Photoresponse of a single poly(p-phenylene vinylene)-CdSe bulk-heterojunction submicron fiber

Article abstract of DOI:10.1039/b923418g

Bulk-heterojunction PPV (poly(p-phenylene vinylene))-CdSe submicron fibers have been prepared by introducing electrostatic interaction between thioglycolic acid capped CdSe nanocrystals and PPV precursor, subsequently with electrospinning and thermal conv

Full text of DOI:10.1039/b923418g

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Photoresponse of a single poly(p-phenylene vinylene)-CdSe
bulk-heterojunction submicron fiberw
Yi Xin,a Zonghao Huang,*a Zijiang Jiang,b Dongfeng Li,c Liang Peng,d
Jiali Zhaid and Dejun Wangd
Received (in Cambridge, UK) 9th November 2009, Accepted 4th January 2010
First published as an Advance Article on the web 23rd January 2010
DOI: 10.1039/b923418g
Bulk-heterojunction PPV (poly(p-phenylene vinylene))-CdSe
submicron fibers have been prepared by introducing electrostatic
interaction between thioglycolic acid capped CdSe nanocrystals
and PPV precursor, subsequently with electrospinning and
thermal conversion. The photoconductive device on a single
PPV-CdSe fiber showed notable photoresponse, good response
speed, wavelength-sensitivity, and reproducibility.
Herein, we report the fabrication of the conjugated polymer
PPV (poly(p-phenylene vinylene))-CdSe nanocrystals bulk-
heterojunction submicron fiber and the remarkable perfor-
mance of a photoconductive device based on a single PPV-CdSe
fiber. The organic/inorganic bulk-heterojunction fiber was
prepared as shown in Scheme 1 (details can be found in the
ESIw). CdSe nanocrystals synthesized in alkaline solutions,7
with thioglycolic acid (TGA) as stabilizer, were negatively
charged. The monomer 1 of PPV was absorbed onto the
negatively charged CdSe nanocrystals 2 through electrostatic
interaction to form the adduct 3. Then, NaOH solution was
added to polymerize the monomer in the presence of TGA-
covered inorganic nanocrystals to form PPV precursor-CdSe
composites 4. At the end of the polymerization, the reaction
mixture was neutralized with aqueous HCl. The obtained PPV
precursor-CdSe solution was purified by dialysis against
deionized water, and ethanol subsequently. The resultant
solution was an orange, viscous solution (inset of Fig. 1a).
The individual oriented PPV precursor-CdSe composite
fibers were prepared by a modified electrospinning method
(Fig. 1a). In a typical experiment, a droplet of PPV precursor
or precursor-CdSe solution was placed on an arrow-shaped
tip that acted as an electrospinning source.8 By applying a
potential of 2.9 kV, the polymer jet electrostatically extracted
from the droplet, dried in transit to a rotating aluminium disk
collector, which was placed 2 cm from the tip and rotated at a
relative velocity of about 200 cm sÀ1. The edge of the disk
created a converging electric field to control the deposition
Composite materials of conjugated polymers and inorganic
semiconductor nanocrystals are particularly interesting for
the development of novel optoelectronic devices because of
their combined optoelectronic properties and facile solution
processibility.1 For an efficient light harvesting and photocurrent
generation in an organic/inorganic optoelectronic device, it is
essential to control precisely the dispersion of the inorganic
material in a conjugated polymer host to obtain a so-called
bulk-heterojunction. Several kinds of inorganic nanometre-sized
semiconductors such as TiO2, ZnO, CdS, CdSe, and PdS have
been employed in combination with semiconductor polymers
for fabrication of the organic/inorganic hybrid film photo-
voltaic.2 Recently, one dimensional (1D) semiconductors
have emerged as promising and prospective next-generation
materials for electronic, optoelectronic, and photonic nano-
devices because of their distinctive geometries, novel electrical
and optical properties. Nanodevices based on 1D inorganic
semiconductors, organic semiconductors, and organic/
inorganic semiconductor p–n junctions such as field-effect tran-
sistors (FETs),3 sensors,4 photoswitches,5 and light-controlled
diode6 have thus far been demonstrated. However, how to
combine conjugated polymer and inorganic semiconductors
for producing the uniformity of the size and shape on the nano-
or submicro-scale with unique properties is still a challenge.
And, to our knowledge, there has been no report on the
optoelectronic devices on a single 1D organic/inoganic semi-
conductor bulk-heterojunction nano- or submicro-structure.
a College of Chemistry, Northeast Normal University,
Changchun 130024, P. R. China. E-mail: huangzh295@nenu.edu.cn,
xiny659@yahoo.com.cn; Fax: +86-431-85098768;
Tel: +86-431-85099206
b Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun 130022, P. R. China
c School of Chemistry and Life Science, Changchun University of
Technology, 130012, P. R. China
d College of Chemistry, Jilin University, Changchun 130023,
P. R. China
w Electronic supplementary information (ESI) available: Experimental
details of preparative procedure for PPV-CdSe fiber, solid-state MAS
13C NMR and absorption spectra characterization. See DOI: 10.1039/
b923418g
Scheme 1 Preparation of PPV-CdSe fiber. Reagents and conditions:
(i) tetrahydrothiophene, methanol, 50 1C; (ii) NaOH, N2, 0 1C;
(iii) HCl, dialysis, water, ethanol; (iv) electrospinning, vacuum, 60 1C,
1 h, 230 1C, 3 h.
ꢀc
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location of the fiber. The rotation of the disk helped to stretch
and orient the deposited fiber. This technique indicated the
possibility of precisely and directly locating a fiber on the
electrodes with designed orientation, which is favorable for
some practical applications such as electrical and optical
nanodevices. The diameters of the collected straight fibers
ranged from 200 to 600 nm, with uniformity over lengths up
to several centimetres, as shown in a typical field emission
scanning electron micrograph (FE-SEM) in Fig. 1b. To
examine the photocurrent of a single fiber, a patterned gold
electrode attached on the counter electrode was used to collect
the fiber. The inserts of Fig. 2 show the photograph of the
patterned gold electrode and the schematic illustration of the
photoconductive device. Four 250 nm-thick Au interdigitated
electrodes (IDEs) with 50 nm-thick Cr as an adhesion layer
were patterned on a substrate using photolithography and wet
chemical etching. The substrate was a heavily doped p-type Si
chip capped with a 500 nm oxide layer. An individual fiber was
electrospun onto the IDEs with an interelectrode spacing of
3 mm. After electrospinning, the device was heated at 60 1C in
vacuum for 1 h and at 230 1C for an additional 3 h for
conversion of the precursor to PPV and the removal of TGA
capped on the nanocrystal surfaces. Fig. 2 shows a typical
current–voltage (I–V) curve of the single PPV-CdSe composite
fiber. It shows that the I–V curve was nearly linear, indicating
good physical contact between the structure and the electrodes.
The composite fiber was directly electrospun onto the electrodes
subsequently with in situ thermal conversion, which ensured
the good electrical contact of the PPV-CdSe fiber with the
electrodes. It is worth mentioning that heat treatment can also
enhance the mobility of polymer chains bound to an inorganic
surface,9 and make an intimate contact between PPV and
CdSe nanocrystals.
Fig. 2 Typical I–V curve of a single PPV-CdSe bulk-heterojunction
fiber under light irradiation (l = 500 nm, 0.25 mW cmÀ2). Insets:
photograph of the patterned gold electrode and schematic of the single
PPV-CdSe bulk-heterojunction fiber photoconductive device.
polymer PPV)11 and the stretching of –OH in spectrum a. The
spectrum of PPV-CdSe composites (Fig. 3c) is consistent with
that of pristine PPV samples (Fig. 3d), and no characteristic
peaks of organic TGA capping agent on the CdSe nanocrystals
surfaces were observed. The absorption band near 963 cmÀ1
was due to C–H out-of-plane bending of trans-configurations of
the vinylene group. The band near 3022 cmÀ1 was assigned to
the trans-vinylene C–H stretching mode. The absorption bands
near 836 and 1516 cmÀ1 were attributed to p-phenylene C–H
out-of-plane bending and C–C ring stretching, respectively.
Solid-state MAS 13C NMR characterization (Fig. S1, ESIw)
also confirmed the successful formation of the PPV-CdSe
composites. There were only two peaks in the spectrum of
PPV-CdSe fibers. From the literature12 the peak at 136 ppm was
assigned to 1,4-phenyl carbons, and the peak at 128 ppm
contained unresolved 2,3-phenyl and vinylene carbons.
Fig. 4 shows a typical transmission electron micrograph
(TEM) of the PPV-CdSe composite fiber. The dark dots
correspond to the CdSe nanocrystals, which have a higher
mass density than PPV. The nanocrystals dispersed homo-
geneously in the composite fiber even with the high con-
centration of 60 wt%. It is known that the CdSe nanocrystals
have an intrinsic tendency to aggregate. This effective dis-
persion of CdSe nanocrystals in the fiber are attributed to the
combination of polymerization and electrospinning method.
The polymerization method eliminates the difficulties associated
with attempted blending of CdSe nanocrystals into polymer
materials such as PPV.13 The large specific surface area of the
electrospun fibers and fast evaporation of the solvents in the
The successful synthesis of PPV-CdSe composites and the
elimination of surface ligands were confirmed by IR analysis
of pristine PPV fibers, CdSe nanocrystals capped with TGA,
and the PPV precursor-CdSe fiber samples before and after
heat treatment (Fig. 3). In the spectrum of CdSe nanocrystals
capped with TGA (Fig. 3a), the peak at 1598 cmÀ1 is attributed
to COOÀ stretch.10 This result implies that the TGA was
chemically modified on the surface of CdSe nanocrystals and
the COOH was in a state of COOÀ, making it possible to
absorb the monomer of PPV on the nanocrystals by electro-
static interaction. The spectrum of PPV precursor-CdSe
composites (Fig. 3b) shows one broad band in the region
of 3200–3500 cmÀ1, which was referenced as the com-
bination water peak of PPV precursor (absent in the eliminated
Fig.
1 (a) Schematic illustration of the electrospinning setup.
Inset: the prepared PPV precursor-CdSe composite ethanol solution.
(b) FE-SEM image of the PPV-CdSe fiber.
Fig. 3 IR spectra of (a) CdSe nanocrystals capped with TGA,
(b) PPV precursor-CdSe, (c) PPV-CdSe, and (d) PPV fibers.
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Fig. 5 (a) Photoresponse characteristics (white light, 100 mW cmÀ2
,
at bias voltage 10 V) of the single PPV-CdSe composite fiber, which
can be switched on/off quickly by photoirradiation. Inset: FE-SEM
image of a part of the measured device. The fiber bridges one 3 mm
gold electrodes gap. (b) Photocurrent spectral response of PPV and
PPV-CdSe bulk-heterojunction fiber device.
Fig. 4 TEM image of the PPV-CdSe fiber.
to absorption directly into the CdSe nanocrystals, producing
free charge carriers above the band gap.
electrospinning process resulted in the uniform distribution of
the CdSe nanocrystals in the fibers.14 For the high concentration
of CdSe nanocrystals in the composite fiber, phase separation
between the nanocrystals is about 10 nm, which is within the
length limitation of exciton diffusion in conjugated polymer.15
Fig. 5a presents the photoresponse behaviors of the single
PPV-CdSe composite fiber under light irradiation. With light
on or off, the device based on the PPV-CdSe fiber could work
between low and high impedance states fast and reversibly
with a switching ratio close to 5 orders of magnitude. The
significant photoresponse of PPV-CdSe composite fiber may
be attributed to the following reasons. First, the rough
surfaces of the electrospun PPV-CdSe fibers lead to a high
surface area-to-volume ratio, and subsequently result in many
photogenerated excitons in the fiber. Second, the homo-
geneous dispersion of nanocrystals in PPV polymer (Fig. 4)
creates a large donor–acceptor interface area which could
improve charge separation efficiency. Charge transfer will take
place efficiently due to the intimate contact of the two com-
ponents in the fiber. Additionally, the nanoscale phase separation
in the PPV-CdSe composite fiber may improve the electron
transport through the nanocrystals. As a result, a high photo-
current was obtained in the bulk-heterojunction fiber device
under a given external bias. Fig. 5b shows the photocurrent
spectral response of the pristine PPV and the PPV-CdSe bulk-
heterojunction fiber device, respectively. The PPV fiber device
has only one response peak at 460 nm. The spectral response
of the composite fiber device shows two peaks at 460 and
500 nm. To clarify the origin of the wavelength-dependent
photocurrent, we measured the absorption spectra of the CdSe
nanocrystals, PPV, and PPV-CdSe fibers (Fig. S2, ESIw). The
optical absorption spectrum of pure PPV fibers has an absorption
peak at 460 nm. For CdSe nanocrystals the absorption starts
a slow rise at around 500 nm. The absorption spectrum of
PPV-CdSe composite fibers is the superposition of the two
types of materials. According to the spectral response of the
devices (Fig. 5b), it is obvious that both absorption in the
polymer and absorption in the nanocrystal contributed to
photocurrent of the PPV-CdSe fiber. Additionally, the inten-
sity of photocurrent response originated from CdSe nano-
crystals is much higher than that from PPV, which must be due
In summary, we designed and fabricated PPV-CdSe bulk-
heterojunction submicron fiber. In the composite fiber, the
homogeneously dispersed CdSe nanocrystals can be in intimate
contact with PPV chains, and separate on a nanometre scale.
The photoconductive device based on the fiber showed notable
photoresponse with a switch ratio close to five orders of
magnitude, good response speed, wavelength-sensitivity, and
reproducibility. It is believed that the novel concept and the
submicron fiber we developed here will open up tremendous
opportunities for the realization of a wide range of novel high-
performance nanostructured devices, including solar cells,
photodetectors, and sensors.
This work was supported by the National Natural Science
Foundation of China (Grant No. 20774017 and 20703020).
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2318 | Chem. Commun., 2010, 46, 2316–2318