Ambient Blade Coating of Mixed Cation, Mixed Halide Perovskites without Drip: 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 the anti-solvent drip and mixed-cation mixed-halide compositions. Solar cell fabrication through low-cost scalable methods, such as blade coating cannot accommodate the anti-solvent drip and needs to be performed in ambient atmosphere. Consequently, their efficiency has lagged behind that of spin-cast devices, fabricated in inert atmosphere and with carefully timed anti-solvent drip 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 ℃, RH ≈ 50 %) without the benefits of anti-solvent drip 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. 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 drip 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 % in similar conditions without the benefit of anti-solvent drip. This study provides valuable insight into the crystallization pathway of mixed-cation mixed-halide formulations without anti-solvent drip in high-temperature processing conditions that enable the translation of perovskites toward upscalable ambient manufacturing in high throughput conditions.