Tuning the carrier injection efficiency for organic light-emitting diodes

Organic light-emitting diodes based on tris-8-(hydroxyquinoline) aluminum and N.N'-diphenyl-N,N'-(2-naphthyl)-(1,1'- -phenyl)-4,4'-diamine (NPB) ordinarily have an operating stability of less than 1000 h. Stability and efficiency are, however, dramatically enhanced by interposing a copper phthalocynanine (CuPc) layer between the indium tin oxide (ITO) anode and the NPB hole-transport layer. To specifically understand the origin of an interlayer-induced increase in light output, we have directly measured and analyzed the effective hole injection efficiency from ITO into NPB as a function of the CuPc interlayer thickness. Because NPB is trap free, a direct and self-consistent measure of the hole injection efficiency can be determined from the measured drift mobility and dark current at the same applied field. By this means, we have been able to demonstrate that the hole transport from the ITO anode to the NPB is injection-limited and that the hole injection efficiency is reduced progressively as the CuPc interlayer thickness is systematically increased in the range from 0 to 30 nm. In fact, the diminution of hole injection from ITO into NPB is already significant for a CuPc layer thickness of as little as 2 nm. The ITO anode used in the present study has a work function of 4.4 eV, as measured by ultraviolet photoemission spectroscopy. The increase in device efficiency induced by insertion of a CuPc layer is correlated with a decrease in hole injection efficiency. As a consequence, it must be attributed to an improved balance between hole and electron currents arriving at the recombination zone.