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

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─PbI₂ 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).