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Selective Reflectors Bring High Efficiency to Lower-Bandgap PV Cell Materials





Berkeley Lab principal investigator Xiang Zhang, along with researcher Majid Gharghi and colleagues at UC Berkeley, have developed an innovative design that uses selective reflectors on a solar cell surface to coax high solar efficiency out of low-bandgap semiconductor material. In the Berkeley Lab design, a selective reflector, which can be a Bragg reflector, an interference filter, or any energy selective mirror, captures and concentrates photons inside the semiconductor. A portion of photons that would otherwise be re-emitted is instead reflected back onto semiconducting material.

The selective reflectors placed on the solar cell surface modify the optics allowing designers to bring the performance of these less expensive earth-abundant semiconductors up to the theoretical Shockley-Queisser (S-Q) limit of ~30%. That is the peak efficiency that can be reached with single-crystal silicon (Si) and gallium-arsenide (GaAs) devices without optical modification. The optimal bandgap to match the solar spectrum for the peak (S-Q) efficiency is in the 1.1-1.5 eV range, well suited for the characteristics of semiconductors such as Si, CdTe, and GaAs. The selective reflector technology does not surpass the S-Q limit. Instead, it allows otherwise poorly performing semiconductor materials to reach that peak, by allowing only photons of certain energy to reach or leave the cell.

Because it can capture and concentrate photons, the research team dubbed the device a “flux capacitor.” In this case, the spectral control of solar radiation shifts the optimal bandgap range for the 30% efficiency peak down to lower energy levels of 0.61-1.0 eV, and so enables solar cell designers to utilize materials otherwise deemed suboptimal for high-efficiency solar cells. That is well within bandgap ranges of less expensive semiconductors such as FeS2, which otherwise have low solar energy conversion efficiency limits of close to 20%.  Also brought up to 30% maximum efficiency by these selective reflectors is the promising, but somewhat more expensive, semiconductor gallium-antimonide (GaSb), which is currently used in infrared applications and thermophotovoltaics.

Although Materials such as GaAs, Si, and cadmium telluride (CdTe) are the current semiconductors of choice for solar cells, they are expensive. Implementation of the Berkeley Lab technology could expand the list of available semiconductor materials that can be used to produce highly efficient solar cells. Use of lower-cost, earth-abundant semiconductor materials could dramatically decrease the cost of solar cells and make them more competitive with fossil fuels, expanding the market for carbon-neutral sources of energy. The technology’s capacity to tune the spectrum can also be useful in tandem multi-junction solar cells, where matching between portions of the solar spectrum absorbed in each junction is crucial for optimal cell operation.

DEVELOPMENT STAGE:  Modeled concept.

STATUS: Patent pending.  Available for licensing or collaborative research.


Niv, A., Abrams, Z.R., Gharghi, M., Gladden, C., Zhang, X., “Overcoming the bandgap limitation on solar cell materials,” Applied Physics Letters, Vol. 100, Issue 8, 2012.


Non-sintered, Layered Nanocrystal Photovoltaic Cells, IB-2511

Field Effect P-N Junctions for Low Cost, High Efficiency Solar Cells and Electronic Devices, IB-3094, and Self-powered Gating and Other Improvements for Screening-engineered Field Effect Photovoltaics, IB-3170


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