Broadening the processing window is of high importance for developing efficient Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Herein, a high efficiency (active‐area: 13.5%) of CZTSSe solar cells achieved from thioglycolic acid (TGA)‐ammonia based water solution method is first reported. The cell performance exhibits the high tolerance to the element composition variations of the CZTSSe film. Film crystallization and chemical mechanism studies reveal that this high composition‐tolerance mainly benefits from an existence of a conductive carbon framework layer with high element accommodating ability and a phase‐segregation growth behavior of CZTSSe grains driven by thermodynamics properties of the heterogeneous film system. It is shown that Sn–TGA coordination capable to induce large metal–organic molecule clusters plays a critical role in forming the mesoscopic carbon layer. These results and related material mechanism provide new routes to regulate crystal growth of the CZTSSe film for further improving cell performance.

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