Thermophotonics is a variation of TPV in which the thermal source heats a luminescent diode rather than a broadband or metallic absorber as in TPV. This diode then illuminates a solar cell with a spectrum strongly peaked just above their common bandgap [Catchpole et al., 2003]. The advantage over TPV is that no additional selective emitter is required as the luminescent diode fulfills this role, (although its bandwidth is wider than an ideal selective emitter). It does, however, require a diode that has a very high luminescent radiative efficiency. Materials for such a device must be able to cool radiatively, i. e. such that they emit as much or more energy than they absorb as heat. Devices with almost the required radiative efficiency have been fabricated from III-V double heterojunctions [Catchpole et al., 2003]. These only have a temperature increase of mK when illuminated in a photoluminescent mode, with a measured external radiative efficiency of 96%. This is very close to the 98% required for cooling. However, for the diode structure required for thermophotonics the electroluminescent threshold of about 65% external efficiency is the more relevant. This more modest target is perhaps more attainable but does require the very pure properties of the undoped heterojunction device to be incorporated in a doped p-n junction.