Environmental Engineering Reference
In-Depth Information
7.4 Discussions
This chapter has been devoted to presenting the advances made
with a new device structure for PV solar cells. Its performance has
beenexperimentallyexploredusingawell-researchedGaAs/AlGaAs
system. Thermalisation losses have been minimised using a graded
bandgap multi-layer solar cell structure. By starting with a large
bandgap material in the front and a gradual reduction of the
bandgap towards the back of the solar cell, a major part of the solar
spectrum can be absorbed while reducing thermalisation losses.
This approach has a cooling effect for the solar cell with effective
absorption of the major part of the solar spectrum and the creation
of enhanced charge carriers.
This multi-layer graded bandgap device structure also utilises a
major part of IR radiation, minimising transmission losses. Close to
the back end of the solar cell, the abundance of naturally occurring
defects plays a very useful role in creating e-h pairs, combining
impurityPVeffectandimpactionisation,inadditiontotakingpartin
the detrimental R&G process. Although the defects take part in two
competing processes, R&G has been suppressed to a certain extent
and the impurity PV effect has been enhanced by the shape of the
device structure. The carriers are readily accelerated and separated
by the slope of the device, and photo-generated carriers are kept
away from each other (see Fig. 7.12), reducing the recombination
even further. The defect levels present at different depths take part
in pumping electrons to the conduction band through the impurity
PV mechanism. These defect levels also combine impurity PV and
impact ionisation due to accelerated electrons in the conduction
band to create more e-h pairs using IR radiation. Therefore, this is
an excellent way of reducing transmission loss by converting the
presence of defects into thedevice's advantage.
Finally, the losses due to both series and shunt resistance
have also been minimised. The series resistance due to materials
have been minimised by growing epitaxial layers with the well-
established MOVPE method, and the contact resistance has been
minimised by using well-established low resistance front and back
contacts to AlGaAs and GaAs [6, 5]. The leakage resistance has been
maximised by minimising leakage paths using epitaxial material
layers.
