Conventional chemical doping processes for conjugated polymers (CPs) often degrade the film morphology or cause unsatisfactory doping efficiency owing to the aggregation formation between charged species or insufficient dopant diffusion. In this work, a new strategic doping method, "hybrid doping," is suggested for maximizing the doping efficiency of CPs without hampering the surface morphology of the CP films. The advantage of hybrid doping is that it combines mixture blending and sequential soaking processes. Based on systemic characterizations including spectroscopic, structural, and electrical analyses, it is revealed that hybrid doping enables whole area doping for the crystalline and amorphous regions of CP films, and thus an unprecedentedly high electrical conductivity of up to 81.5 and 639.1 S cm −1 , for poly(3-hexylthiophene) P3HT and poly (2-([2,2′-bithiophen]-5-yl)-3,8-difluoro-5,10-bis(5octylpentadecyl)-5,10-dihydroindolo [3,2-b]indole) (PIDF-BT), respectively, is achieved. Furthermore, the exceptional electrical conductivity compensates a reduced Seebeck coefficient, resulting in excellent power factors up to 26.8 and 76.1 μW m −1 K −2 for thermoelectric devices based on doped-P3HT and PIDF-BT films, respectively, which is among the highest levels for semiconducting CPs. Hybrid doping is a strategic approach for the simultaneous optimization of electrical conductivity and thermoelectric properties of various CPs.

Intercorrelation of thermoelectric properties of a doped conjugated semiconducting polymer (PIDF-BT) with charge carrier density, conductive morphology, and crystallinity are systematically investigated. Upon being doped with F4-TCNQ by the sequential doping method, PIDF-BT exhibited a high electrical conductivity over 210 S cm–1. The significant enhancement of electrical conductivity resulted from a high charge carrier density, which is attributed to the effective charge–transfer-based integer doping between PIDF-BT and dopant molecules. Based on the systemic characterization on the optical, electrical, and structural properties of doped PIDF-BT annealed at different temperatures, we investigated the characteristic correlations between thermoelectric properties of PIDF-BT films and their four-probe electrical conductivity, charge carrier density, and charge carrier mobility obtained from AC Hall effect measurements. This study revealed that exercising fine control over the crystallinity and conductive migration of the conjugated polymer films can be a strategic approach to suppressing the degradation of the Seebeck coefficient at high charge carrier density and ultimately to maximizing the power factors of organic thermoelectric devices.

Doping capability is primitively governed by the energy level offset between the highest occupied molecular orbital (HOMO) of conjugated polymers (CPs) and the lowest unoccupied molecular orbital (LUMO) of dopants. A poor doping efficiency is obtained when doping directly using NOBF4 forming a large energy offset with the CP, while the devised doping strategy is found to significantly improve the doping efficiency (electrical conductivity) by sequentially treating the NOBF4 to the pre‐doped CP with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquino‐dimethane (F4TCNQ), establishing a relatively small energy level offset. It is verified that the cascade doping strategy requires receptive sites for each dopant to further improve the doping efficiency, and provides fast reaction kinetics energetically. An outstanding electrical conductivity (>610 S cm−1) is achieved through the optimization of the devised doping strategy, and spectroscopy analysis, including Hall effect measurement, supports more efficient charge carrier generation via the devised cascade doping.

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The effect of molecular weight of a series of conjugated polymers (CPs) on the doping efficiency, electrical conductivity, and related thermoelectric properties of doped CPs is studied. Low (L), medium (M), and high (H) molecular weight batches of PDFD-T polymers, based on difluorobenzothiadiazole and dithienosilole moieties, are synthesized and denoted as PDFD-T(L), PDFD-T(M), and PDFD-T(H), respectively. Furthermore, to compare the effects of different donor moieties, donor units of PDFD-T(L) are structurally modified from thiophene to thienothiophene (TT) and dithienothiophene (DTT), denoted as PDFD-TT(L) and PDFD-DTT(L), respectively. After doping the CPs with FeCl 3 , d-PDFD-T(H) exhibits an electrical conductivity of 402.9 S cm −1 , which is significantly higher than those of d-PDFD-T(L), d-PDFD-T(M), d-PDFD-TT(L), and d-PDFD-DTT(L). The highest power factor of 101.1 µW m −1 K −2 is achieved through organic thermoelectric devices fabricated using PDFD-T(H). Through various characterizations, it is demonstrated that CPs with a high molecular weight tend to have a high carrier mobility while maintaining their original crystallinity and good charge transport pathways even after doping. Therefore, it is suggested that optimizing the molecular weight of CPs is an essential strategy for maximal power generation from their doped CP films.