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The major drawback of flexible Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells is the inevitable residual stress in CZTSSe that considerably limits the efficiency and flexibility of these cells. Hence, in this work, TiN layers with varying thicknesses were sputtered between flexible Ti substrates and back contact Mo layers as diffusion barriers. The TiN barrier layer relieved residual stress, facilitated grain growth, and decreased the porosity of CZTSSe, thereby effectively suppressing the formation of carrier recombination paths and improving the mechanical strength of CZTSSe. Meanwhile, the band alignment of the CZTSSe/CdS heterojunction could be significantly tailored, leading to an improved "cliff-like" conduction band offset from −0.49 eV to −0.33 eV. Under the optimized TiN layer thickness of 50 nm, the power conversion efficiency of the fabricated flexible CZTSSe solar cell increased considerably from 3.43% to 4.85% along with high bending stability. Therefore, introducing the TiN diffusion barrier into traditional flexible CZTSSe solar cells improves the efficiency and flexibility of these devices. Moreover, this method could be a promising pathway for the large-scale production of smart, flexible, and portable electronic devices. [ABSTRACT FROM AUTHOR]