Defect Passivation by a Donor–Acceptor–Donor‐Structured Small Molecule via Bidentate Anchoring for Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) have exhibited a tremendous photovoltaic performance over the past few years. However, the ionic nature of perovskite and the solution‐processable fabrication methods lead to various defects (vacancies, interstitials, and antisites) at the perovskite surface. Incorporating interfacial or surface passivation layers has proved to be crucial in passivating these defects. Herein, a novel donor–acceptor–donor (D–A–D)‐based bidentate material, namely, BDTBT, consisting of benzothiadiazole (BDT) as the central acceptor unit and benzothiophene (BT) as a donor end cap unit, is synthesized. The various structural analyses reveal that N and S heteroatoms at BDTBT coordinate effectively to undercoordinated Pb 2+ in perovskite via Pb–N/S bidentate interactions. As a result, the BDTBT‐treated perovskite exhibits an improved power conversion efficiency (PCE) of 20.42% compared with the bare perovskite, having a PCE of 17.18%. The BDTBT incorporation provides favorable band alignment, increased hole transfer, and suppressed nonradiative recombination losses by reducing the surface defect states. In addition, there is significant increase in the device stability and moisture resistance owing to the hydrophobic nature of BDTBT. This study provides a simple and efficient route to obtain stable and highly efficient PSCs by incorporating small molecules as an additional interfacial layer.