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

Flexible kesterite Cu 2 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 OC , J SC , and fill factor. These results highlight rear-interface stabilization as a key strategy for improving flexible kesterite photovoltaics on metal substrates.