Work function engineering of ZnO electrodes by using p-type and n-type doped carbon nanotubes

Transparent electrodes in organic electronic devices are strongly needed in order to replace indium tin oxide (ITO). Some of the best candidates are ZnO films, which have shown both good electronic properties and solution processability compatible with roll-to-roll production of the devices. We present the possibility to engineer the work function of ZnO by blending it with carbon nanotubes (CNTs). B-doped (p-type), N-doped (n-type) and undoped CNTs as well as their blends with ZnO have been characterized by atomic force microscopy (AFM), scanning Kelvin probe microscopy (SKPM) and Raman spectroscopy. The results of Raman spectroscopy demonstrate the substitutional doping of carbon nanotubes, which preserves their covalent structure although increasing the disorder within the nanotubes. The roughness and average shape of grains of ZnO when blended with the doped nanotubes have been measured by AFM. Finally, SKPM shows that the work function of the blends can be engineered from 4.4 ± 0.1 to 4.9 ± 0.1 eV according to the kind of nanotube that is blended even if only a small amount of nanotubes is added to the blend (0.08 wt%).