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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 ∼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.

Quantum-dot (QD) intermediate-band (IB) materials are regarded as promising candidates for high-efficiency photovoltaics. The sequential two-step two-photon absorption processes that take place in these materials have been proposed to develop high-efficiency solar cells and infrared (IR) photodetectors. In this work, we experimentally and theoretically study the interrelation of the absorptivity with transitions of carriers to and from the IB in type II GaSb/GaAs QD devices. Our devices exhibit three optical bandgaps with: E L =0.49 eV, E H =1.02 eV and E G =1.52 eV, with the IB located 0.49 eV above the valence band. These values are well supported by semi-empirical calculations of the QDs electronic structure. Through intensity-dependent twophoton photocurrent experiments, we are able to vary the filling state of the IB, thus modifying the absorptivity of the transitions to and from this band. By filling the IB with holes via E=1.32 eV or E=1.93 eV monochromatic illumination, we demonstrate an increase in the E L-related absorptivity of more than two orders of magnitude and a decrease in the E H-related absorptivity of one order of magnitude. The anti-symmetrical evolution of those absorptivities is quantitatively explained by a photo-induced shift of the quasi-Fermi level of the IB. Furthermore, we report the observation of a two-photon photovoltage; i. e., the contribution of sub-bandgap two-photon absorption to increase the open-circuit voltage of solar cells. We find that the generation of the twophoton photovoltage is related, in general, to the production of a two-photon photocurrent. However, while photons with energy close to E L participate in the production of the two-photon photocurrent, they are not effective in the production of a two-photon photovoltage. We also report the responsivity of GaSb/GaAs QD devices performing as optically triggered photodetectors. These devices exhibit an amplification factor of almost 400 in the IR spectral region. This high value is achieved by minimizing-via doping-the absorptivity in the IR range of the QDs under equilibrium conditions. I. INTRODUCTION

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Abstract-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 E L 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: E G = 1.88 eV, E H = 1.26 eV and E L > 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 E L . The benefits of using thin AlGaAs barriers before and after the quantum-dot layers are analyzed.

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