Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin-film solar cells offer various advantages including excellent optical and electrical properties, nontoxic and earth-abundant raw materials, and a simple fabrication process. However, these devices suffer from a high deficit of the open-circuit voltage (V OC), mainly caused by interface recombination, which increases with increasing surface roughness. In this study, to achieve a high V OC and enhance the overall device performance, an additional heat treatment process was introduced during the fabrication of co-electrodeposited rough CZTSSe solar cells, and its effect on the photovoltaic properties was systematically investigated using various characterization techniques including diode analysis, transient photovoltage decay measurement, evaluation of the temperature dependency of the open-circuit voltage, current-voltage and drive-level capacitance profile analysis, and electrochemical impedance spectroscopy. At the optimized post-heat treatment (PHT) temperature of 200 C, a significant increase in the conversion efficiency (as high as 32%, from 7.11% to 9.40%) was observed owing to the change in the interfacial materials properties (i.e., higher conductivity and reduced interfacial nonradiative recombination), which in turn is a consequence of the diffusion of the Cd ions and the expansion of the Cu-poor/Zn-rich phase. The PHT-applied CZTSSe device exhibited a high conversion efficiency close to the record-high one reported for electrodeposited CZTSSe thin-film solar cells. These findings confirm the

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