CZTS-based materials and interfaces and their effects on the performance of thin film solar cells

Cu2ZnSnS4 (CZTS) and its related materials such as Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(S,Se)4 (CZTSSe) have attracted considerable attention as an absorber material for thin film solar cells due to the non-toxicity, elemental abundance, and large production capacity of their constituents. Despite the similarities between CZTS-based materials and Cu(In,Ga)Se2(CIGS), the record efficiency of CZTS-based solar cells remains significantly lower than that of CIGS solar cells. Considering that the difference between the two lies in the choice of the absorber material, the cause of the lower efficiency of CZTS-based solar cells can be isolated to the issues associated with CZTS-based materials and their related interfaces. Herein, these issues and the work done to understand and resolve them is reviewed. Unlike existing review papers, every unique region of CZTS-based solar cells that contributes to its lower efficiency, namely: (1) the bulk of the absorber, (2) the grain boundaries of the absorber, (3) the absorber/buffer layer interface, and (4) the absorber/back contact interface are surveyed. This review also intends to identify the major unresolved issues and the potential improvement approaches of realizing sizable improvements in the solar cells' efficiency, thus providing a guide as to where research efforts should be focused. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)