Conformation-insensitive ambipolar charge transport in a diketopyrrolopyrrole-based co-polymer containing acetylene linkages

Article abstract of DOI:10.1021/cm5014703

A new donor-acceptor organic semiconducting co-polymer (PDPP-TAT) containing acetylene linkages based on dithienyl-diketopyrrolopyrrole (tDPP) has been synthesized and compared with a tDPP-based co-polymer (PDPP-TVT) containing vinylene linkages. The sp-hybridized carbons in the acetylene linkages result in favorable overlap of the electron wave functions of the tDPP units along the main chain. Further, the π-conjugation of PDPP-TAT was found to be highly insensitive to the chain conformation, in contrast to that of PDPP-TVT. As a result, PDPP-TAT provides favorable charge transport for electrons as well as holes, and enables facile charge transport in amorphous and tie-molecular regions connecting its crystalline domains. PDPP-TAT exhibits ambipolar characteristics with a high electron/hole mobility ratio (μ_e/μ_h) of ～0.3 in field-effect transistors, whereas PDPP-TVT exhibits unipolar characteristics with a μ_e/ μ_h value that is a factor of 30 lower. Our results demonstrate that the conformation sensitivity of charge transport is a vital factor in the electrical performances of actual organic transistor devices.

Full text of DOI:10.1021/cm5014703

Article
pubs.acs.org/cm
Conformation-Insensitive Ambipolar Charge Transport in a
Diketopyrrolopyrrole-Based Co-polymer Containing Acetylene
Linkages
Hui-Jun Yun,‡,∇ Hyun Ho Choi,†,∇ Soon-Ki Kwon,* Yun-Hi Kim,* and Kilwon Cho*
,‡
,§
,†
†Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH) and Center for Advanced Soft
Electronics (CASE), Pohang 790-784, Korea
‡
School of Materials Science and Engineering, Research Institute for Green Energy Convergence Technology (REGET), Gyeongsang
National University, Jinju 660-701, Korea
§
Department of Chemistry, Gyeongsang National University and Research Institute of Nature Science (RINS), Jinju 660-701, Korea
*
S Supporting Information
ABSTRACT: A new donor−acceptor organic semiconducting
co-polymer (PDPP-TAT) containing acetylene linkages based
on dithienyl-diketopyrrolopyrrole (tDPP) has been synthe-
sized and compared with a tDPP-based co-polymer (PDPP-
TVT) containing vinylene linkages. The sp-hybridized carbons
in the acetylene linkages result in favorable overlap of the
electron wave functions of the tDPP units along the main
chain. Further, the π-conjugation of PDPP-TAT was found to
be highly insensitive to the chain conformation, in contrast to
that of PDPP-TVT. As a result, PDPP-TAT provides favorable
charge transport for electrons as well as holes, and enables facile charge transport in amorphous and tie-molecular regions
connecting its crystalline domains. PDPP-TAT exhibits ambipolar characteristics with a high electron/hole mobility ratio (μ /μ )
e
h
of ∼0.3 in field-effect transistors, whereas PDPP-TVT exhibits unipolar characteristics with a μ /μ value that is a factor of 30
e
h
lower. Our results demonstrate that the conformation sensitivity of charge transport is a vital factor in the electrical performances
of actual organic transistor devices.
INTRODUCTION
LUMO) of the co-polymers. The use of the diketopyrrolo-
pyrrole (DPP) moiety as an acceptor unit in such co-polymers
is of particular interest, because the thiophene-coupled DPP
moiety (tDPP) has been reported to have remarkable
■
Ambipolar organic field-effect transistors (ambipolar FETs)
based on both p-type and n-type channels have recently
attracted great interest, because of the simplicity of their circuit
1
8−22
1
−6
aggregation properties.
The favorable intramolecular
design and fabrication.
For example, a complementary
sulfur−oxygen interactions of the tDPP moiety mean that it
is nearly co-planar, which results in high planarity with low
dihedral angles between DPP and the adjacent thiophene units.
This planarity is expected to allow long conjugation length
along the intrachain and facilitate strong intermolecular π−π
interaction, which is associated with good charge transport
properties.
metal-oxide-semiconductor (CMOS)-like inverter can easily be
constructed by combining two identical ambipolar FETs.
Several types of high-performance ambipolar FETs have been
7
−15
developed.
Most ambipolar FETs based on a dual-nature
organic semiconductor are superior to unipolar component
systems (i.e., to bilayers or bulk-heterojunction blends), in
terms of the aforementioned requirements.
Recent research into dual-nature organic semiconductors has
developed low-bandgap co-polymers based on alternating
conjugated donor−acceptor (D-A) dyads as the building blocks
of the polymer backbones, which are expected to exhibit
Janssen and Li groups synthesized the homopolymer of
23,24
tDPP and found it to exhibit ambipolar FET properties.
However, the reported FET mobilities for both hole and
electron were quite low. In attempts to improve the properties
of the tDPP homopolymer, additional donor units composed of
thiophene or selenophene have been introduced between the
tDPP components. The DPP acceptor unit typically has
branched alkyl side chains which hinder close intermolecular
16−18
ambipolarity and highly π-extended conjugation.
The
intramolecular charge transfer within these D−A systems,
namely, the push−pull effect, can be readily manipulated to
yield high charge carrier mobilities. Moreover, the low
bandgaps of these co-polymers mean that the charge injection
from electrodes to their HOMO and LUMO levels is facile,
because of the low-energy offsets between the metals’ Fermi
levels and the frontier energy levels (i.e., the HOMO and
Received: April 23, 2014
Revised: June 18, 2014
©
XXXX American Chemical Society
A
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Chemistry of Materials
Article
2
5,26
37
π−π stacking at thiophene sites.
By inserting additional
1,4-dione were prepared according to literature procedures. All
solvents were further purified prior to use.
thiophene (T) or selenophene (S) units, the resultant donor
components act as spacers that permit closer intermolecular
π−π stacking. The π−π distance of the tDPP homopolymer is
Synthesis of Poly(2,5-bis(2-tetradecyloctadecyl)-3-(thio-
phen-2-yl)-6-(5′-(thiophen-2-ylethynyl)-2,2′-bithiophen-5-yl)-
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (PDPP-TAT). The poly-
mer was prepared using a palladium-catalyzed Stille coupling reaction.
2
3,24
3.75−3.82 Å,
whereas co-polymers with inserted thiophene,
bithiophene, or terthiophene units (with monomer structures
T[tDPP], TT[tDPP], or TTT[tDPP], respectively) have closer
π−π distances in the range of 3.65−3.75 Å.
facilitated intermolecular charge transport can be obtained in
thin films of tDPP-based co-polymers.
However, this approach can hinder intrachain electron
transport and, thus, eliminate the ambipolarity. Intrachain
overlap of the electron wave functions of the tDPP acceptor
units is required for ambipolarity, but the long donor
components separate these units and localize the electrons.
For example, the T[tDPP]- and S[tDPP]-structured co-
polymers contain short donors (T denotes thiophene, S
denotes selenophene) and exhibit ambipolar characteristics in
3
,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-tetradecyloctadecyl)pyrrolo-
[3,4-c]pyrrole-1,4(2H,5H)-dione (0.30 g, 0.221 mmol) and other1,2-
bis(5-(trimethylstannyl)thiophen-2-yl)ethyne (114.14 mg, 0.221
mmol) were dissolved in dry toluene (8.5 mL) and dimethyl
formamide (DMF) (1.5 mL). The monomer synthesis is described
in Supporting Information. After degassing under nitrogen for 60 min,
tetrakis(triphenylphosphine)palladium(0) Pd(pph3)4 (20 mg) was
added to the mixture and was stirred for 12 h at 90 °C. 2-
Bromothiophene and tributyl(thiophen-2-yl)stannane were injected
sequentially into the reaction mixture for end-capping, and the
solution was stirred for 6 h after each addition. The polymer was
precipitated in methanol. The crude polymer was collected by
filtration and purified by Soxhlet extraction with methanol, acetone,
hexane, toluene, and chloroform, successively. The PDPP-TAT was
obtained by precipitation in methanol. Yield: 63%. (Number-average
mass (M ) = 73 000, weight-average mass (M ) = 96 000, PDI = 1.31).
2
7−29
Thus,
2
7,30−32
FET devices,
tDPP]-, and SSS[tDPP]-structured co-polymers contain long
donors and exhibit hole-accumulated unipolar character-
whereas TT[tDPP]-, SS[tDPP]-, TTT-
[
n
w
1
H NMR (CDCl , 500 MHz), δ (ppm): δ 8.75−7.79 (br, 4H), 7.65−
3
2
8,29,33−36
7.22 (br, 2H), 6.65−6.40 (br, 2H), 4.53−4.12 (br, 4 H), 2.20−2.11
br, 2 H), 1.78−1.00 (br, 112 H), 0.92−0.85 (br, 12H).
istics.
Recently, our group reported record FET
(
mobilities in devices using the TT[tDPP]- and SS[tDPP]-
structured co-polymers: tDPP−thiophene−vinylene−thio-
phene (PDPP-TVT) and tDPP−selenophene−vinylene−sele-
1
13
Characterization. H NMR and C NMR spectra were recorded
with a Bruker Model Avance-300 spectrometer. The thermal analysis
was performed on a TA Instruments Model TGA 2100 thermogravi-
metric analyzer in a nitrogen atmosphere at a rate of 10 °C/min.
Differential scanning calorimetry (DSC) was conducted under
nitrogen on a TA Instruments Model 2100 DSC system. The sample
was heated at a rate of 10 °C/min, from 30 °C to 300 °C. UV−vis
absorption spectra were measured by a Model UV-1650PC
spectrophotometer. Molecular weights and polydispersities of the co-
polymers were determined by gel permeation chromatography (GPC)
analysis with polystyrene standard calibration (Waters Model M515).
Cyclic voltammetry (CV) was performed on an EG and G Parc Model
2
5,34
nophene (PDPP-SVS), respectively.
The introduction of
TVT or SVS as donor units with rigid vinylene (V) linkages can
extend the polymer coplanarity, and results in not only a long
intrachain π-conjugation length but also in enhanced
intermolecular π−π stacking. However, as is consistent with
previous studies of TT[tDPP]- and SS[tDPP]-structured co-
polymers, the long distance between the acceptors in the T
T[tDPP] and SS[tDPP] structures prevents electron trans-
port and results in unipolar charge transport.
2
73 Å potentiostat/galvanostat system with a three-electrode cell in a
Here, we propose a method for inducing ambipolarity in
tDPP-based co-polymers with a long donor system. A triple-
bonded acetylene (A) linkage is inserted into the long donor
component of the TT[tDPP] system to produce a final
architecture of TT[tDPP] (PDPP-TAT). The acetylene
linkage is highly rigid and, similar to the vinylene linkage,
extends the π-electrons. We found that the acetylene linkage
acts as a bridge in the delocalization of the electrons at tDPP
sites, because of the electron acceptor characteristics of its sp-
hybridized carbons. The rigid acetylene linkages produce fluent
π-electron densities with cylinder-like forms and, therefore, are
also expected to preserve the co-planarity of the thiophene
moieties, as well as enhance the intermolecular interactions.
Furthermore, the π-conjugation of the synthesized polymer is
highly insensitive to the chain conformation, which results in
facile charge transport in the amorphous regions connecting its
crystalline domains. In contrast to the T−T[tDPP]-, S−
S[tDPP]-, TT[tDPP]-, and SS[tDPP]-structured co-
polymers such as PDPP-TT, PDPP-SS, PDPP-TVT, and
PDPP-SVS, the TT[tDPP]-structured PDPP-TAT exhibits
solution of 0.1 M tetrabutylammonium perchlorate (Bu NClO ) in
acetonitrile at a scan rate of 50 mV/s. A Pt wire was used as the
4
4
counter electrode, and an Ag/AgNO (0.1 M) electrode was used as
3
the reference electrode. The morphologies and thickness of the thin
films were analyzed with a Veeco NanoScope IIIa atomic force
microscope. The thickness of the thin films are 50−80 nm. Their
molecular ordering was analyzed using two-dimensional grazing-
incidence X-ray diffraction (2D-GIXD) at the 3C Beamline of Pohang
Acceleration Laboratory. Density fiunctional theory (DFT) calcu-
lations were performed using the Gaussian 09 package with the
nonlocal hybrid Becke three-parameter Lee−Yang−Parr (B3LYP)
function and the 6-31G basis set to elucidate the HOMO and LUMO
levels after optimizing the geometry of PDPP-TAT, using the same
method. All devices were characterized using a Keithley Model 2636A
−3
SourceMeter under vacuum (10 Torr).
Device Fabrication. All procedures were performed in N -purged
2
glovebox. A highly doped n-Si wafer with a 300-nm-thick thermally
grown SiO layer was used as a substrate. A 40−45-nm-thick Cytop
2
(
Asahi Glass) was spin-coated onto it; hence, Cytop/SiO double
2
−2
layers work as the gate dielectric, whose capacitance is 8.1 nF cm at 1
MHz. In order to form well-coated PDPP-TVT or PDPP-TAT thin
films on Cytop, we accelerated solvent evaporation (chloroform)
during the spin coating. At first, the solutions and substrates were
preheated to 55 and 70 °C, respectively, on a hot plate. Next, the
substrates were transferred to a spin coating stage. They were finally
cooled to 55−60 °C due to heat transfer. Next, a warm chloroform
solution (10 mg mL at 55 °C) containing PDPP-TVT or PDPP-
TAT was deposited onto the warm substrate dropwise and spin-
coated. The spin speed was 4000−5000 rpm. Note that the spinning
was performed immediately after the solution was deposited dropwise
onto the substrate. The time interval between solution deposition and
spinning steps was found to result in a significant spatial variance of
1
3
ambipolarity in FET devices, a 10 −10 times higher electron
2
−1 −1
mobility of 0.38 cm V s , and an electron/hole mobility
ratio of ∼0.26.
−1
EXPERIMENTAL SECTION
■
Materials. All reactions were carried out in a nitrogen atmosphere,
using the usual Schlenk techniques. All chemical reagents were
purchased from Aldrich and used as received. 1,2-Di(thiophen-2-
yl)ethyne, and 3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-
B
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Chemistry of Materials
Article
a
Scheme 1. Synthetic Routes for PDPP-TVT and PDPP-TAT
a
The synthetic route for PDPP-TVT is that of ref 28. All the details of the synthesis of PDPP-TAT can be found in the Experimental Section and the
Supporting Information.
FET performance. The films were thermally annealed at 100, 150, 200,
Information). Table 1 summarizes the electrochemical and
optical absorption properties of PDPP-TVT and PDPP-TAT.
or 250 °C for 15 min in N environment. They were not dewetted and
2
maintained their homogeneity after thermal annealing. The thermally
deposited 50-nm-thick Al layer was used as source-drain electrodes;
the channel width (W) and length (L) were 1000 μm and 150 μm,
respectively.
Table 1. Energy Levels of PDPP-TVT and PDPP-TAT:
Optical Bandgap (E ), LUMO Levels (ELUMO), and HOMO
g
Levels (EHOMO) As Derived from the UV−vis Absorption
Spectra and Cyclic Voltammetry Curves
RESULTS AND DISCUSSION
■
UV−vis peak (nm)
Eg (eV)
ELUMO (eV)
EHOMO (eV)
In this paper, we synthesized PDPP-TVT as well as PDPP-
TAT, and then compared their physical, chemical, and electrical
properties. PDPP-TAT (Scheme 1) was synthesized by
performing the palladium-catalyzed Stille coupling reaction
between 1,2-bis(5-(trimethylstannyl)thiophen-2-yl)ethyne and
PDPP-TVT
a
7
80
1.22
1.50
4.15
5.37
b
7
7
96, 721
PDPP-TAT
a
7
61
3
,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-tetradecyloctadecyl)-
3.89
5.39
b
65, 692
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione with long branched
C32 alkyl groups. The syntheses of the monomers and
polymers are described in detail in the Supporting Information
and the Experimental Section. PDPP-TVT (Scheme 1) was
a
b
Solution samples. Thermally annealed thin films.
According to the UV−vis absorption spectra in Figure 1,
PDPP-TVT and PDPP-TAT thin films have optical band gaps
of 1.22 and 1.50 eV, respectively, which were calculated from
the onset absorptions of thin films of the polymers. The
HOMO levels of PDPP-TVT and PDPP-TAT estimated from
cyclic voltammograms were 5.37 and 5.39 eV, respectively.
PDPP-TAT with its sp-hybridized acetylene linkages has a
28
prepared according to an established procedure. After each
polymer had been purified by performing successive Soxhlet
extractions with acetone, methanol, and hexane to remove the
metal catalyst, it was dissolved in chloroform and precipitated
into methanol. The structures of the monomers and polymers
1
13
related to PDPP-TAT were confirmed with H NMR and C-
2
NMR (see the Supporting Information). The number-average
slightly deeper HOMO level than PDPP-TVT with sp -
38−41
molecular weight (M ) of PDPP-TAT was found to be 73.0
hybridized vinylene linkages
because of the electron-
n
kDa with a polydispersity index (PDI) of 1.31. The M of
withdrawing nature of the acetylene linkages. However, the
absorption of PDPP-TAT is blue-shifted, relative to that of
PDPP-TVT, which implies that the “push−pull” effect between
donor and acceptor moieties is somewhat disrupted by the
n
PDPP-TVT was determined to be 82.1 kDa with a PDI of 2.3.
Although PDPP-TAT has high rigidity, which is due to the
introduction of the acetylene groups, the long branched C32
alkyl chain means that it has high solubility; it is soluble in the
solvents chloroform, chlorobenzene, and dichlorobenzene at
4
0,41
acetylene linkages.
It seems possible from these considerations that the
acetylene linkages could negatively impact the intramolecular
charge transfer between tDPP and TAT. However, the actual
charge transport properties are quite different. Top-contact and
bottom-gate FETs were fabricated. To enable favorable
electron injection and achieve ambipolar FETs, aluminum
was used for the source-drain electrodes instead of gold. Cytop-
−1
concentrations of 10 mg mL .
The thermal properties of PDPP-TAT were characterized by
performing thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). A weight loss of 5% was observed
at 393 °C, and a sharp endothermic peak occurred at 250 °C
during heating. These results imply that PD PP-TAT has good
thermal endurance and is crystalline (see the Supporting
−2
coated SiO with a capacitance of 8.1 nF cm was used as the
2
C
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Chemistry of Materials
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Figure 1. Normalized UV-absorption spectra of (top) PDPP-TVT and
(bottom) PDPP-TAT.
gate dielectric. Cytop is a well-known material that forms
defect-free surfaces, so Cytop-based FETs are expected to
exhibit the near-intrinsic electrical properties of the synthesized
4
2−45
polymers.
Figure 2a shows transfer characteristic curves
(
drain current I − gate voltage V with a fixed drain voltage
D
D
G
V ) of the PDPP-TVT and PDPP-TAT FETs. To investigate
ambipolar charge transport in these FETs, both p-mode (VG,
V < 0, red squares) and n-mode (V , V > 0, blue circles)
D
G
D
sweeps were performed in the saturation region. The curve
shapes and on-current levels indicate that the PDPP-TAT FETs
produce ambipolar V-shaped transfer curves, in contrast to the
PDPP-TVT FETs, which produce unipolar L-shaped transfer
curves. The hole (μ ) and electron (μ ) FET mobilities were
h
e
obtained by using the following equation for the saturation
regime:
W
2
I =
Ciμhore(V − V )
D
G
th
(1)
2L
where W/L is the channel width/length, C is the capacitance of
i
the Cytop/SiO gate-dielectric, and V is the threshold voltage.
2
th
Figure 2. (a) Transfer characteristic (I −V ) curves of (left) PDPP-
D
G
The FET mobilities were calculated from the slopes in the |
TVT and (right) PDPP-TAT FETs. The polymer thin films were
annealed at 150 °C for 10 min. p-mode sweeps (red square) and n-
1
/2
I | − V curves (the black lines in Figure 2a). Figure 2b and
D
G
Table 2 show the hole and electron mobilities of the devices for
mode sweeps (blue circle) were performed at V = −100 and +100 V,
D
1/2
various thermal annealing (TA) temperatures. The PDPP-TVT
respectively. The black solid lines are |I | −V curves whose slopes
D
G
2
FETs exhibit high μ values with a maximum value of 5.10 cm
V
are proportional to the FET mobilities. (b) The range of electron and
hole mobilities of (left) PDPP-TVT and PDPP-TAT (right) FETs
according to the thermal annealing (TA) temperature (100, 150, 200,
and 250 °C). (c) The average electron/hole mobility ratio (μ /μ ) of
h
−1
−1
34
s , which is consistent with the results of Kang et al. and
28
Chen et al. PDPP-TVT FETs also exhibit electron channel
2
−1 −1
e
h
accumulation, but μ (0.02 cm V s ) is much lower than μ .
e
h
PDPP-TVT (gray) and PDPP-TAT (black) FETs. (d) Output
The PDPP-TAT FETs exhibit μ values that are 2−2.5 times
h
characteristic (I −V ) curves of PDPP-TAT FETs.
D
D
lower than those of the PDPP-TVT FETs with a maximum
2
−1 −1
value of 2.19 cm V s , which is consistent with our
expectation from the UV−vis spectrum. However, the PDPP-
Figure 2d)) are typical of ambipolar FETs. Both polymer FETs
exhibited almost hysteresis-free curves in p-mode sweep,
whereas they exhibited noticeable hysteresis in n-mode sweep
(see Figure S13 in the Supporting Information). Furthermore,
the PDPP-TVT FETs exhibited a rather wider hysteresis gap in
n-mode sweep than the PDPP-TAT FETs. These results
indicate that both PDPP-TVT and PDPP-TAT FETs contain
several electron trap sites, which are considered to hinder the
formation of electron-accumulated channel and increase the
threshold voltages in n-mode sweep. However, from the result
TAT devices exhibit μ values that are 15 times higher than
e
those of the PDPP-TVT FETs, with a maximum value of 0.38
2
−1 −1
cm V s . As a result, the μ /μ ratios shown in Figure 2c
e
h
are low for the PDPP-TVT FETs (∼0.008), whereas those of
the PDPP-TAT FETs are almost 30 times higher (∼0.26),
which means that the PDPP-TAT FETs are possible to form
both hole- and electron-accumulated channels with high
mobilities, in contrast to the PDPP-TVT FETs. The output
curves of the PDPP-TAT FETs (I − V , with a V step (see
D
D
G
D
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Chemistry of Materials
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Table 2. Field-Effect Electron and Hole Mobilities (μ and
e
μh, Respectively) of PDPP-TVT (up) and PDPP-TAT
(
down) FETs for Various Thermal Annealing (TA)
a
Temperatures
Electron Mobility, μ
Hole Mobility, μ
e
h
2
−1 −1
2
−1 −1
(
cm V
s
)
(cm V
s )
(average)
(max)
(average)
(max)
μe/μh
PDPP-TVT
as-spun
TA100
TA150
TA200
TA250
0.009
0.014
0.021
0.018
0.014
0.014
0.020
0.025
0.023
0.015
1.09
3.30
4.05
4.31
1.95
1.94
3.59
4.64
5.10
2.31
8.3 × 10−
4.2 × 10−
5.1 × 10−
4.2 × 10−
6.5 × 10−
3
3
3
3
3
PDPP-TAT
2.5 × 10−
1
1
1
1
1
as-spun
TA100
TA150
TA200
TA250
0.15
0.25
0.33
0.31
0.10
0.19
0.30
0.38
0.37
0.12
0.61
1.10
1.71
1.21
0.44
0.65
1.32
2.19
1.36
0.45
−
2.3 × 10
2.0 × 10−
−
2.6 × 10
2.3 × 10−
a
The ambipolarities (μ /μ ) are calculated from the average values.
e
h
of hysteresis gap, the PDPP-TAT FETs are considered to
contain less electron trap sites than the PDPP-TVT FETs.
To investigate the molecular ordering of the polymer thin
films, we compared their two-dimensional grazing incident X-
ray diffraction (2D-GIXD) patterns (see Figure 3). In the out-
of-plane patterns of both thin films, (h00) diffraction peaks are
dominant. The observed interlayer distances of (h00) are much
shorter than that of the fully extended alkyl side chains, and are
similar to each other, even though the alkyl chain lengths are
rather different: d(h00) = 21.2 Å for PDPP-TVT and 21.0 Å for
Figure 3. (a) Two-dimensional grazing-incidence X-ray diffraction
(2D-GIXD) patterns of PDPP-TVT (left) and PDPP-TAT (right)
PDPP-TAT, as calculated from their (100) peaks at q = 2.96
z
−
1
thin films. TA150 is a film post-annealed at 150 °C for 15 min. (b)
Out-of-plane (left) and in-plane (right) X-ray diffraction patterns of
PDPP-TVT (red) and PDPP-TAT (blue) thin films. The profiles were
obtained from the 2D-GIXD patterns. (100), (200), and (300)
correspond to molecular stacking in the direction of the alkyl side
chain, and (010) corresponds to π−π stacking of the main chain.
and 3.00 nm . This similarity might be due to the
interdigitation of the alkyl side chains. A side chain on one of
these polymers can easily interdigitate with the side chains of an
adjacent polymer, because of the lack of aliphatic side groups in
the donor components. Moreover, this interdigitation does not
hinder the coplanarity of the main chain. The (010) diffraction
−1
peaks are present at q = 16.82 and 17.36 nm for PDPP-TVT
xy
thought to be PDPP-TAT crystalline domains, are present in
the as-spun films and in those annealed at 100 and 150 °C, but
start to disappear above 200 °C (see Figures 4a−c). The 2D-
GIXD and AFM results and the FET mobilities suggest that
there is a strong correlation between the crystallinity and the
and PDPP-TAT, respectively, so the π−π stacking distances are
3.74 Å for PDPP-TVT and 3.62 Å for PDPP-TAT. These
values are quite small and can only be achieved when the co-
planarity of the main chain is preserved. This co-planarity is one
advantage of tDPP-based co-polymer systems with rigid
linkages. Furthermore, the smaller characteristic distance of
PDPP-TAT in both the (100) and (010) planes is thought to
originate from the cylinder-like forms of the rich π-electrons in
the acetylene linkages. Each carbon atom in the acetylene
linkages contains an additional p-orbital, which is perpendicular
to the other p-orbital. We speculate that not only do these
additional p-orbitals enrich π-electrons in the acetylene
linkages, enhancing the π−π interactions along [010], but
their direction also assists attracting neighbor molecules located
on [100]. For both the PDPP-TVT and PDPP-TAT thin films,
the diffraction peaks (h00) and (0k0) are predominantly
observed along the out-of-plane and in-plane, respectively,
which indicates that the polymers have predominantly edge-on
47−49
FET mobilities.
In general, several factors determine FET performances, such
as molecular weight, process conditions, morphology, and
crystallinity. In this research, we tried to synthesize similar
molecular weights and use the same fabrication processes in
comparing PDPP-TAT with PDPP-TVT. Besides, both
polymer thin films exhibit similar morphologies with needle-
like features, have similar d-spacing of (100) and (010), and
exhibit the same edge-on dominant structures. However,
PDPP-TAT has slightly longer side chains than PDPP-TVT,
and another quantitative information about crystallinity such as
portion and size of crystalline phases can be slightly different.
These might be the origin of the 2−2.5 times lower hole
mobility of PDPP-TAT FETs, but they are not sufficient for
our discussion of the remarkably different electron FET
mobility and ambipolarity. Note that:
46
orientations, with respect to the substrate. The intensities of
both peaks increase and become saturated with increases in the
annealing temperature, which is consistent with the variations
in μ and μ . From the height and phase profiles obtained with
(1) TT[tDPP]-structured co-polymers typically only exhibit
h
e
2
5,28,29,34,35
atomic force microscopy (AFM), needlelike features, which are
p-type FET characteristics,
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Figure 4. AFM phase images of PDPP-TAT films: (a) as-spun thin
film, (b) TA150, and (c) TA250. TA150 and TA250 are the films
post-annealed at 150 and 250 °C for 15 min, respectively. (d)
Schematic diagram of the complex microstructure of the PDPP-TAT
thin film, which consists of needle-shaped crystalline domains and
disordered amorphous regions. Tie-molecules connecting crystalline
domains are highlighted in red. The molecular orientations of the
crystalline domains do not exactly correspond to those in the diagram.
Figure 5. DFT-optimized geometries and charge-density isosurfaces
for the LUMO and HOMO levels of (a) PDPP-TVT and (b) PDPP-
TAT (B3LYP/6-31G, isovalue = 0.028). The optimized geometries of
both polymers are almost co-planar.
(
2) the π-conjugation length of PDPP-TAT is likely to be a
little shorter than that of PDPP-TVT according to the
UV−vis spectra, and, most importantly,
Thus, the results of our calculations suggest that both PDPP-
TVT and PDPP-TAT can be used to produce ambipolar FETs.
However, this conclusion assumes the presence of the
optimized co-planar state. Even though needlelike network
morphologies were observed in both PDPP-TAT and PDPP-
(
3) localization/delocalization of electron wave functions
2
1,50
also has crucial influence on electron transport.
28
TVT films, conjugated polymer chains such as PDPP-TVT
It is likely that the origin of the ambipolarity lies on the
molecular level. Several papers have explained the ambipolarity
of synthesized polymers in terms of the delocalized charge-
density isosurfaces of both their highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
and PDPP-TAT inevitably have amorphous phases and tie
molecules with many degrees of conformational freedom,
which results in complex microstructures in the solid state
(Figure 4d). The amorphous hollows in the 2D-GIXD patterns
(Figure 3a) also indicate the presence of polymeric chains in
various conformations. Very recently, Salleo and Podzorov
5
1−53
(LUMO) levels.
Hence, computational calculations using
density-functional theory (DFT, B3LYP/6-31G) have been
emphasized the importance of polymeric chains that connect
5
4
55−57
performed. A comparison of the distributions of the electron
wave functions near the vinylene and acetylene linkages is vital
to this discussion, so we constructed a model of the “tDPP−
donor−tDPP” system. Since the side alkyl chains do not
contribute significantly to the variations in the charge-density
isosurface, they were replaced by methyl groups in our
calculations. Energy optimizations were performed for PDPP-
TVT and PDPP-TAT.
crystalline domains.
Even though the co-planar forms of
PDPP-TVT and PDPP-TAT are energetically favorable, small
twists of the PDPP-TVT and PDPP-TAT monomer units are
also energetically possible via thermal energy (see Figure S14 in
the Supporting Imformation), and tie molecules cannot
maintain the co-planar state in order to connect crystalline
domains.
Hence, it is possible that some polymeric chains are present
in a tie-molecular phase (as a result of changes in chain
conformation), so we investigated in detail the charge density
isosurfaces of the HOMO and LUMO levels. Figures 6 and 7
show the charge density isosurfaces for the HOMO/LUMO
levels of PDPP-TVT and PDPP-TAT, respectively, as functions
of the dihedral angle. The vinylene and acetylene linkages are
considered to be the key positions determining ambipolarity, so
we focused on the thiophene−vinylene (T-V) and thiophene−
Figure 5 shows the DFT-calculated charge density isosurfaces
for the HOMO and LUMO levels of the two molecules. These
results show that co-planarity is favored for PDPP-TVT and
PDPP-TAT. These co-planar forms are likely to facilitate
intermolecular π−π interactions and the formation of
crystalline domains. The crystalline peaks in the 2D-GIXD
patterns and the needlelike crystalline domains probably
originate from the stacking of the optimized co-planar
molecules. However, the distributions of charge density in the
HOMO and LUMO levels cannot explain their very different
ambipolarities (μ /μ ) in FETs; the charge densities of the
acetylene (T-A) bonds; the dihedral angles (ϕ for PDPP-
T‑V
TVT and ϕT‑A for PDPP-TAT) were varied from the co-planar
state (ϕ = 0°), as shown in Figures 6a and 7a). Interestingly, we
found that the π-conjugation of PDPP-TVT is sensitive to the
chain twist (Figure 6b), whereas that of PDPP-TAT is highly
insensitive (Figure 7b) to the chain twist. In the case of PDPP-
e
h
HOMO and LUMO levels of both PDPP-TAT and PDPP-
TVT are delocalized over the entire conjugated backbones in
the co-planar state.
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Figure 6. (a) Molecular structure of PDPP-TVT model system and the definition of dihedral angle ϕ at the C1234 carbon group. The twist occurs at
thiophene−vinylene (T-V) bond. (b) Views of spirally twisted tDPP−TVT−tDPP at twist angles (ϕ) of 0°−75° (left). DFT-calculated charge-
density isosurfaces for the LUMO (middle) and HOMO (right) levels as a function of ϕ (B3LYP/6-31G, isovalue = 0.028). ϕ = 0° is the co-planar
state.
Figure 7. (a) Molecular structure of PDPP-TAT model system and the definition of dihedral angle ϕ at the C1234 carbon group. The twist was
performed at thiophene−acetylene (T-A) bond. (b) Views of spirally twisted tDPP−TAT−tDPP at twist angles(ϕ) of 0−90° (left). DFT-calculated
charge-density isosurfaces for the LUMO (middle) and HOMO (right) levels as a function of ϕ (B3LYP/6-31G, isovalue = 0.028). ϕ = 0° is the co-
planar state.
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Chemistry of Materials
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TVT, the well-distributed LUMO and HOMO isosurfaces
become monotonically localized when ϕT‑V is in the range of
0°−60° (Figure 6b), which indicates that charge transfer in the
LUMO and HOMO states is interrupted by the twisting of the
T-V bonds. In particular, note that the charge transfer at the
LUMO is more prone to be interrupted than that at the
HOMO; the delocalization of the LUMO is more sensitive to
the dihedral angle ϕ than that of the HOMO (see LUMO/
HOMO at ϕT‑V = 45°−60°). This difference arises because the
bonding interactions of the LUMO are located on the T-T and
V-T bonds and those of the HOMO are located within T and V
1
,58
(
see Figure 5).
Thus, hole transport along the intrachain is
more insensitive to the chain twist than electron transport. In
the case of PDPP-TAT, however, the well-distributed LUMO
and HOMO isosurfaces are maintained, even at high ϕT‑A
(Figure 7b). Thus, the π-conjugation and charge transfer at the
LUMO and HOMO states are highly insensitive to the twisting
of the T-A bonds. This remarkable difference between PDPP-
TVT and PDPP-TAT explains the differences between the
electron transport in their actual FET devices. In the
amorphous and tie-molecular regions within the actual thin
films, the T-V bonds in PDPP-TVT and T-A bonds in PDPP-
TAT are twisted by changes in chain conformation. In the case
of PDPP-TVT, the electron transport between the crystalline
domains along twisted chains is interrupted by diminished π-
conjugation. However, for PDPP-TAT, twisting around the
acetylene linkages does not diminish the π-conjugation, which
means that the PDPP-TAT FETs exhibit μ values 15 times
higher than those of the PDPP-TVT FETs.
e
Figure 8. p-orbitals of the carbon atoms in (a) PDPP-TVT and (b)
PDPP-TAT when ϕ ≠ 0°. The carbon atom in the vinylene linkage
The twist-insensitive π-conjugation of PDPP-TAT originates
from the sp-hybridized carbon atoms in the acetylene linkages.
Figures 8a and 8b show the p-orbitals of the carbon atoms in
PDPP-TVT and PDPP-TAT when ϕ ≠ 0°. The twists were
performed between the A carbon atom (in thiophene) and the
B carbon atom (in the vinylene or acetylene linkages). In
contains only a p -orbital, whereas that in acetylene linkage has an
z
additional p
-orbital. (c) The shifts in the internal potential energy
y
(
(
ΔE) of PDPP-TVT and PDPP-TAT as functions of the twist angles
ϕ) of thiophene−vinylene (T−V), thiophene−acetylene (T−A), and
thiophene−thiophene (T-T). The twisting of the T−V and T−T
bonds is performed to PDPP-TVT and that of T−A bond is
performed to PDPP-TAT.
PDPP-TVT, pz(A) is not present on the same plane as pz(B)
because of the twisting; hence, the interaction pz(A)−p
,
z(B)
.
becomes monotonically weakened with increases in ϕT‑V
the variations in the internal energy shifts as a function of ϕ of
the two co-polymers mean that the electron transport
properties of their amorphous and tie-molecular phases are
different. In PDPP-TVT and PDPP-TAT, most changes in
chain conformation arise as a result of twists of the T−T, T−V,
and T−A bonds. The T−T bond has a π-conjugation structure
that is similar to that of the T−V bond, so the π-conjugation of
the T−T bond is also rather unstable, with respect to twisting.
As shown in Figure 8c, twists of the T−T and T−V bonds are
energetically similar and the twist of the T−A bond is
energetically the most favorable. Based on these results, we
suggest that in PDPP-TVT both T−T and T−V bonds twist for
the conformational changes of the polymer backbones, and
intrachain electron transport is gradually interrupted at these
sites. In the case of PDPP-TAT, however, most changes in
conformation arise through the twists of T−A bonds and slight
twists of T−T bonds. As a result, the reductions in electron
transport along the T−T bonds of PDPP-TAT in the
amorphous and tie-molecular phases are less upon changes in
conformation.
However, in PDPP-TAT, each carbon atom in the acetylene
linkage has one more p-orbital (py(B)), so the pz(A)−py(B)
interaction increases with ϕT‑A. This combination of pz(A)
−
pz(B) and pz(A)−p
interactions means that the π−π
y(B)
interactions are sustained, regardless of variation in the ϕ of
PDPP-TAT.
Actually, high twist angles such as ϕ = 45°−90° are not
energetically possible. Under these low twist angles of the T−V
bonds in PDPP-TVT, the HOMO and LUMO isosurfaces can
maintain their delocalization in a monomer unit, but the
delocalization of the LUMO must be more weakened than that
of the HOMO. This difference will be noticeably increased as
the number of the twisted monomer unit increases. Because a
large twist of a polymeric chain in tie-molecular regions can
arise as the sum of the small twists of the monomer units, the
LUMO isosurfaces in tie-molecular regions lose their
delocalization in, whereas the HOMO isosurfaces can still
maintain their delocalization. It results in unfavorable electron
transport between the crystalline domains and low values of μe
and μ /μ for the PDPP-TVT FETs. In contrast, twists of the
The presence of ambipolarity in the PDPP-TAT FETs
prompted us to investigate the properties of CMOS-like
inverters consisting of two identical ambipolar FETs with a
e
h
T−A bonds do not significantly affect the intrachain electron
transport, and, thus, the electron transport between crystalline
domains is much more favorable in PDPP-TAT, with high
values of μ and μ /μ . Even though further in-depth analysis
common gate as the input voltage (V ) (Figure 9a). Figures 9b
in
and 9c show the output voltage (V ) as a function of the input
e
e
h
out
and study are required, we suggest that the differences between
voltage (V ) at a constant supply bias (VDD). The slopes of the
in
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Figure 9. (a) Circuit diagram of the complementary inverter structure based on two identical PDPP-TAT FETs. (b, c) CMOS-like inverter
characteristics of the inverter.
inverter curve indicate maximum gains of ∼10 for the p-mode
and ∼7 for the n-mode. The n-mode exhibits smaller gain and
more unstable operation, which might originate from electron
charge trapping during operation, the origin of which remains
to be explained.
ACKNOWLEDGMENTS
■
This work was supported by a grant (Code No.2011-0031628)
from the Center for Advanced Soft Electronics under the
Global Frontier Research Program of the Ministry of Science,
ICT & Future Planning, Korea.
CONCLUSION
■
ABBREVIATIONS
In summary, we have demonstrated that a tDPP-based co-
polymer with a TT[tDPP] structure can exhibit ambipolarity.
The synthesized co-polymer PDPP-TAT contains acetylene
linkages within the donor components, which are more
electron-withdrawing than vinylene or ethane linkages. Even
though the push−pull effect of PDPP-TAT is somewhat weaker
than that of PDPP-TVT, the intrachain electron transport of
PDPP-TAT is much more insensitive to the chain con-
formation than that of PDPP-TVT. As a result, PDPP-TAT
exhibits much more favorable electron transport in its
amorphous and tie-molecular phases and its FET devices
■
DPP, diketopyrrolopyrrole
tDPP, dithienyl-diketopyrrolopyrrole
T, thiophene
S, selenophene
V and =, vinylene linkage
A and ≡, acetylene linkage
T-V, single bond between thiophene and vinylene linkage
T-A, single bond between thiophene and acetylene linkage
T-T, single bond between thiophenes
2
−1 −1
exhibit a high electron mobility (0.38 cm V s ) and a high
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Corresponding Authors
E-mail: skwon@gnu.ac.kr (S.-K. Kwon).
E-mail: ykim@gnu.ac.kr (Y.-H. Kim).
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