All sunshine not created equal

Conversion efficiency, open circuit voltage, and other solar cell performance parameters are critically important to solar cell manufacturers and their customers. These parameters define the financial model for an installation, from the size of the array needed for a given output power to the installation's likely generation revenue. They set a viability threshold that new technologies must cross. Yet measuring them accurately turns out to be surprisingly challenging.

Though the whole point of solar energy is that the sun is readily available, the sun is a terrible light source for accurate measurements of solar cell parameters. Clouds, haze, and the time of day and time of year all cause deviations from the often-quoted 1000 W/sq. meter solar irradiance value, and from the "standard" solar spectrum. Instead, companies use solar simulators.

No solar simulator will precisely match the sun's spectrum, though, which means the cell being tested might respond differently in actual use conditions. Correcting for spectral mismatch is especially complicated for organic solar cells (as discussed in an article in April's IEEE Spectrum -- free for IEEE members), but several people have warned me not to trust non-certified reports for inorganic thin film cells, either.

With the solar industry's rapid growth, it was only a matter of time before someone stepped up to fill the need. VLSI Standards has introduced an NREL-traceable Solar Reference Cell, using a monocrystalline silicon cell for calibration of solar simulators. The company also offers calibration services for certification of customer reference cells. (This product is so new that it isn't on VLSI's site yet. Contact the company for more information.)