Low-temperature, solution-processed, high-mobility polymer semiconductors for thin-film transistors

Article abstract of DOI:10.1021/ja067879o

Poly(4,8-dialkyl-2,6-bis(3-alkylthiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene) 1 represents a new class of polymer semiconductors which self-assemble into higher structural orders without thermal annealing and provide excellent field-effect transistor performance with mobility up to 0.25 cm²V^-1 s^-1 when used as a solution-processed thin-film semiconductor in thin-film transistors. Copyright

Full text of DOI:10.1021/ja067879o

Published on Web 03/16/2007
Low-Temperature, Solution-Processed, High-Mobility Polymer
Semiconductors for Thin-Film Transistors
Hualong Pan,† Yuning Li,‡ Yiliang Wu,‡ Ping Liu,‡,§ Beng S. Ong,*,‡ Shiping Zhu,†,§ and Gu Xu†
Department of Materials Science & Engineering, McMaster UniVersity, Ontario, Canada L8S 4L7, Materials
Design & Integration Laboratory, Xerox Research Centre of Canada, Ontario, Canada L5K 2L1, and Department
of Chemical Engineering, McMaster UniVersity, Ontario, Canada L8S 4L7
Received November 3, 2006; E-mail: bengong@ntu.edu.sg
Scheme 1. Synthesis of Poly(4,8-dialkyl-2,6-bis(3-alkylthiophen-
2-yl)benzo[1,2-b:4,5-b′]dithiophene) 1 (1a: R ) Hexyl)
Solution-processable, high-mobility organic semiconductors are
required for printing low-cost thin-film transistor (TFT) circuits
via solution deposition/patterning processes. This has been widely
perceived to be a potentially low-cost alternative to traditional
silicon-based technologies to enable ubiquitous flexible electron-
ics.1,2 In this regard, several solution-processable, functionally
capable polymer semiconductors have been developed,3-5 but most,
if not all, require postdeposition thermal annealing at above 100
°C in vacuum or an inert atmosphere to achieve high field-effect
transistor (FET) mobility.4,5 Such a postdeposition annealing step
is time-consuming, thus not amenable to high-speed roll-to-roll
manufacturing processes for low-cost TFT circuits.
1a in reasonable yield. GPC analysis gave a number-average
molecular weight (Mn) of 16 300 with a polydispersity of 3.9 against
polystyrene standards.
Two critical properties of solution-processed polymer semicon-
ductors are needed to enable printing low-cost TFT circuits:2e,4a,5b
(1) an ability to self-organize into a higher structural order for
efficient charge carrier transport; and (2) a sufficient stability to
permit processing under ambient conditions without a costly
protective environmental setup. Several recently reported regio-
regular polythiophene semiconductors, such as poly(3,3′′′-dido-
decylquaterthiophene) (PQT-12) and analogous polythiophenes,
appear to fulfill these requirements. Controlled effective π-conjuga-
tion has been employed to impart sufficient ambient stability to
these polythiophenes for low-cost processing, while strategically
placed long alkyl side-chain substituents have led to increased
solution processability and efficient self-assembly via intermolecular
interdigitation.4a,b Nonetheless, to achieve higher structural orders
via intermolecular interdigitation for improved FET performance,
thermal annealing of these polymer thin-film semiconductors after
deposition is still necessary.
We report here a new solution-processable polythiophene
semiconductor system, poly(4,8-dialkyl-2,6-bis(3-alkylthiophen-2-
yl)benzo[1,2-b:4,5-b′]dithiophene) 1, which exhibited high FET
mobility in TFTs without postdeposition thermal annealing during
fabrication. It would thus enable a high-throughput roll-to-roll mass
manufacturing process for TFT circuits. The close proximity of
alkyl side-chain substituents in 1a (1, R ) hexyl) is reminiscent of
that of regioregular head-to-tail poly(3-hexylthiophene), P3HT 2a
(2, R ) hexyl) in that efficient self-assembly could be achieved at
room temperature via intermolecular end-to-end side-chain align-
ment.3d
The UV-vis spectrum of 1a in a warm 1,2-dichlorobenzene
solution (∼50 °C) showed a strong absorption with λmax at 475
nm, which was similar to that of PQT-12 (λmax ) 480 nm) but was
red-shifted from that of regioregular P3HT (2a, λmax ) 458 nm).
Two new strong absorption peaks with λmax at 533 and 581 nm
appeared when the solution was cooled to room temperature,
demonstrating attainment of higher structural orders at lower
temperatures. A solution-cast thin film of 1a displayed similar but
broader absorption peaks, which were about 20 nm blue-shifted
from those of regioregular P3HT, indicative of a shorter effective
conjugation length of 1a in the solid state. Cyclic voltammetric
measurement of a thin film of 1a provided an ionization potential
which was 0.22 eV higher than that of regioregular P3HT, thus a
greater stability against oxidative doping. DSC profile of 1a showed
a phase transition at 395 °C from backbone melting. The fact that
only one thermal transition was observed was consistent with the
absence of intermolecular side-chain interdigitation.
Two-dimensional grazing-incidence X-ray diffraction (GIXRD,
incident X-ray angle of 2.5°) measurement (Figure 1a) of a spin-
coated thin film (∼100 nm) of 1a from a 1,2-dichlorobenzene
solution on an OTS-8-modified silicon wafer substrate showed only
a strong primary diffraction pattern (100) at 2θ 5.6° (d-spacing of
16.1 Å; Figure 1c), demonstrating a well-organized lamellar layered
structure which was oriented normal to the substrate.3b,6 The (010)
diffraction from π-π stacking was not observed due to blockage
by the substrate. The highly ordered structure of 1a in thin films
was unambiguously demonstrated via two-dimensional transmission
X-ray diffraction of a stack of thin films of 1a, as schematically
depicted in Figure 1b. Figure 1d is the diffraction pattern obtained
when the incident X-ray was normal to the film stack, showing
both a strong π-π stacking (010) at 2θ 22.6° (d-spacing, 3.9 Å)
The synthesis of 1a is described in Scheme 1 (see also Supporting
Information). 4,8-Dihexyl-2,6-bis(3-hexylthiophen-2-yl)benzo[1,2-
b:4,5-b′]dithiophene monomer was prepared by Suzuki coupling
of 2,6-dibromo-4,8-dihexylbenzo[1,2-b:4,5-b′]dithiophene and 3-hex-
ylthiophene-2-boronic acid pinacol ester in good yield. Subsequent
ferric chloride-mediated oxidative coupling polymerization provided
† Department of Materials Science & Engineering, McMaster University.
‡ Xerox Research Centre of Canada.
§ Department of Chemical Engineering, McMaster University.
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10.1021/ja067879o CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. I-V characteristics of an illustrative as-prepared TFT device
using 1a semiconductor (channel length ) 90 µm; channel width ) 1000
µm): (a) transfer curve in saturated regime at a constant source-drain voltage
of -60 V and square root of absolute value of drain current as a function
of gate voltage; and (b) output curves at different gate voltages.
after standing over an extended period of time in an ambient
environment at 20% relative humidity. In sharp contrast, the devices
fabricated with 2a under similar conditions displayed poorer
performance and failed within a few days.4a,d The mobility of the
test device decreased slightly from 0.20 to about 0.16 cm2 V-1 s-1
over a period of 30 days under these conditions.
Figure 1. Schematic illustrations of 2-D GIXRD measurement (incident
X-ray angle at 2.5°) of a thin film of 1a (a) and 2-D transmission XRD
measurement of a stack of thin films of 1a (b); 2-D GIXRD images of a
thin film of 1a (c); 2-D transmission XRD images obtained with the incident
X-ray normal (d) and parallel (e) to the film stack of 1a; (f), (g), and (h)
are respective XRD diffractograms of pattern intensities of (c), (d), and (e)
[obtained by integration of Chi (0-360°) with GADDS software].
In summary, we have designed a new solution-processable, high-
mobility polythiophene semiconductor with enhanced stability for
fabrication of low-cost TFTs. Of particular significance is that this
semiconductor exhibited high FET mobility even without the usual
time-consuming postdeposition thermal annealing step, thus po-
tentially enabling high-throughput, roll-to-roll mass manufacturing
processes for TFT circuits. In addition, recent progress in structural
optimization has led to greatly improved mobility of ∼0.4 cm2 V-1
s-1
.
Acknowledgment. Partial financial supports of this work were
provided by Natural Science and Engineering Research Council of
Canada (NSERC) and Xerox Foundation.
Figure 2. AFM images of an illustrative thin film of 1a: (a) topography
image; (b) phase image showing large domains of 1 µm in width and >1
µm in length.
Supporting Information Available: Instrumentation, details of
experimental procedures, and additional figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
and a weak interlayer (100) diffraction. Strong (100), (200), (300),
and (010) diffraction patterns were detected when the incident X-ray
was parallel to the film stack (Figure 1e), showing the π-π stacking
and interlayer diffraction patterns which were normal to each other.
Accordingly, the molecular organization of 1a in thin films was
essentially similar to that of 2a. The highly organized nature of 1a
in a solution-processed thin film was manifested by the formation
of extraordinarily large domains of ∼1 µm in width and >1 µm in
length, in comparison to those of other polythiophenes,4c,5b as
visualized in the AFM images (Figure 2).
The FET properties of 1a as a solution-processed thin-film
semiconductor were evaluated using a bottom-gate, top-contact TFT
configuration built on an n-doped silicon wafer with evaporated
gold source/drain electrodes and OTS-8-modified SiO2 gate di-
electric (see Supporting Information). Figure 3 shows the typical
output and transfer curves of a representative as-prepared TFT
device without postdeposition thermal annealing. The output
behaviors followed closely the metal oxide-semiconductor FET
gradual channel model with very good saturation and no observable
contact resistance. The transfer characteristics showed near-zero
turn-on voltage, a small threshold voltage of -5.9 V, and a sub-
threshold slope of ∼2 V/decade. The device gave a saturation
mobility of 0.15-0.25 cm2 V-1 s-1 with a current on/off ratio of
105-106 when measured in ambient conditions. Annealing at 100-
150 °C in vacuum did not lead to improved FET performance, in
sharp contrast to the behaviors of most polymer thin-film semi-
conductors such as PQT and its analogues. The TFT devices using
1a as the semiconductor also exhibited relatively stable performance
over time as no significant degradation in mobility was observed
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