An intermediate-band solar cell (IBSC) is a photovoltaic device conceived to exceed the limiting efficiency of single-gap solar cells thanks to the exploitation of the electrical and optical properties of intermediate-band (IB) materials. This type of material takes its name from the existence of an extra electronic band located in between what in ordinary semiconductors constitutes its bandgap, EG. The IB divides the bandgap EG into two forbidden energy intervals (subbandgaps), EL and EH as drawn in figure 7.1. Although in the figure, the lowest of the intervals involved, EL, has been drawn above the IB, to have located it below the IB would have not make any conceptual difference. For reasons that will become clearer shortly, it will also be required for this IB to be half-filled with electrons. This is the reason why, in the diagram in figure 7.1, we have drawn the Fermi level, corresponding to the structure in equilibrium, crossing the IB. Due to this feature, we also refer sometimes to this IB as possessing a metallic character.