The semiconductor bandgap determines the optoelectronic properties of the semi­conductor material. A semiconductor’s bandgap has a significant influence on the properties (including absorption in the case of a solar cell) of devices produced from them. The bandgap defines the minimum amount of energy needed for an electron to jump from the valence band to the conduction band.

The value of the bandgap (Eg) is characteristic of each semiconductor. This value affects the properties of the solar cells produced from each semiconductor (McEvoy et al. 2003). For example, semiconductors are effectively transparent to photons of energy less than the bandgap energy as these photons have insufficient energy to excite an electron from the valence to the conduction band and hence are not absorbed.

The minimum room temperature bandgap energy values for some common semiconductors range from 0.67 eV for germanium (Lide 2005) to 1.35 eV for gallium arsenide (Lide 2005). The semiconductors used for solar cells should ideally have a bandgap energy close to the peak of the energy range of light in the AM1.5 spectrum (1-3 eV). Not all semiconductors are appropriate for the use in solar cells. The most suitable semiconductors will have a bandgap of about 1-1.6 eV(Wenham et al. 1994). Silicon, with abandgap of 1.12 eV (Lide 2005), is a good candidate material use in solar cells. Ideally, a solar cell should have a flat response to irradiance of different wavelengths. However, this is not usually the case as each will respond differently to different parts of the spectrum. A mea­surement known as the spectral response can characterize the quantum efficiency of the solar cell to different wavelengths of light.

The spectral response of a solar cell is defined as the short-circuit current (output current under short-circuit conditions) per unit power of incident monochromatic light, as a function of the wavelength of the incident light (Cuevas et al. 2002). The spectral response measurement shows how the solar cell will perform under dif­ferent spectral conditions and can have implications on which technology is deployed in the field. For example, Ruther et al. (2002) have shown that crystalline cells are more suitable for “red” spectra and that amorphous silicon solar cells are more suitable for “blue” spectra (Ruther et al. 2002). This can contribute to the better performance of a-Si:H cells during summer months and the better perfor­mance of c-Si cells during winter months, due to the seasonal variations in the spectra of light received by the solar cell (Ruther et al. 2002). Comparison and analysis of the spectral response measurements for different solar cell technologies enables the most appropriate solar cell to be deployed given the spectra of light they are likely to encounter.