Poster
Electron-hole transport over surfaces or interfaces in Si, SiC and GaAs based β-voltaic systems
1V. Lashkaryov Institute of Semiconductor Physics, Kyiv, Ukraine
2University of Ontario Institute of Technology, Oshawa, ON, Canada
3Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John's, NL, Canada
The idea of betavoltaic energy converters, the low-power long-lifetime energy sources that use beta-electrons in conjunction with p-n junctions or Schottky diodes, was proposed a long time ago [1]. But only recently did it begin attracting both experimental and theoretical attention due to the variety of possible applications in hostile or inaccessible environments (see, e.g., [2-5]). The physics of carrier transport through the surfaces and interfaces of the generated electron-hole pairs can be explained in terms of a complex interplay between the surface and bulk related processes, scattering dynamics and recombinations. In this presentation we offer a theoretical formalism to model the role of surfaces or interfaces in generated carrier transport and determine the system's maximum attainable conversion efficiency ηlim, which sets an exact target for experimental research. Although our formalism of carrier transport is essentially based on the similarity of betavoltaic and photovoltaic processes [6], several important differences must be properly included in the description of the physical processes of the electron-hole transport in the beta-voltaics systems.
The efficiency of a betavoltaic converter is a product of three terms: η = ηβ · ηc · ηs. Here ηβ=Nβ/N0 is the ratio of the beta-flux Nβ, reaching the semiconductor surface to the total flux N0 emitted by the beta-source; ηc is the coupling efficiency, depending on the electron reflection from the semiconductor surface and proportional to a collection efficiency of electron-hole pairs Q; finally, ηs is the semiconductor efficiency, similar to that of photovoltaic conversion efficiency. While the bidirectional betavoltaic system design allows getting ηβ close to unity, the second and third terms in the efficiency can be optimised: they depend on the surface and interface associated mechanisms of carriers' transfer, surface recombination, diffusion and appearance of the so-called "dead layer". (The dead layer is the region close to the beta-electron source surface, where the generations of the electron-hole pairs by energetic beta-electrons can be neglected, and its extent depends on the initial beta-electron energy). The realistic experimentally achieved parameters of the betavoltaic systems were included in the analytical formalism developed. The calculated efficiencies indicate a limit to the maximum possible performance of the betavoltaic systems, e.g., η = 8% for tritium based 3H/GaAs system. While being comparable to experimentally achieved efficiencies, our results demonstrate that there is still sufficient room for efficiency increase using optimised materials parameters and the system design.
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