Aging effects on the transport properties in conducting polymer polypyrrole

Abstract

We present electronic transport studies of the aging process in naphtalene--sulfonate-doped polypyrrole. They include in situ conductivity measurements as a function of the aging time up to 1 month, and conductivity and thermoelectric power measurements in the temperature range 300--15 K for different aging times at 120 \ifmmode^\circ\else\textdegree\fi{}C in room atmosphere. We show that while the short-aging-time decay of the conductivity may be accounted for by a law of the type ${\mathrm{\ensuremath{\sigma}}}_{0}$-\ensuremath{\sigma}(${\mathit{t}}_{\mathit{a}}$)\ensuremath{\propto}\ensuremath{\surd}${\mathit{t}}_{\mathit{a}}$, the long-aging-time evolution is well described by a stretched exponential, \ensuremath{\sigma}=${\mathrm{\ensuremath{\sigma}}}_{0}$exp[-(${\mathit{t}}_{\mathit{a}}$/\ensuremath{\tau}${)}^{1/2}$]. Moreover, two distinct temperature dependences have been identified: (i) \ensuremath{\sigma}=${\mathrm{\ensuremath{\sigma}}}_{0}$exp[-(${\mathit{T}}_{0}$/T${)}^{1/2}$] for aged samples and (ii) \ensuremath{\sigma}=${\mathrm{\ensuremath{\sigma}}}_{0}$exp[-${\mathit{T}}_{1}$/T+${\mathit{T}}_{0}$] for as-synthesized or lightly aged samples. The thermal variation of the thermoelectric power can be described by the following law: S(T)=AT+B+C/T, where the relative weight of the linear term, A, appears to be a decreasing function of the aging time. All the results are comprehensively explained in terms of conducting grains separated by insulating barriers in which the conduction is controlled by a hopping process of the charge carriers between the grains. The aging phenomenon is found to consist of a decrease of the grain size, in parallel with a broadening of the barriers, as in a corrosion process. As the aging time increases, the size of the conducting grains decreases and then goes below a critical value that is responsible for a crossover in the transport mechanism and therefore in the time dependence of the conductivity as experimentally observed. In the aged samples, this model leads to the existence of a single expression that accounts for both the temperature and the aging-time dependences of the conductivity, i.e., ln\ensuremath{\sigma}(${\mathit{t}}_{\mathit{a}}$,T)\ensuremath{\propto}-(${\mathit{t}}_{\mathit{a}}$/T${)}^{1/2}$. \textcopyright{} 1996 The American Physical Society.