High-Performance Formamidinium-Based Perovskite Solar Cells via Microstructure-Mediated δ-to-α Phase Transformation

The delta -> alpha phase transformation is a crucial step in the solution-growth process of formamidinium-based lead triiodide (FAPbI(3)) hybrid organic-inorganic perovskite (HOIP) thin films for perovskite solar cells (PSCs). Because the addition of cesium (Cs) stabilizes the alpha phase of FAPbI(3)-based HOIPs, here our research focuses on FAPbI(3)(Cs) thin films. We show that having a large grain size in the delta-FAPbI(3)(Cs) non-perovskite intermediate films is essential for the growth of high-quality alpha-FAPbI(3)(Cs) HOIP thin films. Here grain coarsening and phase transformation occur simultaneously during the thermal annealing step. A large starting grain size in the delta-FAPbI(3)(Cs) thin films suppresses grain coarsening, precluding the formation of voids at the final alpha-FAPbI(3)(Cs)-substrate interfaces. PSCs based on the interface void-free alpha-FAPbI(3)(Cs) HOIP thin films are much more efficient and stable in the ambient atmosphere. This interesting finding inspired us to develop a simple room-temperature aging method for preparing coarse-grained delta-FAPbI(3)(Cs) intermediate films, which are subsequently converted to coarse-grained, high-quality alpha-FAPbI3(Cs) HOIP thin films. This study highlights the importance of microstructure meditation in the processing of formamidinium-based PSCs.