Density functional theory investigation of low dimensional hybrid metal halides incorporating 2-amino (methyl) pyridine for perovskite solar cells application

Light absorber layers incorporating three-dimensional (3D) perovskite structures are widely used in perovskite solar cells (PSCs). However, their long-term instability is primarily due to the presence of volatile organic cations such as methylammonium (MA, CH₃NH₃⁺) at the A-site limits their practical application. This research addresses the issue by exploring low-dimensional hybrid perovskites, substituting MA with a bulkier and more stable organic cation, 2-amino(methyl)pyridine (AMP), a type of bulky organic cation (BOC). The study systematically investigates the structural, electronic, and optical properties of (2-AMP)-based metal halide perovskites using Density Functional Theory (DFT) through the CASTEP computational code, employing LDA-CAPZ, GGA-PBE, and GGA-PBEsol functionals. Among the exchange correlation functionals, GGA-PBEsol produced lattice parameters for (2-AMP)PbI₄ with less than 4% deviation from experimental data. The lead-based compound showed a direct band gap of 1.92 eV, which reduced to 0.98 eV upon inclusion of spin–orbit coupling (SOC). Substitution of Pb with Sn and iodide (I⁻) with bromide (Br⁻) resulted in smaller lattice parameters and shorter bond lengths, confirming a denser crystal structure. Formation energy calculations revealed that (2-AMP)SnBr₄ exhibited the most negative formation energy (–5.3625 eV/atom), indicating enhanced thermodynamic stability compared to the Sn–I compound. This result confirms that dimensional reduction, alongside halide substitution, plays a critical role in enhancing material robustness. Electronic band structure analyses revealed that all studied compounds possess direct band gaps, with narrower gaps in Sn-based systems and slightly wider gaps upon halide substitution with bromide. Optical studies showed strong absorption in the UV-visible range for all compositions. Overall, this first principles study provides key insights into the impact of metal and halide substitution in 2D perovskites, highlighting the potential of (2-AMP)SnBr₄ as a structurally stable and efficient candidate for future photovoltaic applications.