Temperature dependence of charge carrier creation in poly(p-phenylene vinylene) [PPV]

The photocurrent decays measured in poly(p-phenylene vinylene) [PPV] from room temperature (RT) down to 77 K in the presence of air traces have been studied. We have observed long-lived decays, between RT and approximately 200 K, which can be fitted to the well known Kohlrausch's law: ip(t) = ip(0) exp-(t/τ)β characteristic of dispersive transport. A detailed temperature dependence study has been carried out to investigate the behavior of the exponent β and the relaxation time τ in order to understand the origin of the observed complex transport mechanism. At lower temperature, especially below 160 K, a fast component is detected alone, where the slow one is frozen. The associated interpretation is based on a recent work of simulated yield of geminate pair dissociation in an energetically random disorder. Modulated experiments show that both components are superimposed at enough high temperature. In our model, the long-lived contribution is interpreted as being extrinsic due to dissociation of polaron pairs assisted by electronegative defects and the fast one as being intrinsic due to dissociation of excitons assisted by the intrinsic energetic disorder, created by the distribution of conjugation length in PPV.