Impurities Determine Phase Purity of Hollow Perovskite Thin Films

Abstract

Metal halide perovskites have emerged as leading materials in optoelectronics owing to their tunable bandgaps and outstanding charge-transport properties. Conventional bandgap modulation, achieved through halide substitution or incorporation of bulky A-site cations, often entails trade-offs between structural integrity and carrier mobility. Hollow perovskites offer a compelling alternative, accommodating bulky cations such as ethylenediammonium (En) while retaining the three-dimensional octahedral network crucial for efficient charge transport. Translating these superior bulk properties into thin-film architecture is underexplored. Here, we systematically compare two solution-based strategies for forming MAPbI₃-En hollow perovskite thin films: direct precursor mixing and a crystal engineering route utilizing redissolved hollow perovskite crystals. Despite stoichiometric equivalence (verified with ¹H NMR and SEM-EDX), direct-mixed films form lowdimensional EnPbI₄ impurities at En ≥ 40 mol%, evidenced by the presence of ~490 nm PL shoulders and low-angle XRD peaks. In contrast, crystal-engineered films exhibit a continuous hypsochromic shift in photoluminescence (764 ￫ 720 nm across 0-60 mol% En) while maintaining phase-pure 3D frameworks. ¹H/³¹P{¹H} NMR and DFT calculation reveal residual hypophosphorous acid (H₃PO₂, HPA) as the key modulator: preferential interaction with En²⁺ species (E_binding = -0.75 eV) forms spontaneous MAI-enI₂-PbI₂-HPA intermediates with a negative formation energy, suppressing EnPbI₄ nucleation during spin-coating. In-situ absorption spectroscopy during annealing confirms HPA directs 3D crystallization by inhibiting low-dimensional phases at supersaturation onset. This work highlights the role of solution chemistry to achieve phaseselective vacancy-engineered perovskites.