Surface cooling for optimized elemental distribution and improved kesterite solar cells†

One effective approach to achieve high-quality Cu₂ZnSn(S,Se)₄ (CZTSSe) absorbers and efficient CZTSSe solar cells involves adjusting the pre-annealing temperature to regulate the primary and secondary phases within the micro-regions of the CZTSSe absorber. Herein, we inhibit the formation of Cu₂ZnSnS₄ (CZTS) from binary sulfides of the precursor through a surface cooling strategy at the pre-annealing stage, including ZnS, SnS₂, and SnS, which restricts the diffusion of Sn in the absorber with an optimized elemental ratio of (Cu + Ag)/(Zn + Sn) set to 0.8. Meanwhile, optimized precursors exhibit a copper-poor and zinc-rich gradient, resulting in an absorber with high crystalline quality. In comparison to the traditional approach, the CZTSSe solar cell prepared by pre-annealing at 365 °C on the precursor surface displays reduced bulk defect density and increased carrier lifetime. Utilizing the superficial cooling technique on the precursors, this research optimizes the distribution of metal elements in the CZTSSe absorber and achieves a device efficiency of 12.15%. This study offers new insights into the mechanisms influencing the modulation of the microregional phases of the CZTSSe absorber, which can enhance photovoltaic performance.