A new approach is introduced and illustrated for optimizing the performances of photovoltaic cells. A thermal criterion, the minimization of the internal heat sources, is added to the usual criteria that consist of minimizing the optical and electrical losses. A proof of concept is delivered by means of modeling in the case of a standard crystalline silicon (cSi) cell for which the dependence on temperature of optical, electrical, and thermal properties is well known. A numerical code named TASC-1D-cSi simulating the optical-radiative, electrical, and thermal behaviors of cSi solar cells is used. Besides the current-voltage characteristics, this simulation tool provides the spectral variations of the thermalization, recombination, and radiative internal heat sources as well as the equilibrium temperature of the cell which depends on the outdoor conditions. The cell or the anti-reflection coating thickness is varied while the other parameters are prescribed. It is demonstrated in given outdoor conditions that considering the minimization of the internal total heat source in addition to the minimization of the optical and electrical losses modifies significantly the value of each of these parameters that maximizes the output power of the cell. For example, in a thermal condition with natural convection at the front and insulation at the back of the cell, the optimum cell thickness is found to be 55 μm and the optimum antireflection coating thickness 87 nm, instead of, respectively, 75 μm and 80 nm when the cell is maintained at 25 °C. These results suggest that a thermal design rule involving the internal heat source might be included in the improvement of solar cells at use and in the development of the next generation photovoltaics.