Influence of Relative Humidities on Highly Electrically Conductive Polyelectrolyte Multilayer Films

Understanding charge transport mechanisms are crucial when making ultrathin layer-by-layer (LbL) films with high electrical conductivity. We investigate the influence of the relative humidity (RH), and thus, the water content of the films. We study polyelectrolyte multilayers made from PEDOT:PSS (poly-3,4-ethylenedioxythiophen:poly styrenesulfonate) and carbon nanotubes (CNTs) as polyanion layers. All films investigated showed the highest conductivity at RH ≈ 40%. At RH ≤ 20%, UV/vis/IR absorption measurements of PEDOT:PSS films are typical for strongly charged PEDOT (as found in spin-coated PEDOT:PSS films), the direct current (DC) conductivity is low (10³-10⁴ S/m). The conductivity increases by up to 2 orders of magnitude when RH = 40% (≈2.5 × 10⁵ S/m), with absorption typical for moderately charged PEDOT. On further increase of the RH, the conductivity decays exponentially, consistent with charge carrier transport mechanisms by tunneling through nonconductive polymers. The influence of the RH on LbL films made of CNTs is similar, yet the electrical conductivity is lower, and the effects are weaker. By changing the RH between 10 and 40%, the DC electrical conductivity of PDADMA/PEDOT:PSS LbL films could be reversibly changed by 2 orders of magnitude.