Synergistic Isothiourea–Guanidine Additive for Achieving Stable Perovskite Solar Cells with a High Certified Quasi‐Steady‐State Output

Abstract Guanidinium and thiourea derivatives play significant roles in suppressing both shallow‐ and deep‐level defects, regulating perovskite crystallization, and leading to enhanced performance for perovskite solar cells (PSCs). Herein, an asymmetric isothiourea–guanidine hybrid dihydrochloride is designed by merging the two functional motifs onto a thiazole core to overcome the long‐overlooked competition between guanidinium and thiourea additives. Comprehensive characterizations reveal that the isothiourea arm selectively orients crystal growth along the (001) plane while effectively suppressing the formation of dimethylsulfoxide─PbI 2 and other deleterious intermediate phases, whereas the guanidinium counterpart immobilizes iodide ions via N─H···I hydrogen bonding, lowering ion‐migration activation energy. The resulting films exhibit suppressed defect densities, relieved residual strain, and an air‐stable black phase retained after 11 days of ambient aging. Consequently, MA‐free PSC delivers one of the highest certified quasi‐steady‐state output of 26.73% (p–i–n), a conventional 26.18% (n–i–p), and an indoor‐light champion of 44.60% (n–i–p). Notably, the devices retain >90% of their initial efficiency after 4000 h of continuous 1‐sun illumination (international summit on organic photovoltaic stability (ISOS)‐L‐1) and 2000 h of dark storage (ISOS‐D‐1).