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

Flexible kesterite Cu 2 ZnSn (S,Se) (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) 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 OC , J SC , and fill factor. These results highlight rear‐interface stabilization as a key strategy for improving flexible kesterite photovoltaics on metal substrates. An atomically thin graphene interlayer stabilizes the rear interface of flexible CZTSSe solar cells by regulating interfacial stress, suppressing detrimental interfacial reactions, and reducing the back‐contact barrier. This strategy simultaneously improves photovoltaic performance and mechanical durability.