Chalcogenization-Derived Band Gap Grading in Solution-Processed CuIn_xGa_1-x(Se,S)₂ Thin-Film Solar Cells

Significant enhancement of solution-processed CuIn_xGa_1-x(Se,S)₂ (CIGSSe) thin-film solar cell performance was achieved by inducing a band gap gradient in the film thickness, which was triggered by the chalcogenization process. Specifically, after the preparation of an amorphous mixed oxide film of Cu, In, and Ga by a simple paste coating method chalcogenization under Se vapor, along with the flow of dilute H₂S gas, resulted in the formation of CIGSSe films with graded composition distribution: S-rich top, In- and Se-rich middle, and Ga- and S-rich bottom. This uneven compositional distribution was confirmed to lead to a band gap gradient in the film, which may also be responsible for enhancement in the open circuit voltage and reduction in photocurrent loss, thus increasing the overall efficiency. The highest power conversion efficiency of 11.7% was achieved with J_sc of 28.3 mA/cm², V_oc of 601 mV, and FF of 68.6%.