Strain-induced structural distortion, octahedral tilting, and bandgap non-linearity in 2D Butylammonium Tin Iodide Perovskites: A GGA+SOC Approach

Global sustainability challenge requires an immediate shift toward renewable energy resources. Two-dimensional perovskites with long-life stability can be tuned by strain engineering to modulate the structural and electronic properties thus achieving higher solar power conversion efficiency. Herein, we have investigated the influence of biaxial strain (±3% and ±6%) on structural distortion, octahedral tilting, and bandgap tuning of monolayer Butylammonium Tin Iodide (BA₂SnI₄) perovskite through the first-principles density functional theory calculations. The prototype structure has a single sheet of corner shared inorganic SnI₄ metal cages sandwiched between Butylammonium (BA) organic spacer cations offering an optically favorable direct bandgap of 1.457eV, centered at Γ-point. Equivalent biaxial strains (ε_xx and ε_yy) are applied along with the directions [100] and [010] of x- and y-axes respectively, by fractions starting from ε = −6% to ε = +6% using a step size of 3%. The lattice parameters a and b of BA₂SnI₄ are constrained to various values different from their equilibrium lattice parameters, keeping the c parameter invariant. Both tensile and compressive strains (±3% and ±6%) applied along the x-axis result in smaller bandgaps than strains oriented along the y-axis. The cationic and anionic dominance at conduction and valence bands support the defect tolerant tendency, with no trap states in the bandgap. The organic components constitute a larger bandgap than the inorganic part with satisfied quantum and dielectric confinement. Spin-orbit coupling is explored as a more influential phenomenon than the strain effect to tune electronic properties. The strain-induced competing effects of octahedral tilting and structural distortion with bandgap bowing and spin-orbit coupling provide a systematic strategy to tune bandgaps of different optoelectronic device applications.