Li-TFA modulated Li-Sb dual-cation synergy enables efficient and mechanically stable flexible CZTSSe Solar Cells

Abstract

Flexible kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin film solar cells inherently suffer from defect-dominated carrier recombination and poor mechanical flexibility durability. Herein, a Li-TFA-assisted Li-Sb co-doping strategy is proposed for the regulation of various defects. Sb pre-doping acts as a lattice buffer to offset Li-induced lattice expansion. Shallow defects get passivated by Li; meanwhile, Sb functions to eliminate deep recombination centers correlated with Sn. At the optimal 0.08 mol/L Li doping concentration, film residual compressive stress drops from −6.42 GPa to −2.19 GPa, the device achieves a champion efficiency of 5.09% with Urbach energy minimized to 21.43 meV. After 600 cycles of 60° bending, the optimized device retains 90.99% initial efficiency, far exceeding the 81.36% of undoped devices. This work elucidates cation synergistic modulation and offers a feasible route for high-stability flexible CZTSSe solar cells.