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

Abstract

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.

A thermally activated and highly miscible dopant for n-type organic thermoelectrics

“…f Thermoelectric performance comparison of TAM-doped FBDPPV with literature results (refs. 11,24,[26][27][28][29][30][53][54][55][56][57][58] ). Error bars indicate the SD of ten experimental replicates.…”

“…However, these dopants still lack solution stability and miscibility with organic semiconductors 11,24 . To overcome the immiscibility, modification of semiconductor backbone using polar side chain or twisted conjugated building blocks are developed; however, these strategies usually significantly change the charge transport property of organic semiconductors, e.g., leading to much lower charge carrier mobility [25][26][27][28] . Therefore, a simple and efficient approach is desired to solve the stability and miscibility issues.…”

N-type organic thermoelectrics: demonstration of ZT > 0.3

“…After an extensive study of p-type organic thermoelectric materials 1,3,16 , the scientific community has recently turned its focus to the more challenging n-type counterparts because both efficient p-and n-type TE materials are required for practical applications. A large variety of organic semiconductors, including conjugated polymers and small molecules, have been utilized for n-type organic thermoelectrics 12,[17][18][19][20][21][22] . Most n-type organic thermoelectric materials exhibit an electrical conductivity of <2 S cm −1 and power factor (S 2 σ) of <10 µW m −1 K −2 23,24 .…”

Impact of Morphology on Charge Carrier Transport and Thermoelectric Properties of N‐Type FBDOPV‐Based Polymers

“…The CTC formation efficiency can be evaluated from the slope of the linear fit. [ 55 ] An outstanding yield of ≈95% was obtained for doped P(FBDOPV‐F) films, tentatively ascribed to the lower interunit electronic coupling, due to the larger torsion angle compared to the bithiophene‐based copolymer, allowing a higher tolerance of ionized dopant introduction in the vicinity of the reduced polymer chains segments. [ 27 ] Note that as double doping is eventually possible with N‐DMBI (i.e., one mole of N‐DMBI can potentially transfer electrons to two moles of host), a CTC formation yield close to 100% does not necessarily imply a complete ionization of the dopants.…”

“…This phenomenon is commonly observed for molecular‐doped organic layers. [ 12,21,32,55,58 ] The decrease of activation energy with the addition of dopant [ 58 ] is usually counterbalanced by a synergetic effect of dopant segregation and a loss of molecular order. [ 53,54 ] AFM topography images performed on undoped and N‐DMBI‐doped films at different doping concentrations confirmed the formation of round‐shaped dopant aggregates on the top surface of doped films for both polymers (Figure S15, Supporting Information).…”