Formation of Ohmic carrier injection at anode/organic interfaces and carrier transport mechanisms of organic thin films

We have shown that hole mobilities of a wide variety of organic thin films can be estimated using a steady-state space-charge-limited current (SCLC) technique due to formation of Ohmic hole injection by introducing a very thin hole-injection layer of molybdenum oxide (MoO₃) between an indium tin oxide anode layer and an organic hole-transport layer. Organic hole-transport materials used to estimate hole mobilities are 4,4′,4″-tris(N-3- methylphenyl-N-phenyl-amino)triphenylamine(m-MTDATA), 4,4′,4″- tris(N-2-naphthyl-N-phenyl-amino)triphenylamine (2-TNATA), rubrene, N,N′-di(m-tolyl)-N,N′-diphenylbenzidine (TPD), and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4, 4′-diamine (α-NPD). These materials are found to have electric-field-dependent hole mobilities. While field dependence parameters (β) estimated from SCLCs are almost similar to those estimated using a widely used time-of-flight (TOF) technique, zero field SCLC mobilities (μ₀) are about one order of magnitude lower than zero field TOF mobilities.