Large dielectric constant, high acceptor density, and deep electron traps in perovskite solar cell material CsGeI₃

Many metal halides that contain cations with the ns² electronic configuration have recently been discovered as high-performance optoelectronic materials. In particular, solar cells based on lead halide perovskites have shown great promise as evidenced by the rapid increase of the power conversion efficiency. In this paper, we show density functional theory calculations of electronic structure and dielectric and defect properties of CsGeI₃ (a lead-free halide perovskite material). The potential of CsGeI₃ as a solar cell material is assessed based on its intrinsic properties. We find anomalously large Born effective charges and a large static dielectric constant dominated by lattice polarization, which should reduce carrier scattering, trapping, and recombination by screening charged defects and impurities. Defect calculations show that CsGeI₃ is a p-type semiconductor and its hole density can be modified by varying the chemical potentials of the constituent elements. Despite the reduction of long-range Coulomb attraction by strong screening, the iodine vacancy in CsGeI₃ is found to be a deep electron trap due to the short-range potential, i.e., strong Ge–Ge covalent bonding, which should limit electron transport efficiency in p-type CsGeI₃. This is in contrast to the shallow iodine vacancies found in several Pb and Sn halide perovskites (e.g., CH₃NH₃PbI₃, CH₃NH₃SnI₃, and CsSnI₃). The low-hole-density CsGeI₃ may be a useful solar absorber material but the presence of the low-energy deep iodine vacancy may significantly reduce the open circuit voltage of the solar cell. On the other hand, CsGeI₃ may be used as an efficient hole transport material in solar cells due to its small hole effective mass, the absence of low-energy deep hole traps, and the favorable band offset with solar absorber materials such as dye molecules and CH₃NH₃PbI₃.