Polymer-Coordinated PbI₂ in Sequentially Deposited Formamidinium-Based Perovskite Absorbers for Efficient and Stable Solar Cells

Formamidinium (FA)-based perovskite absorbers processed by a two-step method are gaining popularity because of their suitable bandgap and excellent photovoltaic performance. However, the biggest obstacle faced by two-step method is the incomplete reaction between ammonium salts and PbI₂, which results in low crystallinity perovskite films with high defect density and overdosed PbI₂ residues, deteriorating the efficiency and stability of perovskite solar cells (PSCs). Herein, a multifunctional polymer poly(vinyl acetate) (PVAc) is introduced into PbI₂ precursor, and it is demonstrated that abundant carbonyl groups in PVAc inhibit the crystallization of PbI₂ and release its stress, thereby delaying its reaction rate with ammonium salts and regulating the crystallization process of perovskite films. Meanwhile, during the crystal growth process, the nonvolatile PVAc is expelled to the surface and grain boundaries of perovskite films, which effectively passivate the uncoordinated Pb²⁺ defects and further absorb stress from the perovskite lattice through the deformation of polymer chain. As a result, high-quality FA-based perovskite films with high crystallinity, preferred crystal orientation, decreased PbI₂ residues, low defect density, and released tensile stress are obtained, which greatly boost the power conversion efficiency of PSCs to 25.79%, accompanied by significantly enhanced storage, thermal, and operational stability.