Solution processed high performance pentacene thin-film transistors

Article abstract of DOI:10.1039/c2cc31754k

High performance thin-film transistors were fabricated using a new precursor of pentacene through a multiple spin-heat procedure. High quality pentacene thin films can be prepared by this method and hence a FET device can be made in a top-contact configuration. The device exhibited a remarkable field-effect mobility of 0.38 cm² V^-1 s^-1 with an on/off ratio of 10⁶.

Full text of DOI:10.1039/c2cc31754k

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COMMUNICATION
Solution processed high performance pentacene thin-film transistorsw
Ting-Han Chao,ab Ming-Jen Chang,c Motonori Watanabe,b Ming-Hui Luo,c
Yuan Jay Chang,b Tzu-Chien Fang,c Kew-Yu Chen*c and Tahsin J. Chow*b
Received 9th March 2012, Accepted 24th April 2012
DOI: 10.1039/c2cc31754k
High performance thin-film transistors were fabricated using a new
precursor of pentacene through a multiple spin-heat procedure.
High quality pentacene thin films can be prepared by this method
and hence a FET device can be made in a top-contact configu-
ration. The device exhibited a remarkable field-effect mobility of
0.38 cm2 V–1 s–1 with an on/off ratio of 106.
Organic semiconducting materials are of contemporary interest
for fabrication of organic thin-film transistors (OTFTs).1–3
Pentacene has been recognized as a promising material for
application to OTFTs due to its high field effect mobility.4 It
Scheme 1 The synthetic route for precursor 1.
exhibits a strong tendency to form highly ordered films on various
substrates under different growth conditions.5 An optimal mobility
coated on the top without defects. Solution processed top-
contact devices were less commonly seen in the literature.12
In this communication, we describe the synthesis of a new
precursor which can be spin-coated into a highly uniformed
thin film, and a high quality FET device can be made in a top-
contact configuration. These devices performed comparable to
the best reported previously using bottom-contact designs.8a,11a
The structure of a new precursor (1), as shown in Scheme 1, is a
cycloadduct of pentacene and a ketone. It was prepared by
heating pentacene (2) with an excess amount (B10 equiv.) of
diethyl ketomalonate (3) in toluene to reflux without the need of a
catalyst. The cycloaddition proceeded smoothly, while the dark
purple color of the solution turned yellow gradually. Compound 1
was collected as colorless solids in a nearly quantitative yield and
was purified through a silica gel column chromatograph. The
structure was confirmed by spectroscopic data (SI). It is
considerably soluble in most organic solvents, such as dichloro-
methane (55 g L–1), chloroform (50 g L–1), and acetone (45 g L–1).
The thermal fragmentation of 1 can be closely monitored by
a thermogravimetric analysis (TGA) as shown in Fig. 1a. An
initial weight loss was observed at 210 1C, which corresponds
to the expulsion of 3 (39%). At the same time, the colorless
solid turned purple (inset of Fig. 1a), indicating the formation
of pentacene. In UV-vis spectra the strong p–p* absorption of
the naphthalene chromophore at 250 nm diminished, while a
new peak at 301 nm of the pentacene chromophore arose. The
low energy transitions at 450–600 nm with vibronic progressions,
characteristic of pentacene, also appeared after heating (Fig. 1b).
In IR spectra, the strong absorption peak at 1744 cmÀ1
(Fig. S3, ESIw) disappeared completely after heating.
in excess of 5 cm2 VÀ1 sÀ1 has been achieved.6 However, pentacene
has a few drawbacks, such as its poor solubility and high
sensitivity to oxidation, leading to difficulties in processing. Many
attempts have been made to prepare soluble pentacene precursors
in order to overcome the solubility problem. A unique group of
precursors has been designed by forming cyclo-adducts of a
small volatile dienophile with pentacene.7–9 They have exhibited
excellent air stability and higher solubility, and are easily converted
to pentacene via retro-Diels–Alder reactions.
A critical issue concerning the device performance is the
surface morphology of the organic material.10,11 The thin film
made through a solution process is generally not as uniform as
those made by vapor deposition.9,11a As a consequence, the
devices made with solution processes were fabricated more
readily in a bottom-contact configuration, where the organic
material was spread on top of the source–drain metal electrodes.
The molecular packing pattern at the bottom of the pentacene
layer may have been altered by the metal. Through surface
modifications and careful practices, charge mobilities as high as
0.2711a and 0.428a cm2 VÀ1 sÀ1 have been reported. To fabricate
top-contact devices, however, the surface of the pentacene film
has to be smooth enough so that the metal electrodes can be
a Department of Chemistry, National Taiwan Normal University,
Ting-Chow Road, Taipei, 11677, Taiwan
b Institute of Chemistry, Academia Sinica, No.128, Academia Road
Sec 2, Taipei, 11529, Taiwan. E-mail: tjchow@chem.sinica.edu.tw;
Fax: +886-2-27884179; Tel: +886-2-27898552
c Department of Chemical Engineering, Feng Chia University, 40724,
Taichung, Taiwan. E-mail: kyuchen@fcu.edu.tw;
Fax: +886-4-24510890; Tel: +886-4-24517250
A thin film of 1 was prepared by using a multiple spin-heat
technique. A saturated solution was spin-coated on top of an
SiO2 substrate, upon drying micro-crystalline colonies spread
w Electronic supplementary information (ESI) available: Detailed
synthetic procedures, FET devices fabrication, characterizations,
AFM images and spectra. See DOI: 10.1039/c2cc31754k
c
6148 Chem. Commun., 2012, 48, 6148–6150
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lithographically patterned with gold (B50% coverage)
(Fig. S4, ESIw). The WAXRD spectra of pentacene on such a
surface exhibited a distinctively different pattern. A strong diffrac-
tion peak appeared at 28.281, corresponding to a d spacing of
3.15 A (Fig. 2, blue line). It implies that pentacene is lying down on
the surface with the a-axis standing vertically on the surface.15
This experiment indicates that in a bottom contact design, the
gold electrodes lying below the pentacene may interfere the
packing pattern of pentacene, thereby reducing the charge
mobility of the devices.
Fig. 1 (a) Thermogravimetric analysis curve for the retro Diels–Alder
reaction of precursor 1 (heating rate: 10 1C min–1). The inset shows the
microscopic images before (left) and after (right) heating. (b) UV-Vis
spectra of 1 (black line) and the pentacene product (red line) in dichloro-
methane. The inset shows the weak bands in the region 400–650 nm.
The OTFT devices were fabricated using a heavily n-doped
Si wafer as the substrate and the gate, using a 300 nm thermally
grown SiO2 film as the gate insulator. Compound 1 was spin-
coated onto the substrate according to the multiple spin-heat
sequence we have developed. Au (100 nm) was then deposited on
top of the pentacene film as the source–drain electrodes with
W = 500 mm and L = 50 mm. The output characteristics, i.e., the
plot of drain current (IDS) vs. drain–source voltage (VDS) at
various gate voltages (VG), are shown in Fig. 3a and c (device 1).
The OTFT operates in the p-type enhancement mode and exhibits
a hard saturation. The field-effect mobility m was calculated based
on data in the saturation regime according to eqn (1)
evenly on the surface. The substrate was then heated to 210 1C for
7 minutes, while the conversion of 1 to pentacene was complete.
The time of heating was optimized by the signal intensity of X-ray
diffraction spectra (Fig. S5 and S6, ESIw). The substrate was then
subjected to spin-coating again with the same solution of 1. This
time the microcrystals tended to fill up the gaps of pentacene
colonies which were formed in the previous step (Fig. S7 and S8,
ESIw). This process was repeated six times, until a relatively
smooth surface was achieved. The AFM image of the film showed
a polycrystalline structure, which is composed of small grains
(B2 mm wide) with a root mean square roughness of B70 nm,
significantly smaller than those prepared by a single spin-
coating step (Fig. 2).9,11a The average thickness of pentacene
films was estimated to be in the range of 1.6–1.9 mm.
The pentacene film prepared from 1 was examined by out-of-
plane wide-angle X-ray diffractometry (WAXRD) (Fig. 2). The
(00l) series diffractions (where l = 1, 2, 3 and 4) of pentacene bulk
phase crystals are clearly visible at 6.16, 12.33, 18.51 and 24.771,
which correspond to an interplanar d spacing of 15.4 A.13 The
single-crystal structure of pentacene is known to be triclinic with a
long-axis length of 16.01 A.14 The d spacing fits well with the long
axis of pentacene, that is usually tilted to an angle of 171 from the
surface normal. The absence of other (hkl) reflections indicated
that the crystallites are randomly oriented around the surface
normal. These phenomena were consistent with those of the
pentacene films prepared by vacuum deposition.
IDS = (W/2L)mCi (VG À VT)2
(1)
where m is the charge carrier mobility, Ci is the gate channel
capacitance per unit area (1.15 Â 10À8 F cmÀ2), IDS is the source–
drain current, VG is the gate voltage and VT is the threshold voltage.
The corresponding transfer characteristics, i.e., log (IDS) vs. VG for
V
DS = À80 V, IDS1/2 vs. VG in the saturation mode, are shown in
1/2
Fig. 3b and d. From IDS vs. VG in the saturation mode, the
apparent field-effect mobility m is estimated to be 0.12 cm2 VÀ1 sÀ1
with an on/off current ratio of about 106 (Table 1).
It was known that adding a gate dielectric layer on top of
SiO2 improves the polycrystalline morphology of pentacene,
therefore reduces trap side and increases current stability. In
another device (device 2) a layer of hexamethyldisilazane (HMDS)
was covered on the surface of the SiO2. The field-effect mobility m
In order to examine the effect of metal on the surface of
SiO2, the same film was deposited on an SiO2 substrate
Fig. 2 (left) AFM topographic images of pentacene converted from 1;
(right) out-of-plane wide-angle X-ray diffraction patterns of pentacene film:
on bare SiO2 (red line), on SiO2 pre-treated with HMDS (green line), and
on SiO2 lithographically patterned with gold (blue line). The three colour
lines are shifted slightly for better illustration, actually they are at the same
position as the red line. The (00l) series diffraction lines of pentacene
crystals clearly appeared on the SiO2 or HMDS surface, but (l00) series
diffraction line was found on the surface partly covered with gold.
Fig. 3 (a) Typical output and (b) transfer characteristics of device 1;
(c) typical output and (d) transfer characteristics of device 2 (channel
width of 500 mm and channel length of 50 mm, and IDS1/2 vs. VG in the
saturation mode).
c
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Chem. Commun., 2012, 48, 6148–6150 6149

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Table 1 OTFT performance data
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Mobility/
Ion/
Device S/D configuration Dielectric
cm2 VÀ1 sÀ1 Ioff
VT/V
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Top contact
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Bottom contact SiO2
Bottom contact HMDS/SiO2 0.0013
SiO2
0.12
B106
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À28
HMDS/SiO2 0.38
B106
2.9 Â 10À5 7.1 Â 103 À6
1.3 Â 104 À14
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readily by heating pentacene with diethyl ketomalonate in the
absence of a catalyst. The solubility of this material in most
organic solvents was in the range of 50 g L–1. High quality
pentacene thin films can be prepared by a repeated sequence of
multiple spin-heat. Top-contact OTFT devices made in this way
displayed a remarkable charge mobility of 0.38 cm2 VÀ1 sÀ1
with an on/off ratio of about 106. To the best of our knowledge,
this is the best performance of a pentacene device configured in
a top-contact design and fabricated by using a solution process.
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6150 Chem. Commun., 2012, 48, 6148–6150
This journal is The Royal Society of Chemistry 2012