ABSTRACT
Theoretical calculations using density functional theory methods show that germanium halide perovskites have high absorption coefficients as well as similar absorption spectra and carrier transport properties as the lead analogs. Potential candidate for the substitution of lead in the perovskite structure is the group-14 metalloid germanium. Germanium halide perovskites, however, have rarely been investigated experimentally, which is presumably due to the chemical instability upon oxidation of the divalent germanium cation. Due to the reduced inert electron pair effect, this oxidation stability issue is even more prominent in germanium-based perovskites than in tin-based ones. CsGeI3 and CH3NH3GeI3 have already been implemented as absorber materials in meso-structured perovskite solar cells yielding power conversion efficiency values of 0.11 and 0.20%, respectively. Goldschmidt tolerance factor calculations support the formation of germanium halide perovskites, as shown for CH3NH3GeX3 (X = Cl, Br, I) compounds with tolerance factor values of 1.005, 0.988, and 0.965, which coincide with t values reported for the ideal perovskite structure.
