3 years ago

A Silicon Ratchet to Produce Power from Below-Bandgap Photons

A Silicon Ratchet to Produce Power from Below-Bandgap Photons
Ofer Kedem, Mark A. Ratner, Bryan Lau, Emily A. Weiss, Mohamad Kodaimati
This paper computationally demonstrates a new photovoltaic mechanism that generates power from incoherent, below-bandgap (THz) excitations of conduction band electrons in silicon. A periodic sawtooth potential, realized through elastic strain gradients along a 100 nm thick Si slab, biases the oscillatory motion of excited electrons, which preferentially jump and relax into the adjacent period on the right to generate a net current. The magnitude of the ratchet current increases with photon energy (20, 50, and 100 meV) and irradiance (≈MW cm−2), which control the probability of photon scattering, and peaks as a function of the well depth of the ratchet potential, and the dominant mode of energy loss (the 62 meV intervalley phonon). The internal power conversion efficiency of the ratchet has a maximum of 0.0083% at a photon energy of 100 meV, due to inefficiencies caused by isotropic scattering. This new photovoltaic mechanism uses wasted below-bandgap absorptions to enhance the directional diffusion of charge carriers and could be used to augment the efficiency of traditional photovoltaics. A new intraband photovoltaic scheme based on silicon is presented. An asymmetric, periodic “ratchet” potential rectifies intraband excitations of electrons in the conduction band. Unlike previous examples of light-powered ratchets, this work uses incoherent, unpolarized THz radiation, which opens the possibility of using this new intraband photovoltaic mechanism to enhance traditional photovoltaics.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/aenm.201701000

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