Enhanced Stability and Band Gap Tuning of α‑[HC(NH₂)₂]PbI₃ Hybrid Perovskite by Large Cation Integration

We report room-temperature synthesis of lead- and iodide-deficient α-[HC­(NH₂)₂]­PbI₃ perovskites (abbreviated d-α-FAPI, FA⁺ = formamidinium), with the general formula (A′,FA)_1+x[Pb_1–xI_3–x] (with A′ = hydroxyethylammonium (HEA⁺) or thioethylammonium (TEA⁺) cations, 0.04 ≤ x ≤ 0.15). These materials retain a 3D character of their perovskite network despite incorporation of large HEA⁺ or TEA⁺ cations, demonstrating that the Goldschmidt tolerance factor can be bypassed. We found that thin films of (TEA,FA)_1+x[Pb_1–xI_3–x] (x = 0.04 and 0.13) show exceptional α-phase stability under ambient conditions, 1 order of magnitude higher than α-FAPI and α-(Cs,FA)­PbI₃ thin films. d-α-FAPI phases are shown to maintain a direct band gap, which increases monotonously for x ranging from 0 up to 0.20, with characteristics of a p-type semiconductor for low concentrations of vacancies (x ≤ 0.13) and n-type for larger ones. They offer alternatives to reach the methylammonium- and bromine-free stable α-FAPI-type phase and open new avenues in the field of perovskite solar cells, up to band gap tuning desirable for tandem solar cells.