On the effects of optical constants and physical parameters on the omittance of thin-film selective emitters for thermophotovoltaic applications

A parametric study of the thermal radiative properties of thin-film selective emitters used in thermophotovoltaic applications has been performed, It has been found that the effective directional emittance depends strongly on the physical thickness, refractive index and spectral extinction coefficient of the films. The emissivity exhibits a saturation, for an optical depth greater than unity, which is dependent on the refractive index and extinction coefficient of the material, namely the higher the refractive index and extinction coefficient, the lower the saturation value. For transparent films, the thickness required to achieve saturation is of the order of tens of micrometres whereas much lower thicknesses are required for absorbing films. Significantly, an anomalous increase in emittance at low thickness for high-index films has been observed, which might be of particular interest in thermophotovoltaic applications. The emittance has also been found to be strongly dependent on the emissivity and reflectivity of the substrates and on the direction of propagation of radiation. The total hemispherical emittance of the films has been found to increase with increasing temperature and total directional emittance, for the wavelength range 0.9-1.5 mu m. The data presented provide a design model for the optimum thickness of a thin-film thermophotovoltaic emitter.