Environmental Engineering Reference
In-Depth Information
thick CdTe and a negligibly thin (50-100 nm) CdS layer grown on
the CG substrate. Therefore, the main body of this device contains
1500 nm of CdTe, and if this material is assumed to be p-type, the
most sensible doping is with a p-type dopant in CdTe to reduce the
resistance.AllthreeNa,Cu,andAgarewellestablishedp-dopantsin
CdTe, as well documented in de Nobel's thesis [32] and by Zanio in
1978 [33], and these elements have been added with fine control
to the CdTe layers. The most unexpected results were observed
with an increased shunt resistance, increased series resistance of
the diodes, and, hence, a drastic reduction of the e
ciency mainly
due to loss in the FF. This clearly shows that the main body of
n-type CdTe material becomes resistive due to self-compensation
during doping with p-type dopants such as Na, Cu, and Ag. This
experimental evidence confirms the new model proposed for this
device and cannot be explained using the assumed p-n junction
model.
The positive effects on device performance of CdCl
2
treatment
[34] of both CdS and CdTe layers have been puzzling researchers
during the past few decades. In the CdCl
2
treatment, according to
the new model, chlorine clearly helps the n-type doping of both
semiconductor layers, reducing the series resistance and, hence,
improving the FF drastically. This treatment also helps to keep the
CdTe surface rich in Cd, which is necessary for producing high-
quality Schottky barriers at metal/n-CdTe interfaces (see section
4.3.3).Theremayalsobeotherbenefits,suchasthecementingeffect
of chlorine to form larger grains, reducing grain boundaries, but
there are contradictory reports indicating substantial improvement
of e
ciency without any observable grain size improvements [18,
19]. There are also reports on the basis of PL measurements that
halogens are effective in reducing mid-gap defect levels in CdTe.
All previous reports did not even consider the possibility of n-
type doping of the CdTe layer by chlorine since all the explanations
are based on the already accepted p-n junction device model. It
should be noted that chlorine (a halogen) is a well established and
reliable n-type dopant for CdTe material [32, 33]. This most crucial
CdCl
2
treatment needed for drastic device improvement, therefore,
provides strong supporting evidence forthe new model.
