Study on phase formation mechanism of non- and near-stoichiometric Cu ₂ZnSn(S,Se)₄ film prepared by selenization of Cu-Sn-Zn-S precursors

The appearance of impurity phases in a Cu₂ZnSnS₄ (CZTS) layer is a big problem and challenge for high-performance CZTS photovoltaic devices. In spite of some experimental observations of secondary impurity phases, their growth mechanisms are not well established, especially in an S and Se coupled Cu₂ZnSn(S,Se)₄ (CZTSSe) system. This work carried out a systematic experimental study on the formation of secondary phases in CZTSSe, i.e., Cu_xSe and SnSe on the top surface and ZnS at the bottom. Interestingly, a clear reaction pathway for the formation of uniform CZTSSe film was revealed through a temperature dependent annealing of a near-stoichiometric sample. A further reaction thermodynamic analysis was applied to explore the fundamentals of the secondary phase formation mechanisms. Besides, a diagrammatic CZTSSe film growth model was proposed to illustrate the detailed phase development procedures during selenization of a sulfur-contained precursor. A deeper insight in the formation of CZTSSe film can not only provide a better understanding of the experimental phenomena but also guide possible improvement routes of the film formation strategy to achieve a more promising absorber of a thin film solar cell.