Tailoring Addition Sequence of Metal Ions in Precursor Solution Drives Highly Efficient Kesterite Solar Cells

The unsatisfactory material quality of absorber is an important factor limiting the development of Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells, including poor crystallization, multi-layer structure, and various defects. Here, an effective strategy to enhance the quality of kesterite absorber and improve the performance of solar cells is proposed by tailoring the additional sequence of metal ions in precursor solution. With an addition sequence SSnCuZn (STCZ), higher values of Cu+/Cu2+ and Sn4+/Sn2+ ratios are obtained, and the surface grain growth is modulated into an inclined growth mode. Consequently, the decomposition of CZTSSe bulk is suppressed and its final morphology presents mainly as large grains spanning across the absorber. The quality of absorber and its interface is enhanced with activating the desirable shallow acceptor and reducing recombination center defects. An encouraging efficiency of over 12% for CZTSSe solar cells with increased short-circuit current density and fill factor is realized. This novel strategy provides insight into crystallization mechanism of kesterite photovoltaic material and has guiding significance for further improving material crystallization and absorber quality. It is discovered that tailoring addition sequence of metal ions can modify the crystallization mechanism of kesterite thin films for the first time. The physical origin that addition sequence of metal ions enhances photovoltaic performance is revealed and the common layering phenomenon of kesterite is explained.image