Pyromellitic diimides: Minimal cores for high mobility n-channel transistor semiconductors

Article abstract of DOI:10.1021/ja805746h

Three pyromellitic diimides were synthesized in high yields by one conventional reaction between pyromellitic dianhydride and various amines. The films made from these pyromellitic diimides derivatives exhibit a mobility up to 0.079 cm²/(V.s). In addition, the on/off ratios of n-channel devices are as high as 1000000. Copyright

Full text of DOI:10.1021/ja805746h

Published on Web 10/09/2008
Pyromellitic Diimides: Minimal Cores for High Mobility n-Channel Transistor
Semiconductors
Qingdong Zheng, Jia Huang, Amy Sarjeant, and Howard E. Katz*
Department of Materials Science and Engineering and Department of Chemistry, The Johns Hopkins UniVersity,
103 Maryland Hall, 3400 North Charles Street, Baltimore, Maryland 21218
Received July 23, 2008; E-mail: hekatz@jhu.edu
The past decade has witnessed an increasing interest in organic
thin film transistors (OTFTs) due to their applications in light-
emitting displays, radio frequency identification tags, and sensors.1-3
Organic semiconductors can function either as a p-channel or
n-channel charge carrier. While many organic materials can be used
for p-channel OTFTs, there is only a limited number of organic
materials that can be used for n-channel OTFTs.1,2,4 Therefore, there
is a need to explore more n-channel materials with high mobility
and stability, to allow for combining n-channel and p-channel
transistors in complementary circuits that have the advantages of
low power dissipation, low noise, and greater operational stability.5
Since the report of naphthalenetetracarboxylic diimides as air stable
n-channel materials with mobility up to 0.1 cm2/(V.s), a large
Figure 1. Crystal structure (left) and crystal packing diagram of 1 (right).
number of n-channel materials have been based on either naph-
thalene or perylene tetracarboxylic diimides.6,7 Recently, several
Scheme 1. Synthetic Route to Pyromellitic Diimide Derivatives
anthracene tetracarboxylic diimides were synthesized by Marks et
al. through multistep reactions and found to be good n-channel
materials with high on-off ratios.8 The common structural features,
the tetracarboxylic diimides, were flanked on both sides of the planar
aromatic rings, which were varied from two-ring naphthalene to
three-ring anthracene, and finally to five-ring perylene.
Though pyromellitic dimides are best known as segments of
highly insulating polyimide dielectrics, it is nevertheless quite
surprising that no attempt has been made to fabricate transistors
from pyromellitic diimide derivatives, which have the simplest
aromatic ring (benzene) in the center, and the tetracarboxylic
diimides on both sides of the benzene ring. Pyromellitic diimide
derivatives can be easily prepared by one-step reaction between
pyromellitic dianhydride and various amines. Compared to naph-
thalene or perylene tetracarboxylic diimides, the facile synthesis
of pyromellitic diimide derivatives offers an advantage in large scale
synthesis due to its high conversion yield and ease of purification.
Thus, it is possible to screen a large number of imide side chains
and investigate the impact of side chains on the mobility and
environmental stability of the pyromellitic diimide derivatives. In
this Communication, we report synthesis and transistor behaviors
of three pyromellitic diimide derivatives using fluorinated side
chains. The fluorinated side chains are introduced to stabilize the
n-channel OTFTs in air, as has been previously observed.7c,f,g
The synthesis of pyromellitic diimide derivatives involves one
simple conventional reaction (Scheme 1). Amines and pyromellitic
dianhydride were heated in dimethylformamide (DMF) in 110 °C.
After the solution was cooled down, the crystallized product was
collected by filtration, then washed with methanol, and finally
sublimed under vacuum. Elemental analysis results indicate the high
purity for the sublimed compounds. The synthesis and characteriza-
tion details for 1-3 are described in Supporting Information.
Single crystals of 1 were obtained by slowly cooling hot saturated
DMF solutions. The unit cell of the single crystal is monoclinic
with a ) 10.24 Å, b ) 11.53 Å, c ) 9.28 Å, R ) 90°, ꢀ ) 97.78°,
γ ) 90°. The crystal structure and crystal packing diagram of 1
are shown in Figure 1. Interestingly, the crystal structure of 1
exhibits a close π-π packing between the side chain benzene ring
and the pyromellitic diimide core. X-ray diffraction (XRD) spectra
of the sublimed thin films for compounds 1, 2, and 3 showed layer
spacings of 18.6, 36.8, and 22.6 Å in that order, which is consistent
with their molecular lengths. Molecules are believed to orient
perpendicular to the substrate, though there is a modest degree of
tilting. The 18.6 Å d-spacing observed for 1 reveals that the thin-
film packing of 1 is different from its single crystal packing. For
1, several orders of the layer diffraction were observed for the
samples with octadecyltrichlorosilane (OTS) surface treatment and
deposited at 65 °C, indicating its high crystallinity in thin film phase
(see Supporting Information).
The semiconducting films of 45 nm thickness were deposited at
substrate temperatures of 25-115 °C, with SiO2 dielectrics, and
evaporated gold top contact source and drain electrodes. The
channel widths and lengths were approximately 6.5 mm, and 270
µm, respectively. Some substrates were treated with OTS to form
a self-assembled monolayer. Devices on compound 1 were tested
under different conditions to investigate the influence of substrate
temperature and surface treatment on its mobility.
The measured current-voltage characteristics for all the devices
show well-defined gate modulation. As an example, Figure 2 depicts
the current-voltage characteristics of 1 prepared by sublimation
9
14410 J. AM. CHEM. SOC. 2008, 130, 14410–14411
10.1021/ja805746h CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Mobilities, On-Off Current Ratios and Threshold Voltages of Pyromellitic Diimides Based OTFTs Prepared by Vapor Depositiona
compounds
Ts (°C)
substrate treatment
µ (cm2 V-1 s-1
)
max µ (cm 2V- 1s-1
)
Vth (V)
on/off
no. devices tested
1
1
1
1
1
2
3
25
25
65
65
70
OTS
N
OTS
N
OTS
OTS
OTS
0.036 ( 0.001
0.004 ( 0.0004
0.055 ( 0.001
0.029 ( 0.001
0.070 ( 0.003
0.069 ( 0.004
0.023 ( 0.004
0.038 (0.025)
0.0045
0.056
18.1 (28.5)
18.6
14.9
105 (103)
104
12
8
4
106
0.030
29.2
106
4
0.074 (0.039)
0.079 (0.054)
0.030 (0.013)
22.4 (31.2)
14.4 (15.4)
13.7 (21.6)
106 (105)
106 (104)
105 (105)
14
16
7
115
25
a Values in parenthesis are measured in air. Ts, substrate temperature’ OTS, octadecyltrichlorosilane; N, no treatment; Vth, threshold voltage.
two reversible one-electron reductions, which are in agreement with
reported observations for pyromellitic diimide derivatives.9
In summary, a novel family of n-channel materials based on
pyromellitic diimide derivatives have been synthesized and fabri-
cated for organic field effect transistors. The field effect electron
mobility of these materials is found to be as high as 0.079 cm2/
(V.s). In addition, the on/off ratios of pyromellitic diimide based
transistors can reach a high value of 1 000 000. With its shorter
π-conjugation length, the pyromellitic diimides are relatively
transparent to visible light compared to naphthalene or perylene
tetracarboxylic diimides with the same side chain, which makes
them promising candidates for transparent electronics.
Figure 2. Current-voltage characteristics of 1 prepared by sublimation at
Acknowledgment. We are grateful to AFOSR (Award No.
substrate temperature of Ts ) 70 °C: (a) plot of Id versus Vd; (b) plot of
Id-sat versus Vg.
FA9550-06-01-0076) for support of this work.
Supporting Information Available: Details of synthesis, experiment
at substrate temperature of Ts ) 70 °C. Mobilities were calculated
procedures, DSC traces, UV-vis/electrochemical data for compound
1 and 3, X-ray diffraction graphs, AFM images, current-voltage
characteristics of 2 and crystallographic data of 1 in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
from the saturation regime and fitted in the regions of highest slope,
and the results are listed in Table 1. Table 1 also lists a summary
of device performance of 1 deposited under some other conditions,
and of 2 and 3 deposited at 115 and 25 °C, respectively. The results
show that the charge carrier mobility for 1 increases when there is
a self-assembled OTS thin layer between the gate dielectrics and
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