Environmental Engineering Reference
In-Depth Information
contacts. If the device is a simple p-n junction, Cu should always
produce a good ohmic contact to p-CdTe and should improve
ohmic behaviour with aging, due to in-diffusion of this p-type
dopant, instead of forming highly resistive contacts with aging.
These experimental observations provide supporting evidence for
the proposed alternative model, and the p-n junction model fails to
explain the results. Because of the p-type doping of Sb and the low
diffusivity through the material due to its high chemical bonding,
Sb-containing electrical contacts should form superior and stable
contactstothissolarcell.ThishasbeendemonstratedforSb/n-CdTe
Schottky contacts, as discussed in section 4.3.3.
If group III elements, such as indium or aluminium, are used as
the contact metal, PV activity may still be observed with a large
series resistance and, hence, with a very small fill factor (FF). This
is due to the compensation effect introduced by n-type dopants (In
or Al) in the p-type CdTe surface layer. A series resistance of 2 M
has been observed for indium contacts, when copper-containing
contacts have shown only
∼
50
series resistance for 2 mm
diameter devices [8, 30]. In many situations, if the p-type surface
layer is very thin and the interactions of indium and aluminium
are considerable during the metallisation step, the metal contacts
completely consume the surface p-layer, reaching the n-type CdTe,
producing an ohmic contact. In these situations, there is no PV
activity observed for the device and the I-V curves show nearly
ohmic behaviour with low series resistance. These observations
provideparamountinformationtomoveawayfromthep-njunction
model and towards the proposed new device concept. This nearly
ohmic behaviour also indicates the weak rectifying property of the
n-CdS/n-CdTe hetero-junction.
4.5.7
Doping of CdS and CdTe Layers
The main improvement necessary in this type of device is the
reduction of series resistance to increase the FF. Very thoughtful
experiments have been carried out by some groups, on the basis
of the assumed p-n junction model in the past, and reported by
Dennison in 1994 [31]. A typical device contains about 1.5
μ
m
