Highly sensitive phototransistor with crystalline microribbons from new π-extended pyrene derivative via solution-phase self-assembly

Article abstract of DOI:10.1039/c1cc11589h

A new pyrene-cored π-conjugated molecule has been synthesized through Sonogashira coupling reaction. The single-crystalline microribbon-based FET exhibited the highest mobility of 0.7 cm² V^-1 s ^-1 (I_on/I_off > 10⁶). Single-crystalline microribbons were employed to operate in an organic phototransistor (OPT) under very low light intensity (I = 5.6 μW cm ^-2).

Full text of DOI:10.1039/c1cc11589h

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 8907–8909
www.rsc.org/chemcomm
COMMUNICATION
Highly sensitive phototransistor with crystalline microribbons from new
p-extended pyrene derivative via solution-phase self-assemblyw
Youn Sun Kim,a Suk Young Bae,a Kyung Hwan Kim,a Tae Wan Lee,a Jung A Hur,a
Mai Ha Hoang,a Min Ju Cho,a Sung-Jin Kim,b Youngmee Kim,b Minsik Kim,a
Kwangyeol Lee,a Suk Joong Lee*a and Dong Hoon Choi*a
Received 19th March 2011, Accepted 17th May 2011
DOI: 10.1039/c1cc11589h
A new pyrene-cored p-conjugated molecule has been synthesized
through Sonogashira coupling reaction. The single-crystalline
microribbon-based FET exhibited the highest mobility of
0.7 cm2 VÀ1 sÀ1 (Ion/Ioff 4 106). Single-crystalline microribbons
were employed to operate in an organic phototransistor (OPT)
under very low light intensity (I = 5.6 lW cmÀ2).
obtain single crystals so as to provide a deeper understanding
in structural studies for organic semiconducting materials.
Another interesting feature is the ability to exhibit photo-
responsivity to increase the source–drain current (IDS) in
OFET devices under light irradiation. The development
of new types of optoelectronics, such as photodetectors,
photoswitches and photocontrolled memory devices, is highly
dependent on the control of charge density and charge transport
properties in the active layer of organic phototransistors
(OPTs) through both a gate bias and light illumination.
Herein, we report on J-aggregated single crystalline
microribbons from a highly soluble pyrene-cored conjugated
molecule and its unique electronic and optoelectronic properties
along with its single-crystal structure. When designing the
molecular building blocks, face-to-face stacked or slip-stacked
arrangement in the solid state was considered to strongly
improve the carrier mobility. Therefore, we have synthesized
p-extended 1,3,6,8-tetrakis((4-hexyl phenyl)ethynyl)pyrene
(PY-4(THB)) and fabricated crystalline microribbons for
FET and OPTs. Peripheral ethynyl groups would extend
conjugation and facilitate the planarity of the whole molecule,
that improves the stacking in solid state, since the flat pyrene
core is believed to aid strong p–p interactions in the solid state
such as in crystalline films and single crystals.10–12 Thereby,
well-defined p-stacked molecular arrangement can facilitate
charge transport.
High performance organic field effect transistors (OFETs)
have received much attention in the development of novel
organic semiconductors.1–4 The translation of well-designed
organic semiconductors into nano- and micro-scopic objects
such as nanowires, nanoribbons and nanobelts has attracted
significant interest for potential applications in electronics and
optoelectronics.5–9 Their electronic properties are highly
influenced by their structural features such as molecular
structure and packing. Most high performance organic semi-
conductors are based on highly ordered molecular packing
with strong p–p interactions and, thus, show high crystal-
linities when they are assembled in the solid state. Therefore,
structural studies at the molecular level are highly desirable for
the fundamental understanding of device performance.
However, profound studies on structural packing of organic
semiconductors have been rarely exploited due to the difficulties
encountered for single crystal growth. Recent achievement on
OFETs based on self-assembled nano- or micro-crystalline
objects have demonstrated their improved device performances
due to high crystallinity compared to those of thin film
transistors (TFTs), although there are only a few crystal
structures reported for the interpretation of their performance.5–9
Good solution processibility may give an opportunity to
PY-4(THB) was readily obtained in high yield (60%) via
Sonogashira coupling of 1,3,6,8-tetrabromopyrene and 1-ethynyl-
4-hexylbenzene (ESIw).13 PY-4(THB) shows high solubility in
various organic solvents and good self-film-forming property.
Thermal gravimetric analysis (TGA) shows the stability of
PY-4(THB) up to 380 1C (Fig. 1S, ESIw). Density functional
theory (DFT) calculation of PY-4(THB) using the Spartan’06
program at the B3LYP/6-31G* level reveals that the pyrene core
carried peripheral ethynyl benzene units retaining the planarity
although with some disorder around hexyl substituents. The
calculated HOMO, LUMO and bandgap energy (EgDFT) were
found to be À4.75, À2.25 and 2.5 eV, respectively (Fig. 2S, ESIw),
while experimental HOMO, LUMO and bandgap energy (Eg)
values for the film were À5.55, À3.61 and 1.94 eV, respectively.
Upon layering MeOH over a solution of PY-4(THB) in
chloroform and allowing the resulting mixture to stand for
a Department of Chemistry, Research Institute for Natural Sciences,
Korea University, 5 Anam-dong, Sungbuk-gu, Seoul, 136-701 Korea.
E-mail: dhchoi8803@korea.ac.kr, slee1@korea.ac.kr;
Fax: +82-2-924-3141; Tel: +82-2-3290-3140
b Department of Chemistry and NanoScience,
Ewha Womans University, Seoul 120-750, Korea
w Electronic supplementary information (ESI) available: Complete
experimental details of syntheses, compound characterization,
UV-Vis absorption and photoluminescence spectra, DFT calculation,
single crystal analysis, fluorescence lifetime measurements and detailed
device characterizations. CCDC 809193. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1cc11589h
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8907–8909 8907

View Article Online
As expected, the powder X-ray diffraction (PXRD) patterns
for the microribbons are identical to that obtained from a
single crystal of PY-4(THB) grown statically by layering for
2–7 days (Fig. 1g). Further investigation with selected-area
electron diffraction (SAED) patterns of microribbons with
transmission electron microscopy (TEM) clearly shows a
consistent crystallinity in both microribbons and single
crystals (Fig. 1f).
In order to investigate the electrical characteristics of the
microribbons, OFETs (bottom-gate, top-contact) were fabricated
on SiO2/Si substrate with N-doped polycrystalline silicon as
gate electrode and n-octyltrichlorosilane (OTS)-treated SiO2
surface layer as dielectric gate insulator followed by deposition
of gold electrodes using shadow masks (channel length = 40 mm).
The one-dimensional (1-D) crystalline microribbons with a
width of 6.9 mm was characterized by optical microscopy.
The resulting microribbon OFET which were tested at room
temperature and in air exhibited typical p-channel FET
characteristics. The output characteristics show very good
saturation behaviors and clear saturation currents that are
quadratically related to the gate bias (Fig. 2b). The optical
microscopy image of the microribbon FET shows that it
contains a relatively longer channel length compared to other
reported devices.6–8 From the source (S)–drain (D) current–
voltage curves (IDS vs. VG), the maximum field effect mobility
was found to be 0.7 cm2 VÀ1 sÀ1 with on/off current ratio
of 4106 and threshold voltage of À10 V and low gate
leakage current (IGS) (Fig. 2c). The mobility ranged from
0.1–0.7 cm2 VÀ1 À1. Fluctuation in mobility may be due to
s
Fig. 1 SEM image (a) of microribbons obtained from PY-4(THB)
(inset) and optical microscopy image of a single microribbon under a
cross-polarizers. (b) Rotation of the microribbon showed an alternate
appearance of birefringence at every 451 to the polarizer. (c) Crystal
packing diagram and (d and e) cofacial packing in the adjacent
molecules (viewed perpendicularly to the pyrene plane and showing
clear J-aggregations with adjacent phenyl rings. (f) TEM image
and corresponding SAED patterns of crystalline microribbon and
(g) PXRDs of microribbons and that simulated from the CIF.
two days, narrowly dispersed microribbons are obtained that
are B35–100 mm in length and B3–5 mm in width (Fig. 1a).
Birefringence analysis under a polarized optical microscope
shows the effects of rotating the single microribbon on the plate of
optical microscope under crossed polarization. For example,
transmitted light is only observed when microribbon is placed
at an angle of 451 with respect to the optic axis of polarizer,
demonstrating single microribbons show unidirectional orien-
tation of PY-4(THB) (Fig. 1b).
The single-crystal X-ray structure of PY-4(THB) (Fig. 1c)
contains four independent molecules where two of them are nearly
planar with tilting angle between the pyrene and the phenyl planes
of 2.20–4.521, and the remaining two are more tilted with angles of
3.40–4.011 and 24.16–24.511 (Fig. 3S, ESIw). J-aggregated layered
packing is observed in all four molecules (Fig. 4S–7S, ESIw).
Furthermore, the pyrene cores are stacked through p–p inter-
actions (3.68 A) between inter-phenyl planes (Fig. 1d and e). These
four independent J-aggregated layered packing arrangements
are clearly entangled to form wide channels (Fig. 8S, ESIw).
Fig. 2 Optical microscopy image (a) of microribbon based-FET
device, and electrical characterization of microribbon FET; output
(b), transfer and gate leakage current (c) curves (inset: transfer curves
of film FET device, see Fig. 9S, ESIw for magnified version).
c
8908 Chem. Commun., 2011, 47, 8907–8909
This journal is The Royal Society of Chemistry 2011

View Article Online
employed to get the highest photoresponsivity is very low
compared to the light power used in previous reports.14–16
Remarkably, the photoresponsivity (R) of microribbons is
much higher than that of inorganic single-crystalline silicon
TFTs (300 AWÀ1, I = 400 mW cmÀ2).14 Although the PLQY
of the thin film shows an identical response to that of the
microribbon, the R value (B420–430 AWÀ1) in the thin-film
based OPT is found to be much smaller than that of the
microribbon (Fig. 11S, ESIw). It is well known that the longer
exciton lifetime is closely associated with the efficient dissociation
of free charges with applied voltage.17 Since the fluorescence
lifetime (t) of the microribbon (0.44–0.57 ns) is greater than
that of film (0.12–0.16 ns), the large difference in lifetime (t)
supports the effective charge dissociation in microribbons.
To the best of our knowledge, the microribbon-based OPT
displays the highest photoresponsivity and photoswitching
ratio of dark- and photo-current for pyrene-based OPTs to
date under such low light intensity.12
Fig. 3 (a) Transfer curves of microribbon OPT in the dark (black)
and under monochromatic light irradiation (red) of 5.6 mW cmÀ2
(b) Photoresponsivity (R) and photoswitching ratio (P) vs. VG for
the OPT (inset: fluorescence images of thin film and crystalline
microribbons) (lex = 365 nm)).
.
This research was supported by the National Research
Foundation Program (No. 20100025252) and by Priority
Research Centers Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (NRF2011-0018396).
the variable interfacial contact of the microribbon surface to
the dielectric layer. In case of TFT of PY-4(THB) (Fig. 2c,
inset) which was prepared by spin casting of a THF solution of
PY-4(THB) and used without further treatment, the mobility
value is 0.1 cm2 VÀ1 sÀ1 with on/off current ratio of 2.0 Â 106
and threshold voltage (VTH) of À10 V. The significant difference
in mobilities between the microribbon and film is attributed to
the highly crystalline packing structure in microribbon and
more efficient charge transporting channels organized in a slip-
stacked manner induced by J-aggregation, as described above.
Furthermore, the absolute PL quantum yields (PLQY) of
PY-4(THB) turned out to be 0.92, 0.14 and 0.14 in solution,
film and microribbon states, respectively (Fig. 10S, ESIw).
Because of the highly sensitive optical response of the micro-
ribbon, we observed a dramatic increase of the source–drain
current (IDS) with the microribbon-based phototransistor
(width = 8 mm, length = 40 mm) when illuminating incident
Notes and references
1 L. Zhang, L. Tan, Z. Wang, W. Hu and D. Zhu, Chem. Mater.,
2009, 21, 1993.
2 K. H. Jung, S. Y. Bae, K. H. Kim, M. J. Cho, K. Lee, Z. H. Kim,
D. H. Choi, D. H. Lee, D. S. Chung and C. E. Park, Chem.
Commun., 2009, 5290.
3 F. Silvestri, A. Marrocchi, M. Seri, C. Kim, T. J. Marks,
A. Facchetti and A. Taticchi, J. Am. Chem. Soc., 2010, 132, 6108.
4 P. Gao, D. Beckmann, H. N. Tsao, X. Feng, V. Enkelmann,
M. Baumgarten, W. Pisula and K. Mullen, Adv. Mater., 2009,
21, 213.
¨
5 S. J. Kang, I. Bae, Y. J. Park, T. H. Park, J. Sung, S. C. Yoon,
K. H. Kim, D. H. Choi and C. Park, Adv. Funct. Mater., 2009,
19, 1609.
6 Q. Tang, L. Jiang, Y. Tong, H. Li, Y. Liu, Z. Wang, W. Hu, Y. Liu
and D. Zhu, Adv. Mater., 2008, 20, 2947.
7 Z. Wei, W. Hong, H. Geng, C. Wang, Y. Liu, R. Li, W. Xu,
Z. Shuai, W. Hu, Q. Wang and D. Zhu, Adv. Mater., 2010,
22, 2458.
light
dark
light with very low intensity ((IDS
À IDSdark)/IDS
=
1.2 Â 106, VG = À13.5 V, lex = 400 nm, I = 5.6 mW cmÀ2
)
(Fig. 3a). Obviously, photoinduced charge carriers were
generated in the molecular orbital levels through the efficient
absorption of light. The photoinduced electrons may be readily
trapped in the bulk and interface between the active and
insulating layers; then, photoinduced hole transports were
facilitated dominantly. The accumulated charges in the inter-
face regions through the gate bias (VG) and the excited charges
induced by the photo-irradiation both contributed to the rapid
increase of IDS in the active layer.
8 L. Jiang, W. Hu, Z. Wei, W. Xu and H. Meng, Adv. Mater., 2009,
21, 3649.
9 M. Mas-Torrent, P. Hadley, S. T. Bromley, X. Ribas, J. Tarres,
´
M. Mas, E. Molins, J. Veciana and C. Rovira, J. Am. Chem. Soc.,
2004, 126, 8546.
10 H. Zhang, Y. Wang, K. Shao, Y. Liu, S. Chen, W. Qiu, X. Sun,
T. Qi, Y. Ma, G. Yu, Z. Su and D. Zhu, Chem. Commun., 2006,
755.
11 M. Ashizawa, K. Yamada, A. Fukaya, R. Kato, K. Hara and
J. Takeya, Chem. Mater., 2008, 20, 4883.
12 Y. Wang, H. Wang, Y. Liu, C.-A. Di, Y. Sun, W. Wu, G. Yu,
D. Zhang and D. Zhu, J. Am. Chem. Soc., 2006, 128, 13058.
13 G. Venkataramana and S. Sankararaman, Eur. J. Org. Chem.,
2005, 4162.
14 N. M. Johnson and A. Chiang, Appl. Phys. Lett., 1984, 45, 1102.
15 Q. Tang, L. Li, Y. Song, Y. Liu, H. Li, W. Xu, W. Hu and D. Zhu,
Adv. Mater., 2007, 19, 2624.
16 M. Y. Cho, S. J. Kim, Y. D. Han, D. H. Park, K. H. Kim,
D. H. Choi and J. Joo, Adv. Funct. Mater., 2008, 18, 2905.
17 Y. Guo, C. Du, G. Yu, C.-A. De, S. Jiang, H. Xi, J. Zheng, S. Yan,
C. Yu, W. Hu and Y. Liu, Adv. Funct. Mater., 2010, 20, 1019.
As shown in Fig. 3, we estimated the photoresponsivity
(R) for the OPT devices, defined as DIDSphoto/Pinc, where
photo
light
dark
DIDS
intensity. The photoswitching (photocurrent/dark-current)
= IDS
À IDS
and Pinc is the incident light
light
ratio ((IDS
À IDSdark)/IDSdark) was denoted as P. The
average values of the photoresponsivity (R) of microribbon
OPTs were found to be 1980–2000 A WÀ1 (VG = À13.5 V,
I = 5.6 mW cmÀ2). It should be noted that the light intensity
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8907–8909 8909