Impact of Halogen Substitution on the Electronic and Optical Properties of 2D Lead-Free Hybrid Perovskites

Development of hybrid organic–inorganic lead halide perovskites is still hampered by the toxicity of lead and their structural stability in air. Hence, the research of lead-free alternatives is extremely important to promote the development of perovskite-based photovoltaics. Here, we construct lead-free tin (Sn)- and germanium (Ge)- based two-dimensional (2D) hybrid organic–inorganic perovskites, (BA) 2 GeX 4 and (BA) 2 SnX 4 (X = I, Br, and Cl), and explore their geometrical, electronic, and light absorption characteristics via first-principles calculations. The lattice constants will increase from (BA) 2 MCl 4, (BA) 2 MBr 4 to (BA) 2 MI 4 (M = Ge, and Sn), owing to the increase in halogen ionic radius in the order of Cl – (1.81 Å) < Br – (1.96 Å) < I – (2.20 Å). Meanwhile, (BA) 2 GeX 4 materials show less lattice constants by ∼3–4% along a axis and ∼2–4% along b axis, respectively, than those of (BA) 2 SnX 4 materials. Moreover, both the 2D tin and germanium halide perovskites have direct band gaps; (BA) 2 GeX 4 exhibits values of 1.54, 2.10, and 2.55 eV, while (BA) 2 SnX 4 exhibits values of 1.53, 1.98, and 2.49 eV, for X = I, Br, and Cl, respectively. In particular, the estimated effective masses of electrons/holes in 2D-(BA) 2 GeX 4 and (BA) 2 SnX 4 are 0.15–0.23/0.16–0.27 m e and 0.14–0.18/0.14–0.19 m h at Γ–X and 0.16–0.29/0.12–0.14 m e and 0.22–0.35/0.12–0.14 m h at Γ–Y, respectively, about half of those in BA 2 PbI 4 and BA 2 PbBr 4 . Furthermore, the density of states (DOS) and partial DOS results suggest that the contributions to the valence and conduction band are mostly made by the components of halogen p state orbitals and metal p state orbitals, respectively. Furthermore, the band-decomposed charge densities of the conduction band minimum and valence band maximum results imply that the distorted inorganic [GeX 6 ] 4– and [SnX 6 ] 4– octahedra have a direct influence on the band gaps. Our results indicate that this series of homologous 2D tin and germanium halide perovskites are a promising class of stable and efficient light-absorbing materials for photovoltaic and/or photoelectronic applications.