Investigation of charge carrier mobility in 5,6,11,12-tetraphenylnapthacene (rubrene) and coumarin 6 doped Alq3 films

The doping effect on charge carrier mobility in tris (8-hydroxyquinolinato) aluminum (Alq3) was studied by time-of-flight (TOF) measurements. The polar dopant, coumarin 6 (C-6) and extensive ππ conjugated dopant, 5,6,11,12-tetraphenylnaphthacene (rubrene) were used for this study. The co-doped of rubrene (Rb) with C-6 into Alq3 improved the carrier mobility compared to the single doped Alq3:C-6 film. The carrier mobility in single doped Alq3:C-6 film did not follow the linear relationship of Poole–Frenkel (PF) model with applied electric field. The mobility was in agreement with the PF model at two different ranges of electric fields (FF) separated by a critical field Fc≈373(V/cm)1/2. The mobility in co-doped Alq3:(Rb:C-6) film followed the linear relationship with the PF model. The energetic disorder was found as ∼0.32 eV in co-doped films. It was ∼0.55 and ∼0.27 eV before and after the critical field in Alq3:C-6 film. The values of positional disorders in co-doped films were estimated as ∼1.8 and it was ∼2 in Alq3:C-6 film at F≤373(V/cm)1/2. The organic light emitting diode performance of the co-doped film was improved compared to single doped film. The luminescence efficiency was improved tremendously to ∼6  Cd/A in co-doped device at 45 mA/cm2 current compared to Alq3:C-6 film device of ∼1  Cd/A.