Bilateral Anchoring for Enhanced Mechanical Stability and Efficiency in Flexible all-Perovskite Tandem Solar Cells.

Flexible all-perovskite tandem solar cells (TSCs) feature an outstanding power-to-weight ratio, rendering them perfect for building-integrated photovoltaic, wearable electronics, and aerospace applications, owing to their adaptability to flexible and lightweight substrates. However, the weak mechanical adhesion between the perovskite and adjacent functional layers, combined with tin (Sn) oxidation at the buried interface in tin-lead (Sn-Pb) narrow-bandgap (NBG) perovskites solar cells (PSCs), substantially hampers the durability and performance of device. Herein, a bilateral anchoring strategy is proposed by employing 2-bromoethylamine hydrobromide (2-BH) at the NBG perovskite/ hole transporting layer (PEDOT:PSS) interface. The incorporation of 2-BH establishes robust bonds with both PEDOT:PSS and the perovskite layer, thereby enhancing interfacial adhesion and charge transfer. Meanwhile, the morphology and crystallinity of the perovskite films are also improved due to the mitigated oxidation of Sn²⁺. Thus, this approach yields flexible single-junction NBG  with a power conversion efficiency (PCE) of 18.5%, maintaining its 95% efficiency after 3000 bending cycles. When integrated into monolithic flexible all-perovskite TSCs, a certified PCE of 24.01% is achieved.