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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]

• Ga doping is applied to the tailor residual stress of flexible CZTSSe thin film. • Alleviated residual stress contributes a superior microstructure of CZTSSe. • Evaporated 20 nm Ga layer exhibits the best PCE with satisfying bending durability. Ga-doped CZTSSe is achieved by evaporating Ga layer in precursor combined with subsequent selenization to balance the efficiency and bending durability of flexible CZTSSe solar cells in this paper. Ga doping can simultaneously alleviate the residual stress and reinforce the microstructure of CZTSSe, retaining the original lattice structure of CZTSSe and suppressing Sn-related deep level defects. The gratifying residual stress reduction from -5.31 GPa to -3.82 GPa with decreased porosity from 6.24% to 3.24% of CZTSSe thin film spontaneously promotes the device performance after evaporating the 20 nm Ga layer. A desirable efficiency enhancement from 2.61% to 5.04% with preferable bending durability of flexible device is achieved, which strategy provides substantial assistance for the potential application of flexible solar cells. [ABSTRACT FROM AUTHOR]

• TiN-Al 2 O 3 -Cr gradient layer is introduced to tailor residual stress of CZTSSe. • 60 nm sputtered gradient layer contributes the best heterojunction characteristic. • Gradient layer introduced device exhibits more satisfying bending stability. In an effort to promote the mechanical endurance of flexible Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells, a convenient and effective strategy for introducing a TiN-Al 2 O 3 -Cr gradient layer is proposed, which can effectively tailor the residual stress and improve the heterojunction characteristic of solar cell. Benefitted from the optimum TiN-Al 2 O 3 -Cr gradient layer with 60 nm, CZTSSe presents a released residual stress of −3.33 GPa and an optimized heterojunction characteristic with the CBO of −0.28 eV. Furthermore, the PCE of flexible device is remarkably improved from 3.31 % to 5.01 %, which can maintain 85 % of the original efficiency after harsh bending. The proposed gradient layer strategy may pave a thriving way for congener flexible and portable cells. [ABSTRACT FROM AUTHOR]