Selective absorbers for solar thermal energy applications have to show high solar absorptance alpha(s) and low thermal emittance epsilon(h). Stability against oxidation and diffusion is indispensable, especially at high absorber temperatures. The new class of quasicrystalline materials seems to have favourable properties regarding stability. With a genetic algorithm a thin film stack based on dielectric and quasicrystal films was optimized as a selective absorber. A sandwich system dielectric/quasicrystal/dielectric on copper has highly selective properties : alpha(s)=0.86 and epsilon(h) (400 degrees C)=0.051. Even better results can be achieved, at least in theory, by the use of a cermet. The optical constants of cermets with the quasicrystalline material as the metal were calculated with the Bruggeman theory. A system of a cermet film and an additional antireflective coating on copper shows alpha(s)=0.92 and epsilon(h) (400 degrees C)=0.048.