BA₂SnI₄ as a Promising 2D Ruddlesden–Popper Perovskite for Optoelectronic Applications

Lead-free two-dimensional (2D) hybrid metal halide perovskites (MHPs) emerge as promising eco-friendly alternatives to lead-based counterparts, offering excellent thermodynamic stability and environmental compatibility despite lower solar harvesting efficiency. Nonetheless, the dearth of research on tin-based MHPs illustrates the difficulties in their optoelectronic characterization. Herein, we present a computational protocol that integrates ab initio and semiempirical approaches to investigate the electronic, optical, and excitonic properties of the scarcely explored BA2SnI4, where BA represents butylammonium. We apply a cost-effective computational framework that combines a maximally localized Wannier function tight-binding (MLWF-TB) method for electronic states with the Bethe–Salpeter equation (BSE) for excitonic properties. To improve the accuracy of the electronic band gap, we employ a relativistic quasi-particle correction (DFT-1/2) within the density functional theory (DFT) framework, which also includes van der Waals corrections and spin–orbit coupling effects. Our findings reveal that replacing Pb with Sn weakens exciton binding, leading to lower exciton binding energies and improved charge extraction efficiency. These results indicate a band gap of 2.0 eV, an exciton ground state energy of 1.85 eV, and an exciton binding energy of 150 meV. The BSE calculations also predict a redshift in absorption, extending the spectral response further into the visible range, compared to the independent particle approximation (IPA). 2D RP BA2SnI4 perovskite is a promising photovoltaic material since even in ultrathin films smaller than 0.25 μm, this material can achieve PCE values near 25%, close to the Shockley–Queisser limit. Although Sn-based 2D MHPs present advantageous features, more work is required to resolve manufacturing issues and enhance performance stability.