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

ABSTRACT Kesterite Cu ₂ ZnSnS ₄ (CZTS) solar cells offer an earth‐abundant and non‐toxic alternative to Cu(In,Ga)Se ₂ 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.