Environmental Engineering Reference
In-Depth Information
by Potter
et al.
in 2000 [16] and also by X-ray diffraction and
energy dispersive X-ray spectroscopy studies by Terheggen
et al.
[17] in 2001. Fritsche and co-workers [18, 19] reported, in 2001, a
comprehensive and systematic study of the band off-sets at hetero-
junctionsinvolvedinthisdevice.Theirfindingsusingphotoemission
experimentsaresimilartothebanddiagramproposedinthismodel
excepting the most crucial Fermi-level pinning situation at the n-
CdTe/metal interface.
Underoptimumconditions,suchadevicestructuremayproduce
large currents, in excess of the theoretically predicted maximum
current density of
25 mA/cm
2
(Loferski, 1956; M'baye, 1980) for
a p-n junction solar cell based on a single bandgap of CdTe [20, 21].
In addition, this solar cell structure benefits from the impurity PV
effect creating e-h pairs by making use of defect levels within the
bulk of the CdTe and closer to the CdTe/metal interface. Photons
with energy less than the bandgap (1.45 eV) are, therefore, able to
create e-h pairs, as shown in Fig. 4.9. In this case, since there is a
strong electric field for charge carrier separation within the device,
R&G centres are favourably used to create more charge carriers to
produce a higher current in the external circuit. In fact, the ladder
of defect levels (E
1
-E
5
) at the back metal/CdTe interface creates
useful charge carriers using low-energy IR photons through the
process of multi-step charge carrier promotion. According to this
new model, CG forms an ohmic contact to n-CdS and the back metal
contact forms a large Schottky barrier at the metal/CdTe interface.
In contrast, the p-n junction model requires a completely opposite
formation of an ohmic contact at the back metal/CdTe interface.
∼
4.5 Description of Experimental Results Using the
Two Models
It is now worth comparing the two device concepts, as shown
in Fig. 4.10, to describe and understand experimentally observed
results from different techniques. This will allow the identification
ofkeyinformationnecessarytorecognisetheneedofanewconcept
for deep understanding and, hence, further development of this
device.
