Electrical and optical characterization of conducting poly-3-methylthiophene film by THz time-domain spectroscopy

Abstract

Using THz time-domain spectroscopy ͑THz-TDS͒, we have measured the absorption and dispersion of 7.5/͑⍀ cm͒ conducting poly-3-methylthiophene film from low frequencies up to 4 THz. From these measurements the complex conductance that increases with increasing frequency was obtained over this frequency range. The results do not follow simple Drude theory and are not comparable with normal metal and semiconductors; the results were well fit by the localization-modified Drude theory. © 2001 American Institute of Physics. ͓DOI: 10.1063/1.1427754͔The frequency-dependent complex conductivity of conducting polymers is one of the most important properties of these materials, 1-4 associated with the 2000 Nobel Prize in Chemistry. THz time-domain spectroscopy ͑THz-TDS͒ has recently been shown to be the ideal tool to characterize the carrier dynamics in conducting polymers, 3 similar to the previous THz-TDS characterizations of semiconductors.5-10 It is not possible to electrically characterize conducting polymers by simple electrical measurements using mechanical contacts, e.g., Hall effect measurements. For such characterization ohmic contacts would have to be fabricated on the polymer itself. The previous far-infrared Fourier transform spectroscopy reflectivity measurements for conducting polymers require the Kramers-Kronig analysis to calculate the absorption and dispersion of the samples.1,2 In our THz-TDS measurements we directly measure both the absorption and dispersion of the conducting polymer film.In the simple Drude theory picture of conduction the key parameters describing the dynamics of free carriers in a material are the plasma frequency p and the carrier damping rate ⌫ϭ1/, where is the carrier collision time. Because p and ⌫ characteristically have THz values, measurements spanning the values of these parameters must be performed in the THz frequency range.3,5-10 The experiments described in this letter present such THz-TDS measurements on a free standing 15 m thick film of 7.5/͑⍀ cm͒ low conductivity poly-3-methylthiophene, prepared electrochemically at Ϫ40°C. Our results determine, for the first time, the absorption and index of refraction from low frequencies to beyond 4 THz. The fact that the measured frequency-dependent absorption and index of refraction are mostly due to the free carriers allows the complex conductivity to be determined from the measurements over the full frequency range. In contrast to the previous THz-TDS measurement of the 215/ ͑⍀ cm͒ high conductivity polypyrrole which was well fit by Drude theory, 3 here for a 30 times lower conductivity 7.5/ ͑⍀ cm͒ sample of poly-3-methylthiophene, the results can only be fit by the localization-modified Drude theory. For the THz-TDS system, a GaAs transmitting antenna with a simple coplanar transmission line geometry and a silicon on sapphire receiving antenna consisting of a micron size dipole antenna embedded in a coplanar transmission line are optoelectronically driven by 16 mW average power beams of 65 fs pulses from a mode-locked Ti:sapphi...

Photoinduced Charge Carrier Generation in a Poly(3-hexylthiophene) and Methanofullerene Bulk Heterojunction Investigated by Time-Resolved Terahertz Spectroscopy

“…The mobility, µ, and carrier density, N, can be obtained if the effective mass, m*, of the carriers is known; µ ) eτ/m*, and N ) 0 ω p 2 m*/e 2 . We take the average effective mass of the electron and hole in P3HT to be 1.7, 17 noting that uncertainties in the effective mass lead directly to uncertainties in the extracted values of the carrier density and mobility. For 2 of the 16 cases shown in Table 3, the P3HT and the 0.8 films at Table 2.…”

“…[17][18][19][20][21] TRTS is a unique experimental probe that allows for the determination of the photonto-carrier generation efficiency because it is sensitive to the charge carriers (or charged species called polarons in conducting polymer) and can probe on time scales short enough to resolve ultrafast carrier generation dynamics. Meanwhile, tightly bound excitonsselectron-hole pairs on a single polymer chain generated from intrachain photoexcitationscontribute to the TRTS response at a much reduced level compared to charge carriers.…”

Nondestructive measurement of carrier mobility in conductive polymer PEDOT:PSS using Terahertz and infrared spectroscopy

“…However, their carrier transport mechanisms are not fully understood. Several groups reported THz complex conductivity of the conductive polymers using THz-TDS [4,5]. Their results show the partially localized carrier behavior and very high carrier scattering rate Γ.…”

Effect of solvent on carrier transport in poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) studied by terahertz and infrared-ultraviolet spectroscopy

“…[8][9][10] These studies obtained the fitting parameters of Drude-like models for THz conductivity to determine the carrier density and mobility by assuming the effective mass m*. However, the carrier damping rate C/2p in such disordered systems is much higher than the measurable frequency range of a conventional THz-TDS system, which results in multiple solutions for the fitting without information of the background dielectric constant in the absence of carriers e bTHz in the THz region.…”