Cu2SnS3 (CTS) semiconductor material owns very interesting absorbing properties that allow it to be effectively utilized in many thin-film-based heterojunction solar cells. In this work, using the ball milling method, a mixture of pure elemental powders is used to synthesize CTS material. Homemade undoped and Li-doped target pellets are used to form CTS-doped and CTS-undoped thin films. The crystallinity investigation of synthesized targets and thin films confirms the formation of CTS tetragonal phase. Compositional, morphologic, and optic studies are also made to examine the effect of Li atom incorporation on different properties of CTS thin films. These CTS thin films are also incorporated into heterojunction solar cells (Al/n-Si/CTS/Ag and Al/n-Si/Li0.03CTS/Ag). In addition, J–V characteristics of both CTS and Li0.03CTS solar cells are demonstrated and discussed in details. The effect of CTS thin films' thickness on the performance is studied using Solar Cell Capacitance Simulator (SCAPS-1D) program. The results of the calculation are discussed in details and compared with the experimental results. An increase in efficiency is achieved in solar cells based on Li-doped CTS thin films, and it is discussed in detail along with SCAPS-1D simulation that is carried out depending on the thickness of CTS thin films.