FIELD OF INVENTION:
The present invention relates to the field of measurement of electrical
properties in general and to measurement of current-voltage(I-V) characteristics
of solar cell of varying sizes at different temperatures in dark and under
illumination.
BACKGROUND AND PRIOR ART:
Solar cells are essentially large area diodes having certain added features
that enable generation of electric power on being exposed to visible light. The
efficiency of solar cells, a direct measure of its ability to convert incident light
into electricity, is of paramount importance in most of the applications and
therefore needs to be measured as accurately as possible. In these
measurements, apart from the light intensity, the cell temperature also needs to
be precisely kept constant, as solar cell performance is highly dependent on
temperature. However, there are a host of device parameters apart from the
efficiency that also need to be determined while characterizing a solar cell
completely. Such study is also necessary for development of new solar cell
technologies as it is important to know how solar cell performance varies with
variations in process conditions. For full electrical characterization of solar cells, it

is often useful to determine dark I-V characteristics in addition to the light I-V
characteristics and derive the relevant device parameters from the data. Also,
measurements at different illumination levels and temperatures allow one to
derive information that helps understand the functioning of the device in the
field. From these, one can derive the temperature and illumination coefficients
for important solar cell parameters and predict their behaviour under different
conditions of field exposure.
Solar cell testing equipment has been known ever since solar cells were
invented. A list of patents in this area is annexed below. Most of the test
apparatus invented so far deal with only light I-V testing of solar cells and even
where dark I-V is tested, the data has not been used for derivation of important
device parameters. Also, very few have been used with 4-wire method of
measurement.


However, the prior art suffers from the drawback that merging of I-V data, also
called I-V characteristics obtained under different conditions of temperature
and/or illumination for the purpose of comparison is not available. In other
words, the prior art dose not offer any test set up or equipment for complete
characterization of solar cells. The present invention overcomes this drawback of
the prior art.
The present invention discloses a test set-up that has been developed to enable
accurate measurement of I-V characteristics of solar cells of varying sizes at
different temperatures in dark and under illumination. The data obtained is used
to derive a set of device parameters that characterize the solar cell fully and also
throws light on the functioning of the device, which is very useful in improving
efficiency of solar cells. OBJECTS OF THE INVENTION:
An object of the invention is to provide a test set-up for accurate
measurement of I-V characteristics of solar cells in dark and under illumination
and also to derive from the measurements a host of device parameters that
completely characterize the solar cell.
Another object of the invention is to provide a set-up for plotting
complete characteristics of solar cells.
Yet another objective of the invention is to provide test set-up for
merging together of independent characteristics of solar cells under different
conditions of temperature and illumination for comparison purposes.

DESCRIPTION OF THE INVENTION:
The set up essentially comprises a light source, a temperature controlled chuck
with vacuum hold for solar cells, a set of spring-loaded probes for making
contact to the cell, an electronic load with a power supply and a microprocessor-
based electronic controller module interacting on one hand with the electronic
load and a PC on the other. In auto mode, the PC controls entire operation of the
measurement system and also employs the embedded software to automatically
derive a host of device parameters from measured I-V data. The results of
measurement are graphically displayed on the PC monitor and also stored in the
PC in the form of individual files.
The invention will be explained in details with help of the accompanying
drawings
BRIEF DESCRIPTIONOF ACCOMPANYING DRAWINGS:
Fig.1 shows a schematic of the test set-up in accordance with the invention.
Fig.2 shows an interactive block diagram of the PC, controller and power supply
of the test set-up according to the invention.
Fig.3 shows the block diagram for the power supply module.
Fig.4 shows the block diagram for the electronic load module.

Fig.5 shows the block diagram of the controller module.
Fig.6 shows a typical view of the PC monitor screen depicting light and dark I-V
characteristics obtained in the test set-up.
Fig.7 shows forward light I-V and forward/reverse dark I-V characteristics of a
typical solar cell using the test set-up according to the invention.
Fig.8 shows merged I-V characteristics of a typical solar cell at different
illumination levels according to the invention, for comparison purposes.
Fig.9 shows comparison of dark forward and reverse I-V characteristics of a
typical solar cell at three different temperatures, for comparison purpose.
A schematic of the automated test set-up (1.0) is shown in Fig. 1. Solar cell (1.1)
to be tested is firmly held on a nickel-plated, temperature controlled vacuum
chuck (1.2) and measurement of current and voltage made with 4-wire
arrangement (2 wires for current and 2 separate wire for voltage), by means of
spring- loaded current probes (1.5) and voltage probes (1.6). An air operated
probe assembly (1.7) is provided for manipulation of the probes.
The measurements performed are:
I-V characteristics in the forward direction under illumination
I-V characteristics in the forward direction in dark
I-V characteristics in the reverse direction in dark
For all measurements, the intensity of light from light source (1.3), called
insolation level, is first ensured as per set value and displayed on the screen of

PC (2.1). Also the temperature of the chuck is continuously monitored using a PT
100 probe inserted horizontally inside the chuck and a digital temperature
indicator (1.4) and kept constant by temperature controlled water bath (1.8).
The temperature of the chuck to be maintained constant is set on the digital
temperature indicator (1.4). The I-V measurement system comprises a controller
module (2.2) which interfaces with a power supply / electronic load (2.3) on one
hand and a PC (2.1) on the other from where the parameters of measurements
are set. The system operates with the help of embedded software. The
interactive block diagram is shown in Fig.2.
Electronic load and power supply module (2.3): The power supply provides
the required bias voltage to the cells and the electronic load provides dynamic
load to the cells for accurate variation of current and voltage. The power supply
is used to deliver minimum electronic load input voltage and also to compensate
wire drop in light I-V test. The same power supply is also used for supplying
reverse voltage to the solar cell in dark reverse test. The electronic load is used
to draw the current through Photocell as per Programmed current value. Both
the power supply and the electronic load are connected in series during I-V
measurements.
The block diagrams at Figs. 3 and 4 illustrate respectively different sections of
the power supply and the electronic load modules.

The power supply module comprises a step-down transformer, bridge rectifier
with filter capacitors, transistor array for regulation of the output voltage and an
error amplifier for comparing the reference voltage with the feedback voltage
and driving the transistor array accordingly.
The electronic load module has high precision shunt and amplifier for measuring
and amplifying the voltage signal produced due by the input current signal from
the controller module, transistor array for drawing the regulated output current
to the programmed value and error amplifier for comparison of the output
current signal from the controller with the feedback current and driving the
transistor array accordingly.
Microprocessor based controller module (2.2): The controller unit, as
shown in Fig.5, consists of Micro controller, Program memory - EPROM, Data
Memory, RS232 Buffer DAC, ADC, Reference section and Control signal section.
It act as an intelligent bridge processor between PC and Power Supply/
Electronic Load module (PSEL). It not only transfers data between the two points
but also does the signal transformation from analog to digital and vice versa.
The controller can be operated in either manual or remote mode (through a PC).
In remote mode, the controller sends data to PSEL as per the data received from

the PC and ignores the local panel settings. In manual mode, the controller skips
all the setting commands from the PC and sends only monitoring data such as
voltage and current read back to PC on request.
Software: The instrument is operated in the auto mode from a computer
terminal. The software based on Visual Basic enables the I-V plots and the
derived results of all tests to be viewed on the monitor as shown in Fig.6. The
entire set of data together with the graph as well as the results of measurements
is automatically stored in separate files.for future reference and use.
Forward light I-V measurement: During the forward light I-V measurement,
solar cell (1.0) to be tested is connected in series with the electronic load (2.3)
and the power supply and the bias voltage range is selected in the low range (0
- 5 V at 10 A max).
The entire I-V curve domain is divided in four current segments, namely 10% of
Isc, 85% of Isc, 97% of Isc and 105% of Isc with a user-defined number of test
points in each segment. The controller starts loading the current from 0 to Isc in
these steps and records voltage', current and temperature values in each step.
From the captured data, the controller plots the I-V graph and calculates as well
as displays the following parameters: Isc, Voc, Im, Pm, Ff, Eff, light intensity, Rs,
Rsh, the average temperature Tav, as well as temperature corrected values of
Voc at 25C, Isc, at 25C and Pm at 25C. The entire set of data as well as the

results of measurement is automatically stored in separate files for future
reference and use. A typical light I-V graph along with the derived parameter is
shown in Fig.7 (top)
Forward dark I-V measurements: During the forward dark I-V
measurements, solar cell (1.1) to be tested is connected in series with the
electronic load (2.3) and the power supply and the bias voltage range is selected
in the low range (0 - 5V at 10 A max.) Here also, the current range is divided
into four segments and the number of readings in each segment can be user-
defined so that a smooth curve is obtained. Before start of measurement, the
controller prompts the user to ensure that the cell is fully covered. Controller
applies the voltage from 0 to preset Vmax in steps and records voltage, current
and temperature in each step.
From the captured data, the controller plots the graph of Log J vs V and
calculates the following parameters: diode quality factors nl and n2, Rs. Rsh,
reverse saturation currents Ioi and I02 as well as the average temperature Tav.
The user can also define the straight-line portions of the graph to obtain the best
results for nl, n2,10i and 02-
Reverse dark I-V measurements: During the forward dark I-V
measurements, solar cell (1.1) to be tested is connected in series with the
electronic load (2.3) and the power supply and the bias voltage range is selected

in the high range (0 - 15 V at 1 A max). During this test, the output terminals of
the electronic load (2.3) are shorted so that it is put out of action. Here also, the
current range is divided into four segments and the number of readings in each
segment can be predefined so that a smooth curve is obtained. From the
captured data, the controller plots the graph of Log J vs V and calculates the
following parameters: shunt resistance from the linear portion of the average
temperature.
A typical dark I-V graph (both forward and reverse) along with the derived
parameters in shown in Fig.7 ( bottom).
Merging of I-V plots: The software enables automatic merging of to three I-V
or Log J-V graphs on the same screen and corresponding sheets. This enables
comparison of light I-V characteristics of a solar cell under three different levels
of illumination and at three different temperatures. Similarly, dark characteristics
(forward and reverse) of a solar cell are available for viewing on the same screen
or can also be plotted on the same sheet. These graphs are very useful from the
point of view of comparison of the solar cell performance under various
conditions of illumination and temperature. Typical merged plots for light and
dark I-V measurements are shown in Figs. 8 and 9 respectively.
Complete electrical characterization of solar cells is thus carried out with the help
of the test set-up and measurement scheme described above.

WE CLAIM:
1. An automated test set-up for complete and accurate electrical characterization of
solar cells (1.0) comprising:
a nickel plated temperature
controlled vacuum chuck (1.2), a light source (1.3), a digital temperature
indicator (1.4), a solar cell (1.1) which is to be tested being positioned on said
vacuum chuck and firmly held by said chuck, spring loaded current probes (1.5)
and voltage probes (1.6) being manipulated by air operated probe
assembly (1.7) and positioned on said solar cell, the solar cell output (1.9) being
fed to electronic load (2.3) which is connected to a PC (2.1) through controller
module (2.2), a printer (1.10) being connected to the PC (2.1), characterized in
that a plurality of measurements are conducted on the set-up, from which a
plurality of I-V characteristics of the solar cell can be plotted and the
characteristics can then be merged to be displayed on the same screen and
plotted on the same sheet for comparison.
2. A test set-up (1.0) as claimed in claims 1, wherein the temperature of the
vacuum chuck (1.2) is controlled by temperature controlled water bath (1.8) and
kept constant at the temperature value set on the digital temperature
indicator (1.4).

3. A test set-up (1.0) as claimed in claims 1 & 2, wherein the measurement of
the I-V characteristics are made with 4-wire arrangement, the said
characteristics being of the following types:
I-V characteristics in the forward direction under illumination,

ABSTRACT

AN AUTOMATED TEST SET UP FOR COMPLETE AND ACCURATE
ELECTRICAL CHARACTERIZATION OF SOLAR CELLS
The present invention discloses a test set-up that has been developed to enable
accurate measurement of I-V characteristics of solar cells of varying sizes at
different temperatures in dark and under illumination. The data obtained is used
to derive a set of device parameters that characterize the solar cell fully and also
throws light on the functioning of the device, which is very useful in improving
efficiency of solar cells.