PEDOT percolated

Transparent SWNT/insulating polymer nanocomposites have not been made with high conductivity values, mainly because of the intrinsic charge localization arising from insulating polymer dispersant. In a direct comparison, SWNTs were stabilized with insulating surfactant (SDS) or conductive poly(3,4-ethylenedioxythiophene) (PEDOT) PSS. " Both SWNT dispersions were introduced into a polystyrene matrix via a latex-based route. Composites with PEDOT PSS-stabilized SWNTs showed a percolation threshold of 0.18 wt% and a conductivity value of 500 S m compared with 3.8 wt% and 20 S m for the SDS-based composites, respectively. The authors attributed the conductivity enhancement to conduction bridge formed by the conductive polymer between adjacent SWNTs. 

Figure 6.8 Electrical conductivity and surface resistivity comparison. Upper panel electrical conductivity results of P3HT/SWNT composite films depending on (left) different amounts of pre-separated ( ) and separated metallic (O) nanotube samples, and (right) their corresponding effective metallic SWNT contents in the films (dashed line the best fit in terms of the percolation theory equation). Lower panel Surface resistivity results of PEDOT PSS/SWNT films on glass substrate with the same 10 wt% nano tube content (O pre-separated purified sample and T separated metallic SWNTs and for comparison, blank PEDOT PSS without nano tubes) but different film thickness and optical transmittance at 550 nm. Shown in the inset are representative films photographed with tiger paw print as background.

PEDOT PSS, 20-24, 20-45-20-46, 20-49 substituted PEDOTs, 20-25 PEDOT/PSS nanowires, 16-5 Peierls instability, 17-3 Pentacene, 2-2, 2-15, 2-16, 2-17, 2-18 Percolation models, 16-2 Percolation transition, 15-12 Pemigraniline, 7-25 Perturbation, 19-3, 19-11, 19-14 Phase diagram, 17-11-17-12, 17-15, 17-24-17-27 Phenyl-fused EDOT, 13-15-13-16 Phonon scattering, 15-13, 15-30 Phonon-assisted tunneling, 16-4 Phonon-induced delocalization, 15-8, 15-12—15-13, 15-41, 15-51, 15-69 Phosphonic add, 9-18, 9-19... 

Resistivity measurements of sensor coatings were undertaken before and after consolidation (Fig. 16.16) in order to determine the percolation threshold of PEDOT PSS... 

The percolation thresholds observed for composites prepared with a PS matrix and SDS- and PEDOT PSS-stabilized SWCNTs are shown in Figure 6.4 [Q. The linear fittings oft and cpp (ii) using the statistical percolation law are given in Figure 6.4 (ii). The statistical percolation law is given as ... 

Figure 6.4 (i) Percolation thresholds for composites prepared with SDS-stabilized SWCNTs (squares) and PEDOT PSS-stabilized SWCNTs (stars). Arrows indicate applicable axes and the horizontal line corresponds to the measured conductivity of PEDOTrPSS. (ii) Linear fittings of t and (Pp (R values are 0.99). (Reprinted with permission from RSC Publishing). 

Figure 6.6 Representation of Scenario one along with the new percolation equation (6.3] where q)p and q)p are, respectively, the percolation thresholds for the control system (SDS] and the system including PEDOT PSS.

Scenario two We define as the diameter of a bare SWCNT and D2 as the new effective diameter of the SWCNT. The PEDOT PSS layer is now included in the hard-core diameter. This scenario describes the case in which the PEDOT PSS layer is taken to be an impenetrable layer, which simply increases the apparent hard-core diameter of the SWCNT. This would be a reasonable approximation if the mobility of the layer is low in the melt state [minimal flow]. The change in the percolation threshold is still viewed in terms on the SWCNTs themselves, and not in terms of the additional PEDOT PSS. For this reason, the original connectedness criterion remains unchanged. This situation is schematically illustrated in Figure 6.7, along with the Equation 6.1 rewritten for scenario two. 

If it is assumed that f is not affected by the presence of PEDOT PSS, i.e., = 2/ there is no theoretical change in the percolation threshold... 

The co-operative nature of the SWCNTs and PEDOT PSS in the SWCNT/PS/PEDOT PSS system is investigated by altering the ratio of the two components via two methods. The resultant percolation thresholds were evaluated and models proposed to explain the cooperative behavior. From this point, the two routes followed are referred to as methods 1 and 2. 

The values for the theoretical predictions were determined using Equation 6.3 with values for f of 1 and 2 nm. These predictions model a gradual decrease in the overall percolation threshold upon the addition of the SWCNTs. From the experimental data a steep drop in the percolation threshold with the introduction of a small fraction of SWCNTs can be observed. The gradient of this drop is larger than that observed for the addition of PEDOT PSS. This could... 

Figure 6.12 Experimental co-operative behavior of PEDOT PSS latex particles and HiPCO SWCNTs (circles). Pitting of data using a multi-component continuum connectedness percolation theory (line). ...

Composites with varying loadings of SDS- and PEDOT PSS-stabilized Carbolex SWCNTs were prepared. Percolation thresholds, along with the control system (PS/PEDOT PSS blend), were constructed and are given in Figure 6.15. 

Figure 6.15 Percolation threshold of PEDOT PSS-stabilized (stars) and SDS-stabilized (squares) Carbolex SWCNTs, and the insulator-conductor transition of PEDOT PSS/PS (control, circles). Arrows indicate applicable axes. (Reprinted with permission from ACS Publishing).

The inclusion of a conductive polymeric component, namely PEDOT PSS, in PS/SWCNT composites to reduce the non-contact resistivity limiting is shown to reduce the percolation threshold and simultaneously increase the ultimate composite conductivity. The ability of PEDOT PSS to stabilize SWCNT dispersions (individualized SWCNTs] was shown. PEDOT PSS/PS/SWCNT composites showed lower percolation thresholds as compared to PS/SWCNT composites. This reduction was modeled assuming a homogeneous deposition of PEDOT PSS over the SWCNT surface. 

An investigation into the co-operative behavior demonstrates that the percolation threshold can be modeled using a multi-component continuum connectedness percolation theory. A deeper investigation into the co-operative nature of the two conductive components revealed that the contribution of the SWCNTs to the overall composite conductivity is minimal, and that the role of the SWCNTs is more morphological and likely to be that of a kind of template or scaffold for the deposition of a connected PEDOT PSS phase. 

A different explanation was proposed by Hsu et al. Dedoping of PEDOT was ruled out as the total number of injected ions from out of the gate into the PEDOTPSS channel was too low to explain the observed current decrease. Instead a model was favored assuming a change of percolation paths caused by rendered ion positions. A small fraction removal of mediated hopping states near the Fermi level on charge transport paths causes carriers to hop over longer distance to conduct current and therefore fp is reduced. 

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