Perovskite solar cells (PSC) offer solution-processible low-cost fabrication in photovoltaics and are in focus to provide efficiencies similar to Silicon-based solar cells. This study proposes an optimized design for FA0.85Cs0.15Pb(I0.85Br0.15)3 photoactive layer-based PSC, and reports the highest power conversion efficiency (PCE) for this absorber layer so far for which incorporating formamidinium (FA+) and cesium (Cs+) cations reduces defect density in the bulk and at contact interfaces, improves stability, enlarges grain size, and minimizes nonradiative recombination losses. SCAPS-1D was employed to simulate device performance by using different electron transport layers (ETL) and hole transport layers (HTL), film thicknesses, defect density, and doping concentrations. ZnO and MoO3 were identified as optimal ETL and HTL, respectively, for the considered perovskite. Optimal thicknesses for the electron and hole transport layer and absorber layers were found to be 100 nm, 100 nm, and 1000 nm, respectively. Further optimization of absorber thickness, defect density, and doping concentration showed an optimal combination of the device FTO/ZnO/ FA0.85Cs0.15 Pb(I0.85Br0.15)3/MoO3/Au with a PCE of 27.35%, short circuit current density of 24.23 mA/cm2, open circuit voltage of 1.25 V, and a fill factor of 89.70%. The proposed framework thus provide a valuable pathway for systematically optimizing the performance of PSCs.