Light extraction enhancement of organic lightemitting diodes using aluminum zinc oxide embedded anodes Ching-Ming Hsu,* Bo-Ting Lin, Yin-Xing Zeng, Wei-Ming Lin, and Wen-Tuan Wu Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, 1, Nan-Tai St., Yung-Kang District, Tainan City 710, Taiwan * [email protected]

Abstract: Aluminum zinc oxide (AZO) has been embedded onto indium tin oxide (ITO) anode to enhance the light extraction from an organic lightemitting diode (OLED). The embedded AZO provides deflection and scattering interfaces on the newly generated AZO/organics and AZO/ITO interfaces rather than the conventional ITO/organic interface. The current efficiency of AZO embedded OLEDs was enhanced by up to 64%, attributed to the improved light extraction by additionally created reflection and scattering of emitted light on the AZO/ITO interfaces which was roughed in AZO embedding process. The current efficiency was found to increase with the increasing AZO embedded area ratio, but limited by the accompanying increases in haze and electrical resistance of the AZO embedded ITO film. ©2014 Optical Society of America OCIS codes: (290.5880) Scattering, rough surfaces; (230.3670) Light-emitting diodes; (230.4000) Microstructure fabrication; (310.6860) Thin films, optical properties.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, and S. R. Forrest, “Management of singlet and triplet excitons for efficient white organic light-emitting devices,” Nature 440(7086), 908–912 (2006). H. Sasabe, J. Takamatsu, T. Motoyama, S. Watanabe, G. Wagenblast, N. Langer, O. Molt, E. Fuchs, C. Lennartz, and J. Kido, “High-efficiency blue and white organic light-emitting devices incorporating a blue iridium carbene complex,” Adv. Mater. 22(44), 5003–5007 (2010). N. Greenham, R. H. Friend, and D. D. C. Bradley, “Angular dependence of the emission from a conjugated polymer light-emitting diode: implications for efficiency calculations,” Adv. Mater. 6(6), 491–494 (1994). R. Meerheim, M. Furno, S. Hofmann, B. Lussem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97(25), 253305 (2010). S. Mladenovski, S. Hofmann, S. Reineke, L. Penninck, T. Verschueren, and K. Neyts, “Integrated optical model for organic light-emitting devices,” J. Appl. Phys. 109(8), 083114 (2011). K. Arunandan, R. Srivastava, D. S. Mehta, and M. N. Kamalasanan, “Surface plasmon enhanced blue organic light emitting diode with nearly 100% fluorescence efficiency,” Org. Electron. 13(9), 1750–1755 (2012). S. Möller and S. Forrest, “Improved light out-coupling in organic light emitting diodes employing ordered microlens arrays,” J. Appl. Phys. 91(5), 3324–3327 (2002). M. Thomschke, S. Reineke, B. Lüssem, and K. Leo, “Highly efficient white top-emitting organic light-emitting diodes comprising laminated microlens films,” Nano Lett. 12(1), 424–428 (2012). H. J. Peng, Y. L. Ho, X. J. Yu, and H. S. Kwok, “Enhanced coupling of light from organic light emitting diodes using nanoporous films,” J. Appl. Phys. 96(3), 1649–1654 (2004). K. P. Kim, K. S. Lee, T. W. Kim, D. H. Woo, J. H. Kim, J. H. Seo, and Y. K. Kim, “Enhancement of the light extraction efficiency in organic light emitting diodes utilizing a porous alumina film,” Thin Solid Films 516(11), 3633–3636 (2008). J. H. Zhou, N. Ai, L. Wang, H. Zheng, C. Luo, Z. Jiang, S. Yu, Y. Cao, and J. Wang, “Roughening the white OLED substrate's surface through sandblasting to improve the external quantum efficiency,” Org. Electron. 12(4), 648–653 (2011). Y. J. Lee, S. H. Kim, J. Huh, G.-H. Kim, Y.-H. Lee, S.-H. Cho, Y.-C. Kim, and Y. R. Do, “A high-extractionefficiency nanopatterned organic light-emitting diode,” Appl. Phys. Lett. 82(21), 3779–3781 (2003). Y. R. Do, Y. C. Kim, Y. M. Song, C.-O. Cho, H. Jeon, Y.-J. Lee, S.-H. Kim, and Y.-H. Lee, “Enhanced light extraction from organic light-emitting diodes with 2D SiO2/SiNx photonic crystals,” Adv. Mater. 15(14), 1214– 1218 (2003).

#223036 - $15.00 USD Received 12 Sep 2014; revised 10 Oct 2014; accepted 16 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1695 | OPTICS EXPRESS A1695

14. W. H. Koo, S. Y. Boo, S. M. Jeong, S. Nishimura, F. Araoka, K. Ishikawa, T. Toyooka, and H. Takezoe, “Controlling bucking structure by UV/ozone treatment for light extraction from organic light emitting diodes,” Org. Electron. 12(7), 1177–1183 (2011). 15. H.-W. Chang, J. Lee, S. Hofmann, Y. Hyun Kim, L. Müller-Meskamp, B. Lüssem, C.-C. Wu, K. Leo, and M. C. Gather, “Nano-particle based scattering layers for optical efficiency enhancement of organic light-emitting diodes and organic solar cells,” J. Appl. Phys. 113(20), 204502 (2013). 16. Y. Zhang and H. Aziz, “Very high efficiency phosphorescent organic light-emitting devices by using rough indium tin oxide,” Appl. Phys. Lett. 105(1), 013305 (2014).

1. Introduction Since the internal quantum efficiency of an organic light-emitting diode (OLED) can readily reach 100% [1,2], the degree of light extraction from light-emitting area becomes the major factor to determine the external quantum efficiency. It is well known that the emitted light is trapped in the stacked layers of an OLED mostly due to optical total reflections at refractiveindex-mismatched interfaces and optical absorptions by surface plasmon poraritons near the surface of a metallic electrode [3–6]. For these reasons, the external quantum efficiency of an OLED generally remains below 20%. A number of approaches have been proposed to extract light that is trapped by the total reflection at refractive-index-mismatched interfaces. Optical total reflection at glass/air interface is called glass mode total reflection and can be effectively minimized by attaching a micro-lens film out of glass substrate [7,8]. Light extraction efficiency improved up to 70% has been demonstrated, and commercial available micro-lens film can reach a light enhancement factor around 30%. Attaching films with nanoparticles or porous structure or roughing glass substrate can also enhance light extraction efficiency [9–11]. This is because these structures generate light scattering centers that can redistribute the propagating directions of light. Optical total reflection at indium tin oxide (ITO)/glass interface is called waveguide mode total reflection and can be reduced by applying a specific structure at the interface. Structures having a form of photonic crystals, optical gratings and bucking at the interface allow emitted light to be guided away from its total reflection critical angle, leading to an enhanced light extraction [12–14]. The same effect was observed when a scattering layer was internally embedded in the stacked structure of an OLED [15]. However, there are more concerns in reduction of waveguide mode total reflection than glass mode, as the internally introduced structure directly contact with organics and generally cause deteriorated surface topography. New structures for reducing waveguide mode total reflection are still highly interested and need to be further addressed. This article reports the fabrication of aluminum zinc oxide (AZO) embedded ITO anodes for OLED devices to reduce the waveguide mode total reflection and enhance light extraction efficiency without seriously changing the surface topography of ITO. We expected two mechanisms that could lead to an enhanced light extraction efficiency: (1) the newly formed organic/AZO/ITO structure instead of the traditional organic/ITO structure would potentially change the propagating direction of light away from its total reflection critical angle; (2) the embedded AZO/ITO interfaces could provide scattering centers to redistribute light and allow part of the light to escape from the device. 2. Experimental details To fabricate AZO embedded ITO anodes, commercially available ITO films with a thickness of 250 nm deposited on glass were cleaned, photolithography patterned and chemically etched to a depth of 100 nm, to generate recessed surfaces with circular patterns. The circular patterns were designed to have diameters of 4 μm and 6 μm and pitches of 10 μm and 12.5 μm, giving the recessed area a coverage ratio of 7.9%, 14.1%, 20.9%, and 27.7%. Some ITO films were etched to a depth of 150 nm and 200 nm to investigate the thickness effects of embedded AZO. A 300-nm-thick AZO layer was then deposited on top of the recessed ITO using planar magnetron sputtering, followed by a chemical mechanical polishing (CMP) process to remove AZO to a stage that the remaining ITO thickness was 200 nm. AZO embedded ITO films were completed after the CMP polishing and careful cleaning. Figures

#223036 - $15.00 USD Received 12 Sep 2014; revised 10 Oct 2014; accepted 16 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1695 | OPTICS EXPRESS A1696

1(a)–1(c) schematically illustrate how AZO was embedded onto the ITO surface. AZO embedded ITO films were measured using a Haze meter (NHD-5000) and a 4-point prober (4 Dimensions 280SI) to examine their optical and electrical properties, respectively.

Fig. 1. Schematics of process flow for fabricating AZO embedded ITO film (a) recessed ITO processing, (b) AZO sputter deposition, and (c) CMP polishing of AZO/ITO stacked film.

OLED devices with a glass/ITO/N,N”-di-phenyl-N,N”-di-[4-(N,N-di-phenylamino)pheny]benzidine (NPNPB, 75nm)/n-Propyl bromide (NPB, 10nm)/4,4',4”-Tri(9carbazoyl)triphenylamine (TCTA, 5nm)/WHP401(Lumitech Corp., 37.5nm) doped 7% Tris(2-phenylpyridinato) iridium(III) (Ir(ppy)3)/(10-Hydroxybenzo[h]quinolinate)beryllium complex (BeBQ2, 30nm)/LiF(0.5nm)/Al(100nm) structure were fabricated on the AZO embedded ITO films. The AZO embedded ITO substrate, serving as an anode for OLED, was first surface treated in 100W oxygen plasma ambient for 10 mins to clean the surface and elevate ITO surface work function. Organic, LiF and Al layers were then thermally evaporated in sequence to complete the device. All the complete devices were packaged in a glove box for current-voltage (I-V) and luminance-voltage (L-V) characterizations using a source-measure unit (model 237, Keithly Instruments Inc.) and an optical spectrometer (PR650, PhotoResearch). It is noted that device luminance was measured at the perpendicular angle to the substrate and device efficiency was calculated based on this perpendicular luminance. Performances of OLED with and without AZO embedded ITO anodes were examined. Characteristics of OLED devices dependent on AZO coverage ratio were discussed. Table 1. Total optical transmittance, haze ratio and sheet resistivity of AZO embedded ITO films at various AZO coverage ratios. AZO embedded area ratio Optical transmittance (ITO, %) Haze (%) Electrical resistivity (x10−4 Ω cm/)

0% 90.7 0.43 1.35

7.9% 88.9 1.34 1.6

14.1% 90.7 1.43 1.79

20.9% 90.7 1.98 2.23

27.7% 91.1 2.22 2.25

3. Results and discussions Table 1 lists the total optical transmittance, haze and sheet resistivity of AZO embedded ITO films. It can be seen the optical transmittance of AZO embedded ITO (90.7% ~91.1%) is higher than that of the initial ITO film (90.1%) except in the 7.9% embedded case (88.9%), attributed to that AZO embedded ITO film has a thinner thickness (200 nm) than the initial ITO (250 nm). The hazes of AZO embedded ITO films (1.3% ~2.2%) are all higher than that of the initial ITO (0.4%) and increase with the embedded area ratio, indicating the incident

#223036 - $15.00 USD Received 12 Sep 2014; revised 10 Oct 2014; accepted 16 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1695 | OPTICS EXPRESS A1697

light diffuses more seriously as more AZOs are embedded. The sheet resistivity of ITO film increases with the AZO embedded area ratio from 1.5 x 10−4 (0%) to 2.3 x 10−4 Ω⋅cm/ (27.7%), and this is due to that AZO has higher electrical resistivity than ITO. Figure 2 shows the scanning electron microscope (SEM) images of ITO at the corresponding stages in Fig. 1. For clearness, the recess depth is 200 nm in this case. Figures 2(a1) and 2(a2) clearly show that an array of circular recesses has been formed on ITO surface and the recess wall is tapered. The surface roughness of etched surface on recess bottom, determined by an atomic force microscope (AFM, Burleigh Instruments METRIS 2001), is 5.3 nm which is much higher than the initial ITO surface of 0.6 nm. Figures 2(b1) and 2(b2) show the AZO layer deposited on top of the recessed ITO surface is conformal, but with a number of particles on the surface. These particles were removed after CMP processing, as can be seen in Figs. 2(c1) and 2(c2). From Fig. 2(c2) one can also see that CMP polished AZO and ITO surfaces are flat, but with a height difference of around 25 nm, indicating a slight dishing effect exists due to the polishing rate discrepancy between AZO and ITO.

Fig. 2. SEM images of (a) recessed ITO, (b) AZO coated on recessed ITO, and (c) AZO embedded ITO after CMP polishing.

Figure 3(a) shows the I-V characteristics of OLEDs. It can be seen that all AZO embedded devices perform deteriorated I-V characteristics compared with the planar ITO device when V > 3.5 V. This is because the electrical resistance of the planar ITO film is lower than that of the AZO embedded ITO film. Another possible factor is that AZO has a lower work function, typically 4.3 eV, compared to ITO (4.6 eV). AZO embedded devices also exhibit relatively high leakage current except the one with 20.9% embedded area ratio, indicating the CMP polished AZO/ITO surfaces are not as flat as the initial ITO. Figure 3(b) shows the L-V characteristics of OLED devices. All devices exhibit similar L-V characteristics except the one with 14.1% embedded area ratio which is apparently worse. From Figs. 3(a) and 3(b), the current efficiency as a function of driving voltage can be obtained and is shown in Fig. 3(c). It clearly shows all AZO embedded devices perform improved current efficiency. At V = 3.5 V, the current efficiency is improved by up to 64% when comparing the 20.9% embedded device (η = 34.1 cd/A) with the planar device (η = 20.8 cd/A). Since all the samples were loaded to the thermal evaporator for device fabrication at the same time, it is believed there is little discrepancy in the device structure for all the devices. Internal quantum efficiency is then assumed to be the same for all devices and device performances should thus be merely dependent on the status of ITO anode. Figure 3(c) also reveals that the higher embedded area ratio is, the better current efficiency, but it seems to saturate at high AZO embedded area ratio, in this case 20.9%. To explain the phenomena in Fig. 3(c), an optical mechanism was proposed.

#223036 - $15.00 USD Received 12 Sep 2014; revised 10 Oct 2014; accepted 16 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1695 | OPTICS EXPRESS A1698

Fig. 3. (a) I-V, (b) L-V, and (c) η–V characteristics of OLEDs with AZO embedded ITO anodes at various AZO coverage ratios.

Figure 4 shows the possible optical routes for the embedded AZO structures, in which the refractive indices of AZO and ITO, measured by an ellipsometer (J.A. Woollam, M-2000D), are 1.85 and 1.68, respectively. Four regions (A1 – A4) are discussed. In region A1, the emitted light propagating to the left would proceed as it is in a planar device and no light extraction enhancement is expected. In region A2, those to the right and supposedly to be totally reflected in a planar device now have chances to hit on the curved AZO/ITO interface of the side wall at which optical reflection, deflection and scattering can take place. The reflected light can escape from the device if its direction is close to the normal of the substrate. A small portion of the scattered light can propagate to the bottom of AZO where it conducts the second scattering due to the roughed AZO/ITO interfaces and partly escapes from the stacked structure. The light having a more oblique incident angle will be deflected to the bottom of AZO and scattered and out-coupled from the device as described above. It is noted that the scattering effect on a surface with a roughness of a few nanometers is not expected prominent. However Aziz etc [16]. has recently reported a light out-coupling enhancement up to 40% by roughing ITO surface from Ra = 3.8 nm to 8.5 nm. We therefore reckoned slight scattering may take place at the AZO/ITO interfaces, particular for light incident from an oblique angle. In region A3, the emitted light traveling to the left will propagate to the bottom AZO/ITO interfaces where again the roughed interfaces would enhance the light out-coupling. In region A4, the light propagating to the right and supposedly totally reflected in a planar device will partially reflect at the curved AZO/ITO interface and escape from the device as it does in the region A3. A small portion of the light scatters at the curved AZO/ITO interface, and part of it can then travel towards the normal of substrate and escape from the device. According to this mechanism, the degree of improvement in current efficiency should be embedded-area dependent, because more reflection and scattering can occur for a large AZO embedded area. Indeed from Fig. 3(c), the current efficiency at 3.5 V is 28.7, 33.2, and 34.1 cd/A for 7.9%, 20.9%, and 27.7% embedded devices, respectively. The efficiency is improved by 37.4%, 63.7%, and 59.4% correspondingly compared to that of the planar device (η = 21.3 cd/A). The 14.1% embedded device is not discussed here for its instability on I-V behavior

#223036 - $15.00 USD Received 12 Sep 2014; revised 10 Oct 2014; accepted 16 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1695 | OPTICS EXPRESS A1699

that is probably caused by not fully cleaned AZO/ITO surface after CMP processing. At high voltages (> 5 V), the current efficiency improvement factor remains high with 42.8%, 61.3%, and 60.6% for 7.9%, 20.9%, and 27.7% embedded devices, respectively, suggesting the optical mechanism remains effective at high optical density. However, we noticed that 20.9% and 27.7% embedded devices exhibit similar I-V, L-V and η-V characteristics. Besides, their electrical resistivity and total optical transmittance of AZO embedded ITO anode are close, 2.23 × 10−4 Ω cm/ and 91.1% for the 20.9% embedded and 2.25 × 10−4 Ω cm/ and 90.7% for the 27.7% embedded. The difference is that 27.7% embedded ITO film has a higher haze (2.25%) than the 20.9% embedded film (1.98%). We further examined the device performances of 20.9% AZO embedded film with recess depths of 150 nm and 200 nm. The current efficiency was found deteriorated with the increasing AZO thickness, η = 22.5 and 10.1 cd/A for devices with 150 nm and 200 nm AZO embedded ITO anodes, respectively. The 150 nm AZO embedded ITO has a haze of 2.32%, total optical transmittance of 91.3% and electrical resistivity of 2.3 × 10−4 Ω cm/, while the corresponding values for the 200 nm embedded are 0.8%, 91.6% and 3.5 × 10−4 Ω cm/. For the 150 nm device, the AZO embedded ITO film has electrical and optical properties close to those of the 100 nm embedded except for its higher haze (1.98% → 2.32%). For the 200 nm device, the AZO embedded ITO film has lower haze (1.98% → 0.8%) and higher electrical resistivity (2.23 → 3.5 × 10−4 Ω cm/, 60% increased). From these observations, we suggested that high haze of an AZO embedded ITO film has little effect for the light extraction enhancement, while high resistivity of the film would limit the efficiency enhancement. The major factor enhancing the current efficiency is considered to be the curved AZO/ITO interfaces on the AZO side wall, which redirect the light supposedly to be trapped in a planar device to escape from the device. Whereas, the scattering effect for the light directly hit on the roughed AZO/ITO bottom and contributes to an increased haze is considered to be a minor factor for the enhanced efficiency. Further optical simulation to support the mechanism proposed above is necessary.

Fig. 4. Schematics of possible optical routes for OLED devices with an AZO embedded ITO anode. Solid line represents light escaped and the dotted line represents light trapped.

4. Conclusions This work has demonstrated that the current efficiency of OLEDs can be enhanced by up to 63.7% using AZO embedded ITO anodes. The enhancement is mainly attributed to the improved light extraction by additionally created optical reflection and scattering on the curved AZO/ITO interfaces. The current efficiency was found to increase with the increasing AZO embedded area ratio, but limited by the accompanying increases in haze or electrical resistance of the AZO embedded ITO film. Further studies on using various materials to realize the effects of refractive index and work function of embedded materials are of interest. Acknowledgments The authors would like to thank the Ministry of Science and Technology for financially supporting this work under contract grant NSC-102-2221-E218-028.

#223036 - $15.00 USD Received 12 Sep 2014; revised 10 Oct 2014; accepted 16 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1695 | OPTICS EXPRESS A1700

Light extraction enhancement of organic light-emitting diodes using aluminum zinc oxide embedded anodes.

Aluminum zinc oxide (AZO) has been embedded onto indium tin oxide (ITO) anode to enhance the light extraction from an organic light-emitting diode (OL...
2MB Sizes 0 Downloads 7 Views