Ambient blade coating of mixed cation, mixed halide perovskites without dripping: in situ investigation and highly efficient solar cells†
Perovskite photovoltaics have made extraordinary strides in efficiency and stability thanks to process and formulation developments like anti-solvent dripping and mixed-cation mixed-halide compositions. Solar cell fabrication through low-cost scalable methods, such as blade coating, cannot accommodate anti-solvent dripping and needs to be performed in an ambient atmosphere. Consequently, their efficiency has lagged behind that of spin-cast devices, fabricated in an inert atmosphere and with carefully timed anti-solvent dripping to control nucleation and growth. In this study, we demonstrate formamidinium (FA)-dominated mixed-halide mixed-cation perovskite solar cells fabricated by blade coating in ambient air (T = 23 °C and RH ≈ 50%) without the benefits of anti-solvent dripping or a moisture-free environment. We investigated the solidification process during blade coating of single-cation (FAPbI3) and increasingly complex mixed-cation mixed-halide (FA0.8MA0.15Cs0.05PbI2.55Br0.45, MA is methylammonium) perovskites in situ using time-resolved grazing incidence wide-angle X-ray scattering (GIWAXS). We found that the perovskite precursor composition and the blade coating temperature profoundly influence the crystallization mechanism and whether halide segregation occurs or not. The inclusion of Br− suppresses the non-perovskite 2H phase, promoting instead PbI2 together with the intermediate 6H phase and 3C phase of FAPbI2.55Br0.45. Addition of Cs+ suppresses these intermediates and promotes the direct crystallization of the perovskite 3C phase FA0.8MA0.15Cs0.05PbI2.55Br0.45 when coating at elevated temperature, unlike when anti-solvent dripping is used at room temperature. Through control of ink formulation and coating conditions, we demonstrate blade coated perovskite solar cells with a champion power conversion efficiency (PCE) of 18.20% as compared with FAPbI3 perovskites, which yield a PCE of 12.35% under similar conditions without the benefit of anti-solvent dripping. This study provides valuable insight into the crystallization pathway of mixed-cation mixed-halide formulations without anti-solvent dripping under high-temperature processing conditions that enable the translation of perovskites toward upscalable ambient manufacturing under high throughput conditions.