The impact of charge carrier relaxation, electron trapping and oxygen p-doping on the photocurrent transients of a conjugated polymer probed by the Time of Flight method

Most conjugated polymers have been considered so far to be primarily unipolar (hole or electron transporting) materials, even though there is no intrinsic reason why the mobility of holes and electrons should be different in organic semiconductors. We experimentally demonstrated by the Time of Flight method that a prototypical conjugated polymer, poly[2-methoxy 5 (2 ethylhexoxy)-1,4-p-phenylenevinylene], does not show any preference for the transport of electrons or holes, i.e. an ambipolar charge transport. Furthermore, the presence of traps and (unintentional) p-doping makes it difficult to address the intrinsic nature of the charge transport mechanism. However, by disentangling the dependence of the mobility from extrinsic factors, such as molecular oxygen, we were able to reveal a non-dispersive conduction for both holes and electrons. We also reveal a transition from trap-control to thermalization-control in the electrons' charge transport mechanism. We used the Gaussian disorder model to analyze the experimental data, and we found that the positional disorder parameter is negligible for holes, and only the energetic disorder governs their charge transport, while the electrons' conduction is governed by the interplay of both energetic and positional disorder.