Environmental Engineering Reference
In-Depth Information
structures were fabricated side by side, close to each other, on
the same sample. These results were obtained for 2 mm diameter
circular contacts.
The thickness of the CaF
2
layer was only
∼
30 A, as measured
using a quartz crystal thickness monitor, so as not to affect
J
sc
and
FF. For ultra-thin CaF
2
layers of this type, the overall e
ciency is
enhanced due to the improvement of
V
oc
,butthee
ciencywas
observed to be reduced as the thickness of CaF
2
increased due to
introduction ofadditional series resistance to the device.
There is an independent report published by Karpov
et al.
in 2003 confirming these MIS-type electrical contacts improving
deviceparameters[42].Thisgroupelectrodepositedpoly-anilineon
the CdTe surface before metalisation and observed a considerable
enhancement of the
V
oc
value. The same group also dipped a
glass/CG/CdS/CdTe surface in red wine and applied a small DC
voltage across the sample with respect to an anode. After this
process, the completion of the device was made by drying the
surface and metalisation. This procedure enhanced the
V
oc
values
considerably by increasing to
∼
750 mV from their initial values of
∼
200 mV. In this case, the flavonoid particles (insulating organic
particlescontainedinredwine)havedepositedontheCdTesurface,
formingan insulatinglayer. Inboththese cases, poly-anilineandthe
flavonoidlayer act as insulatinglayers in these MIS structures.
4.6.3
A Multi-Layer Graded Bandgap Approach
The introduction of a third layer with an intermediate energy
bandgap will enable the strengthening and smoothing of the slope
of the energy band diagram, which is the internal electric field, for
the enhancement of the charge carrier collection [8]. A schematic
diagram and an energy band diagram of such a device are shown
in Figs. 4.17 and 4.18, respectively, and CdSe, with an intermediate
bandgap of 1.70 eV, would be an ideal candidate since the element
Cd is common to all three semiconductors used. During annealing
treatments, the intermixing at hetero-junctions will take place and
the device structure will become a graded bandgap multi-layer
tandemdevicewithbettercollectioncapabilities.Thesedeviceswill
improve at their hetero-junctions with aging and absorb a major
