MAJORITY CARRIER INJECTION FROM ITO ANODES INTO ORGANIC LIGHT-EMITTING-DIODES BASED UPON POLYMER BLENDS

The current-field characteristics of organic light-emitting diodes (LE Ds), formed by sandwiching blends of hole-transporting molecules and v arious polymer binders between indium-tin oxide (ITO) and Al contacts, have been measured. It is concluded that the rate-limiting step is ho le injection at the ITO anodes. At high fields, temperature-independen t tunneling injection prevails, the efficiency of which scales with th e mobility of the charge carriers in the transport layer. Increasing t he polarity of the matrix enhances injection because of barrier loweri ng. The low field behaviour (E<10(6) V cm(-1)) is controlled by a supe rposition of thermionic injection and fluctuation induced tunneling, t he latter being more pronounced with polar systems.