The Seebeck coefficient (S) and electrical conductivity (σ) are measured and systematically investigated for two n-channel conjugated polymers: a polymer recently synthesized in our lab, poly(PyDI-ethynylene), and commercially available Polyera N2200. N2200 is a blue polymer and shows ambipolar conductivity, whereas poly(PyDI-ethynylene) is yellow and observed to be unipolar. The purpose of this study is to characterize the terms in the thermoelectric power factor (SSσ) for these materials in order to understand their potential for the n-channel leg of thermoelectric generators, and to determine fundamental behavior regarding the enhancement of power factor in these systems, including hybrids with inorganic particles.
We characterized two chemically similar versions of poly(PyDI-ethynylene), but which have drastically different molecular weights. Unlike the low-MW polymer (4 kDa, 1.76 PDI), the polymer with higher MW (20 kDa, 3.47 PDI) also contains a flourinated phenyl end-cap. The poly(PyDI-ethynylene) with increased MW shows slightly improved conductivity in its undoped form (0.0001 to 0.001 S/cm), and a much greater S (-40 to -220 μV/K). N2200 (100 kDa, 3-6 PDI) shows still greater conductivity (0.01 S/cm), as expected due to higher intermolecular overlaps of diimide cores, but only slightly greater power factor due to lower intrinsic S (-130 μV/K). Power factors are 0.0012 µW/mK for low-MW poly(PyDI-ethynylene), 0.014 µW/mK for higher MW poly(PyDI-ethynylene), and 0.029 µW/mK for N2200. To our knowledge, these are the first reports of intrinsic n-type thermoelectric behavior in organic polymers.
We also investigated hybrid thin-films utilizing inorganic additives. Here we focus on two additives, tin (II) chloride or sodium niobate, utilizing 20 and 80 wt% in polymers. Power factors of pristine polymers are enhanced by additives in all cases. We observe that tin (II) chloride is a weak dopant for both poly(PyDI-ethynylene) and N2200. S increases greatly for both polymers with 20 wt% tin, but more for N2200 (-1000 to -1500 μV/K), and increases further for higher tin concentration (-2000 to -2500 μV/K). For sodium niobate blends, S increases to around -500 μV/K for both polymers with lower concentration and rises above -5000 μV/K for 80 wt% blends, while conductivity is unaffected. The greatest power factor was obtained using the ionic ceramic at high concentration, and was similar for both n-channel polymers, around 10 μW/mK, followed by 80 wt% tin (II) chloride blends, with around 2-7 μW/mK, within one order of magnitude of high-performing p-type polymer composites. Leveraging further recent advances in n-polymer doping, we anticipate equivalent power factors from both composite polarities to be achievable in the near future, and thus application of flexible thermoelectric generators for remote light-power applications.