As the 37th IEEE Photovoltaics Specialist Conference (PVSC) gets underway in Seattle, Alta Devices plans to discuss how it achieved record cell efficiencies of 27.6% last year as well as its overall best efficiency rating of 28.2%. Both results were verified by the National Renewable Energy Laboratory with Alta disclosing that the high efficiencies they reached were due in part to a new discovery involving the light emitted and collected by a solar cell.
The company points out that the accepted thought on a solar device’s efficiency was that its efficiency was calculated on the amount of external light that could be captured and retained. However, Alta Devices points out that this did not justify for the need to amplify the number of photons being produced within the device so that it could achieve high efficiencies.
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“Up until now it was understood that to increase the current from our best solar materials, we had to find ways to get the material to absorb more light,” said Alta co-founder Eli Yablonovitch, director of the NSF Center for Energy Efficient Electronics Science and professor at the University of California at Berkeley. “But, the voltage is a different story. It was not recognized that to maximize the voltage, we needed the material to generate more photons inside the solar cell. Counter-intuitively, efficient light emission is the key for these high efficiencies.”
The scientists at Alta Devices came to a simple conclusion as they saw the efficiency ratings reach higher levels, “a great solar cell also emits light and collecting that light increases cell performance.” The company refers to the Schockley-Queisser Limit, which shows that the theoretical maximum solar cell efficiency limit for a single junction device is 33.5%. Even though the company acknowledges that attaining this level of efficiency is proving to be a lengthy process, Alta President and CEO Christopher Norris stated that the company’s scientists and engineers are reaching new cell efficiency improvements about once every two months.
“In 2009, our team came to me with an aggressive timeline for solar cell efficiency advances, but with a few caveats: they were butting up against what appeared to be entrenched, practical limits,” stated Norris. “Nevertheless, over the past two years, the team has succeeded in meeting each of its milestones. We are committed to using new scientific understanding, such as internal light generation and extraction, to push the limits of solar cell and module efficiencies while simultaneously driving production costs down through other important developments. The goal of achieving the US$1 per installed watt target set by the Department of Energy has energized our entire company.”
Alta Devices will be presenting its paper entitled, “27.6% Conversion Efficiency, A New Record for Single-Junction Solar Cells Under 1 Sun Illumination,” with Dr. Brendan Kayes explaining how Alta accomplished its efficiency results at the IEEE PVSC conference June 19 through 24.
