Abstract
We report a record thermoelectric power factor of up to 160 μW m–1 K–2 for the conjugated polymer poly(3-hexylthiophene) (P3HT). This result is achieved through the combination of high-temperature rubbing of thin films together with the use of a large molybdenum dithiolene p-dopant with a high electron affinity. Comparison of the UV–vis–NIR spectra of the chemically doped samples to electrochemically oxidized material reveals an oxidation level of 10%, i.e., one polaron for every 10 repeat units. The high power factor arises due to an increase in the charge-carrier mobility and hence electrical conductivity along the rubbing direction. We conclude that P3HT, with its facile synthesis and outstanding processability, should not be ruled out as a potential thermoelectric material.
Highlights
Conjugated polymers currently receive considerable attention as thermoelectric materials because they are composed of abundant elements and offer ease of processing, low weight, and mechanical robustness.[1−3] The thermoelectric efficacy of a material can be described by either the dimensionless figure of merit ZT = α2σT/κ or the power factor α2σ, where α is the Seebeck coefficient, σ and κ are the electrical and thermal conductivity, and T is the absolute temperature
Power factors reported for unoriented conjugated polymers reach values of μW m−1 K−2 4−8 and in the case of aligned materials more than μW m−1 K−2.9,10 The majority of studies focus on polythiophenes because they are widely available and can be synthesized with a wide range of molecular weights, sidechain lengths, and regioregularities
The archetypal semicrystalline conjugated polymer is regioregular poly(3-hexylthiophene) (P3HT), which has been employed in a considerable number of studies related to organic thermoelectrics
Summary
Power factors reported for unoriented conjugated polymers reach values of μW m−1 K−2 4−8 and in the case of aligned materials more than μW m−1 K−2.9,10 The majority of studies focus on polythiophenes because they are widely available and can be synthesized with a wide range of molecular weights, sidechain lengths, and regioregularities. Regardless, the highest thermoelectric power factor reported for P3HT has remained well below 102 μW m−1 K−2 (see Table 1), as a result of which many researchers question the relevance of structure−property relationships established with this material. Kdo−p2.i1n9gInwiathddFitei(oTnF, sSoI)m3etoofreuaschhaavepouwseedr tensile drawing of bulk samples[20] or high-temperature rubbing of thin films[9,21] to orient P3HT, achieving a power factor of 16, 21, and 56 μW m−1 K−2 upon subsequent doping with Mo(tfd-. COCF3)[3] (see Figure 1 for the chemical structure), which due to a high electron affinity of EA ∼ 5.6 eV25,26 offers a large driving force for the oxidation of P3HT
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