Counterintuitive “LED-type” solar cell breaks efficiency record

When you think of a solar cell, you probably think of something designed to absorb as much sunlight as possible. What you probably don't think of is something that is also capable of emitting light. Nonetheless, that’s exactly what a new prototype device designed more like an LED does, and it recently set an efficiency record for flat-plate single junction solar cells. The cell was created by Alta Devices, a California-based company co-founded by University of California, Berkeley professor of electrical engineering Eli Yablonovitch. Together with graduate student Owen Miller, he worked out a mathematical principle which states that “the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce.”

The prototype cell incorporates a gallium arsenide semiconductor. Electrons within that material are knocked loose to flow freely by the energy of photons in sunlight. As the electrons are knocked loose by the existing photons, other photons are generated in the process. So far, that’s the same principle that’s at work in most conventional solar cells. However, the prototype cell is designed to let these new photons escape as easily as possible (in part, via a highly-reflective rear mirror surface), which in turn raises the voltage that it is able to produce. The technology allowed the prototype to achieve an efficiency of 28.3%, which broke the previous record of 26%. Yablonovitch hopes that researchers building on his work may be able to achieve a figure of 30% within a few years. Although the record-breaking device is a flat-plate single junction cell (meaning it is only capable of absorbing light waves within a given frequency), the principle it uses is said to apply to all types of solar cells.