It is demonstrated that the n-type thermoelectric performance of donor-acceptor (D-A) copolymers can be enhanced by a factor of >1000 by tailoring the density of states (DOS). The DOS distribution is tailored by embedding sp -nitrogen atoms into the donor moiety of the D-A backbone. Consequently, an electrical conductivity of 1.8 S cm and a power factor of 4.5 µW m K are achieved. Interestingly, an unusual sign switching (from negative to positive) of the Seebeck coefficient of the unmodified D-A copolymer at moderately high dopant loading is observed. A direct measurement of the DOS shows that the DOS distributions become less broad upon modifying the backbone in both pristine and doped states. Additionally, doping-induced charge transfer complexes (CTC) states, which are energetically located below the neutral band, are observed in DOS of the doped unmodified D-A copolymer. It is proposed that charge transport through these CTC states is responsible for the positive Seebeck coefficients in this n-doped system. This is supported by numerical simulation and temperature dependence of Seebeck coefficient. The work provides a unique insight into the fundamental understanding of molecular doping and sheds light on designing efficient n-type OTE materials from a perspective of tailoring the DOS.

Molecular doping of organic semiconductors creates Coulombically bound charge and counterion pairs through a charge-transfer process. However, their Coulomb interactions and strategies to mitigate their effects have been rarely addressed. Here, we report that the number of free charges and thermoelectric properties are greatly enhanced by overcoming the Coulomb interaction in an n-doped conjugated polymer. Poly­(2,2′-bithiazolothienyl-4,4′,10,10′-tetracarboxydiimide) (PDTzTI) and the benchmark N2200 are n-doped by tetrakis (dimethylamino) ethylene (TDAE) for thermoelectrics. Doped PDTzTI exhibits ∼10 times higher free-charge density and 500 times higher conductivity than doped N2200, leading to a power factor of 7.6 μW m–1 K–2 and ZT of 0.01 at room temperature. Compared to N2200, PDTzTI features a better molecular ordering and two-dimensional charge delocalization, which help overcome the Coulomb interaction in the doped state. Consequently, free charges are more easily generated from charge–counterion pairs. This work provides a strategy for improving n-type thermoelectrics by tackling electrostatic interactions.

The ‘phonon-glass electron-crystal’ concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly ‘phonon glasses’. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being ‘electron crystals’. Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic ‘PGEC’ thermoelectric material. This thermoelectric material exhibits an excellent electrical conductivity of >10 S cm−1 and an ultralow thermal conductivity of <0.1 Wm−1K−1, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use ‘arm-shaped’ double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of organic semiconductors as thermoelectric materials.

polymers have also emerged as potential candidates for organic thermoelectrics, [7,8] potentially delivering flexible, large-area, and low-cost energy generation or heatingcooling devices for appealing applications, e.g., wearable energy harvesting, that are currently not possible for traditional brittle and usually either toxic or rare inorganic crystalline thermoelectric materials. Thermoelectric materials are evaluated by the dimensionless figure of merit ZT = S 2 σT/κ, where S, σ, T and κ represent the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively. Most conjugated polymers are characterized with low κ values, intrinsically contributing to a high ZT. This point has been verified by recent extensive thermoelectric studies based on p-type conjugated polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) with ZT > 0.25. [9,10] The performance of p-type and n-type thermoelectric materials should pair with each other ahead of any practical applications. However, n-type conjugated polymer-based thermoelectric devices are still far inferior to their p-type counterparts in terms of power factor (S 2 σ). [11,12] Therefore, the development of efficient The ORCID identification number(s) for the author(s) of this article can be found under

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