Interface-limited injection in amorphous organic semiconductors - art. no.085201

We examine electron transport in the archetype amorphous organic material t ris(8-hydroxyquinoline) aluminum (Alq(3)). It is established that for Al, L iF/Al, and Mg:Ag cathodes, injection processes at the metal/organic contact dominate the current-voltage characteristics. We find that transport is al so injection-limited at low temperatures, but that the cathode dependence o f current-voltage characteristics at T=30 K is substantially reduced, raisi ng doubts over metal-to-organic injection models that depend on the cathode work function. Given that ultraviolet photoelectron spectroscopy measureme nts show a shift in the vacuum potential at the metal/Alq(3) interface of s imilar to1 eV, we investigate the impact of interfacial dipoles on adjacent molecules in the organic film. Consequently, we propose that injection is limited by charge hopping out of interfacial molecular sites whose energy d istribution is broadened by local disorder in the interfacial dipole field. We derive a general analytic model of injection from interfacial states an d find that it accurately predicts the current-voltage characteristics of t ransport in Alq(3) over many orders of magnitude in current and over a wide range of temperatures. The model is extended to other amorphous organic se miconductors and is found to be applicable to both polymers and small molec ular weight organic compounds.