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

Abstract

Recent optically detected magnetic resonance studies of poly ( p-phenylene vinylenes) (PPVs) and poly(3-alkyl thiophenes) (P3ATs) suggest that excitation at visible wavelengths λ ex 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 λ ex 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 C 60 - counterions apparently enhance the generation of these trapped polarons as well as intersystem crossing from the singlet to the triplet manifold. At short λ ex 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 l 1B u 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).