Flexible low-voltage organic thin-film transistors enabled by low-temperature, ambient solution-processable inorganic/organic hybrid gate dielectrics

Article abstract of DOI:10.1021/ja107079d

We report here on the design, synthesis, processing, and dielectric properties of novel cross-linked inorganic/organic hybrid blend (CHB) dielectric films which enable low-voltage organic thin-film transistor (OTFT) operation. CHB thin films (20-43 nm thick) are readily fabricated by spin-coating a zirconium chloride precursor plus an α,ω-disilylalkane cross-linker solution in ambient conditions, followed by curing at low temperatures (～150 °C). The very smooth CHB dielectrics exhibit excellent insulating properties (leakage current densities ～10^-7 A/cm²), tunable capacitance (95-365 nF/cm²), and high dielectric constants (5.0-10.2). OTFTs fabricated with pentacene as the organic semiconductor function well at low voltages (<-4.0 V). The morphologies and microstructures of representative semiconductor films grown on CHB dielectrics prepared with incrementally varied compositions and processing conditions are investigated and shown to correlate closely with the OTFT response.

Full text of DOI:10.1021/ja107079d

Published on Web 11/18/2010
Flexible Low-Voltage Organic Thin-Film Transistors Enabled
by Low-Temperature, Ambient Solution-Processable
Inorganic/Organic Hybrid Gate Dielectrics
Young-geun Ha,† Sunho Jeong,†,§ Jinsong Wu,‡ Myung-Gil Kim,†
Vinayak P. Dravid,‡ Antonio Facchetti,*,† and Tobin J. Marks*,†,‡
Department of Chemistry, Material Science Engineering, and Materials Research Center,
Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 United States
Received August 6, 2010; E-mail: a-facchetti@northwestern.edu; t-marks@northwestern.edu
Abstract: We report here on the design, synthesis, processing, and dielectric properties of novel cross-
linked inorganic/organic hybrid blend (CHB) dielectric films which enable low-voltage organic thin-film
transistor (OTFT) operation. CHB thin films (20-43 nm thick) are readily fabricated by spin-coating a
zirconium chloride precursor plus an R,ω-disilylalkane cross-linker solution in ambient conditions, followed
by curing at low temperatures (∼150 °C). The very smooth CHB dielectrics exhibit excellent insulating
properties (leakage current densities ∼10-7 A/cm2), tunable capacitance (95-365 nF/cm2), and high dielectric
constants (5.0-10.2). OTFTs fabricated with pentacene as the organic semiconductor function well at low
voltages (<-4.0 V). The morphologies and microstructures of representative semiconductor films grown
on CHB dielectrics prepared with incrementally varied compositions and processing conditions are
investigated and shown to correlate closely with the OTFT response.
Introduction
Organic thin-film transistor (OTFT)-based electronics per-
forming simple operations offer unique attractions compared
to conventional inorganic technologies, including low cost,
large-area coverage, and low processing temperatures suitable
for flexible substrates.1 A typical bottom-gate top-contact OTFT
structure is shown in Figure 1A. In this device architecture, the
current between the source and the drain contact is modulated
by both the source-drain voltage (VSD) and the source-gate
voltage (VG). When the device is in the off state (VG ) 0.0 V),
the channel current must be very low, whereas in the on state
of the source-gate voltage relationship (VG * 0.0 V), large,
abrupt current increases must be achieved. The saturation current
in organic thin-film transistors is generally described by the
following equation:
Figure 1. (A) Schematic of a bottom-gate/top-contact OTFT device
geometry and the dielectric layer employed in this study. (B) Chemical
structures of the ZrO2 and siloxane cross-linkers employed and the CHB
dielectric densification process.
W
2L
µCi(VG - VT)2
(1)
where µ is the field-effect charge carrier mobility, Ci is the
capacitance per unit area of the dielectric, VT is the threshold
voltage, and W and L are the OTFT channel width and length,
respectively. Despite recent impressive progress in developing
new organic semiconductors,2 large OTFT operating voltages,
reflecting the intrinsically low mobilities of organic versus
conventional inorganic semiconductors, remain one of the major
challenges to overcome.3 For low-power applications such as
RF-ID tags, flat panel displays, and portable electronics, it is
IDS
)
† Department of Chemistry.
‡ Department of Materials Science and Engineering.
§ Current address: Korea Research Institute of Chemical Technology,
19 Sinseongno, Yuseong, Daejeon 305-600 (Korea).
(1) (a) Daniele, B.; Gilles, H. AdV. Mater. 2009, 21, 1473. (b) Roberts,
M. E.; Mannsfeld, S. C. B.; Queralto´, N.; Reese, C.; Locklin, J.; Knoll,
W.; Bao, Z. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 12134. (c) Cho,
J. H.; Lee, J.; Xia, Y.; Kim, B.; He, Y. Y.; Renn, M. J.; Lodge, T. P.;
Frisbie, C. D. Nat. Mater. 2008, 7, 900. (d) Luisa, T.; Gianluca, F.;
Francesco, M.; Cristina, T.; Hassan, O.; Ludovico, V.; Francesco, B.;
Francesco, P.; Giorgio, Z.; Francesco, N. Nat. Mater. 2008, 7, 412.
(e) Murphy, A. R.; Frechet, J. M. J. Chem. ReV. 2007, 107, 1066. (f)
Klauk, H. Organic Electronics: Materials, Manufacturing, and Ap-
plications; Wiley-VCH: Weinheim, Germany, 2006. (g) Lim, B.; Baeg,
K.-J.; Jeong, H.-G.; Jo, J.; Kim, H.; Park, J.-W.; Noh, Y. Y.; Vak, D.;
Park, J.-H.; Park, J.-W.; Kim, D.-Y. AdV. Mater. 2009, 21, 2808.
(2) (a) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C. E.; Quin, J.; Dolz, F.;
Kastler, M.; Facchetti, A. Nature 2009, 457, 679. (b) Zaumseil, J.;
Sirringhaus, H. Chem. ReV. 2007, 107, 1297. (c) Sun, Y.; Rogers,
J. A. AdV. Mater. 2007, 19, 1897. (d) Sirringhaus, H. AdV. Mater.
2005, 17, 2411. (e) Baeg, K.-J.; Kim, D.; Kim, D.-Y.; Koo, J. B.;
You, I.-K.; Choi, W. S.; Noh, Y. Y. Thin Solid Films 2010, 518, 4024.
(f) Noh, Y. Y.; Sirringhaus, H. Org. Electron. 2009, 10, 174.
9
17426 J. AM. CHEM. SOC. 2010, 132, 17426–17434
10.1021/ja107079d  2010 American Chemical Society

OTFTs Enabled by Inorganic/Organic Dielectrics
A R T I C L E S
mandatory to achieve high thin-film transistor (TFT) drain
currents (ISD) at acceptably low operating voltages. Without
changing the device geometry (W and L) and semiconductor
material (µ), equivalent OTFT ISD parameters should be achiev-
able at lower operating voltages by increasing the gate dielectric
capacitance Ci, given by the following equation:
high-k metal oxide films, particularly the polycrystalline ones,
are too brittle for flexible electronics applications. Another
strategy to increase the k/d ratio and mechanical flexibility is
to use polymer/high-k inorganic nanoparticle composites.6
However, the dielectric constants of these composite materials
are generally dominated by the relatively low-k polymeric
component. To increase composite k values, large nanoparticle
loadings are necessary; however, this generally results in greatly
enhanced surface roughness and poor mechanical flexibility.
Last, hybrid gate dielectrics composed of self-assembled mono-
layers or multilayers containing ultrathin inorganic oxides show
promise for low-voltage OTFTs;7 however, low-cost pathways
for integration into large-volume coating processes are still under
development.
k
Ci ) ε
(2)
0d
where ε0 is the vacuum permittivity, k the dielectric constant,
and d the thickness of the dielectric layer.
From eq 2, note that operating bias reduction can be achieved
by either increasing the dielectric constant (k) or decreasing the
thickness (d) of the gate dielectric layer. An increase in the k/d
ratio is also essential for efficient device scalability, a prereq-
uisite to enhancing low-power TFT operation. In addition to
an increased k/d ratio, it is also necessary for the gate dielectric
to be processable from solution and at low temperatures to
enable compatibility with flexible plastic substrates and circuitry
production via roll-to-roll and related fabrication technologies.
To date, cross-linked polymer films, metal oxides, polymer/
high-k nanoparticle composites, and hybrid inorganic/organic
dielectrics are proven candidates for low-voltage TFTs.4-7
However, all of these materials have some limitations in
achieving practical, flexible, low-voltage TFTs. First, cross-
linked polymeric materials typically have relatively low k values,
and thus, TFT drain currents (ISD) at low operating voltages
are frequently insufficient.4 An alternative approach is to employ
high-k materials such as metal oxide (MO) films.5 However,
high-quality MO dielectric films typically require high growth/
annealing temperatures (>400 °C) and/or vacuum deposition
(e.g., atomic layer or chemical/physical vapor deposition) to
ensure acceptably low leakage currents. Furthermore, most
To address the above issues, optimum gate dielectric films
should have high k values and good insulating properties and
be fabricable via solution-processing techniques at low tem-
perature under ambient conditions. Here we report a new class
of cross-linked inorganic/organic hybrid blend dielectric materi-
als (Figure 1B) compatible with plastic substrates and deposited
via a solution-phase process, which affords high gate capaci-
tances, low leakage current densities, and smooth surfaces when
integrated into TFT structures. In addition to very large dielectric
strength, the combined properties of both soft-matter and hard-
matter components enables these new materials to be mechani-
cally flexible and thermally/environmentally robust. Further-
more, these densely cross-linked hybrid blends ensure that
subsequent device layers can be solution-processed without
dissolution of the dielectric layer.
The new composites are based on high-k zirconium oxide (k
) 16-25)8 as the inorganic component, derived from com-
mercially available ZrCl4. Note that zirconium oxide has a wide
band gap (5.8 eV),9 which is potentially useful for transparent
device applications. The organic component of this hybrid gate
dielectric is an R,ω-disilylalkane cross-linking agent. Spin-
coated formulations of these two components result in dielectric
films exhibiting high k values (5-10), high capacitances
(95-365 nF/cm2), and low leakage current densities (4 × 10-7
to 6 × 10-6 A/cm2 at 2 MV/cm). Furthermore, it will be shown
that these dielectrics function far more effectively than neat ZrOx
films which are fabricated in parallel for control experiments.
Finally, pentacene OTFTs fabricated on plastic substrates with
these new hybrid dielectrics are shown to operate at low voltages
(3) (a) Ortiz, R. O. P.; Facchetti, A.; Marks, T. J. Chem. ReV. 2010, 110,
205. (b) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J.
AdV. Mater. 2009, 21, 1407. (c) Facchetti, A.; Yoon, M. H.; Yan, H.;
Marks, T. J. AdV. Mater. 2005, 17, 1705. (d) Veres, J.; Ogier, S.;
Lloyd, G. Chem. Mater. 2004, 16, 45434555.
(4) (a) Roberts, M.; Queralt, N.; Mannsfeld, S.; Reinecke, B.; Kno, W.;
Bao, Z. Chem. Mater. 2009, 21, 2292. (b) Kim, C.; Wang, Z.; Choi,
H.-J.; Ha, Y.-G.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008,
130, 6867. (c) Jang, Y.; Kim, D. H.; Park, Y. D.; Cho, J. H.; Hwang,
M.; Cho, K. Appl. Phys. Lett. 2006, 88, 7072101. (d) Yoon, M. H.;
Yan, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127,
10388. (e) Noh, Y. Y.; Zhao, N.; Caironi, M.; Sirringhaus, H. Nat.
Nanotechnol. 2007, 2, 784.
(5) (a) Choi, S. J.; Lee, S.; Han, K. K.; Lee, K.; Kim, D.; Kim, J.; Lee,
H. H. Appl. Phys. Lett. 2007, 90, 063507. (b) Wang, G.; Moses, D.;
Heeger, A. J.; Zhang, H.-M.; Narasimhan, M.; Demaray, R. E. J. Appl.
Phys. 2004, 95, 316. (c) Lee, J.; Kim, J. H.; Im, S. Appl. Phys. Lett.
2003, 83, 2689. (d) Javey, A.; Kim, H.; Brink, M.; Wang, Q.; Ural,
A.; Guo, J.; McIntyre, P.; McEuen, P.; Lundstrom, M.; Dai, H. Nat.
Mater. 2002, 1, 241. (e) Tate, J.; Rogers, J. A.; Jones, C. D. W.; Vyas,
B.; Murphy, D. W.; Li, W.; Bao, Z.; Slusher, R. E.; Dodabalapur, A.;
Katz, H. E. Langmuir 2000, 16, 6054.
(7) (a) Acton, O.; Osaka, I.; Ting, G.; Hutchins, D.; Ma, H.; McCullough,
R. D.; Jen, A. K.-Y. Appl. Phys. Lett. 2009, 95, 113305/1. (b)
DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J. J. Am.
Chem. Soc. 2009, 131, 7158. (c) Ha, Y. G.; Facchetti, A.; Marks, T. J.
Chem. Mater. 2009, 21, 1173. (d) Acton, O.; Ting, G.; Ma, H.; Ka,
J.; Yip, H.; Tucker, N. M.; Jen, A.K.-Y. AdV. Mater. 2008, 20, 3697.
(e) Klauk, H.; Zschieschang, U.; Pflaum, J.; Halik, M. Nature 2007,
445, 745. (f) Lee, B. H.; Ryu, M. K.; Choi, S. Y.; Lee, K. H.; Im, S.;
Sung, M. M. J. Am. Chem. Soc. 2007, 129, 16034. (g) Yoon, M. H.;
Facchetti, A.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102,
4678. (h) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm,
C.; Schutz, M.; Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci,
F. Nature 2004, 431, 963. (i) Schlitz, R. A.; Yoon, K. H.; Fredin,
L. A.; Ha, Y.-G.; Ratner, M. A.; Marks, T. J.; Lauhon, L. J. J. Phys.
Chem. Lett. 2010, 1, 3292–3297.
(6) (a) Guo, N.; DiBenedetto, S. A.; Tewari, P.; Lanagan, M. T.; Ratner,
M. A.; Marks, T. J. Chem. Mater. 2010, 22, 1567. (b) Kim, P.; Zhang,
X.-H.; Domercq, B.; Jones, S. C.; Hotchkiss, P. J.; Marder, S. R.;
Kippelen, B.; Perry, J. W. Appl. Phys. Lett. 2008, 93013302. (c) Jung,
C.; Maliakal, A.; Sidorenko, A.; Siegrist, T. Appl. Phys. Lett. 2007,
90062111. (d) Zirkl, M.; Haase, A.; Fian, A.; Scho¨n, H.; Sommer,
C.; Jakopic, G.; Leising, G.; Stadlober, B.; Graz, I.; Gaar, N.;
Schwo¨diauer, R.; Bauer-Gogonea, S.; Bauer, S. AdV. Mater. 2007,
19, 2241. (e) Guo, N.; DiBenedetto, S. A.; Kwon, D.-K.; Wang, L.;
Russell, M. T.; Lanagan, M. T.; Facchetti, A.; Marks, T. J. J. Am.
Chem. Soc. 2007, 129, 766. (f) Maliakal, A.; Katz, H.; Cotts, P. M.;
Subramoney, S.; Mirau, P. J. Am. Chem. Soc. 2005, 127, 14655. (g)
Schroeder, R.; Majewski, L. A.; Grell, M. AdV. Mater. 2005, 17, 1535.
(h) Chen, F. T.; Chu, C. W.; He, J.; Yang, Y.; Lin, J. L. Appl. Phys.
Lett. 2004, 85, 3295.
(8) (a) Hardy, A.; Elshocht, S.; Adelmann, C.; Conard, T.; Franquet, A.;
Douheret, O.; Haeldermans, I.; D’Haen, J.; De Gendt, S.; Caymax,
M.; Heyns, M.; D’Olieslaeger, M.; Van Bael, M.; Mullens, J. R. Thin
Solid Films 2008, 516, 8343. (b) Chang, J. P.; Lin, Y. S. Appl. Phys.
Lett. 2001, 79, 3666.
(9) Robertson, J. Rep. Prog. Phys. 2006, 69, 327.
9
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A R T I C L E S
Ha et al.
(<-4.0 V) and to offer high on/off current ratios (>105) as well
as substantial hole mobilities (0.2-1.5 cm2/(V·s)).
Experimental Section
Materials. The precursors for the present dielectric films were
prepared using zirconium(IV) chloride (99.5%, Aldrich) plus 1,6-
bis(trimethoxysily)hexane (98%, Gelest) or 1,8-bis(triethoxysily)-
octane (98%, Gelest). Heavily n-doped Si wafers (Montco Silicon
Tech) for gate electrodes were cleaned according to standard
procedures. Aluminum was deposited on Kapton (Dupont) as a gate
electrode for flexible devices. Pentacene for the p-type organic
semiconductor was commercially available and was purified by
published procedures.4b Multiple high-vacuum temperature gradient
sublimed PDIF-CN2 from Polyera Corp. was chosen as the n-type
organic semiconductor.
Figure 2. (A) XRR plots for ZrO2 thin films fabricated on Si substrates
and annealed at the indicated temperatures. (B) XRR-derived ZrO2 film
thicknesses as a function of the annealing temperature.
analysis (10 °C/min). Attenuated total reflectance Fourier transform
infrared (ATR-FTIR) spectra were acquired using a Thermo Nicolet
Nexus 870 Fourier transform infrared spectrometer. Cross-sectional
transmission electron microscopy (TEM) samples were prepared
and imaged using a JEOL-2100F scanning/transmission electron
microscope and a Hitachi HD-2300A scanning electron microscope,
with both bright-field (BF) and high-angle annular dark-field
(HAADF) detectors. The thicknesses of the CHB films were
analyzed by profilometry (Veeco Dektak 150 surface profiler). The
morphologies of all thin films were evaluated by atomic force
microscopy (AFM) using a JEOL-5200 scanning probe microscope
with silicon cantilevers operating in the tapping mode. MIS direct
current measurements and OTFT measurements were carried out
under ambient conditions using a Signatone probestation interfaced
to a Keithley 6430 Sub-Femtoamp remote source meter and a
Keithley 2400 source meter with a locally written LabVIEW
program. An impedance analyzer (HP 4192A) was used for
capacitance measurements.
Dielectric Film and OTFT Fabrication. An absolute ethanol
solution (0.10 M) in zirconium(IV) chloride was prepared, followed
by the addition of a mixture of nitric acid and deionized (DI) water
(molar ratio ZrCl4:HNO3:H2O ) 1:10:10). The resulting zirconium
precursor solution was then heated to 50 °C for 3 h to accelerate
hydrolysis. For the control zirconium oxide (ZrO2) film fabrication,
the above zirconium precursor solution was spin-coated onto
substrates at 5000 rpm for 30 s and then cured at a temperature of
150, 300, 400, or 500 °C for 1 h. For cross-linked inorganic/organic
hybrid blend (CHB) film fabrication, 0.10 M 1,6-bis(trimethoxysi-
ly)hexane or 1,8-bis(triethoxysily)octane solutions in EtOH, referred
to in the rest of this paper as BTH and BTO, respectively, were
added to the hydrolyzed zirconium chloride solution in a 1:0.5,
1:0.2, or 1:0.1 ZrCl4:cross-linker molar ratio. These solutions were
then spin-coated at 5000 rpm for 30 s, followed by curing in a
vacuum oven at 150 °C for 2 h. All dielectric precursor solutions
were filtered through a 0.2 µm pore size PTFE membrane syringe
filter prior to spin-coating and were spin-coated under a controlled
atmosphere of less than 10% relative humidity (measured with a
Fisher Scientific traceable hygrometer-thermometer-dew point
probe). Before dielectric layer deposition, the heavily doped n+-
silicon (Montco Silicon Technologies, Inc.) substrates were cleaned
in EtOH (Aldrich, absolute, 200 proof) by sonication for 2 min
and then dried under flowing nitrogen, followed by oxygen plasma
treatment for 5 min to remove organic contamination and to improve
the wettability. After dielectric layer deposition, bottom-gate/top-
contact OTFTs were completed by vacuum deposition of pentacene
(50 nm, 5 × 10-6 Torr, 0.05 nm/s growth rate) onto each ZrO2-
and CHB-dielectric-coated substrate, followed by gold source
(S)-drain (D) electrode vacuum deposition (50 nm, 0.02 nm/s
growth rate) through a shadow mask to yield S-D contacts of
dimensions L ) 100 µm and W ) 2000 µm. For metal/insulator/
semiconductor (MIS) test structures, gold electrodes of dimensions
200 µm × 200 µm were deposited directly onto the Si-gate
dielectric films through a shadow mask. Flexible OTFTs were
fabricated using a top-contact geometry on plastic substrates
(Kapton). Thus, aluminum was thermally evaporated onto the
Kapton films as the gate electrode, and the CHB gate dielectric
and a 50 nm thick pentacene semiconducting layer were then
deposited onto the aluminum layer as described above. Gold was
thermally evaporated onto the pentacene films through a shadow
mask to define the source and drain electrodes. These OTFTs had
a channel length of 100 µm and channel width of 2000 µm.
Dielectric Film and OTFT Characterization. The film thick-
nesses of the ZrO2 dielectric films after annealing at different
temperatures were analyzed by X-ray reflectivity (XRR), and
glancing incidence X-ray diffraction (GIXRD) scans were measured
on a Rigaku ATX-G thin-film diffraction workstation using Cu KR
radiation. The thermal stability of the CHB precursors was assessed
using a Shimadzu TGA-50 thermogravimetric analysis (TGA)
instrument. For the TGA experiments, BTH (with 5 wt % H2O
added) or CHB-zirconia precursor mixtures were deposited onto
a TGA pan followed by drying in vacuum to ensure that all the
solvent was removed. The resulting residue was subjected to TGA
Results
The control ZrO2 films and CHB dielectric films reported here
consist of purely inorganic or inorganic plus organic compo-
nents, respectively, with the latter containing various molar ratios
of ZrCl4 and organic R,ω-disilylalkane cross-linkers (Figure 1B).
It will be seen that high-k ZrO2 is the final inorganic component
and that the cross-linkers function as binders between the organic
and inorganic phases. Importantly, the spin-coated formulations
of these two components result in smooth, robust, and mechani-
cally flexible dielectric films at low processing temperatures
(∼150 °C) and exhibit enhanced electrical insulating properties
versus ZrO2 films alone. Especially, they enable significantly
reduced OTFT operating voltages (vide infra).
ZrO2 Film Fabrication and Characterization. The control
ZrO2 dielectric films are prepared by spin-coating a solution of
the appropriate inorganic precursor using the procedures de-
scribed in the Experimental Section. To assess the influence of
the ZrO2 film annealing temperature, four different annealing
temperatures (150, 300, 400, and 500 °C) were investigated after
spin-coating the films. The X-ray reflectivity-derived thicknesses
of the ZrO2 films using 0.10 M Zr precursor solutions are ∼17.0,
12.1, 11.0, and 10.9 nm after 150, 300, 400, and 500 °C
annealing, respectively (Figure 2). Clearly higher annealing
temperatures (>400 °C) afford thinner, more dense films. For
solution-processed metal oxide films, it is known that metal
hydroxides are progressively converted to the oxides via
thermally driven condensation processes (e.g., tZrsOH +
OHsZrtf tZrsOsZrt + H2O) and that the extent of oxide
formation depends primarily on the annealing temperature.10
The phase nature of the present sol-gel-derived ZrO2 films was
confirmed by GIXRD measurements. Figure 3 shows GIXRD
patterns obtained for ZrO2 films on Si annealed at 150, 300,
400, and 500 °C for 30 min. For the ZrO2 films annealed at
9
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OTFTs Enabled by Inorganic/Organic Dielectrics
A R T I C L E S
Figure 3. Glancing incidence X-ray diffraction patterns obtained from ZrO2
layers annealed at 150, 300, 400, and 500 °C.
lower annealing temperatures (e300 °C), the X-ray diffraction
profiles exhibit broad amorphous-like features located at around
2θ ) 30°. However, the peak shapes and intensities change
substantially at higher annealing temperatures (400 and 500 °C),
with the higher annealing temperatures yielding a sharp crystal-
line Bragg reflection at 2θ ) 30.3°. Under these annealing
conditions, it is expected that ZrO2 will grow as the tetragonal
phase, which is more favored at 400-700 °C than the mono-
clinic phase,11 and the feature at 2θ ) 30.3° can be attributed
to the (011) reflection of the tetragonal zirconia phase, which
has a larger dielectric constant.12 Using full width at half-
maximum analysis of the 2θ ) 30.3° reflection and the
Debye-Scherrer formula,13 the average crystallite size for the
present sol-gel-derived ZrO2 films grown on Si is estimated
to be ∼2 nm after annealing at 400-500 °C.
Figure 4. (A) Low-magnification STEM image of the Si interface of a
film annealed at 500 °C. Labels indicate the layer identities. The inset shows
the corresponding [120] nanodiffraction pattern of the crystalline tetragonal
ZrO2 layer. (B) TEM image of the Si/SiO2/ZrO2 interface of the film
annealed at 150 °C. Labels indicate the layer identities. The inset shows
the corresponding nanodiffraction pattern with a diffuse amorphous ring.
(C, D) High-resolution TEM images of the Si/SiO2/ZrO2 interfaces of films
annealed at 500 °C and 150 °C, respectively.
The amorphous or crystalline nature of the ZrO2 films at
various annealing temperatures is further confirmed by TEM.
Parts A and B of Figure 4 show cross-section TEM images for
500 °C and 150 °C annealed ZrO2 films, respectively. The
thickness estimations are in good agreement with the XRR
results. Nanobeam electron diffraction which can collect dif-
fraction patterns from a nanoscale area (i.e., 20 nm in diameter)
was used to identify the phase, as shown as the insets in Figures
4A,B. The nanodiffraction patterns of the ZrO2 films annealed
at 150 °C have a diffuse amorphous ring structure, while the
ZrO2 films annealed at 500 °C can be indexed in the tetragonal
ZrO2 crystalline phase. These data demonstrate that the ZrO2
films at low annealing temperatures (<300 °C) are essentially
amorphous whereas the ZrO2 films subjected to higher annealing
temperatures (>400 °C) are crystalline, in agreement with the
Figure 5. (A) Leakage current density vs electrical field for ZrO2 dielectric
films annealed at temperatures of 150 °C (black), 300 °C (red), 400 °C
(blue), and 500 °C (green). (B) Transfer plots for pentacene OTFTs based
on the ZrO2 dielectric films of panel A.
GIXRD experiments. Additional TEM images after annealing
at various temperatures are presented in the Supporting Informa-
tion.
To quantify the ZrO2 dielectric properties, MIS sandwich
structures were fabricated using vapor-deposited 200 µm × 200
µm Au contacts on dielectric-coated Si substrates. Figure 5A
shows typical current density versus electric field (J-E) plots
for MIS structures fabricated with the ZrO2 dielectric films
annealed at increasing temperatures. The leakage current density
of these films progressively decreases as the annealing temper-
ature is increased. Furthermore, high annealing temperatures
(g300 °C) are required to obtain adequate dielectric strength
(<10-6 A/cm2 at 2 MV/cm) for efficient OTFT operation (vide
infra). The data are summarized in Table 1. The ZrO2 dielectric
in the n+-Si/dielectric/Au MIS devices can be modeled as two
capacitors in series, 1/Ci ) 1/CZrO + 1/CSiO . The capacitance
(10) (a) Ehrhart, G.; Capoen, B.; Robbe, O.; Boy, Ph.; Turrell, S.;
Bouazaoui, M. Thin Solid Films 2006, 496, 227. (b) Shimizu, H.; Sato,
T.; Konagai, S.; Ikeda, M.; Takahashi, T.; Nishide, T. Jpn. J. Appl.
Phys. 2007, 46, 4209. (c) Chang, S.; Doong, R. Thin Solid Films 2005,
489, 17. (d) Nishide, T.; Honda, S.; Matsuura, M.; Ide, M. Thin Solid
Films 2000, 371, 61. (e) Shane, M.; Mecartney, M. L. J. Mater. Sci.
1990, 25, 1537. (f) Brinker, J., Scherer, G. W. Sol-Gel Science: The
Physics and Chemistry of Sol-Gel Processing; Academic Press: San
Diego, CA, 1990.
(11) (a) Shimizu, H.; Konagai, S.; Ikeda, M.; Nishide, T. Jpn. Appl. Phys.
Lett. 2009, 48, 101101. (b) Krumov, E.; Dikova, J.; Starbova, K.;
Popov, D.; Blaskov, V.; Kolev, K.; Laude, L. D. J. Mater. Sci: Mater.
Electron. 2003, 14, 759. (c) Wang, J. A.; Valenzuela, M. A.; Salmones,
J.; Vazquez, A.; Garcia-Ruiz, A.; Bokhimi, X. Catal. Today 2001,
68, 21.
2
2
of the native oxide on the Si bottom electrode is 1380 nF/cm2
assuming a 2.5 nm thick (from the TEM image) SiO2 layer and
k ) 3.9. The Ci of the ZrO2 is measured as 395-700 nF/cm2,
yielding a dielectric constant of 11-17, depending on the
particular annealing temperature. The dielectric constants ob-
tained for ZrO2 are somewhat smaller than in bulk, highly
(12) (a) Sayan, S.; Nguyen, N. V.; Ehrstein, J.; Emge, T.; Garfunkel, E.;
Croft, M.; Zhao, X. Y.; Vanderbilt, D.; Levin, I.; Gusev, E. P.; Kim,
H.; McIntyre, P. J. Appl. Phys. Lett. 2005, 86, 152902. (b) Gomez,
R.; Lopez, T.; Bokhimi, X.; Munoz, E.; Boldu, J. L.; Novaro, O. J.
Sol-Gel Sci. Technol. 1998, 11, 309.
(13) Patterson, A. L. Phys. ReV. 1939, 56, 978.
9
J. AM. CHEM. SOC. VOL. 132, NO. 49, 2010 17429

A R T I C L E S
Ha et al.
Table 1. Summary of Dielectric and Film Properties for ZrO2 and
Cross-Linked Hybrid Blend Dielectricsa
dielectric
film
Zr:cross-linker
molar ratio
film thickness J (A/cm2) at
b
cross-linker
Ta (°C)
(nm)
2 MV/cm
ε
ZrO2
none
1:0
150
300
400
500
17.0
12.1
11.0
10.9
35
23
19
43
27
∼5 × 10-5 10.8
∼1 × 10-6 13.2
∼3 × 10-8 17.5
∼2 × 10-8 17.7
∼4 × 10-7 5.9
∼1 × 10-6 8.9
∼4 × 10-6 10.2
∼1 × 10-6 5.0
∼3 × 10-6 7.1
∼6 × 10-6 10.1
CBTH
CBTO
BTH
BTO
1:0.5
1:0.2
1:0.1
1:0.5
1:0.2
1:0.1
150
150
20
Figure 7. Cross-section TEM image of the Si/SiO2/CHB (1:0.5 ratio)
interface of a film annealed at 150 °C. Labels indicate the layer identities.
The inset shows the corresponding nanodiffraction pattern with a diffuse
amorphous ring.
a Measured on MIS structures. b Ta ) annealing temperature.
which is essentially complete by ∼90 °C. This result is
supported by ATR-FTIR experiments carried out on CBTH films
annealed at different temperatures. As shown in Figure 6B, the
intensity of the Zr-OH stretching mode (∼3300 cm-1) de-
creases when the temperature is increased to ∼100 °C and then
stabilizes. These data clearly demonstrate that CHB cross-linking
is complete by 150 °C. Figure 7 shows cross-section scanning
transmission electron microscopy (STEM) images for a 150 °C
annealed ZrO2-based CHB film collected by the annular dark-
field detector, which shows z contrast. Thickness measurements
are in good agreement with the profilometry results, and the
nanodiffraction patterns of the CHB film annealed at 150 °C
have a diffuse amorphous ring structure. These data demonstrate
that the CHB films processed at low annealing temperatures
are essentially amorphous, similar to the ZrO2 films subjected
to low annealing temperatures (150 °C).
To assess the influence of a CHB microstructure on dielectric
properties (Figure 1), two different cross-linkers, BTH and BTO,
and three different ZrCl4:cross-linker molar ratios (1:0.5, 1:0.2,
and 1:0.1) were investigated in CHB film fabrication. It will be
seen that this process affords materials with properties substan-
tially different from those of neat ZrO2 films of similar or greater
thicknesses and which are characterized here by AFM, as well
as by MIS structure leakage current and capacitance measure-
ments. When a new cross-linker is employed, it is necessary to
optimize the Zr:cross-linker molar ratio. The goal is to achieve,
for a specific film thickness range, maximum capacitance along
with smooth dielectric surfaces. Since a smooth dielectric/
semiconductor interface is a prerequisite for efficient charge
mobility in the TFT channel, AFM data provide important
information on surface roughness for initial material evalua-
tion.15 Table 1 summarizes the dielectric properties of 20-43
nm thick CHB films prepared with different zirconium precur-
sor:cross-linker molar ratios. Figure 8 shows AFM images of
CBTH- and CBTO-derived films prepared from ZrCl4 and the
indicated cross-linkers. Independent of the ZrCl4:cross-linker
molar ratio, BTH is found to be a more effective cross-linking
reagent, affording very smooth CBTH surfaces (rms roughness
F ≈ 0.2 nm). In contrast, BTO cross-linker-derived dielectric
films are found to be relatively rough (F ) 0.6-5 nm). BTO
has a more extended hydrocarbon linker, offering greater degrees
of motional freedom than that of BTH, and this phenomenon
Figure 6. (A) TGA curves of a single component (BTH cross-linker) and
mixed components (BTH-Zr precursor). (B) ATR-FTIR spectra of CBTH
films as a function of annealing temperature.
crystalline zirconia or in thick films (16-25),8 and the present
films are likely less dense than those grown using more elaborate
high-vacuum and high-temperature techniques.14
Cross-Linked Hybrid Dielectric Film Fabrication and
Characterization. The new CHB dielectric films (Figure 1B)
are prepared by spin-coating a solution of a zirconium oxide
sol-gel solution combined with either of two different orga-
nosilane cross-linking reagents (Figure 1B) using the procedures
described in the Experimental Section. This procedure yields
20-43 nm thick films as established by profilometry. Films of
greater thicknesses, if required, can be obtained by varying the
processing conditions or by multiple spin-on depositions since
the CHBs are insoluble in the mother solutions at all stages of
curing. In this chemistry, a CHB dielectric network is formed
by condensation of the zirconia gel and the moderately reactive
-Si(OR)3 groups of the cross-linking reagents. Previous all-
organic cross-linked polymer blends (CPBs) based on chlorosi-
lane cross-linkers afford relatively rough dielectric film surfaces
due to the high reactivity of chlorosilanes, and alternative cross-
linkers affording smoother film morphologies were subsequently
developed.4b In the present investigation, the relatively slow
condensation between the cross-linker alkoxy groups and the
zirconia gel hydroxyl groups results in smooth, dense, and robust
CHB films with excellent leakage current properties. Figure 6
shows the thermal behavior of the BTH cross-linker and of a
BTH-Zr precursor mixture as determined by TGA. For the
cross-linker component alone, the broad weight loss occurring
between 50 and 125 °C can be attributed to methanol loss,
resulting from slow CHB consensation-cross-linking. In the
case of the two-component mixture, the abrupt weight loss
observed at ∼90 °C is assigned to methanol evaporation after
efficient cross-linking. These data suggest that far more acidic
ZiOx catalyzes the condensation/polymerization chemistry,
(15) (a) Yang, H.; Yang, C.; Kim, S. H.; Jang, M.; Park, C. E. ACS
Appl. Mater. Interfaces 2010, 2, 391. (b) Yang, H.; Shin, T. J.; Ling,
M.-M.; Cho, K.; Ryu, C. Y.; Bao, Z. J. Am. Chem. Soc. 2005, 127,
11542.
(14) You, H.; Ko, H.; Lei, T. J. Electrochem. Soc. 2006, 153, 94.
9
17430 J. AM. CHEM. SOC. VOL. 132, NO. 49, 2010

OTFTs Enabled by Inorganic/Organic Dielectrics
A R T I C L E S
used in OTFTs and are superior to those of conventional SiO2
(k ) 3.9) and a great many polymer/inorganic nanoparticle
nanocomposite films.6 Since the present CHB films exhibit
excellent dielectric properties, they were next investigated as
gate dielectrics in archetypical OTFTs.
Pentacene Thin-Film Transistor Fabrication and Characteriza-
tion. Bottom-gate top-contact pentacene OTFTs were next
fabricated using both neat ZrO2 and the CHB films as the gate
dielectrics, as described in the Experimental Section. From the
I-V data, the average pentacene field-effect mobility is calcu-
lated in the saturation regime (VG < VDS ) -4.0 V) by plotting
the square root of the drain current versus gate voltage (eq 1).
Figure 5B shows representative transfer plots for pentacene
OTFTs fabricated with the neat ZrO2 control dielectric films
which were annealed at various temperatures in the range
150-500 °C. Note that these devices require annealing of the
dielectric at or above 300 °C to function properly. Thus, when
the ZrO2 dielectric films are annealed from 150 to 500 °C, the
TFT hole mobility increases from 0.1 to 0.4 cm2/(V·s). OTFTs
were next fabricated on the various CHBs again using pentacene
as the active semiconducting layer. In contrast to the above ZrO2
results, pentacene TFTs fabricated with the CHB dielectric films
all exhibit excellent linear and saturation regime characteristics
and minimal hysteresis when operated at -4.0 V. Furthermore,
since all of these dielectric films are very smooth, the device
yields approach 98% (39 out of 40) with very narrow device
performance spreads and with essentially every device perform-
ing similarly. The transistor performance characteristics of these
devices are collected in Table 2. Figure 10 shows representative
transfer plots for OTFTs fabricated with the CBTH gate
dielectric annealed at 150 °C. Clearly these devices require far
lower annealing temperatures to operate properly than those
fabricated with the neat ZrO2 films. Pentacene TFTs fabricated
on CBTH (ZrCl4:BTH ) 1:0.5) exhibit exceptional device
characteristics with a pentacene mobility of 1.5 cm2/(V·s), a
Figure 8. AFM images of CHB dielectric films as a function of the
indicated ZrCl4:cross-linker molar ratio and using either BTH (A) or BTO
(B) as the cross-linker. The space bar indicates 1 µm.
affords relatively rougher CBTO surfaces, with smoother CBTO
films obtained at lower BTO cross-linker concentrations.
Capacitance and leakage current density measurements
(Figure 9) were next performed on these MIS structures. CBTH
and CBTO films fabricated with BTH and BTO afford at least
10× lower leakage current densities (<1 × 10-6 A/cm2) than
the ZrO2 films (∼1 × 10-5 A/cm2) at the same bias window (2
MV/cm). As the cross-linker content is increased, the leakage
current density falls. This is the result of the improved network-
forming capacity, affording denser films versus the inorganic-
only films when annealed at low temperatures. Note, however,
that capacitance Vs. voltage (C-V) measurements reveal that
increased cross-linker contents depress the film capacitance. This
is the result of dilution of the ZrOx content (expected from
effective medium theory3b,6a) of the blend as well as increased
film thickness. Nevertheless, capacitances in the range 95-365
nF/cm2, far greater than that of a 300 nm thick thermal SiO2
layer (∼11 nF/cm2), are readily achieved. From the accumulation
regime capacitances, k values of 5.0-10.2 are obtained for the
various cross-linkers and cross-linker molar ratios. These metrics
compare favorably with inorganic dielectric materials typically
Figure 9. Capacitance vs voltage plots measured at 10 kHz (left) and capacitance vs frequency plots measured at 3.0 V (right) for CBTH (A) and CBTO
(B) films. Leakage current density vs electric field plots for CBTH (C) and CBTO (D) films. The ZrCl4:cross-linker molar ratios are indicated in the plot
legends in parentheses.
9
J. AM. CHEM. SOC. VOL. 132, NO. 49, 2010 17431

A R T I C L E S
Ha et al.
Table 2. Zirconium Chloride:Cross-Linker Molar Ratio, Annealing Temperature (Ta, °C), Capacitance (Ci, nF/cm2), Dielectric rms Roughness
(F, nm), OTFT Carrier Mobility (µsat, cm2/(V·s)), and Current On/Off Ratio (Ion/Ioff) Data for MIS and OTFT Devices Fabricated Using
Pentacene as the Organic Semiconductor and Various ZrO2 and CHB Dielectrics
P5
dielectric film
cross-linker
substrate
Zr: cross-linker molar ratio
Ta
Ci
F
µsat
Ion/Ioff
VTH
ZrO2
none
n+-Si
1:0
150
300
400
500
395
567
700
700
135
275
365
95
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.6
1.3
5
0.1
0.3
0.4
0.4
1.5
1.0
0.7
0.2
0.3
0.6
1.6
104
105
105
105
105
105
104
103
104
104
105
-0.7
-1.0
-1.1
-1.1
-1.2
-1.1
-1.0
-1.8
-1.5
-1.1
-1.6
CBTH
CBTO
CBTH
BTH
BTO
BTH
n+-Si
1:0.5
1:0.2
1:0.1
1:0.5
1:0.2
1:0.1
1:0.5
150
n+-Si
150
150
200
345
132
Kapton/Al
0.2
current on/off ratio of ∼105, and a threshold voltage of -1.2
V. In addition, the TFTs based on the CBTO gate dielectric
exhibit good performance with µ approaching 0.6 cm2/(V·s), a
current on/off ratio of ∼104, and a threshold voltage of -1.1
V. These carrier mobilities are far larger than those of control
devices fabricated with a conventional 300 nm thick SiO2 gate
dielectric (mobility ∼0.3 cm2/(V·s)). The operating voltage is
only -4.0 V for the CHB-based devices vs -100 V for SiO2
as a result of the greater capacitance of the CHB vs SiO2 gate
dielectric. The influence of the ZrCl4:cross-linker molar ratio
on transistor performance is also shown in Table 2. The optimal
Zr:cross-linker molar ratio for CBTH is found to be 1:0.5 and
for CBTO 1:0.1. For TFTs based on CBTH, when the ZrCl4:
cross-linker molar ratio is increased, the gate capacitance
increases; however, the dielectric strength falls owing to the
lower film density. Therefore, the TFT performance suffers as
a result of the greater gate leakage. In contrast, the OTFT
performance of the CBTO-based devices increases when the
Zr:cross-linker molar ratios increase from 1:0.5 to 1:0.1 as a
result of the smoother film morphology. Note that the threshold
voltage variations of the devices based on different cross-linker:
Zr precursor ratios are not due to different dielectric interface
trap densities but are only the result of the different CBTH/
CBTO capacitances due to dielectric thickness variations. Using
a similar process, we also demonstrated an n-type organic
transistor using a perylene diimide semiconductor (PDIF-CN2)
(see the Supporting Information, Figure S2, Table S1). These
devices also operate at low voltages (<-4.0 V) and exhibit very
large electron mobilities (up to 0.6 cm2/(V·s)) and acceptable
on/off current ratios (up to103).
Figure 11 shows representative transfer and output plots for
flexible pentacene TFTs based on CBTH (1:0.5 molar ratio)
fabricated on the plastic substrate Kapton. These plots exhibit
reproducible I-V characteristics at low operating voltages
(<-4.0 V) as well as excellent linear/saturation behavior. These
flexible pentacene TFTs exhibit hole mobilities of 1.6 ( 0.2
cm2/(V·s) and current on/off ratios of ∼105. These properties
are quite comparable to those of TFTs fabricated on CBTH-
coated silicon substrates. The data are summarized in Table 2.
Discussion
CHB Morphology and TFT Response Characteristics. All of
the cross-linking reactions employed in this study yield robust,
smooth, adherent, and insoluble CHB films. The major differ-
ences between the various CHBs involve details of film
morphology (film roughness) originating from the different
cross-linkers. Since OTFT charge transport is confined to the
nanoscopic region at the semiconductor/dielectric interface, the
smoothness of the dielectric film surface is clearly a prerequisite
for optimizing the OTFT performance. In the case of pentacene
TFTs, several groups have reported that rough gate dielectric
surfaces induce the growth of smaller pentacene grains than in
the case of very smooth substrates16 and that roughness leads
to a poor OTFT response. It is known that grain boundaries
between crystallites are correlated with interfacial charge
trapping sites which disrupt carrier drift.17 Figures 12 and 13
Figure 10. Transfer (left) and output (right) plots for pentacene OTFTs
fabricated with the CHB gate dielectric using ZrCl4:BTH cross-linker molar
ratios of 1:0.5 (A) and 1:0.2 (B).
Figure 11. Transfer (A) and output (B) plots for representative flexible
pentacene OTFTs fabricated with a CBTH gate dielectric on Kapton/
aluminum substrates.
9
17432 J. AM. CHEM. SOC. VOL. 132, NO. 49, 2010

OTFTs Enabled by Inorganic/Organic Dielectrics
A R T I C L E S
Table 3. Device Parameters for the Flexible OTFTs Tested
Before, During, and After Bending along a Curvature Parallel to
the Channel Length
current
on/off
ratio
εsb
(%)
VDS
(V)
µ
VTH
(V)
(cm2/(V·s))
before bending
-3.0
-3.0
-3.0
-3.0
-3.0
1.5
1.2
0.7
0.3
1.3
105
105
104
104
104
-0.7
-1.3
-1.4
-1.4
-1.3
bending, r ) 13 mma
bending, r ) 10 mma
bending, r ) 8 mma
after bending
0.45
0.63
0.78
Figure 12. θ-2θ X-ray diffraction patterns of pentacene films grown on
CBTH (A) and CBTO (B) dielectrics with the indicated ZrCl4:cross-linker
molar ratios.
a r ) bending radius. b εs ) tensile strain, calculated from εs ≈ ds/2r,
where ds is the substrate thickness () 125 mm for Kapton used in this
study).
reveal similar trends in pentacene film grain size vs the CHB
surface roughness. Rough gate dielectric surfaces are known to
impede lateral pentacene molecular diffusion, affording smaller
grain sizes versus growth on very smooth substrates, and in the
present case drastically suppress ordered pentacene molecular
nucleation.16b
OTFT Mechanical Flexibility. To investigate the mechanical
properties of the present flexible TFTs, we performed bending
tests in the longitudinal direction (i.e., along the channel
transport axis) by measuring the electrical characteristics of the
devices while wrapped around glass tubes having different
circular cross-sections, as shown in Figure 14C. This test
evaluates the TFT performance before, during, and after bending.
To induce maximum tensile strain (εs)18 during bending, a
relatively thick (∼125 µm) Kapton substrate was used in
fabrication. Parts A-C of Figure 14 show the transfer charac-
teristics and IDS1/2-VG plots of identical devices tested before,
during, and after bending, and performance parameters are
compiled in Table 3. As shown in Figure 14C, the channel
direction is oriented parallel to the bending direction. The
effective device mobilities were evaluated at calculated strains
of 0.45%, 0.63%, and 0.78%, which correspond to bend radii
of 13, 10, and 8 mm, respectively. When the device is bent at
εs ) 0.45% (r ) 13 mm), Ion decreases slightly, along with a
slight decrease in µ. Further bending to larger εs values of 0.63%
(r ) 10 mm) and 0.78% (r ) 8 mm) significantly alters the
performance and leads to decreases in both Ion and µ. Such
bending induces strain in pentacene devices, which can alter
the intrinsic mobility, as is well-known.19 Note that performance
is almost completely recovered after relaxation of the strain,
similar to flexible carbon nanotubes, Si nanoribbons, and other
Figure 13. AFM images of pentacene films grown on CBTH (A) and
CBTO (B) dielectrics prepared with the indicated ZrCl4:cross-linker molar
ratios. Images are of areas 5 µm × 5 µm in size.
show the XRD patterns and AFM images, respectively, of the
pentacene films grown on CHB surfaces. The XRD data for
pentacene films on CBTH-based dielectrics at all ZrCl4:cross-
linker ratios reveal the presence of the pentacene thin-film phase,
characterized by the first reflection located at 2θ ) 5.7°. Three
characteristic features arising from the (00l) Bragg planes of
the thin-film phase dominate the XRD patterns. These films are
highly crystalline since the first reflection is sharp and intense.
In contrast, pentacene films grown on rougher CBTO surfaces
are far less textured. However, the film crystallinity increases
as the BTO cross-linker concentration in the blend decreases,
which affects the dielectric surface roughness. Rougher films
are observed for CBTO vs CBTH, presumably because the more
extended hydrocarbon linker offers greater degrees of motional
freedom. Similar trends were observed for CPB dielectrics based
on styrenic polymer/silane cross-linker mixtures.4a AFM images
of pentacene films on each of the present CHB-based devices
Figure 14. (A) Transfer characteristics and (B) IDS1/2-VG plots of flexible TFTs tested before, during, and after bending along a curvature parallel to the
channel length. The legend indicates the bending radii employed. Inset: Output characteristics which are restored to the initial planar condition after bending.
(C) Optical image of an array of OTFTs with CBTH (1:0.5) dielectric on a Kapton (125 mm thick) substrate, wrapped around a test tube with a radius of
8 mm, and image of the device during measurement.
9
J. AM. CHEM. SOC. VOL. 132, NO. 49, 2010 17433

A R T I C L E S
Ha et al.
organic transistors.20 Remarkably, the after-bending device
exhibits Ion ) 8.6 × 10-6 A and µ ) 1.3 cm2/(V·s) (85% of
the original values). The performance loss is probably the result
of degradation of the crystalline pentacene film order upon
mechanical stress. This phenomenon has been observed in
several studies where pentacene TFTs were fabricated on flexible
substrates.20f
that low-leakage, high-capacitance, cross-linked inorganic/
organic hybrid films are readily accessible and that OTFT
devices implementing efficient, solution-process fabrication
methodologies offer low voltage and low power operation.
Acknowledgment. This research was supported by AFOSR
(Grant FA9550-08-1-0331), the ONR MURI Program (N00014-
05-1-0766) and the NSF-MRSEC program (Grant DMR-0520513)
at Northwestern University. Microscopy studies were performed
in the EPIC and NIFTI facilities of the NUANCE Center at
Northwestern University. The NUANCE Center is supported by
NSF-NSEC, NSF-MRSEC, the State of Illinois, and Northwestern
University.
Conclusions
We have demonstrated here that blending in situ generated
high-k inorganic materials with the appropriate difunctional
organosilane cross-linking agents affords robust, smooth, adher-
ent, flexible, insulating, pinhole-free, high-capacitance, ultrathin
dielectric materials. These films are readily deposited from
solution at low temperatures and adhere strongly to a variety
of conducting substrates, including mechanically flexible plas-
tics. The resulting OTFTs function at low operating voltages
for an inorganic-based gate dielectric. These results demonstrate
Supporting Information Available: Additional TEM images
at various annealing temperatures and n-type organic transistor
device performance data for chosen dielectric materials. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA107079D
(16) (a) Kim, C.; Facchetti, A.; Marks, T. J. Science 2007, 318, 76. (b)
Kim, C.; Facchetti, A.; Marks, T. J. AdV. Mater. 2007, 19, 2561. (c)
Yang, S. Y.; Shin, K.; Park, C. E. AdV. Funct. Mater. 2005, 15, 1806.
(17) (a) Rivnay, J.; Jimison, L. H.; Toney, M. F.; Noriega, R.; Marks, T. J.;
Facchetti, A.; Salleo, A. Nat. Mater. 2009, 8, 952. (b) Fritz, S. E.;
Kelly, T. W.; Frisbie, D. J. Phys. Chem. B 2005, 109, 10574. (c)
Steudel, S.; DeVusser, S.; DeJonge, S.; Janssen, D.; Verlaak, S.; Genoe,
J.; Heremans, P. Appl. Phys. Lett. 2004, 85, 4400. (d) Knipp, D.; Street,
R. A.; Volkel, A. R. Appl. Phys. Lett. 2003, 82, 3907.
(20) (a) Kim, H.-S.; Won, S. M.; Ha, Y.-G.; Facchetti, A.; Marks, T. J.;
Rogers, J. A. Appl. Phys. Lett. 2009, 95, 183504. (b) Cao, Q.; Hur,
S. H.; Zhu, Z. T.; Sun, Y. G.; Wang, C. J.; Meitl, M. A.; Shim, M.;
Rogers, J. A. AdV. Mater. 2006, 18, 304. (c) Artukovic, E.; Kaempgen,
M.; Hecht, D. S.; Roth, S.; Gru¨ner, G. Nano Lett. 2005, 5, 757. (d)
Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai,
T. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 9966. (e) Liu, J.; Buchholz,
D. B.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. AdV. Mater. 2010,
22, 2333. (f) Roberts, M.; Queralt, N.; Mannsfeld, S.; Reinecke, B.;
Kno, W.; Bao, Z. Chem. Mater. 2009, 21, 2292.
(18) Briseno, A. L.; Tseng, R. J.; Ling, M.-M.; Falcao, E. H. L.; Yang, Y.;
Wudl, F.; Bao, Z. AdV. Mater. 2006, 18, 2320.
(19) Smith, C. S. Phys. ReV. 1954, 94, 42.
9
17434 J. AM. CHEM. SOC. VOL. 132, NO. 49, 2010