Homologous post-treatment strategy enabling phase-pure α-FAPbI3films

Abstract

Formamidinium lead triiodide (FAPbI₃) is considered as the prospective light-absorbing layer on account of the close-to-ideal bandgap of theα-phase, wide optical absorption spectrum and good thermal stability. Therefore, how to realizeδtoα-phase transition to obtain phase-pureα-FAPbI₃without additives is important for FAPbI₃perovskite films. Herein, a homologous post-treatment strategy (HPTS) without additives is proposed to prepare FAPbI₃films with pureα-phase. The strategy is processed along with dissolution and reconstruction process during the annealing. The FAPbI₃film has tensile strain with the substrate, and the lattice keeps tensile, and the film maintains in anα/δhybrid phase. The HPTS process releases the tensile strain between the lattice and the substrate. The process of strain release realizes the phase transition fromδtoα-phase during this process. This strategy can accelerate the transformation from hexagonalδ-FAPbI₃to cubicα-FAPbI₃at 120 °C. As a result, the acquiredα-FAPbI₃films exhibit better film quality in optical and electrical properties, accordingly achieving device efficiency of 19.34% and enhanced stability. This work explores an effective approach to obtain additive-free and phase-pureα-FAPbI₃films through a HPTS to fabricate uniform high-performanceα-FAPbI₃perovskite solar cells.

Keywords: FAPbI3; homogeneous; perovskite solar cell; phase transition; strain.