Flexible High Performance Hybrid Azo/Ag-Nanowire/Azo Sandwich Structured Transparent Conductors for Flexible Cu(In,Ga)Se2 Solar Cell Applications

Abstract

Transparent conducting oxides (TCOs) are an important class of materials used in a wide range of optoelectronic technologies, including solar cells, flat-panel displays and organic light emitting diodes, and presently there is a strong drive toward the development of solar cell technologies on flexible platforms that are robust, portable and flexible enough to conform to unusual shapes, particularly building structures. Tin-doped indium oxide (ITO) is currently the commonly chosen TCO for use in non-flexible applications; however, its brittleness limits its application on flexible substrates, whilst the scarcity of indium maintains an undesirably high cost for use in solar cell applications. As such, it is important that ITO alternatives are developed with suitably flexible mechanical and electronic properties, which can also be fabricated at reasonable costs. The chalcopyrite absorber, copper indium gallium selenide, Cu(In,Ga)Se2, has a very high absorption coefficient (~104 cm-1), requiring only thin-films (~1 μm) for efficient solar cell fabrication, making it thin enough for flexible applications. However the lack of a high quality flexible TCO has, to date, limited its use in such applications. In this work, we present a new approach for the fabrication of high performance transparent electrodes suitable for flexible CIGS solar cell applications, using a stacked Al-doped ZnO/Ag-nanowire/Al-doped ZnO (AAA) sandwich structure.1 In our AAA electrode the Al-doped ZnO (AZO) layers act as a broad area charge carrier collector and protective encapsulant for the Ag-nanowire network. Concurrently, the Ag-nanowires provide low-resistance, long-range pathways that aid the extraction of charge carriers, but more importantly, they aid in maintaining a low resistivity for our total electrode structure, by maintaining charge transport pathways across cracks that develop in the AZO during repeated strenuous bending. By optimising of our Ag-nanowire network density, AZO deposition and AAA stack structure we are able produce a transparent electrode with comparable resistivity and transparency performance to that of ITO. These combined benefits enable us to fabricate CIGS solar cells that are able to maintain 95% of their initial power conversion efficiency, following 1000 bending cycles. In comparison, devices fabricated using equivalent thickness AZO and ITO electrodes are only able to maintain 57 and 5%, respectively, due to crack formation and delamination of the films. This AAA sandwich structure electrode could therefore serve as a high-performance electrode for numerous flexible optoelectronic applications. 1. W.-C. Tsai, S. R. Thomas, C.-H. Hsu, Y.-C. Huang, J.-Y. Tseng, T.-T. Wu, C.-h. Chang, Z. M. Wang, J.-M. Shieh, C.-H. Shen and Y.-L. Chueh, Journal of Materials Chemistry A, 2016.