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 ${\mathrm{OC}}_1{\mathrm{C}}_{10}$-poly($p$-phenylene vinylene) [${\mathrm{OC}}_1{\mathrm{C}}_{10}$-PPV] doped by ${\mathrm{FeCl}}_3$ and ${\mathrm{PF}}_6$, and ${\mathrm{PF}}_6$-doped polypyrrole (PPy-${\mathrm{PF}}_6$), 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 $\left(c^{*}\right)$ has been observed from the electrochemical gated transistor data. In PPy-${\mathrm{PF}}_6$, when compared to doped ${\mathrm{OC}}_1{\mathrm{C}}_{10}$-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 $\log \sigma \propto T^{-1∕4}$ as in variable range hopping, for highly doped PPy-${\mathrm{PF}}_6$ the usual interpretation of the hopping parameters leads to seemingly too high values for the density of states.