Environmental Engineering Reference
In-Depth Information
Table 8.2 Experimentally observed defect levels in MOVPE Al
x
Ga
(1
−
x
)
As
layers [8, 9]
Approximate Concentration (cm
−
3
)
Al Content (x)
Activation Energy, E
a
,
(eV)
10
14
-10
16
0.05-0.11
0.41
10
15
0.15
0.50
10
15
0.20
0.55
(0.00-0.25)
(0.80-0.90)
The varying EL2level according
to the value of x
device, especially at the back end, and enhances the combined PV
effect of the device. The production of charge carriers using IR
photons depends on native defect levels, especially those occurring
close to the growth substrate at the rear of the solar cell.
It is appropriate to look at the deep energy levels present in
the MOVPE-grown Al
x
Ga
(1
−
x
)
As layers. This material is the most
researchedsemiconductornexttoSi,andthethedeeplevelspresent
arethoroughlystudiedanddocumentedintheliterature[8,9].Some
oftheseareshowninTable8.2;therefore,itisclearthatthereexists
a ladder of deep levels in the bandgap. The concentration of these
defects must be higher, closer to the GaAs substrate. These levels,
therefore,mustbehelpingtocreatechargecarriersusingmulti-step
generation (or the impurity PV effect), utilising IR radiation from
surroundings.
Imagine the surrounding IR radiation promoting charge carriers
to higher defect levels and accelerated photo-generated charge
carriers promoting these electrons from defect levels to the
conduction band. This will be another form of impact ionisa-
tion taking place in the device to create more photo-generated
charge carriers in these devices. With this mechanism taking
place when illuminated, each incoming photon is capable of
producing more than one e-h pair. Therefore, this type of solar
cells should show quantum e
ciencies greater than unity when
illuminated.
The detrimental recombination and generation (R&G) process
also takes place in parallel, but the shape of this device allows
the impurity PV effect to dominate and positively contribute to
