PROOF COPY [SSL-12-0652] 005201SSL ECS Solid State Letters, 1 (1) Q1-Q3 (2012)

2162-8742/2012/1(1)/Q1/3/$28 © The Electrochemical Society, Inc.

Q1

Air-Stable Inverted Organic Solar Cells with an Ultrathin Electron-Transport Layer Made by Atomic Layer Deposition

1 2

Yong-Jin Kang,a Chang Su Kim,a,z Won-Sub Kwack,b Seung Yoon Ryu,a Myungkwan Song,a Dong-Ho Kim,a Suck Won Hong,c Sungjin Jo,d Se-Hun Kwon,b,z and Jae-Wook Kanga,z

3 4

a

Department of Material Processing, Korea Institute of Materials Science, Changwon 641-831, Korea National Core Research Center for Hybrid Materials Solution, Pusan National University, Busan 609-735, Korea c Department of Nanomaterials Engineering, Pusan National University, Miryang 627-706, Korea d School of Energy Engineering, Kyungpook National University, Daegu 702-701, Korea

5

b

6 7 8 9

11 12 13 14 15 16 17

Manuscript submitted March 2, 2012; revised manuscript received March 27, 2012. Published 00 00 2012

18

21 22 23 24 25 26 27 28 29 30 31 32 33

Au th o

34

Organic solar cells (OSCs) have attracted much attention recently because of their advantages in solution processes, their low cost, and their compatibility with flexible substrates.1–5 In general, OSCs with conventional structures are composed of a photoactive layer sandwiched between an indium tin oxide (ITO) electrode and a top metal electrode with a low work function. The performance of such devices is degraded quickly because of the hygroscopic nature of the hole-transport layer of poly(3,4-ethylene dioxythiophene doped with polystyrene sulfonate) (PEDOT:PSS),6,7 and the oxidation of the top metal electrode during exposure in ambient conditions; the resulting oxide layer has been shown to produce an insulating barrier that reduces the conductivity of the electrode, effectively increasing the serial resistance of the device.8,9 For these reasons, an OSC device with an inverted geometry has been proposed.10,11 The inverted OSC showed better long-term air stability under ambient conditions through the use of inorganic materials functioning as the electron-transport layer interposed between the ITO cathode and the photoactive layer of zinc oxide (ZnO),12,13 titanium oxide (TiOx ),14,15 or aluminum oxide (Al2 O3 ).16 Inorganic materials are used as electron transport layers on top of the ITO cathode because of their large bandgaps and good electron-extraction properties. Here, we employed an air-stable inverted OSC structure using ZnO prepared by atomic layer deposition (ALD) as the electrontransport layer. The use of the ALD deposition method is appropriate for controlling the thickness because it uses a self-limiting process that allows layer-by-layer growth, and therefore, the growth of layers with very precise thicknesses can be achieved on the nanometer scale. In addition, ALD is expected to produce defect-free, uniform, and highly conformal films.17–19 We studied the structural, electrical, and optical properties of ZnO layers with different thicknesses in inverted OSCs. The photovoltaic performance was found to be dependent on the thickness of the ZnO layer. Air-stable inverted solar cells with an optimized ZnO thickness reached a power conversion efficiency of 2.91%, and the device performance remained at approximately 80% of the original value even after storage in air for thirty days. ZnO thin films were prepared on ITO glass substrates using traveling-wave-type ALD (Lucida D100, NCD Technology, Korea) at a deposition temperature of 150◦ C and a pressure of 0.5 torr. Diethylzinc (DEZ, Mecharonics Co. Ltd.) and H2 O were used as the zinc precursor and oxygen reactant, respectively. For a uniform supply, both DEZ and H2 O were cooled to 10◦ C using a Peltier module, and were delivered to the reactor with N2 carrier gas at a flow rate of 50 sccm. Since the vapor pressures of DEZ (15.95 torr) and H2 O (23.8 torr) are very high even at room temperature (25◦ C), they needed to

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49 50 51

52

53

54

55

56 57 58 59 60 61 62

be cooled down for the deposition pressure to be maintained during ALD. One deposition cycle of ZnO ALD consisted of four steps: (i) exposure to the DEZ precursor, (ii) a purge pulse with 50-sccm N2 , (iii) exposure to H2 O vapor mixed with N2 , and (iv) another purge with 50-sccm N2 . This cycle was repeated as many times as necessary to obtain the desired film thickness. The poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend solution was prepared in a 1:1 mass ratio in 1,2-dichlorobenzene (20 mg/mL P3HT and 20 mg/mL PCBM) without any additives. The active material was then coated on the ZnO layer with an average thickness of ≈250 nm using a spin-coating process (spin speed of 600 rpm for 40 s) in a glove box. Subsequently, the solvent was evaporated over a period of 2 h, and pre-annealing was carried out at 150◦ C for 20 min in a glove box. As the hole-selective layer, a PEDOT:PSS aqueous dispersion solution was spin-coated at 5000 rpm onto the active layer. The 120-nm Ag electrode was evaporated below 10−6 torr on the PEDOT:PSS layer through a shadow mask that defined the effective area as 0.38 cm2 . The current voltage (J–V) characteristics were measured with a Keithley 2400 source meter in the dark or under 100-mW/cm2 (AM 1.5G) irradiation from a solar simulator (Pecell Technologies Inc., PEC-L11 model). For each set of experimental conditions, we fabricated over 10 devices, and we report the average PCE parameters together with their standard deviations. The film thickness and coverage were measured by high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2100 microscope, Japan) at an accelerating voltage of 200 kV. The crystalline structure of ZnO was investigated using HRTEM and X-ray diffraction (XRD, D8 ADVANCE, Bruker, Germany) with Cu-Kα radiation at 1.5405 Å. The film composition was analyzed using 9.0-MeV He2+ Rutherford backscattering spectroscopy (RBS, 6-SDH, NEC, Japan) and Auger electron spectroscopy (AES, SAM4300, Perkin Elmer, USA). From the AES and RBS analyzes, the carbon impurity in the as-deposited ZnO film was found to be below the detection limit (<1 at%). First, we studied the dependence of the growth rate of the ZnO films on the DEZ and H2 O injection times on the ITO substrate. For a precise thickness measurement, all the film thicknesses were measured using HRTEM. Figure 1a shows the dependence of the ZnO film thickness on the number of ALD cycles on the ITO substrate at a deposition temperature of 150◦ C. The saturated growth rate (0.078 nm/cycle) of ZnO films was obtained when both the DEZ and H2 O pulse times were longer than 1.5 s and 1 s, respectively. As expected, the thicknesses of the ZnO films increased linearly with the number of ALD cycles, which is one of the inherent characteristics of ALD. However, the extrapolated line did not pass through the origin, but intercepted the x-axis with a perceptible positive value for ZnO ALD. This means that there is an extended period with a lower growth rate in the early stages of film growth in conventional ALD. Generally, this early stage

rP

19 20

ro of

We report on the photovoltaic properties of air-stable inverted organic solar cells in which zinc oxide (ZnO) of varying thicknesses is formed as the electron-transport layer by an atomic layer deposition (ALD) method. The device performance was found to be dependent on the ZnO thickness. Air-stable inverted solar cells with an optimized ZnO thickness reached a power conversion efficiency of 2.91%. This efficiency was found to be comparable to those of conventional organic solar cells. The use of the ZnO electron-transport layer led to improved air stability: the power conversion efficiencies of unencapsulated organic solar cells remained above 80% of their original values even after storage in air for thirty days. © 2012 The Electrochemical Society. [DOI: 10.1149/2.005201SSL] All rights reserved.

10

z

E-mail: [email protected]; [email protected]; [email protected]

63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109

ECS Solid State Letters, 1 (1) Q1-Q3 (2012)

ro of

Q2

Figure 1. (a) Dependence of ZnO film thickness on the number of ALD cycles; ZnO films prepared on ITO substrates at a deposition temperature of 150◦ C. (b) XRD patterns of ZnO films depending on the film thickness, prepared on ITO substrates. The scan axis was 2θ mode, and the scan rate was 3◦ /min.

111 112 113 114 115 116 117 118 119 120 121 122 123 124 125

Au th o

126

is called the “transient region,”20 and is known to be due to the low probability of adsorption of the metal-organic precursor on the starting substrate compared to on the growing homogeneous film. Indeed, the growth rate of ZnO was about 0.057 nm/cycle up to 60 cycles, which was slightly lower than the growth rate (0.078 nm/cycle) obtained in the linear region. Figure 1b shows the effect of the thickness on the crystallinity of the ALD-ZnO films. When the thickness of the ZnO film was 1.5 nm, the film was almost amorphous. As the thickness of the ZnO film increased from 3.4 to 7.2 nm, ZnO (100) peaks started to appear, although the peaks are not clear. However, ZnO (100), (002), and (101) peaks were clearly observed for the 14.2-nm ZnO films, and these ZnO peaks were identified as wurtzite structures according to the Joint Committee on Powder Diffraction Standards (JCPDS) files. Figure 2 shows HRTEM images of the ZnO films with thicknesses of (a) 1.5 nm, (b) 3.4 nm, and (c) 7.2 nm, prepared on ITO substrates. The upper images show enlarged views of the HRTEM images for identification of the detailed microstructures of the ZnO films. All the ZnO films were continuous and uniformly deposited on the ITO substrates, even for the thickness of 1.5 nm. The HRTEM image of the 1.5-nm ZnO in Fig. 2a shows that the film had an amorphous structure. Although the crystallinities were not clear from the XRD results, the ZnO films with thicknesses of 3.4 and 7.2 nm were well crystallized, as seen in the lattice fringes in the HRTEM images (upper images) of Figs. 2b and 2c. Thus, the crystal structure of ZnO was changed from amorphous to polycrystalline by increasing the film thickness, and the transition thickness for crystallization was between 1.5 and 3.4 nm. Similar thickness-dependent crystallization behavior was also observed in the case of ALD-TiO2 films using titanium tetraisopropoxide (TTIP) and H2 O.21 From this observation, the thickness-dependent crystallization behavior of the ZnO films was

127

128

129

130

131

132

133

134 135

136

137

138

139

also responsible for the low growth rate of the ZnO films in the early stages of film growth. Therefore, the film thicknesses need to be controlled carefully through consideration of the early stages of film growth when the film thickness is rather small, even though the ALD technique is used. To study the dependence of the photovoltaic performance on the thickness of the ZnO layer in devices, we fabricated inverted OSCs with the general structure glass/ITO/ZnO/P3HT:PCBM/ PEDOT:PSS/Ag. Figure 3a shows the J–V characteristics of such devices with varying thicknesses of the ZnO electron-transport layer. The 1.5nm ZnO device exhibited a power conversion efficiency of 2.23% ± 0.12 with a fill factor (FF) of 44%. The short-circuit current density (Jsc ) and open-circuit voltage (Voc ) were 9.59 mA/cm2 and 0.53 V, respectively. However, the 3.4-nm ZnO device exhibited a power conversion efficiency of 2.91% ± 0.23, with Jsc = 10.01mA/cm2 , Voc = 0.59 V, and FF = 49%. The devices possessing a crystallinephase ZnO layer exhibited better photovoltaic performances than those derived from an amorphous ZnO layer; this is attributed to the increased electron mobility and decreased charge recombination between the ITO electrode and the P3HT:PCBM active layer.22,23 Therefore, the 3.4-nm ZnO layer is expected to work as an efficient electron-collecting layer. Devices with a thicker 7.2-nm ZnO layer show a smaller Jsc value of 9.09 mA/cm2 and a power conversion efficiency of 2.63% ± 0.10. It can be seen clearly that there are prominent changes in the short-circuit current density (compared to the device with 3.4-nm ZnO, which showed a 10% decrease). The trend in current density is consistent with the variation in the transmittance of the ZnO layers used in the devices. The optical transmittance spectra of ZnO layers of different thicknesses are shown in the inset of Fig. 3a. Finally, we examined the air stability of our best-performing inverted OSC after its exposure to air. Figure 3b shows the changes in power conversion efficiency and current density with time for the inverted OSC with a 3.4-nm ZnO layer. The devices were stored under ambient conditions without encapsulation for stability testing according to the recently reported protocol (ISOS-D-1).24,25 The power conversion efficiency of the inverted OSC with ZnO as the electron-transport layer remained at approximately 80% of its original value, even after storage in air for thirty days. The stabilities of our inverted devices are comparable to those of the recently reported devices based on metal-oxide electron-transport layers.26,27 The use of ZnO as the electron-transport layer in OSCs yields a significant improvement in their air stability. In summary, efficient and air-stable OSCs with the inverted structure, using ZnO prepared by the ALD method as the electron-transport layer with varying thicknesses, were fabricated. The device performances were found to be dependent on the ZnO thickness. Air-stable inverted solar cells with an optimized ZnO thickness reached a power conversion efficiency of 2.91%. The power conversion efficiency of the inverted OSC with ZnO as the electron-transport layer remained at approximately 80% of its original value, even after storage in air

rP

110

Figure 2. HRTEM images of ZnO films prepared on ITO substrates with thicknesses of (a) 1.5 nm, (b) 3.4 nm, and (c) 7.2 nm. The upper images are views of the HRTEM images enlarged for identification of the film crystallinity.

140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188

Q3

ro of

ECS Solid State Letters, 1 (1) Q1-Q3 (2012)

Figure 3. (a) J–V characteristics for inverted OSCs with various thicknesses of the ZnO electron-transport layer. Inset: Transmission spectra of ZnO layers depending on the film thickness. (b) Changes in power conversion efficiency and current density with time for inverted OSC with a 3.4-nm ZnO electron-transport layer. Inset: Schematic diagram of the inverted OSC configuration incorporating ZnO as the electron-transport layer.

191

for thirty days. Our results indicate that the engineering of improved anode–interface materials is an important advancement for achieving efficient and stable OSCs.

192

Acknowledgments

190

193 194 195 196 197 198

This study was supported by a grant from the New and Renewable Energy Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Ministry of Knowledge Economy, and by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References

199 200 201

1. C. E. Small, S. Chen, J. Subbiah, C. M. Amb, S. W. Tsang, T. H. Lai, J. R. Reynolds, and F. So, Nature Photon., 6, 115 (2012). 2. S. W. Liu, C. F. Lin, C. C. Lee, W. C. Su, C. T. Chen, and J. H. Lee, J. Electrochem. Soc., 159(2), H191 (2012). 3. T. S. Kim, S. I. Na, S. H. Oh, R. Kang, B. K. Yu, J. S. Yeo, J. Lee, and D. Y. Kim, Sol. Energy Mater. Sol. Cells, 98, 168 (2012). 4. Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan, and A. J. Heeger, Nature Mater., 11, 44 (2012). 5. H. Cheun, C. F. Hernandez, Y. Zhou, W. J. Potscavage, S. J. Kim, J. Shim, A. Dindar, and B. Kippelen, J. Phys. Chem. C, 114, 20713 (2010). 6. M. P. de Jong, L. J. van Ijzendoom, and M. J. A. de Voigt, Appl. Phys. Lett., 77, 2255 (2000). 7. K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D. D. C. Bradley, and J. R. Durrant, Sol. Energy Mater. Sol. Cells, 90, 3520 (2006). 8. F. C. Krebs and H. Spanggaard, Chem. Mater., 17, 5235 (2005). 9. H. Yip, S. K. Hau, N. S. Baek, H. Ma, and A. K. Y. Jen, Adv. Mater., 20, 2376 (2008).

Au th o

202

203

204

205

206

207

208

209

210

211

212 213 214 215

10. J. B. Kim, C. S. Kim, Y. S. Kim, and Y. L. Loo, Appl. Phys. Lett., 95, 183301 (2009). 11. C. S. Kim, S. Lee, L. L. Tinker, S. Bernhard, and Y. L. Loo, Chem. Mater., 21, 4583 (2009). 12. Y. J. Kang, K. Lim, S. Jung, D. G. Kim, J. K. Kim, C. S. Kim, S. H. Kim, and J. W. Kang, Sol. Energy Mater. Sol. Cells, 96, 137 (2012). 13. T. Kuwabara, Y. Kawahara, T. Yamaguchi, and K. Takahashi, ACS Appl. Mater. Interfaces, 1, 2107 (2009). 14. C. Waldauf, M. Morana, P. Denk, P. Schilinsky, K. Coakley, S. A. Choulis, and C. J. Brabec, Appl. Phys. Lett., 89, 233517 (2006). 15. H. Sun, J. Weickert, H. C. Hesse, and L. S. Mende, Sol. Energy Mater. Sol. Cells, 95, 3450 (2011). 16. Y. Zhou, H. Cheun, W. J. Potscavage, C. F. Hernandez, S. J. Kim, and B. Kipplelen, J. Mater. Chem., 20, 6189 (2010). 17. J. C. Wang, W. T. Weng, M. Y. Tsai, M. K. Lee, S. F. Horng, T. P. Perng, C. Ch. Kei, C. C. Yu, and H. F. Meng, J. Mater. Chem., 20, 862 (2010). 18. W. D. Kim, G. W. Hwang, O. S. Kwon, S. K. Kim, M. Cho, D. S. Jeong, S. W. Lee, M. Seo, C. S. Hwang, and Y. S. Min, J. Electrochem. Soc., 152, C552 (2005). 19. Yu. Tseng, A. U. Mane, J. W. Elam, and S. B. Darling, Sol. Energy Mat. Sol. Cells, 99, 235 (2012). 20. S. H. Kwon, O. K. Kwon, J. H. Kim, H. R. Oh, K. H. Kim, and S. W. Kang, J. Electrochem. Soc., 155(5), H296 (2008). 21. W. D. Kim, G. W. Hwang, O. S. Kwon, S. K. Kim, M. Cho, D. S. Jeong, W. S. Lee, M. H. Seo, C. S. Hwang, Y. S. Min, and Y. J. Cho, J. Electrochem. Soc., 152, C552 (2005). 22. C. S. Kim, S. S. Lee, E. D. Gomez, J. B. Kim, and Y. L. Loo, Appl. Phys. Lett., 94, 113302 (2009). 23. Y. J. Kang, C. S. Kim, D. S. You, S. H. Jung, K. Lim, D. G. Kim, J. K. Kim, S. H. Kim, Y. R. Shin, S. H. Kwon, and J. W. Kang, Appl. Phys. Lett., 99, 073308 (2011). 24. M. O. et al. Reese, Sol. Energy Mat. Sol. Cells, 95, 1253 (2011). 25. M. Jørgensen, K. Norrman, S. A. Gevorgyan, T. Tromholt, B. Andreasen, and F. C. Krebs, Adv. Mater., 24, 580 (2012). 26. F. C. Krebs, Sol. Energy Mat. Sol. Cells, 92, 715 (2008). 27. F. C. Krebs, J. Fyenbo, D. M. Tanenbaum, S. A. Gevorgyan, R. Andriessen, B. Remoortere, Y. Galagan, and M. Jørgensen, Energy Environ. Sci., 4, 4116 (2011).

rP

189

216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250