Semiconductor thermionics for next generation solar cells: photon enhanced or pure thermionic?

Abstract

Semiconductors have been used in solar energy conversion for decades based on the photovoltaic effect. An important challenge of photovoltaics is the undesired heat generated within the device. An alternative approach is thermionics, which uses the thermal excitation of electrons from an emitter to a collector across a vacuum gap. If the emitter is a p-type semiconductor, the photogeneration-induced quasi-Fermi level splitting can reduce the effective barrier for electron emission—a mechanism used by a photon enhanced thermionic emission device. Here, we evaluate the prospects of this alternative solar conversion technology considering different semiconductor materials and thermionic device configurations. We also reveal that whether such a device operates in the photon enhanced or purely thermionic mode, depends on the complex interplay among materials properties, device physics and solar concentration level.

The effects of electronic and photonic coupling on the performance of a photothermionic-photovoltaic hybrid solar device

“…To obtain R , we implemented the radiative, surface, Auger and Shockley-Reed-Hall recombination processes using theories valid under both low and high injection levels [37]. The detailed implementation of the emitter model can be found in [11].…”

“…These radiative exchanges can be enhanced due to nonequilibrium radiative recombination processes. Thermal radiative exchange with the ambient is calculated using the Stefan-Boltzmann law considering the spectral emissivity of the PT emitter material [11]. The radiative exchange between the PT emitter and the PV sub-device strongly depends on the vacuum gap width, and we calculate it using fluctuational electrodynamics as explained above.…”

“…Similarly to the PV effect, this approach exploits the quantum nature of solar photons by generating electron-hole pairs. On the other hand, like CSP plants, it also utilizes any excess energy (which is lost by the thermalization process in the PV mechanism) of the absorbed above-bandgap solar photons [11]. However, PT devices have their own limitations.…”

Hybrid thermionic-photovoltaic converter with graphene-on-semiconductor heterojunction anode for efficient electricity generation

“… Schwede et al., (2010) developed photon-enhanced thermionic emission (PETE) of p-type semiconductor cathode, which reduces the electron emission barrier by the difference between photo-induced electron’s quasi-Fermi level and Fermi level, so that the output current density is enhanced. Very recently, Rahman and Nojeh ( Rahman and Nojeh, 2021 ) proposed that the PETE mode could only operate under specific conditions. Otherwise, it still operates at pure thermionic mode.…”

Preparation of graphene aerogel and application in photon-enhanced thermionic emission

“…Compared with silicon solar cells, a PETE device has higher theoretical efficiency, 2–5 and its limit of efficiency was estimated to be as high as 40–52%. 6 If the thermal cycle is considered, 8,9 a total efficiency of 70% 7 can be reached. Previously, research into PETE has mainly been theoretical, 10,11 and this paper will focus on the experiment itself.…”