Stepwise Pre‐Treatment Enables Tunable Phase Evolution for High‐Efficiency Co‐Evaporated and Sulfurized Cu 2 ZnSnS 4 Solar Cells

ABSTRACT Controlling the phase evolution during the sulfurization of co‐evaporated Cu 2 ZnSnS 4 (CZTS) is crucial for developing efficient solar cells. Here, a stepwise pre‐treatment strategy combining pre‐alloying and pre‐sulfurization was developed to precisely regulate the phase‐evolution kinetics of CZTS absorbers. During the pre‐alloying stage, elemental distribution was homogenized and atomic diffusion pathways were established, while the subsequent pre‐sulfurization process enabled uniform sulfur incorporation throughout the precursor film. This resulted in the precursor transforming into CZTS in a single step, without the formation of secondary phases. Compared with conventional high‐temperature sulfurization, this strategy yielded highly dense, through‐thickness columnar grains with a reduced defect density. In addition, a Zn x Sn 1‐x O (ZTO) buffer layer was employed to substitute for the conventional CdS layer, achieving a completely Cd‐free architecture. Consequently, we report a Cd‐free solar cell with a high efficiency of 10.62%, representing the state‐of‐the‐art for co‐evaporated CZTS devices. This work provides important insights into the sulfurization mechanism and phase‐evolution kinetics of kesterite absorbers, offering an effective strategy for the development of high‐performance thin‐film photovoltaics.