NREL Invention Speeds Solar Cell Quality Testing for Industry

NREL Invention Speeds Solar Cell Quality Testing for Industry

A solid-state optical system invented by the National Renewable Energy Laboratory (NREL) measures solar cell quantum efficiency (QE) in less than a second, enabling a suite of new capabilities for solar cell manufacturers.

QE is a measurement of how cells respond to light across the solar spectrum, but traditional methods for measuring QE had been too slow, limiting its application to small samples pulled from the production line and analyzed in laboratories. NREL’s technique, commercialized by Tau Science as the FlashQE™ system, uses a solid-state light source, synchronized electronics, and advanced mathematical analysis to parallel-process QE data in a tiny fraction of the time required by the current method, allowing its use on every solar cell passing through a production line.

The FlashQE system uses an array of light-emitting diodes (LEDs), each emitting a different wavelength of light. The LEDs illuminate the cell simultaneously, rather than the serial approach of the conventional system. The key to the technology is that all the LEDs are flashed on and off at different frequencies, thereby encoding their particular response in the solar cell. High-speed electronics and advanced mathematics cleverly extract the encoded information to reveal a full-spectrum QE graph of the cell. A wide variety of information is gathered in less than a second—information about the ability of the front surface of the cell to absorb high-frequency light, the quality of the thin-film surface coatings, the ability of the middle region of a cell to absorb a wide range of wavelengths, how well the back surface absorbs lower-energy light, and the ability of the back surface to collect electrons.

Some of this is new information for manufacturers. Solar cell manufacturing lines test each cell to determine useful cell parameters, such as how much current and voltage is generated. But conventional tests give no information about how the cell responds to each color of light in the solar spectrum. Flash QE's ability to also test for each cell’s response to color allows crucial extra information to be fed back into the production line. It does it so fast that cells of the same current and the same response to particular colors can be sorted into bins. From these sorted bins, spectrally matched modules can be made to optimize the energy produced throughout a day.

NREL’s ingenious approach, in which parallel processing allows all of the QE data points to be measured simultaneously to produce a QE graph in 1 second, is more than 1,000 times faster than the industry’s current state-of-the-art technique.