A new method is proposed to focus and trap sunlight inside thin-film solar cells. In this method, a nano-structure metasurface-based flat lens is integrated inside the silicon thin-film solar cell to focus the light in the active area of the cell. The flat lens is designed using generalized Snell’s laws, and its characteristics are investigated using three-dimensional numerical full-wave analysis. The designed flat lens is integrated in the solar cell, and the performance of the cell is numerically studied at different wavelengths as well as different angles of incidence, for both TM and TE polarizations. The numerical results show that the proposed method enhances the absorption in the cell in the visible and infrared ranges of the solar spectrum. The short-circuit current is numerically calculated, and the results show that it is increased for the solar cell with plasmonic metasurface by a factor of 1.22 and 1.15 for TM and TE polarizations, respectively. It is also shown that the short-circuit current enhancement reaches values of 1.47 and 1.25 for TM and TE polarizations, respectively, when using a dielectric metasurface lens in the solar cell.