Compositional engineering is considered one of the recent interesting techniques used in the field of perovskite solar cells (PSCs). In this method, more than one material was used in a specific cation in the perovskite structure. This work aims to simulate the cesium-containing triple-cation perovskite (TCP) via the SCAPS-1D simulation program with a device structure of ITO/SnO2/TCP/Spiro-OMeTAD/Au. First, we studied the effect of interface defects on the PCSs with respect to experimental results and found that when no interface defects occur, the power conversion efficiency (PCE) reaches a value of 22.16% which is higher than the reported PCE, implying that the fabricated cell suffers from the interface defects as a main effect on cell degradation. Incorporating interface defects into the simulation results in a very good match between the experimental and simulated data with a PCE of 17.92%. Further, to provide possible routes to enhance the performance of the solar cell under investigation, impacts of absorber layer thickness, conduction band offset (CBO), surface recombination velocity, and light intensity were explored. In addition, hole transport layer (HTL)-free design was investigated to alleviate the instability issues associated to the organic HTL, leading to a PCE of 18.28%, for a surface velocity of 104 cm/s, which is interestingly higher than the initial cell. The provided study reveals the critical role of interface defects and other key design factors and suggests potential solutions to alleviate the subsequent degradation mechanisms, thereby enhancing the overall cell performance.