The role of tuning Se/(S + Se) ratio in the improvement of Cu₂MnSn(S, Se)₄ thin films properties and photovoltaic device performance

The beneficial effects of Se/(S + Se) ratio were widely reported in Cu₂ZnSn(S, Se)₄ thin films and solar cells, while very limited works were reported about that on Cu₂MnSn(S, Se)₄ (CMTSSe) thin films or/and solar cells. In this work, CMTSSe thin films were fabricated via selenizing the Cu-Mn-Sn-S precursors under different amount of selenium powder and atmosphere pressure. Changing amount of selenium powder can tune the ratio of Se/(S + Se) form 0 to 0.28, while tuning the selenization pressure increases the ratio up to 0.81. The shift of (1 1 2) diffraction peak to lower degree and the change of the major phase from Cu₂MnSnS₄ to CMTSSe indicate that the Se atom gradually substitute for S atom in the CMTSSe films. The Increase on Se/(S + Se) ratio enlarges the lattice parameters, dramatically improves the crystallinity, and lowers the bandgap. As the Se/(S + Se) ratio increasing from 0.28 to 0.81, the efficiency (PCE) boost from 0.21% to 0.58% is mainly attributed to the increase of short current density (J_sc) (from 2.41 mA⋅cm⁻² to 5.09 mA⋅cm⁻²) and fill factor (FF) (from 26% to 39%). Moreover, the calculated voltage deficit of our solar cells shows a decrement of over 60 mV, which can be attributed to the effect of bandgap and defect density reduction.