Naphthodithiophenediimide–Bithiopheneimide Copolymers for High‐Performance n‐Type Organic Thermoelectrics: Significant Impact of Backbone Orientation on Conductivity and Thermoelectric Performance

Abstract

The development of n‐type conjugated polymers with high electrical conductivity (σ) has continued to pose a massive challenge in organic thermoelectrics (OTEs). New structural insights into the charge‐carrier transport are necessitated for the realization of high‐performance OTEs. In this study, three new n‐type copolymers, named pNB, pNB‐Tz, and pNB‐TzDP, consisting of naphthodithiophenediimide (NDTI) and bithiopheneimide (BTI) units, are synthesized by direct arylation polymerization. The backbone orientation is altered by incorporating thiazole units into the backbone and tuning the branching point of the side chain. The alteration of the backbone orientation from face‐on to bimodal orientation with both face‐on and edge‐on fractions significantly impacts the σ and the power factors (PFs) of the polymers. As a result, pNB‐TzDP, with the bimodal orientation, demonstrates a high σ of up to 11.6 S cm−1 and PF of up to 53.4 µW m−1 K−2, which are among the highest in solution‐processed n‐doped conjugated polymers reported so far. Further studies reveal that the bimodal orientation of pNB‐TzDP introduces 3D conduction channels and leads to better accommodation of dopants, which should be the key factors for the excellent thermoelectric performance.

n-Type organic semiconducting polymers: stability limitations, design considerations and applications

“…The NDTI unit was first reported in 2013, 106 and has since been explored with a variety of comonomers and sidechains. 58,107,108 The closest comparison that can be drawn between the NDI and the NDTI unit is with the polymers P(NDI2OD-T2) and PNDTI-BT-DT, which aside from the additional fused thiophenes, only differs by two carbon atoms on each sidechain. The NDTI unit has the desired effect of increasing the EA from 3.91 eV to 4.40 eV (Fig.…”

“…Similarly to n-type OECTs, the introduction of an extrinsic charged component into the OSC, in the case of OTEs an n-type dopant, means again for this application often polar side chains are exploited to aid the miscibility of the dopant and OSC. In order to gain some insight into the structureproperty relationships of these emerging n-type OTE materials, we have again grouped them into five general categories: NDIbased derivatives, 171,174,181 polylactam/lactone derivatives, 66,185 NDTI-based copolymers, 58,107 DPP-based copolymers 182,186 and fully fused A-A derivatives. 63,171 4.3.1.…”

Brønsted Acid Doping of P3HT with Largely Soluble Tris(pentafluorophenyl)borane for Highly Conductive and Stable Organic Thermoelectrics Via One‐Step Solution Mixing

“…The Mott's polaron hopping model has first been employed, but it could not properly explain our experimental results, similar with the cases of other doped organic semiconductors (see Supporting Information for more detailed explanation). [39,62] Therefore, we focused on the investigation into the temperature dependences of σ. increasing temperature, indicating thermally activated hopping dominant charge transport. The activation energies (E A ) extracted from the Arrhenius equation fitting significantly decreased with increasing dopant ratio for both dopants because of reduced hopping distances and Coulomb binding energies.…”

Controlling n-Type Molecular Doping via Regiochemistry and Polarity of Pendant Groups on Low Band Gap Donor–Acceptor Copolymers

“… 22 Wang and Takimiya reported an acceptor–acceptor n-type copolymer consisting of naphthodithiophenediimide and bithiopheneimide building blocks, showing an impressive n-type conductivity value of up to 11.6 S cm –1 and PF up to 53.4 μW m –1 K –2 . 23 To the best of our knowledge, most of the previous work has focused exclusively on the effects of the backbones (i.e., the electronic structure) on n-doping of conjugated polymers, relegating modifications of the pendant groups to empirical observations about solubility and film formation; pendant groups affect the processing and packing of conjugated polymers, improving charge transport. 24 , 25 However, the interplay between pendant groups and molecular doping remains poorly understood.…”