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Balancing the efficiency and mechanical endurance of flexible Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin film solar cells is challenging its potential applications of the day. Here, a straightforward strategy is developed to prepare flexible solar cells with the coexistence of photovoltaic performance and flexibility by introducing a novel SbCl 3 in precursor solution. Sb-doped CZTSSe thin film with progressive grain growth from nanoscale to microscale and strengthened toughness with porosity from 8.13 % to 2.44 % is obtained. By exploiting the benefits of upgrading the band matching and tailoring the residual stress, SbCl 3 solution with a concentration of 0.30 mol/L achieves the optimum heterojunction characteristic and mechanical performance. The final structured device with improved efficiency from 3.27 % to 4.98 % and indomitable mechanical endurance is attained, which can reserve 93 % of its original efficiency after bending 200 times. The present work indubitably paves a convenient way toward efficient and flexible photovoltaic devices. • Sb doping is applied to the tailor residual stress of flexible CZTSSe thin film. • Alleviated residual stress contributes to a superior microstructure of CZTSSe. • 0.30 mol/L SbCl 3 exhibits the best PCE with satisfying bending durability. [ABSTRACT FROM AUTHOR]