high rotation speeds over the temperature range from 4
to 40 C, while it suddenly stopped rotating at temperatures
greater than 40 C, thereby suggesting that the current
and voltage outputs from the DSSC significantly
decrease due to the increased loss in photogenerated electrons
resulting from enhanced charge transfer kinetics at
higher operating temperatures.
Since the intensity of irradiated light on solar cells is
directly related to variation in the cell temperature, we also
examined the effects of light intensity in conjunction with
operating temperature on the photovoltaic performance
of DSCCs. From Fig. 5a, we note that the Jsc values exhibit
a significant increase with increase in the light intensity.
However, no appreciable change in Jsc values for different
operating temperatures is observed at a given light intensity.
This is because of the increase in power input caused
by higher light concentration. From Fig. 5b, we note that
the Voc value logarithmically increases with increasing light
intensity. The Voc value relatively steeply increases for low
illumination conditions (i.e. 6100 mW cm2), and subsequently,
it gradually increases with increasing light intensity
for values greater than 100 mW cm2. In general, Voc
is proportional to the temperature, while the enhanced
charge recombination rate resulting from the reduction in
liquid electrolyte viscosity leads to a significant decrease
in Voc due to increase in the Io value, as can be inferred
from Eq. (1). The net result of these two contrasting effects
eventually leads to a significant decrease in Voc values at
elevated temperatures, as shown in Fig. 5b. The expression
for Voc is given as
V oc
nkT
q
ln
IscðXÞ
Io
ð1Þ
Here n denotes the diode quality factor, k the Boltzmann
constant, T the absolute temperature, q the electronic
charge, X the light intensity, Isc(X) the short circuit current
in light intensity of X, and Io the reverse saturation current.
Fig. 5c shows the variation in the fill factor (FF) values
as a function of light intensity. We note that the FF values
significantly decrease with increasing light intensity at all
operating temperatures. The reduction in FF values at high
light intensities appears to occur due to the enhanced