Tailoring Crystallization Dynamics and Phase Evolution of Co-evaporated Pure α-FAPbI3 for Enhanced Device Performance

Abstract

Formamidinium lead iodide (FAPbI₃) is among the most promising perovskite composition for high-efficiency photovoltaics. However, its thermal co-evaporation remains technically challenging due to inherent phase instability and thermal decomposition of the organic precursors. In this work, we demonstrate an additive-free thermal co-evaporation of pure FAPbI₃, by sequentially employing multiple sources of formamidinium iodide (FAI). This strategy effectively mitigates process instabilities of FAI during prolonged evaporation, enabling the growth of thick perovskite films with improved morphology. Furthermore, we show that the introduction of an intermediate annealing step significantly enhances crystallization dynamics during thickness scaling, resulting in highly ordered and coherent film structures. A systematic investigation of substrate temperature during deposition highlights its crucial role on phase purity and crystallinity. Notably, films grown at 60 °C exhibit superior structural order and effective suppression of the undesired hexagonal non-perovskite phases, as confirmed by X-ray diffraction analysis. We also compare two widely used hole transport layers, MeO-2PACz and PTAA, and show that, although MeO-2PACz promotes α-phase formation, it poses challenges related to film morphology and processing reproducibility. Finally, devices employing PTAA in a p–i–n architecture achieve a power conversion efficiency of 18.5% using pure, thermally evaporated FAPbI₃.