The thermal stability of copper substrate material used in solar thermal collectors was investigated with and without atomic layer deposited (ALD) hafnium oxide barrier films at temperatures of 200-400 degrees C. HfO2 films were studied as barriers against thermal diffusion of copper substrate atoms. The ALD HfO2 thin films were deposited in a thermal process at 200 C using Tetrakis(Dimethylamido)Hafnium(Hf(NMe2)(4)) and H2O precursors, with 200, 400, and 600 cycles. The Cu substrates with and without HfO2 thin films were aged by means of heat treatment in air. The influence of the HfO2 barriers was determined by optical, microstructural, and morphological analyses before and after the ageing procedures. The optical performance of the HfO2 barriers as a part of solar absorber stack was modelled with CODE Coating Designer. The copper surface without a HfO2 barrier thin film oxidized significantly, which increased thermal emittance and surface roughness. 200 cycles of HfO2 deposition did not result in a completely continuous coating and only provided a little protection against oxidation. Films of 200 and 400 cycles gave continuous coverage and the thickest HfO2 thin film studied, which was deposited from 600 ALD cycles and had a thickness 50 nm, prevented Cu oxidation and diffusion processes after 2 h heat treatment in air at 300 C, and retained low thermal emissivity. At 400 C, diffusion and formation of copper oxide hillocks were observed but the HfO2 thin film significantly retarded the degradation when compared to a Cu substrate without and with thinner barrier layers.