In this paper, we report a study of electron-transfer-layer (ETL)-free perovskite solar cells, which included the structure of the fluorine-doped tin oxide/absorber/hole transfer-layer (HTL)/contact. Interface defect layers were introduced at the absorber interfaces to take the effect of interface defect densities into account. The simulations in this work were performed using solar cell capacitance-simulator (SCAPS-1D) software based on recent experimental and theoretical data. Along with the analysis and optimization of the thickness and defect density, special attention was paid to the effect of the energy band offset (BO) of the absorber/HTL interface and the effect of carrier mobility in the HTL on the performance of the solar cells. The optimum thickness for the absorber was 500 nm. The energy BO of the absorber/HTL interface significantly affected the charge-transport properties of the device. A BO of -0.05 eV to 0.2 eV was determined to be optimum to establish effective carrier transport. However, the tradeoff between the BO of the absorber/HTL interface and carrier mobility was found to be crucial to achieving high performance. The power conversion efficiency increased from 13.83% to up to 19.58% for HTLs with carrier mobilities of 5 x 10(-6) cm(2) V(-1)s(-1) and 2 cm(2) V(-1)s(-1), respectively. Our investigations and findings will support the understanding of carrier transport mechanisms in ETL-free perovskite solar cells and the material development and selection of such devices.