Terahertz complex conductivities of carriers with partial localization in doped polythiophenes

We have investigated the complex conductivity spectra (sigma) over tilde(omega) of two p-doped polythiophenes-poly(3-hexylthiophene) and poly(3,4-ethylenedioxythiophene)-with various carrier densities by using terahertz time-domain spectroscopy. The real part of (sigma) over tilde(omega) is found to gradually decrease with decreasing frequency omega and to approach a finite value for omega -> 0 unlike the Drude conductivity behavior, suggesting that carriers in polythiophenes have a partially localized nature. By reproducing both the measured real and imaginary parts of (sigma) over tilde(omega) with the Drude-Smith model, we show that carriers become less localized with increasing carrier density up to similar to 1.8x10(20) cm(-3). (C) 2010 American Institute of Physics. [doi:10.1063/1.3466916]