Intragap-trapped-carriers enhancement of the low-temperature delayed phosphorescence in Alq3

Time resolved electroluminescence of Alq3 based organic light emitting devices was measured on a millisecond time scale. The millisecond delayed electroluminescence (DEL) was detected at 80 K and 26 K, while no long lived electroluminescence was observed at room temperature. The DEL spectrum consists of two components centered at ∼2.3 and ∼1.9 eV. The shape of the high energy band is similar to the prompt fluorescence emission peak, although a systematic red shift of about 30 meV is reproducibly found. The low-energy band position corresponds to the phosphorescence of Alq3. The time decay of both bands is nonexponential, with the low energy band decaying slower. To explain dependencies of intensities of both components on the delay and the injection-current density a mechanism based on enhancement of the triplet-exciton radiative decay by carriers trapped in deep intragap traps is proposed.