Conductivity of n-doped polyacetylene : dependence on doping level and temperature

Abstract

In order to perform reliable and reproducible conductivity versus temperature measurements on n-doped (CH) x , we have designed and built a special electrochemical cell which allows the doping (or dedoping) process and the temperature variation to be made alternatively.Using an oriented film (obtained by the liquid crystal technique) and for the maximum concentration (16%) of K dopant, we have measured a room temperature conductivity of 10 4 S/cm. This conductivity decreases by less than 30% when T is decreased down to liquid helium temperature. We have also investigated, in the same cell, lower concentrations during doping and dedoping cycles. We present preliminary results showing a double network of curves σ = f(T) with y constant and σ = f(y) with T constant. At each doping level, we can correlate our results with the structural evolution of the system and its electronic properties (8) by measuring the open circuit voltage. Our principal aim is to study the possibility of a correlation between the variation σ(y) (i.e. the crystalline and electronic structure) and σ(T) (i.e. the principal transport mechanism).These results are discussed in the light of existing theoretical models such as: i) fluctuation induced tunneling, ii) variable range hopping, iii) transport by activation to a mobility edge, iv) weak localization in disordered metals. The last cannot be used to fit our data. The others can be used in specific doping level ranges (y 6%).We will also discuss differences in σ(T) between single phase systems and two-phase systems. We will study to what extent the evolution of σ(y > T) of our system can be described by modelling the conductivity of an heterogeneous system with two compounds having each different electronic properties and different intrinsic σ(T).