We investigate the absorptance and the power conversion efficiency of a perovskite thin-film solar cell at a specular and Lambertian reflection on the cell's back-surface reflector. The analysis is done by means of the Monte-Carlo ray tracing simulations, complemented by the transfer-matrix method to account for the interference phenomenon in the local generation rate G (z) of carriers inside the semiconductor. This function is employed further in the transport equations to calculate the current-voltage characteristics of the cell. Taking the perovskite absorber CH 3 NH 3 PbI 3 as an example, we analyse its efficiency, altering the absorber thickness in the interval 50÷ 1000 nm. We find that in the cell (∼ 200 nm) the enhanced photon absorption due to the Lambertian reflection yields the efficiency increase on 2.2% as opposed to the specular reflection case. Moreover, the maximum efficiency, obtained for the perovskite layer of the thickness∼ 150 nm, exceeds the one obtained for the perovskite layer of the thickness∼ 500 nm with the specular reflection.