Presented at the 4th Annual USF College of Engineering Undergraduate Research Symposium

1
CdTe Thin Film Solar Cell Characteristicsas a
Function of Temperature Andrew Quecan, Sara
Harrison, Vijay Padma, and Dr. Chris Ferekides
REU 2006 Poster EE.5
ABSTRACT As energy costs continue to rise and
sources of energy still have a profound impact on
international politics, alternative energy
development is an important, if not essential,
research discipline. Studied by The Lewis Group
at The California Institute of Technology, on a
global scale, solar energy has the potential to
practically produce over six hundred terawatts of
power. Of course, individual steps to this end
goal are being researched each day. To develop
more cost-efficient CdTe (cadmium telluride)
solar cells, the characteristics of those cells
are needed to enhance overall understanding. To
acquire a range of CdTe characteristics such as
IV curves and fill factors, a vacuum-chambered
cylindrical device was developed for testing a
range of temperatures (100K-400K). The device
uses a liquid nitrogen pump and flexible heaters
to vary the temperature of the solar cell while
conducting four-point measurements to obtain the
needed data for IV curves, fill factors, and
other useful characteristics. As expected, the
open-circuit voltage of CdTe cells increased as
temperature decreased. As more cells with are
tested with varying light intensities, more
characteristics will be attained to lead to a
more efficient solar cell.
METHODOLOGY 1. A single solar cell is placed
within the vacuum chamber device and connected to
a four-point instrument. 2. The chamber is
sealed and a rough vacuum with a rotary vane pump
is created to remove any moisture within the
device. 3. Testing begins after flexible heaters
raise the cells temperature to approximately
400K. A liquid nitrogen pump, connected to the
chamber, gradually reduces the temperature of the
cell. Temperature is reduced by varying the speed
of the liquid nitrogen pump. Temperature is
increased by varying the voltage of the flexible
heaters. 4. The J-V data is collected using a
four-probe measurement technique and a computer
controlled electrometer/power supply.
Measurements are taken in intervals of 10K. A DC
voltage sweep is applied at each interval for
voltages from -1.5V to 1.5V. 5. To obtain light
J-V characteristics of the solar cells, a light
source is introduced after initial voltage is
applied. The light source is held constant during
the DC voltage sweep. 6. To obtain C-V
characteristics of the solar cells, a frequency
of 3KHz is applied during a voltage sweep from
-2.5V to .5V. Same process described above is
used to obtain light C-V characteristics.
CdTe SOLAR CELLS Cadmium telluride (CdTe) solar
cells are heterojunction semiconductors, heavily
doped, so that it behaves as a one-sided pn
junction. Since Na gtgt Nd , for a p-type
semiconductor like CdTe, the total space charge
width is approximately equal to the space charge
width in the low-doped (n-region) space charge
width. Using these assumptions, junction
capacitance reduces to Therefore,
determination of capacitance will lead to doping
concentration. Solar cell performance
characteristics and efficiency are given by VOC,
ISC, and fill factor (FF). Open circuit voltage
(VOC) occurs when I0. Conversely, short circuit
current (ISC) occurs when V0. In addition,
optimal performance is when power is maximum
PMIMVM. Fill factor relates the rectangular
areas formed as a ratio As shown in Fig. 1
below, a CdTe solar cell is fabricated on a glass
substrate. The thin-film (SnO2F) increases
photoconductivity and reduces reflection. Cadmium
sulfide (CdS) is part of the heterojunction.
DEVICE DEVELOPMENT Initial design
considerations included a clean environment
created by a low pressure vacuum environment,
cooling and heating system, variability of the
intensity of light, and four-point measurements
for J-V data. A T-shaped vacuum chamber was
selected to alternate the three main variables on
each flange voltage, temperature, and light
intensity. Each flange was adjusted to provide
for these variables a glass flange was made for
light, while the other two flanges were drilled
and epoxied for the copper tubes and wires. A
brass cylinder was selected primarily for its
thermal conductivity and designed with a testing
platform for the solar cells and probes. Copper
tubing was installed within the cylinder and is
used as the cooling system while the liquid
nitrogen flows through the tubing and pumping
system. Flexible heaters were wrapped around the
cylinder to provide a heating mechanism. The
cylinder is placed inside the vacuum chamber,
which is pressurized by a rotary vane pump. A
four-point measurement system was developed after
designing probes that would be sufficient for
solar cells.
SUMMARY In order to obtain a better
understanding of cadmium telluride solar cells, a
cylindrical vacuum-chambered device was developed
to measure a range of CdTe characteristics.
Experimental results support the theory that the
open-circuit voltage of CdTe cells will increase
as temperature decreases. Further testing is
needed on CdTe cells in order to acquire their
complete range of characteristics. A better
understanding of CdTe cells will lead to the
development of a more cost-efficient solar cell.