The design and optimization of photovoltaic (PV) solar-energy-converting systems are strongly determined by the sunlight spectrum and by the fact that there is no back connection between a receiver and the Sun. However, there is a possibility of varying the operating concentration ratio (in other words, the operating current density of the pn-junction). In contrast to this, in thermophotovoltaic (TPV) systems, the optimization may imply choosing the emitter spectrum and returning the radiation from the receiver which has not been used back to the emitter surface supplying it with ‘additional’ power. Nevertheless, there are many common features in PV and TPV systems.