Simple Flashlamp I-V Testing of Solar Cells
William Keogh, Andrés Cuevas
Centre for Sustainable Energy Systems
Faculty of Engineering
The Australian National University
Canberra, ACT 0200
AUSTRALIA
Telephone:
+61 (0)2 62494914
Facsimile:
+61 (0)2 62490506
E-mail:
William.Keogh@anu.edu.au
Andres.Cuevas@anu.edu.au
Abstract
A new approach to the design of a flash tester has been developed. In this approach, the bias voltage
on the cell is held constant and the light level varied. This is the reverse of conventional flash testers.
Compared to conventional flash testers, the new approach makes use of simpler, commercial
equipment and extracts more information about the cell performance. A prototype of the new system
has been built and successfully used for testing concentrator cells on a pilot production line.
1
INTRODUCTION
Flashlamp systems have been used for testing solar cells for some time and their advantages are well known. The
primary benefit is that the light shines on the cells for a very short time and so does not heat them up significantly. This
is convenient for 1 sun cells and essential for concentrator cells that have not been mounted on a heatsink.
Unfortunately, commercial testers are mostly designed for production use with few options for more sophisticated
characterisation.
The conventional approach to flash testing, as described in Lipps et al (1995), Mueller (1993) and Sturcbecher et al
(1994), is to maintain a nearly constant light intensity while rapidly changing the bias voltage on the cell to sweep out
the I-V curve. Typically about 1000 I-V data points are acquired in less than 1 ms. An alternative method is presented
here that utilises simpler, cheaper, commercial equipment and extracts more information about the cell performance.
2
NEW METHOD
Our approach is to allow the light intensity to vary over the course of the flash while maintaining a nearly constant bias
voltage. Compared to the conventional approach, this simplifies the flashlamp and the cell bias circuitry. The flashlamp
can be a standard commercial flash because it does not have to produce a constant light output. The cell bias circuitry
only needs to provide a constant voltage; in our design it consists of a large capacitor and a relatively low speed, low
current amplifier.
A family of I-V curves can be measured by doing several flashes, each at a different bias voltage. The curves can then
be used to plot the performance of the cell as a function of light intensity. This is useful not only for concentrator cells
but also 1 sun cells. To achieve a similar effect with a conventional flash tester would require changing the flash-to-cell
distance or adding neutral density filters.
In operation, the bias voltage is applied to the cell and capacitor and sufficient time allowed for the capacitor to charge.
The flash is triggered and the light intensity, cell current and cell voltage are measured for the duration of the flash.
Then an interpolation process, as shown in Figure 1, is used to extract an I-V point for each intensity at which an I-V
curve is desired. For the desired light intensity, E, the time at which this intensity occurs is determined. The cell current
and voltage at this time are then taken from the I(t) and V(t) data and the I-V point is added to the existing data for the
appropriate I-V curve. This process is repeated for different bias voltages to measure the entire I-V curve.
Once a complete set of curves have been measured, various indicators of cell performance (Isc, Voc, FF, efficiency,
etc.) are calculated and plotted as a function of concentration. Additional physical parameters such as ideality factor,
saturation current, and series and shunt resistance can be obtained from the data. Figure 2 shows some typical results.
Simple Flashlamp I-V Testing of Solar Cells
William Keogh, Andrés Cuevas
E(t)
I
E1
V
(V2,I2)
E2
V1
V2
t
V(t)
(V1,I1)
t
I(t)
I2
I1
t
Figure 1 - Conversion of Flash data to I-V Points
Figure 2 - Example of results produced by tester
2
Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society
Paper 110
Simple Flashlamp I-V Testing of Solar Cells
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William Keogh, Andrés Cuevas
IMPLEMENTATION
As indicated in the previous section, the hardware required
to implement the design is simple. Most of the complexity
Labview Software
is in a commercial multi-purpose data acquisition card (12
bit 80 ksample/s A to D, 12 bit D to A, digital I/O). A
A/D, D/A, digital I/O card
block diagram of the system is shown (see Figure 3).
The flashlamp presently used is a commercial disco strobe,
Bias
Cell
Cell
Flash
Light
though we have also used photographic flashlamps. The
Voltage
Current Voltage
Trigger
Intensity
duration of each flash is about 8ms. The variation between
maximum and minimum intensity allows a cell to be tested
Ref
Test
over approximately a decade range of light intensity
Flash
x1
Cell
Cell
without any mechanical movement of the light source. The
strobe can flash up to 10 times per second, allowing a
complete set of I-V curves to be obtained in a few seconds.
Figure 3 - Block diagram of Flash Tester
The spectrum of a xenon flashlamp changes with discharge
current, as described in Lipps et al (1995) and EG&G
(1997), and this can cause an error due to the spectral mismatch change. However, the error can be made small by using
reference and test cells with a similar spectral response.
Compared to conventional flash testers, the cell biasing circuitry is quite simple because the tester maintains a constant
voltage on the cell during each flash. The circuitry consists of a large electrolytic capacitor (~1 Farad) and a relatively
small power supply which are permanently connected across the cell. In operation, the desired bias voltage is applied to
the cell and capacitor while in the dark and sufficient time is allowed for the capacitor to charge. During the flash the
capacitor sinks the current delivered by the solar cell (which can be as much as 100A), not the power supply. This
avoids the need for a high-speed high-current bipolar supply. A block diagram of the system is shown in Figure 3.
The cell-testing block uses a 4 point contact scheme with large area contacts to closely simulate a tabbed busbar.
Temperature control of the cell testing block is achieved by use of a thermoelectric cooler (Peltier device) and a digital
process controller. The temperature can be quickly set to any value in the range –10ºC to 100ºC. This system is
considerably smaller, lighter and cheaper than the commonly used water chiller.
The A/D card used presently is a relatively low cost unit that does not include a sample-and-hold on each input channel.
This means that the light intensity, cell current, and cell voltage are measured at slightly different times. To reduce the
severity of this problem, a software interpolation is used to estimate what the signals would have been if they were truly
simultaneous. An A/D card with sample-and-holds would be slightly more accurate though more expensive.
The software is programmed using National Instruments’ LabVIEW©, which is a widely available, high-level
instrumentation programming language. It provides excellent support for data acquisition, processing and display.
4
ACCURACY & CALIBRATION
4.1
Possible Transient Effects
The conventional flash testing of solar cells and modules can produce anomalous I-V curves if the sweep rate of the
electronic load is too fast. The problem is due to the need to acquire the full I-V curve in about 1ms, the time that the
flash delivers a nearly constant light intensity. A hump in the maximum power region of the curve can appear when
sweeping from Voc towards Isc for sweep rates higher than about 50 V/s for 1 ohm cm devices (King et al, 1988).
In our approach there is no forced change of the cell voltage, which remains nearly constant (it changes slightly due to
the resistance of the external wiring), so the above mentioned problem can be expected not to occur. However, a
different kind of error is possible. The light intensity of our flash varies with a time constant of a few milliseconds. As
the light intensity changes, a new balance between generation and recombination of carriers is established within the
device, resulting in a different output current. The new balance takes a finite time to establish, and a lag between current
and light intensity could result, producing an error.
The situation is very similar to that of a quasi-steady state measurement of the photoconductance (Sinton et al, 1996),
and it is in fact identical for open-circuit conditions. When the sample or device’s effective minority carrier lifetime is
less than 60 microseconds and the time constant of light variation is about 2 milliseconds, the possible error is less than
one percent. The error increases to 10% for 230-microseconds minority-carrier effective lifetimes (Sinton et al, 1996).
A typical silicon solar cell with a one sun Voc of 650mV has an effective minority carrier lifetime of about 60
microseconds. Open circuit is a worst case condition and we can conclude that the error due to the rate of decay of the
present flash should be less than 1% for most solar cells.
Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society
Paper 110
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Simple Flashlamp I-V Testing of Solar Cells
4.2
William Keogh, Andrés Cuevas
Superlinear Cell Current
Simple solar cell theory predicts that short circuit current should be proportional to light intensity; in other words,
current is linear with light intensity. More sophisticated theories predict that, depending on the cell construction, the
current will be either slightly less than linear (sublinearity), or slightly more than linear (superlinearity).
Our system consistently measures short circuit current as being approximately 7% superlinear over a decade range of
light intensity. This is significantly more than theory predicts, so we suspect that it is an artifact. At the time of writing,
the cause is unknown. Possible explanations being investigated are: spectral mismatch changes, inductance in the
current sense resistors, phase errors due to non-simultaneous sampling in the A/D card, and light uniformity changes
during each flash.
At present the problem is avoided by assuming linearity between short circuit current and light intensity, which is
reasonable for most crystalline silicon solar cells.
4.3
Calibration against Steady State System
The accuracy of the system has been checked by measuring a cell with a reliable 1 sun continuous illumination system
and then measuring the cell with the flash tester at 1 sun. In both cases, the cell being tested was used to set the light
intensity. FF and Voc are in agreement to within 2% at 1 sun (Table 1). Current is not reported because of the
previously mentioned superlinearity problem.
Table 1 - Comparison of Steady State and Flash Test results for a typical cell
Cell A43bb
Steady State Test
Flash Test
FF
0.787
0.799 (+1.5%)
Voc
0.619
0.621 (+0.2%)
5
APPLICATION
The system has been used for testing
concentrator solar cells on a pilot
production line. The cells are 5x4 cm
and produce ~20 A at 40 suns.
More than 100 cells have been tested
straight off the production line, without
bonding them to a heatsink. Providing
good heatsinking at 40 suns is not a
trivial process, so it is desirable that the
test results are of electrical performance
only and do not have errors due to
thermal problems.
6
Flash
Ref Cell
Test Cell
CONCLUSION
In summary, the proposed I-V testing
method has the following advantages:
• Simple, low cost hardware
• Measures cell performance over a
Cell Bias Capacitor
range of light intensities
• Particularly appropriate for
Figure 4 - Picture of Flash Tester
concentrator cell testing
• Fast enough to be used for production testing
• Allows full physical characterisation of the solar cell, including saturation currents, ideality factor, series and shunt
resistance etc.
Although it is customary to report solar cell operation under standard 1 sun conditions only, the measurement of a set of
I-V curves at different illumination levels provides a more complete evaluation of cell performance under real
conditions. This is particularly important for semiconductor devices and materials whose properties change with light
intensity. The proposed I-V testing method conveniently provides this higher level of characterisation.
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Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society
Paper 110
Simple Flashlamp I-V Testing of Solar Cells
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William Keogh, Andrés Cuevas
ACKNOWLEDGMENTS
We would like to thank to R. Sinton for useful suggestions. Thanks are due to ERDC (the Energy Research and
Development Corporation) for providing William Keogh’s PhD Scholarship. Additional funding has been provided by
the Australian Research Council.
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REFERENCES
EG&G, EG&G Linear Flashlamps Design Guide, EG&G Electro-Optics, Salem MA, USA, 1997
D. L. King, J. M. Gee, and B. R. Hansen, presented at Twentieth IEEE Photovoltaic Specialists Conference, Las
Vegas, 1988.
F. Lipps, A. Zastrow, and K. Bucher, I-V Characteristics of PV Modules with a msec Flash Light Generator and a
2MHz Data Acquisition System, presented at 13th European Photovoltaic Solar Energy Conference and Exhibition,
Nice, France, 1995.
R. L. Mueller, The Large Area Pulsed Solar Simulator (LAPSS), JPL, Pasadena JPL Publication 93-22, 15 Aug
1993 1993.
J. J. Sturcbecher and J. C. Larue, The mini-flasher: a solar array test system, Solar Energy Materials and Solar
Cells, vol. 36, pp. 91-98, 1994.
R. A. Sinton and A. Cuevas, Contactless Determination of Curent-Voltage Characteristics and Minority-Carrier
Lifetimes in Semiconductors from Quasi-Steady-State Photoconductance Data, Appl. Phys. Lett., vol. 69, pp. 2510,
1996.
Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society
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