Facile, Noncyanide Based Etching of Spray Deposited Cu2ZnSnS4 Thin Films for Secondary Phase Removal

Abstract

The coexistence of secondary phases in the quaternary compound kesterite Cu2ZnSnS4 (CZTS), a promising photovoltaic absorber, is a major problem while synthesizing under Zn or Cu rich conditions. A large variety of secondary phases, such as CuS, Cu1.8S, Cu2S, Cu2SnS3, and ZnS exist on the surface unless they are not removed by dedicated surface treatment before the annealing step. Under a carefully chosen concentrations of the starting precursors (usually, Zn-poor and Sn-rich) for spray pyrolyzed CZTS, the fraction of ZnS is minimized. However, under such growth conditions, binary Cu-sulfides become dominant. In this work, a selective noncyanide based chemical etching procedure is demonstrated prior to the deposition of the buffer layer. The absorber surface was treated with hydrogen peroxide that is known to remove Cu1.8S, Cu2S, and allied secondary phases to a large extent as compared to conventional KCN based techniques. By this treatment, the optical band gap is changed from 1.8 eV to most suitable 1....