Conjugated Polymer Blends for Organic Thermoelectrics

Abstract

A major attraction of organic conjugated semiconductors is that materials with new, emergent functionality can be designed and made by simple blending, as is extensively used in e.g. bulk heterojunction organic solar cells. Here, we critically review doped blends based on organic semiconductors (OSC) for thermoelectric applications. Several experimental strategies to improve thermoelectric performance, measured in terms of power factor (PF) or figure-ofmerit ZT, have been demonstrated in recent literature. Specifically, density-of-states design in blends of 2 OSCs can be used to obtain electronic Seebeck coefficients up to ∼2000 µV/K. Alternatively, blending with (high-dielectric constant) insulating polymers can improve doping efficiency and thereby conductivity, as well as induce more favorable morphologies that improve conductivity while hardly affecting thermopower. In the PEDOT:PSS blend system, processing schemes to either improve conductivity via morphology or via (partial) removal of the electronically isolating PSS, or both, have been demonstrated. Although a range of experiments have at least quasi-quantitatively been explained by analytical or numerical models, a comprehensive model for organic thermoelectrics is lacking so far.Strategies to increase thermoelectric performance of doped organic semiconductors by blending are reviewed. Experimental results are, where possible, compared to analytical and numerical models. Several promising strategies to increase conductivity and/or thermopower are experimentally identified in recent literature. In contrast, formal understanding, especially of the role of morphology, is somewhat lagging behind.

Correlating charge and thermoelectric transport to paracrystallinity in conducting polymers

“…Another interesting finding in P3HT and PBTTT is that w values are smaller (0.1−0.3 eV) than the ones found for PEDOT: PSS (w~0.8 eV), which is consistent with the fact that polymers with side chains tend to aggregate in crystallized domains, and hence narrower DOS tails could be obtained. It is worth mentioning that, in our analysis, E F exceeds E t at most by~0.4 eV, which falls in the same range as DOS broadening, which is physically reasonable 21,22 . Furthermore, the fitting curves align well with the experimental results of PEDOT:PSS as σ > 1 S/cm, while they deviate from the ones of PEDOT:Tos at σ < 0.3 S/cm (Fig.…”

Non-Wiedemann-Franz behavior of the thermal conductivity of organic semiconductors

“…5, meaning that rates and energies were only recalculated for the moving particle, which significantly reduces calculation time. This approximation is accurate up until relative doping levels of 10 −2 , as shown by Zuo et al [19].…”

“…The kMC model has been extensively described before [18,19]. In brief, the kMC simulations account for variablerange hopping on a random lattice with a mean intersite distance of a NN = N −1/3 0 = 1.8 nm, with N 0 the total site density.…”

“…(3)- (6). In contrast, assuming, e.g., a δ function for σ (E ) ∝ δ(E − E * ) as typically done in Mott-type percolation models [19,22] leads to κ el = 0.…”

Experimentally Validated Hopping-Transport Model for Energetically Disordered Organic Semiconductors

“…(3), which we refer to as the eGDM, or the mobilities from the analytical model that is introduced next. The analytical VRH model is an extension of the Mott-Martens model, as reviewed by Coehoorn et al [24], along the lines previously used by Zuo et al for doped organic semiconductors [25,26]. The conductivity is given by a Miller-Abrahams-type expression,…”