Influence of the dielectric constant of a polyvinyl phenol insulator on the field-effect
mobility of a pentacene-based thin-film transistor
Yunseok Jang, Do Hwan Kim, Yeong Don Park, Jeong Ho Cho, Minkyu Hwang, and Kilwon Cho
Citation: Applied Physics Letters 87, 152105 (2005); doi: 10.1063/1.2093940
View online: http://dx.doi.org/10.1063/1.2093940
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APPLIED PHYSICS LETTERS 87, 152105 共2005兲
Influence of the dielectric constant of a polyvinyl phenol insulator
on the field-effect mobility of a pentacene-based thin-film transistor
Yunseok Jang, Do Hwan Kim, Yeong Don Park, Jeong Ho Cho,
Minkyu Hwang, and Kilwon Choa兲
Department of Chemical Engineering, Pohang University of Science and Technology,
Pohang 790-784, Korea
共Received 1 June 2005; accepted 31 August 2005; published online 5 October 2005兲
The mobility of pentacene thin-film transistors 共TFTs兲 is correlated with the dielectric properties of
their insulators. We varied the dielectric properties of the poly共4-vinylphenol兲 insulators of such
TFTs by changing the molar ratio of the prepolymer/cross-linking agent while keeping the surface
potential of the insulator surface constant. It was found that the field-effect mobility of the pentacene
TFTs increases with increases in the dielectric constant of the insulators. A small increase in the
dielectric constant of the insulator 共a 20% increase, 3.6–4.3兲 was found to result in a dramatic
increase in the field-effect mobility of pentacene TFTs by a factor of 3 共0.26 to 0.81 cm2 / V s兲.
© 2005 American Institute of Physics. 关DOI: 10.1063/1.2093940兴
The insulator is a crucial component of organic fieldeffect transistors 共OTFTs兲. This is especially true in the case
of “bottom gate” architectures in which the semiconductor is
deposited on top of the insulator and the surface characteristics of the insulator strongly influence the quality of the
insulator-semiconductor interface, significantly affecting the
performance of the device. In the last decade, a considerable
number of studies have been devoted to the fabrication of
OFETs 共Refs. 1–6兲 using organic insulators7–13 for use as
switching devices for flexible displays. However, although
great advances have been made in the improvement of
OFETs using polymer insulators, there has been relatively
little research into the effects of the dielectric properties of
polymer insulators on device performance.13–15 Peng et al.
showed a correlation between the device field-effect mobility
and the dielectric constant of the insulator.14 However, the
relationship between the dielectric constant of the insulator
and the electric properties of the device was not sufficiently
clear in their system, because they used different types of
polymer insulators with different surface potentials. In the
case of semicrystalline organic semiconductors such as pentacene, the morphology and molecular ordering of the semiconductor at the insulator-semiconductor interface have a
significant influence on the field-effect mobility of the device, and is determined by the quality of the insulatorsemiconductor interface, especially by the surface potential
of the insulator.4,16–20 In their study, the effects of surface
potential and those of the dielectric properties of the insulator were not distinguished.
In the present study, in order to investigate the effects of
varying the dielectric constant of the insulator on the electric
properties of FETs, we used only one kind of polymer insulator, polyvinyl phenol 共PVP兲, and varied the molar ratio of
prepolymer and cross-linking agent, thus changing only the
dielectric constant while keeping the surface energy of the
polymer insulator constant.
The M w of the polyvinyl phenol prepolymer was
⬇20 kg/ mol. We used poly共melamine-co-formaldehyde兲
a兲
Author to whom correspondence should be addressed; electronic mail:
kwcho@postech.ac.kr
关PMF兴 as the cross-linking agent. PVP and PMF were dissolved in propylene glycol methyl ether acetate 关PGMEA兴
共about 13– 15 wt. %兲 with mole ratios of 20:1, 5:1, 7:1, and
1:1. The insulator films were formed by spin coating the
prepared solutions at 3000 rpm on heavily doped n-type Si
wafers as gate electrodes, with subsequent cross-linking for
1 h at 200 ° C in a vacuum oven.
The rms roughnesses measured with AFM and the surface energies of the polymer insulators are shown in Table I.
The surface energies of the insulators were determined from
contact angle measurements using distilled water and diiodomethane as a probe liquid and the geometric mean
equation,21
共1 + cos ␪兲␥ pl = 2共␥sd␥dpl兲1/2 + 2共␥sp␥ ppl兲1/2 ,
where ␥s and ␥ pl are the surface energies of the sample and
the probe liquid, respectively, and the superscripts d and p
refer to the dispersion and polar 共nondispersion兲 components
of the surface energy, respectively. As can be seen in Table I,
the prepared polymer insulators have similar rms roughnesses, 0.3– 0.4 nm, and surface energies, ⬇42 mJ/ m2.
Table I shows the dielectric constants k of the polymer
insulators, as obtained with the following equation:22
k=
Ct
,
A␧0
where C is the capacitance of the insulator, A is the dot area
共0.005 cm2兲, t is the insulator thickness 共⬃350 nm兲, and ␧0
is the permittivity in vacuum 共8.85⫻ 10−14 F cm−1兲. The capacitance, C, was obtained from C-V measurements 共Agilent
4284A兲 and the thicknesses of the polymer insulators were
measured with ellipsometry.
Table I shows that the dielectric constant decreases with
increases in cross-linking density and varies from 3.6 to 4.3
with increases in the molar ratio. The dielectric constant reveals significant information about the chemical and physical
state of a polymer, since it is drastically affected by the presence of another polymer or a dopant.23,24 Therefore the variation of the dielectric constant with molar ratio is caused by
the presence of another polymer 共the cross-linking agent兲 in
PVP and the number of hydroxyl groups of PVP in the insu-
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87, 152105-1
© 2005 American Institute of Physics
141.223.173.110 On: Tue, 21 Apr 2015 07:14:39
152105-2
Appl. Phys. Lett. 87, 152105 共2005兲
Jang et al.
TABLE I. Properties of pentacene devices with PVP insulators prepared
with various PVP/PMF ratios.
Insulator
Device
Field-effect On/off Threshold
Equivalent
Surface
rms
ratio
roughness energy Dielectric mobility current voltage
ratio
共V兲
共PVP:PMF兲
共Å兲
共mJ/ m2兲 constant 共cm2 / V s兲
20:1
5:1
1.7:1
1:1
3
3
4
4
42.5
42.6
42.3
42.2
4.3共±0.1兲
4.1共±0.1兲
3.8共±0.1兲
3.6共±0.1兲
0.81
0.62
0.34
0.26
3.5⫻ 103
8.5⫻ 102
1.9⫻ 104
8.1⫻ 103
−14.6
−15.9
−20.3
−21.5
lator. Thus we were able to successfully vary the dielectric
constant of the polymer insulator in the range 3.6–4.3 while
keeping the surface potential of the insulator constant.
The 26 nm pentacene film was deposited on top of the
insulator surface at a deposition rate of 0.4 Å s−1 at 70 ° C by
using the thermal evaporation method. Figure 1 shows AFM
images of the pentacene films deposited on the polymer insulators prepared with different 共PVP:PMF兲 ratios. The surface morphologies and grain sizes of the pentacene films on
the polymer insulators are very similar 共⬃200 nm in diameter兲. This result indicates that the pentacene crystal grain
size is not an important parameter in our system.
Figure 1 shows the reflective x-ray diffraction patterns of
pentacene films grown on polymer insulators prepared with
various 共PVP:PMF兲 ratios. The typical x-ray diffraction pattern of pentacene includes two distinct crystalline phases, the
“thin-film phase” and the “single-crystal phase,” with correand 14.5 Å,
sponding d001-spacings of 15.4 Å
respectively.25,26 All samples produced sharp first-order diffraction peaks at 5.72° corresponding to a lattice spacing of
15.4 Å, which indicates the presence of only a single “thinfilm phase.” The diffraction results also clearly demonstrate
that the ordering of the pentacene molecules grown on the
four different polymer insulators with different 共PVP:PMF兲
ratios is the same. This result is expected from the results for
the surface energies and rms roughnesses of the insulator
surfaces, and is consistent with the results for the surface
morphologies.
FIG. 2. The drain current 共ID兲 vs drain voltage 共VD兲 characteristics of a
pentacene TFT prepared with a PVP:PMF ratio of 1:1.
A top-contact thin film transistor 共TFT兲 with a gold electrode 共channel width 800 ␮m, length 40 ␮m兲 was used for
analyzing the electrical characteristics of the pentacene
TFTs. Current-voltage measurements were carried out in air
at room temperature using Keithley 236 voltage source units.
Figure 2 shows the drain current 共ID兲 vs drain voltage 共VD兲
curve obtained for the OTFT fabricated with a PVP:PMF
ratio of 1:1. This figure shows a typical output characteristic
of transistors. In Fig. 3, the square root of 兩ID兩 in the saturation region at VD = −40 V is plotted as a function of gate
voltage VG for the prepared devices. From each linear fit in
Fig. 3, the threshold voltage Vth and field-effect mobility ␮ in
the saturation region were determined with the following
equation:27
兩ID兩 =
W
C␮共VG − Vth兲2 ,
2L
where C is the capacitance of the polymer insulator, W is the
channel width, L is the channel length, Vth is the threshold
voltage, and ␮ is the field-effect mobility.
The characteristic electric properties of the four devices
are summarized in Table I. Field-effect mobilities of
0.26– 0.81 cm2 / V s and threshold voltages of −14.5 to
− 21.5 V were obtained. The field-effect mobility increases
and the absolute value of the threshold voltage decreases
with increases in the dielectric constant of the insulator. This
result agrees with previous results.14,15
Surprisingly, the field-effect mobility increases by a factor of approximately 3 with only a small change 共3.6–4.3, a
20% increase兲 in the dielectric constant, k, of the insulator.
The higher field-effect mobility for insulators with higher
dielectric constants may be due to their enhanced polarization, which increases the number of hole-carriers at the
insulator-semiconductor interface. As a result, for the same
gate field, a higher mobility is obtained for devices with
higher k insulators.28,29
In summary, we have varied the dielectric constant of a
PVP insulator by varying the polymer/cross-linking agent
molar ratio while keeping other parameters such as the
roughness and surface potential of the insulator constant;
these insulators with different dielectric constants produce
similar film morphologies and molecular ordering. The fieldeffect mobility was found to increase by a factor of more
FIG. 1. 共a兲 X-ray diffraction patterns of pentacene films grown on insulathan 3 with small changes 共less than 20%兲 in the dielectric
tors;
共b兲
AFM
micrographs
of
pentacene
on
insulators
prepared
with
various
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constant of the insulator.
共PVP/PMF兲 ratios: 20:1, 5:1, 1.7:1, 1:1.
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152105-3
Appl. Phys. Lett. 87, 152105 共2005兲
Jang et al.
FIG. 3. 冑兩ID兩 vs VG and log10兩ID兩 vs VG
characteristics of pentacene TFTs prepared with various 共PVP:PMF兲 ratios:
共a兲 20:1, 共b兲 5:1, 共c兲 1.7:1, 共d兲 1:1
This work was supported by the National Research
Laboratory Program of the Ministry of Science and Technology of Korea, a grant 共F0004022兲 from the Information Display R&D Center under the 21st Century Frontier R&D Program and the Regional R&D Cluster Project designated by
the Ministry of Commerce, Industry and Energy of Korea,
and the BK21 Program of the Ministry of Education and
Human Resources Development of Korea. We also thank the
Pohang Acceleratoty Laboratory for providing the synchrotron radiation source at the 3C2 and 8C1 beam line.
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