Au/ ZnO(np)/ Al diodes produced by spin coating of ZnO nanoparticle (np) dispersions exhibit abnormal I-V characteristics compared to sputtered Au/ ZnO(sp)/ Al diodes produced by reactive oxygen-ion sputtering (sp) of the ZnO layer. Whereas the reactive ion sputtered diode shows the energetically expected injection from the Al electrode under positive bias (forward bias), the spin-coated ZnO(np) diode displays an inverse I-V characteristic. Since Al is used as bottom electrode in both kind of diodes the expected formation of a native Al₂O₃ layer is unlikely to be responsible for the different I-V characteristics. SEM micrographs of the diode’s cross section indicate that most likely the geometrical structure of the contact of the ZnO nanoparticles with the native Al/Al₂O₃ layer on the one side and the tip-like shape of the Au top-electrode on the other side are responsible for the drastic decrease of the injection probability of electrons under forward bias and for the strong increase of the current under reverse bias. A simple tunnelling model shows that the existance of air filled voids at the ZnO-Al₂O₃/Al interface can explain the strongly reduced injection current under forward bias. The strongly increased injection current under reverse bias (using the nomenclature of the sputtered device) can also be understood by the nanoparticulate structure of the ZnO top surface where deposition of Au onto the ZnO(np) surface leads to an enlarged surface area and a tip-like surface structure in contrast to a smooth interface in the case of sputtered ZnO. These Au-tips lead to high local electric fields which can explain the observed strong increase of the injection current under reverse bias. The same inverse I-V characteristics can be observed for an Au/ZnO(np)/Au device supporting strongly the proposed mode of operation of the nanoparticles on the injection processes. Characteristics of different diodes will be compared and observed diode characteristics will be rationaled by theoretical considerations along the above lines of argumentation. Possible impact on already published and more complex device structures like light emitting diodes and organic solar cells will be discussed comprising all functional nanoparticulate ZnO- layers.