Influence of stacking order and intermediate phase at low temperature on Cu₂ZnSnS₄ thin film formation for solar cell

A stacking order of a precursor is one of the important factors that affects the quality of Cu₂ZnSnS₄ (CZTS) thin film. Understanding of the intermediate phase formation and its reaction mechanism can contribute to the design of a precursor to enhance the performance of CZTS solar cell. In this work, three precursors such as Zn/SnS/Cu, SnS/Cu/Zn, and SnS/Zn/Cu were designed based on previous studies. These precursors were treated for sulfurization process at 560 °C, and annealed at 300 °C for 30 min to investigate the reaction of intermediate phases. The annealed thin film from Zn/SnS/Cu precursor exhibited crystallized and uniform intermediate ternary phase of Cu₂SnS₃. This easy and uniform transformation to Cu₂SnS₃ phase as a whole at low temperature using Zn/SnS/Cu precursor resulted in relatively smooth surface of CZTS thin film after sulfurization at high temperature. In contrast, CZTS thin film from SnS/Cu/Zn or SnS/Zn/Cu precursor showed a rough surface morphology with loss of Zn content. A rough surface will degrade the contact between CZTS absorber and CdS buffer layers, resulting in decreased a device performance. Accordingly, CZTS solar cell device fabricated from Zn/SnS/Cu precursor achieved improved efficiency of 4.20%.