Pyrazine-Flanked Diketopyrrolopyrrole (DPP): A New Polymer Building Block for High-Performance n-Type Organic Thermoelectrics

Article abstract of DOI:10.1021/jacs.9b10107

n-Doped conjugated polymers usually show low electrical conductivities and low thermoelectric power factors, limiting their applications in n-type organic thermoelectrics. Here, we report the synthesis of a new diketopyrrolopyrrole (DPP) derivative, pyrazine-flanked DPP (PzDPP), with the deepest LUMO level in all the reported DPP derivatives. Based on PzDPP, a donor-acceptor copolymer, P(PzDPP-CT2), is synthesized. The polymer displays a deep LUMO energy level and strong interchain interaction with a short π-πstacking distance of 3.38 ?. When doped with n-dopant N-DMBI, P(PzDPP-CT2) exhibits high n-type electrical conductivities of up to 8.4 S cm^-1 and power factors of up to 57.3 μW m^-1 K^-2. These values are much higher than previously reported n-doped DPP polymers, and the power factor also ranks the highest in solution-processable n-doped conjugated polymers. These results suggest that PzDPP is a promising high-performance building block for n-type organic thermoelectrics and also highlight that, without sacrificing polymer interchain interactions, efficient n-doping can be realized in conjugated polymers with careful molecular engineering.

Full text of DOI:10.1021/jacs.9b10107

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Article
Pyrazine-Flanked Diketopyrrolopyrrole (DPP): A New Polymer
Building Block for High-Performance n-Type Organic Thermoelectrics
Xinwen Yan, Miao Xiong, Jia-Tong Li, Song Zhang, Zachary Ahmad, Yang
Lu, Zi-Yuan Wang, Ze-Fan Yao, Jie-Yu Wang, Xiaodan Gu, and Ting Lei
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b10107 • Publication Date (Web): 27 Nov 2019
Downloaded from pubs.acs.org on November 27, 2019
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Pyrazine-Flanked Diketopyrrolopyrrole (DPP): A New Polymer Building
Block for High-Performance n-Type Organic Thermoelectrics
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Xinwen Yan1,3†, Miao Xiong2,3†, Jia-Tong Li1, Song Zhang4, Zachary Ahmad4, Yang Lu2,3, Zi-Yuan
Wang2,3, Ze-Fan Yao2,3, Jie-Yu Wang2,3, Xiaodan Gu4, and Ting Lei1,2,5*
1Department of Materials Science and Engineering, College of Engineering; Peking University, Beijing 100871, China.
2Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China.
3College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
4School of Polymer Science and Engineering, The University of Southern Mississippi, Hattiesburg, MS 39406, USA.
5Beijing Key Laboratory for Magnetoelectric Materials and Devices, Peking University, Beijing 100871, China.
KEYWORDS Conjugated polymers, Organic thermoelectrics, Diketopyrrolopyrrole polymers, n-Doping
ABSTRACT: n-Doped conjugated polymers usually show low electrical conductivities and low thermoelectric power factors, lim-
iting their applications in n-type organic thermoelectrics. Here, we report the synthesis of a new diketopyrrolopyrrole (DPP) deriva-
tive, pyrazine-flanked DPP (PzDPP), with the deepest LUMO level in all the reported DPP derivatives. Based on PzDPP, a donor-
acceptor copolymer P(PzDPP-CT2) is synthesized. The polymer displays a deep LUMO energy level and strong interchain interac-
tion with short π-π stacking distance of 3.38 Å. When doped with n-dopant N-DMBI, P(PzDPP-CT2) exhibits high n-type electrical
conductivities of up to 8.4 S cm−1 and power factors of up to 57.3 μW m−1 K−2. These values are much higher than previously re-
ported n-doped DPP polymers, and the power factor also ranks the highest in solution-processable n-doped conjugated polymers.
These results suggest that PzDPP is a promising high-performance building block for n-type organic thermoelectrics, and also high-
light that without sacrificing polymer interchain interactions, efficient n-doping can be realized in conjugated polymers with careful
molecular engineering.
The past few years have witnessed the rapid improvement
of the charge carrier mobilities of donor-acceptor (D-A) con-
jugated polymers, largely due to the innovation of high-
performance polymer building blocks.18-23 Among them,
diketopyrrolopyrrole (DPP) is one of the most extensively
studied building blocks.20 D-A polymers based on DPP have
exhibited high hole mobilities over 10 cm2 V−1 s−1,24 and high
electron mobilities over 5 cm2 V−1 s−1.22 However, the electri-
cal conductivities of n-doped DPP polymers are always low
(usually 0.1~1 S cm−1), though DPP polymers have shown
comparable or even higher charge carrier mobilities (> 1 cm2
V−1 s−1) than those of p-type conjugated polymers (e.g. ~ 1 cm2
V−1 s−1 for PEDOT or PBTTT etc.).25 Theoretically, electrical
conductivity (σ) is determined by the charge carrier concentra-
tion (n) and charge carrier mobility (μ) (σ = nμq, q is the ele-
mentary charge). Therefore, the low n-type electrical conduc-
tivities of DPP based polymers are mainly due to their low n-
doping levels. Recently, to enhance the n-doping level in DPP
polymer, Chabinyc et al. used a twisted non-planar donor
moiety and demonstrated an improved n-doped electrical con-
ductivity of 0.45 S cm−1.11 Pei et al. used fluorinated donor
moiety to reduce the donor-acceptor character of a DPP based
polymer and achieved currently the highest n-type electrical
conductivity (1.3 S cm−1) in n-doped D-A type polymers and
maximum thermoelectric power factor of 4.65 μW m−1 K−2.9
Nevertheless, these values are still much lower than those of
p-type materials, and more efforts are needed to further im-
prove the doping efficiency and thermoelectric performance of
n-doped conjugated polymers.
Introduction
Conjugated polymers are an intriguing class of semiconduc-
tors for printed optoelectronics, energy conversion and storage
devices since they are solution-processable, light-weight and
can be fabricated into flexible devices.1-3 Compared with inor-
ganic alloys, organic thermoelectrics (OTEs) have shown great
potential as thermoelectric materials due to their low toxicity,
low thermal conductivity, and good solution processability. p-
Type polymer, poly(3,4-ethylenedioxythiophene) (PEDOT),
has exhibited high thermoelectric figure of merit (ZT) over 0.4,
which is already comparable to that of inorganic materials at
low temperature. The good thermoelectric performance and
above-mentioned unique properties of polymers make them
particularly suitable for applications requiring distributed
power generation, such as mobile devices, wearable electron-
ics, and sensor networks.4
To achieve highly efficient thermoelectric modules, both p-
and n-type conjugated polymers with comparable performance
are required. However, the thermoelectric performance of n-
doped conjugated polymers is far inferior to their p-type coun-
terparts. The high ZT values of PEDOT are mainly ascribed to
its high electrical conductivities (> 1000 S cm−1) and high
power factors (> 300 μW m−1 K−2).5 Although high electrical
conductivities over 1000 S cm−1 have been obtained in p-
doped polymers,6 whereas only a few n-doped polymers are
demonstrated to have electrical conductivities approaching or
over 1 S cm−1 with power factors usually below 10 μW m−1
K−2.7-17
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fluoromethanesulfonic anhydride to afford compound 4. The
trifluoromethanesulfonic group can be readily converted to
bromine with the treatment of tetrabutylammonium bromide,
which is also ascribed to the strong electron-deficient nature of
PzDPP, yielding the PzDPP monomer 5 ready for polymeriza-
tion.
Recent studies have suggested that electron-deficient modi-
fication of the donor moiety can enhance the electron affinity
of the D-A polymers, leading to better miscibility with n-
dopants and improved n-doping efficiency.8-9 Thus, 3,3'-
dicyano-2,2'-bithiophene was chosen as the donor moiety, due
to its good planarity and strong electron-withdrawing property
of the two cyano groups. Polymer P(PzDPP-CT2) was syn-
thesized through the Pd-catalyzed Stille coupling reaction
between PzDPP and 5,5'-bis(trimethylstannyl)-3,3'-dicyano-
2,2'-bithiophene in the presence of a small amount of CuI as
the cocatalyst.27 To understand the important role of the
PzDPP, a reference polymer, P(TDPP-CT2), containing the
most studied thiophene-flanked DPP (TDPP) building block
was also synthesized. Both polymers were purified by Soxhlet
extraction, and their chemical structures were verified by high-
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temperature H NMR spectra (details in the Supporting Infor-
mation (SI)). Both polymers have similar molecular weights
Figure 1. (a) Chemical structures and calculated
HOMO/LUMO levels of two mostly studied DPP building
blocks (thiophene-flanked and pyridine-flanked DPP) and
PzDPP (R = Me, B3LYP/6-311+G(d,p)). (b) Structure of the
D-A polymer P(PzDPP-CT2).
(Mn) around 30 kDa, which were evaluated by high-
o
temperature gel permeation chromatography (GPC) at 150 C
using 1,2,4-tricholorobenzene (TCB) as eluent (Table 1). Both
polymers show excellent thermal stability with decomposition
temperature over 380 oC and no observable phase transition in
o
Here we report the synthesis of a new DPP derivative, pyra-
zine-flanked DPP (PzDPP), which has the deepest LUMO
energy level in all reported DPP derivatives (Figure 1a). We
also employed an electron-deficient donor moiety, 3,3'-
dicyano-2,2'-bithiophene, to polymerize with PzDPP and ob-
tained a new donor-acceptor polymer, P(PzDPP-CT2) (Figure
1b). The polymer exhibits a planar and conformation-locked
backbone structure with a deep LUMO (lowest unoccupied
molecular orbital) level down to −4.03 eV. When doped with
n-type dopant N-DMBI (Scheme 1), P(PzDPP-CT2) displays
the highest electrical conductivity of 8.4 S cm−1 and power
factors of up to 57.3 μW m−1 K−2. The conductivity is obvious-
ly higher than other n-doped D-A type polymers, and the pow-
er factor also ranks the highest in n-type solution-processable
polymer thermoelectric materials.8-10
the range from room temperature to 300 C (Figure S1-S3 in
the SI).
Both polymers exhibit similar absorption features (Figure
S4 in the SI). P(TDPP-CT2) has two absorption peaks at 734
nm and 797 nm, while P(PzDPP-CT2) has a blue-shifted
maximum absorption peak at 713 nm with a shoulder peak at
655 nm in solution. In film, both polymers exhibit slightly red-
shifted spectra with increased vibrational peaks, indicating that
both polymers may have more planar backbones in solid state.
Temperature-dependent absorption spectra reveal that both
polymers showed strong aggregation even in dilute (10−5 M)
TCB solutions at high temperature, and the PzDPP polymer
exhibited stronger aggregation compared to the TDPP polymer
(see Figure S5 for detailed discussions). Density functional
theory (DFT) calculations show that similar to the TDPP,
PzDPP also has small dihedral angle between the pyrazine and
the DPP core (ɸ1 = 0.24o) (Figure 2a). Even though the dihe-
dral angle between the two thiophene units slightly rises to
5.58o, the optimized structure of P(PzDPP-CT2) trimer still
exhibits almost coplanar conformation (Figure S9 in the SI).
Calculations also show that the O…H and N…H distances in
the polymer are smaller than the sum of the van der Waals
radii of O, N and H (1.52, 1.55 and 1.20 Å, respectively), indi-
cating that the pyrazine moiety can form intramolecular hy-
drogen bonds along the polymer backbone, leading to a rigid
and planar backbone and efficient intrachain charge transport.
According to the cyclic voltammetry (CV) measurement (Fig-
ure S8 in the SI), PzDPP monomer shows a deep LUMO level
of −3.77 eV, 0.16 eV lower than the pyridine-flanked DPP
(PyDPP). This result is consistent with the theoretical calcula-
tion. To our knowledge, PzDPP is currently the most electron-
deficient DPP building block in literature.20 The LUMO and
HOMO (highest occupied molecular orbital) energy levels of
P(PzDPP-CT2) reach −4.03 and −5.89 eV, respectively. 0.33
and 0.28 eV lower than that of P(TDPP-CT2) (−3.70 and
Results and Discussions
Polymer design, synthesis and characterization
Scheme 1 illustrates the synthetic routes to PzDPP and poly-
mer P(PzDPP-CT2). The synthetic procedure of PzDPP is
quite different from the commonly used thiophene- or pyri-
dine-flanked DPP unit that can be directly synthesized from 2-
26
thiophenecarbonitrile or 5-bromo-2-pyridinecarbonitrile.22,
The bromine atom connected to the pyrazine is highly reactive
and easily substituted with the bases in the reaction system,
including tert-pentoxide or isopropoxide. The high reactivity
of the bromine atom is probably due to the strong electron-
deficient nature of PzDPP. After alkylation, we isolated a mix-
ture of three compounds: 3a (X = two tert-pentyloxy), 3b (X =
one tert-pentyloxy and one isopropoxy), and 3c (X = two iso-
propoxy). To obtain the bromo-substituted PzDPP unit for
polymerization, the mixture was successively treated with
BBr3 and con. HCl to remove the isopropyl and t-pentyl
groups, and the resulting crude product was treated with tri-
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−5.61 eV) (Figure 2b). Clearly, P(PzDPP-CT2) shows strong-
er electron affinity than the reference TDPP polymer.
5
Scheme 1. Synthetic routes to P(PzDPP-CT2) and the reference polymer P(TDPP-CT2).
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Reagents and conditions: (i) Na, FeCl3, 2-methyl-2-butanol, 85 oC, 12 h. (ii) K2CO3, 18-crown-6, DMF, 70 oC, 18 h. (iii) BBr3, di-
o
chloromethane, −78 C ~ r.t., 2 h; con. HCl, 1,4-dioxane, reflux, 0.5 h. (iv) Tf2O, pyridine, dichloromethane, r.t., 1.5 h. (v)
Bu4N+Br−, toluene, 85 oC, 12 h. (vi) Pd2(dba)3, P(o-Tol)3, CuI, DMF/toluene, 110 oC. (vii) Pd2(dba)3, P(o-Tol)3, toluene, 110 oC.
Figure 2. (a) DFT-optimized molecular model of P(PzDPP-CT2) fragment (B3LYP/6-311G(d,p)). Long alkyl chains were re-
placed with methyl groups to simplify the calculation. (b) Cyclic voltammograms of both polymers in thin film. (c) UV-vis-NIR
absorption spectra of pristine and N-DMBI doped P(PzDPP-CT2) in thin film. (d) UPS binding energy of the pristine and the
doped P(PzDPP-CT2) (top) and P(TDPP-CT2) (bottom) films. (e) EPR signals of the pristine and the doped P(PzDPP-CT2) (top)
and P(TDPP-CT2) (bottom) at different dopant/polymer weight ratios. (f) Transfer characteristics for the pristine polymer
P(PzDPP-CT2) (W = 100 µm, L = 5 µm, Ci = 3.7 nF cm−2).
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Table 1. Summary of the molecular weights, energy levels, electron mobilities, and π-π stacking distances of the pristine
polymers; electrical conductivity maxima and PF maxima of the N-DMBI doped polymers.
a
a
a
Mn
[kDa]
28.5
34.0
Eg
EHOMO
ELUMO
μeb
[cm2 V−1 s−1]
0.79
σmax
[S cm−1]
8.4
PFmax
[μW m−1 K−2]
57.3
dπ-π
[Å]
Polymer
PDI
[eV]
1.86
1.91
[eV]
[eV]
P(PzDPP-CT2)
P(TDPP-CT2)
2.90
2.14
−5.89
−5.61
−4.03
−3.70
3.38
3.53
0.32
0.39
9.3
aEstimated from the cyclic voltammetry (CV) measurement. bMaximal mobilities measured using field-effect transistor with a top-
gate bottom-contact configuration.
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Characterization of the doped polymers
cm−3. This result indicates that bipolarons or other species
without radical signals were formed in P(PzDPP-CT2) at
higher doping levels.33-34
N-DMBI was used to dope both polymers due to its strong n-
doping ability.28-30 UV-vis-NIR absorption spectroscopy was
used to evaluate the n-doping efficiency for both polymers
(Figure 2c, Figure S6 & S7 in the SI). No obvious absorption
above 900 nm is observed in their pristine films. After N-
DMBI doping, both polymers exhibit two typical (bi)polaron
absorption bands in the near-infrared region, suggesting that
both polymers can be successfully n-doped using N-DMBI.
For P(PzDPP-CT2), the new absorption bands from 800 to
1300 nm and above 1300 nm can be ascribed to the P2 and P1
absorptions of the (bi)polaron (Figure 2c).31 The (bi)polaron
absorption for n-doped P(TDPP-CT2) can be similarly identi-
fied. The main absorption peak of both doped polymer films
undergo a slightly blue-shift, which also suggests the for-
mation of (bi)polaronic bands.31-32 Compared with P(TDPP-
CT2), P(PzDPP-CT2) shows much stronger (bi)polaron ab-
sorption at each dopant/polymer ratios (Figure S7 in the SI),
suggesting that the PzDPP polymer can be more efficiently
doped than the TDPP polymer. In o-dichlorobenzene (o-DCB)
solution, P(PzDPP-CT2) also shows stronger polaron absorp-
tion than P(TDPP-CT2) after doping (Figure S6 in the SI),
indicating the PzDPP polymer is more easily doped in solution
state.
The ultraviolet photoelectron spectroscopy (UPS) and X-ray
photoelectron spectroscopy (XPS) measurement also supports
the absorption spectra results. The secondary electron cut-off
of P(PzDPP-CT2) shifts by 0.75 eV when doped with 60% N-
DMBI, equivalent to an upward movement of its Fermi level
by 0.75 eV, much larger than the shift of P(TDPP-CT2) (0.06
eV) under the same dopant/polymer ratio (Figure 2d). The
XPS data of the doped polymers also clearly demonstrate that
P(PzDPP-CT2) is more readily n-doped with N-DMBI than
P(TDPP-CT2). As the doping reaction is accompanied by the
generation of N-DMBI+ (402 eV), doping level of the films
could be estimated by the analysis of the N-DMBI+ peak. At
each doping concentration, the peak ratio between N-DMBI+
and other N (1s) peaks in P(PzDPP-CT2) film is larger than
that of the doped P(TDPP-CT2) film (Figure S10 in the SI).
Figure 3. The electrical conductivities, Seebeck coefficients
and power factors of (a) P(PzDPP-CT2) and (b) P(TDPP-
CT2) at different dopant/polymer ratios. The dashed lines
show the original conductivities measured in nitrogen glove-
box. The solid lines are the thermoelectric parameters meas-
ured under a vacuum chamber.
Thermoelectric and charge transport measurement
The thermoelectric properties of the doped P(PzDPP-CT2)
and P(TDPP-CT2) films were studied in a vacuum chamber
to avoid temperature fluctuation and O2/water de-doping. To
accurately evaluate the thermoelectric performance of both
polymers, all devices were fabricated and patterned according
to the criteria proposed by Reenen and Kemerink.35 The films
were encapsulated in case of exposing to oxygen and water
when transferred to the thermoelectric testing instrument. The
conductivity decreased after patterning and transfer (as shown
by the dashed line to the solid line in Figure 3). P(PzDPP-
CT2) showed a maximal conductivity of 8.4 S cm−1 after dop-
ing with 40% N-DMBI. When the dopant ratio increased from
The electron paramagnetic resonance (EPR) spectroscopy
was used to evaluate the numbers of radicals formed in the
polymer after doping. In the EPR spectroscopy, no observable
radical signal was observed in both pristine polymers. After
doped with 20% N-DMBI, P(PzDPP-CT2) showed stronger
radical signal than P(TDPP-CT2). The calculated spin density
of the doped P(PzDPP-CT2) is approximately 1.61 × 1020
cm−3, which is about 5 times that of the doped P(TDPP-CT2)
(3.12 × 1019 cm−3) (Figure 2e). When the N-DMBI fraction
increased to 40%, the spin density of the doped P(TDPP-CT2)
continuously increased to 5.36 × 1019 cm−3, however, the spin
density of the doped P(PzDPP-CT2) decreased to 1.15 × 1020
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5% to 70%, the Seebeck coefficients of both polymers de-
films (Figure 4a, Figure S15 in the SI). The lamellar packing
and π-π stacking distances were calculated to be 29.7 and 3.38
Å for P(PzDPP-CT2), 28.6 and 3.53 Å for P(TDPP-CT2).
The shorter π-π stacking distance of P(PzDPP-CT2) might be
due to its stronger interactions, which is coincident with the
temperature-dependent absorption spectrum study. When N-
DMBI concentration increases, the lamellar packing and π–π
stacking distances remain almost unchanged in both the in-
plane and the out-of-plane GIWAXS diffractions of both pol-
ymers (Figure 4b and Figure S15). As we know, X-ray diffrac-
tion characterization is mainly sensitive to the changes of the
crystalline region of a polymer film but can hardly detect the
changes occurring in the amorphous region. Therefore, these
results indicate that the introduction of dopants do not signifi-
cantly alter the crystalline regions of both polymers. Note that
the π-π stacking distances of the pristine and the 60% N-DMBI
doped P(PzDPP-CT2) films are 3.38 Å and 3.43 Å, respec-
tively. These values are all obviously smaller than those of the
P(TDPP-CT2) (3.53 Å and 3.51 Å respectively), suggesting
that closer π-π stacking distance and stronger interchain inter-
actions in the PzDPP polymer do not impede the efficient n-
doping.
creased because of the negative correlation between Seebeck
coefficient with charge carrier concentration36 (Figure S19 in
the SI). The Seebeck coefficient of P(TDPP-CT2) is higher
than that of P(PzDPP-CT2), which is due to the higher charge
carrier concentration in P(PzDPP-CT2) film at each do-
pant/polymer ratio. To exclude the possible contribution of
ionic Seebeck coefficient, we carried out the test of the long-
time thermovoltage of doped films, and the thermovoltage
remained stable at a steady temperature difference for several
cycles. So the contribution of ionic Seebeck effect and short-
term contributions can be excluded (Figure S17, S19 in the SI).
Power factors (PF) are calculated using the conductivities (sol-
id lines) and Seebeck coefficients measured at the same time
and under the same condition (after transferring to the vacuum
chamber). Maximal power factor of P(PzDPP-CT2) was ob-
tained by varying the dopant fractions, yielding a high value of
57.3 μW m−1 K−2. To the best of our knowledge, this value is
the highest in solution-processable n-doped conjugated poly-
mers. In contrast, the maximal electrical conductivity of the N-
DMBI doped P(TDPP-CT2) is only 0.39 S cm−1, far below
that of P(PzDPP-CT2), thus resulting in a lower maximal
power factor of 9.3 μW m−1 K−2. The conductivities of both
polymers decrease at high doping concentrations (> 40% or
30%), which was also observed in other N-DMBI doped pol-
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AFM height images show that pristine P(PzDPP-CT2) and
P(TDPP-CT2) films have a very smooth surface with the
root-mean-square surface roughness of 0.83 nm and 0.63 nm.
After doping, both polymers exhibit good miscibility. Only
some large aggregates are observed at high doping concentra-
tions (Figure 4d). Since the crystalline regions of both poly-
mers do not show significant changes in GIWAXS, we tenta-
tively propose that the dopants may exist in the amorphous
regions of the polymer films. The large aggregates observed in
AFM at high dopant concentrations may cause the charge car-
rier scattering and large grain boundaries,39 leading to the de-
creased conductivity at higher dopant/polymer ratios.
9
ymer systems.7, This phenomenon is probably due to the
damaging of the charge transport networks after introducing a
large number of dopants.14, 17
The temperature-dependent electrical conductivity of the
doped P(PzDPP-CT2) exhibits weaker temperature depend-
ence compared with that of P(TDPP-CT2), suggesting the
lower hopping barrier in doped P(PzDPP-CT2) film (Figure
S11 in the SI). The field-effect mobilities of the pristine
P(PzDPP-CT2) and P(TDPP-CT2) were measured to under-
stand their charge transport properties. The electron mobility
of P(PzDPP-CT2) was evaluated to be 0.68 ± 0.11 cm2 V−1
s−1 with a near-ideal n-type transport curve and a high on/off
ratio (Figure 2f). After changing the solvent to 1-
chloronaphthalene and optimizing the device fabrication con-
ditions, the electron mobility of P(PzDPP-CT2) can be further
increased to 1.67 cm2 V−1 s−1 (Figure S13), which is compara-
ble to other DPP based unipolar n-type polymers.9, 37 In con-
trast, P(TDPP-CT2) showed ambipolar transport behavior
with electron mobility of 0.27 ± 0.05 cm2 V−1 s−1 (Figure S12
in the SI). Note that compared with other DPP derivatives,
such as thiophene, pyridine, and quinoline flanked DPP that
typically show ambipolar transport behaviors,22, 38 our PzDPP
polymer only shows n-type charge transport behavior with
negligible hole injection. This again demonstrates that PzDPP
is currently the most electron-deficient DPP building block,
and also suggests the promising application of PzDPP as the
unipolar n-type polymer FET building block. Overall, we can
conclude that compared with the TDPP polymer, both the
higher electron mobility and the higher doping level contribute
to the higher n-type electrical conductivity of the PzDPP pol-
ymer.
To further understand the charge transport mechanism after
doping, we measured the mobilities of P(PzDPP-CT2) films
doped with 1% and 2% N-DMBI. The mobility increases as
increasing the N-DMBI concentration (Figure S18). In the
pristine film, the electron mobility is determined to be 0.63 ±
0.23 cm2 V−1 s−1. After doping, the mobility of the polymer
film increased to 0.91 ± 0.15 cm2 V−1 s−1 for 1% doping and to
1.20 ± 0.33 cm2 V−1 s−1 for 2% doping. These results suggest
that electron mobilities could be further improved after lightly
n-doped, which is probably due to the reduced charge trapping
effects after doping.40 Under heavy doping (2% to 40%), if the
crystalline regions of the polymer film were not significantly
damaged due to the above-mentioned “amorphous region dop-
ing”, high electron mobility and high charge carrier concentra-
tion could be simultaneously realized, thereby leading to high
electrical conductivity. In addition, a recent study suggests that
the good miscibility between polymer and dopant and less
energetic disorder in polymer film can lead to higher Seebeck
coefficient and PF,41 which might help to explain the high PF
in our polymer/dopant system.
Previous strategies that using either nonplanar donor unit11
or ethylene glycol side chains10 to enhance the n-doping effi-
ciency in n-type conjugated polymers always result in weaker
interchain interactions or lower electron mobilities, and finally
low conductivities. In this work, our PzDPP polymer realizes
closer π-π stacking, higher electron mobility, and higher elec-
Molecular packing and film morphology
GIWAXS shows that both P(PzDPP-CT2) and P(TDPP-CT2)
films have relatively weak crystallinity in pristine and doped
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†These authors contributed equally to this work.
trical conductivity at the same time, suggesting that without
obviously sacrificing polymer interchain interactions, efficient
n-doping and high thermoelectric performance can be
achieved in PzDPP polymers.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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This work is supported by the Key-Area Research and Develop-
ment Program of Guangdong Province (2019B010934001) and
the Beijing Natural Science Foundation (2192020). The scattering
component in this manuscript is supported by U.S. Department of
Energy, Office of Science, Office of Basic Energy Science under
award number of DE-SC0019361. The authors thank beamline
BL14B1 (Shanghai Synchrotron Radiation Facility) for providing
beam time.
ABBREVIATIONS
N-DMBI,
(4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-
yl)phenyl)-dimethylamine; AFM, Atomic Force Microscope;
GIWAXS, Grazing-Incidence Wide-Angle X-ray Scattering.
REFERENCES
Figure 4. (a) 2D GIWAXS patterns of the 60% N-DMBI
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images of (c) the pristine (RMS = 0.83 nm) and (d) the 60%
N-DMBI doped P(PzDPP-CT2) film (RMS = 0.92 nm). The
circles show the large aggregates observed in the polymer film.
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DPP derivative, PzDPP, that has the deepest LUMO level in
all reported DPP building blocks for conjugated polymers.
With the new building block and careful polymer engineering,
P(PzDPP-CT2) has shown high electrical conductivities of up
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ASSOCIATED CONTENT
Supporting Information.
Device fabrication details; GPC, TGA, and DSC traces; UV−vis-
NIR spectra; XPS data; additional GIWAXS and AFM images;
monomer and polymer synthesis and characterization. This mate-
rial is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
tinglei@pku.edu.cn
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