Precisely tuning Ge substitution for efficient solution-processed Cu₂ZnSn(S, Se)₄ solar cells

The kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells have yielded a prospective conversion efficiency among all thin-film photovoltaic technology. However, its further development is still hindered by the lower open-circuit voltage (V_oc), and the non-ideal bandgap of the absorber is an important factor affecting this issue. The substitution of Sn with Ge provides a unique ability to engineer the bandgap of the absorber film. Herein, a simple precursor solution approach was successfully developed to fabricate Cu₂Zn(Sn_yGe_1-y)(S_xSe_1-x)₄ (CZTGSSe) solar cells. By precisely adjusting the Ge content in a small range, the V_oc and J_sc are enhanced simultaneously. Benefitting from the optimized bandgap and the maintained spike structure and light absorption, the 10% Ge/(Ge+Sn) content device with a bandgap of approximately 1.1 eV yields the highest efficiency of 9.36%. This further indicates that a precisely controlled Ge content could further improve the cell performance for efficient CZTGSSe solar cells.