64°. After the application of a prism sheet onto the device,
a higher luminous enhancement was obtained for larger angular FWMH. The higher luminance came from the increase of extracted photons at larger incident angles and the reduction of the total reflection of the normal incident light. The additional inclined surface could help to couple out the light with large incident angle, but the one in small incident angle would be trapped due to the total internal reflection occurred at the inclined surface. This kind of trade-off concern had been reported in some other works [14]. Using the cavity tuning layer to change the emission profile in cooperation with surface roughening techniques can maximize the out-coupling efficiency of these emission devices. As illustrated in Fig. 4(b), the luminous enhancement ratio increased from 33.1% to 54.4% after the TiOx layer was inserted. Although thicker TiOx layer had a higher extraction improvement, too thick TiOx layer also depressed the electricity and internal quantum efficiency. There is an optimal condition for the TiOx layer with prism sheet attachment. Fig. 5 shows the current efficiency and efficiency enhancement ratio for different thickness of TiOx layers with and without the prism sheet. As depicted in Fig. 5(a), the best efficiency occurred as the thickness of TiOx was 20 nm and the angular FWMH was 62.8°. In this case, the current efficiency increased from 8.0 cd/A to 15.0 cd/A, which indicates that the efficiency enhancement was up to 86.0%.
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Fig. 4. (a) Relative luminous intensity at different viewing angles with constant current injection, 3 mA/cm2 (normalized to the individual luminance in the normal direction) (b) Angular FWHM of emission profile and luminance enhancement ratio under different TiOx deposition thickness.
Fig. 5. (a) Current efficiency under different TiOx deposition thickness with and without the prism sheet. (b) The efficiency enhancement ratio for different configuration of the device.
References 4. Conclusion In summary, the extraction efficiency enhancement is not only influenced by the extraction ability of the microor nanostructure but also the emission profile. We used a high-index n-type material, TiOx to tune the emission profile in cooperation with the prism sheet to achieve the best extraction efficiency. The current efficiency improvement of individual techniques of TiOx layer implementation and prism sheet attachment was 33.1% and 29.3%, respectively. By combining both techniques, the overall current efficiency was increased up to 86.0% higher than the summation of the individual improvement. The optimal enhancement was attributed to the simultaneous improvement of both electron–hole balance and light extraction. Acknowledgement This work was supported by National Science Council, Taipei, Taiwan, under Contract No. NSC-100-2120-M-007006 and NSC-100-2221-E-001-010-MY3.
[1] Y.R. Do, Y.-C. Kim, Y.-W. Song, Y.-H. Lee, J. Appl. Phys. 96 (2004) 7629. [2] C.F. Madigan, M.-H. Lu, J.C. Sturm, Appl. Phys. Lett. 76 (2000) 1650. [3] J.-S. Kim, P.K.H. Ho, N.C. Greenham, R.H. Friend, J. Appl. Phys. 88 (2000) 1073. [4] S. Möller, S.R. Forrest, J. Appl. Phys. 88 (2000) 1073. [5] A. Chutinan, K. Ishihara, T. Asano, M. Fujita, S. Noda, Org. Electron. 6 (2005) 3. [6] S.-Y. Hsu, M.-C. Lee, K.-L. Lee, P.-K. Wei, Appl. Phys. Lett. 92 (2008) 013303. [7] D.K. Gifford, D.G. Hall, Appl. Phys. Lett. 81 (2002) 4315. [8] K. Ishihara, F. Masayuki, I. Matsubara, T. Asano, S. Noda, H. Ohata, A. Hirasawa, H. Nakada, N. Shimoji, Appl. Phys. Lett. 90 (2007) 111114. [9] T. Nakamura, N. Tsutsumi, N. Juni, H. Fujii, J. Appl. Phys. 96 (11) (2004) 6016. [10] J.-H. Li, Y. Yang, Solution processed organic light-emitting diodes with improved cathode interfacial structure, Proc. SPIE 7051 (2008) 705116. [11] Y. Takahashi, Y. Matsuoka, J. Mater. Sci. 23 (1988) 2259. [12] N. Takada, T. Tsutsui, S. Saito, Appl. Phys. Lett. 63 (1993) 2032. [13] K.-Y. Chen, J.-H. Lee, M.-K. Wei, Y.-T. Chang, Y.-H. Ho, J.-R. Lin, H.Y. Lin, J. Soc. Inf. Display 19 (2011) 21. [14] H.-Y. Lin, Y.-H. Ho, J.-H. Lee, K.-Y. Chen, J.-H. Fang, S.-C. Hsu, M.-K. Wei, H.-Y. Lin, J.-H. Tsai, T.-C. Wu, Opt. Express 16 (2008) 11044.