Zinc oxide is attractive for photovoltaic applications due to its properties as Transparent Conductive Oxide (TCO), as well as for rectifying heterojunctions to silicon. The present work investigates the electrical properties of the ZnO/Si heterojunction and the influence of buffer layers; i.e. ZnO/buffer/Si structures. The structures have been produced with buffer thicknesses in the range 5-20 nm, composed of amorphous silicon (aSi), silicon germanium (aSiGe) or silicon carbide (aSiC). The compositional variation is done to alter the band gap of the buffer layer; approximately 1.9 eV was anticipated for aSi and the addition of Ge should decrease this value while an increase is expected from C. The deposition techniques Plasma Enhanced Chemical Vapor Deposition (PECVD) and sputter deposition was used for synthesis, and characterization has primarily been focused on electrical characteristics. Current-Voltage (IV) characteristics show high rectification for most samples, and an improvement is observed for the structures with a buffer layer compared to the samples without buffer layer on p-type substrates. Both IV and Capacitance-Voltage (CV) measurements indicate deviations from thermionic emission theory and the Schottky-Mott model, which is further elaborated on by studying the temperature dependence of these measurements. These deviations in IV and CV data are supported by broad peaks observed in Deep Level Transient Spectroscopy (DLTS) spectra, indicating distributions of defects throughout the band gap. Fermi level pinning is discussed as an explanation for the high barrier heights derived from the IV measurements. Accompanying this, inhomogeneity in the junction properties is considered to explain the difference in the IV and CV results, as well as temperature dependences of these. Finally, as a proof of concept, the samples have been investigated for photovoltaic effect in a solar simulator.