This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 8907–8909 8907

Cite this: Chem. Commun., 2011, 47, 8907–8909

Highly sensitive phototransistor with crystalline microribbons from new

p-extended pyrene derivative via solution-phase self-assemblyw

Youn Sun Kim,aSuk Young Bae,

aKyung Hwan Kim,

aTae Wan Lee,

aJung A Hur,

a

Mai Ha Hoang,aMin Ju Cho,

aSung-Jin Kim,

bYoungmee Kim,

bMinsik Kim,

a

Kwangyeol Lee,aSuk Joong Lee*

aand 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 V�1 s�1 (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 cm�2).

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

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.

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 �4.75, �2.25 and 2.5 eV, respectively (Fig. 2S, ESIw),while experimental HOMO, LUMO and bandgap energy (Eg)

values for the film were �5.55, �3.61 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

aDepartment 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

bDepartment of Chemistry and NanoScience,Ewha Womans University, Seoul 120-750, Korea

w Electronic supplementary information (ESI) available: Completeexperimental details of syntheses, compound characterization,UV-Vis absorption and photoluminescence spectra, DFT calculation,single crystal analysis, fluorescence lifetime measurements and detaileddevice characterizations. CCDC 809193. For ESI and crystallographicdata in CIF or other electronic format see DOI: 10.1039/c1cc11589h

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8908 Chem. Commun., 2011, 47, 8907–8909 This journal is c The Royal Society of Chemistry 2011

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).

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 V�1 s�1 with on/off current ratio

of 4106 and threshold voltage of �10 V and low gate

leakage current (IGS) (Fig. 2c). The mobility ranged from

0.1–0.7 cm2 V�1 s�1. Fluctuation in mobility may be due to

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.

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).

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 8907–8909 8909

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 V�1 s�1 with on/off current ratio of 2.0 � 106

and threshold voltage (VTH) of �10 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

light with very low intensity ((IDSlight � IDS

dark)/IDSdark =

1.2 � 106, VG = �13.5 V, lex = 400 nm, I = 5.6 mW cm�2)

(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.

As shown in Fig. 3, we estimated the photoresponsivity

(R) for the OPT devices, defined as DIDSphoto/Pinc, where

DIDSphoto = IDS

light � IDSdark and Pinc is the incident light

intensity. The photoswitching (photocurrent/dark-current)

ratio ((IDSlight � IDS

dark)/IDSdark) was denoted as P. The

average values of the photoresponsivity (R) of microribbon

OPTs were found to be 1980–2000 A W�1 (VG = �13.5 V,

I = 5.6 mW cm�2). It should be noted that the light intensity

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 AW�1, I = 400 mW cm�2).14 Although the PLQY

of the thin film shows an identical response to that of the

microribbon, the R value (B420–430 AW�1) 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

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).

Notes and references

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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,

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Fig. 3 (a) Transfer curves of microribbon OPT in the dark (black)

and under monochromatic light irradiation (red) of 5.6 mW cm�2.

(b) Photoresponsivity (R) and photoswitching ratio (P) vs. VG for

the OPT (inset: fluorescence images of thin film and crystalline

microribbons) (lex = 365 nm)).

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