High‐Performance Perovskite Solar Cells with Large Grain‐Size obtained by using the Lewis Acid‐Base Adduct of Thiourea

Recently, perovskite solar cells (PSCs) have been rapidly developed, counting as the most promising alternative to the Si solar cells. The fabrication of perovskite films with controlled crystallinity and grain size is critical for highly efficient and stable solar cells. In this work, thiourea (TU) serving as a Lewis acid‐base adduct is introduced into the CH 3 NH 3 PbI 3 precursor. A smooth and large grained perovskite crystal is obtained without the intermediate phase PbI 2  · SC(NH 2 ) 2 using the ideal thiourea amount. Thiourea, through forming MAI · PbI 2  · DMSO · thiourea in perovskite precursor solution, significantly impacts the perovskite crystallinity and morphology, as proved using X‐ray diffraction patterns and infrared spectroscopy. Light harvesting, suppressed defect state, and enhanced charge separation and transport of the perovskite absorber layer are improved. The optimum performance of perovskite solar cells with TU demonstrated a power conversion efficiency (PCE) of 19.80%, an average steady‐state PCE of 18.60% and potent stability under ambient air.