Doping, density of states, and conductivity in polypyrrole and poly(p-phenylene vinylene)

The evolution of the density of states (DOS) and conductivity as a function of well-controlled doping levels in OC₁C₁₀-poly(p-phenylene vinylene) [OC₁C₁₀-PPV] doped by FeCl₃ and PF₆, and PF₆-doped polypyrrole (PPy-PF₆), has been investigated. At a doping level as high as 0.2 holes per monomer, the former one remains nonmetallic while the latter crosses the metal-insulator transition. In both systems, a similar almost linear increase in DOS as a function of charges per unit volume (c*) has been observed from the electrochemical gated transistor data. In PPy-PF₆, when compared to doped OC₁C₁₀-PPV, the energy states filled at low doping are closer to the vacuum level; by the higher c* at high doping, more energy states are available, which apparently enables the conduction to change to metallic. Although both systems on the insulating side show log0.2em0exσ∝T^-1∕4 as in variable range hopping, for highly doped PPy-PF₆ the usual interpretation of the hopping parameters leads to seemingly too high values for the density of states.