Directing intermediate phase crystallographic orientation promotes carbon-based CsPbI3 perovskite solar cells to beyond 20% efficiency

Inorganic CsPbI3 perovskite, known for its high chemical stability and near-ideal bandgap, offers a promising solution to the instability of organic-inorganic hybrid perovskites that limit perovskite solar cells (PSCs) longevity. However, the conventional intermediate phase (dimethylammonium lead iodide, DMAPbI3) templating method suffers from inefficient phase conversion, hindering high-performance PSC development. To overcome this limitation, we engineered the crystallographic orientation of DMAPbI3 to promote rapid volatilization of DMAI and accelerating the transformation. Through in situ anchoring of Pb2+-complexing groups (-F < -Cl < -SO4) on TiO2 substrates during chemical bath deposition, we direct the preferential in-plane growth of Pb-rich (100) planes of DMAPbI3, enhancing its [100] orientation. Crucially, stronger complexing groups yield higher orientation degrees, accelerating thermal conversion into highly oriented CsPbI3 perovskite with higher purity and better optoelectronic properties. This strategy enables carbon-based, hole-transport-layer-free CsPbI3 PSCs to achieve a record 20.72% efficiency (certified as 20.35%). Unencapsulated device retains > 85% of their initial efficiency after 1156 h of continuous maximum power point tracking under 1-sun illumination. Inorganic cesium lead iodide perovskites promise stable solar cells but are limited by slow conversion from a templating phase. Li et al. tuned crystal orientation using surface-bound lead-complexing groups to speed conversion and achieve efficient, durable devices.