Growth of Copper Phthalocyanine Rods on Au Plasmon Electrodes

Jan 11, 2010 - A. Betty C. , Padma N , Shalav Arora , Parth Survaiya , Debarati Bhattacharya , Sipra Choudhury , Mainak Roy. Applied Surface Science 2...
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Growth of Copper Phthalocyanine Rods on Au Plasmon Electrodes through Micelle Disruption Methods Wei-Hung Chen,† Wen-Yin Ko,† Ying-Shiou Chen,† Ching-Yuan Cheng,‡ Chi-Ming Chan,† and Kuan-Jiuh Lin*,† †

Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, ROC and ‡ Experimental Facility Division, National Synchrotron Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan, ROC Received September 14, 2009. Revised Manuscript Received December 23, 2009

To improve the efficiency of the photocurrent conversion process, we have utilized copper phthalocyanine (CuPc) rods, which are capable of enhancing the interfacial area of electron transport and plasmonic gold nanoparticles (Au NPs), which can increase the separation and photogeneration of excitons, to produce a more effective system. In-plane horizontal CuPc rods, with diameters ranging from 0.2 to 1.5 μm, were electrodeposited onto the surface of plasmonic (Au NP) monolayers predeposited onto ITO substrates through electrolytic micelle disruption (EMD) methods.

Introduction Ever since the discovery of photovoltaic effects in organic heterostructures, there has been great interest in utilizing organic thin film semiconductors for applications in photovoltaic (PV) devices.1-10 To date, the most efficient organic heterostructures for organic PV devices have been based on copper phthalocyanine (CuPc) and fullerene (C60); these devices exhibit a power-conversion efficiency, ηp, of up to ∼5%.3,4 CuPc, a molecule possessing a planar structure with four aromatic rings around a porphyrin-like central ring and a copper atom at its center (Figure 1), was a promising candidate for an organic electron donor because of its excellent electronic properties and high thermal and chemical stability. Recently, 1D architectures of CuPc, such as rods or wires, were considered to be an ideal alternative to organic photovoltaics because this structure could greatly enhance the interfacial area for electron transport.5,11-15 However, the fabrication of CuPc rods is difficult. There are only a few methods that *Corresponding author. E-mail: [email protected]. (1) Kumar, H.; Kumar, P.; Chaudhary, N.; Bhardwaj, R.; Chand, S.; Jain, S. C.; Kumar, V. J. Phys. D: Appl. Phys. 2009, 42, 015102. (2) Khodabakhsh, S.; Sanderson, B. M.; Nelson, J.; Jones, T. S. Adv. Funct. Mater. 2006, 16, 95–100. (3) Xue, J. G.; Rand, B. P.; Uchida, S.; Forrest, S. R. Adv. Mater. 2005, 17, 66– 71. (4) Datta, D.; Tripathi, V.; Gogoi, P.; Banerjee, S.; Kumar, S. Thin Solid Films 2008, 516, 7237–7240. (5) Mi, O. Y.; Ru, B.; Lin, C.; Yang, L. G.; Mang, W.; Chen, H. Z. J. Phys. Chem. C 2008, 112, 11250–11256. (6) Yu, H. Z.; Peng, J. B. Org. Electron. 2008, 9, 1022–1025. (7) Yang, F.; Forrest, S. R. Adv. Mater. 2006, 18, 2018–2022. (8) Kim, I.; Haverinen, H. M.; Wang, Z.; Madakuni, S.; Kim, Y.; Li, J.; Jabbour, G. E. Chem. Mater. 2009, 21, 4256-4260. (9) Cheng, C. P.; Chen, W. Y.; Wei, C. H.; Pi, T. W. Appl. Phys. Lett. 2009, 94, 203303. (10) Chen, L.; Tang, Y. W.; Fan, X.; Zhang, C.; Chu, Z. Z.; Wang, D.; Zou, D. C. Org. Electron. 2009, 10, 724–728. (11) Hsiao, Y. S.; Whang, W. T.; Suen, S. C.; Shiu, J. Y.; Chen, C. P. Nanotechnology 2008, 19, 415603. (12) Rusu, M.; Gasiorowski, J.; Wiesner, S.; Meyer, N.; Heuken, M.; Fostiropoulos, K.; Lux-Steiner, M. C. Thin Solid Films 2008, 516, 7160–7166. (13) Xi, H. X.; Wei, Z. M.; Duan, Z. M.; Xu, W.; Zhu, D. B. J. Phys. Chem. C 2008, 112, 19934–19938. (14) Borras, A.; Aguirre, M.; Groening, O.; Lopez-Cartes, C.; Groening, P. Chem. Mater. 2008, 20, 7371–7373. (15) Suzuki, H.; Yamashita, Y.; Kojima, N.; Yamaguchi, M. Jpn. J. Appl. Phys. 2008, 47, 6879–6882.

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can be used to produce CuPc rods; these include organic vaporphase deposition,12,14 a solvent treatment method,13 a molecularbeam epitaxy technique,15 and a method that involves the use of a template.5 Moreover, CuPc rods are usually generated under strict experimental requirements, for example, high vacuum conditions and high reaction temperatures, and the instruments used for the production are expensive. Another way to improve the power-conversion efficiency of PV devices is to increase the fraction of photogenerated excitons by enhancing the light-trapping capability of the active layers to promote absorption. A well-known method is to incorporate metallic nanoparticles into the layers. Metallic nanoparticles can tune the surface plasmon resonance, which can then be exploited for the design of light-trapping layers inside the photoactive material; metallic nanoparticles also have the ability to achieve localized surface plasmon resonance (LSPR), which can lead to electromagnetic field enhancement in the surrounding regions.16-26 For instance, the Yamada group reported that the LSPR of Au NPs can allow longer-wavelength photocurrents from the photoresponsive dye molecules.20 Parak et al. demonstrated that an increase of 15.3% in energy conversion efficiency in InGaP/InGaAs/Ge multijunction solar cells could be attained after utilizing Au NPs.21 Nah and co-workers described that the power-conversion efficiency of organic solar cells increased from 3.05 to 3.69% after the incorporation of Ag NPs.22 In addition, (16) Pillai, S.; Catchpole, K. R.; Trupke, T.; Green, M. A. J. Appl. Phys. 2007, 101, 093105. (17) Schaadt, D. M.; Feng, B.; Yu, E. T. Appl. Phys. Lett. 2005, 86, 063106. (18) Stenzel, O.; Stendal, A.; Voigtsberger, K.; von Borczyskowski, C. Sol. Energy Mater. Sol. Cells 1995, 37, 337–348. (19) Hagglund, C.; Zach, M.; Kasemo, B. Appl. Phys. Lett. 2008, 92, 013113. (20) Akiyama, T.; Nakada, M.; Terasaki, N.; Yamada, S. Chem. Commun. 2006, 395–397. (21) Yang, M. D.; Liu, Y. K.; Shen, J. L.; Wu, C. H.; Lin, C. A.; Chang, W. H.; Wang, H. H.; Yeh, H. I.; Chan, W. H.; Parak, W. J. Opt. Express 2008, 16, 15754– 15758. (22) Kim, S. S.; Na, S. I.; Jo, J.; Kim, D. Y.; Nah, Y. C. Appl. Phys. Lett. 2008, 93, 073307. (23) Lu, Y. L.; Chen, X. B. Appl. Phys. Lett. 2009, 94, 193110. (24) Beck, F. J.; Polman, A.; Catchpole, K. R. J. Appl. Phys. 2009, 105, 114310. (25) Chou, C. S.; Yang, R. Y.; Yeh, C. K.; Lin, Y. J. Powder Technol. 2009, 194, 95–105. (26) Duche, D.; Torchio, P.; Escoubas, L.; Monestier, F.; Simon, J. J.; Flory, F.; Mathian, G. Sol. Energy Mater. Sol. Cells 2009, 93, 1377–1382.

Published on Web 01/11/2010

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Figure 1. Schematic of the CuPc/LSPR-Au/ITO solar cell device.

the fabrication of metallic nanoparticle thin films provides a noteworthy advantage in that the incident angle of the irradiating light does not need to be adjusted for every excitation wavelength because the near-field localized plasmon itself is not dependent on the incident angle.20 With these advantages in mind, we have aimed to improve organic PV devices by making changes in the CuPc morphology and incorporating interfacial metallic nanoparticles. Our method comprises two principal developments: (1) the fabrication of hightransparency plasmonic Au NP electrodes by using the electrodeposition method, which is a simple, quick process for metal nanoparticle formation,27,28 and (2) the crystal growth of CuPc rods by a simple electrolytic micelle disruption (EMD) method, a process that is based on the desorption of surfactant molecules from the desired particles.29 After sputtering an Al film on top of the CuPc film, an organic solar cell device could be obtained, as schematically illustrated in Figure 1. In preliminary experiments, the device was subjected to tests of photocurrent generation through photoelectrochemical cells and an enhancement of the photocurrent capability was observed.

15 h at room temperature and washed with toluene for 1 h. The pale-yellow powder obtained was then refluxed with bromooctadecane in acetonitrile at 85 °C for 2 days. After the sample was washed with chloroform to remove the unreacted bromooctadecane, 2-V-18, which is a yellow product, was obtained. Characterization. Electrochemical experiments were performed on a CH Instruments 672A electrochemical system with the use of a conventional three-electrode system at room temperature with a platinum wire and a Ag/AgCl wire (immersed in a 3 M KCl filling solution saturated with AgCl) as the counter electrode and reference electrode, respectively. All potentials are measured versus the Ag/AgCl electrode. The field-emission scanning electron microscopy (FESEM) images were recorded using a Zeiss Ultra plus field-emission microscope. The particle size of ∼300 particles was analyzed from the FESEM images with SigmaScan Pro 5 software. UV-visible absorbance and photoluminescence (PL) measurements were carried out at room temperature using a PerkinElmer Lambda 990 spectrophotometer and a Thermo’s AMINCO-Bowman Series 2 (AB2) spectrofluorometer, respectively. The X-ray diffraction (XRD) study was recorded using a four-circle diffractometer with a wiggler BL17A beamline (1.333 A˚) at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. Photocurrent measurements were carried out using the CH Instruments 672A electrochemical system. The active area of the electrode was approximately 1 cm2 and was illuminated through the back of the ITO by a projector equipped with a metal halide lamp (OSRAM HMI 575 W/GS). No corrections were performed to take into account the mismatch between the excitation light and standard solar irradiation. Preparation of CuPc/LSPR-Au/ITO. We first fabricated the self-assembled Au nanoparticle monolayer on an ITO electrode (1  2 cm2) prefunctionalized with MPTMS via the electrodeposition method, resulting in a plasmonic Au electrode, namely, LSPR-Au/ ITO. Here, the electrodeposition of the Au NPs was carried out using an aqueous solution of 0.02 M HAuCl4 3 3H2O and 0.021 g of DBSA in 50 mL of Milli-Q water. The deposition potential was controlled at -0.7 V (vs Ag/AgCl) at a deposition time of 100 s. Our method of producing rod-shaped CuPc, unlike other methods with more stringent requirements, involved synthesis at room temperature by a simple micelle disruption method via a commercial potentiostat. The electrolyte used for the CuPc rod deposition consisted of 0.576 g of CuPc and 0.1345 g of 2-V-18 in 50 mL of 0.1 M KCl aqueous solution. This experiment was carried out at -0.65 V and ran for 300 s. The insoluble CuPc was dispersed in the electrolyte due to the adsorption of the 2-V-18 surfactants; in addition, some of the free 2-V-18 surfactants, which were not adsorbed onto the CuPc, were deposited onto the electrode because of the negatively charged sulfonate-functionalized Au nanoparticles. By applying a potential of -0.65 V, the free 2-V-18 was reduced from þ2 to þ1, allowing 2-V-18 to be desorbed from CuPc and subsequently decreasing the dispersion ability of CuPc. Finally, CuPc was released and precipitated onto the electrode to form CuPc/LSPR-Au/ITO.

Experimental Section Chemicals. All chemicals were used as received without further purification. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 3 2H2O), copper(II) phthalocyanine (CuPc), and 1-bromooctadecane were purchased from Sigma-Aldrich. (3-Mercaptopropyl)trimethoxysilane (MPTMS), dodecylbenzene sulfonic acid sodium salt (DBSA), and bromoethane were purchased from Fluka. 4,40 -Bipyridine was obtained from Alfa Aesar. Asymmetric viologen surfactants (1-ethyl-10 -octadecanyl-4,40 -bipyridinium, V2þ, denoted as 2-V-18) were synthesized by the process in which 4,40 -bipyridine was reacted with bromoethane in acetonitrile for (27) Ko, W. Y.; Chen, W. H.; Tzeng, S. D.; Gwo, S.; Lin, K. J. Chem. Mater. 2006, 18, 6097–6099. (28) Ko, W. Y.; Chen, W. H.; Cheng, C. Y.; Lin, K. J. Sens. Actuators, B 2009, 137, 437–441. (29) Shrestha, N. K.; Kobayashi, H.; Saji, T. Langmuir 2007, 23, 1912–1916.

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Results and Discussion The size and density of the Au NPs were determined by FESEM. High yield, high density, and nearly spherical Au NPs homogeneously and separatively distributed on top of the MPTMS-modified ITO were observed, as shown in Figure S1a. The size distribution of the Au NPs is ca. 11.7 ( 2.82 nm (Figure S1b). The UV-vis absorption spectrum reveals an important optical property of the Au NPs (Figure S1c). It is observed that the plasmon absorbance peak is clearly visible and the band position (λmax) is at 564 nm, which exhibits a pronounced red shift compared to that of citrate-stabilized Au nanoparticles that are around 12 nm in size.30 The LSPR red (30) Kumar, V. G.; Grace, A. N.; Pandian, K. Curr. Sci. 2005, 88, 613–616.

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Figure 2. (a) FESEM image of the CuPc rods deposited onto an LSPR-Au/ITO substrate by a micelle disruption method via a commercial potentiostat. (b) Size distribution of the deposited CuPc rods prepared on the LSPR-Au/ITO substrate. (c) XRD pattern of CuPc rods on LSPR-Au/ITO.

shift was attributed to the stronger near-field interparticle coupling effect seen in the Au NPs, which was revealed in the close-packed metallic nanoparticle array.31-33 In addition, the optical transparency of the LSPR-Au/ITO film is >95%, excluding the plasmon band. Moreover, with different deposition times, tunable LSPR spectra of the LSPR-Au/ ITO film in the range of 550-600 nm could be obtained (Figures S2 and S3) through the use of our simple electrodeposition method. For the development of photoresponsive CuPc dye molecules, rodlike CuPc provided an attractive option because of its capacity to enhance the charge carrier transport across the interface. In previous studies, the EMD technique has been successfully employed in the formation of phthalocyanine (Pc) materials,34 for example, Pc films of 0.8 μm thickness, thick (>2 μm) magnesium phthalocyanine (MgPc) films, and metal-free phthalocyanine (H2Pc).35-37 However, there are no reports in the literature in which rod-shaped CuPc has been synthesized using EMD techniques. Here, we have synthesized, for the first time, CuPc rods in large quantities through the use of a simple micelle disruption method via a commercially available potentiostat under mild conditions. In Figure 2a, we present a top-view FESEM image that shows that the rodlike CuPc film was successfully deposited onto the LSPR-Au/ITO electrode and that CuPc tends to form in-plane horizontal structures with diameters of ∼0.2 to 1.5 μm (Figure 2b). Figure 2c shows the XRD pattern of the (31) Haynes, C. L.; McFarland, A. D.; Zhao, L. L.; Van Duyne, R. P.; Schatz, G. C.; Gunnarsson, L.; Prikulis, J.; Kasemo, B.; Kall, M. J. Phys. Chem. B 2003, 107, 7337–7342. (32) Gunnarsson, L.; Rindzevicius, T.; Prikulis, J.; Kasemo, B.; Kall, M.; Zou, S. L.; Schatz, G. C. J. Phys. Chem. B 2005, 109, 1079–1087. (33) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2000, 104, 10549–10556. (34) Hoshino, K.; Saji, T. J. Am. Chem. Soc. 1987, 109, 5881–5883. (35) Harima, Y.; Yamashita, K. J. Phys. Chem. 1989, 93, 4184–4188. (36) Harima, Y.; Yamashita, K. Appl. Phys. Lett. 1988, 52, 1542–1543. (37) Kubota, H. G.; Muto, J.; Itoh, K. M. J. Mater. Sci. Lett. 1996, 15, 1475– 1477.

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CuPc rods. The diffraction peaks can be assigned to a typical β phase of CuPc, which is known to be a monoclinic crystal with unit cell dimensions of a = 19.40 A˚, b = 4.79 A˚, c = 14.62 A˚, and β = 120.93, which means that the CuPc molecules are lying perpendicular to the surface and forming in-plane horizontal CuPc rods.38,39 This result is consistent with our observations of the FESEM image. Figure 3 shows the optical spectra of the CuPc rods grown on the LSPR-Au/ITO thin film. Two important points could be observed in the UV-vis absorption spectra: first, the low-energy band was split into two bands (Davydov splitting) at around 610 and 720 nm, which occurs because of the first and second π-π* transitions on the phthalocyanine macrocycle, known as the Q band.40-42 The Q band of the CuPc rod is broadened and blue shifted with respect to that of CuPc in solution, indicating that the structure of the phthalocyanine polymers is 1D linearly stacked with a van der Waals thickness and a cofacial molecular arrangement (Figure S4).40-42 Second, inside the Au nanoparticles that are placed in the depletion layer of the ITO/CuPc Schottky junction, the formation of an excited electron-hole pair via plasmon decay is also capable of yielding a contribution to the photocurrent,43 making it useful for the improvement of the conversion efficiency of photovoltaic cells. In addition, the photoluminescence (PL) spectrum indicates that the PL intensity in CuPc/LSPR-Au/ITO is significantly quenched, where the integrated emission spectrum is decreased by 70 and 50% at 765 and 824 nm, respectively. This result is indicative of the fact that the incorporation of the Au NPs is beneficial for reducing charge recombination by improving (38) Della Pirriera, M.; Puigdollers, J.; Voz, C.; Stella, M.; Bertomeu, J.; Alcubilla, R. J. Phys. D: Appl. Phys. 2009, 42, 145102. (39) Li, J.; Wang, S. Q.; Li, S.; Wang, Q.; Qian, Y.; Li, X. P.; Liu, M.; Li, Y.; Yang, G. Q. Inorg. Chem. 2008, 47, 1255–1257. (40) Wojdyla, M.; Derkowska, B.; Rebarz, M.; Bratkowski, A.; Bala, W. J. Opt. A: Pure Appl. Opt. 2005, 7, 463–466. (41) Hassan, B. M.; Li, H.; McKeown, N. B. J. Mater. Chem. 2000, 10, 39–45. (42) Hatton, R. A.; Blanchard, N. P.; Stolojan, V.; Miller, A. J.; Silva, S. R. P. Langmuir 2007, 23, 6424–6430. (43) Westphalen, M.; Kreibig, U.; Rostalski, J.; Luth, H.; Meissner, D. Sol. Energy Mater. Sol. Cells 2000, 61, 97–105.

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Figure 3. (a) UV-vis spectrum of the CuPc rods deposited by the micelle disruption method via a commercial potentiostat onto ITO substrates with (2, gray) and without (b, black) Au NPs decoration. (b, c) PL spectra of the CuPc rods with (2, gray) and without (b, black) modification by Au NPs, with excitation at 610 nm.

Figure 4. (a) Current-time response of the CuPc/ITO device with (black) and without (gray) LSPR-Au NPs upon switching the light on and off. The applied potential is 0.5 V. (b) Schematic of the energy-level diagram of the CuPc/LSPR-Au/ITO solar cell device. The energy band gap, the difference between the HOMO and LUMO, of 2-V-18 was calculated from the UV-vis absorption spectrum and the cyclic voltammogram.46

charge separation as well as promoting interfacial charge-transfer processes.44,45 According to the above statements, the possibility of using LSPR-Au to design an organic PV with enhanced photocurrent capabilities could be expected. To investigate whether the photocurrent of CuPc is increased by LSPR-Au, we coated the top of the CuPc rod film with an Al metal film to form an organic PV cell for investigation. The photocurrent measurements were plotted as a function of time for the device with Au nanoparticles and the device without Au nanoparticles, indicated by the black and gray lines, respectively, in Figure 4a. Both of the devices exhibited an increase in the photocurrent under illumination. A remarkable 4fold enhancement in the photocurrent density was observed in the CuPc/LSPR-Au/ITO cell as compared with that of the CuPc/ITO cell. Moreover, the photocurrent “on” and “off” cyclability of the CuPc/LSPR-Au/ITO cells was quite good, whereas that of the CuPc/ITO is very poor.(Figures S5 and S6). We suggest that the photocurrent enhancement mainly arises from suppressing the (44) Barazzouk, S.; Hotchandani, S. J. Appl. Phys. 2004, 96, 7744–7746. (45) Jakob, M.; Levanon, H.; Kamat, P. V. Nano Lett. 2003, 3, 353–358. (46) Roy, M. S.; Balraju, P.; Deol, Y. S.; Sharma, S. K.; Sharma, G. D. J. Mater. Sci. 2008, 43, 5551–5563.

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recombination between photoinduced holes and electrons in CuPc because the quenching effect is much more significant than additional plasmonic absorption. The photoinduced charge transfer might be explained by a schematic energy-level diagram of the composite, which is presented in Figure 4b.11,44 Under illumination, the photogenerated excitons in CuPc would transfer to the Au NPs via the viologen moieties because of the ability of Au to store and shuttle electrons. A quick shuttling of electrons from the Au NPs to ITO would occur when the Fermi level of the Au NPs was increased to be closer to that of ITO because of the accumulation of electrons on the Au NPs.44 However, holes would exit through the opposite Al anode. LSPR-Au therefore enables effective charge transport to the ITO electrode, generating a photocurrent that may be controlled by the morphologies of the CuPc dye.

Conclusion We have successfully synthesized rod-shaped CuPc’s with diameters of ∼0.2 to 1.5 μm at room temperature by an electrolytic micelle disruption (EMD) technique for the first time. The CuPc rods were in-plane horizontally deposited onto LSPR-Au thin films. Our results demonstrate that Au NP decoration provides an attractive method of enhancing the photocurrent of a CuPc rod, with the indication that more excitons are generated under illumination and avoiding charge recombination, which should be very useful for improving the conversion efficiency of solar cells. Further study and improvements to this design are in progress. Acknowledgment. We gratefully acknowledge the financial support of the National Science Council of Taiwan (NSC-972627-M-005-001). Supporting Information Available: Top view of an FESEM image, particle-diameter histogram for the Au NPs, and the UV-vis absorption spectrum of the LSPR-Au/ITO film. UV-vis absorbance spectra of an as-prepared Au NPs film deposited on MPTPS/ITO through the use of electrodeposition at an applied potential of -0.7 V as a function of Langmuir 2010, 26(4), 2191–2195

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deposition time. FESEM images of Au NPs deposited with different deposition times on MPTPS/ITO using electrodeposition at an applied potential of -0.7 V. UV-vis spectrum of the CuPc rods deposited by a micelle disrup tion method via a commercial potentiostat, with and without Au NPs decorating the ITO substrate, compared with

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concentrated sulfuric acid in the CuPc solution. Photocurrenttime measurements for three CuPc/ITO devices prepared in an identical fashion. Photocurrent versus time plot for three CuPc/ LSPR-Au/ITO devices prepared in an identical fashion. This material is available free of charge via the Internet at http:// pubs.acs.org.

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