Altermatt Lecture:   The Solar Spectrum


3.3:  There are a lot of photons in the infrared!

The left graph shows the AM0 spectrum as energy, the right graph as photon flux [1]. Comparing these two graphs, you see that in the infrared (IR) range of the spectrum, there is a very large number of photons. Actually, more than half of all solar photons are in the IR. This is so because photons have less energy in the IR than in the blue. This can be quantified with the relation:

E equals hc on lambda

where E is the photon's energy [eV], ν is its frequency [s–1], ω is the angular frequency 2πν [s–1], λ is the wavelength, c is the speed of light (299,792,458 m/s), h is Planck's constant (4.13566733 x 10–15 eVs), and "hbar" is a handy abbreviation for h/2π.

Hence, per given radiation energy, there are more photons in the IR than in the blue part of the spectrum.

That there are many photons in the IR range of the solar spectrum has important consequences to photovoltaics. We will see in a future lecture on photogeneration that silicon absorbs IR photons only weakly, so any improvement in the IR absorption causes a significant increase in the photo-generated current.

The PV community has developed sophisticated methods to improve silicon's absorption in the IR by means of light trapping.

Spectral irradiance of AM0

Figure: The AM0 spectrum, where spectral irradiance is plotted against wavelength (left) and photon energy (right). Data from [1].


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