Electroluminescence properties of organic light-emitting diodes with a red dye doped into Alq3:rubrene mixed Host

We have studied the electroluminescence (EL) properties of devices with a red fluorescent dye, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), doped into a mixed matrix of tris-(8-hydroxyquinoline)aluminum (Alq$_3$) and rubrene. The devices doped with DCJTB in the Alq$_3$:rubrene mixed host show an efficient red emission from DCJTB with negligible EL emission from both Alq$_3$ and rubrene. The QE increases with increasing rubrene concentration and reaches a maximum of about 3.6 % for a DCJTB doping concentration of about 5 % in the Alq$_3$:rubrene mixed (50:50 % ratio by weight) host, and then decreases at higher rubrene concentration, due to the concentration quenching effect. At a given bias voltage, the current density increases with increasing rubrene doping concentration, but it decreases with increasing DCJTB doping concentration. The results imply that the injected electrons and holes can transport via hopping through the energy levels of rubrene molecules while DCJTB acts as traps