Band transport evidence in PEDOT:PSS films using broadband optical spectroscopy from terahertz to ultraviolet region

Abstract

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a prototypical conducting polymer. When a polar solvent is used during film fabrication, the hole-doped PEDOT oligomers form crystalline clusters in the films, exhibiting high conductivity. However, whether hole carriers exhibit band transport has not been clarified yet. Here, we employ a multilayer spin-coating method using an aqueous solution with ethylene glycol, with additional procedures of dipping the films in ethylene glycol or dropping sulfuric acid onto the films, to achieve a high DC conductivity of ∼1000 S cm⁻¹ or higher. Using terahertz time-domain spectroscopy and far-infrared-to-ultraviolet reflection spectroscopy, we derive complex optical conductivity $\tilde{\sigma }$ spectra, which are reproduced by the sum of the Drude response, and Lorentz-oscillator responses due to phonons. These results demonstrate the band transport, which is further confirmed by the Hall effect measurements. The hole mobility estimated from the spectral analyses is 7–11 cm² V⁻¹ s⁻¹, a significantly large value. The reported evaluation methods for broadband $\tilde{\sigma }$ spectra can help elucidate carrier transport mechanisms in various conducting films.

Hole-doped polymer PEDOT:PSS shows high conductivity but the carrier transport mechanism is not yet clarified. Here, broadband optical conductivity spectra are derived using terahertz time-domain spectroscopy and far-infrared to-ultraviolet reflection spectroscopy, demonstrating band transport of hole carriers.