Perovskite solar cells (PSCs) have experienced a decade of intense investigation as a promising photovoltaic technology, having a power conversion efficiency of 25.5% with the use of lead based light harvesters. Recently, inorganic cesium cation based mixed halide perovskites (CsMI_3−xBr_x, M = Pb or Sn) as absorbers in PSCs gave promising results; in particular CsPbI₂Br demonstrated improved thermal stability and carrier transport properties. However, the performance is far-off from the theoretical limit due to intriguing issues such as high defect density (N_t) and energy level mismatch. Such barriers can be overcome through device modelling and unraveling the kinetics. Here we have employed a computational approach to design and investigate CsPbI₂Br based solar cells by elucidating the role of defect density in the performance and further different types of hole transport layers via optimizing their valence band offset and the barrier height at the back contact was also probed. By optimizing such parameters for CsPbI₂Br, power conversion efficiencies of 17.71, 17.44 and 17.54% using CuSCN, PTAA and spiro-OMeTAD as hole selective layers respectively can be reached. Furthermore, lead free CsSnI_3−xBr_x (0 < x < 3) based PSCs were simulated and the effect of band gap variation as a result of the Br content was studied on the performance. The influence of the defect density of the absorber layer (CsSnIBr₂) at interfaces was studied, and with optimized defect density CsSnIBr₂ based PSCs gave an efficiency of 20.32% with a V_oc of 1.35 V when SnO₂ was used as the electron transport layer and spiro-OMeTAD as the HTM. Our approach suggests ways for experimental design protocols to achieve high performance inorganic Pb and Sn based PSCs.