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.
Highlights
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 main advantage of the near-field thermophotovoltaic (NF-TPV) devices is the enhancement by orders of magnitude of the rate of photon absorption in the photovoltaic (PV) cell[1,2,3]
Two electrical transport models are used: the common one which is valid only when the injection of electrical carriers is low – the Minority Carrier Separation (MCS) model, the other one which is valid for any injection level of electrical carriers – the Full Drift-Diffusion (FDD) model
Summary
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. Even though the near-field enhancement of photon absorption and electron-hole pair (EHP) injection is the backbone of NF-TPV devices, high-injection levels were not considered likely events This may appear as an excessive assumption since radiation fluxes locally absorbed by the PV cell in a NF-TPV device largely exceed the one-Sun illumination level (1 kW·m−2) and may come close (e.g. up to 10 MW·m−2 in the extreme near field in ref.19) to the maximum illumination that solar concentrated PV cells allow Two electrical transport models are used: the common one which is valid only when the injection of electrical carriers is low – the Minority Carrier Separation (MCS) model –, the other one which is valid for any injection level of electrical carriers – the Full Drift-Diffusion (FDD) model– (see Methods) In this manuscript, the analysis is concentrated on the impact of varying the acceptor doping density on the electrical output power in the case of a GaSb p-on-n junction cell
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