Graphene-Engineered Rear Interfaces Enable Efficient Flexible CZTSSe Solar Cells on Metal Foils

Flexible kesterite ${\mathrm{Cu}}_2\mathrm{ZnSn}$ (S,Se) ${}_4$ (CZTSSe) solar cells on metal foils are attractive for lightweight and conformable photovoltaic applications, yet their efficiencies remain significantly lower than those of rigid devices due to severe foil-induced rear-interface losses during high-temperature chalcogenization. These effects include uncontrolled Mo(S,Se) ${}_2$ formation, interfacial instability, and the emergence of blocking back-contact barriers that hinder efficient carrier extraction. Here, we introduce a single-crystal graphene interlayer to engineer the Mo foil/CZTSSe interface. The atomically thin graphene layer decouples absorber growth from the mechanically and chemically non-ideal foil surface, suppressing detrimental interfacial reactions and stabilizing the rear interface. As a result, carrier extraction is enhanced, and the back-contact barrier is reduced toward quasi-ohmic behavior. Flexible CZTSSe solar cells with graphene-engineered interfaces achieve a champion efficiency of 11.6% with simultaneous improvements in $V_{\mathrm{OC}}$ , $J_{\mathrm{SC}}$ , and fill factor. These results highlight rear-interface stabilization as a key strategy for improving flexible kesterite photovoltaics on metal substrates.