Quantitative Disentanglement of Fill Factor Losses in Kesterite Solar Cells Via an Integrated Experimental–Simulation Framework

photovoltaics, yet their efficiencies remain markedly lower, largely due to a persistently low fill factor (FF) in addition to substantial open-circuit voltage deficits. While the open-circuit voltage deficit has been extensively investigated, the physical origins of FF losses remain insufficiently understood due to multiple intertwined mechanisms. Here, we establish an integrated experimental-simulation framework to quantitatively disentangle FF loss channels in solution-processed sulfide CZTS solar cells. By combining temperature-dependent photoluminescence, deep-level transient spectroscopy, impedance spectroscopy, and transient photocurrent/photovoltage measurements with diode-equation-based modeling, the total FF deficit is decomposed into contributions from defect-mediated recombination, Schottky-type back-contact barriers, and carrier extraction inefficiencies. Using Cd-alloyed CZTS as a model system, we find that Cd incorporation suppresses Sn-related defect recombination, eliminates back-contact barriers, and improves carrier collection, collectively enhancing the FF from 53.9% to 65.2%, accompanied by an efficiency increase from 7.3% to 12.1%. Moreover, Ag incorporation further validates the generality of the framework, yielding an FF of 71%, among the highest values reported for pure sulfide kesterite solar cells. Overall, this work establishes a quantitative and generalizable framework for diagnosing and mitigating FF losses in complex multinary semiconductors.