  The radiance of thermal emission, as described by Planck law, depends only on
the emissivity and temperature of a body, and increases monotonically with the
temperature rise at any emitted wavelength. Nonthermal radiation, such as
photoluminescence, is a fundamental light matter interaction that
conventionally involves the absorption of an energetic photon, thermalization,
and the emission of a redshifted photon. Such a quantum process is governed by
rate conservation, which is contingent on the quantum efficiency. In the past,
the role of rate conservation for significant thermal excitation had not been
studied. Recently, we presented the theory and an experimental demonstration
that showed, in contrast to thermal emission, that the PL rate is conserved
when the temperature increases while each photon is blueshifted. A further rise
in temperature leads to an abrupt transition to thermal emission where the
photon rate increases sharply. We also demonstrated how such thermally enhanced
PL, TEPL, generates orders of magnitude more energetic photons than thermal
emission at similar temperatures. These findings show that TEPL is an ideal
optical heat pump that can harvest thermal losses in photovoltaics with a
maximal theoretical efficiency of 70%, and practical concepts potentially
reaching 45% efficiency. Here we move the TEPL concept onto the engineering
level and present Cr:Nd:YAG as device grade PL material, absorbing solar
radiation up to 1 micrometer wavelength and heated by thermalization of
energetic photons. Its blueshifted emission, which can match GaAs cells, is 20%
of the absorbed power. Based on a detailed balance simulation, such a material
coupled with proper photonic management can reach 34% power conversion
efficiency. These results raise confidence in the potential of TEPL becoming a
disruptive technology in photovoltaics.
