Analysis of charge-injection characteristics at electrode-organic interfaces: Case study of transition-metal oxides

The formation of resistance-free or Ohmic contacts at metal/organic interfaces remains a significant challenge for achieving high-performance organic electronic devices such as organic light-emitting diodes. Several oxides have recently been reported to yield extremely low-voltage devices and thus have excited a renewed interest in developing the next generation of contacting electrodes. In this paper, major metal oxides, CuO, Cu2O, Ni2O3, Co3O4, WO3, MoO3, V2O5, and indium tin oxide, have been systematically studied to compare their relative performance as hole injection anodes, as well as to provide an experimental database for theoretical analysis of current-voltage (IV) characteristics with a diverse range of injection barrier heights. Contrary to previous reports in the literature, none of the oxides studied in this work were found to form a true Ohmic contact with commonly used hole transport layers, such as N,N-diphenyl-N, N-bis-1-naphthyl-1-1-biphenyl-4,4-diamine (alpha-NPD). This discrepancy is attributed to incorrect IV data analysis of the quasi-Ohmic injection regime-the region in between space-charge limited current (SCLC) and injection limited current (ILC)-in previous studies. It is found that the quasi-Ohmic regime is much larger (i.e., covers a greater range of injection barrier height) than has previously been expected. A criterion that defines Ohmic, quasi-Ohmic, and injection limited contacts has been quantified based on a time-domain simulation of charge transport across alpha-NPD single-carrier devices. This criterion includes the effects of the electric field dependent mobility, organic layer thickness, and charge-injection barrier height. The effects of the built-in potential on the IV characteristics are also evaluated. A barrier-thickness-voltage phase diagram that defines the regions of SCLC, quasi-Ohmic, and ILC for alpha-NPD is presented.