Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 236

Since within the energy gap model, the energy of the electron is represented on
the y-axis, in Fig. 6.5 c diffusion voltage is shown in opposite direction and does
therefore not match the distribution function shown above (Fig. 6.5 b).
6.1.6 Photovoltaic effect
If photons, the quantum's of light energy, hit and penetrate into a semiconductor,
they can transfer their energy to an electron from the valance band (Fig. 6.5 e). If
such a photon is absorbed within the depletion layer, the region's electrical field
directly separates the created charge carrier pair. The electron moves towards the
n-region, whereas the hole moves to the p-region. If, during such light absorption,
electron-hole-pairs are created outside of the depletion region within the p- or n-
region (i.e. outside of the electrical field), they may also reach the space-charge
region by diffusion due to thermal movements (i.e. without the direction being
predetermined by an electrical field). At this point the respective minority carriers
(i.e. the electrons within the p-region and the holes in the n-region) are collected
by the electrical field of the space-charge region and are transferred to the oppo-
site side. The potential barrier of the depletion layer, in contrast, reflects the re-
spective majority carriers.
Finally, the p-side becomes charged positively while the n-side is charged
negatively. Both, photons absorbed within, and outside, of the depletion layer
contribute to this charging. This process of light-induced charge separation is
referred as p-n-photo effect or as photovoltaic effect.
Thus, the photovoltaic effect only occurs if one of the two charge carriers cre-
ated during light absorption passes the p-n-junction. This is only likely to occur
when the electron-hole-pair are generated within the depletion layer. Outside of
this electrical field there is an increasing likelihood that charge carrier pairs cre-
ated by light get lost by recombination. This is more likely the greater the distance
is between the location of the generation of the electron-hole-pair and the deple-
tion layer. This is quantified by the "diffusion length" of the charge carriers inside
the semiconductor material. The term "diffusion length" refers to the average path
lengths to be overcome by electrons or holes within the area without an electrical
field before recombination takes place. This diffusion length is determined by the
semiconductor material and, in case of the identical material, highly depends on
the impurity content - and thus also on doping (the more doping the lower the
diffusion length) - and on crystal perfection. For silicon the diffusion length var-
ies from approximately 10 up to several 100 µm. If the diffusion length is less
than the charge carrier's distance to the p-n-junction most electrons or holes re-
combine (mathematically spoken: after having overcome a certain diffusion length
the number of light-induced charge carriers is reduced to 1/e ; after having covered
two diffusion lengths it is reduced to 1/e 2 etc. To achieve an effective charge car-
rier separation the diffusion length should be a multiple of the absorption length
of the solar radiation incident on a photovoltaic cell.