A diketopyrrolopyrrole-thiazolothiazole copolymer for high performance organic field-effect transistors

Article abstract of DOI:10.1039/c2cc38811a

A diketopyrrolopyrrole-thiazolothiazole copolymer with a short π-π stacking distance (3.52 ), due to the introduction of heteroaromatic rings, exhibits a high charge mobility above 3.40 cm² V^-1 s ^-1 at a relatively gentle annealing temperature. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cc38811a

Chemical Communications
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Volume 49 | Number 20 | 11 March 2013 | Pages 1973–2072
ISSN 1359-7345
COMMUNICATION
Yunlong Guo, Gui Yu, Yunqi Liu et al.
A diketopyrrolopyrrole–thiazolothiazole copolymer for high performance
organic field-effect transistors
1359-7345(2013)49:20;1-A

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A diketopyrrolopyrrole–thiazolothiazole copolymer for
high performance organic field-effect transistors†
Cheng Cheng,a Chunmeng Yu,ab Yunlong Guo,*a Huajie Chen,a Yu Fang,b Gui Yu*a
and Yunqi Liu*a
Cite this: Chem. Commun., 2013,
49, 1998
Received 8th December 2012,
Accepted 7th January 2013
DOI: 10.1039/c2cc38811a
www.rsc.org/chemcomm
A diketopyrrolopyrrole–thiazolothiazole copolymer with a short p–p introducing heteroaromatic rings could enhance the accumulation
stacking distance (3.52 Å), due to the introduction of heteroaro- of the molecule. Heteroaromatic ring structures have a strong
matic rings, exhibits a high charge mobility above 3.40 cm2 Vꢀ1 sꢀ1 tendency to form p–p stacks with a large overlapping area that is
at a relatively gentle annealing temperature.
favourable for charge-carrier transport.6 In addition, introduction of
the electron deficient heteroaromatic rings into the polythiophene
In their most conspicuous forms, large area flexible organic system is a promising approach to increase the ionization potential
electronics, such as organic radio frequency identity tags, sensor and thereby improve the stability.7
arrays and flexible displays, address a range of applications outside
Here we report the design and synthesis of a solution-processable
those where Si integration is used, or they approach those conven- polymer semiconductor PDPPTzBT (Scheme 1),8 which shows a high
tional applications with a uniquely advantageous performance bene- hole mobility of 3.43 cm2 Vꢀ1 sꢀ1 in OFET devices. In this polymer,
fit.1 Organic field-effect transistors (OFETs) that use solution- we deliberately combined the diketopyrrolopyrrole (DPP) moiety
processed organic semiconductors to replace amorphous silicon as with a donor–acceptor–donor (D–A–D) building block, namely,
the active layer have attracted much interest in recent years, owing to thiophene–thiazolothiazole–thiophene (TzBT), to form an A–D–A–D
the advantages of low cost, light-weight, and potential use of flexible polymer with a repeat unit of DPPTzBT. These two electron deficient
substrates.2 Compared with small-molecule OFETs, solution pro- cores, DPP and TzBT, have been demonstrated to be significant
cessed conjugated polymers could offer better film forming and building blocks for high performance OFETs.9 In this system, the
mechanical properties, such as flexibility and large-area uniformity.3 long alkyl and heteroaromatic rings in TzBT blocks could induce a
However, solution-processed polymer thin films always comprise high order molecular organization through alkyl-assembling, p–p
amorphous regions along with crystalline domains, which is not stacking and heteroatom contact.
conducive to the charge carrier transport.4 Thermal annealing is a
Polymer PDPPTzBT was readily synthesized via Suzuki-coupling
commonly used practice to improve the molecular organization of polymerization between 3,6-bis-(5-(4,4,5,5-tetramethyl-1,3,2-dioxa-
the polymer thin films to obtain higher charge carrier mobility. High borolan-thiophen-2-yl))-N,N0-bis(octyldodecyl)-1,4-dioxopyrrolo[3,4-c]-
field effect mobility has been achieved for a few polymers with the pyrrole and 2,5-bis(3-dodecylthiophen-2-yl)thiazolo[5,4-d]thiazole in
aid of thermal annealing, but the annealing temperature is some-
what high, most are higher than 180 1C and some even more than
250 1C.5 These thermal annealing steps are energy-consuming and
usually need to be conducted under vacuum or in an inert atmo-
sphere. This poses a challenge in the cost-effective production of
printed OFETs. Changing the original structure of the polymer by
a University of Chinese Academy of Science, Beijing National Laboratory for
Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, P. R. China. E-mail: liuyq@iccas.ac.cn, guoyunlong@iccas.ac.cn,
yugui@iccas.ac.cn; Tel: +86 10 62613253
b Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education),
School of Chemistry and Chemical Engineering, Shaanxi Normal University,
Xi’an 710062, P. R. China
† Electronic supplementary information (ESI) available: Synthetic procedures and
characterization, general procedures and experimental details, FET device fabri-
Scheme 1 Synthesis of PDPPTzBT via Suzuki-coupling polymerization.
cation. See DOI: 10.1039/c2cc38811a
c
1998 Chem. Commun., 2013, 49, 1998--2000
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75% yield (ESI,† S1). PDPPTzBT was purified by Soxhlet extraction
To confirm the diffraction pattern and molecular ordering,
using methanol, acetone, and hexane successively to remove the the microstructure of the polymer thin films was studied using
oligomers and other impurities. The polymer was soluble in most grazing incidence X-ray scattering (GIXS). For the GIXS measure-
common organic solvents such as chloroform and toluene. The ments, the polymer thin films were prepared by drop-casting
number average molecular weight was 23.9 kDa, with a polydis- 1,2-dichlorobenzene (DCB) solution of the polymers on silicon
persity index (PDI) of 1.81, which was determined by using gel wafers with the native oxide layer. As shown in Fig. 2, when the
permeation chromatography (GPC) against polystyrene standards films were subjected to thermal annealing, the diffraction patterns
(ESI,† S2 and Fig. S4). The thermal properties of PDPPTzBT were became sharper and multiple diffraction peaks were observed for
examined by thermogravimetric analysis (TGA) and differential the films, which demonstrated that the highly crystalline films were
scanning calorimetry (DSC) techniques. TGA showed a decom- formed in the annealing process. After annealing at 120 1C, the thin
position temperature of 375 1C in air, indicating a high thermal film showed a strong primary peak at 3.541 (100), which corre-
stability of the polymer (Fig. S2, ESI†). DSC characterization was sponds to a d-spacing of 24.9 Å, and a peak at 25.21 (010), which
carried out up to 320 1C under nitrogen (which is 55 1C lower than corresponds to a p–p stacking distance of 3.52 Å. The small p–p
the decomposition temperature) to study the thermal transitions stacking distance implies the stronger intermolecular interactions
of the polymer. During the first heating scan, two endothermic between the fused aromatic DPP unit and the TzBT planar segment
transition temperatures were observed at 170.2 1C and 286.4 1C, comprising the PDPPTzBT polymer backbone.10 Based on the
whereas in the second heating scan the endothermic peaks were d-spacing results, there is no significant difference when dodecyl
not observed, so it is probably due to the melting of the bulk side chains on the thiophene rings of PDPPTzBT were replaced
(Fig. S3, ESI†). The optical properties of PDPPTzBT in chloroform by octyloxy side chains,8 indicating that longer side chains while
and as a thin film were characterized by using UV-vis-NIR increasing solubility and alkyl-assembling do not impede the
spectroscopy. In chloroform, PDPPTzBT exhibited a maximum accumulation of this copolymer.
absorption (lmax) at 772 nm (Fig. 1). The polymer thin film
Bottom-contact, bottom gate OFET devices were fabricated
showed a slight red shift of 34 nm in lmax (806 nm). A vibronic using solution processed PDPPTzBT thin films as the semi-
absorption shoulder at 718 nm was observed, suggesting that the conducting layer on the OTS treated SiO2 substrate (ESI,† S3).
molecule in the thin film underwent molecular organization to The organic semiconductor layer was deposited by spin-casting
form a more ordered structure. The absorption cut-off of 965 nm for 60 s at 2000 rpm from a 3 mg mLꢀ1 DCB solution in the air.
in the solid state corresponds to an optical band-gap (Egopt) of OFET device characteristics were measured under ambient
1.28 eV, indicating that PDPPTzBT is a small band-gap semi- conditions. Typical transfer and output curves for PDPPTzBT
conductor. The electrochemical property of the PDPPTzBT thin based FET devices are shown in Fig. 3. Initially, the field-effect
film was investigated by cyclic voltammetry (CV) in order to mobility of the as-spun film of PDPPTzBT was 0.41 cm2 Vꢀ1 sꢀ1
determine the HOMO/LUMO energy levels (Fig. S1, ESI†). The (Ion/Ioff = 7.1 ꢁ 106) in the saturation regime. Interestingly, the
HOMO value of PDPPTzBT was calculated from the onset oxida- hole mobility was increased to 3.43 cm2 Vꢀ1 sꢀ1 (Ion/Ioff = 7.22 ꢁ
tion potential, which was ꢀ5.28 eV, according to the equation 106) as the annealing temperature increased to 120 1C for 5 min
EHOMO = ꢀe(Eoonxset + 4.4) eV, whereas the LUMO value of ꢀ4.0 eV in air (Table 1). Moreover, when the annealing temperature
was determined from the difference between the HOMO and the increased to 100 1C, the hole mobility was already increased to
optical band-gap. We could note that the HOMO energy level of 2.86 cm2 Vꢀ1
s
ꢀ1, promising a relatively gentle annealing
PDPPTzBT was obviously lower than that of regioregular poly- temperature for high performance polymers.5 The high hole
(3-hexylthiophene) (ꢀ4.76 eV).10 The decreased HOMO energy mobility observed for PDPPTzBT was thought to arise from the
level is expected to be beneficial for increasing the stability of strong intermolecular interactions of this polymer. First, the fused
the polymer against oxidation as well as enhancing the hole DPP and TzBT moieties have a tendency to form strong inter-
injection from a gold source electrode into p-type thin film.
molecular overlapping through enhanced stacking, resulting in
efficient channels for charge carrier transport.11 Second, the
donor moieties (thiophene units) and the acceptor moieties
(DPP and thiazolothiazole units) in the nearest neighbouring
Fig. 2 (a) Grazing incidence X-ray scattering pattern (out-of-plane) of polymer
PDPPTzBT films annealed at different temperatures. (b) In-plane X-ray scattering
pattern of polymer film annealed at 120 1C.
Fig. 1 Normalized UV-vis-NIR absorption spectra of PDPPTzBT in CHCl3 solution
or as fresh thin films on a glass substrate.
c
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the thin films at room temperature. With increasing annealing
temperature, crystal-like grains are observed at 100 1C or
120 1C, which is in good agreement with the GIXS results.
The strong crystallinity and ordered packing in PDPPTzBT
would form highly efficient pathways for charge carrier trans-
port throughout the polymer film.
In conclusion, the diketopyrrolopyrrole–thiazolothiazole
copolymer, PDPPTzBT, with the combination of fused ring aromatic
moieties of DPP and TzBT is a high mobility p-type polymer
semiconductor for OFETs. The small p–p stacking distance implies
that heteroaromatic ring structures have a strong tendency to
form p–p stacks with a large overlapping area that is favourable
for charge-carrier transport. The facile preparation and high
mobility make such polymers very promising for application as
solution-processable semiconductors for organic electronic devices.
This work was supported by the National Natural Science
Foundation of China (20825208, 60911130231, 61101051 and
21021091), and the Major State Basic Research Development
Program (2011CB808403, 2011CB932303, 2009CB623603), and
Chinese Academy of Sciences.
Fig. 3 (a) Devices structure of the polymer FETs. (b) Output and (c) transfer
characteristics of the PDPPTzBT FET device with W = 1400 mm and L = 40 mm,
exhibiting a hole mobility of 3.43 cm2 Vꢀ1 sꢀ1 at VDS = ꢀ80 V. (d) The mobility
distributions based on different channel lengths.
Notes and references
Table 1 Bottom contact devices performance of the polymer FETs
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Annealing
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Mobility
(cm2 Vꢀ1 sꢀ1
)
Ion/Ioff
VT (V)
0
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100
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0.41
1.38
2.86
3.43
7.10 ꢁ 106
5.35 ꢁ 106
1.26 ꢁ 106
7.22 ꢁ 106
ꢀ10.2
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ꢀ2.60
ꢀ2.61
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c
2000 Chem. Commun., 2013, 49, 1998--2000
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