Trapped polarons, free polarons and triplet excitons in π-conjugated polymer films and light-emitting diodes

Recent optically detected magnetic resonance studies of poly (p-phenylene vinylenes) (PPVs) and poly(3-alkyl thiophenes) (P3ATs) suggest that excitation at visible wavelengths λex largely generates trapped polarons and polaron pairs, but UV excitation and carrier injection in LEDs generate free polarons. The photoluminescence (PL)-enhancing polaron resonance at long λex is attributed to non-radiative recombination of trapped polaron pairs, which reduces their population and consequently the rate at which they non-radiatively quench singlet excitons. Interchain coupling, some defects induced by structural disorder, and sites adjacent to C60- counterions apparently enhance the generation of these trapped polarons as well as intersystem crossing from the singlet to the triplet manifold. At short λex a PL-quenching polaron resonance emerges, and is assigned to fusion of photogenerated free polarons to bipolarons, which also quench singlet excitons. These interpretations imply that the l1Bu exciton binding energy, defined as its dissociation energy to free polarons, is the difference between the absorption edge and the excitation energy at which the PL-quenching polaron resonance emerges. This difference is ∼0.8 eV in poly(2,5-dioctoxy-p-phenylene vinylene) and poly (3-hexylthiophene).