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.