EMISSION MECHANISM IN RUBRENE-DOPED MOLECULAR ORGANIC LIGHT-EMITTING-DIODES - DIRECT CARRIER RECOMBINATION AT LUMINESCENT CENTERS

The emission mechanism in molecularly doped organic light-emitting dio des, where the emitting layer is composed of N,N'-diphenyl-N,N' -bis(3 -methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) as the host and 5,6,11 ,12-tetraphenylnapthacene (rubrene) as the dopant, is investigated in terms of energy transfer and direct carrier recombination, Hole trappi ng by rubrene is identified by current versus voltage and mobility mea surements in single layered devices. Shallow traps are formed and are found to be filled by injected holes at electric field above 2 x 10(5) V/cm, Electroluminescence observed in single-layered devices indicate that electrons can be injected directly into the hole transporter, TP D, In double-layered devices composed of TPD and tris-(8-hydroxyquinol inato) aluminum(III) (Alq(3)), the penetration depth of electrons into undoped TPD is determined to be less than or equal to 5 nm from the A lq(3) interface. Upon doping with rubrene, the emission zone is extend ed to 20 nm due to the increase in the electron penetration depth. Thi s is attributed to the transition of the electron hopping sites from T PD to rubrene molecules. At high-rubrene concentration, electron trans port occurs via hopping on the rubrene molecules. The dominant emissio n mechanism in rubrene-doped TPD is attributed to the electron-hole re combination at the dopant molecule. This is maximized by hole trapping and electron transport of rubrene.