  An ideal solar thermal absorber has a sharp transition between high and low
absorptance at the wavelength where the blackbody emissive power begins to
exceed the solar irradiance. However, most real selective absorbers have a
fairly broad transition, leading to both solar absorption and thermal emission
losses. Here, we model, fabricate, and characterize a highly selective
semiconductor-dielectric-metal (Ga0.46In0.54As - MgF2 - Ag) solar absorber with
an extremely sharp transition from high to low absorptance. The thin
semiconductor serves as a selective filter, absorbing photons with wavelengths
shorter than the bandgap and transmitting those with longer wavelengths. The
highly reflective dielectric-metal rear mirror allows the structure to have
very low emittance for longer wavelengths. These characteristics provide the
absorber with a measured solar absorptance >91% below the bandgap wavelength
and infrared emittance <5% at 100 C above the bandgap wavelength. This
transition wavelength can be tuned by modifying the semiconductor composition,
and modeling indicates that the absorber's optical properties should be stable
at high temperatures, making the structure a good candidate for unconcentrated
to highly concentrated solar thermal energy conversion.
