THERMALISSUES
Using the Linkam system has enabled the measurement of electrical properties to become simple and routine. Accurate control of a lab-based experiment is vital to be able to correlate with real-life usage of solar cells
Figure 3: In this first example, current density rises uniformly as a function of increase of temperature.
49
REFERENCES
Figure 4: In this example, the sample voltage falls by nearly half over a range of 200o
C
simple and routine. Accurate control of a lab-based experiment is vital to be able to correlate with real- life usage of solar cells. Setting up the experiments is straightforward too. For example, an optical microscope may be used to position the probes onto the surface of the electrodes.
Efficient liquid nitrogen cooling enabled rapid turnaround between individual tests and hence it was easy to make multiple measurements to gain statistically viable data. The ability to change temperature enables observations of shifts in I/V curves which give much higher reliability to the results obtained with earlier homemade systems.
The study of capacitance change enables the study of defect levels. To observe trapped charge and then following its emission may be tracked as a function of temperature. This helps determine the energy of the trap level and indicates the presence of impurities.
The lower the impurity level means the longer the lifetime of the solar cell. With an increased ability to study the impact of temperature changes on manufacturing outcomes as well as greater control of the temperature environment, solar cell manufacturers have a new tool in their growing arsenal of yield improving tools and processes.
Figure 5: In the final figure, the effect of a series of I/V curves and the effect of rising temperature is shown. At low voltages, the gradient of the curve is flat. However, when increased to greater than 0.2V, the effect of higher temperatures is clearly observed in curves covering nearly 200o (data courtesy of NREL)
C.
Reference: David L Young et al, Applied Physics Letters 86, 262107 (2005): “Strongly temperature- dependent free- energy barrier s measured in a polycrystalline semiconductor.”
ACKNOWLEDGEMENT
Linkam Scientific are most grateful to David L Young of the National Renewable Energy Laboratory for his helpful comments, figures and
photographs in the preparation of this article.
www.solar-pv-management.com Issue III 2010
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