Delocalization and insulator to metal transition in PEDOT:PSS

Charge transport in poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS), with systematic increase in the percentages of dimethyl sulfoxide (DMSO) and ethylene glycol (EG), is investigated as a function of temperature and electric field down to 4.2 K. The conductivity in pristine PEDOT:PSS (1.4 S cm^-1) increased to 1070 S cm^-1with 10% of DMSO, and to 565 S cm^-1with 6% of EG. The conductivity ratio (σr=σ300K/σ4.2K) is substantially reduced to 2.5 in 10% DMSO sample. This gradual transition from insulator (variable range hopping) to disordered metal (T^1/2fit), with an intermediate critical regime (power law), is observed by small variations in the volume fractions of these solvents. The temperature dependence of the ratio of hopping length to localization length (R/Lc)is rather weak for samples (2D & 4EG) near the transition. As the delocalization increases in more conducting samples, larger electric field dependence of conductivity is observed at lower fields (0.1 V cm^-1). The real part of impedance decreases at higher frequency and the peak in the imaginary part of impedance shifts to higher frequency as the sample conductivity increases.