Doping-dependent charge injection into regioregular poly(3-hexylthiophene)

We investigate charge injection from gold into regioregular poly(3-hexylthiophene) (P3HT) as a function of doping, by studying the nonlinear current-voltage characteristics of Au/P3HT/Au devices at different doping levels. The comparison of these characteristics allows us to distinguish contact-limited from bulk-limited transport. We demonstrate that there is a significant barrier to charge-injection from the contacts into the polymer, in spite of the good alignment of the Au work-function relative to the energy gap of P3HT and that the contact limitation is particularly strong at low doping levels. The contact resistance with a Ti electrode is similarly doping-level dependent. Our results show that the ability to control the doping level in organic semiconductors can be used as a tool to investigate the electronic properties of devices prepared from these materials. (C) 2003 Elsevier B.V. All rights reserved.