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A common phenomenon of polymer solar cells with metal oxide electron-transport layers (ETLs), known as “light-soaking” issue, is that the as-prepared device exhibits an anomalous S-shaped J-V characteristic, resulting in an extremely low fill factor (FF) and thus a poor power conversion efficiency. However, the S-shape disappears upon white light illumination with UV spectral components, meanwhile the performance parameters of the device recover the normal values eventually. This behavior appears to be of general validity for various metal oxide layers regardless of the synthesis and fabricating processes. Its origin is still under debate, while the ETL interface problems have generally been claimed to be the underlying reason so far. In this paper, both conventional and inverted cells with using ZnO nanoparticles (NPs) as ETL are fabricated to clarify the interface effect of the ETL on the light soaking procedure. The inverted device shows a typical light-soaking issue with an initial FF less than 20% as expected, whereas the J-V curves of the conventional cell remain regular shapes throughout the test. This result indicates that the ITO/ZnO interface is a key reason of S-shaped J-V characteristics, which is further verified via the use of Cs2CO3/ZnO ETL. The insert of Cs2CO3 layer isolates the ITO electrode from contacting with ZnO layer, and the kink disappears in the as-prepared device with this bi-layered ETL inverted structure. Our explanation for the result above is that the oxygen impurities absorbed onto the surface of ZnO NPs during fabrication process, behave as strong electron traps, and thus increasing the width of the energy barrier (EB) at the interface of ITO/ZnO. Subsequently, photogenerated electrons accumulate in the ZnO layer adjacent to the interface, resulting in extremely poor performance. Upon white light illumination, however, the trap sites are filled by photogenerated carriers within the ZnO layer, and therefore narrowing the EB. As the barrier width becomes thin enough to be freely tunneled through, a good selectivity behavior of ZnO ETL is reached, leading to a fully remarkable recovery in device performances.
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Keywords:
- light-soaking /
- ZnO nanoparticles /
- electron transport layer /
- polymer solar cells
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[1] Service R F 2011 Science 332 293
[2] Chen J W, Cao Y 2009 Acc. Chem. Res. 42 1709
[3] He Z C, Zhong C M, Su S J, Xu M, Wu H B, Cao Y 2012 Nat. Photon. 6 591
[4] Li Y F 2012 Acc. Chem. Res. 45 723
[5] Yang Q Q, Yang D B, Zhao S L, Huang Y, Xu Z, Gong W, Fan X, Liu Z F, Huang Q Y, Xu X R 2014 Chin. Phys. B 23 038405
[6] Liu Z F, Zhao S L, Xu Z, Yang Q Q, Zhao L, Liu Z M, Chen H T, Yang Y F, Gao S, Xu X R 2014 Acta Phys. Sin. 63 068402 (in Chinese) [刘志方, 赵谡玲, 徐征, 杨倩倩, 赵玲, 刘志民, 陈海涛, 杨一帆, 高松, 徐叙瑢 2014 63 068402]
[7] Chen C C, Chang W H, Yoshimura K, Ohya K, You J B, Gao J, Hong Z R, Yang Y 2014 Adv. Mater. 26 5670
[8] Krebs F C, Norrman K 2007 Prog. Photovolt.: Res. Appl. 15 697
[9] de Jong M P, van IJzendoorn L J, de Voigt M J A 2000 Appl. Phys. Lett. 77 2255
[10] Kyaw A K K, Sun X W, Jiang C Y, Lo G Q, Zhao D W, Kwong D L 2008 Appl. Phys. Lett. 93 221107
[11] Krebs F C 2009 Sol. Energy Mater. Sol. Cells 93 465
[12] Hau S K, Yip H L, Acton O, Baek N S, Ma H, Jen A K Y 2008 J. Mater. Chem. 18 5113
[13] Chen L M, Hong Z, Li G, Yang Y 2009 Adv. Mater. 21 1434
[14] Tan Z A, Zhang W Q, Zhang Z G, Qian D P, Huang Y, Hou J H, Li Y F 2012 Adv. Mater. 24 1476
[15] Schmidt H, Zilberberg K, Schmale S, Flgge H, Riedl T, Kowalsky W 2010 Appl. Phys. Lett. 96 243305
[16] Trost S, Zilberberg K, Behrendt A, Riedl T 2012 J. Mater. Chem. 22 16224
[17] Sun Y M, Seo J H, Takacs C J, Seifter J, Heeger A J 2011 Adv. Mater. 23 1679
[18] Sondergaard R, Helgesen M, Jorgensen M, Krebs F C 2011 Adv. Energy Mater. 1 68
[19] Kim C S, Lee S S, Gomez E D, Kim J B, Loo Y L 2009 Appl. Phys. Lett. 94 113302
[20] Lilliedal M R, Medford A J, Madsen M V, Norrman K, Krebs F C 2010 Sol. Energy Mater. Solar Cells 94 2018
[21] Sista S, Park M H, Hong Z R, Wu Y, Hou J H, Kwan W L, Li G, Yang Y 2010 Adv. Mater. 22 380
[22] Lin Z H, Jiang C Y, Zhu C X, Zhang J 2013 ACS Appl. Mater. Interfaces 5 713
[23] Kim J, Kim G, Choi Y, Lee J, Park S H, Lee K 2012 J. Appl. Phys. 111 114511
[24] Trost S, Zilberberg K, Behrendt A, Polywka A, Görrn P, Reckers P, Maibach J, Mayer T, Riedl T 2013 Adv. Energy Mater. 3 1437
[25] Manor A, Katz E A, Tromholt T, Krebs F C 2012 Sol. Energy Mater. Solar Cells 98 491
[26] Qian L, Zheng Y, Xue J, Holloway P H 2011 Nat. Photon. 5 543
[27] Manor A, Katz E A, Tromholt T, Krebs F C 2011 Adv. Energy Mater. 1 836
[28] Liao H H, Chen L M, Xu Z, Li G, Yang Y 2008 Appl. Phys. Lett. 92 173303
[29] Li Q H, Gao T, Wang Y G, Wang T H 2005 Appl. Phys. Lett. 86 123117
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