Optically Detected Magnetic Resonance (ODMR) Studies of Trions in Organic Light-Emitting Materials and OLEDs, and Their Possible Relation to Long-Term OLED Degradation - art. no. 70510G

Some recent photoluminescence (PL)- and electroluminescence (EL)-detected magnetic resonance (PLDMR and ELDMR, respectively) studies of the negative (PL- and EL-quenching) spin 1/2 resonance are reviewed. These include the resonances in poly[2-(N-carbazolyl)-5-(2'-ethyl)-hexoxy-1,4-phenylenevinylene] (CzEh-PPV) films, rubrene films, tris(quinolinolate) Al (Alq(3)) OLEDs, rubrene-doped Alq(3) OLEDs, and fac tris(2-phenylpyridine) iridium [Ir(ppy(3))]- doped poly(N-vinyl carbazole) (PVK) polymer LEDs (PLEDs). The resonances are all assigned to quenching of SEs by trions, which are bipolarons stabilized by a counterpolaron or counterion. As bipolarons arc spinless, their formation from two like-charged polarons is spin-dependent, and hence enhanced at resonance. This enhanced formation, and the resulting enhanced quenching of SEs, yields the negative spin 1/2 PLDMR and ELDMR. As previously shown, since trion formation also reduces the mobility of the trapped carriers, this process also results in a negative spin 1/2 electrical Current-detected magnetic resonance (EDMR). Importantly, since the counterpolaron is usually trapped, e.g., at organic/cathode or organic/organic interfaces, or at impurity sites such the oxygen center in rubrene, it is suspected that the trions might be responsible for the long term degradation of OLEDs and PLEDs associated with abrupt junctions or impurities.