where T_\mathrm{pV} is the temperature of the pV cell and T_{p,\mathrm{O}} is the temperature of the optical phonon population, as dictated by the Bose–Einstein statistics. This efficiency is reduced the smaller the band gap is in comparison to the thermal energy (k_\mathrm{B}T, where k_\mathrm{B} is the Boltzmann constant and T is the temperature). Indeed, the p-n junction efficiency is approximately
