Rate conservation in photoluminescence at elevated temperatures offers new thermodynamic ideas for solar cells
Nimrod Kruger, Matej Kurtulik, Assaf Manor, Tamilarasan Sabapathy and Carmel Rotschild
Department of Mechanical Engineering, Technion − Israel Institute of Technology, Haifa 32000, Israel
The radiance of thermal emission, as described by Planck’s law, depends only on the emissivity and temperature of a body, and increases monotonically with temperature rise at any emitted wavelength. Nonthermal radiation, such as photoluminescence (PL), is a fundamental light–matter interaction that conventionally involves the absorption of an energetic photon, thermalization, and the emission of a red-shifted photon. In this quantum process, radiation is governed by the photon rate conservation and thermodynamically described by the chemical potential. Until recently, the role of rate conservation when thermal excitation is significant had not been studied in any nonthermal radiation, leaving open many questions; for example, what is the overall emission rate if a high quantum efficiency PL material is heated to a temperature where it thermally emits a rate of 50photons/sec at its bend edge, while in parallel, the PL is excited at a rate of 100photons/sec? Here we experimentally demonstrate that the answer is an overall rate of 100 blue-shifted photons/sec. In contrast to thermal emission, the PL rate is conserved if the temperature increases, while each photon is blue-shifted. A further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also demonstrated how endothermic-PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. These findings show that PL is an ideal optical heat pump, and can harvest thermal losses in photovoltaics with theoretical maximal efficiency of 70%. Solutions of the rate equations for non-ideal quantum efficiency, experimentally measured absorption spectrum and available cavities for photon recycling predict a practical device that aims to reach 48% efficiency.
 A. Manor, N. Kruger, T. Sabaphati and C. Rotschild, Thermally-Enhanced Photoluminescence for Heat Harvesting in Photovoltaics, Nat. Commun. DOI:10.1038/ncomms13167 (2016). Also Optics & Photonics news December (2016)