On the synergistic interplay between annealing temperature and time and additive concentration for efficient and stable FAPbI3 perovskite solar cells

Formamidinium-based lead triiodide perovskite (FAPI) is the workhorse candidate for highly efficient perovskite solar cells owing to its ideal bandgap and phase purity. An essential condition to that is the formation of a pure α-black phase, commonly triggered by external additives such as methylammonium chloride (MACl). This results in the suppression of unwanted crystallization phases and defect reduction. However, there is no consensus on the exact experimental conditions (i.e. annealing time, temperature and concentration) for processing of the additives neither a defined relationship between them nor the resulting film morphology, leading to scattered knowledge on it and lack of thin film reproducibility. Here, we provide a systematic investigation on the simultaneous effect of MACl concentration, annealing time, and annealing temperature on the FAPI crystallization, and ultimately FAPI perovskite solar cell efficiency and stability. Finally, the optimized combination of such parameters, based on tests with 230 devices, leads to the realization of 22.7 % efficient inverted FAPI perovskite solar cells with enhanced stability.