Robot-Assisted Study on the Crystallization Pathway of FAPbI3 in Air

Organic-inorganic halide perovskites allowed for the most rapid increase in power conversion efficiency of single junction solar cells ever seen from any photovoltaic technology. For some time now, it has become a major goal for the community to fabricate and stabilize pure α-FAPbI3 due to its - compared to other perovskites - superior photovoltaic properties, such as small bandgap, long carrier lifetime and low defect state density. Unfortunately, the photoactive cubic α-phase is only metastable at room temperature and transforms into the photoinactive hexagonal δ-phase over time. Recent efficiency improvements of α-FAPbI3-based perovskite solar cells have been enabled by insights and application of compositional, intermediate phase and dimensionality engineering, all enhancing the phase purity and stability of α-FAPbI3.<br/><br/>In this talk, I will report on the investigation of the mutually influencing effects of precursor composition, thin film synthesis conditions, and annealing profile, on the crystallization process of FAPbI3 in a sequential two-step spin coating process in air. Thereby, we first screened the high-dimensional parameter space using a robotic spin coating platform for automated thin film synthesis, annealing and in-line optical characterization (UV/vis, photoluminescence). We paired the platform with external X-ray diffraction measurements and neural networks which are trained on the time-dependent annealing process to optimize the synthesis protocol for crystal quality and optical properties in a closed-loop.<br/>In a second step we used multimodal in-situ characterization techniques (GIWAXS, UV/vis, absorption) for certain composition/process conditions and annealing profiles to better understand the effects of described parameters on the crystal phase formation and phase stability of FAPbI3. These insights may help to develop a robust and reproducible synthesis protocol for high quality and stable α-FAPbI3 in air, which further propels the commercialization of perovskite solar cells.