High-injection effects in near-field thermophotovoltaic devices

Abstract

In near-field thermophotovoltaics, a substantial enhancement of the electrical power output is expected as a result of the larger photogeneration of electron-hole pairs due to the tunneling of evanescent modes from the thermal radiator to the photovoltaic cell. The common low-injection approximation, which considers that the local carrier density due to photogeneration is moderate in comparison to that due to doping, needs therefore to be assessed. By solving the full drift-diffusion equations, the existence of high-injection effects is studied in the case of a GaSb p-on-n junction cell and a radiator supporting surface polaritons. Depending on doping densities and surface recombination velocity, results reveal that high-injection phenomena can already take place in the far field and become very significant in the near field. Impacts of high injection on maximum electrical power, short-circuit current, open-circuit voltage, recombination rates, and variations of the difference between quasi-Fermi levels are analyzed in detail. By showing that an optimum acceptor doping density can be estimated, this work suggests that a detailed and accurate modeling of the electrical transport is also key for the design of near-field thermophotovoltaic devices.

Thermionic-enhanced near-field thermophotovoltaics

“…The S-matrix method allows calculating radiation emitted by the tungsten and lanthanum hexaboride thermally emitting layers and absorbed by any layer, as a function of the vacuum gap thickness (d), and over spectral intervals of interest [31]. In particular, radiation power absorbed by the receiver constituted of the collector, the p-n junction and the back reflector, properties, the diffusion equations are solved as in [46]- [48] in order to derive the characteristic and associated parameters. In particular, the electrical power output and current density at the maximum power point ( , , ) are respectively used for inferring the TPV converter efficiency and the TIC converter current matching that of the TPV device.…”

New insights into the thermal behavior and management of thermophotovoltaic systems

“…Radiative heat transfer between the radiator and the PV cell is computed within the frame of fluctuational electrodynamics [31,32] in conjunction with the S-matrix approach [33]. The calculated radiative power absorbed by the PV cell is then used to compute the local generation rate of electron-hole pairs G within control volumes formed by the spatially discretization of the PV device [28][29][30]34]…”

“…The optical properties required for the calculations in the range [0.5 µm -30 µm] are measured with ellipsometry for W, HfO 2 , and taken from [36] for Au. For GaSb, the same model as in [ 29,34] is used. In Fig.…”

Micron-sized liquid nitrogen-cooled indium antimonide photovoltaic cell for near-field thermophotovoltaics

“…In addition, absorption by the free carriers and the phonons must be included because it is has tremendous impacts on the efficiency of the converter, especially in the near field because of subbandgap phonon-polariton heat transfer [17]. Radiation transfer and electrical models, available elsewhere [10,15,27], are briefly reminded. In the following section, results are presented and discussed.…”