Proquinoidal-Conjugated Polymer as an Effective Strategy for the Enhancement of Electrical Conductivity and Thermoelectric Properties

Abstract

P-doping of conjugated polymers requires electron transfer from the conjugated polymer to the p-dopant. This implies that the highest occupied molecular orbital (HOMO) of the conjugated polymer has to be higher than the lowest unoccupied molecular orbital (LUMO) of the p-dopant. Although commonly used p-dopants such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) have a low LUMO of −5.24 eV, most conjugated polymers used in high-performance field-effect transistors are donor–acceptor-type polymers with deep HOMO values, making them difficult to be effectively doped by F4TCNQ. Here, we utilized the proquinoidal 2,6-dialkyl-benzo­[1,2-d;4,5-d′]­bistriazole (BBTa26) moiety in conjugated polymers to destabilize HOMO, allowing effective p-doping using very dilute F4TCNQ solutions. The extent of the quinoidal character and hence their intrinsic conductivities and the ability to be doped are dependent on the dihedral angles and aromaticity of the aryl spacer groups along the polymer backbone. Intrinsic conductivities as high as 10–2 S cm–1 were achieved. Upon doping using F4TCNQ, highly delocalized polarons were observed. As such, electrical conductivities of over 100 S cm–1 and an enhancement of the Seebeck coefficient from carrier-induced softening can be achieved. A maximum power factor of 11.8 μW m–1 K–2 was achieved in thin-film thermoelectric devices. These results are among the highest for solution-phase p-doping using F4TCNQ without additional processing.

Open‐Shell Donor–Acceptor Conjugated Polymers with High Electrical Conductivity

“…Using an improved synthetic route and straightforward slow‐drying process, P1 demonstrates a record high σ RT of 8.18 S cm −1 for a charge‐neutral, undoped material. We benchmarked this value with other neutral narrow bandgap conjugated polymers, [ 16,47–50 ] polysquaraines, [ 14,51 ] radical polymers, [ 1,52,53 ] self‐doped polyelectrolytes, [ 54–56 ] and some commercial grades of poly(styrene‐sulfonate)‐doped poly(3,4‐ethylenedioxythiophene) (PEDOT:PSS) ( Figure and Figure S18 (Supporting Information)), [ 11 ] with full details assembled in Table S5 (Supporting Information). Neutral narrow bandgap polymers based on quinoidal poly(isothianaphthene) frameworks achieve σ ≈ 10 −3 –10 −2 S cm −1 , while the highest performing captodatively stabilized zwitterionic polysquaraines achieve σ ≈ 10 −4 S cm −1 .…”

“…As such, there has been a long‐standing interest in neutral narrow bandgap conjugated polymers that promote high σ in their native form; however, decades of research have met with limited success, with σ ranging from ≈10 −10 to 10 −2 S cm −1 . [ 4,14–16 ]…”

Quinoidal bisthienoisatin based semiconductors: Synthesis, characterization, and carrier transport property

“…[ 42 ] The quinoidal resonance through alternative connection of TQBTI and TII units would promote conductive state with singlet‐triplet excitation as in quinoidal materials. [ 43,46,47 ] The delocalized electronic structure and elevated HOMO level of P(TQBTI‐TII) is well evidenced by theoretical estimation and optical and electrochemical examinations.…”

“…The background profile remarkably higher than P(TBTI‐TDPP) is associated with the polaron/bipolaron absorption as typically displayed in doped state of polymer semiconductors bearing high‐lying HOMO level. [ 41‐43 ] The estimated optical energy gaps E g opt of P(TQBTI‐TII) and P(TQBTI‐TDPP) by Tauc plot [ 44,45 ] from the thin‐film absorption spectra, which is commonly proposed to determine energy gap of polymers, were 0.92 and 1.17 eV, respectively. These values are in good agreement with the DFT calculations of the polymers.…”

A Structurally Simple but High‐Performing Donor–Acceptor Polymer for Field‐Effect Transistor Applications

“…This has been addressed by the fusion of additional heterocycles in the 5,6-positions of the BTz ring. [25][26][27][28][29] Alternatively, additional electron-withdrawing groups such as fluorine or nitrile can be added to the BTz to increase the acceptor strength. Such derivatives have been extensively used in the development of polymers for OPV devices, [30][31][32][33][34] while there has been comparatively less work on the use of BTz as a co-monomer in materials for transistor applications.…”