THERMALISSUES
sufficient. Contact to a solar cell is achieved with metal contacts. If the circuit is closed, meaning an electrical load is connected, direct current flows. The energy of the photons occurs in discrete “packages” called “quanta”, and the energy of each quantum depends on the wavelength of the light radiation.
Keeping tabs on the heat
In the subsequent work on temperature control in solar cell manufacturing, the chosen temperature stage is based upon the heating and cooling technology as used by the LTS350 (Figures 1 & 2). The hot stage has excellent temperature control and range for a variety of applications (-196oC to 350oC) while the dynamic heating and cooling performance ensures that most electrical component temperature control applications are covered.
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Figure 1: The experimental set up shows the control modules on the left (from the top, electrical to temperature controller to flow of coolant). The stage is mounted on an optical breadboard while the electronics for the light source are shown on the right. (photograph courtesy of NREL)
This set up may have up to eight positional gold- tipped, tungsten probes (though other types can be used). They are connected using isolated BNC connectors within the sample chamber and enable electrical measurements on the sample to be made simultaneously while accurately controlling the temperature inside a gas-tight environment. Gas ports on the sides of the stage enable purging of the sample environment with inert or humidified gas.
The stage consists of a large area (58x43mm) temperature controlled element with a platinum resistor sensor embedded close to the surface for accurate temperature measurements. The sample is mounted on the heating element and the probes, held on a magnetic base-plate for stability, are moved manually to make contact at the appropriate points.
Historical connections
Solar cells exhibit similar characteristics to those of a diode. Thus, measuring I/V curves where the voltage input is varied and the change in current monitored, it is possible to gain insight into a number of performance properties. These include leakage current detection, drops in resistance, measurement of ideality factor, open circuits and operating efficiency. The effects that we monitor routinely in characterization of samples are shown here. (Figures 3 -5).
Figure 2: The illumination source is situated above the hot stage. When the experiment is running, the whole apparatus is enclosed in a blackout box. (photograph courtesy of NREL)
Using the Linkam system has enabled the measurement of electrical properties to become
www.solar-pv-management.com Issue III 2010
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