Emission properties of dopants rubrene and coumarin 6 in Alq3 films

We have investigated the emission properties of dopants 5,6,11,12-tetraphenylnapthacene (rubrene) and 3-(2′-benzothiazolyl)-7-diethylaminocoumarin (coumarin 6) as well as co-doping of these two dopants in tris (8-hydroxyquinolinato) aluminum (Alq3) films in double-layer organic light emitting diodes (OLEDs). We varied the rubrene (Rb) doping concentration in Alq3:Rb films up to 10 wt%. The maximum luminescence efficiency of ∼6.5 cd/A was observed for Rb doping concentration of ∼0.7 wt% in Alq3:Rb film, which was nearly double efficiency compared to pure Alq3 device. The co-doping of dopants of C-6 and Rb in the ratio of 1:1 and 1:2 in Alq3 films reduced the bias voltage compared to pure Alq3 and Alq3:C-6 devices for the same current density. The maximum luminescence efficiency was improved to ∼7 cd/A in Alq3:{C-6:Rb(1:2)} film OLED. The direct recombination of holes and electrons in the dopant molecules may be responsible for the improvement of the luminescence efficiency. We also observed the shifting of photoluminescence (PL) and electroluminescence (EL) peaks position from ∼515 to ∼562 nm by co-doping of Rb and C-6 in Alq3.