Thermoelectric properties and intrinsic conduction processes in DBSA and NaSIPA doped polyanilines

Abstract

Seeking to gain fundamental understanding of the thermoelectric (TE) behavior of polyanilines (PANIs), structure-property relationships of PANI nanorods, doped with dodecylbenzenesulfonic acid (DBSA) and 5-sulfoisophtalic acid sodium salt (NaSIPA), and prepared by an indirect synthetic route, are discussed in terms of the contribution of the acid concentrations on the thermoelectric properties.The synergistic combination of high doping level and layer structure, accounts for the moderately high electrical conductivities () and low constant Seebeck coefficients (α) of PANI-DBSA. Conversely, the poor doping ability of NaSIPA and low crystallinity degree explain the low electrical conductivities along with significant increases in Seebeck coefficient values. In relation to conduction mechanisms, PANI-DBSA shows a hopping behavior with a carrier concentration of c0.49 (hole type), while PANI-NaSIPA displays a diffusive regime, characteristic of degenerate metallic semiconductors, with an estimated charge carrier density of n ≈3×10 21 e/cm 3 . 1.Thus, a good thermoelectrical material should have high  and α, but poor κ. The three thermoelectrical parameters are interdependent in bulk materials and decoupling these parameters is definitely non-trivial [7].Polyaniline (PANI) as one of the important ICPs has caused a lot of scientists' interests due to high stability, facile synthesis and tunable electronic properties [8]. Electrical conductivity of PANI increases with doping, which may be achieved by an acid-base reaction. As a result of the protonation of the nitrogen sites in the emeraldine base (EB), the cation radical of one nitrogen acts as a polaronic hole and these holes are charge carriers (p-type doped PANI) [9] ( Fig. 1). Electrical conductivity occurs via interpolaron hopping along and across polymer chains [10,11]. Therefore, the carrier density depends on the degree of protonation or doping level [4,12]. Furthermore, in most cases PANIs appear to be amorphous, sometimes with some degree of crystallinity, thus heterogeneous conduction is set up involving "islands" of higher conductivity regions separated by lower conductivity regions. The polaron structures are responsible for electrical conduction through the hopping mechanism in the crystalline region [9], while intergrain resonance tunnelling occurs through the strongly localized states in the amorphous media [13]. These systems are featured with low charge carrier mobility and low electrical conductivities, even at high carrier densities [14,15]. This mixture of metallic and non-metallic behaviour is a key characteristic of all ICPs, including PANI [16].By contrast, the Seebeck coefficient is more complex and difficult to predict. The reduction of the total number of charge carriers increase the Seebeck coefficient while lowering the electrical conductivity, but because the power factor (PF) scales with α 2 , a 3 net increase in the PF can be achieved for certain doping ranges. The identification of conditions conducive to decouple  and α, remains ch...

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