Synthesis, stability and electrical properties of new soluble pentacenes with unsaturated side groups

Article abstract of DOI:10.1039/c4ra11195h

A series of pentacene derivatives with unsaturated hydrocarbon substituents at the 6,13-positions as side groups have been synthesized. The effects of conjugated ??-bonding throughout the side groups on the solubility, oxidative stability, crystallinity, and electrical properties of the compounds were investigated. Here we report that the solubility of pentacene can be greatly improved by the insertion of linear long side groups at the 6,13-positions: the obtained pentacene derivatives were soluble in common organic solvents. Time-dependent UV-visible spectra in chloroform solutions as well as thin films were recorded to investigate the oxidative stability of each pentacene compound under ambient conditions. The photooxidative stability of the pentacene derivatives was remarkably enhanced by increasing the electron conjugation length through the installation of unsaturated hydrocarbon substituents at the 6,13-positions. The solution-processed TFT devices based on 6,13-bis(4-pentylphenyl-ethynyl)pentacene (P5) and 6,13-bis(4-hexylphenylethynyl)pentacene (P6) showed charge transport mobilities of 2.14 ?? 10^-2 cm² V^-1 s^-1 and 3.94 ?? 10^-2 cm² V^-1 s^-1, respectively, and exhibited an on/off ratio of 10⁵. Here we demonstrate that P5 and P6 revealed much more stable electrical performances than the parent pentacene under the oxidative environment.

Full text of DOI:10.1039/c4ra11195h

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Synthesis, stability and electrical properties of new
soluble pentacenes with unsaturated side groups
Cite this: RSC Adv., 2015, 5, 8070
b
b
b
a
Heung Gyu Kim,†ab Hyun Ho Choi,† Eunjoo Song, Kilwon Cho and E-Joon Choi
*
*
A series of pentacene derivatives with unsaturated hydrocarbon substituents at the 6,13-positions as side
groups have been synthesized. The effects of conjugated p-bonding throughout the side groups on the
solubility, oxidative stability, crystallinity, and electrical properties of the compounds were investigated.
Here we report that the solubility of pentacene can be greatly improved by the insertion of linear long
side groups at the 6,13-positions: the obtained pentacene derivatives were soluble in common organic
solvents. Time-dependent UV-visible spectra in chloroform solutions as well as thin films were recorded
to investigate the oxidative stability of each pentacene compound under ambient conditions. The
photooxidative stability of the pentacene derivatives was remarkably enhanced by increasing the
electron conjugation length through the installation of unsaturated hydrocarbon substituents at the
6,13-positions. The solution-processed TFT devices based on 6,13-bis(4-pentylphenyl-ethynyl)pentacene
(P5) and 6,13-bis(4-hexylphenylethynyl)pentacene (P6) showed charge transport mobilities of 2.14 ꢀ
Received 25th September 2014
Accepted 22nd December 2014
10ꢁ2 cm2 Vꢁ1 sꢁ1 and 3.94 ꢀ 10ꢁ2 cm2 Vꢁ1
s
ꢁ1, respectively, and exhibited an on/off ratio of 105. Here
DOI: 10.1039/c4ra11195h
we demonstrate that P5 and P6 revealed much more stable electrical performances than the parent
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pentacene under the oxidative environment.
positions are extremely sensitive to oxygen when exposed to
visible light.9–12
Introduction
To improve both the solubility and photooxidative stability
of pentacene, various research groups are designing and
synthesizing new pentacene derivatives.13–19
In this study, we designed new soluble and stable pentacene
derivatives through the introduction of unsaturated hydrocarbon
substituents into the 6,13-position of a parent pentacene: the
target molecules have been synthesized by the “organolithium
reaction” method, followed by reduction with SnCl2$2H2O. The
resulting six compounds are 6,13-bis(Z-prop-1-enyl)pentacene
Over the last decade, intensive research and development efforts
have focused on the synthesis of organic semiconducting mate-
rials and their applications in light emitting diodes (OLEDs),
photovoltaic cells, sensors, radio frequency identication (RFID)
tags,1 and organic thin lm transistors (OTFTs). Recently, organic
semiconductors have attracted more attention than inorganic
semiconductors such as Si, GaAs, and ZnO, since the latter require
high-temperature and high-vacuum-deposition processes for
device fabrication, while the former can be more easily fabricated
by solution processes such as spin-coating, inkjet printing, and
screen printing. Moreover, future displays are predicted to exhibit
exibility, which can be easily achieved with organic materials.
Therefore, organic semiconductors are considered to be potential
candidates for TFTs.2–5
Pentacene has attracted considerable attention for its use in
high-performance conjugated semiconductors.6–8 However,
pentacene TFTs cannot be prepared using solution deposition
processes due to their limited solubility. Thus, their fabrication
requires high-vacuum deposition techniques. Furthermore,
even though pentacene is quite stable under dark, its 6,13-
(P1);
6,13-bis(E-prop-1-enyl)pentacene
(P2);
6,13-bis(4-
tolylethynyl)-pentacene (P3); 6,13-bis(4-methoxyphenyl-ethynyl)
pentacene (P4); 6,13-bis(4-pentylphenylethynyl)pentacene (P5);
and 6,13-bis(4-hexylphenylethynyl)pentacene (P6), as depicted in
Scheme 1. We investigated the solubility, oxidative stability,
crystallinity, and electrical properties of these pentacene
aDepartment of Polymer Science and Engineering, Kumoh National Institute of
Technology, Gyungbuk 730-701, Korea. E-mail: ejchoi@kumoh.ac.kr; Fax: +82-54-
478-7710; Tel: +82-54-478-7684
bDepartment of Chemical Engineering, Pohang University of Science and Technology,
Gyungbuk 790-784, Korea. E-mail: kwcho@postech.ac.kr
† The authors contributed equally to this work.
Scheme 1 Synthesis of pentacene derivatives.
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derivatives for their application in solution-processable organic an eluent to afford the product. Yield: 45%. IR (KBr pellet,
semiconductors for TFT fabrication.
cmꢁ1): 3040 (aromatic ¼ CH, st), 2950, 2814 (aliphatic CH, st),
1429, 1381 (C]C, st). 1H NMR (CDCl3, d in ppm): 9.22 (4H, s,
ArH), 8.12–8.03 (4H, m, ArH), 7.56–7.48 (4H, m, ArH), 2.55
(6H, s, CH3).
Experimental
Materials and measurements
All reagent-grade starting materials were purchased from
Aldrich and TCI. Commercially available reagents were used
without further purication. All solvents were further puried
prior to use by general methods. 6,13-Pentacenequinone was
synthesized according to reported literature procedures.17 The
structures of all isolated compounds were identied by FT-IR
(Jasco, FT/IR-300E) and 1H NMR spectroscopy (Bruker, DPX
Synthesis of 6,13-di[(E)-prop-1-enyl]pentacene (P2)
An alkylation reaction of 6,13-pentacenequinone (0.30 g, 1.0
mmol) with trans-1-bromo-1-propene (0.47 g, 3.9 mmol) and
reduction with SnCl2$2H2O was carried out following the
procedure described above to afford the product. Yield: 43%. IR
(KBr pellet, cmꢁ1): 3040 (aromatic ¼ CH, st), 2950, 2814
1
1
200 MHz). The H NMR chemical shis are presented in the
(aliphatic CH, st), 1427, 1379 (C]C, st). H NMR (CDCl3, d in
unit of d (ppm) relative to tetramethylsilane (TMS, d ¼ 0) and
referenced to the peaks signals corresponding to the residual
non-deuterated solvent. The absorption spectra of the pen-
tacene derivatives were measured by a UV-visible spectro-
photometer (Mecasys, UV-3220). The current–voltage
characteristics of the devices were measured at room
temperature in dark by using a semiconductor parameter
analyzer (Keithley, S4200). X-ray diffraction (XRD) measure-
ments were obtained by using the synchrotron source at the
Pohang Accelerator Laboratory (PAL) in Korea (10C1). The
optical microscopy (OM) and polarized optical microscopy
(POM) images were observed by using an Axioplan microscope
(ZEISS).
ppm): 9.22 (4H, s, ArH), 8.12–8.03 (4H, m, ArH), 7.56–7.48 (4H,
m, ArH), 2.62 (6H, s, CH3).
Synthesis of 6,13-bis[4-tolylethynyl]pentacene (P3)
An alkylation reaction of 6,13-pentacenequinone (0.30 g, 1.0
mmol) with 1-ethynyl-4-methylbenzene (0.34 g, 2.6 mmol) and
reduction with SnCl2$2H2O was carried out following the
procedure described above to afford the product. Yield: 36%. IR
(KBr pellet, cmꢁ1): 3023 (aromatic ¼ CH, st), 2913, 2854
(aliphatic CH, st), 2183 (C^C, st), 1508 (C]C, st). 1H NMR
(CDCl3, d in ppm): 9.31 (4H, s, ArH), 8.16–8.05 (4H, m, ArH),
7.85, 7.80 (4H, d, ArH), 7.48–7.41 (4H, m, ArH), 7.39, 7.32 (4H, d,
ArH), 2.49 (6H, s, Ar–CH3).
Fabrication of OTFT device
Bottom-gate and top-contact OTFTs prepared using pentacene
derivatives as channel semiconductors were fabricated under
ambient conditions. A highly n-doped silicon wafer was used as
a gate electrode as well as a substrate. A silicon dioxide (SiO2)
layer thermally grown to 300 nm thickness was employed as a
gate dielectric. Then, 0.5 wt% and 0.1 wt% solutions of the
pentacene derivatives in chloroform were used for spin-coating
on HMDS coated surface and drop-casting on bare surface,
respectively. The channel width/lengths of 1500 mm/40 mm were
adopted. As a source/drain electrode, 60 nm Au was thermally
deposited through a shadow mask.
Synthesis of 6,13-bis[(4-methoxyphenyl)ethynyl]pentacene
(P4)
An alkylation reaction of 6,13-pentacenequinone (0.30 g, 1.0
mmol) with 4-ethynylanisole (0.39 g, 2.7 mmol) and reduc-
tion with SnCl2$2H2O was carried out following the proce-
dure described above to afford the product. Yield: 52%. IR
(KBr pellet, cmꢁ1): 3045, 3033 (aromatic ¼ CH, st), 2954, 2925
(aliphatic CH, st), 2183 (C^C, st), 1602, 1510 (C]C, st),
1
1247, 1027 (C–O, st). H NMR (CDCl3, d in ppm): 9.31 (4H, s,
ArH), 8.13–8.09 (4H, m, ArH), 7.92, 7.88 (4H, d, ArH), 7.48–
7.44 (4H, m, ArH), 7.13, 7.09 (4H, d, ArH), 3.97 (6H, s, Ar–O–
CH3).
Synthesis of 6,13-di[(Z)-prop-1-enyl]pentacene (P1)
A 2.0 M solution of n-BuLi in cyclohexane (2.0 mL, 4.0 mmol)
was added dropwise to a solution of cis-1-bromo-1-propene
(0.47 g, 3.9 mmol) in dry THF (20 mL) at ꢁ78 ꢂC under a
Synthesis of 6,13-bis[(4-pentylphenyl)ethynyl]pentacene (P5)
nitrogen atmosphere. The solution was stirred for 50 min. An alkylation reaction of 6,13-pentacenequinone (0.30 g, 1.0
Aer adding 6,13-pentacenequinone (0.30 g, 1.0 mmol), the mmol) with 1-ethynyl-4-pentylbenzene (0.51 g, 2.7 mmol) and
solution was stirred for 1 h at ꢁ78 ꢂC and for an additional 7 h reduction with SnCl2$2H2O was carried out following the
at room temperature. Into the solution, SnCl2$2H2O (2.0 g, 8.8 procedure described above to afford the product. Yield: 36%. IR
mmol) and 50% acetic acid (2.0 mL) were added. The mixture (KBr pellet, cmꢁ1): 3041 (aromatic ¼ CH, st), 2919, 2850
was stirred for 12 h at room temperature, and then poured (aliphatic CH, st), 2179 (C^C, st), 1506, 1457 (C]C, st). 1H
into water (300 mL). To this mixture, dichloromethane (300 NMR (CDCl3, d in ppm): 9.15 (4H, s, ArH), 8.05–8.00 (4H, m,
mL) was added and the organic layer was separated and ArH), 7.86, 7.81 (4H, d, ArH), 7.46–7.42 (4H, m, ArH), 7.40, 7.36
washed with water (300 mL ꢀ 2). Aer removing the solvent by (4H, d, ArH), 2.80–2.73 (4H, t, Ar–CH2–CH2), 1.81–1.76 (4H, m,
evaporation under reduced pressure, the residue was puried Ar–CH2–CH2), 1.51–1.42 (8H, m, CH2–CH2–CH3), 1.03–0.96 (6H,
by column chromatography on silica gel using chloroform as t, CH3).
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pentacene molecules by loosening the p–p stacking interac-
Synthesis of 6,13-bis[(4-hexylphenyl)ethynyl]pentacene (P6)
tions of the fused rings.
An alkylation reaction of 6,13-pentacenequinone (0.30 g, 1.0
mmol) with 1-ethynyl-4-hexylbenzene (0.55 g, 2.7 mmol) and
reduction with SnCl2$2H2O was carried out following the
procedure described above to afford the product. Yield: 42%.
IR (KBr pellet, cmꢁ1): 3027 (aromatic ¼ CH, st), 2921, 2850
(aliphatic CH, st), 1508, 1459 (C]C, st). 1H NMR (CDCl3, d in
ppm): 9.16 (4H, s, ArH), 8.04–8.00 (4H, m, ArH), 7.88, 7.03 (4H,
d, ArH), 7.49–7.46 (4H, m, ArH), 7.40, 7.37 (4H, d, ArH), 2.82–
2.76 (4H, t, Ar–CH2–CH2), 1.82–1.76 (4H, m, Ar–CH2–CH2),
1.51–1.42 (12H, m, CH2–CH2–CH2–CH3), 1.03–0.96 (6H, t,
CH3).
Photophysical properties and oxidative stabilities
UV-vis spectra of obtained pentacene derivatives were recor-
ded in chloroform solutions and thin lms. As shown in
Fig. 1, the pentacene derivatives in chloroform solutions
showed characteristic vibrational nger-like absorbances
similar to the parent pentacene (lmax ¼ 577 nm) and exhibited
signicant red shis. As depicted in Table 1, the pentacene
derivatives show absorbances lmax in the range 580–614 nm
and 631–668 nm in chloroform solutions. Interestingly, P3–P6
exhibited more red shis than P1–P2 due to the relatively
large extended p-electron delocalization in the former.
Moreover, for all compounds, the lm state shows greater red
shis compared with the solution state because the inter-
molecular interaction may increase in the dense state. The
HOMO–LUMO bandgaps were calculated using the absorp-
tion onset obtained in the solution and are also included in
Table 1. The all pentacene derivatives exhibited relatively low
bandgaps (Eogpt ¼ 1.79–1.91 eV) compared to the parent pen-
tacene (Eg ¼ 2.12 eV).
Results and discussion
Synthesis and solubility
Six pentacene derivatives with unsaturated side groups were
synthesized by the “organolithium reaction” method,
followed by reduction with SnCl2$2H2O, as shown in
Scheme 1. They contain Z-prop-1-enyl (P1), E-prop-1-enyl (P2),
4-tolylethynyl (P3), 4-methoxyphenylethynyl (P4), 4-pentyl-
phenylethynyl (P5), and 4-hexylphenylethynyl (P6) side
groups. The structures of the products obtained from each
reaction step were characterized by 1H NMR and FT-IR
spectroscopy. The results were in accordance with the
expected formula.
The photooxidative stability of pentacene derivatives has
been investigated under ambient conditions. The time-
dependent UV-vis spectra were measured to estimate their
oxidative stabilities in solutions (Fig. 2) and thin lms (Fig. 3).
The solubilities of the resulting compounds were investi-
gated to examine potential for the solution process. Three
levels of solubility were classied in a common organic
solvent (e.g., chloroform) at room temperature, and the
results are summarized in Table 1. Triple (+++), double (++),
and single (+) plus signs were used to indicate solubility
levels corresponding to more than 7 mg mLꢁ1 (excellent
solubility), between 4 and 6 mg mLꢁ1 (good solubility), and
less than 3 mg mLꢁ1 (poor solubility), respectively. P1 and P2,
which contain propenyl side groups, exhibited poorer solu-
bility levels than P3–P6, which contain alkylphenylethynyl
side groups. Specically, P5 and P6 with p-pentylphenyle-
thynyl and p-hexylphenylethynyl groups, respectively,
exhibited the highest solubility levels. This indicates that
Fig. 1 UV-vis spectra of various pentacene derivatives obtained in
linear long side groups may inhibit the packing of the chloroform solutions (a) and in thin films (b).
Table 1 Solubility, half-life times, and photophysical properties of various pentacene derivatives
lmax (nm)
t1/2 (day)
Compound
Solubilitya
Solution
Film
lonset (nm)
Eogpt (eV)
Solutionb
Filmc
P1
P2
P3
P4
P5
P6
+
+
++
++
+++
+++
580, 631
580, 631
611, 666
614, 668
611, 666
611, 666
604, 656
606, 661
775
794
762
650
650
688
693
688
688
1.91
1.91
1.80
1.79
1.80
1.80
1.5
1.4
3.3
2.5
2.7
2.7
0.6
0.8
>5
>5
ꢃ5
748
2.9
a
b
c
Classication: +++ (>7 mg mLꢁ1); ++ (4–6 mg mLꢁ1); + (<3 mg mLꢁ1). Concentration: 2 mg mLꢁ1
.
Films coated on glasses.
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ambient atmosphere, light, and temperature. In Table 1, the
pentacene derivatives exhibited remarkably long half-life times
(t1/2 ¼ 1–3 day) in solution compared with the parent pentacene
(t1/2 ¼ ꢃ3 min).16 Interestingly, the derivatives containing
alkylated phenylethynyl groups (P3–P6) exhibited greater
oxidative stabilities than those containing E/Z-propenyl groups
(P1 and P2). In Fig. 2b and 3b, the lmax peaks of the latter
molecules almost disappeared aer 3 days in the solutions and
2 days in the lms, whereas those of the former molecules
remained till 5 days. We infer that the steric hindrance of the
bulkier phenyl groups against attacking of oxygen atoms
prevents the oxidation of the 6,13-positions of P3–P6, and the
extended p-electron delocalization thermodynamically stabi-
lizes the molecular structures. The half-life times determined in
lms are also included in Table 1. Notably, P3–P5 presented
longer half-life times ($5 day) than the values obtained in
solution, whereas P1 and P2 exhibited relatively shorter half-life
times (<1 day). Meanwhile, P6 shows the half-life time in the
lm similar to that in the solution.
XRD studies and TFT performances
Fig. 4 depicts the out-of-plane XRD patterns of P5 and P6
thin-lms deposited by spin-coating or drop-casting
methods. The drop-casted lms show several strong reec-
tion peaks, whereas the spin-coated lms exhibit only very
weak reection peaks. This indicates that the degree of
crystallinity of lms obtained via drop-casting are much
higher than the lms obtained via spin-coating. We suggest
that the molecular stacking dynamics of these pentacene
derivatives strongly correlate with the rate of solvent drying
Fig. 2 Photooxidation stability in chloroform solutions: (a) time-
dependent UV-vis spectra under ambient condition; (b) linear fits for
the time-dependent lmax
.
Solutions of pentacene derivatives in chloroform gradually
became colorless and the peak intensities of lmax in the UV-vis
spectra decreased depending upon the exposure period under
Fig. 3 Photooxidation stability in thin films: (a) time-dependent UV-vis
spectra under ambient condition; (b) linear fits for the time-dependent Fig. 4 Synchrotron radiation X-ray diffraction patterns of P5and P6 on
lmax bare Si/SiO2 substrates.
.
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during lm fabrication. That is, the spin-coating process with
a faster drying rate does not provide enough time to form p–
p stacks with the P5 and P6 molecules, whereas the drop-
casting process with the slower drying rate does. In the
XRD patterns for the drop-casted lms, P6 showed only the
(001) reection peaks, while P5 exhibited the (001) reection
peaks as well as additional Bragg peaks. This implies that the
crystalline structure of P5 may be more complex than that of
P6. Consistent with this X-ray result, we observed that the
drop-casted lms by POM, P5 and P6, exhibited different
optical textures at ambient temperature as shown in Fig. 5.
The d-values for the (001) peaks for P5 and P6 were calculated
˚
˚
to be 23.9 A and 24.2 A, respectively, and the contour lengths of
˚
the side groups for P5 and P6 were estimated to be 12.4 A and
˚
13.7 A, respectively. These values are indicative of the interca-
lation of side groups into the layer spacings. Based on these
results, we conclude that the long axes of the P5 and P6 mole-
cules are perpendicular to the surface and that the molecular
axes of pentacenes stack cofacially, as proposed in Fig. 6. In case
of P5 and P6, a linear long n-alkylated phenylethynyl side group
can discourage the herringbone packing arrangement and
promote the cofacial p-stacked packing arrangement. Mean-
while, P1–P4, which contain a short vinyl side group, might
form imperfect herringbone stacks and thus somewhat distort
the cofacial stacks. These results are in agreement with those of
previous studies that patacene derivatives with bulky side silyl
groups (TIPS- and TES-PENs) promote the cofacial p-stacked
packing arrangement instead of the herringbone packing
arrangement.14
The electrical characteristics of P5 and P6 were measured by
using top-contact bottom-gate OTFTs. Fig. 7a shows the typical
Fig. 6 Proposed molecular structures in drop-casted films for P5 (a)
transfer curves (ID–VG) of P5 and P6 OTFTs, whose quantitative and P6 (b).
performances are summarized in Table 2. The devices exhibited
clearly hole channel-switching characteristics. The eld-effect
hole mobilities for drop-casted and spin-coated devices were
ꢃ10ꢁ2 cm2 Vꢁ1 sꢁ1 and ꢃ10ꢁ3 cm2 Vꢁ1 sꢁ1, respectively. This
indicates that the eld-effect hole mobilities were higher in
drop-casted devices than in spin-coated devices. It is attributed
to the more effective p–p stacking of molecules in drop-casted
lm than in spin-coated lm, aforementioned in Fig. 4. As
adopted the drop-casted lm, the P5-based devices displayed a
eld-effect mobility of 2.14 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 and an on/off
ratio of 105, and the P6-based devices exhibited a eld-effect
mobility of 3.94 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 and an on/off ratio of
105. Unlike P5 and P6, P1–P4 revealed insignicant FET activi-
ties in the solution process. It is presumed that the efficient
overlap of p-orbitals in P5 and P6 causes relatively superior
device properties in OTFTs, compared with P1–P4. Besides, the
formers form relatively effective packing structure, aforemen-
tioned in Fig. 6.
Finally, the oxidative stability of the OTFT devices based on
the P5 and P6 was evaluated. Fig. 7b and c show the time-
dependent transfer characteristics of the OTFTs exposed for 4
days to ambient air and 60 W uorescent light. In Table 2,
compared P5-device with P6-device, the mobility is slightly
Fig. 5 Optical microscopy (OM) and polarized optical microscopy
(POM) images of drop-casted films for P5 (a) and P6 (b).
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consistent with the aforementioned tendency of time-
dependent UV-vis absorption spectra of P5 and P6 thin lms
under oxidative environment. It is noteworthy that only the
structural difference between the two compounds is very minor:
P5 has a pentyl end group and P6 has a hexyl one.
Conclusions
Six new pentacene derivatives P1–P6 were synthesized and
characterized. The resulting compounds were soluble in
common organic solvents such as chloroform, dichloro-
methane, THF, and toluene. We assume that linear unsaturated
side groups may inhibit the packing of molecules by loosening
the p–p stacking interactions of the fused rings. The photoox-
idation stability for the pentacene derivatives were considerably
increased by introducing propenyl (P1 and P2) as well as alky-
lated phenylethynyl (P3–P6) groups at the 6,13-positions. We
assume that the unsaturated side groups may wave aside an
attack by the oxygen atoms at the 6,13-positions through the
steric hindrance and may thermodynamically stabilize the
molecule through the extended p-electron delocalization.
Further, we found that the morphology and crystallinity of these
molecules, which are affected by the casting method, are the
predominant source of the different eld-effect mobilities. In
case of the solution process, the OTFT devices of P5 and P6 (R ¼
pentyl- or hexyl-phenylethynyl) showed the superior charge
transport properties whereas those of P1–P4 exhibited the
inferior properties. Here we demonstrate that P5 and P6
revealed much more stable electrical performances than the
parent pentacene under oxidative environment. Finally, we
conclude that the installation of unsaturated side groups into
pentacene derivatives tended to much more improve oxidative
stability rather than charge mobility. Especially, by using linear
long unsaturated side groups, as exemplied by P5 and P6, the
level of conjugation is improved and results in remarkable
oxidative stability with superior electrical performances.
Fig. 7 (a) Transfer characteristics of top-contact bottom-gate OTFTs
based on P5 and P6 films using drop casting (black square) or spin
coating (red circle); gate electrode and dielectric: n-doped Si and 300
nm-thick SiO2, respectively; source/drain electrode: Au (width: 1500
mm, length: 40 mm). Photooxidation experiments for spin-coated
devices: (b) representative time-dependent transfer characteristics of
the OTFTs exposed to ambient air and 60 W fluorescent light; (c)
dependence of field-effect hole mobilities on the exposure period.
Acknowledgements
higher in the P6 than in the P5 before the exposure to ambient
air and light, but the on-current and eld-effect mobilities were
less decreased in the P5 than in the P6 under oxidative envi-
ronment. This means that the electrical performance was less
diminished in P5-device than in P6-device. This result is
This research was supported by the Basic Science Research
Program (no. NRF-2013R1A1A2012471) through the National
Research Foundation of Korea (NRF), funded by the Ministry of
Education Science and Technology.
Table 2 Field-effect transistor characteristics of selected pentacene derivativesa
Compound
Fabrication method
Spin-coating
Exposure periodb (day)
Mobility (cm2 Vꢁ1 sꢁ1
)
On/off ratio
P5
0
4
0
0
4
0
3.43 ꢀ 10ꢁ3
3.04 ꢀ 10ꢁ4
2.14 ꢀ 10ꢁ2
3.72 ꢀ 10ꢁ3
1.09 ꢀ 10ꢁ4
3.94 ꢀ 10ꢁ2
ꢃ106
ꢃ104
ꢃ105
ꢃ106
ꢃ104
ꢃ105
Drop-casting
Spin-coating
P6
Drop-casting
a
Top-contact bottom-gate OTFTs; gate electrode and dielectric: n-doped Si and 300 nm-thick SiO2, respectively; source/drain electrode: Au (width:
1500 mm, length: 40 mm). b OTFTs were exposed to ambient air and 60 W uorescent light; 0 day stands for an as-prepared device.
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