Copper-indium-gallium-diselenide (CIGS) thin-film solar cells suffer from high recombination losses at the back contact and parasitic absorption in the front-contact layers. Dielectric passivation layers overcome these limitations and enable an efficient control over interface recombination, which becomes increasingly relevant as thin-film solar cells increase in efficiency and become thinner to reduce the consumption of precious resources. We present the optoelectronic and chemical interface properties of oxide-based passivation layers deposited by atomic layer deposition on CIGS. A suitable postdeposition annealing removes detrimental interface defects and leads to restructuring and oxidation of the CIGS surface. The optoelectronic interface properties are very similar for different passivation approaches, demonstrating that an efficient suppression of interface states is possible independent of the metal used in the passivating oxide. If aluminum oxide (Al2O3) is used as the passivation layer we confirm an additional field-effect passivation due to interface charges, resulting in an efficient interface passivation superior to that of a state-of-the-art cadmium-sulfide (CdS) buffer layer. Based on this chemical interface model we present a full-area rear-interface passivation layer without any contact patterning, resulting in a 1% absolute efficiency gain compared to a standard molybdenum back contact. © 2020 authors. Published by the American Physical Society.