ABSTRACT
In order to effectively harvest photons from all spectral regions
(ultraviolet [UV], visible, and infrared [IR]) and combine the impact
ionisation and impurity photovoltaic (PV) effects, a multi-layer
graded bandgap solar cell structure was introduced in chapter
6. This structure is based on an n-type window material layer.
However, improved results can be obtained from device structures
starting with p-type window materials. As indicated in section 1.6
of chapter 1, these two structures are shown in Fig. 1.16. The
structure with p-type window material has additional advantages
over that with n-type windowmaterial. The potential barrier height
(φb) for electron flow through the device structure is determined
by the large bandgap of the window material. This is, therefore,
capable of producing high potential barriers and, hence, large Voc values. In addition, the electrons produced by high-energy photons
can accelerate and create more charge carriers due to impact
ionisation, because of the smaller bandgap present at the rear of the
solar cell. Because of these additional advantages, the experimental
testing was carried out only on this device structure with p-
type window material. This chapter presents the experimental
results achieved within only two growths using a well-researched
metal organic vapour phase epitaxy (MOVPE)-grown GaAs/AlGaAs
material system. GaAs/AlGaAs is the best-researched inorganic
material system next to Si. The author has, therefore, purposely
selected this well-explored system to test this new design proposed
for PV solar cells.
