Band gap engineering of atomic layer deposited Zn x Sn 1-x O buffer for efficient Cu(In,Ga)Se 2 solar cells Raphael Edem Agbenyeke 1,2 , Soomin Song 3 , Bo Keun Park 2 , Gun Hwan Kim 2 , Jae Ho Yun 3 , Taek-Mo Chung 2 , Chang Gyoun Kim 2 , Jeong Hwan Han 4 , David J. Fermin 1 1 University of Bristol, UK, 2 Division of Advanced Materials, Korea Research Institute of Chemical Technology(KRICT), Republic of Korea, 3 Photovoltaics Laboratory, Korea Institute of Energy Research (KIER), Republic of Korea, 4 Department of Materials Science and Engineering, Seoul National University of Science and Technology, Republic of Korea Ternary zinc tin oxide (ZTO) is an attractive buffer material with the promising potential of replacing the n -CdS buffer in chalcocite and kesterite solar cells. Besides its non-toxic elemental composition, it offers important electrical and optical/bandgap engineering opportunities that are yet to be fully explored. In this study, ZTO thin films were grown by atomic layer deposition and systematically characterized with the aim of establishing correlations between film compositions and properties. Using a series of characterization techniques, the effect of Zn/Sn ratio on film growth rate, crystal structure, majority carrier concentrations and optical band gap was uncovered. Most importantly, a parabolic correlation was observed between bandgap and Zn/Sn composition, which allowed for band offset tuning in CIGSe solar cells. Device Voc’s increased by more than twofold from 299 mV for a pure ZnO buffer to 627 mV for ZTO buffer with 16 atomic percent of Sn. Band alignment studies revealed an upward shift in conduction band minimum of ZTO with Sn incorporation, which favors the formation of a spike-type conduction band offset at the CIGSe/ZTO interface and reduces interfacial recombination. The 13.9% champion conversion efficiency of the ZTO incorporated cell relative to the 14.4 % of the CdS reference cell highlighted the promising potential of ZTO buffer layers. References 1. Lee Y. S, Heo J, Siah S.C, et al. Ultrathin amorphous zinc-tin-oxide bufferlayer for enhancing heterojunction interface quality in metal-oxidesolar cells.Energ. Environ. Sci. 2013;6(7):2112-2118. 2. Wei J, Yin Z, Chen S.C, Zheng Q. Low-temperature solution-processedzinc tin oxide film as a cathode interlayer for organic solar cells.ACSAppl.Mater.Interfaces. 2017;9(7):6186−6193. 3. Gorrn P, Ghaffari F, Riedl T, Kowalsky W. Zinc tin oxide based driverfor highly transparent active-matrix OLED displays.Solid- State Electron.2009;53(3):329-331.
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