Environmental Engineering Reference
In-Depth Information
rather than CG/CdS surfaces. This information is vital to recognise
thattherectificationpropertyrequiredforthePVeffectisnotdueto
the CdS/CdTe hetero-junction but due to the large Schottky barrier
formed at the back CdTe/metal interface. Various other reports
in the literature and authors' recent works highlight that better
performance is achieved with thinner CdS layers, in the order of
50 nm [30]. In fact, the new understanding is that the CdS layer
provides a better nucleation surface for CdTe to grow but the layer
is partially or completely consumed at this interface by the post-
deposition heat treatment of the device. Such a thin layer of 50 nm
of CdS will not survive in the presence of the 1500 nm (1.5
μ
m)
thick CdTe layer, to form an identifiable semiconductor, and hence
providea2.42eVbandgaptoformacleanp-njunction.Theinterface
chemistry taking place during about 5 hours of electrodeposition of
CdTe and post-growth heat treatment at 450
◦
C for 20 minutes will
formmixedphases,CdS
x
Te
(1
−
x
)
atthisinterface.However,iftheCdS
layeristhickenough,thiswillformagradedbandgapinterfacewith
bandgaps varying from 2.42 eV (CdS) to 1.45 eV (CdTe) due to the
partial consumption ofthe CdS layer in the middle.
4.5.6
Results From Electrical Contacting Work
SincetheSchottkybarrieratthemetal/CdTeinterfaceisdetermined
by Fermi-level pining, almost any electrical contact shows the PV
activity with different e
ciencies for this device, provided the
electrical contact used does not completely destroy the p-layer or
the Fermi-level pinning effect on the surface. For example, metals
such as Cu, Sb, and As, p-type dopants of CdTe, help to keep the
Fermi level close to the valence band and, hence, produce low-
resistance contacts. This will provide good results for freshly made
devices, but with aging, metals like Cu diffuse into the n-type CdTe
region (i.e., beyond the surface p-layer) and form a highly resistive
layer due to self-compensation. Therefore, the device will rapidly
deteriorate, showing an increased contact resistance. However, a
contact containing a small amount of Cu will reduce the contact
resistance due to p
+
doping of the surface p-layer without getting
into the n-CdTe region. For this reason, Cu/Ni or Cu/Au contacts
with less than
∼
5% Cu should act as low-resistance electrical
