A Precursor Doping Source Layer Engineering to Suppress Antisite Defects Enabling High Efficiency in Cu 2 ZnSnS 4 Solar Cell

Pure‐sulfide kesterite Cu 2 ZnSnS 4 (CZTS) is an attractive photovoltaic absorber, but its efficiency is largely constrained by antisite defects (e.g., Cu Zn ) and the associated band‐tail‐induced carrier losses. Here, we develop a precursor doping source layer (PDSL) engineering, in which an ultrathin CdS layer deposited on the porous CZTS precursor surface serves as an active Cd dopant during sulfurization. Different from conventional Cd cation doping, this localized PDSL targeted releases dopant during CZTS absorber formation, enabling Cd incorporation to be coupled with thermodynamics of phase evolution and kinetics of grain coalescence. Such reaction‐coupled Cd delivery stabilizes Zn‐site occupation and weakens Cu/Zn interchange, thereby mitigating Cu Zn ‐dominated defect responses and disorder‐induced band tails. As a result, trap‐assisted recombination and energetic fluctuations are reduced, leading to improved carrier transport and collection. The obtained pure‐sulfide CZTS/ZTO photovoltaic device delivers an efficiency of 12.23% and a large‐area (1.00 cm 2 ) efficiency of 10.31%, reaching the leading level of efficiency reported for non‐alloyed pure‐sulfide kesterite thin‐film solar cells. This work highlights PDSL design as an effective approach for overcoming defect‐limited performance in kesterite photovoltaics.