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 se lective emitters used in thermophotovoltaic applications has been perf ormed, It has been found that the effective directional emittance depe nds strongly on the physical thickness, refractive index and spectral extinction coefficient of the films. The emissivity exhibits a saturat ion, for an optical depth greater than unity, which is dependent on th e 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 muc h lower thicknesses are required for absorbing films. Significantly, a n anomalous increase in emittance at low thickness for high-index film s has been observed, which might be of particular interest in thermoph otovoltaic applications. The emittance has also been found to be stron gly 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 tem perature and total directional emittance, for the wavelength range 0.9 -1.5 mu m. The data presented provide a design model for the optimum t hickness of a thin-film thermophotovoltaic emitter.