Dopant-Induced Morphology Changes and Their Impact on Thermoelectricity in Polymers

Conducting polymers have attracted interest as thermoelectric materials because of their processability, light weight, flexibility, low toxicity and cost. In terms of performance, it is their intrinsically low lattice thermal conductivity (typically 0.2 – 0.5 Wm^-1K^-1) which is of particular interest to the thermoelectric community. However, can we assume that the thermal conductivity is always low in these systems? Doping these systems is required to obtain useful electrical conductivities, but causes significant morphological changes in order to incorporate the large molecular dopants and further structural adjustments occur to accommodate the polaronic charge carriers. How do these morphological changes impact the thermal conductivity of polymers? How does this impact zT?<br/><br/>This presentation aims to shed some light on these questions, by investigating the cases of four polymers, poly(3-hexyl thiophene) [P3HT], poly(3,4-ethylenedioxythiophene) [PEDOT], poly[6,9-dihydro-6,9-dioxobisbenzimidazo[2,1-b:1′,2′-j]benzo[1mn]phenan-throline-2,12-diyl] [BBB] and poly[17-oxo-7,10-benz[de]-imidazo[4′,5′:5,6]-benzimidazo[2,1-a]isoquinoline-3,4:10,11-tetrayl)-10-carbonyl]) [BBL]. We measure in-plane thermal conductivity of different P3HT morphologies produced by spin-coating, drop casting and aligned by mechanical rubbing. We also compare doped and undoped analogues of these films. We do find an effect of morphology on thermal conductivity in aligned polymer films that we can link to structural changes. However, by far the biggest effect on lattice thermal conductivity comes from electrical doping, which induces a subtle change to the morphology but a significant increase in thermal conductivity. We find thermal conductivity states in the range of 0.2 to &gt; 1 Wm^-1K^-1.[1] In PEDOT, we de-dope the polymer to improve the Seebeck coefficient, but also aim to maintain or improve charge transport. We observe significant morphological changes in this process which we link to the macroscopic observables.<br/><br/>In the case of BBB and BBL we investigate the effect of ladderisation on polymers (BBL being a ladder-type polymer and BBB it’s non-ladder type analogue). Ladder-type polymers should be more planar and more rigid than non-ladder types. Correspondingly, we find that the thermal conductivity in the ladder-type polymers can be &gt;3 times that in the non-ladder-type analogue. When doped the lattice thermal conductivity increases to remarkably large values &gt;2 Wm^-1K^-1.<br/><br/>[1] Degousée et al. <b><i>J. Mater. Chem. A</i></b>, 2021,<b>9</b>, 16065-16075