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 ( E p , O ≃ ≃ Δ Δ E e , g ≫ ≫ k B T p V ) . Furthermore, the pV cell requires a material wherein a hot optical phonon prefers to produce an electron rather than multiple low energy acoustic phonons ( γ γ ˙ ˙ e − − p ∗ ∗ → → 1 ).