We consider near-field heat transfer with non-zero chemical potential for photons, as can occur between two semiconductor bodies, held at different temperatures with at least one of the bodies under external bias. We show that the dependence of radiative heat flux on chemical potential enables electronic control of both the direction and magnitude of near-field heat transfer between the two bodies. Moreover such a configuration can operate as a solid-state cooling device whose efficiency can approach the Carnot limit in the ideal case. Significant cooling can also be achieved in the presence of inherent non-idealities including Auger recombination and parasitic phononpolariton heat transfer. a shanhui@stanford.edu

A heated semiconductor light-emitting diode at low forward bias voltage V

We derive the thermodynamic limits of harvesting power from the outgoing thermal radiation from the ambient to the cold outer space. The derivations are based on a duality relation between thermal engines that harvest solar radiation and those that harvest outgoing thermal radiation. In particular, we derive the ultimate limit for harvesting outgoing thermal radiation, which is analogous to the Landsberg limit for solar energy harvesting, and show that the ultimate limit far exceeds what was previously thought to be possible. As an extension of our work, we also derive the ultimate limit of efficiency of thermophotovoltaic systems.

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We consider a near-field thermophotovoltaic device with metal as the emitter and semiconductor as the photovoltaic cell. We show that when the cell is a III-V semiconductor, such as GaSb, parasitic phonon-polariton heat transfer reduces efficiency in the near-field regime, especially when the temperature of the emitter is not high enough. We further propose ways to avoid the phonon-polariton heat transfer by replacing the III-V semiconductor with a non-polar semiconductor such as Ge. Our work provides practical guidance on the design of near-field thermophotovoltaic systems for efficient harvesting of low-quality waste heat. V