Altermatt Lecture:   The Solar Spectrum


4.1:  The influences of Earth's atmosphere on the solar spectrum

The AM0 spectrum applies only to solar cells mounted on satellites (when normalised to S = 1366.1 W/m2) or on space crafts (when at a distance r to the sun, there holds Sr = S⋅1AU/r2). However, solar cells for terrestrial applications face a rather different solar spectrum, which cannot be approximated by the AM0 spectrum or the black body spectrum.

This is so because, when light penetrates the earth atmosphere, the AM0 spectrum is changed due to the following main mechanisms:

  • Absorption due to molecules and particles in the earth atmosphere.
  • Scattering at molecules and particles in the atmosphere.
  • Reflection from the earth surface.
  • The "air mass" the light goes through, depending on the elevation of the sun over the horizon.

Accordingly, the terrestrial solar spectrum is variable, as we know from daily life: the sun changes color and intensity all the time. For these reasons, the photovoltaic community has adopted standard spectra for measuring the efficiency of solar cells. Without standards, a cell that responds well to red light would show a diminished performance when tested under a bluish solar spectrum.

Check your initial knowledge before proceeding to these topics by clicking on the 'Quiz' tab. Knowing both your right and wrong assumptions beforehand helps you to learn more effectively.


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