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

Novel method to evaluate the density and mobility of carriers in conductive polymers is presented. The method utilizes the broadband spectral information of THz and infrared region. Terahertz-time domain spectroscopy revealed that the strong carrier scattering induced the weakly localized carrier behavior in conductive polymer poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate acid) (PEDOT:PSS) thin films. However, it is impossible to determine the carrier density and mobility uniquely only from the spectral information in THz range (0.1-4THz) when the carrier scattering rate is much larger than the THz frequency. We successfully determine the mobility of carriers in PEDOT:PSS thin films by the simultaneous fitting of THz conductivity and infrared reflectivity spectra. The dc conductivity increase by the secondary doping can be attributed to the mobility enhancement induced by the improvement of the disorder in PEDOT:PSS thin films. Under the assumption of the effective mass m*=0.3, the obtained carrier mobility of PEDOT:PSS thin film with and without the secondary dopant of ethylene glycol are 9.77 and 0.77 cm²/Vs, respectively.