Altermatt Lecture:   The Solar Spectrum


3.2:  Intensity or photon flux?

The amount of radiation can be quantified in various ways. In photovoltaics, we usually use either

  • light intensity, also called energy flux or irradiance, which is energy per area per time, typically [W/m2] or [mW/cm2]; or
  • photon flux, which is the number of photons per area per time [cm–2s–1].

→ How to convert a spectrum from intensity to photon flux.

The figures below show the AM0 spectrum plotted either as intensity (top) or photon flux (bottom).

It is convenient to quantify the photon flux Nph in the equivalent units of electric current, Jph = q⋅Nph [mA/cm2], because Jph equals the electric current that a cell can deliver if it converts every photon into a free electron–hole pair.

Spectral irradiance and photon flux of AM0

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


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