In-doping collaboratively controlling back interface and bulk defects to achieve efficient flexible CZTSSe solar cells

Focusing on the low open circuit voltage (V_OC) and fill factor (FF) in flexible Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells, indium (In) ions are introduced into the CZTSSe absorbers near Mo foils to modify the back interface and passivate deep level defects in CZTSSe bulk concurrently for improving the performance of flexible device. The results show that In doping effectively inhibits the formation of secondary phase (Cu(S,Se)₂) and V_Sn defects. Further studies demonstrate that the barrier height at the back interface is decreased and the deep level defects (Cu_Sn defects) in CZTSSe bulk are passivated. Moreover, the carrier concentration is increased and the V_OC deficit (V_OC,def) is decreased significantly due to In doping. Finally, the flexible CZTSSe solar cell with 10.01% power conversion efficiency (PCE) has been obtained. The synergistic strategy of interface modification and bulk defects passivation through In incorporation provides a new thought for the fabrication of efficient flexible kesterite-based solar cells.