Theoretical Investigation of the Role of Anion and Trivalent Cation Substitution in the Physical Properties of Lead-Free Zero-Dimensional Perovskites

Zero-dimensional (0D) perovskites have received in-creased attention in the last years due to their wide range of compositionsand suitable optoelectronic properties for several technologicalapplications. Here, we performed density functional theory calculationsto investigate the role of halide anion (X = Cl, Br, I) and trivalent cation(B = Bi, Sb) substitution in the structural, energetic stability,optoelectronic, and excitonic properties of lead-free 0D perovskitesusing the C20H20N4B2X10and C30H25N6BX6middot2H2Ocompoundsasprototype structures. We found a crucial role of the replacement of thehalide species in the investigated physical properties; that is, mostproperties change (decrease or increase) almost linearly from Cl -> Br -> I, while the effects induced by B are less pronounced. For example, 0Dperovskites with smaller halide species have higher energetic stability. The valence band is dominated by the X p-states, in which theelectron binding decreases from 3p -> 4p -> 5p, and hence, it yields a reduction of the band gap energy as Cl is replaced by Br andthen by I. The electronic band structures are veryflat, which can be explained by the 0D nature of the crystal structures (i.e., isolatedinorganic octahedra). As expected, spin-orbit coupling is more significant for I-based systems. As a consequence of quantumconfinement, the exciton binding energies of 0D perovskites (up to 456.95 meV) are about 2 orders of magnitude higher than theirconventional 3D counterparts. Finally, we found power conversion efficiencies to be less than 14% for all systems