Anthracenedicarboximides as air-stable N-channel semiconductors for thin-film transistors with remarkable current on-off ratios

Article abstract of DOI:10.1021/ja073306f

A new n-type semiconductor family for organic field-effect transistors (FETs), based on core-unsubstituted and core-cyanated anthracenedicarboximides, is reported. By tuning electron affinity, these materials exhibit good electron-transport properties and

Full text of DOI:10.1021/ja073306f

Published on Web 10/13/2007
Anthracenedicarboximides as Air-Stable N-Channel Semiconductors for
Thin-Film Transistors with Remarkable Current On-Off Ratios
Zhiming Wang, Choongik Kim, Antonio Facchetti,* and Tobin J. Marks*
Department of Chemistry and the Materials Research Center, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois 60208
Received May 18, 2007; E-mail: a-facchetti@northwestern.edu; t-marks@northwestern.edu
Thin-film transistors based on organic semiconductors (OTFTs)
have attracted great scientific and technological interest in the quest
for “plastic” electronics.1 The basic OTFT structure includes three
contacts (source, drain, gate), a dielectric, and a semiconductor
(Figure 1), with the latter functioning as either a p-channel (hole-
transporter)2 or n-channel (electron-transporter) charge carrier.3
OTFT p-channel semiconductors have been widely studied and have
achieved acceptable device performance and stability. For example,
OTFTs based on acene films2d and single-crystals4 can attain hole
mobilities µh > 1 cm-2/(V s) in ambient. In contrast, n-channel
Figure 1. Schematic representation of OTFT device components and
chemical structures of the ADI semiconductors.
cores are easily n-dopable electrochemically8 and those of Wasielews-
ki on cyanated rylenes.5a,9 The ER1 of ADIR (Figure 1), where R
) bulky 4-t-butylphenyl, was reported to be ca. -1.1 V (vs SCE)7
and, as in the case of perylenedicarboximide (PDI) derivatives,
should be relatively insensitive to N-substitution. From our recent
studies,6 ADIRs should have the electronic energetic characteristics
of stable n-channel semiconductors but the corresponding OTFTs
should not operate in air. However, DFT computations10 predict
that CN functionalization at the anthracene 9,10 positions should
displace ER1 to ca. -0.2/-0.3 V, more negative than core-cyanated
perylenes (ER1 ≈ 0.0 V), but within the air stability window. We
report here that this design strategy achieves the aforementioned
goals.
The synthesis of core-unsubstituted ADIRs (Scheme 1) involves
two simple steps: (a) 1,2,4,5-tetramethylbenzene bromination, (b)
Diels-Alder cycloaddition/aromatization of 1,2,4,5-tetrakis(dibro-
momethyl)benzene with the requisite N-alkylmaleimide. The ad-
vantages include mild reaction conditions, good yields, and
straightforward product purification via reprecipitation/sublimation.
The structures and purities of the new ADIRs were verified by
elemental analysis, 1H NMR, and mass spectrometry (see Support-
ing Information).
The electrochemical properties of these new anthracenedicar-
boximides reveal important aspects of electronic structure and
substituent effects (Table 1). Thus, ADI8, ADICy, and ADI1Ph
exhibit comparable ER1s (-1.1 to -1.2 V, Figure S2)smuch less
negative than that of parent anthracene (-1.9 V). As expected, such
redox potentials are still close to the overpotentials required for
reactions involving O2 (∼1 V), strongly suggesting that ADIR-
based FETs will not operate in ambient conditions.11
Top-contact OTFTs were fabricated on p+-Si/SiO2 substrates
(Figure 1). Typical current-voltage plots are shown in Figures 2
and S2, with carrier mobilities calculated in saturation from the
equation µsat ) (2ISDL)/[WCox(VSG - Vth)2]. The positive gate and
source-drain voltages demonstrate that these ADIRs are n-channel
materials. Tables 1 and S1 summarize TFT response for HMDS-
treated and untreated Si/SiO2 substrates, respectively. Electron
mobilities as high as 0.02 cm2/(V s) and Ion/Ioff ≈ 107 are achieved
in vacuum. Photoconductivity measurements of anthracene single
crystals reveal comparably high hole and electron mobilities12 but,
organic semiconductors remain problematic because of the inherent
electron trapping tendencies of many materials, especially at the
semiconductor-dielectric interface.5 In principle, there are three
approaches to mobilize/stabilize field-effect-derived electrons:3,5 (a)
use strongly electron-deficient π-conjugated cores; (b) employ
p-channel materials but eliminate deep electron trapping sites by
passivating the dielectric surface; (c) functionalize conventional
p-channel cores with powerful electron-withdrawing and/or hydro-
phobic substituents. Indeed, high mobility n-channel semiconductors
have recently been realized with these approaches.3c,f Some of these
materials exhibit a combination of excellent TFT performance both
in vacuum (µe ≈ 0.3-0.6 cm2/(V s); Ion/Ioff ≈ 107-109; Vth ≈ +30
to 50 V) and in ambient (µe ≈ 0.1-0.6 cm2/(V s); Ion/Ioff ≈ 104-
105; Vth ≈ -30 to 15 V). However, a critical characteristic of high-
mobility air-stable n-channel materials has been the relatively low
Ion/Ioff ratios and large negative threshold voltage shifts versus the
corresponding air-sensitive systemssthey are difficult to “turn
off”.3,6 The large electron affinities of known air-stable n-channel
cores which prevent electron trapping also enhance sensitivity to
electron-doping from the metal contacts and/or donor sites in the
dielectric. An empirical first reduction potential (ER1) window for
both stable TFT electron conduction and low doping levels is
derived by analyzing the redox properties of several rylene/
oligothiophene-based n-channel semiconductors developed in our
group.3f,6 When ER1 j -0.6 V (vs SCE), the material may be an
n-channel semiconductor but not air-stable. When ER1 ) -0.6 to
-0.4 V, the onset of n-channel stability begins. However, for ER1
> 0.0 V, significant doping becomes evident and device current
modulation is difficult to control. Therefore, semiconductors with
an ER1 ranging from -0.4 to 0.0 V should result in TFTs exhibiting
both stable electron transport in air and minimal doping (low Ioff).
Note that semiconductor film morphology optimization may also
play a role in stabilization of TFT transport.7
In this Communication we report a new electron-deficient
semiconductor family based on the anthracenedicarboximide (ADI)
core. The goal here is two-fold: (1) demonstrate a new n-channel
semiconductor family for OTFTs; (2) tune electron affinity to
achieve air stability while maintaining low Ioff currents at VSG
)
0.0 V - enhance Ion/Ioff ratios. This work finds inspiration in the
pioneering studies of Miller showing that linear acenedicarboximide
9
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J. AM. CHEM. SOC. 2007, 129, 13362-13363
10.1021/ja073306f CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Synthesis of Anthracenedicarboximides
both stable electron transport in air and low Ioff. Similarly to other
rylene TFTs, the present devices are sensitive to the combined action
of humidity and sunlight. However, the performance of devices
stored in air with exclusion of light remains stable for at least 4
months after fabrication (Figure S5). Finally, ADIR core cyanation
strongly enhances solubility. Films of ADI8-CN2 can be spin-cast
on a solution-processed 100 nm-thick CPB gate dielectric5a afford-
ing µe ≈ 0.001 cm2/(V s); Ion/Ioff > 105; Vth ≈ 0 V.
Table 1. Electrochemicala and OTFTb Data for ADI Derivatives
E(1)
(V)
µ
Vth
S
compound
(cm2/(V s))
I
on/Ioff
(V)
(V/dec)
In summary, we report a new n-type TFT semiconductor family
based on anthracenedicarboximides. ADI8-CN2-based TFTs ex-
hibit good electron mobility (µe up to 0.02 cm2/(V s)), very high
Ion/Ioff > 107 in ambient conditions as a consequence of balanced
electron affinity. Studies are underway to enhance µe by variation
of N-substituents. Note that within the core-cyanated perylene
family, optimized µe varies by >10× with proper N-substitution.6
anthracene
ADI8
ADICy
ADI1Ph
ADI8-CN2
-1.92
-1.17
-1.17
-1.12
-0.33
0.02c
0.02
0.01
0.01
0.03 (vac.)
0.02 (air)
∼104
-10
+45
+35
+45
+10
+15
4 × 107
5 × 106
2 × 107
6 × 106
2 × 107
2.0
2.9
4.1
1.9
1.9
a In CH2Cl2 (vs SCE) solution (0.1 M Bu4NPF6 electrolyte), Pt electrode.
Scan rate: 100 mV/s. Fc/Fc+ (0.52 V vs SCE) internal reference. b Film
growth temperature is 90 °C. c Single crystal (ref 13).
Acknowledgment. We thank the Northwestern NSF-MRSEC
(Grant DMR-0520513) at Northwestern University and Polyera
Corp. for support of this research.
Supporting Information Available: Synthesis of ADIs, device
fabrication details, UV-vis/electrochemical/FET data. This material
is available free of charge via the Internet at http://pubs.acs.org.
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in contrast to other acenes,4b anthracene-based FETs perform poorly
unless single crystals are employed for p-channel semiconductor.12
Therefore, compared to anthracene, introduction of the diimide
groups and N-hydrocarbon functionalization strongly affects frontier
MO energies, enabling n-channel conductivity and film self-
organization on the insulator for efficient charge transport.
We next synthesized 9,10-core-cyanated ADIs, starting with
ADI8-CN2. Since attempts to brominate ADI8 were unsuccessful,
we developed a synthesis starting from tetramethylbenzene (Scheme
2). Interestingly, bromination of the dibromotetramethylbenzene
does not proceed to the tetrakis(dibromomethyl) derivative (5),
probably because of steric hindrance. Cycloaddition of 4 with 2a
affords core 6, which is aromatized with Br2/Et3N to 7, and finally
dicyanated to afford ADI8-CN2 in 25% overall yield. OTFTs based
on ADI8-CN2 operate in air (vacuum) and exhibit µe ) 0.02 (0.03)
cm2/(V s); Ion/Ioff > 107 (∼107); Vth ≈ +15 (+10 V) (Figure 2).
Note the very high Ion/Ioff ratios and positive Vth for this system, in
agreement with the enhanced electron affinity. I-V hysteresis is
negligible (very small) for the devices measured in vacuum
(ambient) (Figure S3). From cyclic voltammetry, ER1 of ADI8-
CN2 is -0.33 V (Figure S4), in agreement with theoretical
expectations10 and the ambient TFT stability characteristics.
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window between ER1 ) -0.4 and 0.0 V is essential to achieve
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