Flexible Cu2ZnSn(S,Se)4 solar cells over 7% power conversion efficiency

Abstract

The development of flexible solar cells is necessary for achieving market competitiveness through the implementation of low cost solar cells and for applying customized business models, such as Building Integrated Photovoltaics (BIPV). Cu2ZnSn(S,Se)S4 (CZTSSe) thin films have attracted attention as low-cost absorber materials for solar cells; however, further studies are required to develop flexible solar cells from this material and to achieve a high power conversion efficiency. In this work, the carrier separation characteristics of flexible CZTSSe thin-film solar cells on Mo foil substrates were identified. The surface electrical characteristics of the CZTSSe absorber layer indicate that the topography, surface potential, and local current are proportional to one another, in contrast to the characteristics exhibited by CIGS cells. For the CZTSSe absorber layer, an upward band bending-like band structure formed at and near the GBs, and a current path formed along the IGs as holes collected toward the GBs, and electrons collected toward the IGs. In contrast to that in CIGS cells, the carrier separation induced by the formation of the upward band bending-like band structure at the GBs is crucial in CZTSSe thin-film solar cells. Furthermore, the positive potential distribution at the IGs and average potential difference showed a proportional relationship to the local current and PCE. Through the implementation of these carrier separation characteristics, a flexible CZTSSe thin-film solar cell on a Mo foil substrate achieved a PCE of over 7%.