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Organic charge transfer complexes (CTCs) or electron donor–acceptor complexes have been intensively studied as organic semiconductors or organic conductors in organic electronics. Herein, the composite of CTCs and single‐walled carbon nanotubes (SWCNTs) is studied as both p‐type and n‐type thermoelectric materials for flexible thermoelectric generators. CTCs are formed by [1]benzothieno[3,2‐b][1]benzothiophene (BTBT) derivatives with different side chains as electron donors and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) as electron acceptors. The thermoelectric properties of the composites as well as the effect of side chains in BTBT derivatives are investigated. It is found that the PhBTBT–F4TCNQ/SWCNT composite film shows the highest p‐type power factor of 244.3 µW m−1 K−2; while the C8BTBT–F4TCNQ/SWCNT composite film exhibits the highest n‐type power factor of 105.1 µW m−1 K−2. The thermoelectric module based on five p–n junctions of C8BTBT–F4TCNQ/SWCNT exhibits the highest output voltage of 13.1 mV and output power of 340 nW under a 38 K temperature gradient. This device performance is mainly generated from the moderate carrier concentrations and low film defects in the n‐type C8BTBT–F4TCNQ/SWCNT composite film.

Polymer and carbon nanotube composites have aroused extensive attention for thermoelectric materials owing to the combination of low thermal conductivity of polymer and high electrical conductivity of carbon nanotubes. Surface properties of the substrate are of great importance for the charge transport behaviors of semiconducting thin films, which are less explored in thermoelectric applications. Herein, self‐assembled monolayers (SAMs) are used to modify the substrate for thermoelectric polymer composites. The trifluoromethyl (CF3)‐terminated SAM is beneficial for an improved electrical conductivity; while the SAM with amino group is found to improve their Seebeck coefficient and decrease the electrical conductivity. As a result, polymer composites on CF3‐SAM‐modified substrate show a high room‐temperature power factor of 285 µW m−1 K−2 and a large output power of 2.36 µW for thermoelectric generator at a temperature gradient of 50 K. This work demonstrates that surface modification by SAMs is a promising strategy for improving performance of thermoelectric materials and devices.

Conjugated polymers have attracted considerable attention for thermoelectric applications in recent years due to their plentiful resources, diverse structures, mechanical flexibility, and low thermal conductivity. Herein, we demonstrate a new strategy of modulating charge carrier concentration of chemical-doped polymer films by modifying the substrate with self-assembled monolayers (SAMs). The SAM with a trifluoromethyl terminal group is found to accumulate holes in the polymer thin films, while the SAM with an amino terminal group tends to donate electrons to the polymer films. Thermoelectric thin films of conjugated donor–acceptor copolymer exhibit high power factors of 55.6–61.0 μW m–1 K–2 on SAMs with polar terminal groups. These power factors are 49% higher than that on the SAM with the nonpolar terminal group and 3 times higher than that on pristine substrate. The high power factor is ascribed to the modulated charge carrier concentration and improved charge carrier mobility as induced by SAMs.

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