Improved photovoltaic performance of inverted polymer solar cells ...

Improved photovoltaic performance of inverted polymer solar cells through a sol-gel processed Al-doped ZnO electron extraction layer Jun Young Kim,1,5 Eunae Cho,2 Jaehoon Kim,1 Hyeonwoo Shin,1 Jeongkyun Roh,1 Mariyappan Thambidurai,1 Chan-mo Kang,4 Hyung-Jun Song,6 SeongMin Kim,2 Hyeok Kim,1,2,7 and Changhee Lee1,3,8 1

Department of Electrical and Computer Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea 2 Peer review, CAE Group, Samsung Advanced Institute of Technology, 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do, 443-803, South Korea 3 Global Frontier Center for Multiscale Energy Systems, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea 4 IT Convergence Technology Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-700, South Korea 5 OLED Advanced Technology Team, LG Display, 245, LG-ro, Wollong-myeon, Paju-si, Gyeonggi-do, 413-779, South Korea 6 Chemistry division, Los Almos National Laboratory, Los Alamos, New Mexico 87544, USA 7 [email protected] 8 [email protected]

Abstract: We demonstrate that nanocrystalline Al-doped zinc oxide (nAZO) thin film used as an electron-extraction layer can significantly enhance the performance of inverted polymer solar cells based on the bulk heterojunction of poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM). A synergistic study with both simulation and experiment on n-AZO was carried out to offer a rational guidance for the efficiency improvement. As a result, An n-AZO film with an average grain size of 13 to 22 nm was prepared by a sol-gel spin-coating method, and a minimum resistivity of 2.1 × 10−3 Ω·cm was obtained for an Al-doping concentration of 5.83 at.%. When an n-AZO film with a 5.83 at.% Al concentration was inserted between the ITO electrode and the active layer (PCDTBT:PC70BM), the power conversion efficiency increased from 3.7 to 5.6%. ©2015 Optical Society of America OCIS codes: (350.6050) Solar energy; (160.5470) Polymers; (310.6845) Thin film devices and applications; (160.4236) Nanomaterials.

References and links 1. 2. 3. 4. 5. 6. 7.

C. J. Brabec, V. Dyakonov, J. Parisi, and N. S. Sariciftci, Organic Photovoltaics: Concepts and Realization (Springer, 2003). C. N. Hoth, P. Schilinsky, S. A. Choulis, and C. J. Brabec, “Printing highly efficient organic solar cells,” Nano Lett. 8(9), 2806–2813 (2008). Y. Galagan, J.-E. J. M. Rubingh, R. Andriessen, C.-C. Fan, P. W. M. Blom, S. C. Veenstra, and J. M. Kroon, “ITO-free flexible organic solar cells with printed current collecting grids,” Sol. Energy Mater. Sol. Cells 95(5), 1339–1343 (2011). G. Li, R. Zhu, and Y. Yang, “Polymer solar cells,” Nat. Photonics 6(3), 153–161 (2012). J.-D. Chen, C. Cui, Y.-Q. Li, L. Zhou, Q.-D. Ou, C. Li, Y. Li, and J.-X. Tang, “Single-junction polymer solar cells exceeding 10% power conversion efficiency,” Adv. Mater. 27(6), 1035–1041 (2015). Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, and H. Yan, “Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells,” Nat. Commun. 5, 5293 (2014). Z. He, B. Xiao, F. Liu, H. Wu, Y. Yang, S. Xiao, C. Wang, T. P. Russell, and Y. Cao, “Single-junction polymer

#243417 © 2015 OSA

Received 22 Jun 2015; revised 26 Aug 2015; accepted 2 Sep 2015; published 15 Sep 2015 21 Sep 2015 | Vol. 23, No. 19 | DOI:10.1364/OE.23.0A1334 | OPTICS EXPRESS A1334

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

solar cells with high efficiency and photovoltage,” Nat. Photonics 9(3), 174–179 (2015). P. A. Staniec, A. J. Parnell, A. D. F. Dunbar, H. Yi, A. J. Pearson, T. Wang, P. E. Hopkinson, C. Kinane, R. M. Dalgliesh, A. M. Donald, A. J. Ryan, A. Iraqi, R. A. L. Jones, and D. G. Lidzey, “The nanoscale morphology of a PCDTBT:PCBM photovoltaic blend,” Adv. Energy Mater. 1(4), 499–504 (2011). L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li, and Y. Yang, “Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer,” Nat. Photonics 6(3), 180–185 (2012). B. R. Aïch, J. Lu, S. Beaupré, M. Leclerc, and Y. Tao, “Control of the active layer nanomorphology by using coadditives towards high-performance bulk heterojunction solar cells,” Org. Electron. 13(9), 1736–1741 (2012). T. Stubhan, H. Oh, L. Pinna, J. Krantz, I. Litzov, and C. J. Brabec, “Inverted organic solar cells using a solution processed aluminum-doped zinc oxide buffer layer,” Org. Electron. 12(9), 1539–1543 (2011). H. Oh, J. Krantz, I. Litzov, T. Stubhan, L. Pinna, and C. J. Brabec, “Comparison of various sol-gel derived metal oxide layers for inverted organic solar cells,” Sol. Energy Mater. Sol. Cells 95(8), 2194–2199 (2011). V. Shrotriya, G. Li, Y. Yao, C.-W. Chu, and Y. Yang, “Transition metal oxides as the buffer layer for polymer photovoltaic cells,” Appl. Phys. Lett. 88(7), 073508 (2006). P. de Bruyn, D. J. D. Moet, and P. W. M. Blom, “A facile route to inverted polymer solar cells using a precursor based zinc oxide electron transport layer,” Org. Electron. 11(8), 1419–1422 (2010). E. L. Ratcliff, J. Meyer, K. X. Steirer, N. R. Armstrong, D. Olson, and A. Kahn, “Energy level alignment in PCDTBT:PC70BM solar cells: solution processed NiOx for improved hole collection and efficiency,” Org. Electron. 13(5), 744–749 (2012). I. Hancox, L. A. Rochford, D. Clare, M. Walker, J. J. Mudd, P. Sullivan, S. Schumann, C. F. McConville, and T. S. Jones, “Optimization of a high work function solution processed vanadium oxide hole-extracting layer for small molecule and polymer organic photovoltaic cells,” J. Phys. Chem. C 117(1), 49–57 (2013). A. Kumar, G. Lakhwani, E. Elmalem, W. T. S. Huck, A. Rao, N. C. Greenham, and R. H. Friend, “Interface limited charge extraction and recombination in organic photovoltaics,” Energy Environ. Sci. 7(7), 2227–2231 (2014). J. Xiong, B. Yang, J. Yuan, L. Fan, X. Hu, H. Xie, L. Lyu, R. Cui, Y. Zou, C. Zhou, D. Niu, Y. Gao, and J. Yang, “Efficient organic photovoltaics using solution-processed, annealing-free TiO2 nanocrystalline particles as an interface modification layer,” Org. Electron. 17, 253–261 (2015). T. Stubhan, I. Litzov, N. Li, M. Salinas, M. Steidl, G. Sauer, K. Forberich, G. J. Matt, M. Halik, and C. J. Brabec, “Overcoming interface losses in organic solar cells by applying low temperature, solution processed aluminum-doped zinc oxide electron extraction layers,” J. Mater. Chem. A Mater. Energy Sustain. 1(19), 6004– 6009 (2013). C. G. Shuttle, B. O’Regan, A. M. Ballantyne, J. Nelson, D. D. C. Bradley, and J. R. Durrant, “Bimolecular recombination losses in polythiophene: Fullerene solar cells,” Phys. Rev. B 78(11), 113201 (2008). Y. Sun, J. H. Seo, C. J. Takacs, J. Seifter, and A. J. Heeger, “Inverted polymer solar cells integrated with a lowtemperature-annealed sol-gel-derived ZnO Film as an electron transport layer,” Adv. Mater. 23(14), 1679–1683 (2011). S. K. Hau, H.-L. Yip, and A. K. Y. Jen, “A review on the development of the inverted polymer solar cell architecture,” Polym. Rev. (Phila. Pa.) 50(4), 474–510 (2010). M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis, and D. S. Ginley, “Inverted bulk-heterojunction organic photovoltaic device using a solution-derived ZnO underlayer,” Appl. Phys. Lett. 89(14), 143517 (2006). C. S. Kim, S. S. Lee, E. D. Gomez, J. B. Kim, and Y.-L. Loo, “Transient photovoltaic behavior of air-stable, inverted organic solar cells with solution-processed electron transport layer,” Appl. Phys. Lett. 94(11), 113302 (2009). C.-H. Hsieh, Y.-J. Cheng, P.-J. Li, C.-H. Chen, M. Dubosc, R.-M. Liang, and C.-S. Hsu, “Highly efficient and stable inverted polymer solar cells integrated with a cross-linked fullerene material as an interlayer,” J. Am. Chem. Soc. 132(13), 4887–4893 (2010). M. Campoy-Quiles, T. Ferenczi, T. Agostinelli, P. G. Etchegoin, Y. Kim, T. D. Anthopoulos, P. N. Stavrinou, D. D. C. Bradley, and J. Nelson, “Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends,” Nat. Mater. 7(2), 158–164 (2008). Z. Xu, L.-M. Chen, G. Yang, C.-H. Huang, J. Hou, Y. Wu, G. Li, C.-S. Hsu, and Y. Yang, “Vertical phase separation in poly(3-hexylthiophene): fullerene derivative blends and its advantage for inverted structure solar cells,” Adv. Funct. Mater. 19(8), 1227–1234 (2009). H.-H. Liao, L.-M. Chen, Z. Xu, G. Li, and Y. Yang, “Highly efficient inverted polymer solar cell by low temperature annealing of Cs2CO3 interlayer,” Appl. Phys. Lett. 92(17), 173303 (2008). C. Tao, S. Ruan, X. Zhang, G. Xie, L. Shen, X. Kong, W. Dong, C. Liu, and W. Chen, “Performance improvement of inverted polymer solar cells with different top electrodes by introducing a MoO3 buffer layer,” Appl. Phys. Lett. 93(19), 193307 (2008). S. Bai, Z. Wu, X. Xu, Y. Jin, B. Sun, X. Guo, S. He, X. Wang, Z. Ye, H. Wei, X. Han, and W. Ma, “Inverted organic solar cells based on aqueous processed ZnO interlayers at low temperature,” Appl. Phys. Lett. 100(20), 203906 (2012). P. K. Nayak, J. Jang, C. Lee, and Y. Hong, “Effects of Li doping on the performance and environmental stability of solution processed ZnO thin film transistors,” Appl. Phys. Lett. 95(19), 193503 (2009). K.-S. Shin, K.-H. Lee, H. H. Lee, D. Choi, and S.-W. Kim, “Enhanced power conversion efficiency of inverted organic solar cells with a Ga-doped ZnO nanostructured thin film prepared using aqueous solution,” J. Phys.

#243417 © 2015 OSA


Chem. C 114(37), 15782–15785 (2010). 33. H. Karaagac, E. Yengel, and M. Saif Islam, “Physical properties and heterojunction device demonstration of aluminum-doped ZnO thin films synthesized at room ambient via sol–gel method,” J. Alloys Compd. 521, 155– 162 (2012). 34. S. Y. Park, B. J. Kim, K. Kim, M. S. Kang, K.-H. Lim, T. I. Lee, J. M. Myoung, H. K. Baik, J. H. Cho, and Y. S. Kim, “Low-temperature, solution-processed and alkali metal doped ZnO for high-performance thin-film transistors,” Adv. Mater. 24(6), 834–838 (2012). 35. Z. Zhang, C. Bao, W. Yao, S. Ma, L. Zhang, and S. Hou, “Influence of deposition temperature on the crystallinity of Al-doped ZnO thin films at glass substrates prepared by RF magnetron sputtering method,” Superlattices Microstruct. 49(6), 644–653 (2011). 36. Z. Q. Xu, H. Deng, Y. Li, Q. H. Guo, and Y. R. Li, “Characteristics of Al-doped c-axis orientation ZnO thin films prepared by the sol–gel method,” Mater. Res. Bull. 41(2), 354–358 (2006). 37. B. D. Cullity, Elements of X-ray diffraction, 2nd ed. (Addison-Wesley, Reading, MA, 1978). 38. Y. Caglar, M. Caglar, and S. Ilican, “Microstructural, optical and electrical studies on sol gel derived ZnO and ZnO:Al films,” Curr. Appl. Phys. 12(3), 963–968 (2012). 39. G. Kresse and J. Hafner, “Ab initio molecular dynamics for liquid metals,” Phys. Rev. B Condens. Matter 47(1), 558–561 (1993). 40. P. E. Blöchl, “Projector augmented-wave method,” Phys. Rev. B Condens. Matter 50(24), 17953–17979 (1994). 41. J. P. Perdew, K. Burke, and M. Ernzerhof, “Generalized gradient approximation made simple,” Phys. Rev. Lett. 77(18), 3865–3868 (1996). 42. P. Palacios, K. Sánchez, and P. Wahnón, “Ab-initio valence band spectra of Al, In doped ZnO,” Thin Solid Films 517(7), 2448–2451 (2009).

1. Introduction Organic photovoltaic devices based on the bulk-heterojunction (BHJ) structure of conjugated polymer:fullerene have been widely studied over the last decade for use in next-generation thin-film solar cells [1–4]. Multilateral efforts such as the development of new donor and acceptor materials, optimal control of the BHJ film morphology, and improvement in the device structure with better contacts and interfaces have led to high power conversion efficiencies (PCE) of 7 up to 11% [4–10]. The interfacial properties between electrodes and the organic active layer play a tremendously crucial role in the overall performance of organic photovoltaic cells (OPVs) [11–19]. Hole- and electron-extracting layers are often used to reduce energy barriers at the electrode interfaces and to increase the charge-carrier collection [11–19]. Such interlayers are employed to improve the power conversion efficiency (PCE) of OPVs because the improvement in the charge extraction capability increases the short-circuit current (Jsc) and decreases the bimolecular recombination loss [14,16]. Because the bimolecular recombination loss is known as one of the limiting factors of the open-circuit voltage (Voc) and fill factor (FF) in the organic BHJ solar cells [16,20], the reduced bimolecular recombination loss results in higher Voc and FF, thereby leading to higher PCE. In addition, the Voc can be augmented due to the reduced voltage loss at the electrode interfaces with an appropriate charge extraction layer. In a typical device structure for BHJ OPVs, an indium tin oxide (ITO) substrate, coated with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a hole transport buffer layer, works as a hole-collecting anode, and a low work function metal plays the role of an electron-collecting cathode. It is well known that acidic PEDOT:PSS can etch the ITO, and the low work function metals can also be easily oxidized, leading to device instability [21]. To overcome these problems, an inverted device structure has been developed [22–25]. The inverted structure reverses the polarity of the electrodes by using the ITO electrode with an electron-extraction layer as the cathode and high-work-function metals as the anode, with molybdenum trioxide (MoO3) instead of acidic PEDOT:PSS as the hole transport layer [14]. Furthermore, the inverted structure is known to have improved device stability with the additional benefit of a self-encapsulation effect arising from the verticalphase separation and concentration gradient in the active layer [26,27]. In inverted polymer solar cells, several n-type electron-extraction layers, such as cesium carbonate (Cs2CO3) [28], titanium dioxide (TiO2) [29], zinc oxide (ZnO) [22,23,30], were

#243417 © 2015 OSA


used due to their low work function, relatively high electron mobility, environmental stability and transparency. Recently, it was reported that doped ZnO thin films with metals, such as Li, Al, or Ga, are an effective way to modify the grain size and orientation of ZnO crystallites to greatly influence their structural, optical and electrical properties [31–34]. The Al-doped ZnO (AZO) films are attractive due to their good conductivity, relatively low cost, and high transparency for most of the solar spectrum used in photovoltaic devices. In addition, it was also reported that thick AZO films can be processed at low temperature without the significant loss of efficiency [11,19]. However, there have been few researches employing systematic calculation for AZO thin film characterization. In this work, we investigated the effects of Al doping on the structural and electrical properties of ZnO thin films with a firstprinciples calculation and observed greatly enhanced photovoltaic performance of inverted polymer solar cells with an ITO/AZO/PCDTBT:PC70BM/MoO3/Al structure. 2. Experimental section The AZO thin films were prepared using a sol-gel spin-coating method. Zinc acetate dihydrate and aluminum nitrate nonahydrate were dissolved into the co-solvents ethanol and ethanolamine. The AZO films were spin-coated onto ITO substrates using the prepared solution. After annealing at 450 °C for 2 h, these films form nanocrystalline Al-doped ZnO thin films approximately 35 nm thick. The Al concentration was varied from 0 to 10%. On top of the AZO layer, the PCDTBT:PC70BM blend solution (1:4 weight ratio in 1,2dichlorobenzene) was spin-coated to form a film approximately 70 nm thick. Subsequently, the MoO3 (10 nm) and Al electrodes (100 nm) were deposited under high-vacuum conditions at 10−6 Torr. The crystalline structure of the undoped ZnO and Al-doped ZnO films were analyzed using an X-ray diffractometer (New D8 Advance). The high-resolution transmission electron microscope (HRTEM) micrographs of the films were obtained using a JEM-3010 type HRTEM. Electrical resistivity measurements were carried out using a current–voltage (IV) measurement method. The photocurrent density-voltage characteristics of the devices were measured using a Keithley 237 source-measurement unit under illumination (AM 1.5G 100 mW/cm2) from a 300 W solar simulator (Newport 91160A). The incident photon to charge conversion efficiency (IPCE) spectra were measured by using a lock-in amplifier (Model 7265, Signal Recovery) with light from a xenon lamp directed through a monochromator (SpectroPro-150, Acton Research Corporation). 3. Results and discussion Figure 1(a) shows the X-ray diffraction pattern of undoped ZnO and AZO films with various Al concentrations. All the diffraction peaks can be indexed to the hexagonal phase of ZnO with lattice constants of a = 3.251 Å and c = 5.210 Å, which are in good agreement with the standard values (JCPDS No. 36-1451). The (002) diffraction peak is shifted to a higher angle in the AZO thin films compared to undoped ZnO due to the lattice distortion introduced by the substitution of Al3+ for Zn2+. The lattice constants of the AZO thin films (a = 3.231 Å and c = 5.193 Å) are observed to be slightly smaller than those of undoped ZnO because the ionic radius of Al (0.054 nm) is smaller than that of Zn (0.074 nm) [35]. Al-doping concentrations correspond to lower intensity and increased full width at half maximum (FWHM) of the diffraction peaks, indicating deterioration of the film crystallinity. The reduced crystallinity arises from the incorporation of Al into ZnO [36]. The grain size of ZnO and AZO was determined using Scherrer’s equation [37]. In accordance with the Scherrer’s equation, the 0.9λ where D is grain size, λ grain size is determined by the following equation, D = β cos Θ is the wavelength of the X-rays, β is the full width at half-maximum of the diffraction peaks and Θ is the Bragg angle. Using diffraction peak (002), the grain size was found to be 22.5

#243417 © 2015 OSA


nm for undoped ZnO and 15.2, 14.7, 13.0, 12.5 and 11.6 nm for the AZO films with the Al concentrations of 1.64, 3.97, 5.83, 7.58 and 9.91 at.%, respectively. Figures 1(b)-1(e) show the high-resolution transmission (HRTEM) images of the undoped ZnO and the AZO films with 5.83 at.% Al-doping concentration. Figures 1(b) and 1(d) indicate that the grains were spherical in nature with little agglomeration and good crystallinity. The

Fig. 1. (a) X-ray diffraction patterns of the undoped ZnO and AZO films with different Aldoping concentrations. (b, c) HRTEM images of the undoped ZnO and (d, e) 5.83 at.% AZO.

average grain sizes of the undoped ZnO and the 5.83 at.% AZO film were found to be approximately 22 nm and 13 nm, respectively, which is consistent with the XRD result in Fig. 1(a). The interplanar lattice spacing for the undoped ZnO [Fig. 1(c)] was found to be 2.59 Å, corresponding to the (002) plane of hexagonal ZnO. It is slightly smaller (2.53 Å) for the 5.83 at.% AZO [Fig. 1(e)], owing to the smaller ionic radius of Al3+ compared to that of Zn2+.

#243417 © 2015 OSA


Fig. 2. Variation of resistivity with doping concentration of AZO films.

Figure 2 displays the variation of resistivity with respect to doping concentration of the AZO films. Overall, the resistivity of AZO is found to be lower than that of undoped ZnO. It is observed that the resistivity decreases first as the Al-doping concentration increases up to 5.83 at.% (from 8.3 × 10−3 Ω·cm to 2.1 × 10−3 Ω·cm). These results signify that one free electron is obtained from one zinc atom replacement, and the carrier concentration increases with Al concentration because a small amount of Al is ionized into Al3+ and replaces Zn2+. Then, the resistivity increases with a further increase in the Al-doping concentration because the extra Al atoms may not occupy the correct places inside the ZnO lattice due to the limited solubility of aluminum inside the ZnO matrix [38]. This introduces lattice distortion, and the electrons are scattered.

Fig. 3. (a) Current density-voltage characteristics, (b) IPCE spectra of the inverted solar cell with the undoped ZnO and AZO with different Al-doping concentrations, (c) PCE and Jsc and (d) Voc and FF as a function of the Al-doping concentration.

Figures 3(a) and 3(b) exhibit the photocurrent density-voltage characteristics and the IPCE spectra of the inverted PCDTBT:PC70BM solar cells with undoped ZnO and with the AZO #243417 © 2015 OSA


layer as an electron-extraction layer. The JSC, VOC, FF, and PCE are plotted in Figs. 3(c) and (d) as a function of the Al-doping concentration. Table 1 summarizes all the PV parameters, including the series resistance (RS) and shunt resistance (RP) for various Al-doping concentrations. The overall device performance of AZO was found to be improved compared to that of the undoped ZnO device. In particular, when the ZnO was doped with 5.83 at. % of Al, at the minimum resistivity, the device exhibited the highest JSC, FF and PCE values: JSC = 9.94 mA/cm2, VOC = 0.91 V, FF = 61.95%, and PCE = 5.60%. Moreover, the RP increased from 497.21 to 957.78 Ω·cm2, while the RS was almost same as that of the device fabricated with undoped ZnO. The FF was ameliorated from 48.95 to 61.59 when 5.83 at.% AZO was employed. The improvement of the PV performance of the devices with the AZO as an electron-extraction layer can be attributed to the combined effects of the low resistivity and enhanced electron-extraction capability. Table 1. Device performance of the inverted solar cell with ZnO and AZO with different Al-doping concentrations under AM1.5G illumination with 100 mW/cm2 intensity. ZnO

1.64 at.%

3.79 at.%

Al-doped ZnO 5.83 at.%

7.58 at.%

9.91 at.%

PCE (%)

3.71

4.98

5.36

5.60

5.34

4.73

JSC (mA/cm2)

8.80

9.56

9.79

9.94

9.65

8.75

VOC (V)

0.86

0.90

0.91

0.91

0.91

0.90

48.95

58.27

60.46

61.95

60.88

59.84

6.53 479.22

7.12 671.23

6.21 907.41

6.59 957.78

6.40 830.51

7.78 801.05

FF (%) 2

RS (Ω·cm ) Rp (Ω·cm2)

Fig. 4. (a) The density of states (DOS) for the AZO with various Al concentrations, where the Fermi level is set to zero, (b) the sum of conduction bottom states in AZO.

To understand the effect of Al concentration on the electronic structures, a first-principles calculation was carried out using VASP [39]. The electron-ion interactions were described by the projector-augmented wave pseudopotentials [40], while the generalized-gradient approximation was employed for the exchange–correlation energies between electrons [41]. To study the AZO structures, the unit cell of ZnO was expanded to a supercell with 96 atoms. A substitutional Al atom was taken into account to simulate the doped system, and the doping concentration was varied from 2 at.% to 10 at.%. Figure 4(a) shows the rigid shift of the #243417 © 2015 OSA


Fermi-level towards the conduction band and the increase of the DOS at the Fermi-level according to the Al concentration augmentation. From the analysis of the spatial charge distribution, we found that the states near the conduction bottom were delocalized over the whole system and mainly distributed over the Zn p and Al s orbitals. The integration of the DOS at the conduction bottom is shown in Fig. 4(b), which is proportional to the number density of electrons; therefore, an enhancement of electrical conductivity is expected. As a result, the higher the Al-doping concentration is compared to ZnO, the more energy states can be found at the conduction band edge. This creates a higher number density of electrons and results in conductivity augmentation [42]. Although the energy states of electrons at 10 at.% are higher than other cases in the first-principles calculation, the highest conductivity is shown at 5.83 at.% in the experimental results. This deviation from the calculation occurs because conductivity is also limited by the electron mobility, which is affected by the electron scattering, and electron mobility at 10 at.% is lower than at 5.83 at.%. 4. Concluding remarks The AZO was synthesized by a sol-gel spin-coating method with varying Al-doping concentrations. The XRD and HRTEM results show that the grain size of the AZO thin films is smaller than those of the ZnO. Moreover, the electrical properties of the ZnO are found to be improved as a result of Al doping, which clearly follows the simulation results for the DOS by a first-principles calculation. Particularly, 5.83 at.% AZO exhibited the decreased resistivity of 2.1 × 10−3 Ω·cm. The performance of an inverted polymer solar cell fabricated with 5.83 at.% AZO was observed to perform the best, with a PCE of 5.6%. This constitutes a remarkable over 51% improvement in PCE (from 3.7 to 5.6%), and is important due to its potential application in high performance OPVs with other functional donors. These results also suggest a route to achieve decent efficiency via the introduction of Al into a ZnO electron-extraction layer. Acknowledgments This work was financially supported by the Korea Ministry of Science, ICT & Future through the Global Frontier R&D Program on Center for Multiscale Energy System (2011-0031567) and the Human Resources Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry, and Energy (No. 20124010203170).

#243417 © 2015 OSA
