Lattice softness regulates recombination and lifetime of carrier in Germanium doped CsPbI2Br perovskite: First principles DFT and NAMD simulations

Perovskite materials are characterized by high concentration of intrinsic defects. These defects lead to non-radiative recombination of electron and hole pairs, which is one of the major factors leading to the poor performance of the optoelectronic devices fabricated from these materials. Using the first-principles method, we have demonstrated that energy loss due to non-radiative recombination is suppressed in defective CsPbI2Br (D-CsPbI2Br) with the doping of Germanium (Ge) in CsGexPb1-xI2Br (x ​= ​0.15). Bromine vacancy (VBr) in CsPbI2Br introduces localized states as seen in the PDOS, however, doping of Ge eliminates these states. Using nonadiabatic molecular dynamics (NA-MD) and time-domain density functional theory, we show that the lifetime of the charge carriers is extended in Ge-doped CsPbI2Br (D-GeCsPbI2Br) by a factor of 3 and the dephasing time shortens to 6.2 ns. The electron localization function (ELF) plots of bulk CsPbI2Br (B–CsPbI2Br) show that charge localization increases around the Ge atom and it decreases the interaction between Pb atoms. In D-CsPbI2Br, phonons from a wide range of frequencies participate in scattering, and weak non-adiabatic (NA) coupling between electron and phonon in association with phonon modes in the low-frequency spectrum is responsible for extending the lifetime of charge carriers. Strong dynamic disorder in molecular dynamics (MD) simulations points to softening of lattice in D-GeCsPbI2Br. The increase in lattice softness of the Ge-doped is confirmed by the reduced value of bulk modulus (13.2 ​GPa) in D-GeCsPbI2Br as compared to the value in D-CsPbI2Br (13.6 ​GPa). Computing inverse participation ratio (IPR) gives the degree of localization of charge in the valence-band maximum (VBM) and conduction-band minimum (CBM). We have investigated the behavior of IPR and carrier lifetime with temperature, and based on these results, we found that lattice softness and a lifetime of charge carriers increase in D-GeCsPbI2Br.