THERMALISSUES
Temperature Control
The variety of materials and processes in PV and solar manufacturing require specific environments to be used effectively and efficiently. Although thermal control has been part of manufacturing for some time, it has generally being restricted to bulky
furnaces. Ian Pearce of Linkam
Scientific Instruments discusses how the companies products were discovered to be excellent tools for temperature related property characterization of solar cells.
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t is well known that many performance properties of electronic devices are dependent on both time and temperature events. A change in temperature during the manufacturing time can lead to a range of faults that reduces yield and quality. As solar cell manufacturers seek any advantage in a demanding market place and improvement on yield and manufacturing practices is most welcome. Whilst thermal control through manufacturing has a long history in
microelectronic’s manufacturing it has not being as sophisticated in solar cell manufacturing.
Early studies of the electrical properties of solar cells used to be restricted to the use of homemade furnace systems where the temperature could be raised and properties measured. However, in 2000, some Japanese workers presented data using a Linkam hot stage that could not only provide rapid and repeatable control of heating but also controlled cooling with equal accuracy. This stage could also control cooling with equal accuracy. It had the further advantage of being easy to use and as a commercial system, was well supported with spares and service available quickly from the factory and through local distributors.
The Linkam LTS350 hot stage turns out to be an ideal size for the solar cell specimens. The open architecture design of the hot stage has enabled the users to make critical modifications to enable the best electrical contact for high sensitivity measurements. In normal operation, the window of the hot stage enables users to view the sample specimen and watch it as other physical properties change as a function of changing temperature. In this application, samples are illuminated through the window using different wavelengths of light. By choosing the right frequencies and intensities, it is possible to investigate photo capacitance as a function of current/voltage (I/V) curves changing with respect to a controlled rise or fall in temperature.
By way of background, a solar cell is a large area semiconductor diode. Due to the photovoltaic effect energy of light (energy of photons), there is a conversion into electrical current. At a p-n junction, an electric field is built up which leads to the separation of the charge carriers (electrons and holes). At the incidence of the photon stream onto the semiconductor material, the electrons are released providing the energy of photons is
www.solar-pv-management.com Issue III 2010
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