Strain-induced bandgap engineering in CsGeX 3 (X = I, Br or Cl) perovskites: insights from first-principles calculations.

Based on density functional theory and following first-principles methods, this paper investigated the electronic structures, densities of states, effective masses of electrons and holes, and optical properties of CsGeX 3 (X = I, Br or Cl) perovskites under triaxial strains of -4% to 4%. The calculated results show that the tuning range of the bandgaps of the CsGeI 3 , CsGeBr 3 , and CsGeCl 3 perovskites are 1.16 eV, 1.64 eV, and 1.63 eV, respectively. This result shows that the bandgap of the CsGeX 3 perovskite is tuned over the entire visible spectrum by applying strain. Also, it is found that the change of the bandgap is caused by the change of the Ge-X long bond. Besides, the optimal bandgaps of CsGeI 3 and CsGeBr 3 can be achieved by applying compressive strains, providing theoretical support for adjusting the bandgaps of CsGeX 3 perovskites. The effective masses of electrons and holes of CsGeX 3 perovskites decrease gradually with the strains changing from 4% to -4%, which is conducive to the transmission of electrons and holes. In addition, the optical properties of CsGeX 3 perovskites change from redshifted to blueshifted under different strains.