Copper zinc tin sulfo-selenide (CZTSSe) is a promising light-absorbing material of thin-film solar cells because of its low material cost especially when it is prepared by cost-effective processes like the electrochemical deposition process. The CZTSSe thin-film solar cells, however, suffer from the relatively low efficiency, mostly because of the significant charge recombination. Given that the surface recombination is one of the major recombination paths, controlling the surface roughness, and thus the interfacial area is one of the key factors for improving their device performances. In this study, we demonstrated a simple but effective strategy for reducing the surface roughness during the electrochemical deposition process of the CZTSSe thin films. By adopting an initial nucleation stage with higher deposition currents ahead of the steady-state galvanostatic deposition, the surface of the copper−zinc−tin (CZT) precursor and CZTSSe thin films became significantly smoother and uniform (ΔR rms : −43.8% for CZT, −28.9% for CZTSSe). The effects of the surface roughness on the photovoltaic properties of the CZTSSe thin-film solar cells have been investigated systematically with various characterization techniques like the diode analysis, lifetime measurement, and the temperature dependency of the open-circuit voltage. The device with the smoother surface exhibited higher open-circuit voltage and fill factor, mostly because of the significantly reduced charge recombination, leading to the high conversion efficiency of 8.64% (active).

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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