Comprehensive picture ofp-type doping of P3HT with the molecular acceptor F4TCNQ
By means of optical spectroscopy, Kelvin probe, and conductivity measurements, we study the $p$-type doping of the donor polymer poly(3-hexylthiophene), P3HT, with the molecular acceptor tetrafluorotetracyanoquinodimethane, F${}_{4}$TCNQ, covering a broad range of molar doping ratios from the ppm to the percent regime. Thorough quantitative analysis of the specific near-infrared absorption bands of ionized F${}_{4}$TCNQ reveals that almost every F${}_{4}$TCNQ dopant undergoes integer charge transfer with a P3HT site. However, only about 5$%$ of these charge carrier pairs are found to dissociate and contribute a free hole for electrical conduction. The nonlinear behavior of the conductivity on doping ratio is rationalized by a numerical mobility model that accounts for the broadening of the energetic distribution of transport sites by the Coulomb potentials of ionized F${}_{4}$TCNQ dopants.