A Multifunctional Molecular Bridging Layer for High Efficiency, Hysteresis-Free, and Stable Perovskite Solar Cells

At present, the dominating electron transport material (ETL) and hole transport material (HTL) used in the state-of-the-art perovskite solar cells (PSCs) are tin oxide and 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (Spiro-OMeTAD). However, the surface hydroxyl groups of the SnO₂ layer and the Li⁺ ions within the Spiro-OMeTAD HTL layer generally cause surface charge recombination and Li⁺ migration, significantly reducing the devices' performance and stability. Here, a molecule bridging layer of 3,5-bis(fluorosulfonyl)benzoic acid (FBA) is introduced onto the SnO₂ surface, which provides appropriate surface energy, reduces interfacial traps, forms a better energy level alignment, and, most importantly, anchors (immobilizes) Li⁺ ions in the ETL, and consequently improves the device power conversion efficiency (PCE) up to 24.26% without hysteresis. Moreover, the device with the FBA passivation layer shows excellent moisture and operational stability, maintaining over 80% of the initial PCE after 1000 h under both aging conditions. The current work provides a comprehensive understanding of the influence of the extrinsic Li⁺ ion migration within the cell on the device's performance and stability, which helps design and fabricate high-performance and hysteresis-free PSCs.