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

CH3NH3PbI3 (MAPbI(3)) 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 MAPbI(3) perovskite thin films with no voids or pinholes. To ensure uniform solid-state conversion (MAI + PbI2 -> MAPbI(3)) 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 MAPbI(3) 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 MAPbI(3) 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.