Environmental Engineering Reference
In-Depth Information
the enhancement of the internal electric field. In device structures
described in this topic, the window material has a bandgap of 2.42
eV(CdS)andtheabsorbermaterialhasabandgapof1.45eV(CdTe).
In addition, there are varying bandgap material layers consisting
of alloys between CdS and CdTe formed during fabrication. As
described in chapters 6, 7, and 8, this device structure is capable
of creating charge carriers using impurity PV effect. This device
structure with optimum conditions is capable of utilising infrared
radiation from surroundings to create charge carriers in the device
(see chapter 8).
Theobservationofhigh
J
sc
isanunusualexperimentalresultthat
can be observed from time to time. When CdS/CdTe solar cells are
fabricated,the
J
sc
valueshowsadrasticvariation.Forsomebatches,
these values can be as low as a few mA/cm
2
. A majority of the
batches produces
J
sc
values in the 20-30 mA/cm
2
range. However,
certain batches exhibit
J
sc
values in excess of 40 and 50 mA/cm
2
[8, 35]. For some iodine-doped devices,
∼
60 mA/cm
2
has been
observed, but consistency and reproducibility remain unsolved to
date. It should be stressed that this is not due to the variation
of light intensity from the solar simulator but genuinely arising
from the device itself under some conditions of materials growth
and device fabrication. This is a typical behaviour of electronic
devices and indicates that these devices can be further developed
once the materials issues and device physics have been thoroughly
understood.
Independent work by Romeo's group [38] provides further
confirmation of doping with halogens. This group grows CdTe using
the close space sublimation and introduces halogen-containing
gases — freon (CHF
2
Cl) — into the chamber during the growth.
This method produces thin-film CdTe, producing small-scale device
e
ciencies of about 16%. The growth of CdTe at an elevated
temperature in the presence of halogen atoms (Cl and F) produces
n-type doping and any other benefits provided by halogens in CdTe
and, hence, produces high-e
ciency devices. These advances by PV
researchers confirm the benefits of halogen doping in CdTe. This
group also use group-V elements such as Sb and As in order to
produce stable back electrical contacts.
