Defect Synergistic Regulations of Li&Na Co-Doped Flexible Cu₂ZnSn(S,Se)₄ Solar Cells Achieving over 10% Certified Efficiency

Ion doping is an effective strategy for achieving high-performance flexible Cu2ZnSn(S,Se)(4) (CZTSSe) solar cells by defect regulations. Here, a Li&Na co-doped strategy is applied to synergistically regulate defects in CZTSSe bulks. The quality absorbers with the uniformly distributed Li and Na elements are obtained using the solution method, where the acetates (LiAc and NaAc) are as additives. The concentration of the harmful Cu-Zn anti-site defects is decreased by 8.13% after Li incorporation, and that of the benign Na-Zn defects is increased by 36.91% after Na incorporation. Synergistic Li&Na co-doping enhances the carrier concentration and reduces the interfacial defects concentration by one order of magnitude. As a result, the flexible CZTSSe solar cell achieves a power conversion efficiency (PCE) of 10.53% with certified 10.12%. Because of the high PCE and the homogeneous property, the Li&Na co-doped device is fabricated to a large area (2.38 cm(2)) and obtains 9.41% PCE. The co-doping investigation to synergistically regulate defects provides a new perspective for efficient flexible CZTSSe solar cells.