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

• Published in: Advanced materials (Weinheim) Vol. 30; no. 44; pp. e1804290 - n/a
• Main Authors: Liu, Jian, Ye, Gang, Zee, Bas van der, Dong, Jingjin, Qiu, Xinkai, Liu, Yuru, Portale, Giuseppe, Chiechi, Ryan C., Koster, L. Jan Anton
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01-11-2018
• Subjects: Backbone; Charge transfer; Charge transport; Computer simulation; Copolymers; Density of states; donor–acceptor copolymers; Doping; electrical conductivity and density of states; Electrical resistivity; Materials science; Nitrogen atoms; n‐type doping; Power factor; Seebeck effect; solution processing; Temperature dependence; donor-acceptor copolymers; electrical conductivity and density of states; solution processing; n-type doping
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.201804290

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 sp2‐nitrogen atoms into the donor moiety of the D–A backbone. Consequently, an electrical conductivity of 1.8 S cm−1 and a power factor of 4.5 µW m−1 K−2 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.
Significantly boosted n‐type organic thermoelectric performance of donor–acceptor (D–A) copolymers by a factor of three orders of magnitude is realized by tailoring the density of states through molecular design. This strategy improves the electrical conductivity effectively and reduces the loss of the Seebeck coefficient, leading to a very good power factor of 4.5 µW m−1 K−2 for doped D–A copolymers.