The fabrication and characterization of nickel oxide films and their application as contacts to polymer/porous silicon electroluminescent devices

Porous silicon electroluminescent devices have been fabricated from n-type substrates using indium tin oxide, hole-transporting poly(9-vinyl carbazole) and p-type nickel oxide films as hole injecting contacts. The addition of the polymer layer, which increases the contact area by penetrating into the porous microstructure, leads to an increase in the device quantum efficiency of two orders of magnitude. The replacement of indium tin oxide by nickel oxide, formed by a thermal evaporation process, lowers the device switch-on voltage from 55-60 V to 10-15 V. The p-type nature of the nickel oxide film allows holes to be injected from the valence band of the contact, and hence at a lower applied voltage than that required for indium tin oxide contacts. The luminescence power output from both devices is similar, and we suggest that the limiting factor in the luminescent output is the rate of carrier flow throughout the device nanostructures.