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

摘要

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 $sigma@-enhancing. The PL-enhancing resonance is attributed to nonradiative recombination of trapped polaron pairs, which reduces their population and consequently the rate at which they nonradiatively quench singlet excitons; the EL-enhancing resonance is tentatively assigned to the same mechanism in the CN-PPV layer, but other mechanisms are not ruled out. Interchain coupling, some defects induced by structural disorder, and sites adjacent to dopant molecules (e.g., C$-60$/) 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.