High and balanced hole and electron mobilities from ambipolar thin-film transistors based on nitrogen-containing oligoacences

Article abstract of DOI:10.1021/ja107046s

We demonstrate a strategy for designing high-performance, ambipolar, acene-based field-effect transistor (FET) materials, which is based on the replacement of C-H moieties by nitrogen atoms in oligoacenes. By using this strategy, two organic semiconductors, 6,13-bis(triisopropylsilylethynyl) anthradipyridine (1) and 8,9,10,11-tetrafluoro-6,13- bis(triisopropylsilylethynyl)-1-azapentacene (3), were synthesized and their FET characteristics studied. Both materials exhibit high and balanced hole and electron mobilities, 1 having μ_h and μ_e of 0.11 and 0.15 cm²/V·s and 3 having μ_h and μ_e of 0.08 and 0.09 cm²/V·s, respectively. The successful demonstration of high and balanced ambipolar FET properties from nitrogen-containing oligoacenes opens up new opportunities for designing high-performance ambipolar organic semiconductors.

Full text of DOI:10.1021/ja107046s

Published on Web 10/27/2010
High and Balanced Hole and Electron Mobilities from Ambipolar Thin-Film
Transistors Based on Nitrogen-Containing Oligoacences
Yi-Yang Liu,‡,† Cheng-Li Song,‡,† Wei-Jing Zeng,‡ Kai-Ge Zhou,‡ Zi-Fa Shi,‡ Chong-Bo Ma,‡
Feng Yang,‡ Hao-Li Zhang,*,‡ and Xiong Gong§
State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering,
Lanzhou UniVersity, Lanzhou, 730000, China, and Department of Polymer Engineering, College of Polymer Science
and Engineering, The UniVersity of Akron, Akron, Ohio 44236, United States
Received August 18, 2010; E-mail: Haoli.zhang@lzu.edu.cn
and in nitrogen, respectively. However, when operated under the same
Abstract: We demonstrate a strategy for designing high-perfor-
mance, ambipolar, acene-based field-effect transistor (FET) materi-
als, which is based on the replacement of C-H moieties by nitrogen
atoms in oligoacenes. By using this strategy, two organic semicon-
condition, i.e., in a nitrogen glovebox, the hole mobilities were several
times lower than the electron mobilities, i.e., down to 0.065 cm2/V·s.
Here we demonstrated high and balanced hole and electron mobilities
from 6,13-bis(triisopropylsilylethynyl)anthradipyridine (1) and 8,9,10,11-
ductors, 6,13-bis(triisopropylsilylethynyl)anthradipyridine (1) and
tetrafluoro-6,13-bis(triisopropylsilylethynyl)-1-azapentacene (3), where
8,9,10,11-tetrafluoro-6,13-bis(triisopropylsilylethynyl)-1-azapenta-
1 has µh and µe of 0.11 and 0.15 cm2/V ·s and 3 has µh and µe of 0.08
cene (3), were synthesized and their FET characteristics studied.
and 0.09 cm2/V ·s, respectively.
Both materials exhibit high and balanced hole and electron mobilities,
1 having µh and µe of 0.11 and 0.15 cm2/V·s and 3 having µh and
µe of 0.08 and 0.09 cm2/V·s, respectively. The successful demon-
stration of high and balanced ambipolar FET properties from
nitrogen-containing oligoacenes opens up new opportunities for
designing high-performance ambipolar organic semiconductors.
Scheme 1. Synthetic Route to Molecules 1-3a
In recent years, organic semiconductors have attracted consider-
able attention because of their potential for creating low-cost,
portable electronic and optoelectronic devices such as organic light-
emitting diodes (OLED), organic field effect transistors (OFETs),
flexible displays, and sensors.1-3 Most organic semiconductors
reported so far, such as oligoacene and oligothiophene derivatives,4,5
exhibit p-type transport properties, whereas more efforts have been
devoted to the design and synthesis of n-channel materials in recent
years. The majority of organic semiconductors give only unipolar
FET characteristics, and few organic semiconductors have been
found to exhibit ambipolar characteristics.6 Moreover, to the best
of our knowledge, very few organic semiconductors7-9 exhibit
balanced hole and electron transport characteristics.
Ambipolar materials are strongly desirable for logic circuit design
because they enable the simple fabrication of complementary
circuits like inverters without advanced patterning techniques to
deposit p- and n-channel materials separately. Moreover, ambipolar
materials work for both positive and negative voltages, which
simplifies the circuit design. An ideal ambipolar material is expected
to give balanced performance in both negative and positive voltage
regimes; therefore, it is strongly desired that the ambipolar materials
could have balanced n- and p- channel performance. However,
among the few reported ambipolar organic semiconductors, their
mobilities are either low or unbalanced.10-13 For example, Yoon
et. al reported that dihexanoylquaterthiophene gave good ambipolar
transport behavior, with µh and µe values of ∼0.01 and 0.1 cm2/
V·s, respectively, in a vacuum.14 Recently, Bao et al.7 reported a
high-performance ambipolar material, fluorinated tetraceno[2,3-b]-
thiophene, which has µh and µe of up to 0.12 and 0.37 cm2/V ·s in air
a Reagents and conditions: (a) 1,4-cyclohexanedione, KOH, EtOH, 60
°C, 42%; (b) 1,4-anthraquinone or 5,6,7,8-tetrafluoro-1,4-anthraquinone, NaI,
DMF, 75 °C, 40%; (c) i, n-BuLi, THF, TIPS-acetylene, 0 °C; ii, SnCl2,
60 °C, 24%.
The molecular design is based on 6,13-bis(triisopropylsilylethy-
nyl)pentacene (TIPS-PEN), which is one of the most remarkable
soluble p-type semiconductors that has high hole-transport
performance15,16 and improved stability compared to pentacene.
Our novel design of high-performance, ambipolar, acene-based FET
materials is based on the replacement of C-H moieties in
oligoacenes by nitrogen atoms. This is different from the most
common approach to obtain ambipolar characteristics through
chemical modification of originally p-type materials by introducing
electron-withdrawing groups (i.e., -F,7 -CN,13 -Cl,8 alkanoyl14)
to the conjugated cores in order to lower the lowest unoccupied
molecular orbital (LUMO) levels and reduce electron injection
barriers, hence bringing about n-channel properties. Nitrogen
replacement of the C-H moiety in the aromatic rings17-19 can
efficiently lower the molecule’s LUMO level and enhance electron
transport propeties.17 Scheme 1 shows our synthetic strategy. The
6,13-anthradipyridinequinone was obtained from 2,3-pyridinedi-
carbaldehyde and 1,4-cyclohexanedione by aldol condensation.20
The one-nitrogen-containing pentacene-6,13-quinones were syn-
thesized by Diels-Alder reaction21 between 2,3-bis(dibromometh-
yl)pyridine and 1,4-anthraquinone or 5,6,7,8-tetrafluoro-1,4-
‡ Lanzhou University.
§ The University of Akron.
† These authors contributed equally.
9
10.1021/ja107046s  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 16349–16351 16349

C O M M U N I C A T I O N S
Table 1. Summary of Parameters of Top-Contact OFETs
Fabricated from 1-3 with Au Electrodes, W/L ) 20
anthraquinone. The one- or two-nitrogen-containing quinones were
then reduced by SnCl2 to give 1, 6,13-bis(triisopropylsilylethynyl)-
1-azapentacene (2), and 3. We noticed that the traditional reduction
condition (H+/SnCl2) was unfavorable in the reduction of nitrogen-
containing quinones and could not give the corresponding target
compounds, most likely due to the protonation of the nitrogen atoms
under acidic conditions. Reducing the quinones under a neutral
condition using SnCl2 ·2H2O afforded the target compounds in
yields of 20-30%. All the nitrogen-containing TIPS-PENs exhibit
very good solubility in common organic solvents, like chloroform
and toluene, and can be recrystallized from CH3OH/CH2Cl2.
compd
conditions
µh; µe (cm2/V·s) on/off
VT (V)
carrier
1
rt, OTS, Ar
Tc > 80 °C, OTS, Ar
0.008
0.11; 0.15
103 -50
h
104 -55; +70 h; e
2
3
rt, OTS, Ar
80 °C, OTS, Ar
0.02
0.22
103 -28
104 -55
h
h
rt, OTS, Ar
80 °C, OTS, Ar
0.0015
0.08; 0.09
103 -65
h
104 -60; +75 h; e
treated with octadecyltrimethoxysilane (OTS).24 The molecules
(purified by chromatography on silica gel and recrystallized twice)
were thermally evaporated onto the substrates under high vacuum
(10-6 Torr) to a thickness of 40 nm, as measured in situ by a quartz
crystal microbalance. Au electrodes were then deposited through a
shadow mask. Table 1 summarizes the OFET parameters for
compounds 1-3. The reported mobility data are the average values
of around 20 devices for each material, and device statistics are
provided in the Supporting Information. As expected, no n-channel
behavior was observed on bare SiO2, due to the unpassivated
hydroxyl groups serving as electron traps.25 Like most n-type
organic semiconductors, no n-channel property was obtained in
ambient conditions because of the thermodynamic instability of the
radical anion toward oxygen in the presence of water.26 However,
inside the argon glovebox, on OTS-treated substrates, molecules 1
and 3 exhibit favorable ambipolar characteristics: for 1, µh ) 0.11
cm2/V·s, µe ) 0.15 cm2/V·s, on/off ratio ) 104 (Figure 1); for 3,
µh ) 0.08 cm2/V·s, µe ) 0.09 cm2/V ·s, on/off ratio ) 104.
Compared to the previously reported ambipolar compounds,7,10-15
our synthesized molecules showed excellent performance in both
p- and n-channel conditions and gave similar hole and electron
mobilities under the inert atmosphere. Under the same device
fabrication conditions, molecule 2 showed high hole transport
performance (µh ) 0.22 cm2/V ·s, on/off ratio ) 104) but no electron
transport behavior.
Molecular orbital levels were measured using cyclic voltammetry
in CH2Cl2 at room temperature. UV-vis spectra were recorded in
solution and on the thin films (Supporting Information). DFT
calculation at B3LYP/6-31G (d) level was used to study the
molecular orbital levels. The results are summarized in Table 2.
The reason that compounds 1 and 3 showed ambipolar character-
istics while compound 2 gave only p-type transport properties could
be attributed to their different LUMOs and the highest occupied
molecular orbital (HOMO) energy levels (Table 2). The LUMO
energy of compound 2 is higher than those of 1 and 3 by 0.11 and
0.27 eV, respectively, as suggested by DFT calculation. Therefore,
electron injection to the LUMO of 2 is more difficult than that to
1 and 3, and hole transport properties are dominant. Actually, using
the same top-contact device structure and employing the low work
function metal (Ag) as drain and source electrode material, we also
observed ambipolar transport from compound 2, although the
electron mobility (0.005 cm2/V·s) was more than 1 order of
magnitude lower than the hole mobility (0.07 cm2/V·s) (see
Supporting Information). This result suggests that precise tuning
of the molecular levels is important to achieve high and balanced
ambipolar properties, and incorporating a nitrogen atom into the
pentacene framework is an effective means to achieve this. The
LUMO levels of 1 and 3 are between -3.2 and -3.4 eV. To obtain
air-stable electron-transport properties, further reducing the LUMO
level is required to avoid quenching of the anions by atmospheric
oxidants.26 However, whether further reduction of the LUMO level
Figure 1. Typical output curves for 1 (top) and 3 (bottom) deposited at
80 °C on OTS-treated SiO2 for L ) 150 µm, W/L ) 20. The gate voltage
ranges from -120 to 120 V. Inset: 2D slip-stacks of 1 and 3 in the single-
crystal view along the axis that is formed by two ethynyl substituents, which
does not correspond to any crystallographic axis.
Single crystals of 1 and 3 have been obtained, and X-ray
crystallography revealed that they pack into highly desirable 2D
brick-layer structures (Figure 1), similar to TIPS-PEN.2 Indeed,
introduction of nitrogen atoms at the terminal ring(s) reduced the
π-π stacking distance between the conjugated acene backbones.
The distances between the aromatic backbones are 3.35 Å for 1
and 3.30 Å for 3, compared to 3.43 Å for TIPS-PEN.22 The reduced
packing distance in the nitrogen-containing TIPS-PENs may be
attributed to negatively polarized nitrogen atoms of one molecule
located over positively polarized C(H) atoms of another.23 Atomic
force microscope (AFM) images of the thin films deposited on
octadecyl(trimethoxy)silane (OTS)-treated SiO2 surfaces showed
grain structures (Supporting Information). The grain sizes increased
from tens of nanometers to nearly 1 µm when the substrate
temperatures were increased from room temperature to 80 °C. Large
grain size is expected to improve the FET properties, which is
confirmed by the device characteristics (Table 1).
Top-contact transistors were fabricated on a 400 nm thermally
grown SiO2 layer on a heavily doped Si gate. Some substrates were
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C O M M U N I C A T I O N S
(2) Anthony, J. E. Chem. ReV. 2006, 106, 5028–5048.
will still give balanced hole and electron transport properties needs
to be further investigated.
(3) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004, 104, 4891–
4946.
(4) Murphy, A. R.; Fre´chet, J. M. J. Chem. ReV. 2007, 107, 1066–1096.
(5) Di, C.-a.; Yu, G.; Liu, Y.; Zhu, D. J. Phys. Chem. B 2007, 111, 14083–
14096.
Table 2. Summary of HOMO, LUMO, and Band Gap of
Compounds 1-3
(6) Meijer, E. J.; Leeuw, D. M. d.; Setayesh, S.; Veenendaal, E. v.; Huisman,
B.-H.; Blom, P. W. M.; Hummelen, J. C.; Scherf, U.; Klapwijk, T. M.
Nat. Mater. 2003, 2, 678–682.
CVsa
Egap
solutionb
DFTd
compd
LUMO
HOMO
Egap
filmc
LUMO
HOMO
Egap
(7) Tang, M. L.; Reichardt, A. D.; Miyaki, N.; Stoltenberg, R. M.; Bao, Z.
J. Am. Chem. Soc. 2008, 130, 6064–6065.
1
2
3
-3.40 -5.42 2.02
-3.33 -5.31 1.98
-3.46 -5.49 2.03
1.84
1.81
1.83
1.63 -3.24 -5.09 1.85
1.57 -3.13 -4.95 1.82
1.63 -3.40 -5.25 1.85
(8) Tang, M. L.; Oh, J. H.; Reichardt, A. D.; Bao, Z. J. Am. Chem. Soc. 2009,
131, 3733–3740.
(9) Noro, S.-i.; Takenobu, T.; Iwasa, Y.; Chang, H.-C.; Kitagawa, S.;
Akutagawa, T.; Nakamura, T. AdV. Mater. 2008, 20, 3399–3403.
(10) Anthopoulos, T. D.; Leeuw, D. M. d.; Cantatore, E.; Setayesh, S.; Meijer,
E. J.; Tanase, C.; Hummelen, J. C.; Blom, P. W. M. Appl. Phys. Lett. 2004,
85, 4205.
a In CH2Cl2 solution (0.1 M Bu4NBF4 electrolyte), Pt electrode. Scan
rate: 100 mV/s. Fc/Fc+ (-4.8 eV to vacuum) internal reference. b Band
gaps estimated from UV-vis absorption edge in solution. c Band gaps
estimated from UV-vis absorption edge in a 25 nm thin film on quartz.
d Computational study at the B3LYP/6-31G(d) level using the Gaussian
03 package. All values are given in eV.
(11) Singh, T. B.; Meghdadi, F.; Gu¨nes, S.; Marjanovic, N.; Horowitz, G.; Lang,
P.; Bauer, S.; Sariciftci, N. S. AdV. Mater. 2005, 17, 2315–2320.
(12) Takahashi, T.; Takenobu, T.; Takeya, J.; Iwasa, Y. Appl. Phys. Lett. 2006,
88, 033505.
(13) Usta, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 8580–
To conclude, we have demonstrated that the incorporation of
nitrogen atoms into the framework of TIPS-PEN is an effective
strategy to produce oligoacene-based ambipolar OFET materials.
Compounds 1 and 3 exhibited high and balanced mobilities and
good on/off ratios. Air-stable and solution-processed high-
performance ambipolar OFETs are currently being pursued.
8581.
(14) Yoon, M.-H.; DiBenedetto, S. A.; Facchetti, A.; Marks, T. J. J. Am. Chem.
Soc. 2005, 127, 1348–1349.
(15) Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV. Mater.
2003, 15, 2009–2011.
(16) Kim, D. H.; Lee, D. Y.; Lee, H. S.; Lee, W. H.; Kim, Y. H.; Han, J. I.;
Cho, K. AdV. Mater. 2007, 19, 678–682.
(17) Tang, Q.; Liang, Z.; Liu, J.; Xu, J.; Miao, Q. Chem. Commun. 2010, 46,
2977–2979.
Acknowledgment. This work is supported by Program for New
Century Excellent Talents in University (NCET), National Natural
Science Foundation of China (NSFC. 20872055, 21073079,
50828301), and 111 project.
(18) Bunz, U. H. F. Chem.sEur. J. 2009, 15, 6780–6789.
(19) Miao, Q.; Nguyen, T.-Q.; Someya, T.; Blanchet, G. B.; Nuckolls, C. J. Am.
Chem. Soc. 2003, 125, 10284–10287.
(20) Bruckner, V.; Tomasz, J. Acta Chim. Hung. 1961, 28, 405–408.
(21) Swartz, C. R.; Parkin, S. R.; Bullock, J. E.; Anthony, J. E.; Mayer, A. C.;
Malliaras, G. G. Org. Lett. 2005, 7, 3163–3166.
(22) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15–18.
(23) Winkler, M.; Houk, K. N. J. Am. Chem. Soc. 2007, 129, 1805–1815.
(24) Ito, Y.; Virkar, A. A.; Mannsfeld, S.; Oh, J. H.; Toney, M.; Locklin, J.;
Bao, Z. J. Am. Chem. Soc. 2009, 131, 9396–9404.
(25) Chua, L.-L.; Zaumseil, J.; Chang, J.-F.; Ou, E. C.-W.; Ho, P. K.-H.;
Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194–199.
(26) Usta, H.; Risko, C.; Wang, Z.; Huang, H.; Deliomeroglu, M. K.;
Zhukhovitskiy, A.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2009,
131, 5586–5608.
Supporting Information Available: Details of experimental pro-
cedures, UV-vis spectra, X-ray diffraction graphs, transfer curves, and
AFM images. This material is available free of charge via the Internet
at http://pubs.acs.org.
References
(1) Coropceanu, V.; Cornil, J.; Filho, D. A. d. S.; Olivier, Y.; Silbey, R.; Bre´das,
J.-L. Chem. ReV. 2007, 107, 926–952.
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