Inverse I-V Injection Characteristics of ZnO Nanoparticle-Based Diodes

Simple Al/ZnO(NP)/Au diodes produced by spin coating of ZnO nanoparticle dispersions (ZnO(NP)) on Al/Al2O3 and Au substrates and subsequent Au deposition have been investigated to understand electron injection properties of more complex devices, incorporating ZnO(NP) as injection layer. Inverse I-V characteristics have been observed compared to conventional Al/ZnO(SP)/Au diodes produced by reactive ion sputtering of ZnO. SEM micrographs reveal that the void-containing contact of ZnO(NP) with the bottom Al electrode and the rough morphology of the top Au electrode are likely to be responsible for the observed injection and ejection probabilities of electrons. A simple tunneling model, incorporating the voids, explains the strongly reduced injection currents from Al whereas the top electrode fabricated by vapor deposition of Au onto the nanoparticle topology adopts the inverse ZnO(NP) morphology leading to enlarged injection areas combined with Au-tip landscapes. These tips in contrast to the smooth sputtered ZnO(SP) lead to electric field enhancement and strongly increased injection of electrons in reverse direction. The injected charge piles up at the barrier generated by voids between ZnO(NP) and the bottom electrode forcing a change in the barrier shape and therefore allowing for higher ejection rates. Both effects in combination explain the inverse I-V characteristic of nanoparticle based diodes.