Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses

ORGELE 2492

No. of Pages 7, Model 3G

2 April 2014 Organic Electronics xxx (2014) xxx–xxx 1

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

2

Letter

7 4 8

Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses

5 6 9 10 12 11 13 1 2 5 9 16 17 18 19 20 21 22 23 24 25 26 27 28

Q1

Dong-Young Kim, Chung Sock Choi, Jin Yeong Kim, Do Hong Kim, Kyung Cheol Choi ⇑ Department of Electrical Engineering, KAIST, Daejeon 305-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 30 October 2013 Received in revised form 11 March 2014 Accepted 15 March 2014 Available online xxxx Keywords: Transparent organic light emitting diodes Surface plasmon Nanostructure Outcoupling Transmittance Haziness

a b s t r a c t In this work, we fabricated nanostructured transparent organic light-emitting diodes (TrOLEDs) using phosphorescent materials and a WO3 layer with various periods of perforation, to improve light extraction. Using these nanostructured TrOLEDs, higher external quantum efficiency (EQE) values were achieved, of 7.8% (bottom emission), and 2.0% (top emission) at 100 mA/cm2. Compared to conventional TrOLEDs, these were 28% and 33% higher for bottom and top emission, respectively. In addition, by varying the periods of the nanostructures, we found that the extraction of the trapped surface plasmon mode was mainly responsible for enhancing outcoupling efficiency. When adopting light extraction methods in TrOLEDs, one should consider the influence of the optical clarity of devices. The nanostructured TrOLEDs in this study showed good optical clarity as the total transmittance was consistent with direct transmittance. Photographs of the TrOLEDs also showed neither optical blur nor haziness. Lastly, the total transmittance of the nanostructured TrOLEDs was similar to that of a conventional TrOLED except for two points where light coupling to the surface plasmon mode and waveguide mode occurred. Ó 2014 Elsevier B.V. All rights reserved.

30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

47 48

1. Introduction

49

In recent years, research on organic light-emitting diodes (OLEDs) has increased rapidly, and they are now beginning to replace liquid crystal display (LCD) and plasma display panel (PDP) displays in the display market. The progressive development of OLEDs has led to a novel area of research, transparent OLEDs (TrOLEDs). However, some issues need to be addressed before TrOLEDs can be deployed on a commercial scale, including improvements in efficiency, transmittance, and haziness. TrOLEDs can be fabricated by replacing the top metallic cathode of an OLED with transparent conductive oxides (TCOs) or thin metal films [1–3]. This causes light to emit

50 51 52 53 54 55 56 57 58 59 60

⇑ Corresponding author. Tel.: +82 42 350 3482; fax: +82 42 350 8082. E-mail address: [email protected] (K.C. Choi).

from both the top (cathode) and bottom (anode). Since this decreases the amount of light emitted in one direction for a given current, high external quantum efficiency (EQE) and power efficiency are essential in TrOLEDs. To improve TrOLED efficiency, internal quantum efficiency (IQE) and outcoupling efficiency need to be considered, just as in conventional OLEDs [4]. The IQE can reach almost 100% when phosphorescent materials [5,6], hole injection layers (V2O5 [7], CuO [8], MoO3 [9], WO3 [10]), and an electron injection layer (LiF [11], MgO [12], CsCO3 [13]) are adopted. On the other hand, the outcoupling efficiency remains at 20% due to light dissipation caused by loss mechanisms such as surface plasmon mode, waveguide mode, and substrate mode [14]. Thus, there have been a number of studies to increase the outcoupling efficiency which have employed the following methods: (1) the use of a low refractive index substrate [15,16]; (2) the use of patterned

http://dx.doi.org/10.1016/j.orgel.2014.03.010 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: D.-Y. Kim et al., Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.010

61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

ORGELE 2492

No. of Pages 7, Model 3G

2 April 2014 2

D.-Y. Kim et al. / Organic Electronics xxx (2014) xxx–xxx

100

ITO [17]; (3) the use of a refractive modulation layer [18]; (4) the use of a micro lens array attached to the glass substrate [19]; and (5) the use of photonic crystal [20]. In conventional OLEDs, these methods are effective increasing outcoupling efficiency based on optical scattering [4]. However, because these methods cause the scattering of incident light, and that scattering in turn causes optical haziness, these methods are not suitable for TrOLEDs. For this reason, techniques are needed that can enhance the efficiency of TrOLEDs without producing haziness and image blur. In a previous work, our group succeeded in increasing TrOLED efficiency without causing image blur by employing colloidal lithography [21]. However, that TrOLED showed low EQE, power efficiency, and transmittance. Also, we could not clarify whether the enhanced outcoupling efficiency was caused by light extraction from the trapped surface plasmon or the trapped waveguide mode. In this study, we fabricated a TrOLED with higher efficiency and transmittance. By measuring the effect of a WO3 layer perforated with various periods we determined that the outcoupling enhancement was mainly due to the light extraction of the trapped surface plasmon mode.

101

2. Experimental section

102

2.1. Fabrication of the perforated WO3 layer with various diameters

78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124

125 126 127 128 129 130 131 132

Nanostructured TrOLEDs were fabricated in a manner similar to that previously described [22]. The perforated nanostructure was fabricated by colloidal lithography. Polystryrene (PS) nanoparticles with diameters of 265 nm, 330 nm, and 390 nm were prepared. These PS particles were synthesized by emulsion polymerization [23], and we could easily obtain different diameters by controlling the weight of the added emulsifier. We prepared 150-nm-thick indium tin oxide (ITO)-coated glass which was sonicated in acetone and isopropyl alcohol (IPA) before use. The Langmuir–Blodgett (LB) method [24] was used to generate well-packed hexagonal arrays of PS nanoparticles on the ITO coated glass, without affecting their sizes. After this coating, air plasma etching was performed for varying amounts of time to obtain loosely packed nanostructures (Plasma cleaner-PDC-32G-2, Harrick Plasma). A 25-nm-thick WO3 layer was deposited on the reduced PS particles by thermal evaporation. After WO3 deposition, residual PS particles were removed by sonication in ethanol and plasma cleaning to make a well patterned perforated layer (Fig. 1). 2.2. Fabrication of nanostructured TrOLEDs and measurements of characteristics Organic layers and cathode material were thermally evaporated one after another on the prepared nano-structured substrate. A 40-nm-thick N, N0 -diphenyl-N, N0 -bis (1-naphthyl)-1, 10 -biphenyl-4, 400 -diamine (NPB) layer for the hole transporting layer, a 3% tris(1-phenylisoquinoline) iridium (III) (Ir(piq)3) doped in 30-nm-thick

bis(10-hydroxybenzo(h)quinolinate) beryllium (Bebq2) layer for light emission, and a 20-nm-thick Bebq2 layer for electron transport were deposited. In addition, a 1-nm-thick lithium fluoride (LiF) layer, a 1-nm-thick aluminum (Al) layer for electron injection, a 15-nm-thick silver (Ag) cathode layer, and 50-nm-thick NPB optical capping layer were deposited. The devices were encapsulated with UV curable resin and glass in a glove box under a nitrogen atmosphere to prevent degradation of the devices during measurement. The current–voltage characteristics of the devices were measured using a Keithley 2400 source meter, and the luminance and emission spectra were measured using a spectroradiometer (CS-2000, Konica Minolta). Angular emissions were measured at various emission angles from 60° to 60° with increments of 10° to calculate the EQE and power efficiency. All the transmittance spectra were obtained using a UV–Visible spectrophotometer (UV-2550, Simadzu), and an integrating sphere was used to measure scattering in the total transmittance.

133

3. Results and discussion

153

3.1. Bragg scattering condition in nanostructured TrOLEDs

154

To elucidate the reason for the observed device outcoupling enhancement we fabricated conventional TrOLEDs, and three types of nanostructured TrOLEDs, with nanostructure periods of 265 nm, 330 nm, and 390 nm, as shown in Fig. 1(a) and (b). Fig. 1(c–e) shows SEM images of the perforated WO3 layer with various periods. These perforated layers were fabricated by colloidal lithography using the PS particles of 265 nm, 330 nm, and 390 nm diameter. Each image shows a well patterned hexagonal nanostructured WO3 layer without any change in period. The organic layers and metal cathode layer have been deposited sequentially on the patterned WO3 layer, and the corrugated nanostructures are embedded in the metal cathode [22]. It is well known that there is a significant amount of power loss as a waveguide mode and SP mode in small molecule OLEDs [14]. The energy loss which trapped between the organic layer and metal layer could be reduced by a periodic nanostructure when it satisfies the Bragg scattering condition [25–30]. In normal emission, the Bragg scattering condition can be calculated using the following equation. The fabricated nanostructures were considered to be a hexagonal grating structure based on the SEM images.

155

jkguide j  Gmn ¼ 0

Gmn

4p pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ pffiffiffi m2 þ n2 þ mn 3a

where kguide is the wave vectors of the waveguide mode or the surface plasmon mode, and Gmn is a grating vector of the nanostructure. Here, m and n are integers, and a is the period of the hexagonal grating structure. The values calculated by the transfer matrix are shown as Fig. 2. Fig. 2(a) shows the normalized E-field profiles of the TrOLEDs. It indicates that the double peaks of the TM0 mode (surface plasmon mode) exist near the Ag cathode layer. One peak is below the Ag cathode layer, which has

Please cite this article in press as: D.-Y. Kim et al., Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.010

134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152

156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177

178 180 181 182 183 184 185 186 187 188 189

ORGELE 2492

No. of Pages 7, Model 3G

2 April 2014 D.-Y. Kim et al. / Organic Electronics xxx (2014) xxx–xxx

3

Fig. 1. Schematics of (a) conventional TrOLEDs and (b) nanostructured TrOLEDs. (c), (d), and (e) are SEM images of the perforated WO3 layer with various periods of 265 nm, 330 nm, and 390 nm, respectively. Inset: Magnified SEM image of the perforated WO3 layer.

190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210

211 212 213 214 215 216 217

stronger intensity due to its proximity to the exciton. The other peak, above the Ag cathode layer, appears because of the NPB organic capping layer. Additionally, the TE0 and TM1 modes (waveguide modes) exist in the ITO substrate and organic layer. In the supplemental information, the distribution of electric field intensity for the conventional TrOLEDs is illustrated using the Finite Difference Time Domain Technique (FDTD) technique (Fig. S1). The simulation results show that strong electric field intensity exists near the Ag cathode layer as the TM mode. Fig. 2 (b–d) shows the relation between the emission wavelength and grating periods, which can extract the trapped waveguide mode and surface plasmon mode to the light. Based on these values, we can calculate the emission wavelength where the outcoupling enhancement can occur. The wavelength of the peak emission intensity of TrOLEDs is near 625 nm, and it matches well with the TM0 mode coupling condition when the grating period is 265 nm. However, when the grating period is 330 nm or 390 nm, the TM0 mode coupling condition deviates from the peak emission intensity. 3.2. EL intensity and light extraction for the nanostructured TrOLEDs Fig. 3(a) shows the EL intensity spectra of the bottom emission for the conventional TrOLEDs and nanostructured TrOLEDs with various periods. All spectra of the TrOLEDs were measured in normal emission where the emission angle was 0°. Compared to the conventional TrOLEDs,

higher EL intensity could be obtained when the grating period was 265 nm. However, when the grating period was 330 nm or 390 nm, there was a slight enhancement of EL intensity near the wavelengths of 700 nm or 750 nm. This is because the wavelength at which Bragg scattering occurred was far from the peak EL intensity of the emitting layer; thus, the increase in EL intensity was smaller than when the grating period was 265 nm. Fig. 3(b) shows the EL spectra of the top emission for all devices, and it shows an increase similar to the bottom emission. These results show that the wavelengths of increased EL intensity were well matched to the calculation for the Bragg scattering condition when the grating period was 265 nm, 330 nm, or 390 nm (Fig. 2). In particular the wavelengths of increased EL intensity correspond to the wavelength at which Bragg scattering of the TM0 mode occurred. Meanwhile, the increased EL intensity caused by the Bragg scattering of the waveguide mode was insignificant, although the wavelength was near the peak EL intensity when the grating period was 390 nm. Therefore, it is considered that the enhancement of outcoupling efficiency of the nanostructured TrOLEDs is mainly the result of reduced surface plasmon loss due to extraction of the trapped surface plasmon mode.

218

3.3. Performance of the nanostructured OLEDs

242

The characteristics of the device were measured with varying emission angles (h) from 60° to 60° in increments of 5° at a fixed azimuthal angle (U). There was no angular

243

Please cite this article in press as: D.-Y. Kim et al., Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.010

219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241

244 245

ORGELE 2492

No. of Pages 7, Model 3G

2 April 2014 4

D.-Y. Kim et al. / Organic Electronics xxx (2014) xxx–xxx

Fig. 2. (a) Normalized E-field intensity profiles of the surface plasmon mode and waveguide mode in TrOLEDs at 625 nm (main emission wavelength of Ir(piq)3 doped in Bebq2). (b), (c) and (d) show the relation between the grating periods and outcoupled emission wavelength with grating periods of 265 nm, 330 nm, and 390 nm, respectively. The intersection points are marked with dotted circles, and the corresponding values at the x-axis are the wavelengths at which Bragg scattering occurs.

Fig. 3. Measured EL spectra of conventional TrOLEDs and nanostructured TrOLEDs with various periods. (a) bottom (anode side) emission and (b) top (cathode side) emission at an emission angle of 0°.

246 247 248

dependency on the azimuthal angle because the nanostructured device was fabricated using colloidal lithography, which tends to form domains with random

orientation over a scale of tens of micrometers (Fig. 1). The radiant intensity profiles also showed no angular dependency when the nanostructured TrOLEDs were

Please cite this article in press as: D.-Y. Kim et al., Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.010

249 250 251

ORGELE 2492

No. of Pages 7, Model 3G

2 April 2014 D.-Y. Kim et al. / Organic Electronics xxx (2014) xxx–xxx

5

Fig. 4. (a) Current density (J)–voltage (V) characteristics of conventional TrOLEDs and nanostructured TrOLEDs. EQE as a function of current density characteristics for (b) bottom (anode side) emission and (c) top (cathode side) emission. Power efficiency versus current density for (d) bottom (anode side) emission and (e) top (cathode side) emission.

252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282

measured with varying emission angles and azimuthal angles [21]. The current density–voltage characteristics of the nanostructured TrOLED are similar to those of the conventional device, as shown in Fig. 4(a). This is due to the WO3 layer, which has a good hole injection property. This insertion layer could reduce the effect of the nanostructure layer, which may have an insulating character or may change the hole pathway. Fig. 4(b) and (c) shows the EQE of the conventional TrOLED and the nanostructured TrOLEDs for bottom and top emission. In comparison to the conventional TrOLED, the nanostructured TrOLED with a 265 nm grating period showed substantial enhancement for bottom and top emissions. The increase was 28% for the bottom emission and 33% for the top emission at a current density of 100 mA/cm2. This enhancement ratio is similar to that observed in a previous work [21], but the absolute value of EQE was greatly increased (6.1% to 7.8% at 100 mA/cm2 for the bottom and 1.5% to 2.0% at 100 mA/ cm2 for the top). This was possible because the TrOLEDs were fabricated using highly efficient phosphorescent materials for the emitting layer, while the previous work employed a fluorescent material. However, the EQE of the nanostructured TrOLEDs with 330 nm and 390 nm grating periods showed slight or no increase in emission compared to conventional devices. As seen in Fig. 4(d) and (e), the power efficiency showed the same tendency as the EQE because the value was calculated from EQE and voltage (a log scale for the current is provided in Fig. S2). These EQE and power efficiency results correspond to the calculation for the E-field profile and dispersion relation as shown above.

3.4. Transmittance and optical haziness in nanostructured TrOLEDs

283

When an additional nanostructure layer is embedded in conventional TrOLEDs, transmittance and image blur are affected. The nanostuctured layer decreases transmittance or causes haziness due to scattered transmittance (Ts). Since total transmittance (Tt) consists of direct transmittance (Td) and scattered transmittance, the smaller the difference between Tt and Td, the less image blur occurs. Fig. 5(a) and (b) shows the total transmittance (Tt) spectra and direct transmittance (Td) spectra. The Td spectra of the nanostructured TrOLED were almost identical to the Tt spectra. This means that the proposed nanostructured TrOLEDs have little scattered transmittance, and this was also confirmed by photographs. Fig. 6 shows photographs in which letters are clearly visible without blurring when viewed through the nanostructured TrOLEDs. As seen in Fig. 5(c), the peak transmittance of the nanostructured TrOLEDs with grating periods of 265 nm was 67% around 550 nm, and this transmittance value is over 10% higher than that achieved in a previous work [21]. This is due to the thinner Ag cathode layer (15 nm), which reduces the light reflection and absorption. In the spectra, there is no significant difference between the nanostructured TrOLEDs and conventional TrOLEDs except at two sites. In the nanostructured TrOLEDs with a grating period of 265 nm, the different sites (around 450 nm and 625 nm) are the minima sites in transmittance. The nanostructured TrOLEDs with other periods also had two different minima points compared to the conventional TrOLEDs as shown in

285

Please cite this article in press as: D.-Y. Kim et al., Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.010

284

286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312

ORGELE 2492

No. of Pages 7, Model 3G

2 April 2014 6

D.-Y. Kim et al. / Organic Electronics xxx (2014) xxx–xxx

Fig. 5. (a) Total transmittance and (b) direct transmittance of bare glass, ITO-coated glass, TrOLEDs with planar WO3 layer, TrOLEDs without WO3 layer, and TrOLEDs with various periods of WO3 layers. (c), (d), and (e) show the direct transmittance grouped by the nanostructure period to find the difference between conventional TrOLEDs and nanostructured TrOLEDs.

Fig. 6. Photographs of turn off and turn on images at various current densities: (a) conventional TrOLEDs, (b) nanostructured TrOLEDs.

313 314 315 316 317 318 319

Fig. 5(d) and (e). This slight decrease of transmittance is due to the light coupling of the TM0 mode and TM1 mode. When light coupling occurs, the incident light may be absorbed or re-scattered to the ambient under the identical Bragg scattering condition. Therefore, this result also indicates that the increased TrOLEDs efficiency is due to extraction from the trapped surface plasmon mode and

waveguide mode when the nanostructure periods satisfy the Bragg scattering condition.

320

4. Conclusion

322

In conclusion, we fabricated phosphorescent nanostructured TrOLEDs with a perforated WO3 layer having varying periods, to overcome the limitations observed in previous work. When the nanostructures’ periods were varied, the site at which Bragg scattering occurred changed, and that affected the enhancement of EQE and the power efficiency. In particular, the enhancement of EL intensity matched the site of the extraction for the surface plasmon mode; therefore, we concluded that the main reason for the outcoupling enhancement was light extraction from the trapped surface plasmon mode. The EQE, power efficiency, and transmittance were also improved by using phosphorescent materials for the emitting layer, and thinner Ag film for the cathode. Moreover, when structures for outcoupling enhancement are typically employed, they can produce optical haziness or image blur due to scattered transmittance; however, these nanostructured TrOLEDs showed no scattered transmittance, which indicates that haziness and image blur do not occur in these devices. Consequently, we expect the suggested method and the results of this work to be a fundamental solution for the improvement of TrOLEDs efficiency.

323

Please cite this article in press as: D.-Y. Kim et al., Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.010

321

324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345

ORGELE 2492

No. of Pages 7, Model 3G

2 April 2014 D.-Y. Kim et al. / Organic Electronics xxx (2014) xxx–xxx 346

Acknowledgments

347

351

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (CAFDC-2007-0056090) and LG Display Co., Ltd. also supported this research.

352

Appendix A. Supplementary material

353 355

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.03.010.

356

References

357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398

[1] J. Lee, S. Hofmann, M. Furno, M. Thomschke, Y.H. Kim, B. Lussem, K. Leo, Influence of organic capping layers on the performance of transparent organic light-emitting diodes, Opt. Lett. 36 (2011) 1443– 1445. [2] G. Gu, V. Bulovic´, P.E. Burrows, S.R. Forrest, M.E. Thompson, Transparent organic light emitting devices, Appl. Phys. Lett. 68 (1996) 2606. [3] H. Cho, J.M. Choi, S. Yoo, Highly transparent organic light-emitting diodes with a metallic top electrode: the dual role of a Cs2CO3 layer, Opt. Express 19 (2011) 1113–1121. [4] K. Saxena, V.K. Jain, D.S. Mehta, A review on the light extraction techniques in organic electroluminescent devices, Opt. Mater. 32 (2009) 221–233. [5] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Highly efficient phosphorescent emission from organic electroluminescent devices, Nature 395 (1998) 151– 154. [6] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Very high-efficiency green organic light-emitting devices based on electrophosphorescence, Appl. Phys. Lett. 75 (1999) 4. [7] X.L. Zhu, J.X. Sun, H.J. Peng, Z.G. Meng, M. Wong, H.S. Kwok, Vanadium pentoxide modified polycrystalline silicon anode for active-matrix organic light-emitting diodes, Appl. Phys. Lett. 87 (2005) 153508. [8] G.B. Murdoch, M. Greiner, M.G. Helander, Z.B. Wang, Z.H. Lu, A comparison of CuO and Cu2O hole-injection layers for low voltage organic devices, Appl. Phys. Lett. 93 (2008) 083309. [9] M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, A. Kahn, Role of the deep-lying electronic states of MoO3 in the enhancement of holeinjection in organic thin films, Appl. Phys. Lett. 95 (2009) 123301. [10] J. Li, M. Yahiro, K. Ishida, H. Yamada, K. Matsushige, Enhanced performance of organic light emitting device by insertion of conducting/insulating WO3 anodic buffer layer, Synth. Met. 151 (2005) 141–146. [11] H. Heil, J. Steiger, S. Karg, M. Gastel, H. Ortner, H. von Seggern, M. Stößel, Mechanisms of injection enhancement in organic lightemitting diodes through an Al/LiF electrode, J. Appl. Phys. 89 (2001) 420. [12] J.W. Ma, Z. Liang, C. Jin, X.Y. Jiang, Z.L. Zhang, Enhanced power efficiency for white OLED with MoO3 as hole injection layer and optimized charge balance, Solid State Commun. 149 (2009) 214– 217.

348 349 350

354

7

[13] Y. Li, D.-Q. Zhang, L. Duan, R. Zhang, L.-D. Wang, Y. Qiu, Elucidation of the electron injection mechanism of evaporated cesium carbonate cathode interlayer for organic light-emitting diodes, Appl. Phys. Lett. 90 (2007) 012119. [14] R. Meerheim, M. Furno, S. Hofmann, B.R. Lüssem, K. Leo, Quantification of energy loss mechanisms in organic light-emitting diodes, Appl. Phys. Lett. 97 (2010) 253305. [15] Y. Sun, S.R. Forrest, Enhanced light out-coupling of organic lightemitting devices using embedded low-index grids, Nat. Photonics 2 (2008) 483–487. [16] M. Slootsky, S.R. Forrest, Full-wave simulation of enhanced outcoupling of organic light-emitting devices with an embedded low-index grid, Appl. Phys. Lett. 94 (2009) 163302. [17] T.W. Koh, J.M. Choi, S. Lee, S. Yoo, Optical outcoupling enhancement in organic light-emitting diodes: highly conductive polymer as a low-index layer on microstructured ITO electrodes, Adv. Mater. 22 (2010) 1849–1853. [18] K. Hong, H.K. Yu, I. Lee, K. Kim, S. Kim, J.L. Lee, Enhanced light outcoupling of organic light-emitting diodes: spontaneously formed nanofacet-structured MgO as a refractive index modulation layer, Adv. Mater. 22 (2010) 4890–4894. [19] S. Möller, S.R. Forrest, Improved light out-coupling in organic light emitting diodes employing ordered microlens arrays, J. Appl. Phys. 91 (2002) 3324. [20] K. Ishihara, M. Fujita, I. Matsubara, T. Asano, S. Noda, H. Ohata, A. Hirasawa, H. Nakada, N. Shimoji, Organic light-emitting diodes with photonic crystals on glass substrate fabricated by nanoimprint lithography, Appl. Phys. Lett. 90 (2007) 111114. [21] C.S. Choi, D.-Y. Kim, S.-M. Lee, M.S. Lim, K.C. Choi, H. Cho, T.-W. Koh, S. Yoo, Blur-free outcoupling enhancement in transparent organic light emitting diodes: a nanostructure extracting surface plasmon modes, Adv. Opt. Mater. 1 (2013) 687–691. [22] C.S. Choi, S.M. Lee, M.S. Lim, K.C. Choi, D. Kim, D.Y. Jeon, Y. Yang, O.O. Park, Improved light extraction efficiency in organic light emitting diodes with a perforated WO3 hole injection layer fabricated by use of colloidal lithography, Opt. Express 20 (2012) A309–A318. [23] J.A. Lee, S.T. Ha, H.K. Choi, D.O. Shin, S.O. Kim, S.H. Im, O.O. Park, Novel fabrication of 2D and 3D inverted opals and their application, Small 7 (2011) 2581–2586. [24] X.D. Chen, S. Lenhert, M. Hirtz, N. Lu, H. Fuchs, L.F. Chi, Langmuir– Blodgett patterning: a bottom-up way to build mesostructures over large areas, Acc. Chem. Res. 40 (2007) 393–401. [25] P.A. Hobson, J.A.E. Wasey, I. Sage, W.L. Barnes, The role of surface plasmons in organic light-emitting diodes, IEEE J. Sel. Top. Quantum Electron. 8 (2002) 378–386. [26] J.M. Lupton, B.J. Matterson, I.D.W. Samuel, M.J. Jory, W.L. Barnes, Bragg scattering from periodically microstructured light emitting diodes, Appl. Phys. Lett. 77 (2000) 3340. [27] Y.R. Do, Y.-C. Kim, Y.-W. Song, Y.-H. Lee, Enhanced light extraction efficiency from organic light emitting diodes by insertion of a twodimensional photonic crystal structure, J. Appl. Phys. 96 (2004) 7629. [28] J.Y. Kim, C.S. Choi, W.H. Kim, D.Y. Kim, D.H. Kim, K.C. Choi, Extracting optical modes of organic light-emitting diodes using quasi-periodic WO3 nanoislands, Opt. Express 21 (2013) 5424–5431. [29] K.H. Cho, J.Y. Kim, D.G. Choi, K.J. Lee, J.H. Choi, K.C. Choi, Surface plasmon-waveguide hybrid polymer light-emitting devices using hexagonal Ag dots, Opt. Lett. 37 (2012) 761–763. [30] J.Y. Kim, S.Y. Cho, K.C. Choi, Analysis and structure optimization of nanostructure-embedded organic light-emitting diodes, J. Inf. Display 14 (2013) 73–77.

Please cite this article in press as: D.-Y. Kim et al., Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.010

399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461