Study of bromine substitution on band gap broadening with consequent blue shift in optical properties and efficiency optimization of lead-free CsGeIBr based perovskite solar cells

Lead-based perovskite solar cells have experienced tremendous growth and achieved an outstanding power conversion efficiency (PCE) of 27.4% during the last decade. However, lead poisoning has remained a matter of concern for commercialization. Therefore, researchers are looking for alternative perovskite materials free from lead. Cesium-based perovskite material CsGeIBr may be a promising alternative due to its favorable optical conductivity and light absorption coefficient. To understand the atomic level calculation of perovskite solar cells (PSCs), a detailed model of interaction between the electrons and the interface is strongly anticipated. The optoelectronic property of the perovskite absorber layer has the most significant impact on device performance. Using Density functional theory (DFT), we can precisely predict the behavior of charge transport layers, including the active perovskite layer. In this work, we have done first-principles calculations based on DFT to analyze the electronic and optical properties of lead-free full inorganic CsGeIBr perovskite compounds. In addition, we incorporate DFT-extracted values of the electronic band gap, the effective density of states, and the optical absorption spectrum in the Solar cell capacitance simulator (SCAPS-1D) program to understand the device performance with the variation of thickness and total defect density of the perovskite layer. We obtained the value of the energy bandgap as 1.363 eV for CsGeI, 1.5795 eV for CsGeIBr, 1.7493 eV for CsGeIBr and 1.885 eV for CsGeBr. The CsGeI-based device performs best and achieves maximum power conversion efficiency (PCE) of 27.63%. It was observed that while increasing the doping concentration of Br in CsGeIBr perovskites, the bond length decreases, and consequently, the bandgap increases. Also, as the doping concentration increases, a substantial blue shift was observed in the calculated optical conductivity and absorption spectra.