Aerotaxy: a possible path to relatively low cost direct band gap GaAs PV cells

This abstract of an article published in the Dec 2012 issue of  Nature describes a method of growing crystalline GaAs (gallium arsenide) nanowires in gold nano-particle aerosols at high throughput rates compared to traditional molecular beam epitaxy (MBE) or metalorganic chemical vapour deposition (MOCVD) methods of synthesis. The hope is that a high growth rate will translate to lower costs.

The company Sol Voltaics is hoping to utilize the technology in conjunction with traditional silicon solar cells produce a tandem device with higher efficiency than can be achieved using silicon by itself. GaAs is a direct band gap semiconductor which means that the probability of the absorption of an impinging photon whose energy is near the band gap is much higher than for an indirect band gap semiconductor such as silicon. This high probability of absorption means that GaAs solar cells can be very thin and still effectively absorb solar energy, whereas silicon solar cell have to be much thicker to effectively absorb all of the photons that are capable of being converted to electrical current.

The band gap of GaAs is different than that of silicon so that it absorbs a different part of the solar frequency spectrum. That fact that the GaAs film can be made very thin means that the photons of a frequency that it is not capable of using for the production of electricity can easily pass through and enter the silicon cell where the photon near the band gap of silicon can be absorbed and converted to electricity as well. The combined cell has substantially higher energy production than a silicon cell alone.

The key to the economics of such a tandem cell the the production cost of the GaAs layer. Triple junction cells with two of the junctions based on GaAs films have achieved solar conversion efficiencies in excess of 45%. However the enormous costs of producing crystalline then films of GaAs have prevented the commercialization of this technology except in specialized niche applications where low weight and small surface area is more important than cost (e.g. remote power for military application, solar power for space missions, etc).

Whether or not the cost/performance characteristics of PV cells produced by this technology can enable mainstream commercial application of GaAs remains to be seen. 

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