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

Abstract

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, ${\text{Cu}}_{2}\text{O}$, ${\text{Ni}}_{2}{\text{O}}_{3}$, ${\text{Co}}_{3}{\text{O}}_{4}$, ${\text{WO}}_{3}$, ${\text{MoO}}_{3}$, ${\text{V}}_{2}{\text{O}}_{5}$, 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 ($\ensuremath{\alpha}$-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 $\ensuremath{\alpha}$-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 $\ensuremath{\alpha}$-NPD is presented.