For indoor use the photovoltaic cells must be adopted to the prevailing conditions because the irradiation will be different from outdoor applications. If natural light is present the exposure is likely to be only the diffuse part of the solar spectrum. In the previous section it was shown that this is much less intense and also centered more towards the blue region of the visible spectrum.
Also, indoor applications are often expected to work under artificial
illumination like incadescent, halogen, or fluorescent light sources. The
properties of such sources are much different from sunlight. Below, the
spectral power densities of some typical indoor light sources are compared
Figure1: Compared to sunlight, the metal halide lamp shows superimposed halogen emission lines. The low intensity at long wavelengths is probably due to a broad IR filter. Usually incadescent lamps emit most of their radiation in the red and IR part of the spectrum. This is also the case for metal halide lamps, although their higher operating temperatures shift the intensity maximum slightly towards shorter wavelengths.
Figure 2: The spectra of the fluorescent lamps consist mainly of indvidual emissions of the glow discharge, often neon or noble gas mixtures. The broad background depends on the individual fluorescent bulb coating.
For maximum performance of the solar cell the bandgap of the absorber should be adopoted to the illumination. For solar cells in outdoor applications the range of ideal bandgaps is betwen 1.0 and 1.6 eV.
For indoor applications the absorber material should make use of a higher
bandgap for a better conversion of the short wavelength intensities.
Consequently, a widely used material for indoor photovoltaics is amorphous
silicon (a-Si) with a bandgap of approximately 1.8 eV.