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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.
7.1.1
Incorporation of Impurity PV Effect
In real semiconductors, there are many defect levels present due
to impurities and native defects. It is also natural to have more
energy levels close to the growth substrate due to lattice mismatch
and other surface effects. In this device structure (Fig. 7.1), these
impurity levels exist close to the rear of the device, where the
growth has started from a GaAs substrate, and are used to convert
IR photons into useful charge carriers. Absorption of two or more
IR photons at different absorption stages could create one e-h pair.
In these events, holes in the valence band and the electrons in
the conduction band appear at two different times and are readily
separated by the effective slopes available throughout the device.
This process reduces the probability of detrimental recombination
of e-h pairs due to defects in the device. This new design, therefore,
Figure 7.1
A new design proposed for PV solar cells to maximise
optical absorption and minimise thermalisation and transmission losses by
combining the impurity PVeffect and impact ionisation.
