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• Precursor-heating strategy is introduced to tailor the stress of Cu 2 ZnSn(S,Se) 4. • 100℃ treated precursor contributes maximum stress reduction in Cu 2 ZnSn(S,Se) 4. • Decreased porosity contributes improved mechanical strength of Cu 2 ZnSn(S,Se) 4. • Precursor-treated device exhibits more satisfying bending stability. Co-existing feature with high efficiency and desirable mechanical tolerance for flexible solar cells plays a decisive role in potential industrialization. In this study, we demonstrated a precursor-heating strategy in fabricating flexible Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cell, which effectively decreased residual stress, enhanced mechanical strength and reduced carrier recombination of flexible CZTSSe solar cells. Compared to strategy without precursor-heating, the prepared CZTSSe thin film with 100 °C precursor treatment exhibited a decreased residual stress from −3.39 GPa to −0.71 GPa, featuring an increased efficiency from 3.07% to 4.40% and satisfactory mechanical stability even suffered from various harsh bending. The work paves a forward way for future large-scale application in portable devices. [ABSTRACT FROM AUTHOR]

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]

Antisite defects induced undesirable open-circuit voltage (Voc) and fill factor (FF) have always been considerable factors that plagued the efficiency enhancement of Cu 2 ZnSn(S,Se) 4 (CZTSSe)-based photovoltaic devices. Accordingly, sputtered Cu–Zn–Sn–S precursor with e-beam evaporated CdS layer on its top and bottom followed by selenization process was firstly proposed to obtain Cd-alloyed flexible CZTSSe thin film solar cells. The introduced CdS significantly improved crystalline quality and promoted grain growth of CZTSSe absorber. Besides, captured conduction band offset (CBO) exhibited a "cliff-like" structure with the maximum value of −0.15 eV when CdS layer was evaporated on top of Cu–Zn–Sn–S precursor, which largely relieved the band alignment issue in CZTSSe/CdS heterojunction. Interestingly, for CZTSSe prepared with CdS on top, Cd element shows a better uniform distribution in absorber than that with CdS on bottom. Finally, thanks to the evaporated CdS layer on precursor top, the efficiency of flexible CZTSSe thin film solar cell was improved from 2.31% to 4.52%, featuring a Voc of 343.53 mV, Jsc of 27.94 mA/cm2 and FF of 47.70%. • Cd-alloyed CZTSSe was obtained by evaporating CdS layer on CZTS precursor. • The evaporated CdS layer largely relieved band mismatching of CZTSSe/CdS. • CBO exhibited a maximum value of −0.15 eV when CdS was on precursor top. • Evaporated CdS on top of precursor can better guarantee the device performance. [ABSTRACT FROM AUTHOR]