Reversible Degradation in Hole Transport Layer‐Free Carbon‐Based Perovskite Solar Cells

The rationale for ambient degradation behaviors in hole transport layer (HTL)‐free carbon‐based perovskite solar cells (PSCs) still remains a mystery, although carbon‐based PSCs have been the frontrunner among the emerging next‐generation photovoltaics owing to their low cost of materials and fabrication process, as well as the exceptional durability. Herein, the long‐term stability of HTL‐free carbon‐based PSCs (H‐C‐PSCs) in the ambient air environment is investigated to thoroughly understand and identify the governing chemistry during the degradation. Specifically, a reversible degradation phenomenon is observed along with an anomalous S‐shape of current–voltage curves involving only reduction of fill factor (FF), almost without impact for short‐circuit current density and open‐circuit voltage. Furthermore, a minute‐heating treatment will eliminate the reversible degradation which can be attributed to the reversible formation of intermediate hydrate at the interface between perovskite and carbon contact. This provides a new perspective for the stability issue of H‐C‐PSCs.