Molecular Surface Self‐Accumulation Toward High‐Efficiency and Mechanically Robust Flexible Perovskite Solar Cells

Flexible perovskite solar cells (F‐PSCs) have promising applications in building portable energy systems. However, the efficiency and stability of F‐PSCs still lag far behind their rigid counterparts. The efficiency gap stems mainly from the high roughness of flexible substrates, which aggravates the spatial heterogeneity within transporting interfaces and thus reduces the carrier transport efficiency. Moreover, dynamic stress induces cracks and interfacial delamination in F‐PSCs, resulting in significant mechanical stability issues. Here, we adopt a molecular surface self‐accumulation strategy to improve the spatial homogeneity of the self‐assembled monolayers (SAMs) by adding halogenated phenothiazine‐based molecules (Br‐4PAPT) into the perovskite precursor solution. The Br‐4PAPT diffuses to the bottom interface of the perovskite film to fill vacancies in the underlying SAMs during perovskite crystallization. This enhanced homogeneity translates to p‐i‐n structured ultra‐thin (9 µm) F‐PSCs achieving a record efficiency of 22.02% on an active area of 1 cm 2 and 24.47% on 0.1 cm 2 . Furthermore, the ultra‐thin large‐area F‐PSCs achieved a power‐per‐weight of 12.71 W/g. The strategy also reduces the Young's modulus at the bottom interface of the perovskite film. Coupled with mechanical neutral plane design, the ultra‐thin small‐area F‐PSCs maintained 91.81% efficiency after 10 000 bending cycles at a bending radius of 2 mm.