Invited paper: Carrier injection barrier formation at metal/organic interfaces

Interfaces play a critical role in determining the performance of organic light-emitting diodes and related organic electronic devices. Various models and mechanisms have been advocated to elucidate the formation of charge-injection barrier at organic/metal interfaces. Charge-injection barrier at organic/metal interfaces has been commonly described as a function of metal work function, although a large scatter exists in experimental data. We introduce the electronegativity concept to characterize the variation of the carrier injection barrier heights at the metal/organic interfaces. We show that the interface barrier heights can be described nicely as a linear function of the metal electronegativity for a number of organic materials. The physical basis for using electronegativity rather than work function is discussed. Barrier height formation is associated with the difference in electronegativity of metals and organ ics that causes charge transfer at the interface between the two solids. The feasibility of extending the concept of electronegativity to compound electrodes (e.g., indium tin oxide) which do not have a well-defined electronegativity is also explored.