Flexible CZTSSe Thin-Film Solar Cells with Over 14% Certified Efficiency Enabled by Lithium Doping

Flexible kesterite solar cells show significant application potential, yet enhancing their power conversion efficiency remains a critical challenge. Herein, we achieved a breakthrough in certified efficiency, exceeding 14% for flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, by incorporating the light alkali metal lithium (Li) into the absorber layer, which substantially narrows the performance gap between flexible and rigid kesterite devices. The introduction of Li into the absorber layer reduces recombination centers in the film and facilitates carrier transport at the grain boundaries. Additionally, Li incorporation modulates the distribution of heavy alkali metal potassium in the CdS buffer layer, thereby enhancing the electrical properties of the CdS layer and optimizing the p–n junction band alignment. These synergistic effects significantly improve charge carrier transport and collection efficiency. This work bridges the efficiency gap between flexible and rigid CZTSSe solar cells, and provides additional insights into the influences and mechanisms of Li doping for high-performance kesterite photovoltaics.