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

We investigated electroluminescent (EL) characteristics of single-layer organic light emitting diodes (SOLEDs). Our SOLED devices are composed of an inert polymer as a binder, in which hole transport molecules, emissive electron transport molecules (ETMs), and highly fluorescent dopants as luminescent centers are dispersed. We examined two typical dopants: rubrene and coumarin 6. These exhibited different charge carrier recombination and emission mechanisms. The dopant concentration dependence of the current density–voltage–luminance relationships clearly showed the importance of carrier trapping by dopant molecules for obtaining high luminance. When the dopant was rubrene, we observed that charge carriers were well trapped by the dopant molecule. This means that direct recombination of holes and electrons occurred on the dopant molecules and trapping significantly enhanced the external EL quantum efficiency ${\Phi }_{\mathrm{EL}}.$ For coumarin 6, on the other hand, we observed that charge carriers primarily recombined at the emissive ETMs and that the energy transfer from the host to the guest coumarin 6 molecule dominated the EL process. A comparison of these distinct processes revealed that carrier trapping by dopant molecules was necessary to enhance ${\Phi }_{\mathrm{EL}}$ in SOLED devices. In our best SOLED device with rubrene as a dopant, we measured luminance of 2800 ${\mathrm{cd/m}}^2$ at $\text{J=100}$ ${\mathrm{mA/cm}}^2,$ which corresponds to ${\Phi }_{\mathrm{EL}}\text{=0.85\%.}$