First-Principles Study on Stability and Photoelectrical Properties of Hybrid Perovskite Systems FA2BBr4–xIx (B = Pb, Sn, Ge; x = 0, 1, 2, 3, 4)

The excellent photovoltaic efficiency of hybrid organic–inorganic perovskite materials has garnered significant attention in the field of solar cells. Herein, we have constructed FA2BBr4–xIx (B = Pb, Sn, Ge; x = 0, 1, 2, 3, 4) perovskites based on the experimentally reported FA2PbBr4 perovskite. The investigations on their geometries, stability, electronic properties, and optical absorption characteristics have been carried out using first-principles calculations. The obtained formation enthalpies and ab initio molecular dynamics simulations confirm that all 15 systems exhibit good thermodynamic stability. The substitution of Sn/Ge on B-site Pb and I doping on X-site makes the bandgap of perovskites narrowed, especially Sn-based ones. The obtained bandgaps of FA2SnBrI3 and FA2SnI4 are 1.44 eV, belonging to an optimal bandgap range used in single-junction solar cells. Moreover, I doping can decrease the effective mass of the carriers. The absorption spectra of perovskites show that I-doping results in a significant redshift of spectra in the visible and near-ultraviolet regions. The replacement of Sn on Pb has expanded the range of optical absorption to cover the entire visible spectrum region. Additionally, the dielectric functions, including its real and imaginary parts, are all increased through B-site Sn/Ge substituent and X-site Br–I hybrid, thus enhancing the optical performance of systems. Among all of 15 systems, FA2SnBr4–xIx perovskites are promising as the leading materials in tandem perovskite solar cells or as the absorbing layer in single-junction perovskite solar cells, and FA2GeBrI3 and FA2GeI4 perovskites might be useful in tandem perovskite solar cells. These results provide some insights into comprehending the diverse properties of mixed halide perovskites.