dc current flowing through a 10–15 nm-thick P3HT film exposed to the fumes of (tridecauoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTS) as a function of exposure time. Dashed line: P3HT on a semi insulating (transparent for IR) Si(111) substrate. Solid line: P3HT on a glass substrate. Measurements are performed with applied between graphite contacts that define a P3HT film. The left inset shows the structure of FTS molecules. The right inset shows a conceptual geometry of 2-probe samples. The apparent large background conductivity in the P3HT/Si device is due to the residual bulk conduction through the Si substrate.
IR response of P3HT film modified by electrostatic doping in an OFET structure and under exposure to FTS fumes. Top panel: mid-IR absorption for a device gated at (black dashed curve) and (gray dashed curve); for FTS modification with no applied bias (blue solid curve). Bottom panel: Change in the absorption coefficient for the same device. Bottom inset: Schematic of FTS-treated OFET device.
IR absorption of four different P3HT-based structures extended down to to include the far-IR range of the spectrum. The blue curve is FTS-induced absorption of the FET device presented in Fig. 2. All other curves are 2-probe (ungated) P3HT/Si structures doped with FTS. The curves are labeled according to their integrated polaron spectral weight (detailed in text) and listed in order of polaron absorption strength. Inset: vibrational spectrum of an FTS-treated P3HT/Si sample and FTS-coated KBr substrate.
IR absorption spectra collected from different spots on a single P3HT/Si sample doped with FTS to saturation. Probed locations are separated by 1 mm.
Effective 2D spectral weight of the polaron band and corresponding 2D carrier density plotted as a function of the applied gate voltage for () and ()-based FET devices (triangles). The blue circles are and associated with the FTS-doped P3HT structures, with error bars due to the uncertainties in film thickness. Dotted and dashed lines indicate doping levels of 5% and 10%, respectively, for P3HT.
Schematic energy level diagram displaying three possible scenarios for an insulator-metal transition in P3HT upon doping: (i) a first-order transition from a bipolaron to a polaron lattice, leading to a partially-filled band; the (ii) polaron or (iii) bipolaron band broadens to merge with the valence band.
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