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

TL;DR

This study investigates how doping affects the density of states and conductivity in polypyrrole and poly(p-phenylene vinylene), revealing differences in their transition to metallic behavior and the nature of their electronic states.

Contribution

It provides detailed experimental analysis of the doping-dependent electronic properties of PPV and polypyrrole, highlighting the conditions under which they transition from insulators to conductors.

Findings

Polypyrrole crosses the metal-insulator transition at high doping levels.

DOS increases linearly with charge density in both materials.

Highly doped PPy shows metallic conduction with variable range hopping behavior.

Abstract

The evolution of the density of states (DOS) and conductivity as function of well controlled doping levels in OC_1C_10-poly(p-phenylene vinylene) [OC_1C_10-PPV] doped by FeCl_3 and PF_6, and PF_6 doped polypyrrole (PPy-PF_6 have been investigated. At a doping level as high as 0.2 holes per monomer, the former one remains non-metallic, while the latter crosses the metal-insulator transition. In both systems a similar almost linear increase in DOS as function of charges per unit volume c* has been observed from the electrochemical gated transistor data. In PPy-PF_6, when compared to doped OC_1C_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) proportional to T^-1/4 as in variable range hopping, for highly doped PPy-PF_6 the usual interpretation of the hopping parameters leads to seemingly too high values for the density of states.