Improving the Open Circuit Voltage through Surface Oxygen Plasma Treatment and 11.7% Efficient Cu₂ZnSnSe₄ Solar Cell

The photovoltaic performance of Cu₂ZnSnSe₄ (CZTSe) solar cells subjected to surface oxygen plasma treatments is investigated. The observed improvements are related to an enhancement of the open circuit voltage V_OC, that is, the suppression of the V_OC deficit. The V_OC monotonically increases with treatment time up to 0.460 V. The origin of this improvement is discussed, and it is concluded that the effectiveness of the surface treatment is not due to oxygen-related alloying but instead to the homogeneous oxidation and removal of the oxidized CZTSe surface layer. The surface oxygen content increases with surface treatment time, although surface oxides are fully removed after ammonia treatment, which is conducted in a similar manner to CdS buffer deposition. The reduction of surface recombination is confirmed by time-resolved photoluminescence measurements, and the minority carrier lifetime deduced using the fast decay component increases with increasing treatment time. The relationship between photovoltaic properties and lifetime is clearly demonstrated. The best-performing CZTSe solar cell obtained using surface oxygen treatment demonstrates a conversion efficiency of 11.7%, which is higher than those of previous reports on CZTSe cells.