Fine-tuning the solid-state ordering and thermoelectric performance of regioregular P3HT analogues by sequential oxygen-substitution of carbon atoms along the alkyl side chains

Article abstract of DOI:10.1039/c8tc05938a

Conjugated polymers in thin films tend to orient their backbones with respect to substrates through self-assembly when processed from a solution. The molecular packing orientation and crystallinity of polymer crystals have been found to play a critical role in terms of their electrical performance. Taking advantage of the shorter bond length and the smaller rotation energy of the CH₂-O bond than those of the CH₂-CH₂ bond in an alkyl chain, a side-chain engineering strategy is reported, which involves sequential oxygen-substitution of the carbon atoms from the γ position outwards along the hexyls in regioregular poly(3-hexylthiophene) (RR-P3HT). The subtle disparity between the CH₂-O and CH₂-CH₂ bonds is found to enable fine tuning of the solid-state organization of the corresponding RR-P3HT analogues, namely poly(3-(2-propoxyethyl)thiophene) (P3POET), poly(3-(3-ethoxypropyl)thiophene) (P3EOPT) and poly(3-(4-methoxybutyl)thiophene) (P3MOBT). The evolution of film microstructures is observed when doping with FeCl₃ in nitromethane at room temperature, and so is their thermoelectric performance. The highest power factor of 19 μW m^-1 K^-2 was observed for the doped P3POET film, which presents a dominant edge-on orientation with the strongest crystallinity and the closest π-π stacking. At the doping time corresponding to the optimized power factors (PFs) for each polymer, the doped P3POET film maintains both relatively high Seebeck coefficient (S) and electrical conductivity (σ) close to their maximum values, while the other polymers show either higher S and much lower σ (P3HT) or higher σ but much lower S (P3EOPT and P3MOBT). The square dependence of the PFs on S together with higher σ amplifies the power factor value of P3POET, which is almost 2 times those of the other polymers (11-13 μW m^-1 K^-2).

Full text of DOI:10.1039/c8tc05938a

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ISSN 2050-7526
PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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DOI: 10.1039/C8TC05938A
Journal Name
ARTICLE
Fine-Tuning Solid-State Ordering and Thermoelectric Performance
of Regioregular P3HT Analogues by Sequential Oxygen-
Substitution on Carbon Atoms along the Alkyl Side Chains
Received 00th January 20xx,
Accepted 00th January 20xx
Liangjun Chen,a Wei Liu,a Yonggao Yan,a Xianli Su,a Shengqiang Xiao,*a Xinhui Lu,*b Ctirad Uherc
and Xinfeng Tang*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Conjugated polymers in thin films tend to orient their backbones in respect to substrates through self-assembly when
processed from a solution. Molecular packing orientation and crystallinity of polymer crystals have been found to play a
critical role on their electrical performance. Taking advantage of the shorter bond length and the smaller rotation energy of
the CH2-O bond than the CH2-CH2 bond in an alkyl chain, a side-chain engineering strategy is reported by sequential oxygen-
substitution on carbon atoms from the γ position outwards along the hexyls in regioregular poly(3-hexylthiophene) (RR-
P3HT). The subtle disparity between CH2-O and CH2-CH2 bond is found to be able to fine-tune the solid-state organization of
the corresponding RR-P3HT analogues, namely poly(3-(2-propoxyethyl)thiophene) (P3POET), poly(3-(3-
ethoxypropyl)thiophene) (P3EOPT) and poly(3-(4-methoxybutyl)thiophene) (P3MOBT). The evolution of film
microstructures is observed when doping with FeCl3 in nitromethane at room temperature, so are their thermoelectric
performance. The highest power factor is 19 μW m-1 K-2 observed for doped P3POET film, which presents the dominant edge-
on orientation with the strongest crystallinity and the closest π - π stacking. At the doping time corresponding to the
optimized power factors (PF) for each polymer, the doped P3POET film maintains both relatively high Seebeck coefficient
(S) and electrical conductivity (σ) close to their maximum values, while the other polymers give either higher S and much
lower σ (P3HT) or higher σ but much lower S (P3EOPT and P3MOBT). The square dependence of PF on S together with higher
σ amplifies the power factor value for P3POET, which is almost 2 times compared to the other polymers (11-13 μW m-1 K-2).
into thermoelectric applications at moderate temperatures has
attracted an increasing attention in recent years.12-17 The
thermal-to-electrical energy conversion efficiency for a given
1. Introduction
Owing to their tunable optoelectronic properties, thermal
stability and mechanical flexibility, conjugated polymers have
been actively employed in the field of organic electronics by
various cost-effective solution-processing technologies.1-6 Such
tunability has been incessantly witnessed by manipulating their
chemical structures and physicochemical properties in
conjunction with device parameters towards expected
performance output. Deep understanding of the fundamental
structure-property-performance correlations has therefore
been significantly advanced over the past years.4, 7-11 The know-
how could also be readily applied in materials design and
fabrication of lightweight, flexible and low-cost organic
thermoelectrics (OTE). Thus, introducing conjugated polymers
semiconducting material is generally evaluated by its
dimensionless figure of merit defined as ZT = S2σT/κ, where S (V
K-1) is the Seebeck coefficient, σ (S m-1) is the electrical
conductivity, T is the absolute temperature in Kelvin, and κ (W
m-1 K-1) is the thermal conductivity.18 The Seebeck coefficient is
directly related to the density of states. The electrical
conductivity is expressed as σ = nqμ, where n (cm-3) is the carrier
density, q is the carrier charge, and μ (cm2 V-1 s-1) is the carrier
mobility. The combined contribution from the lattice (κL) and
electron (κE) to heat transport constitutes the overall thermal
conductivity. The product of S2σ is defined as the power factor
(PF), usually specified in μW m-1 K-2 and indicative of the
electrical power that can be generated by a thermoelectric
material. What is the most exasperating is the strong adverse
interdependence among S, σ, and κ, which imposes restrictions
on how large ZT can be achieved. In particular, as n increases,
the increasing σ and κ are accompanied by a decreasing S.18
Optimizing ZT thus stands as a grand challenge. Considerable
efforts have been exerted on the structure design and/or
processing of materials so as to decouple the interdependence
among parameters of S, σ, and κ by manipulating phonons and
electrons at nanoscale to enhance the ZT value.13, 19 On the
other hand, although the thermal conductivity of organic
a.State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, China. *E-mail:
shengqiang@whut.edu.cn; tangxf@whut.edu.cn
b.Department of Physics, Chinese University of Hong Kong, Hong Kong, P. R. China.
*E-mail: xhlu@phy.cuhk.edu.hk
c. Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA.
Electronic Supplementary Information (ESI) available: [detailed information on
materials, measurement and instrumentation, 1H-NMR spectra, TGA, CV curves,
powder XRD patterns, neutral film GIWAXS profiles and doping time dependent
Raman spectra of the polymers and the collection of the optimized thermoelectric
performance of the polymers at r.t. doped with different concentrations of
FeCl3/CH3NO2 solution]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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semiconductors is much lower (0.1 - 1 W m-1 K-1) compared to
their inorganic counterparts,20-24 it is quite challengeable to be packing and thermoelectric properties of p-doped regioregular
measured, especially in the in-plane direction. Moreover, the poly(3-hexylthiophene) (RR-P3HT) analogues by engineering the side
thermal conductivity of organic semiconductors, including hexyl chains as indicated in Chart 1. RR-P3HT has evolved as a
polymers, is mostly closer to that of a phonon glass.18 benchmark polymer for OTE research as well as PEDOT:PSS due to its
Journal Name
In this contribution, we elucidate the evolutionVoiewf AmrtioclleeOcunllianer
DOI: 10.1039/C8TC05938A
Consequently, it is the PF that is usually determined instead of ready availability and tendency to form ordered lamellar structures
43-49
the ZT to assess the thermoelectric competency of in films by interchain stacking.41,
Herein, sequential oxygen
semiconducting polymers.
substitution on carbon atoms along the side hexyl chains was
Conjugated polymer paradigms within various organic rationally adopted to fine-tune intermolecular interactions of
electronic applications have been reiterating the critical role of polymers and to explore their effects on materials physicochemical
molecular ordering and orientation as a controlling factor to properties, film microstructures, and OTE performance. Commercial
achieve desired organization for high-performance.25-34 RR-P3HT was also employed as a reference to prompt such
Specifically, the edge-on orientated crystalline lamellae could investigations. It has been found that the energy barrier of the
be more favorable for efficient in-plane charge transport as rotation of O-CH2 bonds (E = 0.08 eV) is slightly smaller than that of
desired in organic field effect transistors, whereas the face-on CH2-CH2 bonds (E = 0.11 eV) in an alkyl chain. Meanwhile, the length
orientated lamellae could facilitate the out-of-plane charge of CH2-O σ bond (1.42 Å) is a little bit shorter than that of the CH2-
transport as desired in polymer solar cells. Since the charge CH2 σ bond (1.54 Å).50-52 Such fine disparities could render the singly
transport proceeds predominantly within the lamellae (either O-substituted chains more flexible without transforming its aliphatic
along the π - π stacking direction or along the polymer property relative to the hexyl. A fine evolution of polymer film
backbones), in some cases, an intermixing orientation textile microstructures could be expected when an oxygen atom substitutes
could even reduce the transport anisotropy, leading to a higher a carbon atom outwards along the side hexyl chains within P3HT. To
charge transport in specific directions. Grazing incidence wide- observe the effect of sequential O-substitution along the side chain
angle X-ray scattering (GIWAXS) has become a powerful tool for on OTE properties, the conventional oxidation doping method by
microstructure characterization of polymers in film.35-37 The soaking polymer films in FeCl3 solution in nitromethane for varied
importance of film microstructures has also been demonstrated time durations was used to make them electrically conducting for
in recent reports on polymer thermoelectric properties.38, 39 For OTE measurements at room temperature. Thus, this work has
example, besides introducing charge carriers that may form focused more on the effect of side chain engineering on solid-state
polarons or bipolarons and perhaps even a mixture of the two, ordering and OTE performance than on the doping method. Indeed,
the electrical transport properties also depend on the structural the evolution of aggregation microstructure is observed accordingly
organization of semiconducting polymers spanning the regime in response to such side chain engineering. When immersion-doped
from the molecular scale to the macroscale in solid state.40 with FeCl3 in nitromethane at an optimized concentration at room
However, given the fact that the optimal solid-state ordering of temperature, the film microstructures evolved in comparison to that
conjugated polymers within a specific device application can of their corresponding undoped films. Moreover, the characteristics
only be reached by trial and error, and is largely materials of charge carriers in the polymer films vary with the increasing
dependent, the task to directly attain desired solid-state doping time, and so do the parameters of S,
ordering is quite challenging. The introduction of charge carriers
within OTE devices is unexceptionally implemented by electrical
doping on organic materials so far. There is no secret that
choosing the right dopant and doping method (by vapor or
solution) is critical to the control of film morphology.21 From a
molecular engineering point of view, the conjugated backbone
and side chains (offering the solubility for processing) of a
conjugated polymer have a significant impact on molecular
ordering and orientations in thin films. Janáky et al studied the
σ and PF.
Chart 1. Chemical structures of the P3HT analogues in this work.
effect of side chain length on the supramolecular structure and
the electrical properties.41 Bazan et al studied the effect of
counterions species at side chain and the side chain length on
the conductivity and thermopower of self-doped conjugated
polyelectrolytes.42 How molecular engineering on conjugated
backbone and/or side chain could play the role for OTE
polymers in affecting their electronic structure and film
microstructure involved in thermoelectric conversion still
remains beyond reach. To realize the potential of
semiconducting polymers for thermoelectrics, the relationship
between the chemical structure, morphology, and the resulting
thermoelectric performance needs to be comprehensively
understood.
2. Experimental Section
2.1 Materials and general information on characterizations
3-Bromothiophene was purchased from Zhejiang Shou & Fu
Chemical Co., Ltd.. 2-Propynol was purchased from Beijing
Zhongketuozhan Chemical Technology Co., Ltd.. RR-P3HT was
purchased from Rieke Specialty Conducting Polymers. 2-Bromo-
3-methylthiophene (
2
), 2-bromo-3-(bromomethyl)thiophene
), and 4-(3-thienyl)-3-
(3
), 3-(2-propoxyethyl)thiophene (6
butyne-1-ol (13) were prepared according to standard literature
2 | J. Name., 2012, 00, 1-3
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Journal Name
procedures.53-56 All other chemicals were purchased from J & K 3-(3-Ethyoxyl-1-propynyl)thiophene (10)
ARTICLE
.
The ViepwreArpticalreaOtniloinne
and Energy Chemicals and used as received unless noted procedure was analogous to that foDrOI3: -1(02.1-0p3r9o/pCo8TxCye05th93y8l)A-
otherwise. Dry n-butanol was obtained by drying with NaOH thiophene (6) with 1-bromopropane replaced by ethyl iodide.
pellets under reflux before distillation under nitrogen. Purification by silica gel column chromatography eluted by
Anhydrous THF were prepared by standard distillation petroleum ether/ethyl acetate (30:1, v/v) to furnish the pure
1
procedures with sodium and benzophenone prior to use. product (yield, 47%). H-NMR (500 MHz, CDCl3), δ (ppm): 7.45
General information on instrumentation and characterizations (dd, J = 0.80 Hz, 1 Hz, 1H), 7.25 (dd, J = 3.15 Hz, 3.00 Hz, 1H),
can be found in Electronic Supplementary Information.
7.12 (dd, J = 0.95 Hz, 0.95 Hz, 1H), 4.34 (s, 2H), 3.64 (q, J = 7.05
Hz, 2H), 1.26 (t, J = 7.00 Hz, 3H). 13C-NMR (125 MHz, CDCl3):
129.86, 128.99, 125.19, 121.68, 84.93, 81.02, 65.50, 58.50,
2.2 Synthesis
2-Bromo-3-(butoxymethyl)thiophene (4). Sodium (2.1 g, 91.0 14.96. GC-MS: C9H10OS, [M]+ calc.: 166.05, found: 166.05 (M),
mmol) was added into a round-bottom flask containing 34 mL 121.01 (M-C2H5O).
of dry n-butanol under nitrogen. The mixture was stirred at 3-(3-Ethyoxylpropyl)thiophene
(11)
.
3-(3-Ethyoxyl-1-
room temperature until the sodium was consumed. 2-Bromo-3- propynyl)thiophene (10, 3.54 g, 21.0 mmol), Pb/C (700.0 mg)
bromomethylthiophene (7.58 g, 29.6 mmol) was then added and 20 mL of methanol was mixed in a high-pressure reactor.
dropwise. The solution was stirred at room temperature The reaction was then carried out at 60 oC under hydrogen with
overnight. The resulting solution was poured into water and the pressure of 20 MPa for 48 h. Then the resulting mixture was
extracted with ethyl ether. After drying over anhydrous MgSO4, filtered and the filtrate was concentrated using rotary
the extraction was filtered and concentrated. The crude product evaporator. Pure product (3.08 g) was obtained from the crude
was distilled under reduced pressure to obtain 4.94 g of the residue after purification by silica gel column chromatography
1
pure product (yield, 67%). H-NMR (500 MHz, CDCl3), δ (ppm): eluted by a mixture of ether/ethyl acetate (40:1, v/v) (yield,
7.23 (d, J = 4.10 Hz, 1H), 6.98 (d, J = 3.15 Hz, 1H), 4.43 (s, 2H), 86%). 1H-NMR (500 MHz, CDCl3), δ (ppm): 7.24 (dd, J = 3.35 Hz,
3.46 (t, J = 6.00 Hz, 2H), 1.58 (m, 2H), 1.39 (m, 2H), 0.91 (t, J = 3.30 Hz, 1H), 6.94 (d, J = 4.40 Hz, 2H), 3.48 (q, J = 7.05 Hz, 2H),
7.13 Hz, 3H). 13C-NMR (125 MHz, CDCl3), δ (ppm): 138.60, 3.43 (t, J = 6.45 Hz, 2H), 2.71 (t, J =7.60 Hz, 2H), 1.90 (m, 2H),
128.18, 125.82, 110.80, 70.22, 66.76, 31.74, 19.32, 13.86. GC- 1.21 (t, J = 7.00 Hz, 3H). 13C-NMR (125 MHz, CDCl3), δ (ppm):
MS: C9H13OSBr, [M]+ calc.: 247.99, found: 247.93 (M), 174.75 142.28, 128.21, 125.15, 120.05, 69.74, 66.09, 30.50, 26.76,
(M-C4H9O).
15.20. GC-MS: C9H14OS, [M]+ calc.: 170.07, found: 170.18 (M),
Similar to the 97.06 (M-CH3CH2OCH2CH2).
), the compound 2-Bromo-3-(3-ethyoxylpropyl)thiophene (12). Similar to the
was prepared from 3-(2-propoxyethyl)-thiophene ( ) and preparation of 2-bromo-3-methylthiophene ( ), the compound
2-Bromo-3-(2-propoxyethyl)thiophene (7)
.
preparation of 2-bromo-3-methylthiophene (
2
6
2
purified by distillation under reduced pressure with a yield of was prepared from 3-(3-ethyoxylpropyl)thiophene (11). The
1
69%. H-NMR (500 MHz, CDCl3), δ (ppm): 7.19 (d, J = 4.90 Hz, pure product was obtained by distilling the crude product under
1H), 6.87 (t, J = 2.80 Hz, 1H), 3.59 (t, J = 6.75 Hz, 2H), 3.40 (t, J = reduced pressure (yield, 67%). 1H-NMR (500 MHz, CDCl3), δ
6.40 Hz, 2H), 2.86 (t, J = 6.65 Hz, 2H), 1.59 (m, 2H), 0.91 (t, J = (ppm): 7.18 (d, J = 5.60 Hz, 1H), 6.80 (d, J = 5.55 Hz, 1H), 3.47(q,
7.20 Hz, 3H). 13C-NMR (125 MHz, CDCl3), δ (ppm): 138.47, J = 7.10 Hz, 2H), 3.42 (t, J = 6.48 Hz, 2H), 2.66 (t, J = 7.58 Hz, 2H),
128.63, 125.20, 109.80, 72.60, 69.62, 29.99, 22.90, 10.56. GC- 1.86 (m, 2H), 1.21 (t, J = 7.00 Hz, 3H). 13C-NMR (125 MHz, CDCl3),
MS: C9H13OSBr, [M]+ calc.: 247.99, found: 247.89 (M), 174.80 δ (ppm): 141.12, 128.26, 125.24, 109.10, 69.49, 66.12, 29.64,
(M-C4H9O).
3-(3-Thienyl)-2-propyn-1-ol (9). To a two-neck round-bottom 247.94 (M), 201.93 (M-C2H5O), 174.82 (M-CH3CH2OCH2CH2).
flask was added in Pd(PPh3)Cl2 (1.01 g, 5.0 mmol), PPh3 (2.62 g, 3-(4-Methoxyl-1-butyne)thiophene (14) Similar to the
10.0 mmol), CuI (1.91 g, 10.0 mmol), 3-bromothiophene (16.3 preparation of 3-(2-propoxyethyl)thiophene ( ), the compound
25.98, 15.20. GC-MS: C9H13OSBr, [M]+ calc.: 247.99, found:
.
6
g, 100.0 mmol), and 30 mL of anhydrous dimethyl formamide was prepared from 4-(3-thienyl)-3-butyne-1-ol (13). The pure
(DMF) under nitrogen. After stirring for 10 min, 100 mL of product was obtained by silica gel column chromatography by
anhydrous diisopropylamine and 2-propyn-1-ol (11.2 g, 200.0 using the eluent of the mixture of petroleum and ethyl acetate
mmol) was added in. The mixture was then heated to reflux (40:1, v/v) (yield, 75%). 1H-NMR (500 MHz, CDCl3), δ (ppm): 7.37
overnight. After cooling down to room temperature, the (d, J = 2.10 Hz, 1H), 7.22 (dd, J = 3.00 Hz, 3.05 Hz, 1H), 7.08 (dd,
mixture was filtered and the filtrate was concentrated. After J = 0.8 Hz, 0.8 Hz, 1H), 3.58 (t, J = 6.95 Hz, 2H), 3.4 (s, 3H), 2.67
removal of the residual solvent of DMF by distillation under (t, J = 6.95 Hz, 2H). 13C-NMR (125 MHz, CDCl3), δ (ppm): 129.95,
reduced pressure, the crude product was further purified by 127.98, 124.94, 122.52, 86.17, 76.46, 70.79, 58.68, 20.58, 14.16.
silica gel column chromatography eluted by petroleum GC-MS: C9H10OS, [M]+ calc.: 166.04, found: 151.21 (M-CH3).
ether/ethyl acetate (3:1, v/v) to furnish the pure product (yield, 3-(4-Methoxylbutyl)thiophene (15). Similar to the preparation
75%). 1H-NMR (500 MHz, CDCl3), δ (ppm): 7.45 (d, J = 2.00 Hz), of 3-(3-ethyoxylpropyl)-thiophene (11), the compound was
7.26 (dd, J = 3.05 Hz, 2.40 Hz, 1H), 7.11 (d, J = 3.95 Hz,1H), 4.48 prepared from 3-(4-methoxyl-1-butyne)thiophene (14) and
(s, 2H), 2.04 (s, 1H). 13C-NMR (125 MHz, CDCl3), δ (ppm): 129.77, purified by silica gel column chromatography by using the
129.08, 125.34, 121.50, 86.86, 80.82, 51.53. GC-MS: C7H6OS, eluent of the mixture of petroleum and ethyl acetate (40:1, v/v)
1
[M]+ calc.: 138.01, found: 138.01 (M).
(yield, 86%). H-NMR (500 MHz, CDCl3), δ (ppm): 7.23 (dd, J =
3.05 Hz, 3.05 Hz, 1H), 6.93 (d, J = 4.85 Hz, 2H), 3.39 (t, J = 6.38
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
Hz, 2H), 3.32 (s, 3H), 2.65 (t, J = 7.50 Hz, 2H), 1.69 (m, 2H), 1.62 nitrogen. The substrates were then transferred to an UVOCS
View Article Online
(m, 2H). 13C-NMR (125 MHz, CDCl3), δ (ppm): 142.70, 128.16, ultra-violet ozone cleaning system and exDpOosI:e1d0.1t0o3U9/VC-8oTzCo0n5e93f8oAr
125.10, 119.93, 72.57, 58.51, 30.02, 29.25, 27.06. GC-MS: 10 min prior to use. The polymers were dissolved in
C9H14OS, [M]+ calc.: 170.08, found: 170.12 (M), 110.04 (M- chlorobenzene (CB) respectively with concentration as 30 mg
CH3OCH2CH2).
2-Bromo-3-(4-methoxylbutyl)thiophene (16). Similar to the substrates at 1000 rpm for 1 min and dried naturally under
preparation of 2-bromo-3-methylthiophene ( ), the compound ambient condition to offer film thickness around ~200 ± 40 nm.
mL-1. Then polymer films were spin-coated on the glass
2
was prepared from 3-(4-methoxylbutyl)thiophene (15) and Anhydrous FeCl3 was dissolved in nitromethane with the
purified by distillation under reduced pressure with a yield of concentration of 0.01M. The polymer films were then immersed
1
73%. H-NMR (500 MHz, CDCl3), δ (ppm): 7.18 (d, J = 5.60 Hz, in FeCl3 solutions for certain periods of time, rinsed by
1H), 6.80 (d, J = 5.60 Hz, 1H), 3.39 (t, J = 6.13 Hz, 2H), 3.32 (s, nitromethane, and subsequently dried by an air flow before
3H), 2.60 (t, J = 7.25 Hz, 2H), 1.63 (m, 4H). 13C-NMR (125 MHz, being transferred instantly to a measurement system of ULVAC-
CDCl3), δ (ppm): 141.48, 128.15, 125.24, 109.98, 72.48, 58.56, RIKO ZEM-3 (Ulvac, Japan) at ambient temperature under
29.12, 29.11, 26.30. GC-MS: C9H14OSBr, [M]+ calc.: 247.99, helium atmosphere. The air humidity was controlled at 30% ~
found: 247.92 (M), 168.93 (M-Br), 136.91(M-Br-CH3O).
40%.
Synthesis of polymers. A general procedure for polymerizations
is described below.57 To
a
solution of 2-bromo-3-
3. Results and Discussion
(butoxymethyl)thiophene (4) (6.25 g, 25.2 mmol) in 100 mL of
anhydrous THF was added dropwise with 12.5 mL of 2 M LDA
3.1 Synthesis and characterization of P3HT analogues
o
(25.0 mmol) in anhydrous THF at -78 C under nitrogen. After
The molecular structures of all P3HT analogues and their
synthetic routes are depicted in Scheme 1. The standard
procedure of the Rieke polymerization method was employed
to prepare the polymers due to its strong capability of achieving
high regioregularity of P3HT with desired Mn and low PDI
values.57-59 The molecular weights of the polymers determined
by gel permeation chromatography (GPC) were found to be in
the range of 7.2 - 13.2 kg mol-1 with PDIs in the range of 1.2 - 1.5
o
stirring for 1 h at -78 C, a solution of anhydrous ZnCl2 (4.90 g,
36.0 mmol) in 100 mL of anhydrous THF was transferred in. The
reaction mixture was stirred for an additional hour at -78 oC and
then warmed to room temperature. Ni(dppp)Cl2 (136.0 mg, 0.25
mmol, 1% equiv.) was then added in and the reaction was kept
at room temperature overnight to yield a thick viscous solution.
After the partial removal of the solvent by rotavap, the mixture
was precipitated into 200 mL of methanol and filtered. The
crude product was transferred into a Soxhlet thimble and
extracted sequentially with methanol, hexanes, and
chloroform. The chloroform extraction was then concentrated
and dried in vacuum to furnish 2.20 g of the final polymer
product poly(3-(butoxymethyl)thiophene).
1
except around 2.13 for P3BOMT. H-NMR spectroscopy of the
polymers provided unequivocal evidence for the introduction of
oxygen atoms at the specific positions along the side chain. The
regioregularity of the polymers is typically calculated by
comparing the integrated peaks corresponding to the α-
methylene protons to thiophene unit on the side chains in head-
to-tail (HT) versus head-to-head (HH) linkages. In particular, δ
4.42 for P3BOMT, δ 2.85 for P3POET, δ 2.65 for P3EOPT, δ 2.59
for P3MOBT, δ 2.56 for P3HT (Fig. S1 - S5). This gives 70%, 96%,
95%, 97%, and 95% HT percentage for P3BOMT, P3POET,
P3EOPT, P3MOBT, and P3HT respectively. The much lower
regioregularity (70%) and higher PDI value of P3BOMT relative
to the other polymers could be ascribed to more unbalanced
reactivity of 2- and 5- position of thiophene unit caused by the
electron-withdrawing oxygen atom at the β position, which
probably increases the content of non-HT linkages in the
polymer. The strength of such electron-withdrawing inductive
effect of the strong electronegative oxygen atom on thiophene
units becomes weaker and weaker when oxygen atom is moving
beyond the β position. Much higher HT regioregularities and
uniform PDIs were reasonably achieved for the other three
polymers, which are comparable to that of the commercial
P3HT. The decomposition temperatures Td for polymers are
Poly(3-(butoxymethyl)thiophene) (
P3BOMT, yield, 50%). 1H-
NMR (500 MHz, CDCl3), δ (ppm): 7.24 (s, 1H), 4.58 (s, 2H), 3.58
(t, J = 6.30 Hz, 2H), 1.64 (m, 2H), 1.45 (m, 2H), 0.96 (t, J = 7.00
Hz, 3H). Mn = 9.0 kg mol-1, Mw = 19.1 kg mol-1, PDI = 2.13.
1
Poly(3-(2-propoxyethyl)thiophene)
(P3POET, yield, 57%). H-
NMR (500 MHz, CDCl3), δ (ppm): 7.10 (s, 1H), 3.72 (t, J = 6.90 Hz,
2H), 3.46 (t, J = 6.60 Hz, 2H), 3.31 (t, J = 6.65 Hz, 2H), 1.64 (q, J =
7.05 Hz, 2H), 0.95 (t, J = 7.40 Hz, 3H). Mn = 7.2 kg mol-1, Mw
=
10.9 kg mol-1, PDI = 1.51.
1
Poly(3-(3-ethoxypropyl)thiophene)
(
P3EOPT, yield, 36%). H-
NMR (500 MHz, CDCl3), δ (ppm):7.02 (s, 1H), 3.50 (dt, 4H), 2.90
(t, J = 7.50 Hz, 2H), 1.98 (m, 2H), 1.22 (t, J = 6.95 Hz, 3H). Mn=
12.4 kg mol-1, Mw = 15.7 kg mol-1, PDI = 1.26.
Poly(3-(4-methoxybutyl)thiophene) (
P3MOBT, yield, 50%). 1H-
NMR (500 MHz, CDCl3), δ (ppm): 6.99 (s, 1H), 3.44 (t, J = 6.35 Hz,
2H), 3.35 (s, 3H), 2.84 (t, J = 7.10 Hz, 2H), 1.77 (m, 2H), 1.73 (m,
2H). Mn = 13.2 kg mol-1, Mw = 17.9 kg mol-1, PDI = 1.36.
o
determined by 5% weight loss, 296, 330, 369, 404, and 450 C
2.3 Polymer films for thermoelectric performance measurements
for P3BOMT, P3POET, P3EOPT, P3MOBT and P3HT, respectively
Glass substrates with the size of 2 cm × 2 cm were sonicated in
acetone, isopropanol, ethanol and deionized water sequentially
and washed by the corresponding solvent and dried by a flow of
(Fig. S6).
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DOI: 10.1039/C8TC05938A
Scheme 1. The synthetic routes of the O-substituted P3HT analogues.
3.2 Melting and crystallization behaviors
The effects of sequential oxygen substitution along the alky side
chain from the β to the ω position on the melting and
crystallization behaviors of P3HT analogues were investigated
by differential scanning calorimeter (DSC) measurements. The
heating and cooling thermograms of the second cycle of the
polymers are presented in Fig. S7. For P3BOMT, no melting and
crystallization peak can be clearly detected, which could be
ascribed to its much lower HT regioregularity around 70%. In
the heating cycle, P3HT offered the highest peak melting
o
temperature (Tm) of 222 C, which is in great agreement with
the reported Tm values.60 The peak Tm values of P3POET,
P3EOPT, and P3MOBT were determined to be 177, 200, and 211
oC, respectively, apparently indicating a progressively increasing
trend when oxygen atom moves towards the end of the side
chain. The second cooling scans indicate apparent propensity of
crystallization for the polymers of P3POET, P3EOPT, P3MOBT,
and P3HT. A similar shift of the crystallization temperatures (Tc)
towards higher temperatures can be observed as well. The peak
Tc values of P3POET, P3EOPT, P3MOBT, and P3HT were found
to increase from 117, 148, 174 to 187 oC. A broad crystallization
process can be noticed for P3POET. Although P3BOMT doesn’t
present obvious melting and crystallization behavior, the
increasing trends in the Tm and Tc values indicate that the
thermal property of the P3HT can be effectively controlled by
engineering the position of O atom located at the side chain,
suggesting the existence of varied intermolecular interaction
among the polymers in solid state.
Fig. 1 The melting temperature (Tm) and crystallization temperature (Tc) of the
polymers as a function of the position of O atom along the side chain.
3.3 Optical and electrochemical properties
The solution (1.0 × 10-5 g mL-1 in chlorobenzene) and thin-film
UV-vis absorption spectra of all polymers at room temperature
are depicted in Fig. 2. In diluted chlorobenzene solution, all the
polymers show a single absorption band with slightly varied
maximum wavelength (λmax,sol) around 456 nm as tabulated in
Table 1, which originates from π - π* transitions on the
conjugated backbones. The extinction coefficients (εsol) of these
bands range from 2.5 × 104 to 3.5 × 104 mL cm-1 g-1, as compiled
in Table 1. When going from the solution to the thin film state,
the peak from the π - π* transitions is red-shifted around 58 nm
for P3POET, P3EOPT, P3MOBT and P3HT, while it is only around
25 nm for P3BOMT. It should be noted that the formation of
vibronic shoulders at the lower energy area can be obviously
observed for all the polymers except for P3BOMT. This is
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especially the case for P3POET with the O atom located at the γ
position in the side chain, implying an increased intermolecular
interaction and ordering in the film over the others. The
maximum wavelengths of films (λmax,film) and the corresponding
absorption coefficients (εfilm) are presented in Table 1 as well.
In addition, except for P3BOMT with an optical bandgap (Eg) of
2.11 eV, the other four polymers yield similar band gaps of
around Eg = 1.92 eV.
more easily oxidized when the O atom is moved from the β to
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the ω position along the side chain. ThisDcOaIn: 1b0e.10w39e/lCl 8eTxCp0la5i9n3e8dA
by the electron-withdrawing inductive effect of O atom. When
the O atom moves outwards from the β to the ω position, its
electron-withdrawing inductive effect decreases. The
conjugated backbone becomes relatively more electron-rich
and tends to be more easily oxidized to offer a lower Eoxonset. The
apparent effect of such side chain engineering on redox
properties of the resulting conjugated polymers was seldom
observed. The LUMO energy levels of the polymers were
estimated to range from -3.2 to -3.3 eV according to the
equation of ELUMO = (EHOMO + Eg).
With much lower regioregularity, the UV-vis absorption
spectra of P3BOMT both in solution and in film differ greatly
from the others. Moreover, the HOMO level of P3BOMT is much
deeper due to the existence of electron-withdrawing inductive
effect of the β oxygen atom on the conjugated backbone. In
order to objectively explore the effect of oxygen substitution on
solid-state ordering and thermoelectric performance, P3BOMT
is therefore excluded from the other P3HT analogues in
following investigations. On the other hand, the variation of
molecular weight was also noticed among P3POET, P3EOPT,
P3MOBT and P3HT with comparable regioregularities. Effective
conjugation length (nECL) is an important point from which
further extension of the conjugated chain does not affect a
certain property any more, for example, the maxmium
absorption wavelength and the band gap in the solid state. For
polythiophenes, the effective conjugation length was proven to
Fig. 2 UV-vis absorption spectra of the polymers in PhCl solutions at 1.0 × 10-5 g mL-1
(a) and in films as casted from the corresponding PhCl solutions at 30 mg mL-1 (b).
62
be 20 ~ 22 monomers.61, Here, the effective conjugation
The electrochemical properties were further investigated by
cyclic voltammetry to figure out the energy level of HOMO and
LUMO of all the polymers (Fig. S8). Irreversible oxidative
responses were observed. The HOMO energy levels (EHOMO) at -
5.36, -5.24, -5.15, -5.10, and -5.21 eV were calculated from the
oxidation onset potentials (Eoxonset) as presented in Table 1 for
P3BOMT, P3POET, P3EOPT, P3MOBT and P3HT, respectively.
The O-substituted polymers roughly exhibit a tendency to be
Table 1. Summary of the photophysical and electrochemical properties of all polymers.
length is around 42 monomers for P3POET with the lowest Mn,
which is quite enough for the saturation of photophysical
properties. As can be proven by the absorption spectra depicted
in Fig. 2, the maxmium absorption wavelength of P3POET,
P3EOPT, P3MOBT and P3HT both in solution and in film are
consistent with each other. The effect of molecular weight was
therefore not taken into consideration for the study on solid
state ordering and OTE below.
onset
λmax,sol
(nm)
εsol
λmax,film
(nm)
εfilm
(104 cm-1)
Eg
(eV)
Eox
EHOMO
(eV)
ELUMO
(eV)
Polymer
(mL cm-1 g-1)
(V)
P3HT
460
452
3.5 × 104
3.2 × 104
515, 551, 603
476
7.0, 6.1, 3.1
6.5
1.92
2.11
0.51
0.66
-5.21
-5.36
-3.29
-3.25
P3BOMT
P3POET
455
2.7 × 104
520, 556, 608
8.4, 8.4, 6.1
1.92
0.54
-5.24
-3.32
P3EOPT
460
458
2.5 × 104
3.3 × 104
513, 545, 600
518, 551, 603
5.8, 5.4, 2.8
6.8, 6.4, 4.1
1.94
1.92
0.45
0.40
-5.15
-5.10
-3.21
-3.18
P3MOBT
of 16.66 Å, 15.77 Å, 15.28 Å, and 15.10 Å, respectively,
determined by the interchain stacking of corresponding side
3.4 Solid state ordering of neutral polymers
Powder X-ray diffraction measurements were performed to chains. It is obvious that the stacking between the side chains
gain insight into the structural ordering of the pure polymers in becomes more compact from P3HT and P3POET to P3MOBT. A
solid state. As shown in Fig. S9, a diffraction peak at 2θ of 5.30, diffraction peak at 2θ around 24.08° can be noticed for P3POET,
5.60, 5.78 and 5.85° can be observed for P3HT, P3POET, P3EOPT corresponding to typical backbone π - π stacking distances of
and P3MOBT, respectively, corresponding to distance value (d) 3.69 Å. A similar diffraction peak at 2θ around 23.41° with
6 | J. Name., 2012, 00, 1-3
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relatively larger π - π stacking distance of 3.80 Å can be in a He-filled chamber with the four-probe configuration at
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observed as well for the other three polymers. Moreover, room temperature. The films were dopedDbOyI:i1m0.m10e3r9s/iCn8gTCth0e59m38inA
P3POET shows comparably high and sharp peaks to P3HT, the oxidant solutions of FeCl3 in nitromethane at varied
indicating better crystallinity over the other polymers with O- concentrations over various doping time prior to the
substitution in side chains. Grazing incident wide-angle X-ray measurements. The optimized concentration of the FeCl3
scattering (GIWAXS) was employed as well to probe the solution was found to be 0.01 mol L-1, which resulted in easily
molecular packing in polymer films under neutral state and measurable electrical conductivities for all polymers (Table S1)
.
polymer orientations of crystalline lamellae with respect to the Polarons and bipolarons have been proposed as the dominant
silicon substrate. As shown in Fig. S10, all polymers films exhibit charge carriers in a conjugated polymer at a certain doping level.
preferable edge-on orientated molecular packing. Lamellar Electronic transitions associated with positive polarons and
diffractions in the out-of-plane (OOP) direction determined by bipolarons appear below the band edge of the polymer in the
side chain interactions can be observed at qz around 0.39, 0.40, visible, near-infrared (NIR), and even middle infrared regions
0.42 and 0.43 Å-1 (d =16.10, 15.70, 14.95 and 14.60 Å (MIR), which can be experimentally distinguished by the
accordingly) accompanied with a π - π stacking peak in the in- number of strong absorption bands.64, 65 In particular, a polaron
plane (IP) direction at qr around 1.68, 1.70, 1.64 and 1.64 Å-1 (d exhibits strong two-band feature while a bipolaron exhibits a
= 3.74, 3.69, 3.83 and 3.83Å accordingly) for P3HT, P3POET, strong one-band feature at longer wavelength. Due to the
P3EOPT and P3MOBT respectively, which is in excellent difficulty of determining the charge carrier density, it would be
agreement with powder XRD results. The lamellar stacking very helpful to consider how the absorption of the neutral
distance shrinks while the π - π stacking distance expands when polymer films evolves upon doping. Thus, before the discussion
substituting carbon atoms from the γ to the ω position with O of the thermoelectric property of the respective doped polymer
atom. Interestingly, P3POET almost displays the longest edge- films, optical absorption spectroscopy was utilized to determine
on oriented lamellar stacking distance and the shortest π - π the nature of charge carriers in films with different doping levels
stacking distance among all O-substituted polymers. The achieved by immersing them in a 0.01 mol L-1 solution of FeCl3
evolution of the lamellar stacking distance of the polymer films in nitromethane for different time durations (Fig. 3). For the
is directly correlated with the location of the O atom in the side P3POET film, the π - π* transition in the neutral film at 520 nm
alkyl chains with the according ending propoxyl, ethyoxyl and is found to continuously decrease with the prolonged doping
methoxyl groups. Siegel et al. once reported more favourable time, and is accompanied by a simultaneous appearance of
sandwich π - π stacking of aryl rings with electron-withdrawing absorption bands at 780 nm (1.82 eV) and above 1500 nm. The
substituents reducing the electron density of aryl rings.63 In this absorption intensity of the band at 780 nm reaches the
work, the existence of the γ-positioned O atom leads to a maximum after doping for 2 min. A slight decrease of the
relatively more electron-deficient conjugated backbone for absorption at 780 nm and a continuous increase above 1500 nm
P3POET. The closer π - π stacking existed in the P3POET film can be then observed after further doping, leading to the
could be explained similarly in comparison with the other two formation of two isosbestic points at around 630 and 930 nm,
O-containing polymers. Moreover, the crystallinity of edge-on indicating the formation of positive bipolarons from positive
oriented P3POET is overwhelmingly higher than the crystallinity polarons after doping for 2 min. P3EOPT presents a doping time
of any other O-substituted polymers, even that of P3HT, which dependent absorption feature with two-band absorption and a
agrees well with the film absorption feature as indicated in Fig. single isosbestic point at around 620 nm, indicating that only
2. From the in-plane GIWAXS profiles shown in Fig. S10, partial polarons formed within 60 min. P3MOBT exhibits a similar
face-on orientation can be noticed for the polymers except doping-induced two-band absorption feature as that of the film
P3MOBT without obviously detectable coexistence of face-on of P3POET with two isosbestic points, while the formation of
orientated crystals. Moreover, the face-on orientated lamellar bipolarons starts after doping for 20 min. The time dependent
stacking peak of P3POET is much weaker and broader than doping absorption spectrum of the P3HT film was also recorded
those of P3EOPT and P3HT, suggesting much inferior polymer as a reference. A similar two-band absorption feature to that of
crystallinity in this orientation.
P3POET was observed including two isosbestic points at 620 and
910 nm, and the formation of bipolarons started after doping
for 2 min. Briefly, only positive polarons formed after doping for
3.5 Thermoelectric properties
To evaluate the thermoelectric properties of the films of these within 60 min for the film of P3EOPT, whereas positive
P3HT analogues with such O-substitution on side chains, the bipolarons started to form from positive polarons after doping
electrical conductivity and Seebeck coefficient were measured P3POET and P3HT for 2 min and P3MOBT for 20 min.
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DOI: 10.1039/C8TC05938A
Fig. 3 UV-vis-NIR absorption spectra of polymer films doped at various doping levels by immersing the films in 0.01 mol L-1 solution of FeCl3 in nitromethane at r.t. over different
times.
Raman spectra analysis was also carried out to investigate the electrical conductivity varies from polymer to polymer. The
effect of FeCl3 doping on the structure characteristics of P3HT film present its highest electrical conductivity of 246.1 S
polymer films. Raman spectra of the films excited at 514 nm cm-1 after immersion for 10 min, at which time both polarons
were obtained before and after doping for within 30 min as and bipolarons exist in the film of P3HT. For the P3POET and
presented in Fig. S11. The bands at ~1380 cm-1 (Cβ - Cβ stretching P3MOBT films, the highest electrical conductivity of 193.5 and
vibration) and at ~1450 cm-1 (Cα = Cβ stretching vibration) are 223.1 S cm-1, respectively, was attained after immersion in the
observed for the films of P3POET, P3EOPT, P3MOBT, and P3HT oxidation solution for 40 seconds, with the charge carrier in the
in neutral state.66, 67 Doping treatment have an unremarkable form of polarons. The P3EOPT film gives the highest electrical
effect on the position of the Raman bands, but make the Cα = Cβ conductivity as 250 S cm-1 after immersion for 2 min, with
stretching vibrations broader. The films of P3POET, P3MOBT polarons as the only charge carrier. The Seebeck coefficient of
and P3HT present a slight shift of the Cα = Cβ stretching vibration P3HT film presents a continuously decreasing magnitude right
towards higher frequency, revealing the transition of from the beginning, while the other three films exhibit an initial
conjugated chains from benzoid to quinoid structure as the increase in the Seebeck coefficient, reach a maximum, and then
doping proceeds.68, 69 Such transitions suggest the appearance decrease as the doping time increases. The largest Seebeck
of bipolarons in agreement with the corresponding doping time coefficients are determined to be 34.0, 22.3, 26.3 and 61.5 μV
dependent UV-vis-NIR absorption spectra.
K-1 with the doping time of 120, 120, 120 and 20 seconds for
Thermoelectric parameters of σ, S, and PF of all polymer films P3POET, P3EOPT, P3MOBT, and P3HT, respectively.
with different levels of doping are depicted in Fig. 4. For these Interestingly, relatively high S values can only be attained in the
polymer films, the electrical conductivities initially increase, to early doping state, for immersion times of less than 10 min for
reach the maxima, and then decrease as the doping time all polymers.
increases. The doping time corresponding to the highest
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View Article Online
DOI: 10.1039/C8TC05938A
a
b
350
300
250
200
150
100
50
350
polaron-only
polaron-only
P3POET
P3HT
63
54
45
36
27
18
36
27
18
300
250
200
150
100
50
polaron + bipolaron
polaron + bipolaron
0
0
200 400 600 800 1000 1200 1400 1600 1800
Time (Second)
0
200 400 600 800 1000 1200 1400 1600 1800
Time (Second)
c
d
350
300
250
200
150
100
50
350
300
250
200
150
100
50
P3MOBT
P3EOPT 36
36
27
18
polaron
polaron-only
27
18
polaron-only
+
bipolaron
0
0
0
200 400 600 800 1000 1200 1400 1600 1800
Time (Second)
0
200 400 600 800 1000 1200 1400 1600 1800
Time (Second)
Fig. 4 Electrical conductivities (in orange color) and Seebeck coefficients (in gray color) of (a) P3HT, (b) P3POET, (c) P3EOPT and (d) P3MOBT. Critical points of the doping time
resulting in the initial formation of positive bipolarons from positive polarons are indicated by the dotted cyan lines.
Despite differences in the electrical conductivities and
Seebeck coefficients among all polymers, the integrated
response of the electrical conductivity and the Seebeck
P3HT
20
18
16
14
12
10
8
coefficient to the doping time leads to an increase of the power
factors that reach their own maxima before falling at longer
immersion times (Fig. 5). Due to the fact that the Seebeck
coefficient and the electrical conductivity of the P3POET film are
both high at low doping levels, the PF of the film achieved
almost a factor of two higher value of 19 μW m-1 K-2 after doping
for 40 seconds than any other film. For the films of P3EOPT and
P3MOBT, the maximum PF values were around 12.5 and 11.5
μW m-1 K-2, respectively. The reference P3HT film yielded a
maximum PF around 11.7 μW m-1 K-2. It is worthwhile to note
that all these maximum PF values were obtained in a state of
doping characterized by the dominance of polarons.
P3POET
P3EOPT
P3MOBT
6
4
2
0
0
200 400 600 800 1000 1200 1400 1600 1800
Time (Second)
Fig. 5 Power factors of the doped films of the P3HT analogues in this work.
3.6 Solid state ordering of doped polymer films with optimized PFs
GIWAXS measurements were performed again as indicated in
Fig. 6 to examine changes in the local molecular organization of
the films doped analogously by immersion in a solution of FeCl3
in nitromethane over the time that gave the optimized PFs,
namely 40 seconds for P3POET and P3MOBT, and 2 min for
P3HT and P3EOPT. All doped polymers formed semi-crystalline
films with strong texturing. The scattering patterns were
broadened slightly and the conformation of preferentially edge-
on orientated crystalline lamellae remained upon doping. The
OOP (100) lamellae distances (~16.10 Å) became very close
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among all polymers except for P3MOBT (~15.32 Å), while the IP structure order of the P3POET film was not disturbed much
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(010) π - π stacking distances were almost similar (~3.81 Å), after doping for 40 seconds. MeanwhiDleO,I:d1o0.p10in3g9/Cw8iTtCh05F9e3C8lA3
except for P3POET (~3.71 Å, remaining the shortest). makes molecular packing in the three polymer films of P3EOPT,
Meanwhile, the crystallinity of OOP (100) lamellae diffractions P3MOBT and P3HT comparable and eliminates the differences
were improved greatly for doped P3MOBT and P3POET, which observed in their molecular packing states when undoped.
were much weaker in their undoped form. A significant change Practically, the maximum electrical conductivities for all
is that the crystallinity of the face-on oriented crystalline polymers are fairly close and in the range of 200 - 250 S cm-1
lamellae was strengthened for P3EOPT, P3MOBT and P3HT although they were achieved at different doping times. The
compared with their undoped films (Fig. S10), with the obvious attainable similar highest electrical conductivities of all films
appearance of OOP (010) diffractions at the qz around 1.65 Å-1 could be attributed to their retention of the preferred edge-on
(with d space ~3.81 Å) corresponding to vertical π - π stacking. orientation, which facilitates the in-plane charge transport after
Evolution of the film structure with varied doping levels has doping at a certain level. However, at the immersion doping
been observed in quite a few solution doping examples due to time corresponding to the maximum PF for each polymer, only
the incorporation of counterion from the dopants, depending the doped P3POET film maintains both S and σ relatively high
on chemical structure of the materials and the doping level.38, and close to the maximum values, while the other polymers
70, 71
Overall, the changes of in-plane scattering features are yield either high S but much lower σ (P3HT) or high σ but much
quite similar in P3EOPT, P3MOBT and P3HT polymer films, but lower S (P3EOPT and P3MOBT) as shown in Fig. 4. The square
not in P3POET, which shows the strongest crystallinity of the dependence of PF on S together with higher σ amplifies the
edge-on orientated crystal lamellae (out-of-plane scattering) difference on the PF of P3POET from the other polymers. The
after immersion doping for 40 seconds. Moreover, the dominant edge-on orientation with stronger crystallinity and
crystallinity of the face-on orientated crystal lamellaes (in-plane closer edge-on oriented π - π stacking might be responsible for
scattering) of P3POET was the weakest with barely detectable the higher integrated PF observed in this polymer at an
OOP (010) π - π stacking diffraction. It seems that the local appropriate doping level.
Fig. 6 GIWAXS profiles (a) and patterns (b) along out-of-plane (OOP) and in-plane (IP) directions of all polymer films on Si substrate immersed in 0.01M FeCl3/CH3NO2 solution for
the doping time corresponding to the maximum power factor (P3HT: 2 min, P3POET: 40 seconds, P3EOPT: 2 min and P3MOBT: 40 seconds).
characteristic absorptions upon increasing the immersion-
doping time in a FeCl3 solution in nitromethane at room
temperature. The similar highest electrical conductivities of all
4. Conclusions
doped films were attained. Morphology studies of polymer
films with optimized doping time corresponding to the
maximum PFs revealed that all the polymers favored the
formation of edge-on orientated molecular packing with
respect to the substrate. Moreover, the polymer with the γ O-
substitution exhibited the largest proportion and the strongest
crystallinity of edge-on oriented lamellas with the closest π - π
stacking. The attractive molecular packing state of the polymer
enabled its maintenance of both S and σ close to their maximum
values, resulting in the optimized PF almost two times of the
Due to the existence of the shorter bond length and the smaller
rotation energy of the CH2-O bond than the CH2-CH2 bond,
sequential O-substitution carbon atoms from the γ position
outwards along the hexyl side chains of RR-P3HT to the ω
position led to the increase of the melting and the crystallization
temperature of the resultant polymers. The neutral polymer
films feature decreased edge-on oriented lamellar stacking
distance and increased π - π stacking distance. UV-vis-NIR
spectroscopy confirmed the presence of polarons and/or the
combination of polarons with bipolarons by probing their
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17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
Y. Chen, Y. Zhao and Z. Liang, Energy Environ. Sci., 2015, 8,
polymers with O-substitution on the δ and ω positions with high
σ unfavorably coupled with lower S. Although how σ coupled
with S responds to polymer film morphology evolution is not
clear, this research successfully demonstrates a facile strategy
on fine-tuning the solid-state organization and polymer
thermoelectric properties via side-chain engineering of
semiconducting polymers by sequential oxygen-substitution on
carbon atoms. This attractive approach could assist in
establishing the guidance for further structure optimization
toward high performance polymer-based thermoelectric
materials.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors acknowledge the supports from the National
Natural Science Foundation of China (Grant No. 21673170,
51521001 and 51632006) and the 111 Project of China (Grant
No. B07040).
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DOI: 10.1039/C8TC05938A
Table of Contents Entry
Sequential O-substitution along alkyl side chain of regioregular P3HT analogues
fine-tunes their film microstructure and thermoelectric property.