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Infrared signatures of high carrier densities induced in semiconducting poly(3-hexylthiophene) by fluorinated organosilane molecules

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.