High charge carrier mobility, low band gap donor-acceptor benzothiadiazole-oligothiophene based polymeric semiconductors

Article abstract of DOI:10.1021/cm3021929

A series of benzothiadiazole oligothiophene and oligo(thienylene vinylene) donor-acceptor (D-A) copolymers were synthesized and characterized. These low optical band gap materials (～1.5 eV) are capable of absorbing photons in the range of 400-800 nm and exhibit good thermal stability. Their hole mobilities, determined using an organic field-effect transistor (OFET) architecture, vary over a range of 3 orders of magnitude and strongly correlate with the molecular ordering and morphology of the respective thin films. Spin-coated films of the poly(benzothiadiazole-sexithiophene) PBT6, which exhibits a highly crystalline lamellar π-π stacked edge-on orientation on the OFET substrate, possesses a hole mobility of ca. 0.2 cm²/V·s. Vinylene-containing analogs PBT6V2 and PBT6V2′ are amorphous and exhibit very low mobilities. The molecular weight of PBT6 has a strong influence on the electronic properties: a sample with a lower molecular weight exhibits a mobility approximately 1 order of magnitude lower than the high molecular weight homologue, and the absorption maximum is appreciably blue-shifted. The hole mobility of PBT6 is further enhanced by a factor of ca. 3 through fabrication of the OFET by drop casting. OFETs fabricated by this process exhibit mobilities of up to 0.75 cm²/V·s and I_ON/OFF ratios in the range of 10⁶-10⁷. These results demonstrate the potential of incorporating benzothiadiazole units into polythiophene derivatives to develop high-mobility semiconducting polymers.

Full text of DOI:10.1021/cm3021929

Article
pubs.acs.org/cm
High Charge Carrier Mobility, Low Band Gap Donor−Acceptor
Benzothiadiazole-oligothiophene Based Polymeric Semiconductors
Boyi Fu,† Jose Baltazar,† Zhaokang Hu,†,+ An-Ting Chien,‡ Satish Kumar,‡ Clifford L. Henderson,†
David M. Collard,§ and Elsa Reichmanis*,†,‡,§
†School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332-0100,
United States
‡School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, Georgia 30332-0245,
United States
§School of Chemistry & Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332-0400,
United States
S
* Supporting Information
ABSTRACT: A series of benzothiadiazole oligothiophene and oligo(thienylene vinylene) donor−acceptor (D−A) copolymers
were synthesized and characterized. These low optical band gap materials (∼1.5 eV) are capable of absorbing photons in the
range of 400−800 nm and exhibit good thermal stability. Their hole mobilities, determined using an organic field-effect transistor
(OFET) architecture, vary over a range of 3 orders of magnitude and strongly correlate with the molecular ordering and
morphology of the respective thin films. Spin-coated films of the poly(benzothiadiazole-sexithiophene) PBT6, which exhibits a
highly crystalline lamellar π−π stacked edge-on orientation on the OFET substrate, possesses a hole mobility of ca. 0.2 cm2/V·s.
Vinylene-containing analogs PBT6V2 and PBT6V2′ are amorphous and exhibit very low mobilities. The molecular weight of
PBT6 has a strong influence on the electronic properties: a sample with a lower molecular weight exhibits a mobility
approximately 1 order of magnitude lower than the high molecular weight homologue, and the absorption maximum is
appreciably blue-shifted. The hole mobility of PBT6 is further enhanced by a factor of ca. 3 through fabrication of the OFET by
drop casting. OFETs fabricated by this process exhibit mobilities of up to 0.75 cm2/V·s and ION/OFF ratios in the range of 106−
107. These results demonstrate the potential of incorporating benzothiadiazole units into polythiophene derivatives to develop
high-mobility semiconducting polymers.
KEYWORDS: conjugated polymers, hole mobility, organic field-effect transistors
low charge carrier mobility, low ambient stability, and a relatively
large band gap compared to their inorganic counterparts.1a,2
These deficiencies have limited the commercialization of such
conjugated polymers. From a molecular perspective, the charge
carrier transport properties and band gap are determined by
intra- and intermolecular π-electronic coupling, while stability in
air may be enhanced by incorporating electron-deficient groups
1. INTRODUCTION
Organic π-conjugated polymers have attracted significant
attention due to their potential as semiconducting materials in
optoelectronic devices, such as organic field-effect transistors
(OFETs) and organic photovoltaic cells (OPVs).1 In compar-
ison to their oligomeric counterparts, polymeric materials
possess a combination of properties that are advantageous for
the fabrication of large-area, lightweight, and low-cost flexible
plastic electronics.2 Although significant progress has been made,
polymeric semiconductors, such as poly(3-hexylthiophene),
P3HT,3 and poly(2,5-thienylene vinylene)4 typically exhibit
Received: July 12, 2012
Revised: October 9, 2012
Published: October 9, 2012
© 2012 American Chemical Society
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Figure 1. Molecular structures of benzothiadiazole-oligothiophene D−A copolymers (PBT4, PBT6, PBT6V2, and PBT6V2′).
Scheme 1. Preparation of Dibromide and Bis(stannane) Monomers M1−M5
that lead to a reduction in electron density (and thereby lower
the energy of the highest occupied molecular orbital, HOMO).5
The electron-deficient benzothiadiazole (BT) unit has recently
been explored in the development of organic semiconductors for
optoelectronic applications.6 In this study, we incorporated the BT
acceptor into electron-rich (donor) polythiophene and poly-
(thienylene vinylene) chains with the aim of addressing the
deficiencies associated with current materials. Four such donor−
acceptor (D−A) copolymers have been synthesized and charac-
terized: poly(benzothiadiazole-quaterthiophene) PBT4, poly-
(benzothiadiazole-sexithiophene) PBT6, and poly(dithieno-benzo-
thiadiazole-vinylene-quaterthiophene)s PBT6V2 and PBT6V2′ as
shown in Figure 1 (the latter pair of polymers differ in the number of
dodecyl substituents in each repeat unit). The D−A character is
expected to support intramolecular charge transfer (ICT) from the
electron-donating oligothiophene units to the electron-accepting
BT units. This should result in a decrease in electron density
compared to analogous polythiophenes, and thereby improve the
ambient stability of the polymers. This feature should also narrow
the band gap, thus extending the photon absorption range to a
higher wavelength region relative to polythiophene, which is a
crucial factor in the development of efficient OPVs.7 In addition, the
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Scheme 2. Preparation of Bisphosphonate and Dialdehyde Monomers M6−M8
planar nature of the fused heteroaromatic BT, in combination with
the match between the electronic structures of BT and the donor
segments, might facilitate π−π intermolecular interactions between
adjacent polymer chains and benefit intermolecular charge carrier
mobility.8
The design and syntheses of these BT-oligothiophene polymers
are described, along with their optical, thermal, morphological, and
electronic properties. The hole transport performance of these
polymers correlates with molecular order within the films
(crystallinity and orientation), molecular configuration (presence
of side chains and vinylene linkages), and molecular weight. Spin-
coated semicrystalline PBT6 films exhibit hole mobilities of
ca. 0.2 cm2/V·s in an OFET configuration. Fabrication of PBT6
films by drop casting resulted in a further increase in hole mobility
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Scheme 3. Polymerzations of PBT4, PBT6/PBT6(L), PBT6V2, and PBT6V2′
by a factor of ca. 3, up to 0.75 cm2/V·s, and ION/OFF ratios in the
range of 106−107.
Stille polymerization, BDTDTB was subjected to bromination with
N-bromosuccinimide (NBS) to afford dibromo monomer M2, and to
metalation to give distannane M3. Bithiophene monomers M4 and
M5 were prepared according to literature procedures (Scheme 1).2a,10
We envisioned preparing the vinylene polymers PBT6V2 and
PBT6V2′ by Horner−Wadsworth−Emmons polymerizations
of dialdehyde monomer M8 with bisphosphonate monomers
M6 and M7, respectively. Dialdehyde 4 was prepared by a
Suzuki coupling of 4,7-dibromobenzo[c][1,2,5]thiadiazole and
5-formylthiophene-2-boronic acid (Scheme 2). Dialdehyde 4
was reduced to diol 5 upon treatment with NaBH4; 5 was then
converted to bis(chloromethyl) compound 6 upon reaction with
SOCl2. A Michaelis−Arbuzov reaction of 6 with triethyl
phosphite afforded bisphosphonate monomer M6. To construct
bisphosphonate M7, dithienobenzothiadiazole 8 was subjected
to a Vilsmeier−Haack reaction to afford dialdehyde 9,11 which
was converted into the bisphosponate monomer in three steps.
Monomer M4 was subjected to Stille coupling with (4-dodecyl-
2-thienyl)tributylstannane (7) to afford quarterthiophene 12,
and a Vilsmeier−Haack reaction provided dialdehyde monomer
M8 (Scheme 2). Attempts to brominate 12 to give monomer M9
led to the formation of inseparable byproducts as a result of
reaction at the β-carbon of the thiophene rings.
2. RESULTS AND DISCUSSION
2.1. Polymer Design and Synthesis. To evaluate the
impact of the ratio of electron-rich thiophene rings to electron-
deficient BT units on the optical and electronic properties of the
resultant polymers, PBT4 and PBT6 were designed with four
and six thiophene units per BT in each repeat unit, respectively.
PBT6 possesses two additional dodecyl chains per repeat unit
relative to PBT4, which should favorably impact polymer
solubility. In comparison to PBT6, PBT6V2 has an electron-rich
vinylene linkage connecting quaterthiophene and dithienoben-
zothiadiazole (DTB) units that might elevate the electron-
donating properties of the donor segment and thereby favor D−A
interactions.9 The incorporation of side chains at the 4-positions of
the thiophene rings of the DTB units of PBT6V2 to provide
PBT6V2′ is expected to further improve polymer solubility.
Synthetic routes leading to the four polymers are outlined in
Schemes 1−3. The synthesis of PBT6 made use of 4,7-bis(3′-
dodecyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole,
BDTDTB, which was synthesized by Pd(0)-catalyzed Stille coupling
of 2-thienyltributylstannane to 4,7-dibromobenzo[c][1,2,5]-
thiadiazole to afford 1, followed by metalation to afford distannane
M1 and subsequent coupling with 2 (Scheme 1). In preparation for a
PBT4 was prepared by the Stille polymerization of bis-
(stannane) monomer M1 and dibromide monomer M4. Two
routes were explored for the preparation of PBT6: Stille
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Table 1. Photophysical (Absorption and Emission), Electrochemical Properties, and Molecular Weights of Benzothiadiazole-
oligothiophene Polymers and Oligomer
a
λabs (nm)
b
c
d
e
f
sample
PBT4
solution
film
λem (nm)
Eopt (eV)
Eox/HOMO (eV)
Mn (kDa)
Mw (kDa)
PDI
403, 569
428, 535
512, 588
473, 582
348, 499
456, 656
495, 677
560, 622
512, 621
380, 560
699
666
730
743
650
1.52
1.54
1.53
1.53
1.79
1.77/−5.24
1.80/−5.27
1.60/−5.07
1.72/−5.19
1.79/−5.26
5.8
9.7
1.67
2.02
3.13
2.98
PBT6
18.1
11.3
14.3
36.5
35.5
42.5
PBT6V2
PBT6V2′
BDTDTB
a
b
c
Wavelength of peak maxima in UV/vis spectra. Wavelength of maxima in fluorescence spectra (excitation wavelength, λex = 450 nm). Optical
d
band gap estimated from onset point (λonset) of absorption band of thin film: Eopt = 1240/λonset
. Eox was measured in 0.1 M solution of
e
f
[n-Bu4N]+[PF6]− in acetonitrile using CcPF6 as the reference. Number-average molecular weight. Weight-average molecular weight.
copolymerization of (i) bis(stannane) M3 with dibromide M4 and
(ii) dibromide M2 with bis(stannane) M5. Gel permeation
chromatography (GPC) indicated that the molecular weight
(MW) of PBT6 prepared through cross coupling of M3 with M4
(number-average molecular weight, Mn: 18.1 kDa, degree of
polymerization, DP = 13.9) is approximately twice that achieved
from the coupling of M2 and M5 (labeled as PBT6(L), Mn:
8.6 kDa, DP = 6.6). The lower MW of PBT6(L) may be attributed
to the low purity of M5 (which contained ∼14% of the monostannyl
1
derivative of 3, as determined by H NMR spectroscopy) that
impedes formation of high MW polymer during the step-growth
polymerization (Scheme 3). The results of GPC characterization of
PBT4, PBT6V2, and PBT6V2′ are shown in Table 1.
2.2. Photophysical Properties. The optical absorption
spectra of the copolymers (PBT4, PBT6, PBT6V2, and PBT6V2′)
and oligomer BDTDTB are shown in Figure 2. In solution
(Figure 2a), the copolymers exhibit absorption bands at signi-
ficantly higher wavelengths than BDTDTB, thereby demonstrat-
ing an extension of the conjugation length that results from
intramolecular electron delocalization. The same tendency is
apparent upon comparison of the spectra of homologues 4,7-
bis(thien-2-yl)-2,1,3-benzothiadiazole (DTB) (λabs = 308, 446 nm),
oligomer BDTDTB (λabs = 348, 499 nm), and polymer PBT4
(λabs = 403, 569 nm), as shown in Figure S1 (Supporting
Information). A bathochromic shift is also apparent for all five
materials in going from solution (Figure 2a) to thin films
(Figure 2b), indicating enhanced order in the solid state that
may be ascribed to interchain π−π electronic coupling and/or
planarization of the copolymer backbone.12 All of the
materials exhibit two absorption bands, as shown in Table 1.
The bands appearing at the lower wavelength (300−550 nm)
correspond to the π−π* transition of the oligothiophene (in
PBT4, PBT6, and BDTDTB) or oligo(thienylene vinylene)
(in PBT6V2 and PBT6V2′) unit.13 For instance, the CHCl3
solution bands at λabs = 403 and 428 nm for PBT4 and PBT6,
respectively, correspond to the typical absorption bands of
α-quaterthiophene (λabs = 380−396 nm) and α-sexithiophene
(λabs = ca. 426 nm) derivatives.14 We propose that the bands at
higher wavelengths (400−800 nm) result from ICT between
the electron-deficient BT and electron-rich oligothiophene units.
This conjecture is confirmed through observation of solvato-
chromism in the fluorescence and absorption spectra.15 For
example, solutions of PBT6 in solvents having different polarities
and polarizabilities (Table S1) exhibit shifts in λem and λabs
(Figure 3), indicative of the charge transfer nature of this peak.
The ICT results in a narrowing of the optical band gap (Egopt).
The D−A BT-containing conjugated copolymers prepared here
possess lower optical band gaps as calculated from the onsets of
absorbance (ca. 1.5 eV, Table 1), than polythiophenes (1.9−2.1 eV)16
Figure 2. Absorption spectra of PBT4, PBT6, PBT6V2, PBT6V2′, and
BDTDTB in (a) chloroform (CHCl3) solution and (b) as a thin film; and
(c) the absorption spectra of PBT6 (high MW, Mn: 18.1 kDa and PDI:
2.02) and PBT6(L) (low MW, Mn: 8.6 kDa and PDI: 1.72) in solution
(PBT6_Sol, PBT6L_Sol) and solid film state (PBT6_Film, PBT6L_Film) .
and poly(thiophene vinylenes) (1.7 eV).17 The absorption bands of
these new D−A polymers are well matched to harvest photons
throughout the visible region (400−800 nm, Figure 2b), which
makes them particularly interesting candidates for OPV applications.
Additional absorption shoulders were observed at 670 and
690 nm in purple CHCl3 solutions of PBT4 and PBT6 (Figure 2a)
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Figure 3. Shifts in emission [(a), ca. 2 × 10−6 M] and absorption spectra [(b), ca. 3.8 × 10−5 M] of PBT6 in chloroform (CHCl3), chlorobenzene (CB),
1,2-dichlorobenzene (DCB), and 1,2,4-trichlorobenzene (TCB).
at room temperature. These features vanish upon heating and re-
emerge upon cooling (Figure S2 and S4); a similar phenomenon
exhibited by P3HT and related conjugated polymers arises from the
presence of aggregates.3d,13a,18 A thermochromic effect was also
observed during the heating−cooling cycles in which λabs blue-
shifted and shoulders vanished upon heating to afford a pink solution
while λabs red-shifted and shoulders reappeared upon cooling,
returning to a purple solution (Figure S2−S4). The observed aggre-
gates are stable in solution at room temperature and are not affected
by dilution. For example, Figure S5 shows that the shoulder observed
in PBT6 is apparent even in a very dilute solution (10−6 M) and that
the relative absorbance of the shoulder in comparison to the two
main bands remains constant. Compared with PBT6, the low MW
counterpart, PBT6(L) (Mn: 8.6 kDa), displays a much weaker
shoulder in the solution absportion spectrum (Figure 2c), suggesting
more limited aggregation in solution likely due to the increased
solubility relative to the higher MW material. In the solid state, the
λabs of PBT6 is red-shifted by ca. 56 nm relative to PBT6(L)
(Figure 2c), suggesting that the increased MW enhances electron
delocalization within the system. Similar effects have been observed
in P3HT.19 We postulate that the higher MW leads to a decrease
in the number of defects originating from chain ends embedded
within crystalline grains, thus benefiting chain ordering.20
The impact of incorporation of a vinylene linkage within the
polymeric backbone on the conjugation length can be seen from the
spectra in Figure 2a. In solution, PBT6V2 exhibits a red-shifted
absorption compared to PBT6. The introduction of the vinylene
moiety might promote D−A ICT due to its electron-donating
properties.9 However, PBT6V2′ displays a blue shift in the
absorption compared to PBT6V2, which may be attributed to the
two dodecyl chains at the 4-positions of the two thiophenes within
the DTB unit that induce dihedral torsion between the thiophene
and vinylene moieties in PBT6V2′ and thus shorten the
conjugation length.7d,21 In the solid state, the vinylene linkage has
little effect on the absorption maxima, suggesting that the effect of
vinylene on the conjugation length is offset by interchain π−π
interactions. A similar phenomenon is seen upon comparison of a
poly(quarterthiophene)22 and a vinylene-containing analog23 in
which the difference in absorption maxima is much larger in solution
than in the solid state. All the polymers investigated here show
strong photoluminesence with λem in the range of 666−743 nm
(Table 1), representing a red shift compared to that of the oligomer
BDTDTB (λem = 648 nm, Figure S6).
and +1.80 V (versus bis(cyclopentadienyl)cobalt(III) hexafluo-
rophosphate, CcPF6) corresponding to HOMO energy levels
at −5.24 and −5.27 eV, respectively. P3HT exhibits a HOMO of
ca. −5.17 eV under identical conditions, and poly(octylthiophene)s
possess HOMOs in the range of −4.97 to −5.07 eV,24 thereby
indicating the impact of the electron-deficient BT units in lowering
the HOMO energy levels of the copolymers. The observation of
lower HOMO energy levels for PBT4 and PBT6 may allow for
enhanced ambient stability relative to the polythiophene.5e,6b,23
PBT6V2, having the additional vinylene linkage in the polymer
backbone, exhibits an increased HOMO energy (−5.07 eV)
compared to the PBT4 and PBT6 analogs. This result can be
attributed to the incorporation of the vinylene moiety that raises the
electron density due to its electron-donating properties.9,22a,25
PBT6V2′, however, has a lower HOMO of −5.19 eV compared
with PBT6V2. This may be a result of side chain induced steric
hindrance (between the dodecyl group at C-4 of the thiophenes in
DTB and the vinylene linkage) that shortens the conjugation length
and thus offsets the effect of the vinylene unit. No evident reduction
peaks were observed in the cyclic voltmmograms, suggesting that all
these polymers belong to the class of p-type (hole transport)
semiconductors.
2.4. Thermal Properties. The thermal characteristics were
evaluated using thermogravimetric analysis (TGA) and differ-
ential scanning calorimetry (DSC, Table 2). The decomposition
a
Table 2. Thermal Decomposition Temperatures (Td ) and
b
c
Thermal Transition Temperatures (Tm and Tc ) of Polymers
d
and Oligomer
heating
Tm2/°C
cooling
Tm1/°C
Tc1/°C
Tc2/°C
(ΔHc2,
J/g))
(ΔHm1
,
(ΔHm2
,
(ΔHc1,
sample
PBT4
Td /°C
J/g)
J/g)
J/g)
461
438
410
351
438
283 (2.6)
249 (3.7)
202 (9.5)
PBT6
45 (7.8) 200, 217 (9.7) 34 (7.4)
PBT6V2
PBT6V2′
BDTDTB
a
b
Temperature corresponding to 5% mass loss. Melting temperature
c
d
on heating. Crystallization temperature on cooling. Values in
parentheses are transition enthalpies (J/g).
2.3. Electrochemical Properties. Cyclic voltammetry (CV,
Table 1) was used to analyze the redox properties of the
materials. PBT4 and PBT6 have oxidation potentials of +1.77
temperatures (Td), defined as the temperature associated with
5% weight loss, are in the range of 350−450 °C, indicative of high
thermal stability. DSC studies on PBT4 (shown in Figure 4a)
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on heavily doped Si) that had been pretreated with
octadecyltrichlorosilane (OTS-18). Samples were tilted at χ =
90° and 30° to study the X-ray pattern along the qz (out-of-
plane) and qxy (in-plane) axes. PBT6 exhibited (h00) peaks at
2θ = 4.52° (100), 9.08° (200), 13.52° (300), and 18.0° (400) along
the qz axis corresponding to a lamellar d-spacing of 19.53 Å
between polymer chains segregated by dodecyl side chains, and
(010) peak at 2θ = 24.16° along the qxy axis arising from π−π
stacking (3.68 Å) (Figure 5c,d). Unlike P3HT,27 the variation of
intensity at 2θ = ca. 24° along the γ axis indicates a high intensity
of the (010) peak in the in-plane direction, and a very low
intensity in the out-of-plane direction (Figure 5e), suggesting the
formation of a highly ordered, lamellar structure with an edge-on
orientation on the substrate.28 In light of the length of a dodecyl
chain (∼15 Å), the interlamellar spacing of PBT6 is consistent
with an interdigitated packing mode of the side chains. PBT4
exhibited a peak at 2θ = 3.3° along the qz axis corresponding to a
lamellar spacing of 26.91 Å (Figure 5a,b). The peak at 2θ =
24.40° corresponding to the π−π stacking distance (3.64 Å) is
barely perceptible, implying less crystallinity than PBT6. The
largely lamellar spacing of PBT4 (26.91 Å) suggests an end-to-
end mode of side-chain packing, similar to that of poly(3-
dodecylthiophene) having a d-spacing of ∼27.21 Å.3c In contrast
to PBT6, PBT4 exhibits a largely arcing diffraction for lamellar
d-spacing and its π−π stacking peaks are equally weak in both the
in-plane and out-of-plane orientations (Figure S11c,d), signifying no
preferential orientation. PBT6 thus appears to adopt a much more
highly oriented configuration on the substrate than PBT4. Two-
dimensional transmission wide-angle X-ray diffraction (2D-WAXD)
of a PBT6 drop-cast film indicates a π−π stacking distance of 3.65 Å
(Figure S12), comparable to the 2D-GIXS result.
The intensities of lamellar spacing and π−π stacking peaks of
PBT4 were clearly enhanced by thermal annealing of a film at
200 °C for 4 h, as shown by one-dimensional X-ray diffraction
(1D-XRD, Figure S13). For PBT6, a highly crystalline XRD
pattern is obtained on as-deposited films without the need for
extended annealing (Figures 5 and S14). As observed during
DSC analysis, PBT6 undergoes side-chain recrystallization at
Tc1 = 34 °C, which is close to room temperature and hence the
side chains in PBT6 may have sufficient thermal energy to
facilitate assembly into a highly crystalline structure. PBT6 was
annealed at temperatures from 75 °C up to 250 °C (i.e., within
mesophase and isotropic phase) followed by subsequent cooling
to room temperature. The corresponding 1D-XRD patterns
measured at room temperature (Figure S14) indicate that the
high crystallinity of PBT6 is maintained after annealing up to
170 °C, clearly decreased after annealing at 200 °C and could not
be detected after annealing at 250 °C, suggesting that the order
is retained in the solid and mesophases (<180 °C) and that
the high-temperature transition in the DSC corresponds to
melting to an isotropic phase. PBT6V2 and PBT6V2′ display no
XRD patterns, consistent with an amorphous structure as
previously suggested by the lack of thermal transitions in the
DSC analysis.
Figure 4. Thermal transitions in DSC: (a) PBT4, and (b) PBT6, in the
second heating/cooling scan at 10 °C/min.
show the presence of one endothermic peak upon heating, with
one exothermic peak during the cooling process, which are
associated with melting (Tm = 283 °C) and recrystallization (Tc =
249 °C), respectively. Interestingly, PBT6 exhibits three
endothermic peaks (Tm1 = 45 °C; Tm2 = 200 °C and Tm2′
=
217 °C) upon heating and two exothermic peaks (Tc1 = 34 °C
and Tc2 = 202 °C) during the cooling process (Figure 4b),
suggesting the existence of a mesophase between Tm1 and Tm2.
The temperature below Tm1 corresponds to a solid phase while
the temperature above Tm2 corresponds to an isotropic phase. As
has been suggested for poly(3-dodecylthiophene), P3DDT,26
Tm1 and Tc1 may represent the melting and crystallization of the
dodecyl side chains of PBT6, while Tm2, Tm2′, and Tc2 correspond
to the main chain melting and crystallization, respectively.
Polarized optical microscopy (POM) demonstrates that a spin-
coated film of PBT6(L), which melts at ca. 200 °C, displays a
birefringent pattern (Figure S10) at approximately 80 °C upon
subsequent cooling. The birefringence remains, even after
cooling to room temperature. No thermal transitions were
observed for PBT6V2, PBT6V2′, and BDTDTB, suggesting
that thin films of these materials are amorphous.
The surface morphologies of the polymer thin films were
evaluated using atomic force microscopy (AFM) in the tapping
mode. Nanofiberous features with a width of 40−50 nm and
length of 200−300 nm were observed for as-spun films of PBT6
(Figure 6c,d) consistent with a high degree of crystallinity as
observed through GIXS and XRD measurements. This
morphology is largely lost upon annealing at 250 °C followed
by subsequent quenching to room temperature, which correlates with
the loss of diffraction peaks in the XRD after annealing at 250 °C.
2.5. Thin Film Crystallinity and Morphology. The
microstructures of polymer films were investigated by two-
dimensional grazing incidence X-ray scattering (2D-GIXS).
Polymer films were prepared by drop casting chloroform
solutions of the polymers onto Si wafers (300 nm SiO2 dielectric
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Figure 5. 2D-GIXS area detector images of PBT4 drop-cast film (annealed at 200 °C for 4 h) at (a) χ = 30° and (b) χ = 90°; PBT6 drop-cast film
(without annealing) at (c) χ = 30° and (d) χ = 90°. The integrated intensity diffraction pattern corresponding to PBT6 drop-cast film at χ = 30° is shown
in (e); the integrated diffraction patterns for (a), (b), and (d) are shown in Figure S12. The intensity variation scanning at 2θ = ca. 24° along the γ axis
from qz (out-of-plane) to qxy (in-plane) is shown in (c) and (f). The scheme of the sample tilting method is also in Figure S12.
PBT4 films display fewer surface features (Figure 6a,b), and
PBT6V2 and PBT6V2′ are largely amorphous. In combination, the
aggregation was observed in this solvent, in contrast to solutions
in CHCl3 and CB (Figure 3b); TCB was not considered because
its high boiling point severely limits the efficiency of solvent
evaporation. Polymer solutions in DCB (8 mg/mL) were spin-
cast onto OTS-18 pretreated SiO2 dielectric (300 nm) grown
onto heavily doped Si substrates. The channel between gold
source and drain electrodes was 50 μm long and 2 mm wide. The
output and transfer curves for devices fabricated with PBT6 and
PBT4 are shown in Figure 7. PBT6 exhibits a hole mobility (μh)
of 0.2 cm2/V·s, which is ca. 1 order of magnitude higher than its
low MW counterpart, PBT6(L) (0.029 cm2/V·s, Table 3),
indicating the impact of MW on charge carrier transport. Figure
S17 demonstrates that drop-cast films of PBT6 are smoother and
more uniform than those of PBT6(L), supporting the premise
that increased MW facilitates formation of more uniform thin
films, a key factor associated with processing and application of
polymer thin film technologies. PBT4 exhibits a moderate
mobility of 0.01 cm2/V·s, ascribable to its lower crystallinity
compared to PBT6, which may originate from its low MW.
Higher MW analogs could not be obtained due to the limited
solubility of the polymer in the polymerization solvent. The
solubility of PBT4 might be improved by incorporating longer
branched alkyl side chains. Recently reported polymers with the
same main-chain structure as PBT4, but substituted with long
branched 2-octyldodecyl appendages, possess higher MW and
mobilities of 0.02−0.2 cm2/V·s.6b,28c The amorphous vinylene-
containing counterpart PBT6V2 exhibits a low μh of 10−4 cm2/V·s,
and no appreciable field-effect hole or electron transport was
detected for PBT6V2′.
Figure 6. Tapping mode AFM phase images of PBT4, (a) and (b); and
PBT6 (c) and (d). Films were prepared by spin-coating 8 mg/mL
solutions of polymer in DCB onto OTS-18 pretreated Si substrates.
results of analyses by DSC, XRD, and AFM indicate that the
degree of molecular ordering in a polymer thin film architecture
for this series of D−A copolymers follows the sequence: PBT6 >
PBT4 > PBT6V2 ∼ PBT6V2′.
The differences in the hole mobilities are consistent with the
thin film morphologies of the respective polymers. The much
higher degree of molecular ordering of PBT6 and PBT4 relative
to PBT6V2 and PBT6V2′ improves their ability to undergo
2.6. Field-Effected Mobility Characterization. Bottom
gate, bottom contact OFET devices were used to evaluate the
charge transport properties of the D−A copolymers (Figure S15).
DCB was selected as the processing solvent because no
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Figure 7. Output (up row) and transfer (down row, VSD = −80 V) characterizations of OFETs (channel size: 50 μm long × 2 mm wide) based on spin-
coated PBT4 films (a, b), spin-coated PBT6 films (c, d), and drop-cast PBT6 films (e, f).
isotropic phase (as determined by DSC, Figure S18). The values
of μh and current on/off ratio (ION/OFF) measured at room
temperature increased from 0.14 cm2/V·s and 104, respectively,
for samples that were not annealed, up to 0.25 cm2/V·s and 106
after annealing at 70 °C (i.e., within mesophase). However, these
values decreased to 0.075 cm2/V·s and 104 upon annealing in
the isotropic phase (250 °C). This result is consistent with
the observed decrease in crystallinity of PBT6 through XRD
(Figure S14) and AFM (Figure S19) analysis. The threshold
voltage ranged from 4.7 to −18 V upon annealing and no obvious
hysteresis was observed (Figure S20).
The hole transport properties of PBT6 were further
investigated using films drop-cast from DCB. Films formed at
room temperature exhibited a mean value for μh of 0.17 cm2/V·s.
After thermal annealing at 80 °C for 24 h, μh was increased by a
factor of ∼3 (mean 0.5 cm2/V·s) with a maximal value of μh as
high as 0.75 cm2/V·s and ION/OFF of 106 −107.
Table 3. Hole-Transport Properties of Polymers Spin-Coated
(Except “PBT6 (Drop Casting)”) onto OTS-18 Pretreated
OFETs Substrates (Channel Size: 50 μm Long × 2 mm Wide)
hole mobility,
threshold voltage,
a
polymers
μh (cm2/V·s)
ION/OFF
VT (V)
b
PBT4
PBT6
0.01
103
−4.9
−2.3 to −3.2
−0.5 to −5.5
c
0.2 (0.25)
106
d
PBT6 (Drop Casting)
PBT6(L)
0.5 (0.75)
106−107
e
0.029
2.2 × 10−4
105
f
PBT6V2
104
−8.7
PBT6V2′
a
b
Values in parentheses are maximal mobilities attained. Annealed at
200 °C for 1 h. Annealed at 115 °C for 1 h. PBT6 was drop-casted
onto FETs annealed at 80 °C for 24 h. Annealed at 115 °C for 1 h.
Annealed at 200 °C for 1 h.
c
d
e
f
interchain charge hopping, which in turn leads to enhanced
mobilities. The incorporation of the vinylene linkage in the latter
two polymers causes a reduction in the crystallinity of thin films
and a concomitant decrease in mobility. No charge carrier trans-
port was detected in BDTDTB. It is noteworthy that a previously
reported oligomeric compound having the same backbone as
BDTDTB but two dodecyl chains on the α-carbons rather than
β-carbons of the peripheral thiophene units exhibits a liquid
crystalline phase and has a μh of 10−3 cm2/V·s in solution-processed
FETs.6a Using the combined results, it is evident that the relative
position of side chains plays a significant role in determining inter-
molecular interactions and charge carrier transport properties in
these materials.
3. CONCLUSIONS
Four benzothiadiazole-oligothiophene D−A copolymers were
synthesized by Stille coupling (PBT4 and PBT6) or by Horner−
Wadsworth−Emmons polymerization (to prepare analogs
containing vinylene linkages PBT6V2 and PBT6V2′). These
low optical band gap polymers (∼1.5 eV) exhibit the ability to
harvest photons at 400−800 nm and possess good thermal
stability, and PBT6, in particular, displays a mesophase. The hole
transport performance of spin-cast thin films of these polymers in
bottom gate bottom contact OFET devices correlates well with
their propensity to form ordered structures (PBT6 > PBT4 >
PBT6V2 ∼ PBT6V2′) as verified by DSC, XRD, and AFM.
PBT6, having a highly crystalline lamellar π-stacked structure
PBT6 devices prepared via spin casting were annealed at
distinct temperatures within both the mesophase and the
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ASSOCIATED CONTENT
* Supporting Information
■
S
The synthetic details of preparing monomers and polymers,
characteristic methods, OFETs fabrications, and Figures S1−
S20. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ereichmanis@chbe.gatech.edu.
■
Present Address
+SABIC, 1 Lexan Lane, Mt. Vernon, IN 47620, United States.
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Salleo, A.; Kirkpatrick, J.; Ashraf, S.; Anthopoulos, T.; Heeney, M.;
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the contributions of Professor Richard
Jordan, Ge Feng, and Nate Contrella of the Department of
Chemistry at the University of Chicago (high-temperature GPC
characterization); Karthik Nayani and Professor Mohan Srinivasar-
ao of the Georgia Institute of Technology (POM measurement),
Wei-Ming Yeh of Georgia Tech (AFM and XRD measurements),
and Stephen Barlow of Georgia Tech (Solvatochromism). This
research was funded in part by the Georgia Institute of Technology,
the Center for Organic Photonics and Electronics (COPE) at
Georgia Tech, the ACS Petroleum Research Fund (PRF
49158ND7), the National Science Foundation (DMR-1207284),
and the STC Program of the National Science Foundation (DMR-
0120967). 2D-GIXS measurements were carried out by Michael
Manno and Linda Sauer at the Characterization Facility, University
of Minnesota, which receives partial support from NSF through the
MRSEC program.
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