Evaluating the index of refraction of thin films at high temperatures for thermophotovoltaic emitters

In thermophotovoltaics, photovoltaic cells convert heat from a thermal emitter to electricity. One way to obtain high-efficiency devices is to have the emitted spectrum tailored to a solar cell. One way to achieve this is to use a thin film emitter configuration, where one could optimize the geometric configuration. However, the index of refraction of materials is likely to change at ultra-high temperatures, which may change the optimal geometric conditions. In this work, we characterize the index of refraction of various metals and dielectrics at high temperatures. The materials chosen have melting points above 1800ºC. Also, the thermal stability and the mismatch in the thermal expansion coefficient of each material combination are discussed. As a proof of concept, we evaluate the best emitter candidates for five different solar cell technologies. This work can pave the way for high-temperature photonics with a simple emitter design, resulting in higher efficient photoelectronic devices.