Environmental Engineering Reference
In-Depth Information
improve
V
oc
.Butinrealdevices,
J
sc
isdrasticallyreducedand,hence,
V
oc
decreases.
There is another factor which affects the value of
n
of a Schottky
diode. In the case of metal-insulator-semiconductor (MIS)-type
interfaces, the
n
value and
φ
b
increase as the insulating layer is
incorporatedandthiseffecthelpsinincreasingthe
V
oc
ofasolarcell
device [15].
Inaddition,thetemperatureofthecellalsoaffectsthe
V
oc
.Asthe
temperature decreases, the
V
oc
value increases due to minimisation
of the thermal agitation of charge carriers. Therefore, in order to
increasethe
V
oc
ofaSchottkybarrier-basedsolarcell,improvement
of the
φ
b
by the incorporation of an insulator, the reduction of the
temperature of the cell, and the minimisation of the R&G process
within the cell structure are desirable.
1.5.4
How to Maximise J
sc
The short circuit current density depends on the number of photo-
generated charge carriers and their separation and collection rates
in the external circuit. This requires e
cient photon absorption
from almost all regions of the solar spectrum by the materials used,
effective carrier generations, and the high quality of the depletion
region formed and hence the strength of the built-in electric field
created. To increase the photo-generated charge carriers, impurity
PV effect and impact ionisation can be combined together. With the
right device design, the impurity PV effect can be used to create
e-h pairs using surrounding heat energy. These will be described
later, in chapters 6, 7, and 8, and are demonstrated by the latest
experimental results. In addition, the detrimental R&G process and
anyotherleakagepathsmustberemovedfromthedevicestructure.
To maximise the value of
J
sc
, the series resistance of the complete
device must beminimised.
1.5.5
How to Maximise FF
To improve the shape of the I-V curve, and hence the FF, the total
seriesresistanceshouldbeminimised(ideally
R
s
=
0).Thisincludes
resistance introduced by the electrical contacts, interfaces within