Quadrimolecular recombination kinetics of photogenerated positive polarons and C60− radical anions in regioregular poly(3-alkylthiophene)/C60 composites measured by light-induced ESR

Light-induced electron spin resonance (LESR) measurements have been performed to study recombination kinetics of photogenerated charge carriers in the composites of regioregular poly(3-alkylthiophene) (RR-PAT) and C60. Two transient LESR signals due to photogenerated positive RR-PAT polarons and C60− radical anions are remarkably enhanced for the excitation energy of ∼1.8 eV. We observe an ∼Iex0.25 power dependence of the prompt decay LESR component on excitation-light intensity Iex. This behavior is well explained by a model of quadrimolecular recombination (QR) of the photogenerated charge carriers. The QR is dominant at higher Iex while bimolecular recombination (BR) is relatively important at lower Iex. The time decay of the LESR intensity is also well explained by the QR model. The temperature dependence of the QR rate constant shows a crossover from tunneling to hopping behavior with increasing temperature; the activation energy for the hopping of 0.10 eV is obtained. In the regiorandom PAT/C60 composites, only BR is observed.