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 diodes, 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-tetraphenylnaphthacene (rubrene) as the dopant, is investigated in terms of energy transfer and direct carrier recombination. Hole trapping by rubrene is identified by current versus voltage and mobility measurements in single-layered devices. Shallow traps are formed and are found to be filled by injected holes at electric field above 2×10⁵ V/cm. Electroluminescence observed in single-layered devices indicate that electrons can be injected directly into the hole transporter, TPD. In double-layered devices composed of TPD and tris-(8-hydroxyquinolinato) aluminum(III) (Alq₃), the penetration depth of electrons into undoped TPD is determined to be ⩽5 nm from the Alq₃ interface. Upon doping with rubrene, the emission zone is extended to 20 nm due to the increase in the electron penetration depth. This is attributed to the transition of the electron hopping sites from TPD to rubrene molecules. At high-rubrene concentration, electron transport occurs via hopping on the rubrene molecules. The dominant emission mechanism in rubrene-doped TPD is attributed to the electron-hole recombination at the dopant molecule. This is maximized by hole trapping and electron transport of rubrene