Negative mobility dependence on electric field in poly(3-alkylthiophenes) evidenced by the charge extraction by linearly increasing voltage method

Carrier mobility measurements were performed in the regioregular poly(3-octylthiophene) (P3OT) and poly(3-hexylthiophene) (P3HT) samples by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. The samples were produced by the drop-casting and spin-coating methods on the Indium–Tin–Oxide (ITO) substrate and provided with Al contacts on the top.In both materials we had observed the “negative” mobility dependence at low electric fields, i.e., the mobility used to decrease with increasing electric field. Such behaviour was not observed by the CELIV method in P3OT before. Data were analysed within the Poole–Frenkel and Gaussian disorder models. The higher energetic disorder parameters were obtained within the framework of both models in P3OT as compared to P3HT. Probably this could be related to the longer side-chains of P3OT. We had demonstrated that in both materials conductivity grows with increasing temperature due to the thermally activated mobility, and not due to the change of carrier number, as it could be expected in case of their thermal generation.