CdSe-nanocrystal nanocomposites

Materials Letters 61 (2007) 2178 – 2181 www.elsevier.com/locate/matlet

Investigation on photoconductive properties of MEH-PPV/CdSe-nanocrystal nanocomposites Ai-wei Tang, Feng Teng ⁎, Hui Jin, Yin-hao Gao, Yan-bing Hou, Chun-jun Liang, Yong-sheng Wang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing JiaoTong University, Beijing, 100044, China Received 25 June 2006; accepted 17 August 2006 Available online 7 September 2006

Abstract The photoconductive properties of photodiodes based on nanocomposites of water-soluble CdSe nanocrystals and poly[2-methoxy-5-(2ethylhexyloxy-p-phenylenevinylen)] (MEH-PPV) were investigated. The photoluminescence intensity of the nanocomposites decreased with the increasing weight ratios of CdSe nanocrystals to MEH-PPV. By comparing the photocurrent action spectra of the nanocomposite device and the pristine MEH-PPV device, it was found that the nanocomposite device exhibited a wider photocurrent action range. In addition, the nanocomposite device displayed an obvious photovoltaic effect upon illumination. The process of exciton dissociation and charge transfer between the interface of CdSe nanocrystals and MEH-PPV was discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: CdSe; Photoconductivity; MEH-PPV; Nanocomposite

1. Introduction Photoconductivity has been proven to be an important method for providing fundamental information regarding the nature of the photo-excitations [1]. The photoconductive properties of nanocomposites of conjugated-polymer and inorganic nanocrystals are currently the subject of intensive investigation, not only because of fundamental interests in the nature of the electronic excitations in these “one-dimensional” polymer semiconductor materials, but also because inorganic semiconductor nanocrystals have become candidates in a wide range of optical and electronic devices. In recent years, with the development of synthesis and characteristics of semiconductor nanocrystals (such as CdS, CdSe, ZnS, etc), conjugated-polymer/nanocrystal nanocomposites have been investigated extensively for their technological applications in optoelectronics and photovoltaic devices [2–9].

⁎ Corresponding author. Tel.: +86 10 51688605; fax: +86 10 51683933. E-mail address: [email protected] (F. Teng). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.042

A good photoconductive device requires not only efficient charge separation, but also efficient transport of charge carriers to the electrodes. The blends of conjugated-polymer and inorganic nanocrystals have exhibited excellent photoconductive properties because the inorganic semiconductor nanocrystals can efficiently dissociate the photo-generated excitons and have high electron mobility. As a new class of photoconductive materials, composites consisting of conjugated-polymer and inorganic semiconductor nanocrystals, such as CdS nanocrystals and the polymer poly-(N-vinylcarbazole) (PVK), have previously been investigated [3,7,8]. In these previous reports, CdSe nanocrystals were synthesized in organic systems and the surface of nanocrystals may be coated with a surfactant molecule trioctylphosphineoxide (TOPO). However, the presence of TOPO surface layer on the nanocrystals can suppress the charge transfer process [3]. In this paper, CdSe nanocrystals were synthesized in an aqueous solution with 2-mercapto-acetic acid as the stabilizer, which was operated easily and economically. Then the phase-transfer method was used to transfer CdSe nanocrystals into a chloroform solution, which combined with the conjugated-polymer MEH-PPV to fabricate a composite

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a clean ITO-coated glass substrate. The thickness of the composite film is about 120 nm. LiF and Al electrodes were deposited onto the nanocomposite layer in sequence by vacuum thermal evaporation as cathode. For the sake of comparison, the pristine MEH-PPV device was fabricated with the same processes as those for the composite device. 2.3. Measurements

Fig. 1. TEM image of CdSe nanocrystals.

device. In such a device, CdSe nanocrystals can be used as an electrons acceptor and MEH-PPV used as a hole acceptor and a hole-transporting material, respectively. The photoluminescence (PL) spectra of the nanocomposites and the comparison of the photocurrent action spectra between the nanocomposite device and the pristine MEH-PPV device were investigated. The voltage–current characteristics showed that the photocurrent of the nanocomposite device was enhanced significantly compared with the dark current, and an obvious photovoltaic characteristic was observed upon illumination. The photocurrent action spectrum of the composite device demonstrated that the photocurrent response extended further to the longer wavelength region than the pristine MEH-PPV device. 2. Experiment 2.1. Synthesis of CdSe nanocrystals CdSe nanocrystals were synthesized based on the previous reference [10]. In brief, 2-mercapto-acetic acid was added into the solution of Cd(CH2COO)2·2H2O, which was adjusted to pH = 10–11 by using the solution of 2 mol/l sodium hydroxide, followed by adding a fresh solution of Na2SeSO3 with vigorous stirring under nitrogen atmosphere. Then the solution was heated to 100 °C for 2 h, and CdSe nanocrystals were formed. The crude products solution was concentrated by a rotatory evaporator and the rest of the dense solution was separated by centrifugation. The precipitation was purified and washed three times with 2propanol and ethyl ether, and the products were dried in vacuum at 70 °C for several hours.

The photocurrent (Iph) action spectra and current–voltage curves were measured by using a Keithley 2410 source-measure unit both in the dark and under illumination. Monochromatic illumination was produced by the output of a Xe lamp dispersed by a monochromator in SPEX Fluorolog-3 spectrophotometer. Positive voltage was defined as ITO electrode biased positively and the samples were illuminated through the ITO electrode. The UV–Vis absorption spectra were determined by a Shimadzu-UV 3101 spectrophotometer and the PL spectra were measured by the SPEX Fluorolg-3 spectrophotometer. The transmission electron microscopy (TEM) images were taken on a Philips EM-400T transmission electron microscope. An X-ray photoelectron spectroscopic measurement (XPS) was performed on a VG MK-II spectrometer with an Al Kα monochromatized X-ray source. All the measurements were taken at room temperature. 3. Results and discussion Fig. 1 displays a typical transmission electron microscopy (TEM) image of the synthesized CdSe nanocrystals. The particles are nearly spherical and slightly agglomerated with an average size less than 10 nm. Fig. 2 shows the PL spectra of MEH-PPV/CdSe nanocomposites with different nanocrystal concentrations of 50 wt.%, 67 wt.% and 80 wt.% along with those of the pristine MEH-PPV and CdSe nanocrystals. In order to avoid thickness and geometry effects, the PL intensity of all the samples were measured in a chloroform solution in quadrate quartz vessels. The nanocomposites and pristine MEH-PPV solutions excited by the light at 467 nm show the PL peak located at

2.2. Fabrication of the nanocomposite device The CdSe nanocrystals were redissolved in distilled water and a surfactant cetyltrimethyl ammonium bromide (CTAB) was used to precipitate the nanocrystals. And then the precipitation was re-dispersed in chloroform after being dried and combined with the solution of MEH-PPV with the nanocrystal concentrations of 50 wt.%, 67 wt.% and 80 wt.%. The blends of MEHPPVand CdSe nanocrystals in chloroform were spin-coated onto

Fig. 2. Photoluminescence spectra of nanocomposites with different weight concentrations of CdSe nanocrystals, pristine MEH-PPV and CdSe nanocrystals in chloroform.

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Fig. 3. Absorption spectra of the pristine MEH-PPV film, nanocomposite film containing 80 wt.% CdSe and CdSe nanocrystals in chloroform.

about 568 nm, and CdSe nanocrystals excited by the light at 420 nm show the peak at 543 nm. It can be seen that the PL intensity of the nanocomposites decreases with the increasing weight concentrations of CdSe nanocrystals in the nanocomposites, which suggests the occurrence of PL quenching in the nanocomposites [3,11]. This can be explained as follows: CdSe nanocrystals were incorporated into the nanocomposites and acted as electron accepting species. When excitons were produced in the polymer, the photo-generated excitons in MEH-PPV might migrate to the surface of CdSe nanocrystals because of the LUMO of CdSe nanocrystals lower than that of MEH-PPV, and the charge transfer and separation can take place in the nanocomposites [11,12]. The PL spectra of the nanocomposites are similar to that of the pristine MEH-PPV, and no emission from CdSe nanocrystals is observed, which indicates that the luminescence predominantly results from excitons which radiatively recombine in the polymer due to the much lower PL intensity of CdSe nanocrystals compared to that of MEH-PPV [11]. Fig. 3 shows the absorption spectra of CdSe nanocrystals in chloroform, pristine MEH-PPV film and the nanocomposite film containing 80 wt.% CdSe nanocrystals. The absorption peak of CdSe nanocrystals locates at 450 nm, while the absorption of MEH-PPV

Fig. 4. Photocurrent action spectra of a pristine MEH-PPV device and a nanocomposite device containing 80 wt.% CdSe nanocrystals.

Fig. 5. Current–voltage curve for a nanocomposite device containing 80 wt.% CdSe nanocrystals in the dark and under illumination.

extends to 510 nm. The absorption spectrum of the nanocomposite film is observed to be similar to that of MEH-PPV which only shifts to the longer wavelength region. The photocurrent action spectra of the pristine MEH-PPV device and the nanocomposite device are shown in Fig. 4. It is observed that the photocurrent action spectrum of the nanocomposite device shows a response in the range of 420–480 nm, but there is no corresponding absorption for the polymer. This response corresponds to the absorption of CdSe nanocrystals. It can be also seen that there is another response in the range of 510–680 nm in the photocurrent action spectrum, which is in consistent with that of the pristine MEH-PPV device. Therefore, this response can be attributed to the absorption of the polymer. As shown in the photocurrent action spectrum of the pristine MEH-PPV device, however, there is only one response region from 500 nm to 600 nm. The introduction of CdSe into the nanocomposite device widens the photocurrent action region, which improves the matching of the photocurrent action spectrum and the solar radiation. This is because in the composite device the photons are firstly absorbed by the nanocrystals, followed by the holes transfer to the polymer [3,13]. It is also observed in the photocurrent action spectrum of the pristine MEH-PPV device, the response occurs at the longer wavelength than the absorption of MEH-PPV, which has been reported in the previous reference [14]. Due to the poor electron transportation in the polymer, only those electrons generated close to

Fig. 6. Energy level diagram for the nanocomposite device.

A. Tang et al. / Materials Letters 61 (2007) 2178–2181

the aluminum electrode can contribute to the photocurrent by reaching the electrode without recombination [3]. The current–voltage curves of the nanocomposite device containing 80 wt.% CdSe nanocrystals in the dark and under monochromatic illumination at 500 nm are shown in Fig. 5, which are plotted on a logarithmic scale. It can be seen that there is a significant increase in the photocurrent in comparison with the dark current and a clear photovoltaic effect for the composite device upon illumination. This indicates that an efficient charge separation takes place at the polymer/ nanocrystal interfaces [11]. According to the current–voltage characteristics of the composite device in the dark and under illumination, the short-circuit current density (Isc), open-circuit voltage (Voc) and fill factor (FF) [defined as (IV)max / VocIsc, where (IV)max is the area of the largest rectangle under the current–voltage curve between 0 V and Voc] are 2.6 × 10− 6 A/cm2, 0.58 V and 0.28, respectively. Based on the above phenomena, it is important to investigate the mechanism for the photoconductivity of the nanocomposite device. The schematic energy level diagram for the composite device: ITO/MEHPPV/CdSe/LiF/Al is shown in Fig. 6. The work function of ITO and LiF/Al electrodes, and the electron affinities and ionization energies of MEH-PPV and CdSe nanocrystals are taken from the previous reports [3,15,16]. Because the electron affinity of CdSe nanocrystals is larger than that of MEH-PPV, the electron transportation of CdSe is better than that of MEH-PPVand the hole transportation of MEH-PPV is better than that of CdSe. When photons are absorbed by the composite device, the photo-generated holes in the CdSe nanocrystals may migrate to the MEH-PPV molecules and the photo-excited electrons in MEH-PPV can transfer to the surface of the CdSe nanocrystals. As a result, charge separation occurs between the interface of CdSe nanocrystals and MEH-PPV. The electrons therefore assemble at the bottom of the conduction band of CdSe nanocrystals, and the holes assemble at the top of the valence band of MEH-PPV. It can be induced that such interfacial charge transfer and separation process can reduce the rate of the exciton recombination because the process of charge transfer is much faster than recombination, thus it can increase the photoconductivity.

4. Conclusion In summary, we have introduced water-soluble CdSe nanocrystals into the photoconductive device combining with the polymer MEH-PPV. The photoconductive properties of the nanocomposite device were presented and discussed. The results indicated that the nanocomposite device displayed a clear photo-

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voltaic effect upon illumination, and the photocurrent was enhanced in comparison with the dark current. The photocurrent action region of the nanocomposite device was wider than that of the pristine MEH-PPV device, which indicated that the charge transfer and separation occurred not only in the polymer but also in the nanocrystals. The energy level diagram for the nanocomposite device gave the illustration of the mechanisms of the charge transfer and separation in the nanocomposites. Acknowledgements This work was supported by the National Key Project of Basic Research (973 Project, No. 2003CB314707), the NSFC programs (Nos. 90301004, 10434030 and 90401006) and the Beijing NOVA program (No. 2004B10). References [1] N.S. Sariciftci, Primary Photoexcitations in Conjugated Polymers: Molecular Exciton Versus Semiconductor Band Model, World Scientific, London, 1997, p. 37. [2] J.G. Winiarz, L.M. Zhang, M. Lal, C.S. Friend, P.N. Prasad, Chem. Phys. 245 (1999) 17. [3] N.C. Greenham, X. Peng, A.P. Alivisatos, Phys. Rev., B 54 (1996) 17628. [4] H. Jin, Y.B. Hou, A.W. Tang, X.G. Meng, F. Teng, Chin. Phys. Lett. 23 (2006) 693. [5] J. Liu, Y.J. Shi, Y. Yang, Adv. Funct. Mater. 11 (2001) 420. [6] W.U. Huynh, X.G. Peng, A.P. Alivisatos, Adv. Mater. 11 (1999) 923. [7] B.Q. Sun, E. Marx, N.C. Greenham, Nano Lett. 3 (2003) 961. [8] C.L. Yang, J.N. Wang, W.K. Ge, S.H. Wang, J.X. Cheng, Appl. Phys. Lett. 78 (2001) 760. [9] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425. [10] S.M. Shu, H.Q. Guo, Z.H. Zhang, R. Li, W. Chen, Z.G. Wang, Phys. E 8 (2000) 174. [11] Y.Y. Lin, C.W. Chen, J. Chang, T.Y. Lin, W.F. Su, Nanotechnology 17 (2006) 1260. [12] J.X. Cheng, S.H. Wang, X.Y. Li, Y.J. Yan, W.K. Ge, Chem. Phys. Lett. 333 (2001) 375. [13] G.D. Sharma, S. Sharma, M.S. Roy, J. Mater. Sci., Mater. Electron. 15 (2004) 69. [14] N.C. Greenham, X.G. Peng, A.P. Alivisatos, Synth. Met. 84 (1997) 545. [15] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [16] S. Coe, W.K. Woo, M. Bawendi, Nature 420 (2002) 800.