Gap-Filling and Crystallization Kinetics Regulation in VTD/Spray Hybrid Perovskite Solar Cells

While vapor transport deposition (VTD) enables the growth of PbI2 films at an extremely high rate (>10 nm/s) under low vacuum conditions, the resulting PbI2 films exhibit a highly porous microstructure, which is detrimental to the quality of perovskite films formed via the subsequent spray deposition of formamidinium iodide (FAI). In this work, we developed a solvent modulation approach for ultrasonic spray deposition to improve the quality of the fabricated perovskite films. The Lewis base additive 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) was introduced into the isopropanol (IPA)-based precursor solution to modulate the perovskite crystallization process under spray conditions. DMPU was found to promote the formation of a stable DMPU-PbI2 intermediate phase while residing within the porous VTD-PbI2 films. This intermediate phase was also shown to preferentially form smaller particles, which can fill the pores in the PbI2 scaffold and lead to enhance device performance. Utilizing this strategy, an effective method to enhance the quality of perovskite films was successfully identified, thereby boosting the efficiency of perovskite solar cells fabricated using VTD technology from 12.7% to 13.83%. Furthermore, the unencapsulated devices that were optimized by DMPU maintained over 80% of their initial efficiency after 500 h of aging at 40–50% relative humidity, outperforming the nonoptimized devices.