  We report on infrared (IR) absorption and dc electrical measurements of thin
films of poly(3-hexylthiophene) (P3HT) that have been modified by a fluoroalkyl
trichlorosilane (FTS). Spectra for FTS-treated films were compared to data for
electrostatically-doped P3HT in an organic field-effect transistor (OFET). The
appearance of a prominent polaron band in mid-IR absorption data for
FTS-treated P3HT supports the assertion of hole doping via a charge-transfer
process between FTS molecules and P3HT. In highly-doped films with a
significantly enhanced polaron band, we find a monotonic Drude-like absorption
in the far-IR, signifying delocalized states. Utilizing a simple capacitor
model of an OFET, we extracted a carrier density for FTS-treated P3HT from the
spectroscopic data. With carrier densities reaching 10$^{14}$ holes/cm$^2$, our
results demonstrate that FTS doping provides a unique way to study the
metal-insulator transition in polythiophenes.
