Thermo-optical Characterization of TPV Selective Emitter Using Ellipsometry

Thermophotovoltaic (TPV) systems convert thermal radiation into electricity, and their efficiency may depend strongly on the temperature-dependent optical properties of the emitter material. To support the design of high-temperature selective emitters, we characterized the permittivity of aluminum nitride (AlN), silicon carbide (4H-SiC), and aluminum oxide on aluminum nitride (Al2O3/AlN) thin film/substrate pairs from room temperature to 600ºC. Spectroscopic ellipsometry measurements were performed from 240 nm to 2500 nm. Using a general oscillator model, we established accurate room-temperature optical models with mean-square errors of 1.239, 4.08, and 29.89, respectively. The results are consistent with literature values. For higher temperatures, systematic changes in the complex refractive index (n, k), associated with phonon broadening and carrier-induced absorption, were observed. These results elucidate the role of thermal and electronic effects on the optical behavior in wide-bandgap ceramics, and provide key input for emitter design in TPV applications operating up to approximately 1800 °C.