Optically detected magnetic resonance (ODMR) studies of π-conjugated polymer films and light-emitting diodes (LEDs)

The photoluminescence (PL)-, electroluminescence (EL)- and conductivity ((sigma) )-detected magnetic resonance of poly(p-phenylene vinylene) (PPV), poly(p-phenylene ethynylene) (PPE), and PPV/CN-PPV LEDs is reviewed and discussed. In the PPV- and PPE-based LEDs the polaron resonance is EL-quenching, but in the PPV/CN-PPV bilayer diodes it contains both EL-quenching and enhancing components. While the (sigma) -detecting polaron resonance is invariably quenching in the PPV devices, in some of the PPE-based LEDs it is (sigma) -enhancing. The PL-enhancing resonance is attributed to nonradiative recombination of trapped polaron pairs, which reduces their population and consequently the rate at which they nonradiatively quench singlet excitons; the EL-enhancing resonance is tentatively assigned to the same mechanism in the CN-PPV layer, but other mechanisms are not ruled out. Interchain coupling, some defects induced by structural disorder, and sites adjacent to dopant molecules (e.g., C60) apparently enhance the generation of these trapped polarons as well as intersystem crossing from the singlet to the triplet manifold. The EL- and (sigma) - quenching resonances are attributed to the fusion of like-charged free polarons to bipolarons, which is also suspected to be induced by disorder and/or impurities. The LEDs also exhibit half-field EL- and (sigma) -detected triplet exciton resonances. Triplet-triplet fusion to singlets and the role of triplets as quenching sites for singlet excitons are discussed as possible mechanisms leading to the triplet resonances.