Realizing n-Type Field-Effect Performance via Introducing Trifluoromethyl Groups into the Donor-Acceptor Copolymer Backbone

Article abstract of DOI:10.1021/acs.macromol.9b00022

Developing a new strategy to obtain n-type organic semiconductors is crucial for the advance of organic electronics. We herein report the synthesis and investigation of a series of donor-acceptor-type diazaisoindigo-based copolymers, named PAIID-TFBVB-C1,

Full text of DOI:10.1021/acs.macromol.9b00022

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Realizing n‑Type Field-Effect Performance via Introducing
Trifluoromethyl Groups into the Donor−Acceptor Copolymer
Backbone
Congyuan Wei,†,‡ Weifeng Zhang,† Jianyao Huang,† Hao Li,† Yankai Zhou,† and Gui Yu*,†,‡
†Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
‡School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
S
* Supporting Information
ABSTRACT: Developing a new strategy to obtain n-type
organic semiconductors is crucial for the advance of organic
electronics. We herein report the synthesis and investigation
of a series of donor−acceptor-type diazaisoindigo-based
copolymers, named PAIID-TFBVB-C1, PAIID-TFBVB-C3,
and PAIID-BVB-C3. After introduction of the trifluoromethyl
groups with strong electron-withdrawing ability into the
molecular backbone of PAIID-BVB-C3-containing stilbene
units, the obtained polymer PAIID-TFBVB-C3 shows
obvious blue-shifted UV−vis absorption profiles because of
the enlarged band gap revealed by the theoretical simulation.
Meanwhile, both the energy levels of the lowest unoccupied
molecular orbital (LUMO) of PAIID-TFBVB-C1 and PAIID-TFBVB-C3 with trifluoromethyl groups are lower than that of
PAIID-BVB-C3 without trifluoromethyl groups, reducing the energy difference between the LUMO energy level and the work
function of the source-drain electrode and further facilitating electron injection. Consequently, both the PAIID-TFBVB-C1-
and PAIID-TFBVB-C3-based devices show typical n-type field-effect performance with electron mobilities of 0.11 and 0.04 cm2
V−1 s−1, respectively, whereas devices based on PAIID-BVB-C3 exhibit p-type semiconducting performance with a hole
mobility of 0.14 cm2 V−1 s−1. Our results demonstrate that it is a feasible strategy to acquire n-type semiconducting materials by
introducing the electron-withdrawing trifluoromethyl groups into the polymer backbone.
work function of the source−drain electrode dominates the
electron flow from the electrode to LUMO.2 In order to
achieve this goal, the polymer backbone should be electron-
deficient. For example, naphthalene diimide (NDI)-based
materials (small molecules and polymers) are representative
and widely investigated electron-transporting materials which
are mainly ascribed to the four carbonyl groups in the NDI
unit, leading to the molecular backbone electron defi-
ciency.8,13−15 Several facile strategies have been employed to
develop new n-type semiconductor materials. Introducing the
electron-withdrawing units, such as fluorine atoms,16 chlorine
atoms,17 and cyano groups18 into the molecular backbone is a
common methodology (Scheme 1, path I), which generates
ambipolar semiconductors or exclusive n-type semiconductors.
The frontier orbital energy levels could be fine-tuned; the
dipole moment and the distribution of the electrostatic
potential could also be modulated purposely. This strategy
not only changes the carrier transport type (hole and electron)
INTRODUCTION
■
π-Conjugated materials, especially organic semiconducting
polymers, have achieved tremendous attention for their
potential applications in electronic devices such as organic
field-effect transistors (OFETs) and organic photovoltaics over
the past two decades.1−3 Compared with inorganic semi-
conductors, such as silicon, organic materials offer opportunity
for solution processing which enables flexible devices on plastic
substrates.4−6 Moreover, lower cost will realize in large-area
preparation in the future because of the solution processi-
bility.7−9 On the basis of the potential commercialization,
extensive efforts have been dedicated to develop various
organic semiconducting materials and the performance
exceeded the benchmark set by amorphous silicon whose
mobility is around 1 cm2 V−1 s−1.10,11 Even so, the
development of n-type semiconductors, which are crucial for
the advance of organic p−n junctions and complementary
circuits, lags far behind p-type counterparts.12
According to the working principle of OFET, lowered
energy level of the lowest unoccupied molecular orbital
(LUMO) is essential to obtain n-type semiconductors because
the potential barrier between the LUMO energy level and the
Received: January 5, 2019
Revised: March 10, 2019
© XXXX American Chemical Society
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trifluoromethyl groups are rarely investigated because appro-
priate starting materials are unavailable. Herein, (E)-1,2-bis(4-
bromo-2-(trifluoromethyl)phenyl)ethene (TFBVB-Br), a new
monomer, was designed and synthesized. This block features
the incorporation of trifluoromethyl group, and it can generate
intramolecular noncovalent Coulombic interactions. Further
functionalization and copolymerization of the corresponding
block with diazaisoindigo (AIID) afford two D−A copolymers
(i.e., PAIID-TFBVB-C1 and PAIID-TFBVB-C3). In order to
systematically study the effect of trifluoromethyl group, (E)-
1,2-bis(4-bromophenyl)ethene (BVB-Br) and the correspond-
ing copolymer without trifluoromethyl groups (PAIID-BVB-
C3) were also synthesized as references. The optical and
electrochemical tests reveal that trifluoromethylation of the
copolymers broadens the optical gap and decreases the frontier
orbital energy levels simultaneously, and theoretical simu-
lations suggest that the electronic structures exhibit substantial
variation. FET characterizations further prove that the carrier
transport type is transformed from hole for PAIID-BVB-C3 to
electron for both PAIID-TFBVB-C1 and PAIID-TFBVB-C3.
Scheme 1. Representative Strategies Previously Reported in
the Literature and New Methodology in this Work for
Obtaining n-Type Semiconductors
but also facilitates the aggregation state among molecules and
thus boosts mobility. Alternatively, substituting nitrogen
atoms, specifically, sp2-hybridized nitrogen atom, for certain
carbon atoms in the conjugation chain decreases LUMO
energy level and provides n-type semiconductors as well
(Scheme 1, path II).19−21 Owing to the greater electro-
negativity of nitrogen atom than that of carbon atom, this
replacement stabilizes frontier orbitals of the resulting
molecules. Moreover, replacing C−H bonds with sp2-
hybridized nitrogen atoms enforces molecular planarity,
reduces energetic disorder,22 and contributes to a closer
stacking distance by depleting the π-electron density.23 All of
the above advantages promote the arrangement of molecules
and further elevate the performance of the corresponding
devices. Besides, constructing quinoid-type building block with
dicyanomethylene groups is another effective method to
develop n-type semiconducting materials for their high
electron affinities which are attributed to the dual electron-
deficient and quinoidization effect (Scheme 1, path III).24
Molecules with quinoidal structures exhibit high electron
affinities and narrower band gaps, and these types of materials
are regarded as promising n-type semiconductors.25,26
Apart from the strategies discussed above, it is necessary to
expand the scope of electron-transporting materials and to
better understand the structure−property relationships.
Trifluoromethyl (−CF3) substituent, as a strong electron-
withdrawing group (electronegativity of 3.2 on the Pauling
scale), is prevalent in pharmaceuticals and agrochemicals.27
This group exerts a strong inductive and hyperconjugation
effect,28 which might generate substantial effect on the
physicochemical property of the resulting materials. Owing
to the high bond strength, molecular stability was generally
improved after incorporating this electron-withdrawing
group.29 The device stability could also get improved for a
large hydrophobic parameter of the trifluoromethyl group,30
preventing water penetration. More importantly, hydrogen-
bonding interactions formed by fluorine atom and C−H bond
contribute to interactions among molecules and further
facilitate ordered packing mode. On the other hand, for
organic semiconductors, as far as we know, introducing this
group generally reduces the frontier orbital energy levels, and
ambipolar or exclusively n-type performance was obtained.31,32
However, donor−acceptor (D−A) copolymers incorporating
EXPERIMENTAL SECTION
■
Materials and Methods. All of the starting materials and reagents
were purchased from Acros, Sigma-Aldrich, or other commercial
sources and used directly without further purification unless otherwise
noted. The solvents were distilled freshly according to the standard
methods. Monomer AIID was synthesized according to the literature
reported previously by our group.33
General Measurements and Characterizations. 1H NMR and
13C NMR spectra were recorded on a Bruker AVANCE 300
instrument in deuterated chloroform (CDCl3) at room temperature
1
(RT). The high-temperature H NMR spectra were acquired on a
Bruker AVANCE 500 instrument in deuterated 1,1,2,2-tetrachloro-
ethane (d2-C2D2Cl4) at 373 K. High-resolution mass spectra (HR-EI)
were recorded on a Shimadzu QP 2010 gas chromatograph mass
spectrometer, and high-resolution matrix-assisted laser desorption/
ionization time-of-flight mass (HR-MALDI-TOF) spectra were
performed on a 9.4T Solarix FT-ICR mass spectrometer. Elemental
analyses were carried out by using a CARLO ERBA 1106 Elemental
Analyzer. The number-average molecular weight (Mn), weight-average
molecular weight (Mw), and polydispersity index (PDI) of copolymers
were determined by using an Agilent Technologies PL-GPC220 series
gel permeation chromatography (GPC) with 1,2,4-trichlorobenzene
(TCB) as the eluent at 150 °C and calibrated against narrow-
polydispersity polystyrene standards. UV−vis absorption spectra were
measured on a Jasco V-570 spectrophotometer. Cyclic voltammetric
(CV) measurements were performed on a CHI660C electrochemistry
workstation with a scan rate of 50 mV s−1. Thermogravimetric
analyses (TGA) were conducted on a DTG-60 instrument under
nitrogen flow, heating from RT to 550 °C with a heating rate of 10 °C
min−1. The surface morphologies of the thin films were investigated
by using an atomic force microscope (AFM) operated in tapping
mode. Two-dimensional grazing incident X-ray diffraction (2D-
GIXRD) was applied to determine the molecular packing and
crystallinity. The GIXRD measurements were performed at BL14B1
beamline of Shanghai Synchrotron Radiation Facility. The incident
beam energy was of 10.03 keV, and the data were collected with an
incident angle of 0.50° on a MAR225 image-plate area detector.
4-Bromo-2-(trifluoromethyl)benzaldehyde (2). To a two-neck
flask protected with argon, 1,4-dibromo-2-(trifluoromethyl)benzene
(1, 5.00 g, 16.45 mmol) and tetrahydrofuran (THF) distilled freshly
(120 mL) were added in sequence. Then, the system was cooled to
−78 °C, and n-BuLi (17.77 mmol, 7.11 mL) was added to the mixture
dropwise. After stirring for 10 min at the same temperature, N-
formylpiperidine (2.79 g, 24.66 mmol) was syringed into the mixture
in one portion. The temperature was elevated to the RT and reacted
for another 1 h. Then, the mixture was poured into ice water.
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Extraction was conducted with dichloromethane, and the organic
phase was dried with anhydrous sodium sulfate. The mixture was
filtered, and the organic solvent was removed under reduced pressure.
Column chromatography was applied for further purification, and the
final compound was acquired as a white solid (3.50 g, 84%). 1H NMR
(300 MHz, CDCl3, δ, ppm): 10.34 (m, 1H), 8.00 (d, 1H, J = 8.4 Hz),
7.94 (d, 1H, J = 0.9 Hz), 7.85 (d, 1H, J = 8.4 Hz). 13C NMR (75
MHz, CDCl3, δ, ppm): 187.82, 187.78, 187.74, 187.70, 135.66,
132.59, 132.38, 132.36, 132.14, 131.70, 130.59, 129.59, 129.51,
129.44, 129.36, 128.85, 128.24, 124.60, 120.95, 117.31. HR-MS (EI):
calcd for C8H4BrF3O, 251.9398 (79Br), 253.9377 (81Br); found,
251.9368 (79Br), 253.9405 (81Br).
Pd(dppf)Cl2 (124.39 mg, 0.17 mmol) were added successively.
Then, 20 mL of 1,4-dioxane was syringed into the flask. The mixture
was refluxed overnight. After stopping the reaction and cooling to the
RT, the mixture was filtered with a thin diatomite film to remove the
residual catalyst. The filtrate was extracted by dichloromethane and
dried with anhydrous sodium sulfate, after which the mixture was
filtered and the solvent was removed with a rotary evaporator. A
further purification process with column chromatography was
conducted, and the final compound was acquired as a white solid
(218.00 mg, 60%). 1H NMR (300 MHz, CDCl3, δ, ppm): 7.80 (d, J =
8.1 Hz, 4H), 7.53 (d, J = 8.1 Hz, 4H), 7.18 (s, 2H), 1.35 (s, 24H). 13C
NMR (75 MHz, CDCl3, δ, ppm): 139.87, 135.17, 129.58, 125.91,
83.81, 24.90. HR-MS (MALDI): calcd for C26H34B2O4, 432.2643;
found, 432.2638.
(E)-1,2-Bis(4-bromo-2-(trifluoromethyl)phenyl)ethene (3, TFBVB-
Br). Zinc powder (1.40 g, 21.40 mmol) and freshly distilled THF (80
mL) were added in a two-neck flask under argon atmosphere. Cooling
the system to 0 °C with an ice bath, TiCl4 (2.03 g, 10.67 mmol) was
syringed into the flask drop by drop. Then, the system was refluxed
for 3 h, after which the system was cooled to 0 °C again, and
compound 2 (1.80 g, 7.11 mmol) was added in one portion. The
mixture was refluxed overnight. The reaction was stopped, and the
mixture was poured into ice water and further extracted by
dichloromethane. The organic phase was dried with anhydrous
sodium sulfate and filtered, and the solvent was removed with a rotary
evaporator. Recrystallization was conducted to further purify the
target compound, and the product was acquired as a white solid (2.40
General Procedure for Polymerization and Purification. To
a Schlenk tube, monomer AIID (0.10 mmol), compound 4 (or
compound 7, 0.10 mmol), Pd2(dba)3 (dba = dibenzylideneacetone,
4.50 mg), P(o-tol)3 (o-tol = 2-methylphenyl, 12.30 mg), a catalytic
amount of Aliquat 336, and 2 mL of 2 M potassium carbonate
solution were added successively. The tube was charged with argon
through a freeze−pump−thaw cycle three times at −78 °C. The
system was heated at 85 °C for 72 h; phenyboronic and
bromobenzene were then used to end-cap. The reaction was stopped
and cooled to RT. Pouring the mixture into methanol and stirring for
3 h, filtration was conducted and the residue was extracted with
methanol, acetone, and hexane to remove the oligomers and the
residual catalysts. Chloroform was further used as the extraction
solvent to obtain the final product.
1
g, 71%). H NMR (300 MHz, CDCl3, δ, ppm): 7.82 (d, 2H, J = 1.8
Hz), 7.72−7.69 (m, 2H), 7.62 (d, 2H, J = 8.4 Hz), 7.32 (s, 2H). 13C
NMR (75 MHz, CDCl3, δ, ppm): 135.27, 134.53, 129.96, 129.56,
129.38, 129.30, 129.22, 129.15, 129.01, 127.96, 125.15, 121.93,
121.51. HR-MS (EI): calcd for C16H8Br2F6, 471.8897 (79Br2),
473.8876 (79Br 81Br), 475.8856 (81Br2); found, 471.8906 (79Br2),
473.8877 (79Br 81Br), 475.8854 (81Br2).
PAIID-TFBVB-C1. (136.41 mg, 82%). 1H NMR (300 MHz,
CDCl3): δ (ppm) 9.31−7.86 (m), 3.86 (br), 2.50−0.50 (br). GPC:
Mn = 26.06 kDa, PDI = 1.48. Anal. Calcd for C78H112F6N4O2: C,
74.84; H, 9.02; N, 4.48. Found: C, 74.30; H, 8.78; N, 4.23.
1
PAIID-TFBVB-C3. (133.08 mg, 80%). H NMR (500 MHz, 373 K,
(E)-1,2-Bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2-
(trifluoromethyl)phenyl)ethene (4). Compound 3 (340.00 mg, 0.72
mmol), 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-bi(1,3,2-dioxaborolane)
(BPD, 419.00 mg, 1.65 mmol), KOAc (423.00 mg, 4.31 mmol),
and Pd(dppf)Cl2 (109.76 mg, 0.15 mmol) were added into a flask
successively. Then, 1,4-dioxane (15 mL) was syringed into the flask,
and the mixture was refluxed overnight. After stopping the reaction,
the mixture was filtered with a thin diatomite layer to remove the
residual catalyst. The filtrate was extracted with dichloromethane and
dried with anhydrous sodium sulfate. The mixture was filtered, and
the solvent was removed under reduced pressure. The final compound
was obtained after a further purification with column chromatography
d2-C2D2Cl4): δ (ppm) 9.65 (s), 8.58 (s), 8.37 (s), 7.97 (m), 7.61(m),
4.09 (br), 2.00−0.50 (br). GPC: Mn = 32.16 kDa, PDI = 1.95. Anal.
Calcd for C78H112F6N4O2: C, 74.84; H, 9.02; N, 4.48. Found: C,
73.11; H, 8.67; N, 4.20.
PAIID-BVB-C3. (116.09 mg, 76%). 1H NMR (300 MHz, CDCl3): δ
(ppm) 8.82 (br), 8.08 (br), 7.54 (br), 3.83 (br), 2.50−0.50 (br).
GPC: Mn = 55.35 kDa, PDI = 2.32. Anal. Calcd for C76H114N4O2: C,
81.81; H, 10.30; N, 5.02. Found: C, 80.74; H, 9.80; N, 4.69.
X-ray Crystallography. X-ray crystallographic data were acquired
with a Bruker Smart CCD diffractometer by using graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). For intermediate
compound 3 (TFBVB-Br), the data were collected at 173 K with ω
scan in the θ range of 2.785°−27.484°. A total of 5153 reflections
were conducted, in which 1752 were independent reflections [R(int)
= 0.0540]. The structure was resolved by the direct method and
refined by full-matrix least-squares on F2 using the SHELXL-97
program. All nonhydrogen atoms were refined anisotropically, and all
hydrogen atoms were placed using AFIX instructions. Crystallo-
graphic data for single crystal of TFBVB-Br: C16H8Br2F6; FM =
1
(213.00 mg, 54%). H NMR (300 MHz, CDCl3, δ, ppm): 8.11 (s,
2H), 7.97 (d, 2H, J = 7.8 Hz), 7.76 (2H, J = 7.8 Hz), 7.48 (s, 2H),
1.37 (s, 24H). 13C NMR (75 MHz, CDCl3, δ, ppm): 138.31, 132.22,
132.15, 129.11, 127.44, 127.05, 126.66, 126.30, 84.30, 24.87. HR-MS
(MALDI): calcd for C28H32B2F6O4, 568.2391; found, 568.2387.
(E)-1,2-Bis(4-bromophenyl)ethene (6, BVB-Br). Zinc dust (5.30 g,
81.07 mmol) was added into a two-neck dry flask, and 100 mL of
distilled dry THF was syringed into the flask, and the mixture was
cooled to 0 °C with an ice bath. At this temperature, TiCl4 (7.69 g,
40.54 mmol) was added dropwise. Then, the ice bath was removed,
and the system was refluxed at 70 °C for 3 h. After cooling the
mixture to the RT, 4-bromobenzaldehyde (5, 5.00 g, 27.02 mmol)
was added to the system. In addition, the mixture was refluxed
overnight. The reaction was stopped and extracted with dichloro-
methane. The organic phase was further washed with saturated brine
and dried with anhydrous sodium sulfate. Remove the solvent under
reduced pressure and the pure target compound was acquired after a
̅
474.04; triclinic; P1; a = 4.761(3) Å; b = 7.824(5) Å; c = 11.183(7)
Å; α = 104.016(6)°; β = 97.912(7)°; γ = 102.918(6)°; Z = 1; ρcalculated
= 2.040 g/cm3. The refinement was converged to R1 = 0.0310 for I >
2σ(I), R1 = 0.0322, wR2 = 0.0759, GOF = 1.142 for all data. CCDC
1876949 contains the supplementary crystallographic data.
Similar procedure was performed on intermediate compound 6
(BVB-Br). The ω scan was in the θ range of 2.946°−27.476°. In
addition, a total of 4385 reflections were measured, in which 1345
were independent reflections [R(int) = 0.0623]. Crystallographic data
for single crystal of BVB-Br: C14H10Br2; FM = 338.04; orthorhombic;
Pccn; a = 7.3205(15) Å; b = 27.660(6) Å; c = 5.9061(12) Å; α = 90°;
β = 90°; γ = 90°; Z = 4; ρcalculated = 1.877 g/cm3. The refinement was
converged to R1 = 0.0602 for I > 2σ(I), R1 = 0.0696, wR2 = 0.1485,
GOF = 1.186 for all data. CCDC 1876950 contains the
supplementary crystallographic data.
1
further purification with column chromatography (5.75 g, 63%). H
NMR (300 MHz, CDCl3, δ, ppm): 7.48 (d, J = 8.4 Hz, 4H), 7.36 (d, J
= 8.4 Hz, 4H), 7.02 (s, 2H). 13C NMR (75 MHz, CDCl3, δ, ppm):
135.92, 131.88, 128.14, 128.02, 121.66. HR-MS (EI): calcd for
C14H10Br2, 335.9149 (79Br2), 337.9129 (79Br 81Br), 339.9108 (81Br2);
found, 335.9153 (79Br2), 337.9132 (79Br 81Br), 339.9110 (81Br2).
(E)-1,2-Bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
phenyl)ethene (7). To a flask, compound 6 (0.28 g, 0.84 mmol), BPD
(493 mg, 1.94 mmol), KOAc (496.59 mg, 5.06 mmol), and
Device Fabrication and Characterization. Top-gate/bottom-
contact (TG/BC) configuration OFET devices were fabricated to
evaluate the polymer semiconductors. Corning glass substrates were
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Scheme 2. Molecular Structures and Synthetic Routes of PAIID-TFBVB-C1, PAIID-TFBVB-C3, and PAIID-BVB-C3
Figure 1. Chemical structure, single-crystal structure, and the packing mode of TFBVB-Br (a) and BVB-Br (b).
rinsed with deionized water, ethanol, and acetone successively with an
ultrasonic apparatus and dried in an oven before device fabrication.
Then, a 30 nm thick Au electrode (channel width: 8800 μm, channel
length: 80 μm) was deposited by thermal evaporation. All of the
copolymers were dissolved in o-dichlorobenzene (concentration: 6
mg/mL), and the corresponding hot solution was spin-coated on the
pre-patterned substrate at a speed of 2500 rpm for 60 s, forming a
semiconductor layer with the thickness of 40−50 nm. The annealing
process was performed on a hotplate for 10 min. Next, a solution of
poly(methylmethacrylate) (PMMA) dissolved in n-butyl acetate was
spin-coated and dried at 80 °C for 30 min, serving as the dielectric
layer. The devices were completed after a 30 nm thick aluminum layer
was deposited through shadow mask as the gate contact. The
fundamental OFET electrical characteristics were measured with a
Keithley 4200 semiconductor characterization system under ambient
conditions, and the field-effect mobilities were extracted in the
Scheme 2, and the detailed synthetic procedures are provided
in the Experimental Section. The starting materials were
purchased and used without further purification. For the
synthesis of trifluoromethyl-substituted monomer (i.e., com-
pound 4), 1,4-dibromo-2-(trifluoromethyl)benzene was trea-
ted with n-BuLi first and quenched with N-formylpiperidine,
which affords the aldehyde (2) in a high yield of 84%. The
TFBVB-Br (compound 3) was obtained by conducting
McMurry coupling reaction. According to a conventional
method, TFBVB-Br was directly borylated with pinacol
diborane, affording borate in 54% yield as a white crystalline
solid. Similarly, the monomer 7 was also acquired as a white
crystalline solid after coupling under McMurry conditions and
esterification under Miyaura boration conditions with 4-
bromobenzaldehyde as the starting material. The AIID
monomers were prepared according to the methods reported
by our group.33 The Suzuki coupling-based polymerization
between AIID monomers and borates generated PAIID-
TFBVB-C1, PAIID-TFBVB-C3, and PAIID-BVB-C3 in a
yield of 75−85%. The structures of the intermediates and
monomers were confirmed by NMR and MS characterizations,
and the resulting polymers were confirmed by NMR
characterization and elemental analysis.
1/2
saturated regime from the slope of ID versus VG according to the
following equation
2
IDS = (W/2L)Ciμ(VGS − VTH
)
where L and W are the channel length and channel width,
respectively. Ci is the insulator capacitance of PMMA per unit area.
In addition, VGS and VTH correspond to the gate voltage and threshold
voltage, respectively.
RESULTS AND DISCUSSION
Synthesis. The synthetic routes of PAIID-TFBVB-C1,
PAIID-TFBVB-C3, and PAIID-BVB-C3 are shown in
The alkyl chains play a significant role in the solubility,
molecular aggregation state, and thus the performance of
OFETs. In view of this reason, we introduced 2-decyltetradecyl
■
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Figure 2. Molecular structures of the model compounds (a), side view of the theoretical simulated molecular geometry (b), frontier orbital
distribution, and corresponding energy levels (c) for the trimer model compounds of the three polymers.
Figure 3. UV−vis absorption spectra of PAIID-TFBVB-C1, PAIID-TFBVB-C3, and PAIID-BVB-C3 in dilute chloroform solution (a) and as-
spun thin films coated on quartz substrates (b).
and 4-decyltetradecyl groups into the AIID units and obtained
PAIID-TFBVB-C1 and PAIID-TFBVB-C3, respectively. On
the other hand, a contrast material, PAIID-BVB-C3 was
synthesized to fully investigate the effects of trifluoromethyl
groups. We evaluated the weight-average molecular weight
(Mw), number-average molecular weight (Mn), and PDI of all
of the resulting polymers by high-temperature GPC analysis
using TCB as the eluent at 150 °C and calibrated against
polystyrene standards. All of the Mns and PDIs were measured
to be 26.06 kDa/1.48 for PAIID-TFBVB-C1, 32.16 kDa/1.95
for PAIID-TFBVB-C3, and 55.35 kDa/2.32 for PAIID-BVB-
C3.
Thermal Properties. TGAs were employed to study the
thermal behaviors of the polymers. The results demonstrate
that all of the three polymers possess excellent thermal
stabilities with the degradation temperature (Td, 5% weight
loss) above 400 °C (Figure S1). In addition, there is no
discernible phase transition of all of the polymers as evidenced
by differential scanning calorimetry (DSC) thermogram
(second cycle) during the range of RT to 300 °C (Figure S2).
Single-Crystal Structure Analysis. To elucidate the
effect of trifluoromethyl on molecular conformation and
molecular aggregation, single crystals of TFBVB-Br and
BVB-Br were obtained by diffusing methanol into its
corresponding dichloromethane solution, and the results are
shown in Figure 1. Single-crystal analysis reveals that both
BVB-Br and TFBVB-Br molecules have good planarity. In
addition, for TFBVB-Br, the distances between F atoms of the
trifluoromethyl unit and the hydrogens of the vinylene unit are
2.53 and 2.47 Å (Figure 1a), respectively, which are shorter
than the sum of van der Waals radii (2.67 Å),34 indicating the
existence of the intramolecular H···F interactions. The packing
mode reveal that C−H···F and C−H···π intermolecular
interaction exists in the TFBVB-Br and BVB-Br single
crystals, respectively, leading to different packing motifs.
Molecular Geometry and Frontier Molecular Orbital
Computation. On the basis of the single-crystal structures
discussed above, molecular structures of the polymers were
simulated with Gaussian0935 by conducting theoretical
calculation on the model compounds with a hybrid B3LYP
correlation functional and 6-31G(d) basis set and the alkyl side
chains were replaced with methyl groups to simplify the
calculation. Figure 2a shows the structures of the model
compounds (PAIID-TFBVB and PAIID-BVB, trimers of the
corresponding repeat units), and the calculated energy-
minimized molecular geometries are depicted in Figure 2b.
The simulation results reveal that PAIID-BVB has a coplanar
conjugated structure, and the introduction of trifluoromethyl
groups contributes a more planar molecular skeleton. To
further gain insight into the trifluoromethyl effect on the
electronic structure and energy level, Figure 2c shows the
computed molecular orbitals of the model compounds and the
corresponding frontier orbital energy levels. Obviously, both
the highest occupied molecular orbitals (HOMOs) of PAIID-
TFBVB and PAIID-BVB are delocalized along the polymer
backbone, whereas the LUMOs are mainly localized on the
acceptor part of both the model molecules. Moreover,
compared with PAIID-BVB, PAIID-TFBVB exhibits lowered
HOMO and LUMO energy levels as well as a larger energy
gap, which are due to the trifluoromethyl substitution. The
decreased frontier energy levels after trifluoromethylation also
accounts for the conversion of the carrier transport type.
Electrostatic potential analysis further confirmed that the
electrostatic potential distribution has a substantial change
along the molecular backbone after incorporation of the
trifluoromethyl substituents, which will further affect molecular
ordering and the carrier transport (Figure S3). The calculation
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Figure 4. Cyclic voltamograms (left) of polymer thin films and the schematic energy level diagram (right) of PAIID-TFBVB-C1, PAIID-TFBVB-
C3, and PAIID-BVB-C3.
Table 1. Summary of Molecular Weight, Optical, and Electrochemical Properties of All of the Polymers
a
b
c
d
e
f
max
film
onset
film
gap
opt
onset
red
cv
LUMO
onset
oxd
cv
HOMO
cv
gap
polymer
Mn (kDa) /PDI
λ
(nm)
λ
(nm)
E
(eV)
E
(V)
E
(eV)
E
(V)
E
(eV)
E
(eV)
PAIID-TFBVB-C1
PAIID-TFBVB-C3
PAIID-BVB-C3
26.06/1.48
32.16/1.95
55.35/2.32
568
570
636
667
665
680
1.86
1.86
1.82
c
−0.71
−0.79
−0.93
−3.69
−3.61
−3.47
1.83
1.78
1.33
−6.23
2.54
2.57
2.26
−6.18
−5.73
a
b
Determined by GPC at 150 °C. Extracted in the low-energy region. Calculated from the absorption edges of thin films (E = 1240/λofinlmset).
opt
gap
d
e
f
cv
LUMO
onset
red
cv
onset
+ 4.40) eV. Determined by the
oxd
According to the equation: E
= − (E
+ 4.40) eV. According to the equation: E
= − (E
HOMO
cv
cv
LUMO
equation: E = (E
− EcHvOMO) eV.
gap
results demonstrate that the introduction of the electron-
withdrawing trifluoromethyl groups not only decreases the
molecular energy levels and broadens energy gap but also
influences the electrostatic potential distribution.
negligible change of the effective conjugation after annealing
processing.
Electrochemical Characterization. Cyclic voltammetry
tests were conducted to evaluate electronic energy levels of the
polymers. The three electrode system was adopted with the
glassy carbon electrode as the working electrode, Ag/AgCl
electrode as the reference electrode, and a wire of platinum
electrode as the counter electrode. A solution of 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) in ace-
tonitrile was employed as the electrolyte. Cyclic voltammo-
grams of the polymer thin films are shown in Figure 4, and the
energy levels of the polymers are calculated and summarized in
Optical Properties. The aggregation behavior and optical
property of all of the polymers were investigated in chloroform
solution and film state with UV−vis absorption measurements
(Figure 3). For both the chloroform solution and thin-film
state, all of the absorption profiles are characterized with dual
absorption bands: the high-energy bands (300−450 nm) are
mainly attributed to the π−π* transition, whereas the low-
energy bands (450−700 nm) correspond to the interactions
between the acceptor and donor units.36,37 Apparently,
polymers of PAIID-TFBVB-C1 and PAIID-TFBVB-C3
exhibit almost identical absorption figures in both chloroform
solution and thin-film state, suggesting that the alkyl chain
negligibly influences the optical properties. Furthermore,
compared with the spectra of PAIID-BVB-C3, those of
PAIID-TFBVB-C1 and PAIID-TFBVB-C3 are obvious blue
shifted which is attributed to the enlarged energy gap after
introduction of the trifluoromethyl substitution revealed by the
theoretical simulation. For the thin films, the maximum
absorption peaks in the low-energy band of PAIID-TFBVB-
C1 and PAIID-TFBVB-C3 are located at 570 nm with a
shoulder peak at 610 nm, whereas PAIID-BVB-C3 has the
maximum absorption peak at 636 nm with a shoulder peak at
586 nm. Estimated from the thin-film absorption edges, the
optical gaps are 1.86, 1.86, and 1.82 eV for PAIID-TFBVB-C1,
PAIID-TFBVB-C3, and PAIID-BVB-C3, respectively. Com-
pared with the solution, corresponding films exhibit almost the
same absorption curves (Figure S4). We also performed
temperature-dependent UV−vis absorption characterizations
in dilute solution of 1,2-dichlorobenzene (Figure S5).
According to the results, it is worth noting that negligible
shifts appear with the increasing temperature of the solution.
Comparisons between as-spun and annealed thin-film
absorption spectra were also conducted (Figure S6), and
they show almost identical spectra, indicating that there is
Table 1. All of the polymers show apparent reduction waves.
onset
red
Estimated from the onset of the reduction wave (E ), The
LUMO energy levels (EcLvUMO) of PAIID-TFBVB-C1 and
PAIID-TFBVB-C3 are approximatively −3.69 and −3.61 eV,
respectively. In contrast, PAIID-BVB-C3 shows higher LUMO
energy level of −3.47 eV. The lower LUMO energy levels of
PAIID-TFBVB-C1 and PAIID-TFBVB-C3 are ascribed to the
introduction of the electron-withdrawing trifluoromethyl units.
On the other hand, the oxidation peaks of PAIID-TFBVB-C1
and PAIID-TFBVB-C3 are different from that of PAIID-BVB-
C3. Both the oxidative waves of PAIID-TFBVB-C1 and
PAIID-TFBVB-C3 are irreversible, whereas PAIID-BVB-C3
shows a quasi-reversible curve. The onset of the oxidative wave
and calculated HOMO energy level of PAIID-TFBVB-C1,
PAIID-TFBVB-C3, and PAIID-BVB-C3 are 1.83/−6.23,
1.78/−6.18, and 1.33/−5.73 eV, respectively. Correspond-
ingly, the electrochemical gaps are 2.54 eV for PAIID-TFBVB-
C1, 2.57 eV for PAIID-TFBVB-C3, and 2.26 eV for PAIID-
BVB-C3. Apparently, the electrochemical gaps of PAIID-
TFBVB-C1 and PAIID-TFBVB-C3 are larger than that of
PAIID-BVB-C3, which agrees well with the UV−vis profiles
and theoretical calculations. Comparison of PAIID-TFBVB-
C1 and PAIID-TFBVB-C3 frontier orbital energy levels also
indicates that the different branch points in the side chains
attached in the polymer backbone have a marginal effect on the
electronic structure in this series. It should also be noted that
the electrochemical band gap is larger than the corresponding
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Figure 5. Typical output and transfer characteristics of the OFET devices with TG/BC configuration based on PAIID-TFBVB-C1 (a,b), PAIID-
TFBVB-C3 (c,d), and PAIID-BVB-C3 (e,f).
Table 2. Performance Parameters of the Polymer-Based OFET Devices
a
b
polymer
carrier type
electron
Tanneal
μave (cm2 V−1 s−1
)
μmax (cm2 V−1 s−1
)
Ion/off
VTH (V)
PAIID-TFBVB-C1
RT
280 °C
RT
280 °C
RT
140 °C
3.6 × 10−4
0.070
5.9 × 10−4
0.020
3.6 × 10−2
5.9 × 10−4
0.11
8.8 × 10−4
0.040
4.7 × 10−2
0.14
∼104
∼104
∼104
∼105
∼105
∼105
40 ( 5)
40 ( 5)
30 ( 5)
37 ( 5)
−45 ( 5)
−40 ( 5)
PAIID-TFBVB-C3
PAIID-BVB-C3
electron
hole
0.090
b
a
Averaged OFET carrier mobility obtained from at least 10 devices. Maximum OFET carrier mobility.
Figure 6. 2D-GIXRD patterns of PAIID-TFBVB-C1 (a,b), PAIID-TFBVB-C3 (c,d), and PAIID-BVB-C3 (e,f) deposited on the glass substrate
before and after annealing treatment.
optical gap, and the difference is ascribed to the electron−hole
pair binding energy.
completed by further spin-coating a PMMA solution (60 mg/
mL) to serve as the dielectric layer and depositing a 30 nm
thick layer of aluminum (Al) as the gate electrode. To optimize
the device performance, the semiconductor layer was annealed
at various temperatures before spin-coating the dielectric layer
and the optimal annealing temperature was found to be 280 °C
for PAIID-TFBVB-C1 and PAIID-TFBVB-C3 and 140 °C for
PAIID-BVB-C3 (Figure S7). All of the fabrication processes
Charge Transport Characteristics. We further evaluated
the charge transport properties of all of the polymers. TG/BC
devices were fabricated by spin-coating a hot dichlorobenzene
solution of polymers onto Corning glass substrates which were
pre-patterned with a 5 nm thick titanium (Ti) as the adhesion
layer and a 50 nm thick Au as the electrodes. The devices were
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Table 3. Crystallographic Parameters of the Polymer Thin Films Obtained by GIXRD
polymer
T (°C)
out-of-plane (100) d−d spacing (Å)
in-plane (010) π−π stacking (Å)
fwhm (nm−1
)
coherence length (Å)
PAIID-TFBVB-C1
pristine
280
pristine
280
pristine
140
20.74
21.41
21.59
21.41
21.94
22.50
3.55
3.51
3.46
3.45
3.38
3.40
0.626
0.392
0.639
0.385
0.367
0.300
89.3
143
87.5
145
152
186
PAIID-TFBVB-C3
PAIID-BVB-C3
were carried out in a glove box, and the devices were tested
under ambient conditions. Detailed fabrication processes are
provided in the Experimental Section. Figure 5 shows the
typical output and transfer characteristics of the three polymer-
based devices treated at their corresponding optimal annealing
temperature, and the charge carrier mobility, μ, was extracted
in the saturated regime. The electrical characteristics of all of
the polymers are summarized in Table 2. Apparently, devices
based on PAIID-BVB-C3 exhibit exclusively p-type charge
transport performance because of the relatively high LUMO
energy level limiting the electron injection. For the PAIID-
BVB-C3-based transistor without annealing treatment, the
highest hole mobility is 4.7 × 10−2 cm2 V−1 s−1. Annealing
treatment benefits the molecular ordering and crystallinity,38
and there is a three times improvement in mobility of 0.14 cm2
Specifically, 4 orders of diffraction of PAIID-TFBVB-C3 and
PAIID-BVB-C3 thin films were obtained compared to that of
PAIID-TFBVB-C1 having 3 orders in the out-of-plane
direction, suggesting that the thermal annealing process leads
to enhanced structural order and crystallinity. These results
partially explain the elevated mobilities. It is worth noticing
that the weak (100) peak in the in-plane direction of the
PAIID-TFBVB-C1 film disappears and two new peaks, (001)’
and (002)’, formed after thermal annealing (Figure S8). In
contrast, such phenomenon cannot be seen in the other two
materials. We further analyzed the orientation distribution of
the crystallites with respect to the substrate by performing the
pole figures for the (100) peak reflection (Figure S11).40,41
The results also confirmed that the edge-on packing mode is
predominated by all of the polymer thin films. For pristine thin
films, the edge-on packing portions of PAIID-TFBVB-C1,
PAIID-TFBVB-C3, and PAIID-BVB-C3 are 79.99, 85.23, and
83.59%, respectively. After annealing treatment, the proportion
of the edge-on packing mode increases for all of the polymers
and PAIID-TFBVB-C1 has the highest proportion up to
90.93% compared with PAIID-TFBVB-C3 and PAIID-BVB-
C3 of 89.92 and 84.59%, respectively. These results confirm
that considerable molecular rearrangement proceeds and more
ordered packing mode is formed during the annealing process
for PAIID-TFBVB-C1 thin film, accounting for the slightly
higher mobility than that of PAIID-TFBVB-C3-based devices.
Corresponding lamellar packing distances were calculated, and
the results are listed in Table 3. The d−d spacing and π−π
stacking distances were evaluated of (100) and (010) peaks,
respectively, by using the formula: d = 2π/q.42 Because the
side-chain branch points are close to the polymeric main chain
for PAIID-TFBVB-C1, it displays closer d−d spacing distances
than those of PAIID-TFBVB-C3 and PAIID-BVB-C3. On the
other hand, the π−π stacking distances decrease on the order
of PAIID-TFBVB-C1, PAIID-TFBVB-C3, and PAIID-BVB-
C3. Interestingly, PAIID-BVB-C3 shows a π−π stacking
distance of ∼3.40 Å, which is smaller than most of the D−A
copolymers.43−46 The π−π stacking distances of PAIID-
TFBVB-C3 (∼3.45 Å) are larger than that of PAIID-BVB-
C3 by ∼0.05 Å, and it is reasonable attributing this change to
the trifluoromethyl substitution for their identical molecular
backbone and alkyl side chains. Moreover, tuning the
branching point of alkyl chains is an effective strategy for
gradually modulating the π−π distances,47 and our results
substantiate that PAIID-TFBVB-C3 with 4-decyltetradecyl
groups shows shorter π−π distances (∼ 3.45 Å) than that of
PAIID-TFBVB-C1 with 2-decyltetradecyl groups (∼3.53 Å).
We further conducted Scherrer analysis of the (100) peak to
quantify the coherence length (CL) which is related to the
crystalline size by employing the Scherrer equation: CL =
2πK/fwhm, where K = 0.89 is a shape factor and fwhm
represents the full width at half-maximum of a diffraction
peak.48,49 The CL values of the pristine thin films of PAIID-
V
−1 s−1 for the devices annealed at the optimal temperature of
140 °C. In contrast, polymers with the electron-withdrawing
trifluoromethyl group, PAIID-TFBVB-C1 and PAIID-
TFBVB-C3, only exhibit electron transport performance and
the highest mobilities of the pristine devices are 5.9 × 10−4 and
8.8 × 10−4 cm2 V−1 s−1, respectively. Annealing processes were
also conducted, and the corresponding devices contribute to a
dramatically elevated mobility of 0.11 cm2 V−1 s−1 for PAIID-
TFBVB-C1 and 0.04 cm2 V−1 s−1 for PAIID-TFBVB-C3.
These results collectively indicate that the LUMO energy level
approaches the work function of the gold electrodes (5.2 eV)
and reduces the potential barrier of the electron injection by
inserting electron-withdrawing trifluoromethyl units, thus
facilitating electron injection. Significant improvement of the
device performance after annealing process could be accounted
for by the morphology investigations revealed by GIXRD and
AFM characterizations discussed in the following section.
Film Morphology and Microstructural Analysis. The
morphology of semiconductor thin film has an obvious
influence on the OFET performance. 2D-GIXRD and AFM
measurements of the thin films were carried out to study the
relationship between film morphology and transistor perform-
ance. Figure 6 shows the corresponding GIXRD patterns of the
as-cast and annealed copolymer films (280 °C for both PAIID-
TFBVB-C1 and PAIID-TFBVB-C3 and 140 °C for PAIID-
BVB-C3) on glass substrates, and their line-cut profiles are
illustrated in Figures S8−S10. Apparently, as for pristine
polymer films, the three materials tend to adopt dominated
edge-on packing structures determined by the obvious (h00)
peaks in the out-of-plane direction accompanied with a weak
(010) peak in the in-plane direction. In addition, the hump-
shaped scattering peak around q = 13 nm−1 is attributed to the
alkyl side chains.39 Note that both thin films of PAIID-
TFBVB-C3 and PAIID-BVB-C3 show more diffraction dots
than that of PAIID-TFBVB-C1, indicating that better
crystallinity are formed and can be ascribed to the farther
branch points of alkyl chains in the acceptor unit. More
diffraction peaks appeared after thermal annealing process.
H
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Figure 7. AFM height images of the thin films of PAIID-TFBVB-C1 (a,b), PAIID-TFBVB-C3 (c,d), and PAIID-BVB-C3 (e,f) at RT and after
annealing treatment.
TFBVB-C1, PAIID-TFBVB-C3, and PAIID-BVB-C3 are
89.3, 87.5, and 152 Å, respectively. The correspondingly
increased values of 143, 145, and 186 Å are obtained after
thermal annealing treatment. Because the CL value is related to
the crystalline size and lattice order, a larger LC value suggests
fewer grain boundaries and renders better carrier transport.16
To further gain insight into the surface morphologies of
polymer semiconductor thin films, AFM characterizations were
performed. As shown in Figure 7, both the as-spun PAIID-
TFBVB-C1 and PAIID-TFBVB-C3 films show very smooth
film topography with root-mean-square (rms) roughness values
of 0.439 and 0.467 nm, respectively, on the glass substrate. In
addition, distinct variations of larger grains after thermal
treatment were observed with the rms values increased to
0.558 and 0.528 nm. Unlike the thin-film morphology of
PAIID-TFBVB-C1 and PAIID-TFBVB-C3, PAIID-BVB-C3
thin films exhibit fiber-like textures before and after annealing
treatment with the rms values of 0.648 and 0.685 nm,
respectively. These observations illustrate that strong molec-
ular self-aggregation occurs during annealing treatment, which
is consistent with the annealing temperature dependence of
FET performance.
results shed new light on the usefulness of trifluoromethyl
substitution to obtain n-type semiconducting materials. It is
believed that excellent electron-transporting semiconductors
with trifluoromethyl substitutions would be realized by fine-
tuning the electronic structure and solid-state packing.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.9b00022.
TGA and DSC curves of all of the polymers, electrostatic
potential maps of the model compounds, temperature-
dependent absorption spectra, additional UV−vis
absorption spectra, 1-D XRD data, pole figure, and
NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yugui@iccas.ac.cn.
ORCID
CONCLUSIONS
■
Congyuan Wei: 0000-0003-3554-8001
Jianyao Huang: 0000-0003-4177-6393
Gui Yu: 0000-0001-8324-397X
We have designed and synthesized new n-type semiconducting
materials, PAIID-TFBVB-C1 and PAIID-TFBVB-C3, via
introducing trifluoromethyl groups in the backbone of D−A
copolymers. Compared with the reference material PAIID-
BVB-C3, introducing trifluoromethyl units generates more
localized frontier orbitals. The optical and electrochemical
characterizations further reveal that enlarged energy gaps and
decreased frontier orbital energy levels are realized simulta-
neously after introduction of the electron-withdrawing
trifluoromethyl groups. Moreover, for PAIID-TFBVB-C1
and PAIID-TFBVB-C3, the potential barrier between the
LUMO energy levels and the work function of Au electrode are
reduced, facilitating the electron injection. Consequently,
PAIID-BVB-C3-based transistors show typical p-type trans-
porting performance with a hole mobility of 0.14 cm2 V−1 s−1,
whereas PAIID-TFBVB-C1- and PAIID-TFBVB-C3-based
devices exhibit n-type transporting performance with electron
mobilities of 0.11 and 0.040 cm2 V−1 s−1, respectively. Our
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to acknowledge the financial support
from the National Key Research and Development Program of
China (2017YFA0204703 and 2016YFB0401100), the Na-
tional Natural Science Foundation of China (grants 21673258,
21774134, and 21474116), and the Strategic Priority Research
Program of the Chinese Academy of Sciences (CAS)
(XDB12030100). The 2D-GIXRD analyses were performed
at the BL14B1 Station of Shanghai Synchrotron Radiation
Facility (SSRF). The authors are very grateful to the assistance
of scientists from the station during the experiments.
I
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