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Artificial light sources

An important field of photovoltaics are indoor applications because for many small devices solar cells offer a convenient alternative to battery power. In fact, for calculators, remote controls, and watches they are already widely used as source of power. Other than in large scale energy production solar cells are competitive even at today's prices because of the fairly expensive and limited power of small batteries. An additional environmental asset is the saved waste of those batteries that can contain highly toxic materials.

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 to sunlight.


Sunlight
Metal halide lamp
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.
Fluorescent light with three narrow bands
Fluorescent light
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.

Figures 1 and 2 show that the spectral illumination density of typical indoor illumination sources is primarily located at wavelengths below 600 nm, just like the diffuse solar spectrum.

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.


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Additional infomation:

Spectral properties of different artificial light sources
Spectra of halogen and fluorescent lamps
Spectra of Na and Hg lamps