  We present research results on the electronic transport in heterostructures
based on p-type nickel oxide (NiO) with the n-type oxide semiconductors zinc
oxide (ZnO) and cadmium oxide (CdO). NiO is a desirable candidate for
application in (opto-)electronic devices. However, because of its small
electron affinity, heterojunctions with most n-type oxide semiconductors
exhibit conduction and valence band offsets at the heterointerface in excess of
1 eV. ZnO/NiO junctions exhibit a so called type-II band alignment, making
electron-hole recombination the only process by which a current can vertically
flow through the structure. These heterojunctions are nevertheless shown to be
of practical use in efficient optoelectronic devices, as exemplified here by
our UV-converting transparent solar cells. These devices, although exhibiting
high conversion efficiencies, suffer from two light-activated recombination
channels connected to the type-II interface, one of which we identify and
analyse in more detail here. Furthermore, CdO/NiO contacts were studied - a
heterostructure with even larger band offsets such that a type-III band
alignment is achieved. This situation theoretically enables the development of
a 2-dimensional electronic system consisting of topologically protected states.
We present experiments demonstrating that the CdO/NiO heterostructure indeed
hosts a conductive layer absent in both materials when studied separately.
