Rational Design of Non-Centrosymmetric Hybrid Halide Perovskites.

Structural non-centrosymmetry in semiconducting organic-inorganic hybrid halide perovskites can introduce functionalities like anomalous photovoltaics and nonlinear optical properties. Here we introduce a design principle to prepare Pb- and Bi-based two- and one-dimensional hybrid perovskites with polar non-centrosymmetric space groups. The design principle relies on creating dissimilar hydrogen and halogen bonding non-covalent interactions at the organic-inorganic interface. For example, in organic cations like I-(CH ₂ ) ₃ -NH ₂ (CH ₃ ) ⁺ (MIPA), -CH ₃ is substituted by -CH ₂ I at one end, and -NH ₃ ⁺ is substituted by -NH ₂ (CH ₃ ) ⁺ at the other end. These substitutions of two -H atoms by -I and -CH ₃ reduce the rotational symmetry of MIPA at both ends, compared to an unsubstituted cation, for example, H ₃ C-(CH ₂ ) ₃ -NH ₃ ⁺ . Consequently, the dissimilar hydrogen-iodine and iodine-iodine interactions at the organic-inorganic interface of (MIPA) ₂ PbI ₄ 2D perovskites break the local inversion symmetries of Pb-I octahedra. Owing to this non-centrosymmetry, (MIPA) ₂ PbI ₄ displays visible to infrared tunable nonlinear optical properties with second and third harmonic generation susceptibility values of 5.73 pm V ^-1 and 3.45 × 10 ^-18 m ² V ^-2 , respectively. Also, the single crystal shows photocurrent on shining visible light at no external bias, exhibiting anomalous photovoltaic effect arising from the structural asymmetry. The design strategy was extended to synthesize four new non-centrosymmetric hybrid perovskite compounds. Among them, one-dimensional (H ₃ N-(CH ₂ ) ₃ -NH(CH ₃ ) ₂ )BiI ₅ shows a second harmonic generation susceptibility of 7.3 pm V ^-1 and a high anomalous photovoltaic open-circuit voltage of 22.6 V.