The Effect of Self‐Assembled Bridging Layer on the Performance of Pure FAPbI₃‐Based Perovskite Solar Cells

Abstract Pure FAPbI₃‐based, with FA being formamidinium, perovskite solar cells (PSCs) have garnered worldwide recognition for their exceptional efficiency. However, the phase stability of FAPbI₃ is still a big obstacle in this area, because the ordinary strategy using MA⁺, Br⁻, Cs⁺ to stabilize α‐FAPbI₃ phase can cause the bandgap change and ion migration. Herein, a new strategy is introduced to improve the α‐FAPbI₃ phase stability by using a self‐assembled bridging layer at the buried interface of FAPbI₃ perovskite in the n–i–p solar cell structure. A series of multidentate bisphosphonic acid molecules are screened and demonstrate that etidronic acid (EA) with the smallest steric hindrance behaves the best. The four P‐OH groups can first form multidentate anchors on SnO₂ while the remaining unanchored ─OH and P═O groups can form strong interaction between I⁻ and Pb²⁺. Thus, a strong and stable bridging layer is formed, which greatly increases the energy barrier of phase transition of FAPbI₃. As a result, the pure FAPbI₃‐based (MA⁺, Br⁺, Cs⁺‐free system) n–i–p device reached an impressive power conversion efficiency of 24.2% with good stability. Furthermore, the strong interaction between EA and Pb²⁺ can greatly reduce lead leakage in harsh conditions.