Microscopic Understanding of the Anisotropic Conductivity of PEDOT:PSS Thin Films

Abstract

The anisotropic conductivity of thin films of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is correlated to the film morphology as obtained from scanning tunneling and atomic force microscopy images. The material was found to consist of layers of flattened PEDOT‐rich particles that are separated by quasi‐continuous PSS lamella (see figure).

Transverse charge transport in inkjet printed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)

“…Similar decrease in the workfunction of PEDOT:PSS was reported in previous studies, due to the addition of a co-solvent: 120 meV by adding 8 wt% glycerol [29] and 300 meV by adding 10 wt% Sorbitol [19,30]. The next stage was to also investigate the applicability of the nn-H charge transport model in the transverse conduction direction, as this model was found to be best for the transverse conduction in spin coated PEDOT:PSS films [11,24]. In order to assess the hopping distance, L, transverse conductivity measurements were performed under variable field and variable temperature conditions.…”

“…The transverse cross-section samples were examined under atomic force microscopy (AFM) and Figure 8 presents the AFM micrographs of the transverse cross-section for a typical spin coated and a printed PEDOT:PSS sample. The typical AFM topography of PEDOT:PSS surfaces or cross-sections displays elevated PEDOT grains surrounded by a PSS matrix shell [8,24]. …”

“…Their work demonstrated how the action of solvent evaporation in spin coating leads to charge conduction anisotropy generating a disparity between in-plane and transverse conduction models, 3D M-VRH and nearest-neighbour hopping, respectively. Indeed, a number of groups have theorised that the action of spin coating essentially 'freezes' the morphology in a non-thermodynamically favoured state [24,25] whilst others have stipulated that the intense centripetal forces might generate a radially aligned morphology in the in-plane direction [26]. Hence, the fast drying process in spin coating freezes this type of morphology, creating an anisotropy between in-plane flattened PEDOT grains and their transverse, z-direction.…”

“…Nardes et al [11,16,22,24] investigated the conduction mechanisms of spin cast PEDOT:PSS in both in-plane and transverse directions. Their work demonstrated how the action of solvent evaporation in spin coating leads to charge conduction anisotropy generating a disparity between in-plane and transverse conduction models, 3D M-VRH and nearest-neighbour hopping, respectively.…”

Explaining the effects of processing on the electrical properties of PEDOT:PSS

“…In spite of extensive investigation it is still hard to pinpoint the physical reason(s) for such changes beyond the level of 'general morphological changes'. Even if evident changes to the morphology are readily observed by AFM or TEM [10][11][12], it can be problematic to tell which exactly are the changes that are responsible for the observed conductivity increase. E.g.…”