Efficiency limits of solar cells have been studied since a long time ago giving the maximal theoretical efficiency and capacity current generation for different solar photovoltaic (PV) devices, and at the same time, real solar cells are becoming more efficient. However, some gaps are still remaining to be further improved and get closer to the desired theoretical limit. Therefore, the optimization of the PV efficiency is possible only if the intrinsic and extrinsic losses are well defined, identified, quantified and then mitigated. In this paper, a brief overview of the research evolution of thermal losses is presented. A physical-driven model is used to quantify internal heat sources, optical losses and the actual performance of a PV device. The Full Thermal Model for thermal assessment is presented in detail and applied to a bifacial heterojunction solar cell.