Indolo[3,2-b]carbazole-based thin-film transistors with high mobility and stability

Article abstract of DOI:10.1021/ja0456149

Proper functionalization of indolo[3,2-b]carbazole led to a new class of high-performance organic semiconductors suitable for organic thin-film transistor (OTFT) applications. While 5,11-diaryl-substituted indolo[3,2-b]carbazoles without long alkyl side chains provided amorphous thin films upon vacuum deposition, those with sufficiently long alkyl side chains such as 5,11-bis(4-octylphenyl)indolo[3,2-b]carbazole self-organized readily into highly crystalline layered structures under similar conditions. OTFTs using channel semiconductors of this nature exhibited excellent field-effect properties, with mobility up to 0.12 cm² V^-1 s ^-1 and current on/off ratio to 10⁷. As this class of organic semiconductors has relatively low HOMO levels and large band gaps, they also displayed good environmental stability even with prolonged exposure to amber light, an appealing characteristic for OTFT applications.

Full text of DOI:10.1021/ja0456149

Published on Web 12/09/2004
Indolo[3,2-b]carbazole-Based Thin-Film Transistors with High
Mobility and Stability
Yiliang Wu, Yuning Li, Sandra Gardner, and Beng S. Ong*
Contribution from the Materials Design and Integration Laboratory,
Xerox Research Centre of Canada, Mississauga, Ontario, Canada L5K 2L1
Received July 21, 2004; Revised Manuscript Received October 7, 2004; E-mail: Beng.Ong@xrcc.xeroxlabs.com
Abstract: Proper functionalization of indolo[3,2-b]carbazole led to a new class of high-performance organic
semiconductors suitable for organic thin-film transistor (OTFT) applications. While 5,11-diaryl-substituted
indolo[3,2-b]carbazoles without long alkyl side chains provided amorphous thin films upon vacuum
deposition, those with sufficiently long alkyl side chains such as 5,11-bis(4-octylphenyl)indolo[3,2-b]carbazole
self-organized readily into highly crystalline layered structures under similar conditions. OTFTs using channel
2
-1
semiconductors of this nature exhibited excellent field-effect properties, with mobility up to 0.12 cm V
-
1
7
s
and current on/off ratio to 10 . As this class of organic semiconductors has relatively low HOMO levels
and large band gaps, they also displayed good environmental stability even with prolonged exposure to
amber light, an appealing characteristic for OTFT applications.
Introduction
displays), desirable form factors and structural features (e.g.,
flexibility for e-paper), and affordability (e.g., ultralow cost for
Organic thin-film transistors (OTFTs) have received enor-
mous interest in recent years as a low-cost alternative to
amorphous hydrogenated silicon transistors for electronic
ubiquitous RFID tags) are essential. For these applications, field
effect transistor (FET) mobility similar or close to that of
amorphous silicon may be sufficient. To enable low-cost OTFT
circuits, it is critical that useful transistor functionality be
achieved under ambient fabrication conditions and that the
OTFTs possess the required stability for long-term operation.
However, not many organic semiconductors have both the
mobility and stability to enable these practical applications.
Three primary classes of p-channel organic semiconductors are
noteworthy: (1) acenes such as tetracene, pentacene, and their
1
-6
applications.
OTFTs are particularly suited for applications
where large-area circuits (e.g., backplane electronics for large
(
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5
(
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e
conditions, afford excellent FET properties.
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8
(
e.g., photoreceptors, organic light-emitting diodes (OLEDs),
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J. AM. CHEM. SOC. 2005, 127, 614-618
10.1021/ja0456149 CCC: $30.25 © 2005 American Chemical Society

Indolo[3,2-b]carbazole-Based Thin-Film Transistors
A R T I C L E S
Scheme 1. Synthesis of 5,11-Disubstituted
Indolo[3,2-b]carbazoles
a
Reagents and conditions: (i) AcOH/EtOH, 50 °C/1 h; (ii) concentrated
H2SO4, 65 °C/15 min; (iii) 1a, 1-bromooctane, aqueous 50% NaOH,
benzyltriethylammonium chloride, DMSO, 50 °C/2 h; 1b, 1-iodo-4-
octylbenzene, 12-crown-6, Cu, 1,2-dichlorobenzene, reflux/24 h; 1c, 4-io-
dotoluene, 12-crown-6, Cu, 1,2-dichlorobenzene, reflux/24 h.
Figure 1. UV-vis absorption spectra of indolo[3,2-b]carbazoles 1: (a)
dilute solutions in chloroform and (b) thin films.
(
larger band gaps) and possessing reasonable stability against
for this structural series. 1a has two octyl chains but without
the intervening phenylene moieties to provide added resonance
stabilization to radical cations, while indolo[3,2-b]carbazole 1c
has the “radical cation-stabilizing” phenylene moieties but
without the octyl side chains to promote molecular ordering.
A general synthesis of indolo[3,2-b]carbazole 1 is illustrated
in Scheme 1. The parent indolo[3,2-b]carbazole, 2, was prepared
via condensation of 1,4-cyclohexanedione with 2 molar equiva-
lents of phenylhydrazine to form cyclohexane-1,4-dione bis-
(phenylhydrazone), followed by double Fischer indolization.11
5,11-Dialkylindolo[3,2-b]carbazole, such as 1a, was readily
prepared by phase-transfer condensation of 2 with alkyl bromide
and aqueous NaOH in DMSO in the presence of a phase-transfer
catalyst, benzyltriethylammonium chloride. 5,11-Diarylindolo-
[3,2-b]carbazoles such as 1b and 1c, on the other hand, were
obtained by Ullmann condensation of 2 with the corresponding
aryl iodides using excess copper and a catalytic amount of 18-
crown-6 in refluxing 1,2-dichlorobenzene. The 5,11-disubstituted
indolo[3,2-b]carbazoles 1a, 1b, and 1c are soluble in organic
solvents such as toluene, chloroform, and chlorobenzene.
The UV-vis spectrum of a dilute solution of 1a in chloroform
showed absorption peaks at 419, 396, and 343 nm, while those
of 1b and 1c showed absorption peaks at 410, 388, and 340
nm (Figure 1a). The spectral absorptions of vacuum-deposited
thin films of 1 were slightly red shifted, with absorption maxima
at 428, 400, and 327 nm for 1a, 415, 341, and 323 nm for 1b,
and 415 and 341 nm for 1c (Figure 1b). The optical band gaps
estimated from the absorption edges of the thin-film spectra
are, respectively, 2.65, 2.8, and 2.8 eV for 1a, 1b, and 1c. These
are larger than those of most organic semiconductors used in
oxidative doping by atmospheric oxygen (lower lying HOMOs).
However, while these materials may be satisfactory for their
intended applications, most provide low FET mobility in OTFTs
due to their inability to form structurally ordered semiconductor
layers.
We report here our studies of a new class of OTFT
semiconductors based on a tertiary diamine structure, 5,11-
disubstituted indolo[3,2-b]carbazole 1. Similar to triarylamine
and carbazole compounds, 1 has a large band gap and lower
lying HOMO than most semiconductors used in OTFTs, and is
therefore expected to be environmentally far more stable. The
relatively large and planar molecular structure of 1 would also
permit facile establishment of higher molecular orders that are
conducive to charge carrier transport. Accordingly, through
strategic substitution, 1 can be transformed into an efficient
charge transport system that may serve as an excellent,
environmentally stable channel semiconductor for OTFTs.
Results and Discussion
Previously, we reported the use of 5,11-bis(1-naphthyl)indolo-
[3,2-b]carbazole as a hole-transporting/electron-blocking mate-
9
rial for OLEDs. However, due to the presence of sterically
encumbered naphthyl substituents, this compound lacked the
ability to order and formed amorphous films upon vacuum
deposition. It provided very low FET mobility when used as a
channel semiconductor in OTFTs. This is not surprising since
charge transport in organic semiconductors is dominated by
hopping,10 and amorphous materials are not expected to give
high mobility. To be an efficient semiconductor for OTFTs,
the indolo[3,2-b]carbazole system needs to provide (i) proper
molecular ordering for efficient charge carrier transport and (ii)
sufficient stabilization to radical cations (i.e., hole carriers) to
enable efficient hole injection. Accordingly, an indolo[3,2-b]-
carbazole system such as 1b (Scheme 1), which has two
strategically placed long alkyl chains to induce molecular self-
assembly and two conjugated triarylamino hole-receptor sites
capable of providing resonance stabilization to injected holes,
would be an ideal semiconductor system for OTFTs. Indolo-
2-5
OTFTs. Semiconductors of this nature with large band gaps,
which absorb in the UV or near-UV regions are suitable for
fabricating transparent OTFT circuits which would be useful
12
for optoelectronic devices requiring circuit transparency. Cyclic
voltammetric measurements of 1b in 0.1 M Bu4ClO4/CH2Cl2
solution showed a reversible oxidation process with an onset
oxidation potential at 0.74 eV against a Ag/AgCl electrode,
corresponding to an estimated HOMO level of 5.12 eV from
vacuum. This is lower than those of most regioregular poly-
3a,e
[
3,2-b]carbazoles 1a and 1c were also synthesized for studies
3
e,f
thiophenes (4.9-5.0 eV), thus indicating higher oxidative
to validate our hypothesis on the structure-property correlation
stability.
(
9) Hu, N.; Xie, S.; Popovic, Z.; Ong, B.; Hor, A. J. Am. Chem. Soc. 1999,
1
21, 5097-5098.
(11) Robinson, B. J. Chem. Soc. 1963, 3097-3099.
(12) Nomura, K.; Ohta, H.; Ueda, K.; Kamiya, T.; Hirano, M.; Hosono, H.
Science 2003, 300, 1269-1272.
(
10) Horowitz, G.; Delannoy, P. In Handbook of Oligo- and Polythiophenes;
Fichou, D., Ed.; Wiley-VCH: Weinheim, Germany, 1999; pp 283-316.
J. AM. CHEM. SOC.
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A R T I C L E S
Wu et al.
Figure 2. X-ray diffraction patterns of (a) a vacuum-deposited thin film
of 1a, (b) a vacuum-deposited thin film of 1b, (c) a vacuum-deposited thin
film of 1c, (d) a powder of sublimed 1b, and (e) a powder of 1b
recrystallized from toluene.
Figure 3. AFM topographic images of 1b: (a) vacuum-deposited film on
a substrate at 25 °C, (b) vacuum-deposited film on a substrate at 50 °C, (c)
blown-up image from (b) showing terrace-like layered structures, and (d)
step heights of crystalline terrace layers as measured by a profiler along
the dotted line direction in (c).
As expected, both 1a and 1b formed highly crystalline thin
films upon vacuum deposition on a glass substrate with sharp
diffraction peaks in their X-ray diffraction (XRD) patterns. On
the other hand, only featureless broad diffraction peaks were
present in the thin-film XRD pattern of 1c, indicating its
amorphous nature as a result of its inability to undergo proper
molecular organization in the thin film. These results unequivo-
cally affirmed the critical role of long alkyl side chains (e.g.,
octyl chains) in the molecular self-assembly of indolo[3,2-b]-
carbazole compounds during vacuum deposition. The thin-film
XRD pattern of 1a (Figure 2a) displayed a primary diffraction
peak at 2θ ) 5.24° (d-spacing 16.85 Å), with the second- and
third-order diffraction peaks at 2θ ) 10.48° and 15.80°. On
the other hand, the thin-film XRD pattern of 1b (Figure 2b)
showed stronger diffraction intensities, with the primary dif-
fraction peak appearing at 2θ ) 3.62° (d-spacing of 24.41 Å).
Sharp diffraction peaks up to the fifth order were observed
different crystal structure. Through comparison of the powder
XRD patterns of 1b from a sublimed sample (Figure 2d) and a
recrystallized sample (Figure 2e), it is apparent that they did
not have the same crystal structures. The primary diffraction
peak of the sublimed sample appeared at 2θ ) 3.62°, while
that of the recrystallized sample appeared at 2θ ) 5.48°.
Accordingly, compound 1b may exist in different crystal forms
depending on the crystallization and deposition conditions.
Preliminary X-ray results of a single crystal of 1b gave a very
complex diffraction pattern, manifesting its polycrystalline
nature. Definitive assignment of the crystal structures will
require further crystallographic studies of carefully grown
crystals under well-controlled crystallization and deposition
conditions.
(third- and fifth-order diffraction peaks at 2θ ) 10.91° and
1
8.09°, respectively), indicating a high degree of ordering and
The crystalline structures in the vacuum-deposited thin films
of 1b at different substrate temperatures can be directly
visualized through the AFM technique. Parts a and b of Figure
3 are the AFM topographic images of 1b vacuum-deposited on
an octyltrichlorosilane (OTS-8)-treated SiO2 surface at two
different substrate temperatures. Small crystal grains of average
diameter of about 100-200 nm were observed in the thin film
deposited at a substrate temperature of 25 °C. The crystal grains
grew in size with increasing substrate temperature, and at 50
°C, they were relatively large and elongated in shape, with the
average grain size being about 0.2 µm × 1 µm. These crystal
grains showed terrace-like layered structures, with the layers
lying parallel to the SiO2 surface (Figure 3c). A cross-section
of a terraced crystal structure gave step heights of about 2.45
nm, which are identical to the interlayer distance extracted from
XRD data.
crystallinity. The intense diffraction peak at 2θ ) 20.02° (d-
spacing 4.4 Å) in powder XRD of 1b (Figure 2d) is likely due
to the π9π stacking order. Compared to this powder XRD
pattern, the absence of many reflections, particularly π-π
stacking reflections, in thin-film XRD patterns of 1a and 1b
indicates that the molecules preferentially adopted an “edge-
on” orientation relative to the substrate. This led to the formation
of layered structures with interlayer distances of 16.85 and 24.41
Å for 1a and 1b, respectively. This type of edge-on orientation
with the π9π stacking direction parallel to the substrate is known
to be favorable for in-plane charge transport.3d On the basis of
two fully extended peripheral side chains tilted at an angle of
9
approximately 25° against the indolocarbazole molecular plane,
the molecular heights of 1a and 1b were estimated to be,
respectively, about 24 and 30 Å. Thus, the observed shorter
interlayer distances would suggest that either the molecules had
an inclining orientation with respect to the substrate or the
interlayer involves some interdigitation of side chains of the
molecules in the adjacent layers. The diffraction peak of 1b at
To study the FET properties of indolo[3,2-b]carbazole
semiconductors 1, a bottom-gate, top-contact OTFT with a 100
nm vacuum-deposited channel semiconductor was utilized as
our test device structure. All three indolo[3,2-b]carbazole
compounds displayed p-type accumulation FET behaviors when
2
θ ) 5.48° in the thin-film XRD pattern may be due to a
616 J. AM. CHEM. SOC.
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VOL. 127, NO. 2, 2005

Indolo[3,2-b]carbazole-Based Thin-Film Transistors
A R T I C L E S
Figure 4. FET characteristics of an exemplary OTFT with vacuum-
deposited 1b (substrate at 50 °C, channel length 90 µm, channel width 5000
µm): (a) output curves at different gate voltages and (b) a transfer curve in
the saturated regime at a constant source-drain voltage of -40 V and square
root of the absolute value of the current as a function of the gate voltage.
Figure 5. Transfer characteristics of OTFTs (channel length 90 µm, channel
width 5000 µm, VDS ) -60 V) in the saturated regime with (a) 1b as the
channel semiconductor deposited at a substrate temperature of 50 °C and
(b) HT-P3HT as the channel semiconductor spin coated from a 0.7 wt %
solution in chlorobenzene at ambient conditions. Transfer curves were
obtained in the dark (red line) and then measured under white light (blue
line).
Table 1. FET Mobility and Current On/Off Ratio of OTFTs with 1b
Semiconductor Layers Deposited at Various Substrate
Temperatures (Td)
T
d
mobility
on/off
ratio
mobility and current on/off ratio. Similar to what has been seen
(cm2 V- s-1)
1
(
°
C)
2a,b
in many OTFTs (e.g., pentacene TFTs), the variation in FET
performance of 1b with substrate temperature was related to
the change in crystal grain size as noted by AFM analysis; i.e.,
larger crystal grains yielded higher mobility and current on/off
10 106
5
-
2
5
7
5
0
0
0.01-0.02
0.07-0.12
0.007-0.01
6
-
7
5
10 10
10
ratio. As with most other solution-processable organic semi-
used as channel semiconductors in OTFTs. With the semicon-
ductors deposited at a substrate temperature of 25 °C, the
observed FET mobility follows the order 1b > 1a . 1c. As
3a,b
conductors (e.g., oligothiophenes),
the devices with the
solution-deposited semiconductor of 1b also gave reduced
-3
2
-1 -1
5
mobility (∼7.0 × 10 cm V s ) and on/off ratio (5 × 10 ).
An appealing attribute of indolo[3,2-b]carbazole semiconduc-
tors such as 1b is their environmental stability. They exhibited
much better air and photochemical stability than most common
organic semiconductors reported to date. For example, the
transfer characteristics of OTFTs with 1b semiconductor
deposited at a substrate temperature of 50 °C (Figure 5a) were
found to be essentially the same in ambient conditions when
the measurements were carried out in the dark and under white
-5
2
-1
expected, 1c provided the lowest mobility (1.0 × 10 cm V
-1
s ) in this series, and no improvements in FET performance
even at substrate temperatures up to 70 °C. The observed low
mobility of 1c was obviously due to its lack of appropriately
long alkyl side chains to induce molecular organization during
vacuum deposition. On the other hand, 1a, which has two
peripheral octyl side chains, yielded higher crystallinity in thin
-
3
2
films, and thus higher FET mobility ((1.3-3.0) × 10 cm
-1
-1
V
s ) in OTFTs. The mobility of 1a was nonetheless still
7
light. The current on/off ratio was ∼10 in both cases,
low as this structure is devoid of aryl substituents at the 5,11-
positions to provide the needed resonance stabilization to
injected hole carriers. In line with these considerations, we
observed that the OTFTs with 1b semiconductor, which has
N,N-bis(4-octylphenyl) substitutions, exhibited the highest
demonstrating the excellent stability of 1b against oxidative
doping or photochemical degradation. The devices were also
relatively stable over an extended period of time in air under
amber light exposure. Our earlier OTFTs fabricated with 1b at
a substrate temperature of 25 °C had shown only a slight
2
-1 -1
mobility (0.01-0.02 cm V s ). This is the manifestation of
the ability of 1b to self-organize through the octyl side-chain
interactions and to provide sufficient resonance stabilization via
phenylene substituents.
2
-1
decrease in mobility (from 0.01-0.02 to 0.007-0.01 cm V
-1
5
6
4-
5
s ) and current on/off ratio (from 10 -10 to 10 10 ) after
continuous exposure to amber light in ambient conditions for
more than one year. For comparison, the FET performance of
OTFTs with the well-known regioregular head-to-tail poly(3-
hexylthiophene) semiconductor (HT-P3HT) was found to be
very sensitive to air, particularly on exposure to light. When
measured in air in the dark, the HT-P3HT devices gave an
As noted, the FET performance of 1b varied with the substrate
temperature, and optimum performance was achieved when 1b
was deposited at a substrate temperature of 50 °C (Table 1).
The FET behaviors of such a device (Figure 4) conform well
to the conventional transistor models in both the linear and
saturated regimes. The output curve shows very good saturation
behaviors with clear saturation currents. The device gave a
3
average initial current on/off ratio of about 10 , but the current
on/off ratio dropped to less than 10 when the measurements
2
were carried out under white light. On exposure to amber light
in air, the HT-P3HT devices rapidly degraded in performance,
with their current on/off ratio dropping from 4000 to 10 over a
period of just 8 h.
2
-1 -1
mobility up to 0.12 cm V
s
and a current on/off ratio up
7
to 10 , with a threshold voltage of -7 V and a subthreshold
-
1
swing of 1 V dec . These are among the best overall OTFT
properties reported. A slight contact resistance, due to the
energetic mismatch between the semiconductor and electrode
materials, was however observed. The OTFTs fabricated at
substrate temperatures higher than 50 °C showed decreased
Conclusion
We have demonstrated that properly functionalized indolo-
[3,2-b]carbazoles 1 represent a new class of high-performance
J. AM. CHEM. SOC.
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VOL. 127, NO. 2, 2005 617

A R T I C L E S
Wu et al.
p-channel semiconductors with good environmental stability for
organic electronic applications. To achieve high FET mobility,
two critical features need to be present in the indolo[3,2-b]-
carbazole structure: (i) a functionality, such as 5,11-diaryl
substitution, that is capable of providing sufficient resonance
stabilization to injected hole carriers (radical cations) and (ii)
appropriately long peripheral alkyl side chains to promote
molecular self-organization. Another appeal of 5,11-diarylin-
dolo[3,2-b]carbazoles as channel semiconductors for OTFTs is
their environmental stability owing to their relatively low-lying
HOMOs and large band gaps. We anticipate further FET
performance improvements will be made through structural and
device fabrication optimizations.
Acknowledgment. Partial financial support is provided by
the National Institute of Standards and Technology through an
Advanced Technology Program Grant (70NANB0H3033).
Supporting Information Available: Instrumentation, materi-
als synthesis and characterization, and OTFT fabrication and
evaluation (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
JA0456149
618 J. AM. CHEM. SOC.
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VOL. 127, NO. 2, 2005