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N‐Type Organic Thermoelectrics of Donor–Acceptor Copolymers: Improved Power Factor by Molecular Tailoring of the Density of States
Author(s) -
Liu Jian,
Ye Gang,
Zee Bas van der,
Dong Jingjin,
Qiu Xinkai,
Liu Yuru,
Portale Giuseppe,
Chiechi Ryan C.,
Koster L. Jan Anton
Publication year - 2018
Publication title -
advanced materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.707
H-Index - 527
eISSN - 1521-4095
pISSN - 0935-9648
DOI - 10.1002/adma.201804290
Subject(s) - materials science , dopant , seebeck coefficient , thermoelectric materials , thermoelectric effect , doping , acceptor , density functional theory , copolymer , chemical physics , nanotechnology , condensed matter physics , optoelectronics , computational chemistry , thermal conductivity , polymer , thermodynamics , chemistry , composite material , physics
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 2 ‐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.