In this work, zinc oxide films were deposited using an argon-fed expanding thermal plasma with diethylzinc and oxygen admixed downstream. The substrate temperature influence on the films’ electrical and structural properties has been investigated. An increase of crystallinity and surface roughness with increasing substrate temperature was found. The conductivity measurements indicated resistivity values as low as for the films deposited at 200 C. For these films, in situ real-time spectroscopic ellipsometry was employed in order to investigate the film growth, i.e., the thickness evolution and the optical properties. Particular attention was paid to the evolution of the surface roughness, an important property in solar cell applications where a rough front electrode is needed for light trapping. Complementary ex situ diagnostics, such as atomic force microscopy and X-ray diffraction, were used to support the outcome of the ellipsometric investigation. The films deposited at 200 C are found to be polycrystalline, with preferential (002) orientation and high surface roughness (up to 10% of the film thickness) at all investigated stages of growth.