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

Inorganic CsPbI 3 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, DMAPbI 3 ) templating method suffers from inefficient phase conversion, hindering high-performance PSC development. To overcome this limitation, we engineered the crystallographic orientation of DMAPbI 3 to promote rapid volatilization of DMAI and accelerating the transformation. Through in situ anchoring of Pb 2+ -complexing groups (-F < -Cl < -SO 4 ) on TiO 2 substrates during chemical bath deposition, we direct the preferential in-plane growth of Pb-rich (100) planes of DMAPbI 3 , enhancing its [100] orientation. Crucially, stronger complexing groups yield higher orientation degrees, accelerating thermal conversion into highly oriented CsPbI 3 perovskite with higher purity and better optoelectronic properties. This strategy enables carbon-based, hole-transport-layer-free CsPbI 3 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.