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)2GeX4 and (BA)2SnX4 (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)2MCl4, (BA)2MBr4 to (BA)2MI4 (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)2GeX4 materials show less lattice constants by ∼3–4% along a axis and ∼2–4% along b axis, respectively, than those of (BA)2SnX4 materials. Moreover, both the 2D tin and germanium halide perovskites have direct band gaps; (BA)2GeX4 exhibits values of 1.54, 2.10, and 2.55 eV, while (BA)2SnX4 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)2GeX4 and (BA)2SnX4 are 0.15–0.23/0.16–0.27 me and 0.14–0.18/0.14–0.19 mh at Γ–X and 0.16–0.29/0.12–0.14 me and 0.22–0.35/0.12–0.14 mh at Γ–Y, respectively, about half of those in BA2PbI4 and BA2PbBr4. 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 [GeX6]4– and [SnX6]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.