The thermoelectric effect is defined in literature as conversion of flow of thermal energy into flow of electrical energy due to replacing the thermal potential of the flowing thermal energy by an electric potential or vice-versa. Following an entropy approach, it was possible to prove a common nature of the flow of thermal and electric energies as flow electromagnetic waves. According to this entropy approach it is possible to modify the previous definition of the nature of thermoelectric effects. Following the same entropy approach and reviewing experimental results, it is found also in this study modified definitions of the photoelectric and photovoltaic effects.