First-principles insight into the photoelectronic properties of Ge-based perovskites

The crystal configuration, electronic structure, charge-carrier transport, and optical properties of Ge-based MAGeX(3) perovskites (MA = CH3NH3+; X = Cl-, Br-, and I-) and AGeI(3) (A = Cs+, MA, FA (HC(NH2)(2)(+)), MO (CH3C(NH2)(2)(+)), and GA (C(NH2)(3)(+))) were investigated using first-principles theory. The results showed that the increase in Ge-X bonds (from Cl- to I-) in MAGeX(3) increased the volumes, weakened the covalent coupling of Ge-X, lowered the bandgaps, reduced the electron and hole effective masses, and red shifted the absorption spectra. Different A cations in the AGeI(3) systems altered the package of perovskite crystals and thus significantly influenced the electronic and optical properties of those perovskites. Electronic property analyses revealed that the valence band maxima (VBM) of AGeI(3) perovskites were mainly contributed by the I 5p and Ge 4s orbitals, whereas the conduction band minima (CBM) were dominated by Ge 4p orbitals. In AGeI(3) perovskites, the bandgap increased and the absorption spectrum blue shifted in the sequence of Cs+ -> MA -> FA -> MO -> GA. Our results highlighted the effects of A and X on the photoelectronic properties of Ge-based perovskites.