[en] A theoretical approach based on Drift-Diffusion equations is presented to study planar mixed lead halide perovskite solar cells. Updated physical parameters such as permittivity, mobility, effective density of states and doping density is employed in simulations. Current-voltage curve data for two experimental sample is imported and through fitting with the model, density of bulk and interface defects is calculated. We obtain the bulk defect density around 10¹⁶ cm⁻³ and surface recombination velocities in the range of 10 cm/s. These values which are in good agreement with experimental measurements and considerably deviated from previous theoretical studies, verify the model and adopted constants. Shockley-Queisser limit is also presented as the ideal device and the effect of bulk and interface defects are presented as loss factors that cause departure from this limit. Our simulations conclude that the overall efficiency of perovskite solar cells is mainly governed by the open-circuit voltage and also identify the interface defects as the major loss factor in these devices.