Direct and High Frequency Alternating Current Conduction Mechanisms in Solution Cast Polyaniline Films

Abstract

Frequency dependent (ac) and independent (dc) conductivity measurements have been carried out on polyaniline (PANI) films deposited by solution casting technique. Under low electric field (1 × 103 V/cm) condition, the dc conductivity measured in the temperature range of 173–303 K obeys the three-dimensional variable range hopping (3D VRH) formalism. The Mott parameters such as localization length (α−1 ≈ 7 A), density of states [N(EF) = 1.04 × 1019 states/eV cm3], hopping range (Rhop = 60 A) and hopping energy (Whop = 0.38 eV) are computed. The ac conductivity measured in the frequency range 10 kHz–5 MHz and in the temperature range 150–380 K follow a power-law dependence σac ∼ ωs, typical for charge transport by hopping or tunnelling processes. Therefore, the experimental results are analyzed with reference to various theoretical models based on quantum-mechanical tunnelling and classical hopping over barriers. The observed minimum in the temperature dependence of the frequency exponent s strongly suggests that tunnelling of large polarons is the dominant transport process. The polaron radius (rp ≈ 25 A) and barrier height for infinite site separation (WHO ≈ 0.22 eV) are evaluated. The density of states [N(EF)] and tunnelling distances (Rω) are estimated and discussed in terms of frequency and temperature.