10.3.2
10.3.3
absorbed in all layers of the solar cell. All the absorption in layers other than the absorber
layer is loss. It is called the parasitic absorption. Further, due to the limited thickness of
the absorber layer, not all the light entering the absorber layer is absorbed. Incomplete
absorption in the absorber due to its limited thickness is an additional loss that lowers the
energy conversion efficiency. The incomplete absorption loss can be described by the
internal optical quantum efficiency IQE op , which is defined as the probability of a photon
being absorbed in the absorber material. Since there is a chance that a highly energetic
photon can generate more than one electron-hole pair, we also define the quantum
efficiency for carrier generation η G which represents the number of electron-hole pairs
generated by one absorbed photon. Usually η G is assumed to be unity.
Solar cell collection losses
Not all charge carriers that are generated in a solar cell are collected at the electrodes. The
photo generated carriers are the excess carriers with respect to the thermal equilibrium and
are subjected to the recombination. The carriers recombine in the bulk, at the interfaces,
and/or at the surfaces of the junction, as discussed in Chapter 7. The recombination is
determined by the electronic properties of the materials that form the junction, such as
density of states within the bandgap because of trap states. The trap density N T strongly
influences the minority-carrier lifetimes.
The contributions of both the electronic and optical properties of the solar cell
materials to the photovoltaic performance are taken into account in the absolute external
quantum efficiency that we already defined in Chapter 9. The EQE can be approximated
by
where IQE el denotes the electrical quantum efficiency and is defined as the probability that
a photo generated carrier is collected.
When we take the shading losses and the EQE into account, we find the short circuit
current density to be
where the * denotes averages across the relevant wavelength range and J ph is as defined in
Eq. (10.13).
Additional limiting factors
We have seen in Section 10.2 that the V oc of a solar cell depends on the saturation current
10.3.3
absorbed in all layers of the solar cell. All the absorption in layers other than the absorber
layer is loss. It is called the parasitic absorption. Further, due to the limited thickness of
the absorber layer, not all the light entering the absorber layer is absorbed. Incomplete
absorption in the absorber due to its limited thickness is an additional loss that lowers the
energy conversion efficiency. The incomplete absorption loss can be described by the
internal optical quantum efficiency IQE op , which is defined as the probability of a photon
being absorbed in the absorber material. Since there is a chance that a highly energetic
photon can generate more than one electron-hole pair, we also define the quantum
efficiency for carrier generation η G which represents the number of electron-hole pairs
generated by one absorbed photon. Usually η G is assumed to be unity.
Solar cell collection losses
Not all charge carriers that are generated in a solar cell are collected at the electrodes. The
photo generated carriers are the excess carriers with respect to the thermal equilibrium and
are subjected to the recombination. The carriers recombine in the bulk, at the interfaces,
and/or at the surfaces of the junction, as discussed in Chapter 7. The recombination is
determined by the electronic properties of the materials that form the junction, such as
density of states within the bandgap because of trap states. The trap density N T strongly
influences the minority-carrier lifetimes.
The contributions of both the electronic and optical properties of the solar cell
materials to the photovoltaic performance are taken into account in the absolute external
quantum efficiency that we already defined in Chapter 9. The EQE can be approximated
by
where IQE el denotes the electrical quantum efficiency and is defined as the probability that
a photo generated carrier is collected.
When we take the shading losses and the EQE into account, we find the short circuit
current density to be
where the * denotes averages across the relevant wavelength range and J ph is as defined in
Eq. (10.13).
Additional limiting factors
We have seen in Section 10.2 that the V oc of a solar cell depends on the saturation current