Comparison of the electroluminescence of a red fluorescent dye doped into the Alq3 and Alq3:rubrene mixed host

We studied the effect of a mixed host of Alq3 and rubrene on the energy transfer and charge trapping processes in organic light-emitting devices with a red fluorescent dopant of 4-(dicyanomethylene)-2-tert-butyl-6 (1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB). The temperature dependence of electroluminescence (EL) properties is compared for the device with DCJTB doped into the Alq3 only host and that with the Alq3:rubrene mixed host. The device with the Alq3:rubrene mixed host shows an efficient red emission from DCJTB, negligible EL emission from Alq3, and a lower EL drive voltage compared to the device with the Alq3 only host. Upon cooling the device temperature, the EL emission from rubrene increases but the emission from Alq3 is still weak, and the quantum efficiency (QE) is almost temperature-independent for the device with the Alq3:rubrene mixed host. On the contrary, the EL emission from Alq3 increases and the QE decreases for devices with the Alq3 only host at low temperature. The results indicate that recombination of injected electrons and holes occurs on rubrene and subsequent energy transfer to DCJTB dominates in the device with the Alq3:rubrene mixed host.