J. Phys. Chem. 1992,96, 1366-1372
1366
molecular spacer is the subject of our forthcoming paper.28
Conclusioar, (i) The nonionic surfactant Triton X-100 aggregates spontaneously the Ag colloid to form the SERS-active system. Only the vibrational frequencies of the hydrophilic part of Triton X-100 molecule are active in the SERS spectrum of the system. (ii) The surfactant molecules are adsorbed on the surface of Ag colloid via their hydrophilic (PEG) moiety with the hydrophobic part exposed to possible interaction with other molecules studied. (iii) Upon addition of free base TPPC., to the Ag colloid/Triton X- 100 SERS-active system, the surfactant acts as an effective
molecular spacer between the porphyrin and Ag colloid. Its hydrophobic part represents an ‘affine group” capable of hydrophobic interaction with the porphyrin macrocycle. (iv) According to our experience, the surfactant-covered silver surface currently represents the only possibility for obtaining the SERS spectrum of an unaltered free base porphyrin in the system with Ag colloid. Acknowledgment. We thank Dr. Vladimir Krsl of the Institute of Organic Chemistry and Biochemistry, Prague, for the synthesis of free base TPPC4. Registry NO. PEG, 25322-68-3; TPPC, 14609-54-2;Ag, 7440-22-4; Triton X-100,9002-93-1.
Improvement of Photoelectrochemical Properties of Chloroalumlnum Phthalocyanlne Thin Fllms by Controlled Crystallization and Molecular Orientatlon Hisao Yanagi,* Shinya Douko, Yasukiyo Ueda, Michio Ashida, Faculty of Engineering, Kobe University, Rokkodai, Nada, Kobe 657, Japan
and Dieter Wiihrle Organische und Makromolekulare Chemie, Universitiit Bremen, 2800 Bremen 33, F.R.G. (Received: July 2, 1991)
An orientation-controlled thin film of AlPcCl was prepared by vacuum deposition on a (001) surface of a KCI substrate. In the epitaxially grown film on KCl AlPcCl molecules stack cofacially, staggering around the Al-Cl-A1 bonds and holding the molecular planes parallel to the KC1 surface. At a low substrate temperature the deposit was a uniform and closely oriented thin film. As the substrate temperature increased, the film became rugged and cracked due to growth of a large size of crystallites. The photovoltaic properties of the oriented AlPcCl film were measured in a photoelectrochemical cell of ITO/AlPcCl/13-,I-/Pt. The oriented AlPcCl films showed good rectifyingI-Vcurves of ptype conductance and cathodic photocurrents under illumination of white light. As compared to amorphous or polycrystalline AlPcCl films, photovoltaic efficiency was improved especially in increase of I , by the epitaxial orientation. The photocurrent quantum yield of the uniform thin film with epitaxial orientation was 25 times as high as that of the rugged and cracked polycrystalline film with large crystallites. The epitaxial AlPcCl film showed an intense peak at 830 nm in the photocurrent action spectrum, which could be attributed to carrier generation from charge-transfer excitons of the cofacially stacked molecules. It was concluded that the reason for the improvement of the photovoltaic properties was that exciton diffusion and charge carrier mobility were larger through the molecular column in the epitaxial film, while exciton trapping and recombination of carriers were probably caused in grain boundaries between crystallites in the polycrystalline film.
I. Introduction Solar energy conversion is the most important technology for a future energy source because solar energy is pollution-free and inexhaustible. The energy conversion efficiency of silicon semiconductors has been more than 10% however, electrical energy obtained with these cells is expensive. On the other hand, colored molecular organic semiconductors may be good candidates for materials of future solar cells due to their cheapness, low processing costs of thin films, and nearly unlimited variability.* The conversion efficiency of stable organic solar cells has barely come up to 1% in a pn-junction configuration with phthalocyanines and perylene derivatives, which are known as p- and n-type semiconductors, respectively. 1-3 Since the solar energy conversion system in nature, photosynthesis, functions with chlorophyll in which the interior ligand is in structural analogy with phthalocyanines, synthetic organic molecules could be tailored for a biomimetic approach. The chlorophyll molecules are integrated in a crystallographically ordered biological system to give a high (1) Chamberlain, G.A. Solar Cells 1983, 8,47. (2)WBhrle, D.;Meissner, D. Adv. Mater. 1991, 3, 129. (3)Tang, C.W. Appl. Phys. k r r . 1986,48, 183.
efficiency of carrier generation and electron t r a n ~ p o r t . ~ . ~ Therefore, for improvement of the conversion efficiency, especially the photocurrent, of organic solar cells, it is very important to control the crystal growth and the molecular orientation in a thin film. Porphyrins6,’ and phthalocyanines8-’ exhibit an epitaxial growth in vacuum-deposited thin films onto a cleavage surface of alkali metal halides. Their crystal structure and molecular orientation have been examined by high-resolution electron microscopy and electron diffraction.&’* The vacuum-deposited
’
(4) Hubcr, R. Angew. Chem. 1989,101, 849. ( 5 ) Deisenhofer, J.; Michel. H. Angew. Chem. 1989, 101, 872. (6) Yanagi, H.; Takemoto, K.; Hayashi, S.;Ashida, M. J . Crysr. Growrh
1990.99, 1038. (7)Ashida, M.;Yanagi, H.; Hayashi, S.;Takemoto, K. Acra Crysrallogr. 1991, 847, 87. (8) Uyeda, N.; Ashida, M.; Suito, E. J . Appl. Phys. 1965, 36, 1453. (9)Ashida, M.;Uyeda, N.; Suito, E. Bull. Chem. Soc. Jpn. 1966,39,2616. (10)Ashida, M. Bull. Chem. Soc. Jpn. 1966.39.2632. (1 1) Uyeda, N.;Kobayashi, T.; Suito, E.; Harada, Y.; Watanabe, M. J . Appl. Phys. 1972, 43,5181. (12)Uyeda, N.;Kobayashi, T.; Ishizuka, K.; Fujiyoshi, Y. Chem. Scr. 1978-1979, 14,47.
0022-3654/92/2096-1366%03.00/0 0 1992 American Chemical Society
Chloroaluminum Phthalocyanine Thin Films
Figure 1. Molecular structure of AlPcCl.
tetraphenylporphyrin grew epitaxially on a KCl (001) surface, and this orientation-controlled thin film exhibited a higher photoelectrochemical efficiency than a polycrystalline film.13 This enhancement in the photocurrent has been attributed to the molecular orientation, in which the planar molecules stay perpendicular to the substrate surface. Trivalent-metal phthalocyanines (MPcX, M = Al, Ga, X = F, C1, Figure 1) have a onedimensional stacking polymer structure.1c16 These thin films show light absorption in the long-wavelength region of visible light (700-800 nm) and active photoelectrochemical properties due to high efficiency of the charge carrier In order to make use of their one-dimensional columnar structure, it is necessary to control the molecular orientation in a thin film. In the present study chloroaluminum phthalocyanine (AlPcCl) was vacuum-deposited onto substrates of indium-tin oxide (ITO)glass electrodes and cleavage surfaces of KC1. The morphology and the epitaxial growth in the films were examined by electron microscopy and electron diffraction. The photoelectrochemical properties of the vacuumdeposited films were investigated in cells of the configuration ITO/A1PcC1/13-,I-/Pt. Thin films of unsubstituted phthalocyanines behave as pconductors and leads to a cathodic photoreduction of I2in this system by illumination with visible light.21-27 The improvement of photoelectrochemical efficiency was discussed in terms of the crystallization and the molecular orientation.
II. Experimental Section AlPcCl was prepared by modification of a known method.14 Phthalonitrile, 4.0 g (3.1 X mol), and aluminum trichloride, 1.0 g (7.5 X mol), were dissolved in dry distilled quinoline, 20 mL, and were refluxed for 1 h under dry nitrogen. The reaction mixture was cooled to 0 OC and then filtered. The resulting solid was washed with toluene, carbon tetrachloride, and acetone. Then followed an exhaustive Soxhlet extraction with acetone, and (13) Yanagi, H.; Ashida, M.; Harima, Y.; Yamashita, K. Chem. Lctt. 1990, 385. (14) Linsky, J. P.; Paul, T. R.; Nohr, R. S.;Kenny, M.E. Inorg. Chem. 1979, 19, 3131. (15) Nohr, R. S.; Kunesof, P. M.; Wynne, K. J.; Kenny, M. E.; Siebenman, P. G. J . Am. Chem. Soc. 1981,103,4371. (16) Nohr, R. S.; Wynne, K. J. J . Chem. Soc., Chem. Commun. 1981, 1211. (17) Mezza, T. M.; Linkous, C. L.; Shepard, V. R.; Armstrong, N. R. J. Electrochem. 1981, 124, 31 1. (18) Klofta, T. J.; Rieke, P. C.; Linkous, C. A,; Wuttner, W. J.; Nanthakumar, A.; Mewbom,T. D.; Armstrong, N. R. J . Electrochem. Soc. 1985, 132, 2135. (19) Panayotatos, P. Solur Cells 1986, 21, 301. (20) Sims, T. D.; Pemberton, J. E.; Lee, P.; Armstrong, N. R. Chem. Mater. 1989, I , 26. (21) Giraudeau, A,; FanF. F.; Bard, A. J. J . Am. Chem. Soc. 1980,102, 5137. (22) Pemer, G.; Dao, L. H. J . Electrochem. SOC.1987, 134, 1148. (23) Loufty, R. 0.;McIntyre, L. F. Con. J . Chem. 1983, 61, 72. (24) Harima, Y.; Yamashita, K.; Saji, T. Appl. Phys. Left. 1988,52,1542. (25) Belanger, D.; Dodelet, J. P.; Dao, L. H.; Lombos, B. A. J . Phys. Chem. 1984,88,4288. (26) W6hrle, D.; Bannehr, R.; Schumann, B.; Jaeger, N. I. J . Mol. Card 1983, 21, 255. (27) Schlettwein, D.; Kaneko, M.;Yamada, A.; Wahrle, D.; Jaeger, N. I. J . Phys. Chem. 1991, 95, 1748.
1"heJournal of Physical Chemistry, Vol. 96, No. 3, 1992 1367
AlPcCl was dried at 110 OC in vacuo. The purity of AlPcCl was confirmed by electronic, IR, and mass spectra. The substrates (10 X 12 mm) used for vacuum deposition were an indium tin oxide (ITO) coated glass (Nippon Sheet Glass Co., Ltd.) and a cleavage (001) surface of a KCl crystal. The former was provided as a transparent conducting electrode, and the latter was used as a substrate for preparation of an epitaxially grown film. The substrate was mounted on a heater in a vacuum evaporator ( E O L JEE4X) and was kept at temperatures between 20 and 250 OC in a vacuum of 1 X lo4 Pa. AlPcCl was evaporated onto the substrate from a quartz crucible source heated by a tungsten coil. The deposition rate was controlled to be about 5 nm/min in thickness using a quartz crystal microbalance. The photoelectrochemical measurements of the AlPcCl films were performed in a gas-tight 20-mL glass cell using a conventional three-electrode system. The thickness of the AlPcCl films was about 150 nm. The film on the I T 0 substrate was provided as a working electrode as it was. In the case of the film on KC1, the KCl substrate was dissolved away on a water surface, and the AlPcCl film was transferred onto an I T 0 glass electrode. The electrical contact to the AlPcCl/ITO electrode was made with a copper wire and a silver conducting paste and was insulated from tho electrolyte with a glass tube and epoxy resin coverage to remain an dctive area of about 1 cm2. The potentiostatic measurement was carried out with a coiled platinum wire as the counter electrode and an Ag/AgCl (saturated KCl) as the reference. The electrolyte containing a redox couple was 5 mM I2 and 0.1 M KI. The electrolyte was bubbled with argon gas for 15 min to remove dissolved oxygen which can also photoelectrochemically be reThe working electrode was placed at about 5 mm from the cell window to minimize light absorption by the solution. The electrode was always directly illuminated from the AlPcCl front side. The steady-state dark and photocurrent-potential characteristics were measured using a Hokuto Denko HAB-151 potentiostat/galvanostat. The experiments were conducted at a constant scan rate of 5 mV s-'. The white light was illuminated with a Shimadzu AT-100HG halogen lump through a 5-cm water filter. The monochromatic light illumination for action spectra was obtained using a Shimadzu SPG-1OOST monochromator. The incident light intensity was measured with a Advantest TQ8215 optical power multimeter. Optical absorption spectra of the AlPcCl electrodes were recorded by a Shimadzu UV-240 spectrometer. The crystalline morphology and the epitaxial orientation of the AlPcCl films were observed by a JEOL JEM-2OOCX electron microscope and electron diffraction. For electron microscopy the films were reinforced with an evaporated carbon film. The films on an I T 0 glass were exposured to a vapor of hydrofluoric acid for a few seconds and separated from the substrate by dipping on a water surface. The KCl substrate was dissolved away by floating on water. The AlPcCl specimen films were transferred on a copper sheet mesh. High-resolution electron micrographs were obtained using an objective pole piece (C,= 2.0 mm) and a minimum dose system (MDS)28at 200 kV acceleration. An accurate lattice spacing was calibrated using the (1 11) diffraction ring of gold as reference.
III. Results 1. Morphology and Crystal Growth of the AlPcCl Films. The morphology and crystal growth of the AlPcCl films depend remarkably on the substrates used and their substrate temperature. On both substrates, an I T 0 glass and a KCl (001) surface, the crystal growth was advanced at a higher substrate temperature. Figure 2 shows transmission electron micrographs and their electron diffraction patterns of AlPcCl films deposited on I T 0 glasses kept at 20, 150, and 250 OC, respectively. At 20 OC the deposited AlPcCl formed a very uniform thin film. Its electron diffraction pattern shows no intense reflection except a weak ring pattern in the small-angle region, so that the film is composed ~~~~
~
(28) Fujiyoshi, Y.; Kobayashi, T.; Ishizuka, K.; Uyeda, N.; Ishida, Y.; Harada, Y. Ultrumicroscopy 1980, 5, 459.
1368 The Journal of Physical Chemistry, Vol. 96, No. 3* 1992
Yanagi et al.
Figure 2. Electron micrographs and electron diffraction patterns of AlPcCl thin films deposited on IT0 glass substrates kept at 20 (a), 150 (b), and 250 "C (c).
of almost an amorphous structure. At a substrate temperature of 150 'C, the crystals grow to form a polycrystalline film, which can be seen from ring pattern of electron diffraction. As the crystal grow larger, the morphology of the film becomes rugged, and some cracks are caused. As the substrate temperature is increased at 250 'C, the deposited AlPcCl forms island crystals of about a few submicrometers in size. Each crystal gives a single net pattern of electron diffraction spots as shown in Figure 2c, indicating that each island is composed of a single crystal. However, each crystal has no epitaxial relation to the substrate, so that the whole film is polycrystalline. The morphology of the AlPcCl films deposited on the KCI (001) surface is shown in Figure 3. The dependence of the substrate temperature on the morphology is similar to that of the films deposited on the I T 0 glass. At lower temperatures the deposited film is uniform and continuous. As the temperature increases, the deposits are aggregated and the film becomes rugged and fissured. These rugged and cracked films prepared at higher substrate temperature are not preferred for the construction of solid solar cells, in which the organic layer is sandwiched by two electrode layers of different work functions to avoid a short circuit. For that reason the photovoltaic effect of the AlPcCl films was measured in a photoelectrxhemicalwet cell. Work is in progress to prepare a solid cell of AlPcCl films obtained at 20 'C on KCI. 2. Epitaxial Orientation of AlPcCl Deposited on KCI. All AlPcCl films deposited on KCI (001) surfaces show the same epitaxial orientation independent of the substrate temperature. The same electron diffraction pattern, as shown in Figure 4a, was obtained from the AlPcCl films of Figure 3a-c, which were deposited on KCI kept at different temperatures. Trivalent-metal phthalocyanines (MPcX) are known to form one-dimensional bridged polymers ((MPcX),), in which Pc rings are eclipsed and coafacially stacked, as shown in Figure l.I4-l6 The ring-ring separation can be estimated approximately from the sums of the related radii,14*29 although the ionic radius changes slightly depending on the coordination number. Fryer30has reported that the (AlPcF), vacuum-deposited on KCI grows epitaxially and the (29) Schannon, R. D. Acru Crysrullogr. 1916, A32.751. (30) Fryer, J. R. Mol. Crysr. Li9. Crysr. 1W6,137.49.
molecules are packed in the tetragonal unit cell with a = 6 = 1.34 and c = 0.36 nm. According to these findings, the electron diffraction pattern of Figure 4a was indexed as shown in Figure 4b. It is interpreted by the superposition of two single net patterns with hM) reflections which intersect each other at 37'. The unit cell parameters were determined from a single hM) series: a = b = 1.37 nm; the space group is P4/mcc. The ab plane of the AlPcCl crystal is parallel, and the c axis is perpendicular, to the KCI (001) surface. The epitaxial relation between the AlPcCl crystal and the KCI substrate was determined from an electron diffraction pattern using gold particles as directional reference of the KCI crystal, as follows. The gold particles deposited on a KCI (001) surface kept above 350 'C grow epitaxially with the orientation of [loo],,//[ 100]K
