Growth control of compact CH3NH3PbI3 thin films via enhanced solid-state precursor reaction for efficient planar perovskite solar cells

Abstract

CH₃NH₃PbI₃ (MAPbI₃) perovskite thin films that are solution-processed using either a one-step or two-step conventional method typically contain a significant number of defects (voids, pinholes) or PbI₂ impurities, which have a detrimental effect on the performance of planar perovskite solar cells (PSCs) fabricated using those films. To overcome this issue, we show that enhancement of the solid-state reaction between inorganic–organic precursors is an effective route for the growth of compact, phase-pure MAPbI₃ perovskite thin films with no voids or pinholes. To ensure uniform solid-state conversion (MAI + PbI₂ → MAPbI₃) across the entire film thickness, a new successive spin coating/annealing (SSCA) process is used, where MAI is repeatedly infiltrated into a nanoporous PbI₂ film, followed by thermal annealing. The mechanisms involved in the SSCA process are elucidated by monitoring the evolution of the phases during the reaction. Owing to these desirable characteristics (high-purity, full-coverage, enhanced smoothness and compactness) of the SSCA MAPbI₃ films, planar PSCs based on these perovskite thin films delivered a maximum power conversion efficiency (PCE) close to 15%. Furthermore, PSCs fabricated using partially converted nanoporous PbI₂ thin films delivered a surprising PCE approaching 10%, suggesting continuous MAPbI₃ phase formation throughout the entire film at each spin coating/annealing process. The advantages gained from enhancing the solid-state precursor reactions allow better control of the growth of the perovskite making the SSCA process more robust.