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Newly commercialized PEDOT:PSS products CLEVIOS PH1000 and FE-T, among the most conducting of polymers, show unexpectedly higher Seebeck coefficients than older  CLEVIOS P products that were studied by other groups in the past, leading to promising thermoelectric (TE) power factors around 47 μW/m K² and 30 μW/m K² respectively. By incorporating both n and p type Bi₂Te₃ ball milled powders into these PEDOT:PSS products, power factor enhancements for both p and n polymer composite materials are achieved. The contact resistance between Bi₂Te₃ and PEDOT is identified as the limiting factor for further TE property improvement. These composites can be used for all-solution-processed TE devices on flexible substrates as a new fabrication option.

The electrical conductivity of all three PEDOT grades increases significantly when the DMSO level approaches 1%, shows a monotonically continuous increase up to 5% DMSO, then starts to decrease. These results were consistent with electrical property information provided by H.C. Stark, Inc. We found that the Seebeck coefficient started a steep fall when the DMSO level exceeded 0.5%. There is no obvious correlation between DMSO amount and Seebeck coefficient when the DMSO content is more than 1%. Kemerink et al propose that the addition of high-boiling solvents like sorbitol and DMSO to the aqueous dispersion used for film deposition  rearranges PEDOT-rich clusters into elongated domains. This model has been supported by STM and AFM. If this model is correct, a plausible explanation is that when the DMSO level is more than 1%, the DMSO only increased the number of connected elongated PEDOT:PSS grains but did not change any carrier scattering mechanism and the energy barrier which needs to be overcome for a carrier moving from one grain to another, because if any of the three had changed, a change in Seebeck coefficient should have been observed.

 PH1000 products showed a power factor superior to other products at all DMSO levels more than 1%. The best power factors, 47μW/m K², are from 4% and 5% DMSO in CLEVIOS PH1000. This power factor is among the highest power factors for pure organic materials. It is a significant discovery that in the PEDOT:PSS system,  both electrical conductivity and thermopower can be improved from P to PH1000, which are claimed to have very similar chemical composition.

 The mechanism behind this has considerable scientific importance. For most semiconductor polymers, increasing the electrical conductivity will detrimentally affect the Seebeck coefficient, usually leading to a small power factor. There are different models pertaining to the mechanisms of improving the electrical conductivity of PEDOT:PSS. None of them take into account the possibility of increasing Seebeck coefficient as well. Our previous work demonstrates a route for designing thermoelectric materials by which the increases in Seebeck coefficient and conductivity do not cancel each other. The core idea of the design is to blend two semiconductor polymers with similar but nonidentical Fermi levels. This situation is effectively equivalent to having a large derivative of density of states with respect to the Fermi level. 

We also measured the TE properties of the mixture of CLEVIOS PH1000 and n/p-type Bi₂Te₃ ball-milled powders. Bi₂Te₃ powders have a non uniform size distribution from sub-micron to several microns. The power factor of the mixture using PH1000 is generally better than other products due to the best power factor associated with PH1000. The three highest power factors, 131, 113, and 119 μW/m K², are respectively from the samples with 10 volume %, 17 volume % and 23 volume % of PEDOT. The general trend is that the electrical conductivity increased and Seebeck coefficient decreased with increase of PEDOT volume ratio.  If we use the Wiedemann-Frantz-Lorentz relationship to estimate the carrier thermal conductivity, the carrier thermal conductivity of the samples having 10%, 17% and 23 % of PEDOT are 0.043 W/m K, 0.055 W/m K, and 0.073 W/m K respectively, The lattice thermal conductivity can be estimated by considering the lattice thermal conductivity of PEDOT:PSS and Bi₂Te₃ in a parallel model that gives an upper limit for a two-phase system. The lattice thermal conductivity of Bi₂Te₃ is estimated by using total thermal conductivity 1.28 W/m K, which is from the technical information provided by Marlow Inc. subtracting the carrier thermal conductivity 0.73W/m K obtained from Wiedemann-Frantz-Lorentz relationship. The upper limit of lattice thermal conductivity of the three samples 10%, 17% and 23%, are 0.55x0.90+0.2x0.10=0.515W/m K, 0.55x0.83+0.2x0.17=0.498W/m K, 0.55x0.77+0.2x0.23=0.470W/m K respectively. The estimated total thermal conductivity equals 0.558 W/m K, 0.553 W/m K and 0.543 W/m K for 10%, 17% and 23% samples respectively. The real thermal conductivity of these samples could be less than these values because the interface phonon scattering between two materials is not considered here.  The estimated ZT for the three samples should be more than 0.08 (10%), 0.06(17%) and 0.07(23%).