Environmental Engineering Reference
In-Depth Information
glass
n-CdS
n-CdSe
n-CdTe
Back metal contact
Front contact (Transparent Conducting Glass)
Figure 4.17
A schematic diagram of the glass/CG(metal-1)/n-CdS/n-
CdSe/n-CdTe/metal-2 graded bandgap device structure.
part of the solar spectrum due to the effective utilisation of photons
at different regions of the device structure. The photons in the
IR region with energy less than 1.45 eV are also utilised within
the CdTe layer and closer to the back metal contact, as shown
in Fig. 4.18, to create photo-generated charge carriers using the
impurity PV effect. These carriers will be collected e
ciently, with
the existing high internal electric field in these thin-film device
structures. Theoretical e
ciency predicted for this type of devices
is high, andtherefore the future potential of this device is bright.
4.6.4
Dealing with Defects
It has been experimentally shown that there is a drastic effect
of defects on Fermi-level pinning at the metal/CdTe interface.
Although the exact origins of these defects are not yet clear, some
importantexperimentalevidencesareemerging.Whenthesurfaces
are prepared to have Te richness (Cd deficiencies) the mid-gap
defects, 0.65 (E
2
) and 0.72 eV (E
3
) are dominant. Therefore, Fermi-
level pinning produces low Schottky barriers, causing low
V
oc
and
thesolarcelldeviceperformanceispoor.Furthermore,theexistence
ofmid-gapR&Gcentresmaximisestherecombinationprocessinthe
device, minimising
J
sc
values of these devices.
