Lead-free Perovskites TlGeClxBr3-x (x=0,1,2,3) as Promising Materials for Solar Cell Application: a DFT Study

Abstract

This study investigates the structural parameters and the electronic properties of cubic TlGeClxBr3-x (x=0,1,2,3) lead-free perovskites to evaluate their potential as absorbers in perovskite solar cell devices. Density Functional Theory (DFT) embedded in the Quantum Espresso code was used to calculate these properties. The results revealed that the compounds have optimized lattice constants of 5.244 Å, 5.336 Å, 5.416 Å, and 5.501 Å, for TlGeCl3, TlGeCl2Br, TlGeClBr2, and TlGeBr3 perovskites, respectively. In addition, the compounds are direct band gap (R→R) semiconductors with energy gap values of 0.847 eV, 0.683 eV, 0.556 eV, and 0.518 eV for the respective materials. It is important to note that the band gap of the perovskites reduces as a Cl− ion, two and three Cl− ions are replaced by a Br− ion, two and three Br− ions, respectively. The analysis of their projected density of states indicated that near the valence band maximum of the perovskites, Cl-3p and Br-4p states contributes the most to their total DOS. In contrast, the Ge-4p orbital is the most dominant state close to the conduction band minimum. Based on these energy gap values, the studied materials are promising candidates for lead-free perovskite solar cell devices, with TlGeBr3 projected to be more promising than the other three materials.