Environmental Engineering Reference
In-Depth Information
qualities will be a good starting point. Better crystallinity, lowest
possible number of defects, and the doping concentration close
to 10
16
cm
−
3
will be desirable for the window material. In the
case of n-CdS, the doping of the material with group III elements
or group VII elements will enhance the electrical conductivity of
the films. However, the introduction of some of these elements
may disrupt the crystallinity, increasing the number of defects, and
reducing the mobility of charge carriers and, hence, reducing the
required electrical conductivity. Obtaining the optimum electrical
conductivity is a challenging task, but every positive step will
contributetotheenhancementofthedeviceperformance.TheCdCl
2
treatment of the CdS layer has already shown the positive effects
of doping with chlorine. Alternative window materials such as ZnS,
InS, and ZnO should be explored. Any window layer which provides
a better substrate for CdTe to form larger grains, especially in the
forms of columns perpendicular to the substrate, will enhance the
device e
ciency.
The absorber layer (CdTe), of the order of
∼
2
μ
m thickness,
should have the desired semiconducting qualities such as better
crystallinity, larger grains, passivated grain boundaries, minimum
possible R&G centres, and the lowest possible background impu-
rities. All these desired properties will increase the charge carrier
mobility, enhancing the electrical conduction. The material layer
should retain n-type electrical conduction, and, hence, the ideal
dopants will be group III or group VII elements. The achievement
of doping concentrations in the desired region of
10
14
to
∼
5
×
10
16
cm
−
3
will be ideal for this device structure. Many factors
contribute to this ultimate doping concentration and, hence, to the
achievement of the optimum electrical conductivity for this layer.
This requires systematic experimentation on materials growth,
impurity identification and removal or passivation, doping, and
processing steps, including heat treatment. The processing steps,
including chemical treatments with cadmium halide solutions, heat
treatmentinair,andthewet-etchingprocessshouldproduceaCdTe
layerwithoptimumn-typepropertiestogetherwithap-typesurface
layer in the order of a few 100 A, to achieve the Fermi-level pinning
close to the valence bandmaximum.
5
×
