For transparent front contacts of nanostructured silicon solar cells aluminum doped zinc oxide (ZnO:Al), deposited by atomic layer deposition (ALD), is investigated. For this purpose it is crucial that ZnO:Al covers the nanostructures conformally. The ZnO:Al deposition at a temperature of 225°C, compatible with the underlying solar cell structures, yields a resistivity of 1.2•10-3 Ωcm and 85% mean optical transmittance in the Vis-NIR range (< 1300 nm). The complex dielectric function of the ALD-ZnO:Al was determined by fitting the optical spectra with a multi-oscillator model. An investigation of the layer structure revealed a preferential growth in the c-direction of the hexagonal ZnO crystal and 100-200 nm long wedge-shaped crystallites. I-V measurements on planar ZnO:Al/a-Si:H(n/p)/c-Si(n+/p+) test structures showed the ohmic nature of the ZnO:Al contact to both n- and p-type a-Si:H. Simple planar solar cells exhibited an excellent rectification and good open circuit voltages VOC = 620…640 mV. Finally, the feasibility of nanostructure silicon heterojunction solar cells could be demonstrated by showing the conformal coating of deep Si nanowire structures.