Highly ordered crystallization of α-FAPbI3 films via homogeneous seeds for efficient perovskite solar cells

Formamidine lead triiodide (FAPbI3) perovskites have become the most promising photovoltaic materials for perovskite solar cells with record power conversion efficiency (PCE). However, random nucleation, phase transition, and lattice defects are still the key challenges limiting the quality of FAPbI3 films. Previous studies show that the introduction or adding of seeds in the precursor is effective to promote the nucleation and crystallization of perovskite films. Nevertheless, the seed-assisted approach focuses on heterogeneous seeds or hetero-composites, which inevitably induce a lattice-mismatch, the generation of strain or defects, and the phase segregation in the perovskite films. Herein, we first demonstrate that high-quality perovskite films are controllably prepared using α- and δ-phases mixed FAPbI3 microcrystal as the homogeneous seeds with the one-step antisolvent method. The partially dissolved seeds with suitable sizes improve the crystallinity of the perovskite film with preferable orientation, improved carrier lifetime, and increased carrier mobility. More importantly, the α-phase-containing seeds promote the formation of α-phase FAPbI3 films, leading to the reduction of residual lattice strain and the suppression of I− ion migration. Besides, the adding of dimethyl 2,6-pyridine dicarboxylate (DPD) into the precursor further suppresses the generation of defects, contributing to the PCE of devices prepared in air ambient being significantly improved to 23.75%, among the highest PCEs for fully air-processed FAPbI3 solar cells. The unpackaged target devices possess a high stability, maintaining 80% of the initial PCE under simulated solar illumination exceeding 800 h.