Investigation of heavy thallium perovskites TlGeX3 (X = Cl, Br and I) for optoelectronic and thermoelectric applications: A DFT study

The present investigation aims to explore the structural, electronic, elastic, optical and thermoelectric properties of germanium-based heavy thallium halide perovskites TlGeX3 (X = Cl, Br and I) and to evaluate their potential application in optoelectronic and thermoelectric devices. Density Functional Theory (DFT) with GGA-PBE functional was applied within the CASTEP code for investigation of their properties except for the thermoelectric properties, for which the BoltzTraP code integrated with Quantum ESPRESSO was used. Results showed that the materials are chemically stable with optimized lattice constants found to be 5.26 angstrom, 5.54 angstrom, and 5.92 angstrom for TlGeCl3, TlGeBr3, and TlGeI3, respectively. The materials are direct (R -> R) semiconductors with bandgaps of 0.794 eV, 0.522 eV, and 0.419 eV, for the respective compounds using the GGA-PBE functional while the corresponding values of the bandgaps using the HSE06 approximation are 1.027 eV, 0.865 eV, and 0.436 eV, respectively. In addition, they are all predicted to be mechanically stable and ductile in nature according to Born stability criteria and Poisson ratio, respectively. The perovskites also possess high absorption and conductivity as well as low reflectivity, strongly indicating their potential applications in optoelectronics including photovoltaic. A comprehensive investigation of their thermoelectric properties also revealed that the studied perovskites are promising for applications in thermoelectric devices.