Maxwell-Garnett theory analysis of thick PbSAl selective absorbers

Abstract

Previous measurements of the solar absorptance and thermal emittance values of PbSAl tandem solar selective surfaces have resulted in α ϵ ratios significantly higher than predicted for thick (>200 nm) PbS layers, assuming optical constants equal to those of the bulk material. We report calculations of photothermal conversion efficiencies for PbsAl tandem absorbers using the Maxwell-Garnett theory to determine α and ϵ values for porous PbS films. We have also prepared samples with PbS thicknesses up to 550 nm and have determined their α and ϵ values using reflectance measurements. Thermal efficiencies in the range of 0.55 to 0.86 were obtained at 300 K and are consistent with values expected for films with moderate (0.15–0.35) void fractions.