page_title: Phonovoltaic
section_title: The suitability of graphene as a phonovoltaic material
hierarchical_section_title: Phonovoltaic / The suitability of graphene as a phonovoltaic material
caption_attribution_description: English: The phonon energy and density of states for various semiconductors and semimetals.
context_page_description: A phonovoltaic cell converts vibrational energy into a direct current much like the photovoltaic effect in a photovoltaic cell converts light into power. That is, it uses a p-n junction to separate the electrons and holes generated as valence electrons absorb optical phonons more energetic than the band gap, and then collects them in the metallic contacts for use in a circuit. The pV cell is an application of heat transfer physics and competes with other thermal energy harvesting devices like the thermoelectric generator.
  While the thermoelectric generator converts heat, a broad spectrum of phonon and electron energy, to electricity, the pV cell converts only a narrow band of phonon energy, i.e., only the most energetic optical phonon modes. A narrow band of excited optical phonons has much less entropy than heat. Thus, the pV cell can exceed the thermoelectric efficiency. However, exciting and harvesting the optical phonon poses a challenge.
context_section_description: As outlined above, an efficient pV cell requires a material with an optical phonon mode more energetic than the bandgap, which in turn is much more energetic than the thermal energy at the intended operating temperature . Furthermore, the pV cell requires a material wherein a hot optical phonon prefers to produce an electron rather than multiple low energy acoustic phonons ().
  Very few materials offer this combination of properties. Indeed, the vast majority of crystals have  optical phonon energies limited to below 50 meV, and those with more energetic optical phonons tend to have much more energetic band gaps. In general, a material with a first-row element (periodic table) is required to have a highly energetic optical phonon. However, the high electronegativity of a first-row elements tends to create a very large band gap, as in diamond and the boron nitride allotropes. Graphene is one of the few materials which diverges from this trend, with no bandgap and an exceptionally energetic optical phonon mode near 200 meV 
  . Thus, graphene has been the initial target for development of a phonovoltaic material through the opening and tuning of its bandgap.
  Opening and tuning the bandgap of graphene has received substantial attention, and numerous strategies have been suggested and investigated. These include the use of uniaxial strain, electric fields, and chemical doping and functionalization. In general, these mechanisms work by either changing the symmetry of graphene (both Carbon atoms in the unit cell are identical) or hybridization ().
  In the first phonovoltaic material investigations, it has been suggested that the latter technique destroys the electron-phonon coupling while the former preserves it. In particular, these investigations predict that hydrogenating graphene, to produce graphane, reduces the electron-phonon coupling so substantially that the material figure of merit vanishes; and that doping graphene with boron nitride maintains the strong electron-phonon coupling in graphene, such that its figure of merit is predicted to reach 0.65 and enable heat harvesting with twice the efficiency of a typical thermoelectric generator.
caption: The phonon energy and density of states for various semiconductors and semimetals.
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