Organic Cation Orientation Preferences in Hybrid Lead Halide Perovskites

The crucial role of organic cation orientation in shaping the optoelectronic properties of hybrid organic-inorganic perovskites (HOIPs) is revealed through a systematic investigation of a diverse set of organic cations, the orientational preferences of many of which are considered for the first time. 22 different organic cations were incorporated as A-site cations in the cubic, orthorhombic, and tetragonal crystal phases of . To explore a broad configurational space, these cations were randomly rotated, generating over 2500 structural variations, among which approximately 400 exhibited distinct symmetry. Density functional theory (DFT) calculations on these structures revealed that variations in organic A-cation orientation can induce formation energy and bandgap differences of up to 0.32 and 0.64 eV, respectively. Rather than aiming to reproduce exactly known experimental compounds for each perovskite, we used a unified modeling approach to systematically explore how the size and orientation of organic cations govern lattice distortion and electronic structure in perovskite frameworks. Through rotational screening, a new orthorhombic phase of the well-known (MA = ) was identified, in which MA cations are aligned along the [102] and [10 directions. Additionally, a novel pseudocubic triclinic perovskite, (TiZ = ), was discovered and validated as a stable perovskite based on its formation energy, bandgap, effective mass, mechanical properties, and dynamic stability.