Preparation of Interpenetrating pn Organic Pigment ... - ACS Publications

Jan 28, 2003 - Faculty of Engineering, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan, and Central Research Laboratories, Idemitsu Kosan ...
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Langmuir 2003, 19, 2458-2465

Preparation of Interpenetrating pn Organic Pigment Heterostructures with Graded and Mixed Junction Profiles Naoko Ishida,† Tadao Shibuya,‡ Takashi Kitamura,† and Katsuyoshi Hoshino*,† Faculty of Engineering, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan, and Central Research Laboratories, Idemitsu Kosan Company, Limited, 1280 Kami-izumi, Sodegaura, Chiba 299-0293, Japan Received July 3, 2002. In Final Form: October 22, 2002 The film formation behaviors of the photocatalytic deposition method are investigated using copper phthalocyanines (R, β, and  forms of CuPc), metal-free phthalocyanines (R, β, and x forms of H2Pc), and N,N′-3,5-xylyl-3,4,9,10-perylene tetracarboxylic diimide. The detailed analyses based on a diffusion equation reveal that the film deposition is controlled by the diffusion of pigment particles from the dispersion bulk to the substrate. This leads to the finding that the average size of the pigment particles composing the films is smaller than that of the particles composing the dispersions. An additional finding is the fact that the packing density of the films remains constant during their growth. These features of fundamental importance enable the preparation of two types of composite films. One is a double-layered structure with a graded junction profile, and the other is a mixed film with a homogeneously blended concentration profile along the thickness. These structures are characterized by scanning electron microscopic analysis, X-ray photoelectron spectroscopy, and UV-vis spectroscopy.

I. Introduction For the past decade, organic pigment film formation methods using aqueous surfactant media have been extensively studied; these methods involve micellar disruption (MD),1 photocatalytic deposition (PCD),2 aqueous coating,3 thermoinduced deposition,4 photochemical deposition,5 and photoelectrochemical deposition techniques.6 A common feature of these methods is to produce films in which the pigment particles are loosely and randomly packed, being in contrast to sublimed films in which the particles are densely and somewhat regularly packed. However, no study has been devoted to use of such a unique structure as novel advanced materials except for its application in a photoelectrochemical cell: Harima et al.6 prepared phthalocyanine (Pc) electrodes by the MD (Pc(MD)) and vacuum sublimation (VS) methods (Pc(VS)) and found that the energy conversion efficiency of the indium-tin oxide/Pc(MD)/I3--I- solution/ Pt cell was 60 times that of the indium-tin oxide/Pc(VS)/ I3--I-/Pt cell. On the other hand, interpenetrating donor (p-type)acceptor (n-type) organic heterostructures have been expected to offer a promising alternative to inorganic solar cells because they provide both the spatially distributed * To whom correspondence should be addressed. E-mail: [email protected]. † Chiba University. ‡ Idemitsu Kosan Co., Ltd. (1) (a) Saji, T. Chem. Lett. 1988, 693. (b) Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450. (c) Hoshino, K.; Saji, T. J. Am. Chem. Soc. 1987, 109, 5881. (2) (a) Mizuno, H.; Hoshino, K.; Hanna, J.; Kokado, H. Chem. Lett. 1992, 751. (b) Hoshino, K.; Kurasako, H.; Inayama, T.; Kokado, H. J. Electroanal. Chem. 1996, 406, 175. (c) Hoshino, K.; Kato, A.; Kurasako, K.; Kokado, H. J. Soc. Photogr. Sci. Technol. Japan 1996, 59, 340. (3) (a) Hoshino, K.; Yokoyama, S.; Saji, T.; Kokado, H. Chem. Lett. 1989, 1137. (b) Hoshino, K.; Kokado, H. J. Electroanal. Chem. 1994, 371, 127. (4) Hiruta, S.; Hoshino, K.; Yokoyama, S.; Kokado, H. Chem. Mater. 1991, 3, 382. (5) Liu, S.; Fujihira, M.; Saji, T. J. Chem. Soc., Chem. Commun. 1994, 1855. (6) Harima, Y.; Matsumoto, K.; Yokoyama, S.; Yamashita, K. Thin Solid Films 1993, 224, 101.

interfaces necessary for efficient charge photogeneration and good connectivity of the donor and acceptor to the respective correction electrodes:7 indeed, an overall power conversion efficiency as high as a few percent was achieved by using such structures.7c-e The concept of the present study is to prepare the above interpenetrating structure by depositing fine pigment particles (phthalocyanine, p-type conductivity) on a loosely packed film consisting of other larger particles with n-type conductivity (perylene derivative) which should allow the penetration of the former smaller particles into the latter matrix film. Herein, the structure with a graded junction profile is prepared by the PCD method and simulated on the basis of the diffusion equation used in silicon technology. The preparation of another interpenetrating structure, a homogeneously blended film, by the same method is also reported. Additionally, the effect of the physicochemical properties of the pigment dispersions on filmformation behavior is also investigated since it provides the basis for realizing such interpenetrating structures as mentioned above. II. Experimental Section A. Materials. As the film-forming materials, metal-free phthalocyanines (R-, β-, and x-H2Pc; BASF Japan Co.), copper phthalocyanines (R-, β-, and -CuPc; BASF Japan Co.), and N,N′3,5-xylyl-3,4,9,10-perylene tetracarboxylic diimide (PTCD, Hoechst Japan Co.) were used. β-H2Pc was prepared by heating x-H2Pc at 300 °C for 1 h in a furnace (ISUZU model AT-E58), and the resultant powder was checked by X-ray diffraction and UVvis spectroscopy.8 The surfactant used to disperse the pigments in water was (11-ferrocenylundecyl)polyoxyethyleneglycol (FPEG, Dojin Chemical Co.).9 The photocatalyst used was tris(2,2′(7) (a) Miller, J. S. Adv. Mater. 2001, 13, 525. (b) Hiramoto, M.; Fujiwara, H.; Yokoyama, M. J. Appl. Phys. 1992, 72, 3781. (c) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (d) Yu, G.; Gao, J.; Hummelen, C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (e) Granstro¨m, M.; Petritsch, K.; Arias, A. C.; Lux, A.; Andersson, M. R.; Friend, R. H. Nature 1998, 395, 257. (8) (a) Wagner, H. J.; Loutfy, R. O.; Hsiao, C.-K. J. Mater. Sci. 1982, 17, 2781. (b) Sharp, J. H.; Lardon, M. J. Phys. Chem. 1968, 72, 3230. (c) Kitamura, T.; Kokado, H. Electrophotography 1982, 20, 60.

10.1021/la0206054 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/28/2003

Interpenetrating Organic Pigment Heterostructures bipyridine)ruthenium(II) chloride hexahydrate (Ru(bpy)3Cl2‚ 6H2O (bpy ) bipyridine), Strem Chemical Co.), having an intense absorption band at 450 nm in water. The sacrificial agent, chloropentaammine cobalt(III) chloride ([Co(NH3)5Cl]Cl2), was synthesized according to the reported procedure.10 The films were deposited on a transparent indium-tin oxide coated glass (ITO, Geomatech Co., 10 Ω/sq) substrate. B. Deposition Cell and Dispersions. The preparations of the pigment dispersions and reaction cell have already been described elsewhere2b but will be outlined here. Monochromatic light at 450 nm was provided by a 500 W Xe lamp (Ushio Electric Inc., UI-501C) through band-pass and interference filters (Toshiba IRA-25S, Y-43, and KL-45). The deposition cell was constructed by interposing a rubber spacer (1.5 mm thick) between the two ITO plates (30 × 30 × 1.1 mm) in which the conductive surfaces of the ITO plates were directed to the interior side of the cell. Into the small space (20 × 20 × 1.5 mm) formed in the cell, pigment dispersions were placed which consisted of pigments, FPEG (2.0 mM), Ru(bpy)3Cl2‚6H2O (10 mM), [Co(NH3)5Cl]Cl2 (10 mM), and HCl (pH ) 1.6); hence, the area of the ITO exposed to the dispersions was 20 × 20 mm2. The dispersions were prepared by the following successive steps: (1) 7 mM pigment and HCl were added to a stirred FPEG micellar solution (2 mM, 50 mL), (2) the suspension was sonicated for 20 min with ice cooling (600 W, model US-600, Nihonseiki Co., Ltd.), (3) the resultant dispersion was allowed to stand for 24 h at room temperature and then the supernatant was separated, and (4) to the supernatant, Ru(bpy)3Cl2‚6H2O (10 mM) and [Co(NH3)5Cl]Cl2 (10 mM) were added, and stirring for ca. 20 min gave a test dispersion. C. Film Formation Procedures. Monochromatic light irradiation (450 nm, 0.3 mW/cm2) of the dispersion through a photomask (masked area, 20 × 10 mm2) and an ITO generates Ru(bpy)33+ (step 1) which oxidizes FPEG (step 2).2a,b This oxidation is accompanied by the desorption of FPEG+ from the pigment particles (step 3) and then by the flocculation and precipitation of the naked pigment particles (step 4). After a 5 min irradiation, FPEG is completely consumed (oxidized) and no pigment is found in the illuminated area. Further irradiation leads to the accumulation of Ru(bpy)33+ in the illuminated area. This highly oxidizing agent oxidizes FPEG in the unilluminated area through the conductive ITO (step 5). This oxidation is distinct from the bulk oxidation in step 2 in that it occurs at the substrate/ dispersion interface, and hence, pigment particles are deposited on the ITO in the form of a film (step 6) in the unilluminated (masked) area. The area of the film was 20 × 10 mm2, being equal to the masked area of the ITO. Throughout these steps, a bias voltage of 0.2 V versus a counter ITO is applied in parallel with the irradiation using a potentiostat (Hokuto Denko HA151). The bias promotes the deposition in the unilluminated area and depresses the one in the illuminated area.2c These mechanistic features of the PCD method are schematically illustrated in the Supporting Information. The as-grown films were washed with toluene after being air-dried overnight at room temperature and then washed with distilled-deionized water after being dried at 60 °C for 5 h. D. Characterization of Dispersions and Films. The particle size (distribution) of the pigments in the dispersion was measured using a centrifugal particle analyzer (Shimazu Co., model SA-CP4L). Scanning electron microscopic (SEM) measurements were done to observe the morphology of the films (Topcon model ABT-32). The film thicknesses were spectrophotometrically determined. The absorption coefficients were calculated from the slope of the film thickness versus absorbance, where films were prepared by the PCD method and the thicknesses were measured by the stylus method (model alpha step 300, Tencor Instruments Co.). The values of the absorption (9) (a) Yokoyama, S.; Kurata, H.; Harima, Y.; Yamashita, K.; Hoshino, K.; Kokado, H. Chem. Lett. 1990, 343. (b) Takeoka, Y.; Aoki, T.; Sanui, K.; Ogata, N.; Watanabe, M. J. Electroanal. Chem. 1997, 438, 153. (c) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abbott, N. L. Science 1999, 283, 57. (d) Aydogan, N.; Abbott, N. L. Langmuir 2001, 17, 5703. (10) (a) Lehn, J. M.; Sauvage, J.-P.; Ziessel, R. Nouv. J. Chim. 1979, 3, 423. (b) Hynes, W. A.; Yanowski, L. K.; Schiller, M. J. Am. Chem. Soc. 1938, 60, 3053.

Langmuir, Vol. 19, No. 6, 2003 2459 coefficients for the pigments are as follows: R-CuPc, 3.44 × 104 cm-1 (λmax ) 610 nm) and 2.30 × 104 cm-1 (λmax ) 697 nm); β-CuPc, 2.77 × 104 cm-1 (λmax ) 764 nm) and 2.82 × 104 cm-1 (713 nm); -CuPc, 4.26 × 104 cm-1 (λmax ) 610 nm) and 4.14 × 104 cm-1 (λmax ) 764 nm); R-H2Pc, 4.43 × 104 cm-1 (λmax ) 616 nm); β-H2Pc, 4.70 × 104 cm-1 (λmax ) 640 nm) and 4.41 × 104 cm-1 (λmax ) 710 nm); x-H2Pc, 5.93 × 104 cm-1 (λmax ) 619 nm) and 4.75 × 104 cm-1 (λmax ) 780 nm); PTCD, 1.17 × 104 cm-1 (λmax ) 478 nm) and 8.71 × 103 cm-1 (λmax ) 553 nm). The concentration of the pigments in the test dispersion, [pigment], was determined by evaporating the dispersion, dissolving the resultant residue in concentrated H2SO4, and recording the UV-vis absorption spectrum of the solution in which the pigments dissolve in a monomeric form. The following molar absorption coefficients for CuPc, H2Pc, and PTCD were used for the determination: CuPc, 2.33 × 105 M-1 cm-1 (λmax ) 790 nm); H2Pc, 1.30 × 105 M-1 cm-1 (λmax ) 836 nm); PTCD, 1.07 × 105 M-1 cm-1 (λmax ) 600 nm). For the X-ray photoelectron spectroscopic (XPS) measurements, an ULVAC phi ESCA 5400 spectrometer with Al KR radiation was used. The absolute binding energy scale was obtained by setting the C1s peak to 284.6 eV. The etch rate during the Ar+ ion exposure was 0.2 nm/s.

III. Results and Discussion A. Film Formation with Various Dispersions. In our previous studies on the PCD method,2 R-H2Pc had been exclusively used for the elucidation of the filmformation mechanism and the application to image recording. However, successful formation of composite films should require the knowledge of the deposition behaviors of the various pigments. Hence, film formation was carried out using the CuPc, H2Pc, and PTCD dispersions. The dependencies of film thickness on the deposition time, t, for the CuPc, H2Pc, and PTCD films were investigated. Though not shown here, the thicknesses quickly increased with t for ca. the first 20 min independent of the kind and crystal form of the pigments, but beyond this time, their increase was followed by a decrease in the deposition rate. This deposition behavior feature can be expected for the film-formation mechanism which involves the diffusion of pigment-FPEG aggregates to the ITO, desorption of FPEG from the pigment particles and its oxidation at the ITO, followed by deposition of the particles to form the film. If we assume that the film formation is limited by the diffusion, the flux of the aggregates at the ITO surface, J(t), is written as11

J(t) )

D1/2[pigment] π1/2t1/2

(1)

where D is the diffusion coefficient of the aggregate in water. Equation 1 is based on Fick’s first and second laws and involves the assumptions that the dispersion is homogeneous before the deposition experiment, that the regions sufficiently distant from the ITO are unperturbed by the experiment, and that [pigment] ) 0 at the ITO for t > 0. The integral of eq 1 from t ) 0 gives the coverage of the deposited film, Γ(t):

Γ(t) )

2D1/2[pigment]t1/2 π1/2

(2)

Figure 1 shows the plots of thickness versus t1/2 for the CuPc (a), H2Pc (b), and PTCD (c) films. Though the (11) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980; Chapter 5. (b) Delahay, P. New Instrumental Methods in Electrochemistry; Interscience: New York, 1966; Chapter 3.

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Figure 2. Scanning electron micrographs for the film surfaces of R-CuPc (a), β-CuPc (b), -CuPc (c), R-H2Pc (d), β-H2Pc (e), x-H2Pc (f), and PTCD (g). Scale bars: 1 µm. Images a-c were taken at a magnification different from that for images d-g. Figure 1. Dependence of thickness on the square root of the deposition time, t1/2, for R, β, and  forms of CuPc (a), R, β, and x forms of H2Pc (b), and PTCD films (c). Preparation conditions such as the cell structure, light intensity, bias voltage, etc. have been described in sections II.B and II.C of the text.

thickness is plotted on the ordinate instead of Γ, all of the plots gave straight lines independent of the kind and crystal form of the pigments. This demonstrates not only that the film formation is controlled by the diffusion but also that the packing density of the films is kept constant during their growth. Later, a linear dependence of Γ on t1/2 will be shown for the cases of -CuPc and PTCD and discussed. The values of Γ(t ) 60 min) for the -CuPc and PTCD films were 140 and 90 nmol/cm2, respectively, which were determined by dissolving the films in concentrated H2SO4 and taking their UV-vis absorption spectra. These values, when combined with the data on film thicknesses (0.6 µm for -CuPc and 1.15 µm for PTCD at t ) 60 min, see parts a and c of Figure 1, respectively) and densities (1.61 g/cm3 for -CuPc12 and 1.38 g/cm3 for PTCD13), gave packing densities of the -CuPc, 0.84, and PTCD films, 0.34. Such a packing density was reflected in the SEM (12) (a) Linstead, R. P.; Robertson, J. M. J. Chem. Soc. 1936, 1736. (b) Robertson, J. M. J. Chem. Soc. 1935, 615. (c) Ashida, M.; Uyeda, N.; Suito, E. Bull. Chem. Soc. Jpn. 1966, 39, 2616. -CuPc: Colour Index Generic name, C. I. Pigment Blue 15:6; Constitution number, C. I. 74160. (13) PTCD: Colour Index Generic name, C. I. Pigment Red 149; Constitution number, C. I. 71137.

Table 1. Characterization of the CuPc, H2Pc, and PTCD Dispersions pigment

dW (µm)

[pigment] (mM)

tind (s)

R-CuPc β-CuPc -CuPc R-H2Pc β-H2Pc x-H2Pc PTCD

0.18 0.069 0.081 0.15 0.24 0.074 0.42

3.0 6.8 6.7 3.8 1.2 1.7 5.3

10 120 240 22 100 100 120

images of the films (Figure 2) deposited from the CuPc, H2Pc, and PTCD dispersions for which the values of the weight-averaged particle diameter (dw) and [pigment] are listed in Table 1. It appears from Figure 2 that the primary particles composing the films roughly reflect the dw value and that the larger particles such as PTCD (g), R-H2Pc (d), R-CuPc (a), and β-H2Pc (e) give films with a lower packing density. The large difference in the packing density between the PTCD (g) and -CuPc films (c) is one of the reasons for their choice in the preparation of the composite films. However, later detailed investigations on the diffusion process will reveal that the mean particle size in the films is smaller than that in the corresponding dispersions, dw. Another notable feature of the plots in Figure 1 is a positive intercept on the t1/2 axis. The value of t derived from the intercept stands for an induction period (tind)

Interpenetrating Organic Pigment Heterostructures

during which the steps 1-4 described in section II.C and ref 2b occur. The values of tind are given in the last column in Table 1. tind is an indication of the dispersion stability to photo-oxidation (steps 1-4 in section II.C) and may be dependent on the chemistry of the pigment-FPEG aggregates in water. The chemistry should depend on the preparation conditions of the dispersions and films. For example, tind ) 900 s for the R-H2Pc (Tokyo Kasei Co., Ltd., dw ) 0.10 µm) dispersion prepared by sonication (75 W, 30 min),2a and its reduction, 300 s, was brought about by an increase in the sonication power (600 W, 20 min, dw ) 0.086 mm).2b Application of a bias voltage to the latter dispersion further reduced tind to 2.4 s.2c The alternate use of the R-H2Pc source (BASF Japan Co.) gave tind ) 22 s (see Table 1) with the application of a bias voltage. The chemistry of the aggregate stabilization coupled with the redox reactions may be far from simple, and the full account for the difference in tind is not yet clear. As described in section II.C, the reaction processes in the unilluminated area during the film formation (i.e., the processes occurring after the induction period) may involve the diffusion of pigment-FPEG aggregates toward the ITO, desorption of FPEG from the pigment,1b and the oxidation of FPEG at the electrode/dispersion interface. The interfacial oxidation is induced through the ITO by Ru(bpy)33+ photogenerated in the illuminated area2b and can be regarded as an electrode reaction. The dispersion contains a sufficient amount of electrolytes indifferent to the oxidation (HCl, Ru(bpy)33+, and [Co(NH3)5Cl]Cl2), and thereby the effect of migration can be neglected because the indifferent (or supporting) electrolytes decrease the contribution of migration to mass transfer of the aggregates (see Chapter 4 in ref 11a). The oxidation of FPEG can be expected to be fast in view of the fact that the electrode reaction for the ferrocenium/ferrocene redox couple falls into the class of the fastest ones (standard rate constant measured in CH3CN ) 0.7 cm/s).14 The rate constant of the preceding process for the oxidation, that is, the desorption of FPEG from the pigment particle, has not yet been measured. However, if we assume that the rate is comparable to that for the exit of surfactant monomers from their micelles,15 it is in the order of 10-6 s-1 and is therefore quite rapid. On the other hand, the reactions in the illuminated area are the formation of Ru(bpy)33+ (the quenching of 3Ru(bpy)32+ by [Co(NH3)5Cl]2+) and its reduction through the ITO by FPEG in the unilluminated area. The former reaction is known to be very rapid (second-order rate constant ) 7.4 × 108 M-1 s-1).16 The latter reaction is again regarded as a heterogeneous electrode reaction: the standard rate constant for the Ru(bpy)33+/2+ couple in aqueous solution was reported to be 0.065 cm/s,17 which falls under the category of very fast reactions. These mechanistic considerations, though some assumptions are involved, are not inconsistent with our experimental findings that the film formation is controlled by the diffusion of the pigmentFPEG aggregates. B. Preparation of a Film with a Graded Junction Profile. The pigments used for the preparation of the composite films were PTCD and -CuPc. These pigments (14) Saji, T.; Murayama, Y.; Aoyagui, S. J. Electroanal. Chem. 1978, 86, 219. (15) (a) Turro, N. J.; Gra¨tzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980, 19, 675. (b) Cline Love, L. J.; Dorsey, J. G.; Hobarta, J. G. Anal. Chem. 1984, 56, 1132A. (16) Navon, G.; Sutin, N. Inorg. Chem. 1974, 13, 2159. (17) (a) Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Electroanal. Chem. Interfacial Electrochem. 1983, 151, 267. (b) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 5007.

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Figure 3. SEM images of the surface (a) and cross section (b) of an -CuPc/PTCD layered film (film L). Scale bars: 1 µm.

were chosen because some visible absorption peaks of their solid-state films and of their concentrated H2SO4 solution do not overlap with each other and therefore allow quantitative analyses of the film compositions and because a lower packing density of the PTCD film, as was found in section III.A, should allow the penetration of the -CuPc particles into the PTCD matrix film. In addition, the fact that -CuPc and PTCD are p-18 and n-type19 organic semiconductors, respectively, is another reason for their choice to enable the formation of pn heterojunction devices. PTCD and CuPc were sequentially deposited on the ITO in that order. First, PTCD was deposited using the dispersion of PTCD to form a 1.15 µm thick film at a deposition time, t, of 60 min. Second, immediately after the deposition, the dispersion was replaced by the -CuPc dispersion and the deposition of -CuPc (t ) 60 min) was done on the PTCD film; when the latter deposition was conducted on a bare ITO, a 0.6 µm thick film of CuPc was obtained (see Figure 1a,c). An SEM observation revealed that the surface of the composite film (abbreviated as film L) was completely covered with needlelike particles of CuPc (Figure 3a), but no CuPc-PTCD interface was observed in its cross-sectional view over the total thickness of 1.8 µm (Figure 3b), suggesting percolation of -CuPc into the PTCD film. Figure 4 shows the depth profile of the mole fraction of CuPc (X) in film L. The value of X along the thickness was derived from the atomic concentration of Cu measured by an XPS analysis on Cu. The fraction of CuPc is ca. 100% to the depth of 500-600 nm and is progressively lower further into the PTCD film. This is likely based on the diffusion of -CuPc particles into the mesoporous PTCD film. During the growth of the inorganic semiconductor industry, gaseous diffusion of impurities into the host crystal has played a crucial role in the development of graded junctions such as may be found in a Si power device.16 The impurities are distributed in a complementary error function. From analogy of this treatment, the depth profile of X was calculated:20a,c

X(d*, t*) ) erfc(d*/2xD*t*)

(3)

where D* is the diffusion coefficient of the CuPc particles

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Figure 4. Mole fraction of CuPc, X, versus the distance from the free surface of film L. The value of X was derived from the XPS depth profile of the atomic concentration of Cu. The symbol d* in the figure shows the distance from the apparent CuPc/ PTCD interface. Solid circles: experimental points. Curves: A, calculated according to eq 3 for D* ) 1 × 10-10 cm2/s; B, D* ) 1 × 10-11 cm2/s; C, D* ) 5 × 10-12 cm2/s; D, D* ) 4 × 10-12 cm2/s; E, D* ) 3 × 10-12 cm2/s; F, D* ) 1 × 10-12 cm2/s.

in the PTCD film, t* is the length of time the diffusion takes place, 3600 s in the present case, and d* is the depth from the surface of the PTCD film. The curves in Figure 4A-F are the results of the calculation for D* ) 10-10 to 10-12 cm2/s. The profile of the curve is very sensitive to the value of D*, and the data points are well fitted to curve D (D* ) 4 × 10-12 cm2/s), indicating the formation of a graded junction interface. This value is 4 orders of magnitude smaller than that measured in an aqueous medium by the dynamic light scattering method (Ohtsuka Denshi Co., model ELS-8000), 3.0 × 10-8 cm2/s, reflecting the slow diffusion of -CuPc particles into the PTCD solid matrix film. The mesoscale porosity of the inner PTCD layer allowed the diffusion of -CuPc particles during the second deposition and consequent interpenetration of the two pigments. The UV-vis absorption spectrum of film L was the superposition of those of the respective single films (Figure 5a,b). In solid-state systems, the diffusion coefficient generally increases exponentially with temperature.20a If this is also the case for the CuPc-PTCD system, it is expected that the -CuPc particles move further into the PTCD film as temperature or diffusion time, t*, increases and that their total number in the PTCD film will also be increased. Accordingly, the temperature dependence of D* should be required for the development of a wide range of deposition profiles, X, that will find wide application. Experiments to this end are being pursued. C. Preparation of Mixed Films with Homogeneously Blended Composition Profiles. Mixed films of -CuPc and PTCD (film M) were prepared from their mixed dispersions. These dispersions were prepared in a similar manner as described above (section II.B) except that both -CuPc and PTCD were added to the FPEG (2 (18) Yagishita, T.; Ikegami, K.; Narusawa, T.; Okuyama, H. IEEE Trans. Ind. Appl. 1984, IA-20, 1642. (19) Loutfy, R. O.; Hor, A. M.; Kazmaier, P.; Tam, M. J. Imaging Sci. 1989, 33, 151. (20) (a) Neudeck, G. W. The pn junction diode, 2nd ed.; AddisonWesley: Reading, MA, 1989; Chapter 1. (b) Blood, P.; Orton, J. W. The Electrical Characterization of Semiconductors: Majority Carriers and Electron States; Academic Press: London, 1992; Chapter 10. (c) Navon, D. H. Electronic Materials and Devices; Houghton Mifflin: Boston, 1975; Chapter 11.

Figure 5. UV-vis absorption spectra of -CuPc and PTCD single-layered films (a), an -CuPc/PTCD double-layered film (b), and a mixed film of -CuPc and PTCD (film MA) (c). Table 2. Suspended and Added Concentrations of PTCD and E-CuPc in the Mixed Dispersions and Their Coverages in the Mixed Films mixed dispersion

[-CuPc]0 (mM)/ [PTCD]0 (mM)

[-CuPc]S (mM)/ [PTCD]S (mM)

Γ-CuPc (nmol/cm2)/ ΓPTCD (nmol/cm2)

dispersion A dispersion B dispersion C

3.50/3.50 5.25/1.75 1.75/5.25

3.36/2.61 5.07/1.30 1.69/3.79

40.4/25.8 50.0/10.7 16.5/31.7

mM) micellar system in such a way that their total concentration is 7 mM. Table 2 shows the added ([-CuPc]0 and [PTCD]0) and the suspended concentrations ([-CuPc]S and [PTCD]S) of -CuPc and PTCD in the three dispersions, dispersions A, B, and C. Also included in this table are the Γ values of the two pigments (Γ-CuPc and ΓPTCD at t ) 30 min) in films prepared from the dispersions A, B, and C (abbreviated as films MA, MB, and MC, respectively). Figure 6 shows the plots of [PTCD]S/[-CuPc]S versus [PTCD]0/[-CuPc]0 (a) and ΓPTCD/Γ-CuPc versus [PTCD]S/ [-CuPc]S (b). The former plot gave a straight line intersecting the origin and having a slope less than 1. This indicates that the concentration ratio of the suspended to the added PTCD, [PTCD]S/[PTCD]0, is constant and independent of the coexistence of -CuPc or vice versa and that [PTCD]S/[PTCD]0 < [-CuPc]S/[-CuPc]0. Indeed, the former and the latter concentration ratios were nearly constant at 0.73 and 0.96, respectively (see Table 2), and were nearly equal to those of the single dispersions, 0.76 for the PTCD and 0.96 for the -CuPc dispersions. The higher ratio of the suspended to the added -CuPc may be due to its smaller particle size compared with that of PTCD. A similar relationship was observed in Figure 6b, also demonstrating the constant ratios of the deposited to the suspended pigments, ΓPTCD/[PTCD]S and Γ-CuPc/ [-CuPc]S, and ΓPTCD/[PTCD]S < Γ-CuPc/[-CuPc]S. The former and latter ratios were 8.8 × 10-3 and 1.1 × 10-2 cm, respectively. The higher ratio of the deposited to the suspended -CuPc may be due to its faster diffusion compared with that of PTCD; this was supported by the

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Figure 7. Dependence of thickness on t1/2 for the -CuPcPTCD mixed films: b, film MC; O, film MA; 4, film MB. Open and closed squares show the plots for the PTCD and -CuPc single films, respectively.

Figure 6. Concentration ratios of PTCD to -CuPc in their mixed dispersions, [PTCD]0/[-CuPc]0 and [PTCD]S/[-CuPc]S, and in their mixed films, ΓPTCD/Γ-CuPc: a, plot of [PTCD]S/ [-CuPc]S versus [PTCD]0/[-CuPc]0; b, plot of ΓPTCD/Γ-CuPc versus [PTCD]S/[-CuPc]S; c, plot of ΓPTCD/Γ-CuPc versus [PTCD]0/ [-CuPc]0. Dotted lines in the figures show a relationship with the slope equal to 1.

diffusion coefficient values of -CuPc (3.0 × 10-8 cm2/s) and PTCD (1.6 × 10-8 cm2/s) that were measured by the dynamic light scattering method. The combination of parts a and b of Figure 6 provides the plot in Figure 6c, that is, the relationship between ΓPTCD/Γ-CuPc and [PTCD]0/ [-CuPc]0. This working curve allows the formation of mixed films with any concentration ratio when the knowledge of the added pigment ratio is given. Figure 7 shows the plot of thickness versus t1/2 for the films MA (O), MB (4), and MC (b). Included for comparison in this figure are the plots for the PTCD (0) and -CuPc (9) single films. The linear dependencies found for the mixed films again indicate their constant packing density and their diffusion-controlled growth during the deposition. The slope of the plot depends on the mixing ratio of

the two pigments and increases with an increase in the concentration ratio of [PTCD]S/[-CuPc]S. The induction periods for the mixtures, tind ) 86 s, were nearly the same and also equal to that for the PTCD single dispersion. This can be explained by assuming that the stability of the dispersion mixtures to photo-oxidation is determined by PTCD. The values of Γ for -CuPc and PTCD were plotted versus t1/2 for the films MA (a), MB (b), and MC (c) in Figure 8. All the plots gave straight lines and intersected the t1/2 axis at t ) 86 s. The former feature again indicates that the film deposition is controlled by the diffusion of the pigment particles and allows the estimation of the diffusion coefficient values, D, using eq 2. Table 3 shows the estimated values of D for -CuPc and PTCD in the three mixtures. Both D values for the -CuPc and PTCD particles are nearly independent of the mixing ratio of the two pigments. This implies no interaction between the two pigment particles during their diffusion processes. Additionally, the estimated values of D in Table 3 were not consistent with those measured by the light scattering method (3.0 × 10-8 cm2/s for -CuPc and 1.6 × 10-8 cm2/s for PTCD). In the measurements of the former values, smaller pigment particles should contribute to a greater extent to the film formation due to their faster diffusion, that is, the average size of the particles comprising the films should be smaller than that in the dispersion. On the other hand, the latter values are averaged over all pigment particles in the dispersions. This may be the reason for the larger D values of the former than the latter. On the basis of the above features of the mixed-film formation, a film comparable in thickness to film L was prepared from the dispersion A, and its depth profile was investigated. During the film formation, the deposition procedure over the period of t ) 60 min was repeated three times to form a 1.8 µm thick film (total deposition time, 180 min); before each deposition, the dispersion was replaced by a new one. The preparation and washing conditions of the film were the same as those described above. The SEM image of the film surface showed a mixture of needlelike particles of -CuPc and granular particles of PTCD (Figure 9a), and no interface was found in its cross-sectional view (Figure 9b). In this case also, the UV-vis spectrum of the film was the superposition of those of the respective single films (see Figure 5c). The depth profile of X determined by the XPS measurements

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Ishida et al.

Figure 9. SEM images of the surface (a) and cross section (b) of an -CuPc-PTCD mixed film (film MA). Scale bars: 1 µm.

Figure 8. The values of Γ for the PTCD (O) and -CuPc (b) plotted as a function of t1/2: a, film MA; b, film MB; c, film MC. Table 3. Diffusion Coefficient Valuesa of E-CuPc and PTCD Particles in the Mixed Dispersions A, B, and C Determined by Using Equation 2 in the Text pigment

dispersion A

dispersion B

dispersion C

-CuPc PTCD

1.0 × 10-7 6.3 × 10-8

7.0 × 10-8 5.6 × 10-8

8.2 × 10-8 5.8 × 10-8

a

Expressed in units of cm2/s.

is shown in Figure 10. The value of X was nearly constant at 63% and agrees well with the mole percent of CuPc in the film of 61% (see Table 2), demonstrating the successful formation of a homogeneously blended mixed film. Tominaga and Toshima21 have reported the preparation and characterization of the mixed and the double-layered films for two kinds of phthalocyanines which were simultaneously and sequentially deposited on a substrate by the vacuum evaporation technique. They demonstrated that the two phthalocyanines are homogeneously dispersed in the former film but gave no detailed structure (21) Tominaga, T.; Toshima, N. Polym. Adv. Technol. 1995, 6, 197.

Figure 10. Depth profile of X versus the distance from the free surface of the film MA. The dotted line indicates the value of X that was determined by the measurements of Γ for -CuPc and PTCD.

of the interface of the latter film. Such composite films exhibited novel electrochemical properties that the corresponding single films did not. Additionally, as described in section I, interpenetrating organic heterostructures have aroused great interest due to their high light-toelectricity conversion efficiency. In view of these unique and high-performance organic composite films, our interpenetrating pn heterostructures presented here may also be promising for use in thin-film solar cells, electrochromic displays,22 electrophotographic photoreceptors,23 and so forth. Studies to characterize their electronic and electrochemical properties are now under way, the details of which will be reported in a separate paper. IV. Conclusions The films of phthalocyanines and a perylene derivative (PTCD) were prepared on an indium-tin oxide coated glass (ITO) by the photocatalytic deposition method, and (22) See for example: Rosseinsky, D. R.; Mortimer, R. J. Adv. Mater. 2001, 13, 783. (23) Weigl, J. W. Angew. Chem., Int. Ed. Engl. 1977, 16, 374.

Interpenetrating Organic Pigment Heterostructures

the dependence of film thickness or coverage on the deposition time was investigated and analyzed on the basis of a diffusion equation (section III.A). The analyses revealed that the deposition is controlled by the diffusion of pigment particles from the dispersion bulk to the ITO, that the density (or porosity) is kept constant during the deposition, and that smaller particles in the dispersion contribute to a greater extent to the film formation. These findings led us to prepare sequentially deposited (section III.B) and co-deposited composite films using an  form of copper phthalocyanine and PTCD (section III.C). Their depth profiles demonstrated that graded and homoge-

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neously blended structures were formed in the former and the latter films, respectively. Acknowledgment. The authors thank K. Tanaka and K. Yamazaki of Otsuka Electronics Co., Ltd., for the particle analysis data and Professor T. Yamaoka of Chiba University for the measurement of film thicknesses. Supporting Information Available: Film formation mechanism of the PCD method. This material is available free of charge via the Internet at http://pubs.acs.org. LA0206054