Environmental Engineering Reference
In-Depth Information
Figure 6.2 (a) The increased densities of
electrons and holes under illumination can be
described by separate quasi-Fermi levels
photons. Energy runs vertically, with zero at
energy of free electron outside the device. This
could be called a PINdevice, with an (I) insulator
layer. Work function
j n
and
j p for electrons and holes, respectively. It is
the shift of these separate quasi-Fermi levels
that creates the output voltage of the cell, as
indicated in (a). The separation of the quasi-
Fermi levels arises as a competition between
photogeneration and recombination, which
may occur in the bulk or at surfaces.
Recombination is to be avoided. Internal
trapping and absorption of the light photons is
to be maximized. (b) A generalized solar cell
device, in which an interposed absorber layer
creates electrons and holes from incoming light
shows highest energy of
an electron in the device. Symbols x are electron
affinities in the different regions of the device,
while E C and E V , respectively, are the lowest
electron energy and the highest filled valence
band energy. The effect of light is to separate the
quasi-Fermi levels for electrons and holes,
indicating nonthermal light-driven
concentrations of electrons and holes. The
resulting shift of quasi-Fermi energies gives the
output voltage of the illuminated device [64].
j
single reflection occurs with 0.1 probability, the backscattering probability is reduced
to 1%, a large improvement. The photon that enters the silicon now has a larger angle
to the normal, making its path in the silicon longer before encountering the back
surface, and also increasing its chance for internal re
ection at that surface.
The re
ection coef
cient R is calculated at normal incidence by the formula
n 2 ) 2 . Taking values 3.5 and 1 for the index of refraction for silicon
and air, respectively, R
n 2 ) 2 /( n 1
R
¼
( n 1
þ
0.31, which is substantial, and an antire ection coating is
usually applied as mentioned above. The large index of refraction also implies that
¼
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