Majority carrier injection from ITO anodes into organic light-emitting diodes based upon polymer blends

The current-field characteristics of organic light-emitting diodes (LEDs), formed by sandwiching blends of hole-transporting molecules and various polymer binders between indium-tin oxide (ITO) and Al contacts, have been measured. It is concluded that the rate-limiting step is hole injection at the ITO anodes. At high fields, temperature-independent tunneling injection prevails, the efficiency of which scales with the mobility of the charge carriers in the transport layer. Increasing the polarity of the matrix enhances injection because of barrier lowering. The low field behaviour (E < 106 V cm−1) is controlled by a superposition of thermionic injection and fluctuation induced tunneling, the latter being more pronounced with polar systems.