A novel tellurophene-containing conjugated polymer with a dithiophenyl diketopyrrolopyrrole unit for use in organic thin film transistors

Article abstract of DOI:10.1039/c3cc41250d

A new tellurophene-based π-conjugated polymer, PDTDPPTe, was synthesized. PDTDPPTe exhibits a smaller optical band gap (E_g ^opt = 1.25 eV) than thiophene-based PDTDPPT (E_g ^opt = 1.30 eV). Thin-film transistors comprising PDTDPPTe displayed outstanding performance (μ_max = 1.78 cm² V^-1 s^-1, I_on/I_off = 10^5-6).

Full text of DOI:10.1039/c3cc41250d

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A novel tellurophene-containing conjugated polymer
with a dithiophenyl diketopyrrolopyrrole unit for use
in organic thin film transistors†
Cite this: Chem. Commun., 2013,
4
9, 5495
Received 17th February 2013,
Accepted 13th March 2013
Matinder Kaur,z Da Seul Yang,z Jicheol Shin, Tae Wan Lee, Kihang Choi,
Min Ju Cho and Dong Hoon Choi*
DOI: 10.1039/c3cc41250d
www.rsc.org/chemcomm
A new tellurophene-based p-conjugated polymer, PDTDPPTe, was
For example, the electronegativities of S and Se are 2.58 and
2.55, respectively, whereas that of Te is 2.10 (according to the
opt
synthesized. PDTDPPTe exhibits a smaller optical band gap (E
g
=
opt
1
.25 eV) than thiophene-based PDTDPPT (E
g
= 1.30 eV). Thin-film Pauling scale). Tellurium is a metalloid and therefore, tellurium
8
transistors comprising PDTDPPTe displayed outstanding perfor- has the capability to form hypervalent coordination complexes,
2
ꢀ1 ꢀ1
5–6
mance (lmax = 1.78 cm
V
s
, Ion/Ioff = 10 ).
which enable strong interchain interaction that can be used to
further control the structure and properties of the tellurophene-
Currently, conjugated polymers are widespread in soft electronics containing conjugated polymers. Moreover, polytellurophenes have
and optoelectronics due to their intriguing applications mostly in potential advantages as polymer semiconductors; specifically, tell-
organic field effect transistors (OFETs), organic light emitting diodes urophene has a narrow HOMO–LUMO gap and, thus, the optical
1
(
OLEDs), chemical sensors, organic photovoltaic cells, etc. Over the absorptions of polytellurophenes should be red-shifted compared
9
10
last decade, different types of low band gap donor–acceptor type to those of polythiophenes. However, in the literature, there
conjugated polymers incorporating various alkyl chains have been are only a few reports on polytellurophenes, and those include
2
synthesized for use in electronic and optoelectronic devices.
only very limited characterization data. Further, due to the
Among them, diketopyrrolopyrrole (DPP)-based conjugated inaccessibility of appropriate synthetic procedures, the synthesis
copolymers have been investigated by many researchers, owing of polytellurophenes is very challenging. Therefore, their semi-
to their pronounced charge transport properties. DPP is electron- conducting properties have barely been investigated to date.
deficient, has a planar structure, and forms intermolecular
In this communication, we report the design and synthesis
hydrogen bonds, which results in materials with strong p–p of a DPP-based donor–acceptor conjugated polymer containing
stacking interactions that are desirable for high-performance tellurophene and its performance as a semiconductor in
3
OFETs. In particular, variation in the electron donating mono- OTFTs. Further, we compare the properties of PDTDPPTe with
1
1
meric unit affects the charge transport properties significantly. those of thiophene-containing PDTDPPT.
Most recent research suggested that a selenophene containing 2,5-Dihydro-2,5-bis(2-octyldodecyl)-3,6-di-2-thienyl-pyrrolo-[3,4-c]-
conjugated polymer can exhibit better performance in OFET pyrrole-1,4-dione has widely been used as a building block for high-
4,12
than the thiophene-containing conjugated polymer, maintaining mobility semiconductors for OTFTs over the past few years since
4
the identical polymer backbone.
it strengthens p–p intermolecular interactions. Monomers 2 and 3
13
From the chemical perspective, despite the wide range of were synthesized according to literature procedures (see ESI†).
applications of polythiophenes, heteroatomic polymers containing PDTDPPTe, 5, was synthesized via Stille coupling of 1 and 3 in
the next two members of group VI of the periodic table, i.e., toluene at 90 1C using the tetrakis(triphenylphosphine)
5
6
selenium and tellurium, are also very important. The selenium palladium(0) catalyst for 52 h (Scheme 1). The polymer was
atom is very similar to the sulfur atom and therefore many then purified by sequential Soxhlet extractions with acetone,
properties of polyselenophenes are similar or even superior to hexane, and chloroform to remove any residual small mole-
7
those of polythiophenes. However, the properties of the tellurium cules and oligomers resulting in pure 5 in 88% yield. The
atom are significantly different from those of selenium and sulfur. polymers were characterized by NMR and elemental analysis
(
Fig. S3 and S4 in ESI†).
The molecular weights of the copolymers were determined
Department of Chemistry, Research Institute for Natural Sciences, Korea University,
Anam-dong, Sungbuk-Gu, 136-701 Seoul, Korea. E-mail: dhchoi8803@korea.ac.kr;
Fax: +82-2-925-4284; Tel: +82-2-3290-3140
Electronic supplementary information (ESI) available: Synthetic procedure,
NMR, GPC, CV, device fabrication etc. See DOI: 10.1039/c3cc41250d
These authors equally contributed to this work.
5
using gel permeation chromatography (GPC) with a polystyrene
standard at 35 1C. The weight-average molecular weights (M
are 129 000 (g mol ) and 53 700 (g mol ) with a polydispersity
index (PDI) of 2.59 and 2.49 for PDTDPPT and PDTDPPTe,
w
)
†
ꢀ1
ꢀ1
‡
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Chem. Commun., 2013, 49, 5495--5497 5495

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A smaller energy barrier between PDTDPPTe and the gold
electrode in TFTs is expected; thus, hole injection will be more
facile due to the smaller difference between the HOMO level
and work function of the gold electrode.
Since the degree of crystallinity and orientation of crystallites on
the substrate in films are key factors that govern TFT device
performance, the polymer thin films were characterized using
grazing-incidence X-ray diffraction [GI-XRD; 9A (U-SAXS) beam line,
energy = 11.26 keV, l = 1.10107 Å] at the Pohang Accelerator
Laboratory (PAL). The orientation of the polymer chains in the
films annealed at 250 1C was precisely determined, which enables
us to specifically study the polymer chain alignment on n-octyltri-
Scheme 1 Polymerization of PDTDPPT, 4 and PDTDPPTe, 5.
respectively (Fig. S5 and S6 in ESI†). The thermal properties of
the polymers were investigated by thermogravimetric analysis
(
(
TGA), and their thermograms are displayed in Fig. S7 and S8
ESI†). Both PDTDPPT and PDTDPPTe exhibited good thermal
chlorosilane (OTS)-treated SiO /Si (Fig. 2). In Fig. 2a and c, the
2
diffraction patterns of the thermally annealed PDTDPPT film
indicate a bimodal distribution of the diffracted intensity due to
the mixture of face-on- and edge-on-oriented crystallites in the
polymer film. In the out-of-plane profile, the strong (100) reflection
at 2y = 3.001 (d(100) = 21.03 Å) is due to a lamellar structure, and a
weak (010) reflection at 16.771 (d(010) = 3.78 Å) from p–p stacking
was also observed. The annealed PDTDPPTe film exhibits four
orders of inter-lamellar stacking peaks (h00) in the out-of-plane
direction without (010) diffraction: 2y = 3.251 (d(100) = 19.41 Å).
The p–p stacking peak (2y = 15.731; d(010) = 4.02 Å) shows a
weak intensity only in an in-plane profile that accompanies
the diffraction peak from (100) diffraction (Fig. 2b and d). The
GI-XRD results for PDTDPPTe clearly reveal a more pronounced
edge-on orientation of the polymer chains than in PDTDPPT,
which leads us to expect a more facile charge transport in
tellurophene-containing polymers.
stability with 5% weight loss (T
respectively, under a N atmosphere. During the heating scan
of differential scanning calorimetry, none of the polymers
displayed a glass transition temperature (T ) in the range of
0–250 1C (Fig. S9 and S10 in ESI†).
UV-Vis absorption spectra of the polymer samples both in
solution and in film are shown in Fig. 1. Two absorption bands
were observed in the range of 600–1000 nm. In chloroform,
PDTDPPT showed maximum absorptions (lmax) at 806 (741) nm
in solution and 829 (753) nm in film (Fig. 1a). In contrast,
PDTDPPTe exhibited absorption maxima at 862 (784) nm in
solution and 900 (810) nm in film, respectively (Fig. 1b). The
wavelength in the parentheses is the absorption band whose
intensity is smaller than the other. Replacing thiophene with
tellurophene resulted in no significant change in the conjuga-
tion length, but the absorption spectrum shifted to the NIR
region. The optical band gap of PDTDPPT is 1.30 eV, as
d
) at 399 1C and 416 1C,
2
g
3
To investigate the carrier-transport properties of the poly-
mers, top-contact bottom-gate TFT devices were fabricated via a
simple spin-coating method using a 1.0 wt% chloroform
solution of the polymer (W = 1500 mm, L = 100 mm). The field-
effect mobilities were obtained from the source-drain current–
estimated from the onset absorption (B952 nm) of the thin
opt
film; this value is larger than that of PDTDPPTe (E
g
= 1.25 eV).
It is postulated that the smaller band gap is due to the presence
of the more electron-rich tellurium atom, which results in a
bathochromic shift of the absorption spectrum. However, the
optical band gap of PDTDPPTe is still small, suggesting that the
intermolecular interactions are relatively strong and favor high
crystallinity and carrier mobility. The molecular energy levels of
the polymer thin films were determined using cyclic voltammetry
gate voltage curves (IDS vs. V ) in the well-resolved saturation
G
regions of more than 20 devices. All devices comprising PDTDPPT
(
CV). The HOMO energy levels (EHOMO) of PDTDPPT and
PDTDPPTe, as calculated from their oxidative onset potentials, are
5.38 and ꢀ5.13 eV, respectively, versus ferrocene as the standard
Fig. S11 in ESI†). Therefore, the LUMO levels of the two polymers,
ꢀ
(
as calculated from their HOMO levels and band gaps, were
determined to be ꢀ4.08 and ꢀ3.88 eV, respectively.
Fig. 2 2-D GI-XRD patterns of (a) PDTDPPT, 4, and (b) PDTDPPTe, 5, films
annealed at 250 1C. 1-D XRD patterns of (c) PDTDPPT, 4, and (d) PDTDPPTe, 5,
films annealed at 250 1C.
Fig. 1 (a) UV-Vis absorption spectra of PDTDPPT 4 in: (i) solution and (ii) film.
(
b) UV-Vis absorption spectra of PDTDPPTe 5 in: (iii) solution and (iv) film.
5
496 Chem. Commun., 2013, 49, 5495--5497
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polymers incorporating bitellurophene and their applications in
organic electronics are currently under investigation.
The authors acknowledge the financial support from the
National Research Foundation of Korea (NRF2012R1A2A1A01008797
&
20110026418) and from Key Research Institute Program
(NRF20120005860). We are grateful to Pohang Accelerator
Laboratory (Pohang, Korea) for allowing us to conduct the
GI-XRD measurements. In particular, D. S. Yang thanks the
support by the Global Ph.D. Fellowship Program in NRF
(2012H1A2004001).
Notes and references
1
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1
Fig. 3 (a) Transfer and (b) output curves of the TFT device fabricated with the
pristine PDTDPPTe film. (c) Transfer and (d) output curves of the TFT device
fabricated with the thermally annealed (250 1C for 10 min) PDTDPPTe film.
U. Scherf, Angew. Chem., Int. Ed., 2008, 47, 4070; (c) Y.-J. Cheng,
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*
OTS–SiO
2
/Si gate insulator. The device performances were measured in air.
2 (a) A. P. Zoombelt, S. G. J. Mathijssen, M. G. R. Turbiez,
M. M. Wienka and R. A. J. Janssen, J. Mater. Chem., 2010,
V
DS = ꢀ100 V. Inset: AFM images.
2
0, 2240; (b) L. Biniek, S. Fall, C. L. Chochos, D. V. Anokhin,
D. A. Ivanov, N. Leclerc, P. Leveque and T. Heiser, Macromolecules,
010, 43, 9779.
2
and PDTDPPTe films exhibited typical p-channel transistor
behavior. Initially, the hole mobility of the pristine PDTDPPTe
film (Fig. 3a and b) was determined to be 0.48 cm
3
(a) B. Tieke, A. R. Rabindranath, K. Zhang and Y. Zhu, Beilstein J.
Org. Chem., 2010, 6, 830; (b) S.-Y. Ku, M. A. Brady, N. D. Treat,
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2
ꢀ1 ꢀ1
V
s
5–6
2
ꢀ1 ꢀ1
(
I
on/Ioff E 10 , Vth = ꢀ1.0 V, mmax = 0.55 cm V
s ). However,
after subsequent annealing at 250 1C, the mobility value was
measured to be 1.47 cm V
current ratio of 10 (V = 2.2 V, mmax = 1.78 cm V
and d). The devices with PDTDPPT exhibited IDS–V
istics quite similar to those with PDTDPPTe. The mobility of
the OTFT device using an as-spun PDTDPPT film without
2
ꢀ1 ꢀ1
4 (a) H.-W. Lin, W.-Y. Lee, C. Lu, C.-J. Lin, H.-C. Wu, Y. W. Lin, B. Ahn,
Y. Rho, M. Ree and W.-C. Chen, Polym. Chem., 2012, 3, 767;
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22, 2120; (c) J. S. Ha, K. H. Kim and D. H. Choi, J. Am. Chem. Soc.,
s
while maintaining an on–off
5
2
ꢀ1 ꢀ1
s
, Fig. 3c
th
G
character-
2
011, 133, 10364.
5
(a) J. Roncali, Chem. Rev., 1992, 92, 711; (b) Handbook of Oligo
and Polythiophenes, ed. D. Fichou, Wiley-VCH, Weinheim, Germany,
1999; (c) R. D. McCullough, Adv. Mater., 1998, 10, 93.
2
ꢀ1 ꢀ1
4–5
thermal annealing was around 0.24 cm V
s
(Ion/Ioff E 10
) (Fig. S13a and b in ESI†);
after the sample was annealed at 250 1C, the mobility was
,
2
ꢀ1 ꢀ1
6 (a) Handbook of Chalcogen Chemistry: New Perspectives in Sulfur,
Selenium and Tellurium, ed. F. A. Devillanova, Royal Society of
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Mater., 2011, 23, 896.
V
th = 10.1 V, mmax = 0.30 cm V
s
2
ꢀ1 ꢀ1
4–5
determined to be 0.62 cm V
mmax = 0.77 cm V
To elucidate the origin of the higher performance of
the OTFT fabricated using thermally annealed PDTDPPTe, the
topography of the semiconducting layer deposited on the OTS-
s
(I /I E 10 , Vth = 10 V,
on off
2
ꢀ1 ꢀ1
s ) (Fig. S13c and d in ESI†).
8
(a) N. A. G. Bandeira, L. F. Veiros, M. J. Calhorda and J. Novosad,
Inorg. Chim. Acta, 2003, 356, 319; (b) P. C. Srivastava, S. Bajpai,
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2
treated SiO substrate was analyzed and is shown in the insets
9
(a) Y. Narita and K. Takeda, Jpn. J. Appl. Phys., 2006, 45, 2628;
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of Fig. 3. Consistent with annealing-induced crystallinity, a
change in the morphology was clearly observed in the AFM
images of the as-spun and annealed films. The pristine
PDTDPPTe film showed large crystallites that densely covered
the surface (average roughness o3.8 nm). Subsequently, after
thermal annealing, the sizes of the crystallites increased and
1
0 (a) A. Patra, Y. H. Wijsboom, G. Leitus and M. Bendikov, Org. Lett.,
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the film surface consisted of more fibrous crystalline domains 11 (a) X. Zhang, L. J. Richter, D. M. DeLongchamp, R. J. Kline,
M. R. Hammond, I. McCulloch, M. Heeney, R. S. Ashraf,
that were highly packed and densely connected by polymer
J. N. Smith, T. D. Anthopoulos, B. Schroeder, Y. H. Geerts,
chains (average roughness o6.2 nm) (Fig. S15 in ESI†).
D. A. Fischer and M. F. Toney, J. Am. Chem. Soc., 2011, 133, 15073;
In conclusion, we first report the synthesis of a tellurophene-
and DPP-based conjugated polymer using the Stille coupling
reaction (88% yield). The PDTDPPTe polymer exhibited a high
(b) J. S. Lee, S. K. Son, S. Song, H. Kim, D. R. lee, K. Kim, M. J. Ko,
D. H. Choi, B. S. Kim and J. H. Cho, Chem. Mater., 2012, 24, 1316.
2 M. Shahid, R. S. Ashraf, Z. Huang, A. J. Kronemeijer, T. McCarthy-
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1
1
2
ꢀ1 ꢀ1
hole mobility (mmax = 1.78 cm
V
s ), which is attributed
3 (a) T. J. Barton and R. W. Roth, J. Organomet. Chem., 1972, 39, C66;
to the fact that the polymer chains featured better edge-on
orientation and the strong donor ability of the tellurophene unit
induced stronger intermolecular p–p interactions. New conjugated
(
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This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 5495--5497 5497