Synthesis of monodispersed wurtzite structure CuInSe2 nanocrystals and their application in high-performance organic-inorganic hybrid photodetectors

Article abstract of DOI:10.1021/ja1057955

A new facile solution method for the synthesis of high-quality CuInSe ₂ nanocrystals with monodispersed size and uniform hexagonal shape was developed. A high-performance hybrid photodetector based on a hybrid film of CuInSe₂ nanocrystals and poly(3-hexylthiophene) was constructed. The device showed distinct ON and OFF states with a ratio of >100 in photocurrents responding to outside illumination. The high sensitivity and stability of the hybrid device revealed a broad prospect for use of the hybrid material in light detection and signal magnification for the development of large-area, low-cost, lightweight, and foldable products.

Full text of DOI:10.1021/ja1057955

Published on Web 08/16/2010
Synthesis of Monodispersed Wurtzite Structure CuInSe2 Nanocrystals and Their
Application in High-Performance Organic-Inorganic Hybrid Photodetectors
Jian-Jun Wang, Yong-Qing Wang, Fei-Fei Cao, Yu-Guo Guo,* and Li-Jun Wan*
Key Laboratory of Molecular Nanostructure and Nanotechnology and Beijing National Laboratory for Molecular
Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Received June 30, 2010; E-mail: ygguo@iccas.ac.cn; wanlijun@iccas.ac.cn
be photovoltaically active.7,22,23 Herein, we used the hybridization of
inorganic CuInSe2 (CISe) NCs and organic poly(3-hexylthiophene)
(P3HT) as an example to demonstrate the construction of multiple
photodetectors with high performance. P3HT is a classical π-electron-
Abstract: A new facile solution method for the synthesis of high-
quality CuInSe2 nanocrystals with monodispersed size and
uniform hexagonal shape was developed. A high-performance
hybrid photodetector based on a hybrid film of CuInSe2 nano-
crystals and poly(3-hexylthiophene) was constructed. The device
showed distinct “ON” and “OFF” states with a ratio of >100 in
photocurrents responding to outside illumination. The high sen-
sitivity and stability of the hybrid device revealed a broad prospect
conjugated polymer with high electrical conductivity and strong
absorption in the visible range.24 CISe was selected not only because
of its high absorption coefficient and appropriate band gap but also
because of its high radiation stability.9,10
CISe colloidal NCs have attracted much attention because they
are convenient for further processing via spin-casting, dip-coating,
and printing, and more importantly, the corresponding devices
for use of the hybrid material in light detection and signal
magnification for the development of large-area, low-cost, light-
weight, and foldable products.
exhibit high performance due to the quantum confinement
effect.22,25,26 In recent years, great progress has been made in the
synthesis of colloidal CISe NCs.9,10,25,27-34 Many synthetic routes,
including solvothermal,29 thermolysis,25 microwave-assisted,27 and
Photoelectric devices based on organic molecules and polymers
hot-injection methods, have been reported.9,10,32 The above methods
have attracted considerable attention recently because of their unique
properties.1-7 First of all, the fast development of solution-based
need either special apparatus or unique precursors with tedious
synthetic methods for organic materials has enabled their large-scale
production at low cost. In addition, organic chemistry offers ample
techniques for tuning their functions by modifying the structures of
the molecules or the monomers of the polymers.1,2,4,8 In particular,
the exceptional mechanical flexibility of organic materials makes them
perfect candidates for use in lightweight and foldable devices1,2,4,6,7
However, despite the rapid advancement in organic photoelectrics, a
well-recognized problem regarding their practical application lies in
the extremely low carrier mobility and the peculiarly limited light
absorption range, which has greatly deteriorated the performance of
organic material-based photoelectric devices.
complicated synthesis procedures, and the synthesized NCs exhibit
agglomeration or broad size distributions. The poor solubility in
popular solvents of Se powder, which is often used either as the
Se source or to stop the reaction between Cu and In sources, makes
it especially difficult to prepare high-quality CISe NCs with a pure
phase, let alone uniform CISe NCs with shape control. Conse-
quently, it remains a big challenge to develop new routes for the
synthesis of high-quality CISe NCs, especially solution-phase
preparations. Here we report the synthesis of nearly monodispersed
wurtzite structure CISe NCs with hexagonal shape and their high
performance in a hybrid photodetector together with P3HT.
In comparison with their organic counterparts, inorganic nanocrystal-
based photoelectric devices possess high device efficiencies, benefiting
from their superior intrinsic carrier mobilities and broadband
absorption.9-15 Meanwhile the properties of semiconductor nano-
crystals (NCs) have proved to be strongly size-dependent. Their
emission and absorption can be easily engineered by their particle size
as a result of the quantum confinement effect.16,17 Therefore, various
high-performance electronic and optical nanodevices have been
demonstrated, including memories,18 photodetectors,12,14 field-effect
transistors,19 and solar cells.9-11,20 On the other hand, because of the
large surface-to-volume ratio, unpassivated orbitals at the NC surface
and defects introduced by surface modifications detrimentally affect
the photoelectric properties of NCs and reduce the efficiencies of the
photoelectric devices.17 Nonetheless, production of photoelectric
devices with the advantages of both organic materials and inorganic
nanomaterials is promising.
Figure 1. Experimental and simulated XRD patterns of CISe NCs with a
wurtzite structure.
In our experiments, a Cu-oleate complex was first prepared from
the reaction between CuCl2 and sodium oleate, both of which are
environmentally friendly and inexpensive. Diphenyl diselenide was
selected as the selenide source to react with the Cu-oleate complex
and InCl3 [see the Supporting Information (SI)]. The synthesized
NCs were capped with organic ligands, allowing them to disperse
freely in organic solvents such as toluene and hexane. Moreover,
the organic ligands initiate attractive forces with polymer chains,
contributing to the formation of hybrid homogeneous films.22
Hybrid organic-inorganic devices are a logical choice for com-
bining the unique properties of NCs with the easy film-forming
properties of organic polymers.21-23 As the state of the art of organic
chemistry has developed, many semiconducting polymers have been
synthesized. This provides an avenue for constructing a new generation
of hybrid organic-inorganic devices in which both components can
9
12218 J. AM. CHEM. SOC. 2010, 132, 12218–12221
10.1021/ja1057955  2010 American Chemical Society

C O M M U N I C A T I O N S
The structure of the as-synthesized NCs was characterized by
X-ray diffraction (XRD), as shown in Figure 1. It was found that
no existing patterns in the standard JCPDS database can match well
with the one we acquired (Table S1 in the SI). Preliminary
simulation revealed that such NCs hold a wurtzite phase (Figure
S1 in the SI),35 which is totally different from the well-characterized
chalcopyrite type in the JCPDS database. As shown in Figure 1,
the experimental pattern of the NCs matches well with the simulated
one of wurtzite structure (a ) 4.083 Å, c ) 6.727 Å). It is also
noteworthy that a strong (002) diffraction peak was observed for
the NCs, indicating that the as-prepared wurtzite structure CISe
(wz-CISe) NCs have an (002)-textured structure.
Figure 2a,b shows typical transmission electron microscopy
(TEM) images of the CISe NCs. The large-area TEM images
indicate that the as-synthesized NCs are nearly monodispersed with
an average diameter of ∼21.3 nm and that they easily self-assemble
into 2D arrays because of their uniform hexagonal shape. Figure
2c shows a high-resolution TEM (HRTEM) image of an individual
nanocrystal. The lattice fringes show that the NCs are well-
crystallized, and the observed d spacings correspond to the (100)
plane of CISe. The selected-area electron diffraction (SAED) pattern
(Figure 2d) is also well-consistent with the simulated wurtzite
structure of CISe. It should be pointed out that the SAED pattern
contains not diffraction rings but instead diffraction arcs with
hexagonal symmetry, indicating that the orientation of the atomic
lattice of an individual nanocrystal self-assembled into a 2D array
is not random but preferentially lies along the [001] direction.15 In
order to understand the formation process of the monodispersed
products, synthesis experiments in the absence of InCl3 were also
performed. As can be seen from Figure S2, monodispersed CuSe
nanocrystals were obtained. This result indicates that a possible
mechanism for the monodispersion could be that diphenyl diselenide
first reacts with the Cu-oleate complex to generate uniform seed
crystals, which act as templates for the growth of monodispered
CISe NCs after addition of InCl3.
Figure 3. (a) Structure of P3HT. (b) Schematic energy-level diagram for CISe
NCs and P3HT showing the charge transfer of electrons to the CISe NCs and
holes to P3HT. (c) UV-vis spectra of the P3HT film (green), CISe NC film
(blue), and P3HT:CISe hybrid film (∼2:1 weight ratio) (red).
eV in the sample, which is consistent with that of monovalent
copper. The result indicates that Cu2+ was reduced to Cu+ by
oleylamine.36 UV-vis absorption spectroscopy was utilized to
estimate the band-gap energy of the wz-CISe NCs (Figure S5); the
effective band gap was found to be 1.03 eV, which is in good
agreement with the reported value of 1.04 eV for bulk CISe.10,28
Figure 3c shows UV-vis spectra of a P3HT film (green), a CISe
NC film (blue), and a P3HT:CISe NC hybrid film (∼2:1 weight
ratio) (red), which indicate that the inlay of CISe NCs into the P3HT
film significantly broadens the absorption spectrum. A schematic
illustration of a device based on the hybrid material is shown in
Figure 4a. The gap width and electrode length were 35 and 125
µm, respectively. The device was fabricated by dropping 1 µL of
hybrid solution onto precleaned electrodes and drying in air (see
the SI). As shown in Figure S6, the hybrid film was highly smooth
and homogeneous. The high absorption efficiency and the large
interface between CISe NCs and P3HT are favorable for light
absorption and exciton dissociation.
Figure 2. (a, b) TEM images of as-synthesized CISe NCs. (c) HRTEM
image of a single CISe nanocrystal. (d) SAED pattern of as-synthesized
CISe NCs.
To further identify the as-synthesized CISe NCs, the chemical
composition was measured by energy-dispersive X-ray spectroscopy
(EDS) (Figure S3), which confirmed that the NCs had a Cu/In/Se
stoichiometric ratio of ∼1:1:2. X-ray photoelectron spectroscopy
(XPS) analysis showed that all three elements were in their expected
oxidation states (Figure S4) and that the proportion of carbon was
∼66.83 atom %. The large proportion of carbon indicates the
presence of long alkyl groups (i.e., oleylamine) on the surface of
the NCs in addition to the generally adsorbed gaseous molecules
during XPS analysis. The binding energy of Cu 2p3/2 was 931.8
Figure 4. (a) Schematic illustration of the hybrid device. (b) On/off switching
of the hybrid device at an incident light density of 7.63 mW cm-2 and a bias
voltage of 0.4 V. (c) Dark current and photocurrents at different incident light
densities. (d) Photocurrent measured as a function of incident light density at
a bias voltage of 10 V. (e) Typical I-V curves for an CISe NC film in the
dark and at an incident light density of 7.63 mW cm-2. (f) On/off switching of
the device made from a pure P3HT film at an incident light density of 7.63
mW cm-2 and a bias voltage of 0.4 V.
9
J. AM. CHEM. SOC. VOL. 132, NO. 35, 2010 12219

C O M M U N I C A T I O N S
The hybrid device exhibits excellent photoresponse character-
istics, as shown in Figure 4b. An iodine-tungsten lamp was used
as a white-light source. With the light irradiation on and off, the
current in the devices showed two distinct states, a “low” current
in the dark and a “high” current under illumination. The switching
in these two states was very fast and reversible, allowing the device
to act as a high-quality photosensitive switch. In the dark, the current
was only 0.15 pA. However, at an incident light density of 7.63
mW cm-2 and a bias voltage of 0.4 V, the current could approach
17 pA, giving an on/off switching ratio of >100. The high
photosensitivity of the hybrid devices was further confirmed by
photocurrent measurements on the devices at different incident light
densities. When the intensity of the incident light was changed,
the photocurrent of the device remarkably changed accordingly
(Figure 4c), which can be attributed to the change in the photon
density of the incident light at different light densities. It is known
that for hybrid organic-inorganic devices (e.g., MEH-PPV-CdSe
NCs), light can be absorbed through the whole thickness of the
device and that both types of charge carrier run within the device.37
As shown in Figure 4d, the current of the present hybrid photo-
detector exhibited a strong dependence on light intensity, in
agreement with the previous report on hybrid devices,37 and
demonstrated a power dependence of ∼1.34 (i.e., I ∼ P1.34),
indicating superior photocurrent capability of the hybrid material.
These results prove the promising potential of the hybrid device as
a photoswitch and a highly photosensitive detector. It is worth
noting that the applied bias voltage influenced the on/off ratio of
the devices, which is a result of the applied bias voltage dependence
of the exciton dissociation and the background current of the
devices.
The results indicate that both the sufficient light absorption of the
hybrid film and the efficient charge dissociation at the interface of
the hybrid material are critical for the construction of high-
performance photodetectors. It should be noted that the hybrid
photodectector also showed an outstanding stability. No obvious
degradation was observed during extended hundreds of cycles. The
high photoresponse performance together with its high stability
makes the hybrid material a perfect candidate for light detection
and signal magnification with prospects for applications in low-
cost, lightweight, and foldable products.
In summary, a new facile solution method for the synthesis of
high-quality CuInSe2 NCs with monodispersed size, uniform
hexagonal shape, and an optical band gap of 1.03 eV was developed.
XRD, TEM, EDS, and XPS measurements confirmed that the NCs
were pure wurtzite CuInSe2. A high-performance hybrid photode-
tectors based on a P3HT:CISe hybrid film were constructed. The
device showed distinct “ON” and “OFF” states with a photocurrent
ratio of >100 in response to outside illumination. The high
sensitivity and stability of the as-constructed device make the hybrid
material a promising candidate for applications in light detection
and signal magnification for the development of large-area, low-
cost, lightweight, and foldable products.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (Grants 20821003 and
50730005), the National Key Project on Basic Research (Grants
2006CB806100 and 2009CB930400), and the Chinese Academy
of Sciences.
Supporting Information Available: Synthesis of the wurtzite CISe
NCs; corresponding EDS, XPS, UV-vis absorption spectra; and SEM
images of the wz-CISe and the P3HT:CISe hybrid film. This material
is available free of charge via the Internet at http://pubs.acs.org.
The interface of the P3HT:CISe hybrid film also plays a key
role in the charge dissociation and transportation. Exciton dissocia-
tion is well-known to occur efficiently at the interface of two
semiconductors mixed together in a blend film, such as a conjugated
polymer and a fullerene derivative.7,22,23 The photoexcited electrons
can be accepted by the material with the higher electron affinity,
while the hole can be caught by the material with the relatively
lower ionization potential.7,22,23 In our system, the CISe nanocrystal
has a high electron affinity and could act as the photoelectron
acceptor. Meanwhile, the P3HT is perfect for acting as a hole
acceptor as well as an electron donor upon photoexcitation (Figure
3b). In the hybrid film, the CISe NCs are uniformly dispersed in
the P3HT matrix, forming a three-dimensional interconnected
network, which leads to a large interface area for charge separation.
On the other hand, the different carriers have their own specific
pathways to reach their corresponding electrodes; this effectively
alleviates charge recombination, allowing the device to achieve a
long-lived charge separation and high transportation.22 In contrast,
devices based on a film of pure CISe NCs showed no obvious
photoresponse (Figure 4e) but had high resistivity due to the
adsorbed insulating surfactants on their surface. The resistivity can
be considerably reduced by exchanging these surfactants with short
alkyl molecules (e.g., ethanethiol) or removing them (Figure
S7).37,38 In the case of the device made from pure P3HT, the
photocurrent after illumination was relatively low and the on/off
ratio was <2 (Figure 4f), consistent with previously reported results.7
The feature of irreversible and low on/off switching demonstrates
inefficient charge separation in the device without CISe NCs as
the electron acceptors.39 Notably, the OFF current in the hybrid
device (∼0.15 pA) at a bias voltage of 0.4 V was lower than that
of pure CISe (∼0.4 nA) or P3HT (∼0.35 nA) at the same bias,
which could be attributed to the difficult charge transportation
through the interfaces of P3HT and CISe without light illumination.
References
(1) Baldo, M.; Deutsch, M.; Burrows, P.; Gossenberger, H.; Gerstenberg, M.;
Ban, V.; Forrest, S. AdV. Mater. 1998, 10, 1505.
(2) Facchetti, A. Mater. Today 2007, 10, 28.
(3) Jiang, L.; Gao, J.; Wang, E.; Li, H.; Wang, Z.; Hu, W.; Jiang, L. AdV.
Mater. 2008, 20, 2735.
(4) Li, R.; Hu, W.; Liu, Y.; Zhu, D. Acc. Chem. Res. 2010, 43, 529.
(5) Tang, Q.; Li, L.; Song, Y.; Liu, Y.; Li, H.; Xu, W.; Hu, W.; Zhu, D. AdV.
Mater. 2007, 19, 2624.
(6) Tong, L.; Li, C.; Chen, F.; Bai, H.; Zhao, L.; Shi, G. J. Phys. Chem. C
2009, 113, 7411.
(7) Zhu, H.; Li, T.; Zhang, Y.; Dong, H.; Song, J.; Zhao, H.; Wei, Z.; Xu, W.;
Hu, W.; Bo, Z. AdV. Mater. 2010, 22, 1645.
(8) Yan, Z.; Li, G. T.; Mu, L.; Tao, S. Y. J. Mater. Chem. 2006, 16, 1717.
(9) Brian, M.; Dunn, G.; Brian, A.; Korgel, A. J. Am. Chem. Soc. 2008, 130,
16770.
(10) Guo, Q.; Kim, S.; Kar, M.; Shafarman, W.; Birkmire, R.; Stach, E.; Agrawal,
R.; Hillhouse, H. Nano Lett. 2008, 8, 2982.
(11) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310,
462.
(12) Konstantatos, G.; Clifford, J.; Levina, L.; Sargent, E. Nat. Photonics 2007,
1, 531.
(13) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem,
E.; Levina, L.; Sargent, E. H. Nature 2006, 442, 180.
(14) Wang, J.-J.; Cao, F.-F.; Jiang, L.; Guo, Y.-G.; Hu, W.-P.; Wan, L.-J. J. Am.
Chem. Soc. 2009, 131, 15602.
(15) Zhuang, Z.; Peng, Q.; Zhang, B.; Li, Y. J. Am. Chem. Soc. 2008, 130,
10482.
(16) Deng, Z.; Yan, H.; Liu, Y. J. Am. Chem. Soc. 2009, 131, 17744.
(17) Smith, A. M.; Nie, S. Acc. Chem. Res. 2009, 43, 190.
(18) Meister, S.; Peng, H.; McIlwrath, K.; Jarausch, K.; Zhang, X. F.; Cui, Y.
Nano Lett. 2006, 6, 1514.
(19) Peng, H.; Xie, C.; Schoen, D. T.; Cui, Y. Nano Lett. 2008, 8, 1511.
(20) Wu, Y.; Wadia, C.; Ma, W.; Sadtler, B.; Alivisatos, A. P. Nano Lett. 2008,
8, 2551.
(21) Arici, E.; Hoppe, H.; Schaffler, F.; Meissner, D.; Malik, M. A.; Sariciftci,
N. S. Thin Solid Films 2004, 451-452, 612.
(22) Gu¨nes, S.; Sariciftci, N. S. Inorg. Chim. Acta 2008, 361, 581.
(23) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425.
(24) Holdcroft, S. Macromolecules 1991, 24, 4834.
(25) Castro, S.; Bailey, S.; Raffaelle, R.; Banger, K.; Hepp, A. Chem. Mater.
2003, 15, 3142.
9
12220 J. AM. CHEM. SOC. VOL. 132, NO. 35, 2010

C O M M U N I C A T I O N S
(26) Green, M.; Emery, K.; King, D.; Hishikawa, Y.; Warta, W. Prog.
PhotoVoltaics 2007, 15, 35.
(34) Zhong, H.; Li, Y.; Ye, M.; Zhu, Z.; Zhou, Y.; Yang, C. Nanotechnology
2007, 18, 025602.
(27) Grisaru, H.; Palchik, O.; Gedanken, A.; Palchik, V.; Slifkin, M.; Weiss,
A. Inorg. Chem. 2003, 42, 7148.
(35) Pan, D.; An, L.; Sun, Z.; Hou, W.; Yang, Y.; Yang, Z.; Lu, Y. J. Am.
Chem. Soc. 2008, 130, 5620.
(28) Koo, B.; Patel, R.; Korgel, B. J. Am. Chem. Soc. 2009, 131, 3134.
(29) Li, B.; Xie, Y.; Huang, J.; Qian, Y. AdV. Mater. 1999, 11, 1456.
(30) Norako, M. E.; Brutchey, R. L. Chem. Mater. 2010, 22, 1613.
(31) Nose, K.; Omata, T.; Otsuka-Yao-Matsuo, S. J. Phys. Chem. C 2009, 113,
3455.
(32) Tang, J.; Hinds, S.; Kelley, S. O.; Sargent, E. H. Chem. Mater. 2008, 20,
6906.
(33) Xu, J.; Lee, C.-S.; Tang, Y.-B.; Chen, X.; Chen, Z.-H.; Zhang, W.-J.; Lee,
S.-T.; Zhang, W.; Yang, Z. ACS Nano 2010, 4, 1845.
(36) Tang, J.; Konstantatos, G.; Hinds, S.; Myrskog, S.; Pattantyus-Abraham,
A. G.; Clifford, J.; Sargent, E. H. ACS Nano 2009, 3, 331.
(37) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. ReV. B 1996, 54, 17628.
(38) Sun, B.; Marx, E.; Greenham, N. C. Nano Lett. 2003, 3, 961.
(39) Amos, F. F.; Morin, S. A.; Streifer, J. A.; Hamers, R. J.; Jin, S. J. Am.
Chem. Soc. 2007, 129, 14296.
JA1057955
9
J. AM. CHEM. SOC. VOL. 132, NO. 35, 2010 12221