NREL's Six-Junction Solar Cell Nears 50 Percent Efficiency

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Besides the solar market slump, constraints imposed purely by physics have also been the bane of solar cell manufacturers. Many modern commercial cells hover around the 10-20 percent efficiency mark, and boosting efficiency is a major R&D focus for many companies. There are lots of reasons why solar cells can’t reach 100 percent efficiency: blackbody radiation (think of it as ambient energy evaporation), the PV materials used, and their capacity to accommodate electrons. Oddly, photons themselves can also be a roadblock to optimal efficiency.

Traditional solar cells are best at collecting only a narrow band of wavelengths, depending on their PV material, and are constrained by the Shockley-Queisser limit, which dictates that the maximum efficiency of an ideal solar cell can never exceed 33.7 percent. That’s because in traditional single-junction solar cells, the bandgap between two semiconductor materials defines how well the photons are converted into electrons within the cell. In this scenario, capturing photons whose energy is well-matched to the materials’ specific bandgap is crucial.

Fortunately, silicon and other semiconducting materials are pretty well-matched in their bandgaps to harness the sun’s natural spectral distribution. But spreading your photonic net across wavelengths could yield more energy-producing photons. That’s what multi-junction or tandem cells, like those from Alta Devices, aim to do.

In March, Alta Devices announced their solar cells had exceeded 30 percent efficiency, and at Intersolar last week founder and Caltech professor Harry Atwater outlined how the company plans to break the 50 percent efficiency barrier. Photon recycling and epitaxial lift-off (check out this earlier story for an explainer) are the two main factors distinguishing Alta’s cells, whose super thin gallium arsenide films are currently more efficient than traditional PV materials like silicon. They are, however, also more expensive, and may thus best serve niche markets where performance requirements trump cost. Alta is focusing on mobile deployments of its tech, from unmanned aerial vehicles to transportable solar arrays.

According to Atwater, simulations indicate that efficiencies in multi-junction cells can continue to increase, provided the structure of the cells is appropriately tweaked. The stacking of thin film layers and using tuned materials to cover the entire wavelength spectrum are some of the main considerations. By iterating and improving the PV design over these parameters, efficiencies of 50 percent or greater should be achievable.

Spectrum splitting – using optical methods to reflect and redirect incident light to appropriate layers – and using lenses or mirrors as concentrators are two accessory ways to further improve solar efficiency that Alta and others are pursuing. With light concentrators, the argument is that fewer solar cells are then needed, leading to potential cost and area savings. The snag is that you need a device, like the QBotix robot, to track the sun, and you need to funnel the energy of hundreds of suns into the system. Companies that use solar concentrators, like Solar Junction, have achieved over 40 percent efficiency with their cells in this way.