Record Efficiency for Thin-Film Polycrystalline Solar Cells Up to 22.9% Achieved by Cs-Treated Cu(In,Ga)(Se,S)₂

An efficiency of 22.9% for 1-cm²-sized Cu(In,Ga)(Se,S)₂ solar cells has been independently verified, establishing a record device efficiency for thin-film polycrystalline solar cells. The main improvement in the solar cell device is due to a reduction in the deficit of the open-circuit voltage (V_oc), which is notably suppressed by modifying the absorber formation. This is presumably due to reduced defect density, as suggested by the enhanced photoluminescence performance. Such improvement in the absorber quality allowed for an opportunity to benefit from the effects of a wider absorber bandgap. The reverse saturation current density and V_oc were significantly improved. Meanwhile, heavier alkali treatment on the absorber surface using cesium was adopted to further boost the device performance. As a result, the significant enhancements in V_oc and fill factor led to the achievement of this record-breaking efficiency. These findings have been systematically reproduced and will be leveraged to improve the module performance of Solar Frontier's production in the near future.