The types of interaction between matter and radiation as well as the kind of energy conversion are determined by the properties of material. Energy conversion through photovoltaic route requires an increase in potential energy of the electron. It requires that the material possesses various energy levels (or energy bands) separated from each other. Energy differences between levels should be more than the room temperature energy of electrons given by k7 (-26 meV at 300 K). Difference in energy levels keeps the excited electrons in higher energy levels for longer periods than its relaxation time to the ground state (or unexcited state). This increases the probability of charge separation and hence the extraction of work from the device. Metals have continuous energy bands and are not suited for this application. Energy bands in insulators are too far from each other and the sun’s photons that reach the earth surface do not have enough energy to excite electrons to higher energy levels.
Energy bands in materials, called semiconductors, are separated from each other in a special manner. Their conductivity falls between that of metals and insulators. And since the separation between the energy bands varies from one semiconductor to the other, their conductivity can be varied over a large range by adding impurities and by optical excitation. For instance, the energy gap for InAs is small, about 0.36 eV, while the energy gap for CdS is large, about 2.42 eV. Photons in the sun’s spectrum have energy in the range 0.3 eV to 4.5 eV, high enough to excite electrons in semiconductors to higher energy levels. Such control over conductivity makes them suitable for electronic device applications and solar photovoltaics.