Lattice activation for air deposition of micrometer-grain Pb-Sn perovskite film realizing efficient and stable ideal-bandgap solar cells

Lead-tin (Pb-Sn) perovskites with an ideal bandgap of 1.34–1.40 eV show great promise in perovskite solar cells (PSCs). Recently, to address the environmental pollution and Sn²⁺ oxidation problems of dimethyl sulfoxide, methylammonium acetate (MAAc) ionic liquid has been developed as an alternative to fabricate ideal bandgap MAPb_0.7Sn_0.3I₃ (1.36 eV) film via hot-casting in air. However, the spontaneous crystallization of Pb-Sn perovskite initiated by heat-induced supersaturation is fast and random, setting critical challenges in regulating crystal growth during the film-forming process. Herein, a lattice activation strategy is developed to control the crystallization dynamics of MAPb_0.7Sn_0.3I₃ in MAAc to produce films with micrometer-sized grains in air. FA is shown to activate the crystal lattice that facilitates the formation of intermediates and balances the crystal growth of MAPb_0.7Sn_0.3I₃, producing films with a grain size of 2.78 ± 0.17 μm. Furthermore, 4-fluoro-phenethylammonium and phenethylammonium are adopted to passivate the defects in the film and promote the energy level alignment at the top interface, respectively. The optimized PSC device achieved an efficiency of 18.24% with a short-circuit current of 29.84 mA/cm², which are both the highest values in 1.36 eV Pb-Sn PSCs to date. Notably, the unencapsulated devices show excellent storage and air stability under various conditions.