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A single-threshold quantum-utilizing device in which the excited carriers thermally equilibrate among themselves, but not with the environment, can convert solar energy with an efficiency approaching that of an infinite-threshold device. Such a hot-carrier flat-plate device operated under typical terrestrial conditions (AM 1.5 illumination, 300 K) can convert solar energy with an efficiency of 66%, substantially exceeding the 33% maximum efficiency of a quantum device operating at thermal equilibrium, and the 52% maximum efficiency of an ideal thermal conversion device. This high efficiency is achieved in part through an unusual inversion, in which the chemical potential of the excited electronic band is below that of the ground band. This negative potential difference reduces radiation losses, permitting a low threshold energy, and a high Carnot efficiency resulting from a high carrier temperature.
Ross et al. (Sat,) studied this question.