Strain Regulation and Defect Passivation of FA-Based Perovskite Materials for Highly Efficient Solar Cells

Formamidine lead triiodide (FAPbI₃ ) perovskites have attracted increasing interest for photovoltaics attributed to the optimal bandgap, high thermal stability, and the record power conversion efficiency (PCE). However, the materials still face several key challenges, such as phase transition, lattice defects, and ion migration. Therefore, external ions (e.g., cesium ions (Cs⁺ )) are usually introduced to promote the crystallization and enhance the phase stability. Nevertheless, the doping of Cs⁺ into the A-site easily leads to lattice compressive strain and the formation of pinholes. Herein, trioctylphosphine oxide (TOPO) is introduced into the precursor to provide tensile strain outside the perovskite lattice through intermolecular forces. The special strain compensation strategy further improves the crystallization of perovskite and inhibits the ion migration. Moreover, the TOPO molecule significantly passivates grain boundaries and undercoordinated Pb²⁺ defects via the forming of P═O─Pb bond. As a result, the target solar cell devices with the synergistic effect of Cs⁺ and TOPO additives have achieved a significantly improved PCE of 22.71% and a high open-circuit voltage of 1.16 V (voltage deficit of 0.36 V), with superior stability under light exposure, heat, or humidity conditions.