In this paper, the photoluminescence(PL) from thin films of factris(2-phenylpyridine) iridium (Ir(ppy)3) doped poly(N-vinylcarbazol) (PVK) modulated by forward bias and reverse bias was measured for the sake of studying the generation and evolution of triplet exciton. The results show that the electric field induced quenching of PL happens before the intersystem crossing from PVK to Ir(PPY)3. Once the triplet excitons at Ir(PPY)3 are formed by the energy transfer from PVK to Ir(PPY)3, it is difficult to dissociate the excitons at Ir(PPY)3. The triplet excitons locating at Ir(PPY)3 molecules is very stable. The electroluminescence mechanism of Ir(PPY)3 doped PVK is also investigated by . For different forward biases, no obvious difference in the profiles of PL spectra is observed, which excludes the influence of electric field and injected carriers on the intersystem crossing from PVK to Ir(PPY)3. Comparing EL spectra and the electric field modulated PL spectra of Ir(PPY)3 doped PVK, we can more reasonably deduce that the main EL emission from Ir(PPY)3 does not come from the energy transfer between PVK chains and Ir(PPY)3 molecules which obeys Föster rule, but comes from the direct recombination of injected carriers at Ir(PPY)3 molecules which act as both the carrier-trap and recombination centers.
AC electroluminescence from organic thin film electroluminescent (OTFEL) devices based on polymer poly(p- phenylene-vinylene) and molecular compound tris(8- hydroxyquinoline) aluminum (Alq3)was observed. The emission spectra of these devices driven by ac were recorded. The current-voltage (I-V) and brightness-voltage (B-V) were measured. It shows that the performances of OTFEL devices which were driven by ac is as good as those driven by dc. It opens a new application field that organic thin film electroluminescent devices can be driven by both dc and ac.
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