The effect of aromatic ring size in electron deficient semiconducting polymers for n-type organic thermoelectrics

Article abstract of DOI:10.1039/d0tc03347b

N-type semiconducting polymers have been recently utilized in thermoelectric devices, however they have typically exhibited low electrical conductivities and poor device stability, in contrast to p-type semiconductors, which have been much higher performing. This is due in particular to the n-type semiconductor's low doping efficiency, and poor charge carrier mobility. Strategies to enhance the thermoelectric performance of n-type materials include optimizing the electron affinity (EA) with respect to the dopant to improve the doping process and increasing the charge carrier mobility through enhanced molecular packing. Here, we report the design, synthesis and characterization of fused electron-deficient n-type copolymers incorporating the electron withdrawing lactone unit along the backbone. The polymers were synthesized using metal-free aldol condensation conditions to explore the effect of enlarging the central phenyl ring to a naphthalene ring, on the electrical conductivity. When n-doped with N-DMBI, electrical conductivities of up to 0.28 S cm-1, Seebeck coefficients of -75 μV K-1 and maximum Power factors of 0.16 μW m-1 K-2 were observed from the polymer with the largest electron affinity of -4.68 eV. Extending the aromatic ring reduced the electron affinity, due to reducing the density of electron withdrawing groups and subsequently the electrical conductivity reduced by almost two orders of magnitude. This journal is

Full text of DOI:10.1039/d0tc03347b

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Fabiano and I. McCulloch, J. Mater. Chem. C, 2020, DOI: 10.1039/D0TC03347B.
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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC03347B
The Effect of Aromatic Ring Size in Electron Deficient Semiconducting
Polymers for n-type Organic Thermoelectrics
Maryam Alsufyani,a* Rawad K. Hallani,a* Suhao Wang,b Mingfei Xiao,d Xudong Ji,c Bryan D.
Paulsen,c Kai Xu,b Helen Bristow,e Hu Chen,a Xingxing Chen,a Henning Sirringhaus,d Jonathan
Rivnay,c Simone Fabianob and Iain McCullocha,e
aKing Abdullah University of Science and Technology (KAUST), Physical Science and
Engineering Division, Thuwal, 23955-6900, Saudi Arabia
bLaboratory of Organic Electronics, Department of Science and Technology, Linköping
University, Norrköping, SE-60174, Sweden
cNorthwestern University, Department of Biomedical Engineering, 2145 Sheridan Rd, Evanston,
IL 60208, USA
dDepartment of Physics, University of Cambridge, Cambridge CB2 1TN, UK
eDepartment of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford, OX1
3TA
Email: maryam.alsufyani@kaust.edu.sa; rawad.hallani@kaust.edu.sa
1

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DOI: 10.1039/D0TC03347B
Abstract
N-type semiconducting polymers have been recently utilized in thermoelectric devices, however
they have typically exhibited low electrical conductivities and poor device stability, in contrast to
p-type semiconductors, which have been much higher performing. This is due in particular to the
n-type semiconductor’s low doping efficiency, and poor charge carrier mobility. Strategies to
enhance the thermoelectric performance of n-type materials include optimizing the electron
affinity (EA) with respect to the dopant to improve the doping process and increasing the charge
carrier mobility through enhanced molecular packing. Here, we report the design, synthesis and
characterization of fused electron-deficient n-type copolymers incorporating the electron
withdrawing lactone unit along the backbone. The polymers were synthesized using metal-free
aldol condensation conditions to explore the effect of enlarging the central phenyl ring to a
naphthalene ring, on the electrical conductivity. When n-doped with N-DMBI, electrical
conductivities of up to 0.28 S cm-1, Seebeck coefficients of -75 μV K−1 and maximum Power
factors of 0.16 μW m−1 K−2 were observed from the polymer with the largest electron affinity of -
4.68 eV. Extending the aromatic ring reduced the electron affinity, due to reducing the density of
electron withdrawing groups and subsequently the electrical conductivity reduced by almost two
orders of magnitude.
2

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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC03347B
Introduction
Conjugated polymers play an essential role in the development of next-generation organic
electronics.1-3 Recently, they have been heavily exploited in the development of cost-competitive
organic thermoelectric devices (OTE) that could be employed to convert waste thermal energy
collected from mechanical, chemical, and electrical processes, into electricity4. Although
developing OTE materials is still a work in progress especially for n-type OTE, the low cost, ease
of fabrication, and high degree of flexibility offer π-conjugated polymers a potential advantage
over other type of thermoelectric materials such as inorganic alloys (i.e., lead telluride and bismuth
chalcogenide).5 The figure of merit, ZT, expresses the efficiency of heat conversion to electricity
by thermoelectric materials, which is determined by electrical conductivity (σ), Seebeck
coefficient (S), and thermal conductivity (k), (ZT = S2σT/k). However, due to the low thermal
conductivity that conjugated polymers possess,6 their thermoelectric performance is usually
evaluated by the power factor (PF) where PF = S2σ. Since the power factor is proportional to the
electrical conductivity, which is dependent on carrier concentration n (cm−3), (maximized upon
doping), and carrier mobility μ (cm2 V−1s−1), a polymer with high electron mobility and an efficient
doping process would lead to a boost in thermoelectric performance.
Hole conducting polymers have demonstrated high electrical conductivities of over 1000 S cm-1.
As a result, their thermoelectric figures of merit (ZT) exhibited high values in the range of 0.2-
0.4,7-11 comparable to the state-of-the-art inorganic (TE) materials. However, the conductivity of
n-type electron-conducting polymers have been trailing behind their p-type counterparts, with few
examples having achieved conductivities of over 1 S cm-1,12-15 This is predominantly arising from
not exhibiting a large enough electron affinity (too shallow LUMO energy level) to facilitate
electron transfer from n-dopant. A deep LUMO level is required to achieve an energy offset that
3