By the first law of thermodynamics, the excitation driving electron generation in both photo- and phonovoltaic cells, i.e., the photon or phonon, must have more energy than the semiconductor band gap. For a PV cell, many materials are available with a band gap ( Δ Δ E e , g ) well matched to the solar photon spectrum, like Silicon or Gallium Arsenide. For a pV cell, however, no current semiconducting materials have a band gap smaller than the energy of their most energetic (optical) phonon modes ( E p , O ). Thus, novel materials are required with both energetic optical phonon modes ( E p , O ≥ ≥ 100 meV, e.g., graphene, diamond, or boron nitride) and a small band gap ( Δ Δ E e , g < E p , O , e.g., graphene).