Environmental Engineering Reference
In-Depth Information
740 nm, as observed before. The increase in
J
sc
value from 12 to 24
mAcm
−
2
highlights the expected results from this improved device.
The third stage is, therefore, to optimise the doping concen-
tration at the front end of the device, and this must be carried
out using a molecular beam epitaxy (MBE) technique. The MOVPE
method cannot control the background C doping to improve the
collection of charge carriers created by UV and visible light. Since
the built-in electric field is weak in the front region of
∼
0.87
μ
m,
∼
×
10
14
to
∼
the optimisation (reduction of C doping to
5
5
×
10
15
cm
−
3
) of C doping will show a further improvement of
J
sc
. The contribution arising from the IR end is comparatively
small due to the low density of photons available in the solar
spectrum. However, when the doping concentration in the front is
optimised, the contribution to the current density should increase
considerably due to the presence of a large number of photons in
thevisibleregionofthesolarspectrum.TheGaAs-basedp-njunction
devices have produced
J
sc
values of
∼
40 mAcm
−
2
[11]. The device
structures described in this chapter exploiting graded bandgaps,
impact ionisation, and impurity PV effect could, therefore, produce
current densities well beyond 40 mAcm
−
2
. This means that these
devices,onceoptimised,couldeasilyachievee
cienciesabove40%
(
V
oc
=∼
1,170 mV, FF
=∼
0.85, and
J
sc
>
40 mAcm
−
2
). Here lies
anopportunityforanexpertMBEgrowertotestthisthird,andfinal,
stageandproduceahigh-e
ciencysolarcelldevice.Asimilardevice
structure based on InGaN alloy system could even achieve better
results with this approach.
The I-V curve shown in Fig. 7.9 also indicates the possibility of
Fermi-level pinning in GaAs-based solar cells. The second growth in
this series used a GaAs wafer with a different crystal orientation to
that of the first wafer. These devices showed
V
oc
∼
900 mV instead
=
,
±
of expected
V
oc
15 mV. This vast difference is clearly
due to a different potential barrier present within the device. The
independent research by Bauhuis
et al.
[11] has also shown the
production of
V
oc
values in two different bands, 830-880 mV and
1,010-1,040mV.ThisraisesthequestionofwhethertheFermi-level
pinning is equally valid for GaAs-based solar cells, similar to those
observed for CdTe [12-15] and CIGS-based solar cells [16, 17]. This
subject is briefly described in chapter 11, since the experimental
evidence is appearing to indicate a similartrend.
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