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.