Thermoelectric Performance Enhancement of p-Type Pyrrolo[3,2-b:4,5-b′]bis[1,4]benzothiazine Conjugated Ladder Polymer by Pendant Group Engineering

Abstract

Organic thermoelectrics have attracted widespread attention due to the possibility of harnessing ambient thermal energy to power Internet-of-things and wearables due to their flexibility, low cost, and large-scale production, but research progress still lags behind that of inorganic materials. Rigid ladder-type conjugated polymers consisting of doublestranded fused-ring structure exhibit excellent thermal and electrochemical stabilities and thus are promising materials for organic thermoelectrics. However, effective synthesis, characterization, and solution processing are challenging and thus limit its application in thin-film devices. Here, two p-type pyrrolo[3,2b:4,5-b′]bis [1,4]benzothiazine ladder-type conjugated polymers with different pendant groups (methyl: PBBTL-CH 3 and phenyl: PBBTL-Ph) were synthesized, and both can be solution-processed for thin-film thermoelectric applications. PBBTL-CH 3 achieved electrical conductivities as high as 36.29 ± 0.58 S cm −1 and power factors up to 22.7 ± 2.1 μW m −1 K −2 when doped with FeCl 3 , which are significantly higher than of PBBTL-Ph with a conductivity of 1.40 ± 0.16 S cm −1 and a power factor of 1.7 ± 0.2 μW m −1 K −2 . Grazing-incident wide-angle X-ray scattering of the polymers showed PBBTL-CH 3 exhibited exclusive edge-on orientation, while PBBTL-Ph exhibited mixed edge-on and face-on orientation. The present study indicates that pendant groups have important influence on the electrical conductivity and thermoelectric performance for the PBBTL series through alteration of HOMO energy levels, polymer packing, and ease of dopant infiltration.

Ambipolar conjugated ladder polymers by room-temperature Knoevenagel polymerization

“…5,6 These unique structural characteristics make it possible to generate outstanding thermal/chemical stability, strong inter-chain organization, high intra-chain carrier mobility, and extraordinary photo-/electro-luminescence properties. 7–9 They are regarded as the perfect targets for various functional applications, and are already adopted as semiconductor layers in organic light-emitting diodes, 10 organic field-effect transistors (OFETs), 11–16 organic thermoelectrics, 17,18 organic lasers, 19,20 and lithium ion batteries. 21,22 Despite these amazing prospects, development of structurally defined cLPs is still extremely challenging due to the scarcity of effective synthetic strategies and poor solubility.…”

“…36,37 The other strategy for the preparation of cLPs is a single-step ladderization route that involves a simultaneous formation of a two-stranded bond between comonomers. 38 Compared with the two-step strategy, this approach is more facile, only involving one-step condensation of one or more multifunctional monomers via polycondensation reactions 15–18,39,40 or Diels–Alder polymerizations. 41,42 One of the most heavily explored ladder polymers, i.e.…”

Conjugated Ladder Poly(thienobenzothiazine): Synthesis, Electronic Structure, Optical Properties, and Electrical Conductivity of a Narrow Bandgap p-Type Semiconducting Polymer

“…Recently, conjugated ladder poly(pyrrolobenzothiazine)s (LPBTs) were shown to be a promising class of p-type semiconducting polymers by our group 23 and others. 14,24,25 Toward investigation and understanding of the structure−property relationships in CLPs, the ladder benzothiazine backbone 23,24,26,27 offers attractive opportunities in part because in addition to the possibility of side-chain engineering, 23,28 chalcogen substitution is a possibility.…”