New High Efficiency Solar Cell

Spectrolab, a Boeing subsidiary, have developed a new multi-junction solar cell that has demonstrated an efficiency of 40.7% using concentrated sunlight.

The multi-junction solar cell is similar to space based solar cells used on satellites but is based upon metamorphic semiconductors. These are dissimilar semiconductor materials combined.

In solar cells, efficiency is limited by the ability of a semi-conductor junction to utilize only one wavelength of light efficiently. This is based upon the semi-conductor junctions band gap, an energy barrier that requires a specific amount of energy to cause an electron to jump over.

The shorter the wavelength of light, the more energy each photons pack. A typical mono-crystalline silicon solar cell will have a band gap of around 1.1 volts. This corresponds to a wavelength of around 1200 nm, near infrared, just below visible light.

Any photon with a wavelength longer than this will not be able to cause an electron to jump the energy band gap and produce a current in a solar cell. Any wavelength shorter than this will not be utilized efficiently, only the energy corresponding to 1200 nm, 1.1 electron volts, will be utilized, the rest will be wasted as heat. A conventional mono-junction solar cell can only utilize a narrow slice of the light spectrum efficiently.

A solution to this problem is to create a multi-junction solar cell, a stack of junctions each of which can capture a portion of the spectrum with high efficiency. This is the approach used in the new Spectrolab solar cell. Other companies have produced multi-junction solar cells in the past, but they have been either too chemically unstable or too expensive for terrestrial applications.

The 40.7% efficiency demonstrated by the Spectrolab solar cell is the highest I have seen. The companies press release implies these cells will be suitable for terrestrial applications and lend themselves to high volume manufacturing but they do not address the issues of cost or long term stability, either of which could be show stoppers.

If both of these issues are addressed in this new solar cell, then this will represent a major breakthrough in photovoltiac solar energy and will allow it to find applications that are presently cost prohibitive.

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