Solar panels on the market today are almost universally limited to less than 20% efficiency at converting sunlight to electricity, and generally get more expensive as efficiency goes up. Numerous reasons exist for this low photovoltaic efficiency, but one of the most fundamental is the Shockley-Queisser Limit. As Shockley and Queisser explained in their famous 1957 paper, the efficiency of even a perfect semiconductor solar cell is fundamentally limited by how a particular semiconductor material absorbs and emits light. Because of this limit, a single semiconductor material can only very efficiently convert a narrow slice of the sun’s light spectrum. By combining multiple semiconductors in a single solar cell, different layers of the device can be “tuned” to optimally convert a particular part of the total spectrum and, in concert, convert the entire spectrum efficiently. A start-up company called Solar Junction in San Jose, California, has recently broken new ground in this area of “multijunction” solar cells. By adding a small amount of nitrogen to gallium-arsenide-based semiconductors, Solar Junction was able to produce a tri-layer device that converted the solar spectrum to electricity at a record-breaking efficiency of 44% in early 2012. This choice of materials known as “dilute nitrides” allows properly tuned semiconductors to be deposited in a multilayer stack while eliminating the physical mismatch that leads to the formation of defects. These devices, while very high efficiency, are much more expensive to manufacture than cells based on silicon and have traditionally only been used on satellites. Solar Junction believes that, through the use of inexpensive optical methods (simple mirrors and lenses), a cost-effective amount of power is achievable on earth by concentrating large amounts of sunlight on a small solar cell. The challenge of concentrating photovoltaics is the construction of systems far more complex than simple flat panels and the requirements to track the sun as it moves across the sky. If Solar Junction can continue to increase efficiency and find a winning partner to produce the low-cost concentrator, they may produce a disruptive effect on the high-power solar market.
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