Solar photovoltaic cells would be designed to exploit photonic-bandgap (PBG) materials to enhance their energy- conversion efficiencies, according to a proposal. Whereas the energy-conversion efficiencies of currently available solar cells are typically less than 30 percent, it has been estimated that the energy- conversion efficiencies of the proposed cells could be about 50 percent or possibly even greater.
The primary source of inefficiency of a currently available solar cell is the mismatch between the narrow wavelength band associated with the semiconductor energy gap (the bandgap) and the broad wavelength band of solar radiation. This mismatch results in loss of power from both (1) long-wavelength photons, defined here as photons that do not have enough energy to excite electron-hole pairs across the bandgap, and (2) short-wavelength photons, defined here as photons that excite electron- hole pairs with energies much above the bandgap. It follows that a large increase in efficiency could be obtained if a large portion of the incident solar energy could be funneled into a narrow wavelength band corresponding to the bandgap. In the proposed approach, such funneling would be effected by use of PBG materials as intermediaries between the Sun and photovoltaic cells.
The approach involves a thermophotovoltaic principle in addition to the use of PBG materials. The basic idea is to tailor the wavelength- and direction-dependent emissivity of one or more PBG material(s) such that as much as possible of the wavelength-mismatched portion of the incident broad-band solar power would be absorbed — the absorbed power would cause heating, and the resulting thermal radiation would be funneled into a narrow band corresponding to the bandgap of the semiconductor material of a solar cell. Recent experiments unrelated to the development of solar cells have shown that as much as half of the thermal power could be thus re-routed into the bandgap.
The figure depicts two of many conceivable configurations for implementing the proposal. In one configuration, the incident solar radiation would be intercepted by an absorber and absorbed energy would be re-radiated by an emitter. A filter behind the emitter would allow primarily bandgap-energy photons to pass through and would reflect most other photons back into the absorber, helping to keep the absorber hot. A mirror at the rear surface of the solar cell would reflect any remaining nonbandgap- energy photons back to the absorber. The filter would be made of a PBG material: the advantage to be gained by using a PBG filter instead of a traditional optical filter is that a PBG structure could be designed to modify the wavelength distribution of thermal radiation from a conventional blackbody distribution to reduce or increase the spectral power densities at selected wavelengths.
In the other configuration, the functions of the absorber and filter would be combined in a single monolithic PBG absorber/emitter that could comprise, for example, thin absorbing layers alternating with thin non-absorbing, wavelength- selective layers. Optionally, the mirror behind the solar cell could also be made of a PBG material.
This work was done by Jonathan Dowling and Hwang Lee of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.
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