On the nature of trapped polarons in π-conjugated polymers

Optically detected magnetic resonance (ODMR) measurements on a wide array of π-conjugated polymers invariably yield a narrow spin 1/2 photoluminescence (PL)-enhancing polaron resonance at g ~ 2.0023± 0.0010. In typical poly(p-phenylene vinylenes) (PPVs) and poly(3-alkyl thiophenes) (P3ATs) a similar PL-quenching resonance emerges at short excitation wavelengths. The microwave chopping frequency dependence of the PL-enhancing resonance is consistent with two distinct polaron pair lifetimes, of ~30 and ~600μs in PPV and ~10 μs and ~1.8 ms in P3HT. Since the emission from PPVs in the microsecond and longer time scale is negligible, the PL-enhancing resonance is attributed to nonradiative recombination of trapped polarem pairs, which eliminates their contribution to nonradiative quenching of singlet excitons. The PL-quenching resonance is assigned to fusion of free polarons to bipolarons. The two distinct lifetimes of the trapped polaron pairs are discussed in relation to the following sites which might induce or stabilize them: (i) adjacent segments of neighboring chains; (ii) either sides of a conjugation defect on the same chain. The nature of their spin correlation, whether parallel or antiparallel off-resonance, and the implications of a scenario in which the energy of the quenched singlet may dissociate the polaron pair, are also discussed.