Solar Cells Based on Single PN Junctions - Nanophysics of Solar and Renewable Energy - page 131

Figure 6.1 (a) Sketch of N þ P junction solar
cell, designed for illumination from the left side.
The doping and dimensions are typical for a Si
cell, see Figure 317. (b) Light absorption into the
cell follows law I

constant a strongly depends on photon energy.
At statedN þ doping 10 25 m 3 , the Fermi level in
the N þ region actually will lie in the conduction
band, making the overall band bending larger.
(Courtesy of M. Medikonda). ¼ I o exp( a x), where decay

internal to the device defeats this desired outcome. Recombination of photogener-
ated minority carriers is undesirable, and surface recombination can be inhibited by
provision of a back surface field (BSF). In the case of the rear contact to the P-region
shown in Figure 6.1, this could be provided by doping acceptor impurities to produce
aP þ P junction at the rear. This would have the effect of raising the conduction band
edge at the rear of the cell and thus reflect minority electrons back into the bulk,
reducing the chance for surface recombination.
It is important to understand that the active region of the classic PN junction device
extends well beyond the depletion layer W , to include the hole diffusion length L p on the
left, and the (minority carrier) electron diffusion length L n on the right. The action of
light absorption is to greatly increase the minority carrier concentrations ( p n on the left,
n p on the right) leading to an enhanced reverse current density (Equations 3.65 - 3.67).
The photoelectric effect is essentially that a photon of light disappears and gives its