Highly conductive and transparent ZnO:Al films for solar cell applications were deposited by reactive co-sputtering of Zn and Al from separate targets onto silicon substrates. The structure of the films was characterized with FE-SEM and HREM. Films with Al-doping levels up to 5 at.% show a columnar structure. The columnar grains are mainly composed of ZnO in the hexagonal wurtzite structure. The single grains consist of mutually tilted parts. Stacking faults and twins were observed. Al influences the growth of nuclei of columnar grains. Near the interface nanocrystallites of different phases were discovered. At the ZnO/Si interface an amorphous silicon oxide layer was formed. The nanocrystalline region grows with increasing Al-doping level. Above an Al concentration of 5 at.% the films exhibit a nanocrystalline structure.