In this study, we prepared a multi-functional thin-film thermoelectric device for small-scale energy harvesting and self-powered light sensing. A comprehensive optimization was conducted in terms of thermoelectric material, device, and system integration. The thermoelectric thin films in the device were prepared using the sputtering method together with a post-annealing process. A Fresnel lens was incorporated into the thin-film thermoelectric device for energy concentration. In addition, a simulation procedure was used to facilitate the optimization of the heat sink, which can provide fast heat dissipation and mechanical support for the thin-film thermoelectric device. The electricity generation and light sensing performance was then characterized under solar irradiation, followed by a feasibility study for sensor application. The results indicate that our thin-film thermoelectric device exhibits high voltage output and greatly enhanced responsivity. This work presents a significant progress toward an integrated design of an applicable and multifunctional thin-film thermoelectric sensor for detecting sunlight intensity.