Environmental Engineering Reference
In-Depth Information
Figure 1.16
Multi-layer graded bandgap solar cell structures possible on
n-type and p-type window materials.
PV effect to increase charge carriers, minimise R&G mechanisms,
and effectively separate and transport charge carriers through the
external circuit. The best candidate to satisfy all these conditions is
thegradedbandgapmulti-layersolarcellstructure,asshowninFig.
1.16, based on n-type and p-type window materials. Chapters 6, 7,
and 8 are devoted to the discussion of these devices together with
the latest experimental results to indicate their high potential.
It may be useful at this stage to note that the graded bandgap
structures are based on a large number of p-p- or n-n-type hetero-
junctions and two ohmic contacts to the front and the back of the
device. The front of the solar cell has wide bandgap n-type or p-
type material, and the bandgap gradually decreases as it progresses
towardsthebackofthesolarcell.Theelectricalconductiontypealso
graduallychangesfromn-typetop-typeorp-typeton-typethrough
the body of the device structure. The devices based on p-type
window materials have potential barriers for electron transport,
approximately equal to the bandgap of the wide bandgap window
material, and hence the highest achievable
φ
b
. This means that the
structures on p-type window materials are capable of achieving the
highest possible
V
oc
value sinceit is a function of the
φ
b
.
1.7 Summary
PV solar energy conversion consists of four main stages — (a)
absorption of a major part of the solar spectrum, (b) effective
