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

Abstract Pure FAPbI 3 ‐based, with FA being formamidinium, perovskite solar cells (PSCs) have garnered worldwide recognition for their exceptional efficiency. However, the phase stability of FAPbI 3 is still a big obstacle in this area, because the ordinary strategy using MA + , Br − , Cs + to stabilize α‐FAPbI 3 phase can cause the bandgap change and ion migration. Herein, a new strategy is introduced to improve the α‐FAPbI 3 phase stability by using a self‐assembled bridging layer at the buried interface of FAPbI 3 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 SnO 2 while the remaining unanchored ─OH and P═O groups can form strong interaction between I − and Pb 2+ . Thus, a strong and stable bridging layer is formed, which greatly increases the energy barrier of phase transition of FAPbI 3 . As a result, the pure FAPbI 3 ‐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 Pb 2+ can greatly reduce lead leakage in harsh conditions.