• Energy Research

Germanium is a low cost alternative to the highly expensive III-V semiconductors commonly used on thermophotovoltaic (TPV) devices. Despite efficiencies up to 16.5 % have been theorized in previous works, no experimental efficiencies have been reported so far for germanium-based devices. In this work, we present the first TPV germanium efficiency measured in a high view factor calorimetry set up. Efficiencies and power densities up to 11.2 % and 1.43 W/cm2 have been obtained using a graphite emitter heated at 1544 ºC.

Germanium is a low cost alternative to the highly expensive III-V semiconductors commonly used on thermophotovoltaic (TPV) devices. Despite efficiencies up to 16.5 % have been theorized in previous works, no experimental efficiencies have been reported so far for germanium-based devices. In this work, we present the first TPV germanium efficiency measured in a high view factor calorimetry set up. Efficiencies and power densities up to 11.2 % and 1.43 W/cm2 have been obtained using a graphite emitter heated at 1544 ºC.

Thermionic energy converters are heat engines based on the direct emission of electrons from a hot cathode toward a colder anode. Because the thermionic emission is unavoidably accompanied by photonic emission, radiative energy transfer is a significant source of losses in these devices. In this Letter, we provide the experimental demonstration of a hybrid thermionic-photovoltaic device that is able to produce electricity not only from the electrons but also from the photons that are emitted by the cathode. Thermionic electrons are injected in the valence band of a gallium arsenide semiconducting anode, then pumped to the conduction band by the photovoltaic effect, and finally extracted from the conduction band to produce useful energy before they are reinjected in the cathode. We show that such a hybrid device produces a voltage boost of similar to 1 V with respect to a reference thermionic device made of the same materials and operating under the same conditions. This proof of concept paves the way to the development of efficient thermionic and photovoltaic devices for the direct conversion of heat into electricity.

Germanium is a low-cost alternative to the highly expensive III-V semiconductors commonly used on thermophotovoltaic (TPV) devices. Despite efficiencies up to 16.5 % having been theorized in previous works, no experimental efficiencies have been reported so far for germanium-based devices. In this work, we present the first experimental germanium TPV cell efficiency measured under high view factor and high temperature irradiance conditions. Efficiencies and electric power densities up to 11.2 % and 1.43 W/cm2 have been obtained using a graphite emitter heated at 1440ºC.

Intermediate band solar cells (IBSCs) pursue the increase in efficiency by absorbing below-bandgap energy photons while preserving the output voltage. Experimental IBSCs based on quantum dots have already demonstrated that both below-bandgap photon absorption and the output voltage preservation, are possible. However, the experimental work has also revealed that the below-bandgap absorption of light is weak and insufficient to boost the efficiency of the solar cells. The objective of this work is to contribute to the study of this absorption by manufacturing and characterizing a quantum dot intermediate band solar cell with a single quantum dot layer with and without light trapping elements. Using one-dimensional substrate texturing, our results show a three-fold increase in the absorption of below bandgap energy photons in the lowest energy region of the spectrum, a region not previously explored using this approach. Furthermore, we also measure, at 9K, a distinguished split of quasi-Fermi levels between the conduction and intermediate bands, which is a necessary condition to preserve the output voltage of the cell. 9 pages, 6 figures

Current prototypes of quantum-dot intermediate band solar cells suffer from voltage reduction due to the existence of carrier thermal escape. An enlarged sub-bandgap EL would not only minimize this problem, but would also lead to a bandgap distribution that exploits more efficiently the solar spectrum. In this work we demonstrate InAs/InGaP QD-IBSC prototypes with the following bandgap distribution: EG = 1.88 eV, EH = 1.26 eV and EL > 0.4 eV. We have measured, for the first time in this material, both the interband and intraband transitions by means of photocurrent experiments. The activation energy of the carrier thermal escape in our devices has also been measured. It is found that its value, compared to InAs/GaAs-based prototypes, does not follow the increase in EL. The benefits of using thin AlGaAs barriers before and after the quantum-dot layers are analyzed.

In this work, we study the limiting efficiency of the annual energy production of multijunction solar cells both as independently connected (MJSC-IC) and as series connected (MJSC-SC). Our calculations take into account the impact of hourly and daily variations of the solar spectrum and the geographical latitude. The results confirm the lower dependence with the solar spectrum of independently connected MJSCs and suggest that, in order to progress in practice toward higher annual energy production efficiencies, it could be advisable to develop a 5-junction MJSC-IC rather than to increase the number of solar cells from 5 to 6 under the MJSC-SC approach.