Environmental Engineering Reference
In-Depth Information
Bias voltage (V)
Figure 4.14
The capacitance of glass/CG/CdS/CdTe/Au solar cells ob-
served at 1 MHz measurement frequency as a function of bias voltage. Note
the constant capacitance value of
∼
56 pF due to the presence of a fully
depleted solar cell device.
capacitance is observed, further parameters such as
φ
b
or doping
concentrationcannotbeextractedfromconventionalSchottky-Mott
plots.Thishowever,indicatesthelowdopingconcentrations(below
∼
10
15
cm
−
3
) in the CdTe layer, producing a depletion width greater
than the complete device thickness [37].
A remarkably high short circuit current density observed for
some batches of this solar cell deserves a detailed discussion.
The theoretical calculations carried out for p-n junctions of single-
bandgap materials show a maximum possible current density of
∼
25 mA/cm
2
[20, 21]. However, when two or more bandgaps are
present in the device structure and two or more active junctions
are added to enhance the internal electric field of the device, the
situation is completely different. The number of photons absorbed
increases rapidly and avoid the device getting heated. This is due to
the presence of several bandgap materials reducing thermalisation.
Similarly, the separation and collection e
ciency increases due to
