Naphthodithiophenediimide–Benzobisthiadiazole-Based Polymers: Versatile n-Type Materials for Field-Effect Transistors and Thermoelectric Devices

Abstract

New π-conjugated polymers with strong electron affinity, PNDTI-BBTs, consisting of naphtho­[2,3-b:6,7-b′]­dithiophenediimide (NDTI) and benzo­[1,2-c:4,5-c′]­bis­[1,2,5]­thiadiazole (BBT) units, were synthesized. PNDTI-BBTs have low-lying LUMO energy levels (∼−4.4 eV), which is sufficiently low for air-stable electron transport in organic field-effect transistors and for being readily doped by a well-known n-dopant, N,N-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (N-DMBI), affording doped polymer films with relatively high conductivities and Seebeck coefficients. Depending on the solubilizing alkyl groups (2-decyltetradecyl, PNDTI-BBT-DT, or 3-decylpentadecyl groups, PNDTI-BBT-DP), not only the electron mobility in the transistor devices with the pristine polymer thin films (PNDTI-BBT-DT: ∼0.096 cm2 V–1 s–1; PNDTI-BBT-DP: ∼0.31 cm2 V–1 s–1) but also the conductivity and power factor of the doped thins films (PNDTI-BBT-DT: ∼0.18 S cm–1 and ∼0.6 μW m–1 K–2; PNDTI-BBT-DP: ∼5.0 S cm–1 and ∼14 μW m–1 K–2) were drastically changed. The differences in the electric properties were primarily ascribed to the better crystalline nature of the PNDTI-BBT-DP than those of PNDTI-BBT-DT in the thin-film state. Furthermore, UV–vis and ESR spectra demonstrated that doping effectiveness was largely affected by the alkyl groups: the PNDTI-BBT-DP films with better crystalline order prevented overdoping, resulting in ca. 20 times higher conductivity and power factors. From these results, it can be concluded that tuning the intermolecular interaction and consequently obtaining the thin-film with well-ordered polymers by the alkyl side chains is a promising strategy for developing superior thermoelectric materials.

High‐Performance n‐Channel Organic Transistors Using High‐Molecular‐Weight Electron‐Deficient Copolymers and Amine‐Tailed Self‐Assembled Monolayers

“…Compared to the leading class of p‐type semiconducting polymers, high‐performance n‐type (electron‐transporting) semiconducting polymers are relatively rare . However, they are of great significance for realizing several important electronic devices, such as complementary metal‐oxide‐semiconductor—like logic circuits, organic thermoelectrics, and all‐polymer solar cells …”

N‐Type Organic Thermoelectrics of Donor–Acceptor Copolymers: Improved Power Factor by Molecular Tailoring of the Density of States

“…For the practical TE applications, both efficient p‐type and n‐type TE materials are required; however, the development of the latter lags much behind that of the former . For this reason, growing research efforts are being directed towards developing better n‐type TE materials . So far, advanced conjugated‐backbone designs targeting favorable energetics, high mobility, planar structure, and good host/dopant miscibility have been reported .…”

Enhancing the n‐Type Conductivity and Thermoelectric Performance of Donor–Acceptor Copolymers through Donor Engineering

“…The construction of feasible thermoelectric generators or heating–cooling devices requires alternating legs of p‐ and n‐type thermoelectric polymers with high, comparable performances . However, an σ over 1000 S cm −1 has been shown for p‐type polymers, whereas only a handful of n‐type conducting polymers, including the ladder‐type polymers reported by Fabiano et al ., naphthalenedicarboximide (NDI)‐based polymer reported by Takimiya et al ., and benzodifurandione‐based oligo(p‐phenylenevinylene) (BDOPV)‐based polymers reported by us and Katz et al ., are reported to have electrical conductivities approaching or over 1 S cm −1 . Since the Seebeck coefficients of the pre‐eminent p‐ and n‐type polymers are comparable, having values of approximately ± (40–200) µV K −1 at the PF maximum, the low electrical conductivity is thought to be an obstacle for improving the thermoelectric performance of n‐type polymers.…”