Charge carrier trapping effect by luminescent dopant molecules in single-layer organic light emitting diodes

We investigated electroluminescent (EL) characteristics of single-layer org anic light emitting diodes (SOLEDs). Our SOLED devices are composed of an i nert polymer as a binder, in which hole transport molecules, emissive elect ron transport molecules (ETMs), and highly fluorescent dopants as luminesce nt centers are dispersed. We examined two typical dopants: rubrene and coum arin 6. These exhibited different charge carrier recombination and emission mechanisms. The dopant concentration dependence of the current density-vol tage-luminance relationships clearly showed the importance of carrier trapp ing by dopant molecules for obtaining high luminance. When the dopant was r ubrene, we observed that charge carriers were well trapped by the dopant mo lecule. This means that direct recombination of holes and electrons occurre d on the dopant molecules and trapping significantly enhanced the external EL quantum efficiency FEL. For coumarin 6, on the other hand, we observed t hat charge carriers primarily recombined at the emissive ETMs and that the energy transfer from the host to the guest coumarin 6 molecule dominated th e EL process. A comparison of these distinct processes revealed that carrie r trapping by dopant molecules was necessary to enhance FEL in SOLED device s. In our best SOLED device with rubrene as a dopant, we measured luminance of 2800 cd/m(2) at J = 100 mA/cm(2), which corresponds to Phi(EL) = 0.85%. (C) 1999 American Institute of Physics. [S0021-8979(99)09415-3].