Army

Photovoltaic cells utilizing a greenhouse filter and reflective back mirror

Design converts radiative energy into electricity in a more efficient way than conventional cells

Energy

Structure of the nonequilibrium PV device with the filter, n-p junction semiconductor sell, and grid-type contacts.

In photovoltaic (PV) energy conversion devices, the useful energy per photoelectron is given by the photoelectron chemical potential. Design, modeling, characterization, and optimization of conventional PV devices are based on a detailed balance condition, which assumes that photoelectrons and emitted photons are in chemical equilibrium. This approximation works well for conventional PV and thermophotovoltaic (TPV) devices, because photon emission, photon reabsorption, photoelectron accumulation, and photoelectron collection processes, all occur at the same narrow energy range near a semiconductor bandedge. The chemical potential of photoelectrons accumulated near the semiconductor bandedge and the chemical potential of photons emitted by these photoelectrons are the same. The photo-induced chemical potential determines the conversion efficiency.

But, to go beyond theoretical efficiencies, new approaches must be pursued and previous assumptions challenged. One such approach by the Army uses photonic and electronic management to establish chemical non-equilibrium between low-energy photoelectrons that provide electric power and high-energy photoelectrons that determine emission from the photovoltaic device.

This non-equilibrium operating regime lowers the emission losses and increases the energy conversion efficiency. This is accomplished through the use of a reflective optical greenhouse filter (reflective interference filter) positioned on the front surface of a semiconductor PV cell along with a back surface mirror. The filter establishes an optical bandgap which is larger (in terms of energy) or smaller (in terms of wavelength) than the semiconductor bandgap of the semiconductor PV cell. The greenhouse filter passes wavelengths a prescribed amount shorter than the bandgap wavelength of the PV device and reflects wavelengths the prescribed amount longer than the bandgap wavelength of the PV device.

In simple terms, the operation of the non-equilibrium PV device mimics the operation of a greenhouse, where the greenhouse glass reduces thermal emission from the greenhouse, and, in this way, keeps more thermal energy in the greenhouse. The technology combines this relatively simple greenhouse photonic management with electronic management, which allows effective conversion of thermal energy into electricity. The non-equilibrium PV device with a greenhouse filter and a back surface reflector increases the conversion efficiency, potentially above the Shockley-Queisser (S-Q) limit, due to suppression of radiative emission in the cell. For solar light conversion, the non-equilibrium solar cell is expected to improve the conversion efficiency above the S-Q limit, up to around 44%.

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