Effects of Inverted Sulfurization on the Photovoltaic Properties of the Cu₂Sn_1–<i>x</i>Ge_<i>x</i>S₃ Absorber Layer in a Solar Cell

Cu2Sn1–xGexS3 (CTGS) is a promising material for sustainable thin-film photovoltaics owing to its tunable band gap and high absorption coefficient. To improve the efficiency of CTGS solar cells, we developed a modified sulfurization process in which CTGS films were placed in an inverted position within a graphite box. This downward configuration suppressed the surface evaporation of sulfur and tin sulfide, resulting in a more uniform surface composition than that of the conventional upward configuration. As CTGS compounds exhibit defect properties that are highly sensitive to composition, maintaining a Cu-poor surface helps to preserve favorable defect characteristics. The modified process may have contributed to suppressing the formation of Cu-rich secondary phases, which could aid in enhancing device performance. Specifically, the power conversion efficiency increased from 1.50% with the upward configuration to 4.92% in the downward configuration when x = 0.1. These findings highlight the importance of the sulfurization configuration and offer a promising strategy for further improving the performance of CTGS solar cells.