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 (-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 -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 -NPD is presented.