  Modeling radiative recombination is crucial to the analysis of photonic
energy converters. In this work, a local radiative recombination coefficient is
defined and derived based on fluctuational electrodynamics that is applicable
to thin-film cells in both the near field and far field. The predicted
radiative recombination coefficient of an InAs cell deviates from the van
Roosbroeck-Shockley relation when the thickness is less than 10 um and the
difference exceeds fourfold with a 10 nm film. The local radiative
recombination coefficient is orders of magnitude higher when an InAs cell is
configured in the near field. The local radiative recombination coefficient
reduces as the doping level approaches that of a degenerate semiconductor. The
maximum output power and efficiency of a thermoradiative cell would be
apparently overpredicted if the luminescence coefficient (defined in this
letter) were taken as unity for heavily doped semiconductors.
