Synthesis, spectral characterization of CuPcF16 and its application in organic thin film transistors using p-6p as inducing layer

Article abstract of DOI:10.1016/j.jpcs.2011.12.016

An air-stable n-channel semiconductor material, CuPcF₁₆, was synthesized in a slightly modified procedure and characterized by infrared (IR), X-ray diffraction (XRD), UVvis and fluorescence spectra. CuPcF₁₆ showed a monomer character

Full text of DOI:10.1016/j.jpcs.2011.12.016

Journal of Physics and Chemistry of Solids 73 (2012) 589–592
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Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Synthesis, spectral characterization of CuPcF16 and its application in organic
thin film transistors using p-6p as inducing layer
Feng Ma n, Shirong Wang, Xianggao Li n
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
An air-stable n-channel semiconductor material, CuPcF16, was synthesized in a slightly modified
procedure and characterized by infrared (IR), X-ray diffraction (XRD), UV–vis and fluorescence spectra.
CuPcF16 showed a monomer characteristic in THF and pyridine while exhibited an aggregation property
in DMF. The CuPcF16/p-6p (CuPcF16 on p-6p) organic thin film transistors (OTFTs) using CuPcF16 as an
active layer and p-6p as an inducing layer was fabricated by the physical vapor deposition technique.
Received 22 August 2011
Received in revised form
16 November 2011
Accepted 17 December 2011
Available online 27 December 2011
Charge carrier field-effect mobility (m), Ion/Ioff and threshold voltage (VT) of the CuPcF16/p-6p OTFTs
were 0.07 cm2/V s, 105 and 5.28 V, respectively. The charge mobility of the OTFTs was two or even three
times higher than that of the conventional single layer CuPcF16-based OTFTs. The improved perfor-
mance was attributed to the introduction of p-6p to form a highly oriented and continuous film of
Keywords:
A. Semiconductors
A. Thin films
C. X-ray diffraction
CuPcF16 with the molecular
p
–
p
conjugated direction parallel to the substrate.
& 2012 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experiment
Metal hexadecafluorophthalocyanine, such as CuPcF16, is cur-
rently receiving a great deal of attention as one of the few
molecules that exhibit air-stable n-channel semiconducting beha-
vior [1]. Such properties result in a number of studies aiming at
different applications like organic solar cell, OTFTs, gas sensors
and rectifying junction [2–4]. For these organic electronics, a
highly oriented and continuous organic thin film is an ideal active
layer to improve the device performance. However, their thin
films are normally composed of needle-like crystals with random
orientations, which will induce more grain boundaries. The high
defect concentration in grain boundaries of multi-crystal thin
films usually leads to poor carrier transport, especially for
phthalocyanine compounds [5–7].
In this paper, we synthesized CuPcF16 in a slightly modified
procedure and went on investigating its spectral characterization.
Especially we employed p-6p as an inducing layer to fabricate
high-quality CuPcF16/p-6p thin film using the vapor deposition
techniques. The morphology and structure of the thin film was
examined by the XRD and the SEM. Furthermore, we fabricated
CuPcF16/p-6p OTFTs and measured their electrical characteristics
to illustrate the significant role of p-6p in the improvement of
device performance.
The synthesis of CuPcF16: Sublimed tetrafluorophthalonitrie
and anhydrous copper (I) chloride in an equimolar 4:1 ratio was
intensively mixed in a mortar. The mixture was filled in a glass
vessel, three times flushed with nitrogen and vacuum and finally
the glass ampoule was sealed under vacuum (1.33 ꢀ 10ꢁ3 Pa).
After heating for 8 h at 240 1C the blue product was isolated and
washed with ethanol and acetone to remove the soluble organic
admixture. Compared with the synthesis process reported in the
previous literature [8], the reaction time was extended from 7 to
8 h and the reaction temperature was elevated from 220 to
240 1C. Meanwhile, the reaction ampoule contained less water,
which would affect the reaction yield through three times
exchange with nitrogen and vacuum. The resulting purple crystals
with higher yield of 52.0% after concentrated sulfuric acid
purification were identified as copper hexadecafluorophthalocya-
nine having the X-ray diffraction maxima peak (2y) of 6.11 IR
(KBr): 1637 w, 1617 w, 1526 m, 1506 w, 1489 s, 1460 m, 1337 s,
1320 m, 1272 m, 1152 s, 1073 w, 963 s, 945 w, 839 m, 767 w, 749
m, 656 w, 603 m, and 497 w cmꢁ1
.
The CuPcF16/p-6p OTFTs configuration is given in Fig. 1. The
device was prepared with an air-stable n-type semiconductor
CuPcF16 and a rod-like conjugated oligomer p-6p molecule. Their
molecular structures are also given in Fig. 1. CuPcF16 and p-6p
were purified twice by thermal gradient sublimation prior to
processing. A 6 nm thick film of p-6p was first deposited on a SiO2
substrate at 180 1C, and then a 30 nm thick layer of CuPcF16
was deposited on top of the p-6p surface by vacuum deposition.
n Corresponding authors.
E-mail addresses: mafontune@sina.com.cn, lixianggao@hotmail.com (X. Li).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2011.12.016

590
F. Ma et al. / Journal of Physics and Chemistry of Solids 73 (2012) 589–592
F
F
0.9
0.8
0.7
0.6
0.5
0.4
0.3
F
F
F
F
N
F
F
F
N
N
Cu
N
N
N
N
N
F
F
F
F
F
F
F
0.2
CuPcF16
p-6p
0.1
Au
Au
0.0
400
500
600
700
800
CuPcF
p-6p
16
Wavelength (nm)
Fig. 2. UV–vis spectra of CuPcF16 dissolved in THF (dotted line), pyridine (dashed
line) and DMF (solid line).
1.4
1.2
1.0
0.8
0.6
0.4
0.2
SiO2
n++-silicon
Fig. 1. Device configuration and molecular structures used in the study.
The CuPcF16/SiO2 films were fabricated under the same conditions
without introducing the p-6p organic layer. All organic films were
deposited in vacuum (10–4–10–5 Pa) at a rate of 0.50 nm min–1
Finally, a 30 nm Au film acting as source and drain electrodes was
deposited by thermal evaporation through a shadow mask defin-
ing channel width (W) and length (L) of 6000 mm and 200 mm,
respectively. The output and transfer characteristics of the tran-
sistors were measured with two Keithley 236 source-measure-
ment units under ambient conditions at room temperature.
.
300
400
500
600
700
800
Wavemumber(nm)
Fig. 3. UV–vis absorption of different concentration CuPcF16 solution in DMF.
3. Results and discussion
3.1. UV–vis and fluorescence spectra
646 nm appears and the absorption intensity grows while the
intensity of absorption peak at 606 nm declines and even dis-
appears. It is a typical UV–vis absorption pattern for aggregation
behavior. The higher the concentration, the more easily it aggre-
gates in DMF.
The fluorescence spectrums of CuPcF16 in THF, pyridine and
N-methyl pyrrolidone were measured on a CARY Eclipse fluores-
cence spectrophotometer, as shown in Fig. 4. The emission
maxima are observed at 398, 416 and 435 nm corresponding to
Fig. 2 shows the UV–vis absorption spectra of 2.2 ꢀ 10ꢁ5
mol Lꢁ1 CuPcF16 solutions in THF, Pyridine and DMF, respectively.
The absorption spectra were measured by an EVOLUTION 300
spectrometer. Absorption maxima for Q band are seen at 677 nm
with shoulders at 646 and 610 nm for CuPcF16 dissolved in THF
and at 679, 647 and 611 nm for CuPcF16 solution in pyridine. The
absence of aggregation effects indicates monomer behavior of
CuPcF16 in THF and pyridine [9]. And with the polarity of solvents
the purple light emission. The red-shifted emission peak may be
n
increasing, the stability of the excited state is higher than the
caused by the solvent effect that the excited state in the p-p
n
ground state in the
p-p
transition system, so the transition
transition system becomes more stable with the increase of
these solvents polarity leading to the Q band shift to a longer
wavelength.
energy gap decreases, which will induce the Q band shift to a
longer wavelength to some extent. The spectra of CuPcF16
dissolved in DMF differs from the spectrums of this compound
in pyridine and THF solvents since it shows two strong absorption
peaks at 646 and 676 nm. It demonstrates that CuPcF16 aggre-
gates in DMF solvent according to reference literature [9]. Fig. 3
shows the UV–vis absorption curves of different concentrations of
CuPcF16 in DMF. In the low concentration region, there are two
strong absorption peaks at 676 and 606 nm for Q band. With the
concentration of CuPcF16 increasing, a new absorption peak at
3.2. XRD of the CuPcF16/SiO2 and CuPcF16/p-6p films
Fig. 5 shows the XRD spectra of the CuPcF16/SiO2 and CuPcF16
/
p-6p thin films. The X-ray diffraction was performed in a Rigaku
D/max 2500 PC X-ray diffractometer with a Cu K
˚
a radiation
(
l¼1.54056 A). The d200 spacing of the CuPcF16/SiO2 film is
˚
14.92 A. The XRD pattern indicates that CuPcF16 molecules are

F. Ma et al. / Journal of Physics and Chemistry of Solids 73 (2012) 589–592
591
400
450
500
550
Wavelength (nm)
Fig. 4. Fluorescence emission spectra of CuPcF16 dissolved in THF (dotted line),
pyridine (dashed line) and N-methyl pyrrolidone (solid line).
CuPcF16/ SiO2
Fig. 6. (a) SEM of CuPcF16/SiO2 thin films and (b) SEM of p-6p/CuPcF16 thin films.
CuPcF16/ p-6p
the CuPcF16 molecules is parallel to the p-6p layer, which will
facilitate the carrier transportation.
6
8
10
12
14
Because of the liquid–crystal-like behavior of the p-6p mono-
layer film and the good coalescence behavior of the p-6p crystal
domains [13,14], a highly-ordered large-sized and continuous
p-6p ultrathin film can be fabricated on a SiO2 substrate. The
p-6p ultrathin film can supply a high quality substrate (the
inducing layer) for the growth of phthalocyanine molecules.
Dish-like CuPcF16 molecules can be oriented after the employ-
ment of the p-6p inducing layer, which likely result from the
geometrical channel of the surface of the inducing layer and the
domain diffusion direction of phthalocyanine molecules on the
surface of the inducing layer [15].
2 Theta [deg]
Fig. 5. X-ray diffraction spectra of CuPcF16/SiO2 and p-6p/CuPcF16 thin films.
grown upright on the SiO2 substrate with the metastable
˚
a-phase
[5,6,10,11]. For CuPcF16/p-6p film, d200¼14.78 A, which has a
slight deviation from the diffraction peak of CuPcF16/SiO2 thin
film. It possibly originates from the difference between the
molecule–substrate interaction and intermolecular interaction
[5,10,12]. These results also demonstrate that the CuPcF16 mole-
cules are approximately standing-up on the p-6p layer.
3.3. SEM of the CuPcF16/SiO2 and CuPcF16/p-6p films
3.4. Current–voltage characteristics
Fig. 6 shows the SEM of CuPcF16/SiO2 and CuPcF16/p-6p thin
films. The SEM images were obtained by a FEI Nanosem 430
Typical output characteristic curves of the CuPcF16/p-6p OTFTs
are shown in Fig. 7 at different gate–source voltages (VG) from 0 to
50 V. The positive voltage signals imply an electron-accumulated
process in these OTFTs. With the increase of VD, the linear region
and the saturation region can be observed. For lower VDS from 0 to
10 V, ID is almost linearly increased with increasing VDS. In
contrast, for higher VDS above 10 V, ID tends to saturate. Fig. 8
shows the typical transfer characteristics of the CuPcF16/p-6p
OTFTs with different gate voltages at a fixed VD of 50 V. The field
effect mobility were extracted from Fig. 8 in the saturation region
electron microscopy instrument. The surface image of CuPcF16
/
SiO2 thin film is shown in Fig. 6a. The CuPcF16 growth on the SiO2
substrate displays worm crystal grain morphology and arranges
on the substrate disorderly, lacking continuity. Compared with
the image of the CuPcF16/SiO2 film, the CuPcF16/p-6p thin film in
Fig. 6b shows better connectivity and fewer grain boundaries and
traps in the CuPcF16 bulk. The image shows a strip grain pattern of
CuPcF16/p-6p film. It implies that the p–p interaction direction of

592
F. Ma et al. / Journal of Physics and Chemistry of Solids 73 (2012) 589–592
two or even three times higher than the single CuPcF16 layer
1.6x10-5
1.4x10-5
1.2x10-5
1.0x10-5
8.0x10-6
6.0x10-6
4.0x10--6
0v
OTFTs devices without p-6p as the inducing layer [16,17]. The
field effect mobility of CuPcF16/p-6p thin film transistor has been
greatly improved due to the introduction of p-6p layer having
induced the formation of a high quality CuPcF16 thin film.
10v
20v
30v
40v
50v
4. Conclusion
In summary, highly oriented organic thin film of CuPcF16 with
the molecular
p–p conjugated direction parallel to the substrate
2.0x10-6
was fabricated. By applying this film-forming technique, the
performance of the n-channel CuPcF16/p-6p OTFTs is greatly
improved. Employing p-6p as an inducing layer is an effective
and a simple method to fabricate the high performance OTFTs for
practical application.
0.0
0
10
20
30
40
50
VD(V)
Fig. 7. Output characteristics of CuPcF16/p-6p at varied VG from 0 to 50 V (at a step
of 10 V).
Acknowledgments
The authors are grateful to Donghang Yan research group in
Changchun Institute of Applied Chemistry of Chinese Academy of
Sciences for support of devices’ fabrication. The work has been
partially supported by National Natural Science Foundation of
China (no. 21176180) and Research Fund for the Doctoral Pro-
gram of Higher Education of China (no. 20100032110021).
10-4
10-5
0.004
10-6
0.003
10-7
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10-8
10-9
0.002
0.001
0.000
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2
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¼
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CiðVGꢁVT Þ
ð1Þ
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m
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From the transfer characteristics, we extract a mobility
m of
0.07 cm2/V s, threshold voltage VT of 5.28 V and on–off current
ratio Ion/Ioff of 105 for CuPcF16/p-6p transistors. The mobility was