A Silicon Ratchet to Produce Power from Below-Bandgap Photons

Scientific Achievement

We show, with a 3D model of silicon under incoherent THz irradiation, that below-bandgap excitations (free carrier or intraband absorptions) can do useful work by exciting, i.e. flashing, electrons in and out of a structurally embedded ratchet potential.

Significance and Impact

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.

Top: Schematic of the device simulated. Multiple strips of silicon-germanium alloy could create the strain gradients (shown as graded shaded regions) in an experimental system. Middle: the ratchet potential (orange) that results from gradients in parallel strain along the x-direction. Bottom: Current density as a function of the average depth of the wells in the ratchet potential, ∆E, for irradiation of the Si ratchet with 100 meV photons and a series of irradiances (MW/cm2).

Research Details

  • 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 peaks as a function of the well depth of the ratchet potential, and the dominant mode of energy loss (the 62 meV intervalley phonon)

Work performed at Northwestern University