Terahertz complex conductivities of carriers with partial localization in doped polythiophenes

We have investigated the complex conductivity spectra σ̃(ω) 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 σ̃(ω) is found to gradually decrease with decreasing frequency ω and to approach a finite value for ω→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 σ̃(ω) with the Drude–Smith model, we show that carriers become less localized with increasing carrier density up to ∼1.8×1020 cm−3.