Photocurrent Generation in Heterostructured Ultrathin Films

Langmuir , 2005, 21 (4), pp 1584–1589. DOI: 10.1021/la047989d. Publication Date (Web): January 19, 2005. Copyright © 2005 American Chemical Society...
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Langmuir 2005, 21, 1584-1589

Photocurrent Generation in Heterostructured Ultrathin Films Fabricated by Layer-by-Layer Deposition of Polyelectrolytes Bearing Tris(2,2′-bipyridine)ruthenium(II) and Ferrocene Moieties Toshiki Fushimi, Akimichi Oda, Hideo Ohkita, and Shinzaburo Ito* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan Received August 11, 2004. In Final Form: November 15, 2004 The photoelectrochemical properties of single-component and heterostructured layer-by-layer deposited films bearing tris(2,2′-bipyridine)ruthenium(II) (Ru) moieties were investigated by photocurrent measurements in solutions in the presence of sacrificial reagents. The photocurrent increased with an increase in the thickness of the films and then had a maximum at a thickness of 10 nm. This increase demonstrates a light-harvesting effect based on excitation energy migration among the Ru moieties to the film/electrolyte interface. A cathodic photocurrent was observed for a heterostructured film where bilayers bearing ferrocene (Fc) moieties and bilayers bearing Ru moieties were deposited on an indium tin oxide (ITO) substrate in the order (ITO/Fc/Ru). On the other hand, an anodic photocurrent was observed for the reverse order film (ITO/Ru/Fc). These results show that the direction of the photocurrent is determined by the gradient of the redox potentials formed in the heterostructured films. The internal quantum efficiency for the ITO/ Ru/Fc film was twice that for the single-component film (ITO/Ru). This enhancement of the quantum efficiency is due to suppression of charge recombination by successive electron transfers in the heterostructured film.

Introduction Conversion of photon energy to a current is an important subject not only in the field of basic research but also in the field of application.1-4 Absorption of a photon, excitation energy transport to a charge separation site (lightharvesting), and multistep bipolar charge transport to a cathode or anode are primary processes in the photoncurrent conversion. In particular, the transport of charges generated by photoinduced electron transfer is an important process. Most of the electrons and the holes would recombine before they reach the electrodes when the hole and electron mobility in the film is considerably low. Even if some charges survive, they would not result in a current flow without unidirectional charge transport. One method for realizing unidirectional charge transport is successive electron transfer along a gradient of potential. Since the transfer of excitons5-10 and electrons11-14 takes place on a scale of nanometers, the arrangement of molecules with * Corresponding author. E-mail: [email protected]. Fax: +81-75-383-2617. (1) Kuhn, H. J. Photochem. 1979, 10, 111-132. (2) Mo¨bius, D. Acc. Chem. Res. 1981, 14, 63-68. (3) Nelson, J. Curr. Opin. Solid Mater. Sci. 2002, 6, 87-95. (4) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 7, 3693-3723. (5) Fo¨rster, Th. Ann. Phys. 1948, 2, 55. (6) Ohmori, S.; Ito, S.; Yamamoto, M. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 815-824. (7) Ohmori, S.; Ito, S.; Yamamoto, M. Macromolecules 1991, 24, 23772384. (8) Dexter, D. L. J. Chem. Phys. 1953, 21, 836-850. (9) Hisada, K.; Ito, S.; Yamamoto, M. Langmuir 1996, 12, 36823687. (10) Hisada, K.; Ito, S.; Yamamoto, M. J. Phys. Chem. B 1997, 101, 6827-6833. (11) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265322. (12) Ohkita, H.; Ishii, H.; Ito, S.; Yamamoto, M. Chem. Lett. 2000, 1092-1093. (13) Ohkita, H.; Ishii, H.; Ogi, T.; Ito, S.; Yamamoto, M. Radiat. Phys. Chem. 2001, 60, 427-432.

nanometer-scale precision is essential for constructing an artificial system for the photon-current conversion. Heterostructured ultrathin film is a system in which molecules are arranged on a scale of nanometers in the direction normal to the film plane. Many studies have been devoted to unidirectional electron transfer and photocurrent response in heterostructured films prepared by various methods.1,2,15-19 Murray et al. reported a rectifying effect on a Faraday current in heterostructured polymer films prepared by sequential electropolymerization of vinyl monomers bearing redox groups.15 Miyashita et al. reported directional control of photocurrent in heterostructured Langmuir-Blodgett (LB) films bearing tris(bipyridine)ruthenium(II) (Ru) moieties and ferrocene (Fc) moieties.17 These studies demonstrate that heterostructured ultrathin films made of polymers are attractive systems for the energy conversion. The layer-by-layer deposition technique has attracted a great deal of attention as an alternative method for fabricating ultrathin polymer films.20-32 This method is (14) Ohkita, H.; Ogi, T.; Kinoshita, R.; Ito, S.; Yamamoto, M. Polymer 2002, 43, 3571-3577. (15) Abrun˜a, H. D.; Denisevich, P.; Uman˜a, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 1-5. (16) Aoki, A.; Miyashita, T. J. Electroanal. Chem. 1999, 473, 125131. (17) Aoki, A.; Abe. Y.; Miyashita, T. Langmuir 1999, 15, 1463-1469. (18) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129-9139. (19) Xue, J.; Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004, 84, 3013-3015. (20) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321-327. (21) Decher, G. Science 1997, 277, 1232-1237. (22) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1998, 14, 2768-2773. (23) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Thin Solid Films 1998, 333, 5-8. (24) Shimazaki, Y.; Ito, S.; Tsutsumi, N. Langmuir 2000, 16, 94789482.

10.1021/la047989d CCC: $30.25 © 2005 American Chemical Society Published on Web 01/19/2005

Photocurrent Generation in Layer-by-Layer Films

extremely versatile compared with the LB method, because a variety of materials are available for the fabrication of nanostructured multilayer films (polyelectrolytes and charged nanoparticles such as molecular aggregates, clusters, or colloids).30-32 Owing to the versatility, layer-by-layer films can be fabricated not only on charged flat substrates but also on charged particles.21,31 Recently, a roll-to-roll processing has been developed.33 On porous substrates made of charged particles, the amount of deposited material will greatly increase, leading to enhancement of the optical density of sensitizing dye molecules in films. Thus, layer-by-layer films have the potential for an efficient energy conversion system. Previously, we fabricated heterostructured layer-bylayer films bearing tris(bipyridine)ruthenium(II) (Ru) moieties and Fc moieties.29 Tris(bipyridine)ruthenium(II) is an attractive dye because of its strong absorption in the visible region and remarkable photochemical stability.34 We demonstrated efficient triplet energy migration among the Ru moieties in the fabricated films. The triplet excitation energy efficiently migrates to an interface between the Ru and Fc layers followed by charge separation.29 In other words, the layer-by-layer films bearing the Ru moieties have the two advantages of strong absorption in the visible region and efficient energy transport, which are required for a light-harvesting film. On the other hand, Fc is a stable redox group known by its quasi-reversible oxidation of iron(II), fairly high stability under illumination, and reductive quenching of excited states.25,29,35 In the present study, we discuss the photocurrent response in the layer-by-layer films bearing the Ru and Fc moieties on the basis of our previous studies. Experimental Section Materials. The synthesis of the polycations, P(CM-Rux) and P(CM-Fcy), used in this study has been described elsewhere.25,29 Chart 1 shows the chemical structures. The molar fractions of the Ru moiety (x) and the Fc moiety (y) in the copolymers were evaluated from UV-visible absorption measurements: the value of x was 4 or 18%, and the value of y was 21%. Poly(acrylic acid) (PAA, Mw ) ∼100 000, Aldrich) was purchased and was used as received. Potassium chloride (99.9%, Wako Pure Chemical Industries, Ltd.), methyl viologen dichloride hydrate (MVCl2, Aldrich), and L(+)-ascorbic acid (AsA, guaranteed reagent, Nacalai Tesque, Inc.) were purchased and were used without further purification. Water was purified by deionization, distillation, and then by passing it through a filtering system (Barnstead Nanopure II). Slide glasses coated with indium tin oxide (ITO) (10 Ω per square) were used as conductive substrates. The ITO substrates were washed by ultrasonication in toluene, acetone, and ethanol for 15 min each in this order and were then boiled in a mixture of 25% aqueous ammonia, 30% aqueous H2O2, and water (v/v 1:1:5) for 20 min. (25) Fushimi, T.; Oda, A.; Ohkita, H.; Ito, S. Thin Solid Films, submitted for publication. (26) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 71077114. (27) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552-1557. (28) Baur, J. W.; Rubner, M. F.; Reynolds, J. R.; Kim, S. Langmuir 1999, 15, 6460-6469. (29) Fushimi, T.; Oda, A.; Ohkita, H.; Ito, S. J. Phys. Chem. B 2004, 108, 18897-18902. (30) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065-13069. (31) Kaschak, D. M.; Johnson, S. A.; Waraksa, C. C.; Pogue, J.; Mallouk, T. E. Coord. Chem. Rev. 1999, 185-186, 403-416. (32) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (33) Hammond, P. T. Adv. Mater. 2004, 16, 1271-1293. (34) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85-277. (35) Fery-Forgues, S.; Delavaux-Nicot, B. J. Photochem. Photobiol., A 2000, 132, 137-159.

Langmuir, Vol. 21, No. 4, 2005 1585 Chart 1. Chemical Structures of P(CM-Rux) and P(CM-Fc21)

Preparation of Layer-by-Layer Films. Layer-by-layer films were prepared by repetitive cycles of immersion of the substrates into solutions in the following order: an aqueous solution of P(CMRux) or P(CM-Fc21) with a concentration of 10 mM for 5 min, water for 3 min, an aqueous solution of PAA with a concentration of 10 mM for 5 min, and water for 3 min. The concentrations are based on the molecular weight of their repeated units. The PAA solution was adjusted to a pH of 6.5 with sodium hydroxide. The substrates were dried in the air for 5 min after each immersion under a relative humidity of 50-60%. This drying process was omitted in the preparation of single-component layer-by-layer films consisting of P(CM-Ru18) and PAA in order to control the thickness finely. The characterization of the layer-by-layer deposited films has been described elsewhere.25,29 Hereafter, we will use the abbreviation “Fc21-4/Rux-n”, which refers to a heterostructured layer-by-layer film prepared by sequential deposition of four bilayers of P(CM-Fc21)/PAA and then n bilayers of P(CM-Rux)/PAA. The thickness of the Ru layers was evaluated from their absorption at 290 nm. Measurements. Absorption spectra of the films were measured with a spectrophotometer (Hitachi, U-3500) at room temperature. The photocurrent of the films was measured with a potentiostat (Perkin-Elmer, 273A), a three-electrode electrochemical cell with a window for mounting an ITO electrode (Perkin-Elmer, Flat Cell K0235), and a 500 W Xe lamp (Oriel) with a monochromator (Oriel, Cornerstone 130). The counter electrode was a Pt net, the reference electrode was an Ag/AgCl electrode in a saturated KCl aqueous solution, and the working electrode was an ITO electrode mounted on the window with an area of 0.95 cm2. The potential of the working electrode was kept at 0.0 V versus the reference electrode. The electrolyte solution was a 0.2 M KCl aqueous solution. Before the measurement, dissolved oxygen in the solutions was removed by bubbling of nitrogen gas for 30 min. Two optical filters, L-38 and IRQ-80, were used for cutting off second-order diffracted light and stray light at wavelengths shorter than 380 nm and longer than 800 nm. The wavelength and the power of the irradiation light were 450 nm and 12 mW cm-2, respectively, if not otherwise specified.

Results and Discussion Characterization of Layer-by-Layer Films Bearing the Ru Moieties. First, we will discuss the electrochemical and photophysical properties of the singlecomponent films bearing Ru moieties. The electrochemical properties were investigated by cyclic voltammetry. Figure 1 shows the cyclic voltammograms for Ru4-n films (n ) 4, 8, 12, or 16) at a scan rate of 50 mV s-1. An anodic current peak at 1.15 V and a cathodic current peak at 1.10 V were ascribed to the oxidation from Ru2+ to Ru3+ and the

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Figure 1. Cyclic voltammograms for Ru4-n (n ) 4, 8, 12, or 16) films at a scan rate of 50 mV s-1.

Figure 2. Surface concentration of the Ru moitey in Ru4-n (n ) 4, 8, 12, or 16) films. The circles and the solid line stand for the values of the surface concentration evaluated from the cyclic voltammograms of the films on ITO substrates and absorption spectra of the films on quartz substrates, respectively.

reduction from Ru3+ to Ru2+, respectively.16,34 The peak current density increased linearly as the number of bilayers increased. This result indicates that Ru moieties separated from the electrode can participate in the electrode reaction via electron transport in the films. The mechanism of the electron transport is not translational diffusion of the Ru moieties but probably electron hopping among the Ru moieties in the films because the Ru moieties are attached to the polymer chain with a covalent bond.36 The surface concentration of the electrochemically active Ru moiety was evaluated from an integral of the anodic current in the cyclic voltammograms. Figure 2 shows the surface concentration of the Ru moiety in Ru4-n (n ) 4, 8, 12, or 16) films. The open circles and the solid line stand for the values of the surface concentration evaluated from the cyclic voltammograms of the films on ITO substrates and from UV-visible absorption spectra of the films on quartz substrates, respectively. The values evaluated from the cyclic voltammograms agreed well with those from the absorption spectra. Since the thickness per bilayer is independent of substrates after a few cycles of alternate deposition,32 this agreement shows that almost all of the Ru moieties in the films are electrochemically active. In other words, even holes generated at the interface of the film/electrolyte were transported to the other interface of the electrode/film by the electron hopping. The photophysical properties of the films were investigated by absorption spectroscopy. Figure 3 shows the absorption spectra of Ru18-n films on quartz substrates. The absorption bands at 290 and 460 nm are ascribable to the ligand centered π f π* transition and the metalto-ligand charge transfer (MLCT) d f π* transition of the Ru moiety, respectively.34 The absorbance at 290 and 460 nm increased linearly as the number of bilayers increased. The thickness of the films was determined from the (36) Blauch, D. N.; Save´ant, J.-M. J. Am. Chem. Soc. 1992, 114, 33233332.

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Figure 3. Absorption spectra for Ru18-n (n ) 2, 4, 6, 8, or 10) films on quartz substrates. The inset shows the relationship between the number of bilayers and the thickness evaluated from the absorbance at 290 nm.

Figure 4. Photocurrent response in a Ru18-12 film in 0.2 M aqueous solution with (solid line) and without (dashed line) 40 mM MV2+.

absorbance at 290 nm, by using a molar absorption coefficient of 8.07 × 104 M-1 cm-1, a density of 1.36 g cm-3 for the bilayer, and a Ru molar fraction of 0.18. The inset of Figure 3 shows the relationship of the number of bilayers and the thickness. The thickness per bilayer was evaluated to be ∼2 nm from the slope in the inset. An atomic force microscopy (AFM) image of a Ru18-3 film on a quartz substrate (not shown) showed that the quartz surface was completely covered by the film. The surface morphology of the film was uniform and pinhole-free. Thus, the increase in the Faraday current with an increase in the number of bilayers is not due to an increase in surface coverage but is due to the increase in the number of electrochemically active Ru moieties. On the basis of the cyclic voltammograms, the absorption spectra, and the AFM image, we conclude that the thickness of Ru18-n films on ITO substrates is the same as that on quartz substrates. Photocurrent Generation in the Ru Films. The solid and dashed lines in Figure 4 show the photocurrent response in a Ru18-12 film with and without 40 mM MV2+ as a sacrificial electron acceptor, respectively. A steady cathodic photocurrent was observed for the Ru18-12 film with 40 mM MV2+, whereas only a small photocurrent was observed for the Ru18-12 film without MV2+ after a spikelike transient current. The difference in the photocurrent shows that the electron transfer from the excited species to MV2+ is essential for the photocurrent generation. The small photocurrent of the Ru18-12 film without MV2+ may be due to some electron acceptors in the electrolyte solution such as impurity or residual oxygen. The spikelike transient current may be attributed to a capacitive current based on the change in the dielectric constant of the film due to the formation of the excited state.17 The action spectrum of the Ru18-12 film with 40 mM MV2+ is shown in Figure 5. The closed circles and the solid curve stand for the photocurrent density in the Ru1812 film on an ITO substrate and the absorption spectrum for a Ru18-10 film on a quartz substrate. The current density was normalized by the number of incident photons

Photocurrent Generation in Layer-by-Layer Films

Langmuir, Vol. 21, No. 4, 2005 1587 Scheme 1. Redox Potentials and Interfacial Electron Transfers in the ITO/Ru/MV2+ System

Figure 5. Action spectrum for a Ru18-12 film with 40 mM MV2+. The closed circles and the solid line stand for the experimental data and the absorption spectrum of a Ru18-10 film on a quartz substrate. The current density was normalized by the number of incidental photons at 440 nm, where the power of the excitation light was 10 mW cm-2.

at 440 nm, where the power of the excitation light was 10 mW cm-2. The action spectrum agreed well with the absorption spectrum of the Ru moiety over the wavelength range from 400 to 560 nm. As described above, this absorption is ascribed to the MLCT excitation of the Ru moiety. Therefore, we can safely conclude that the photocurrent is generated by the MLCT excitation of the Ru moiety in the film. Excitation of Ru(bpy)32+ molecules leads to the formation of the lowest excited triplet state 3 [Ru(bpy)32+]* after very fast intersystem crossing from the lowest excited singlet state 1[Ru(bpy)32+]* with a short lifetime of 10 nm. The excitation energy formed in the films needs to migrate to the film/electrolyte interface where Ru3+ and MV+ are formed through the electron transfer from 3(Ru2+)* to MV2+. At the same time, the hole on the Ru3+ moiety needs to migrate to the ITO electrode to generate the photocurrent. In other words, the excitation energy migration as well as the hole transport in the films should play important roles in the mechanism for the photocurrent generation. The increase in the photocurrent density is probably due to the effect of excitation energy migration to the film/electrolyte (37) Bhasikuttan, A. C.; Suzuki, M.; Nakashima, S.; Okada, T. J. Am. Chem. Soc. 2002, 124, 8398-8405.

interface. The triplet excitation energy has been reported to migrate among Ru(bpy)32+ moieties and its derivatives by the Dexter mechanism.38,39 We previously evaluated the root-mean-square distance of one-dimensional diffusion of the 3(Ru2+)* excitation energy in Ru18-n films to be as long as 36 nm from phosphorescence spectroscopy.29 The distance is long enough compared with the thickness of the films examined in this study. Thus, the number of Ru moieties in the excited state, which reach the interface and generate the photocurrent, increases with an increase in the thickness of the films. The enhancement of the photocurrent in the multilayered films demonstrates a light-harvesting effect due to the energy migration in the films. Since the thicknesses examined in Figure 6 were smaller than the diffusion distance, we conclude that the decrease in the photocurrent in the film with d > 10 nm is mainly due to inefficient hole migration through the film, which results in an increase in the recombination of Ru3+ with MV+. From the Marcus theory, we can deduce that the hole transport in the films is slower than the energy migration because the reorganization energy of charged species is larger than that of neutral species.11 We will discuss the directional control of the photocurrent by the structure of films in the next section.

Figure 6. Dependence of the photocurrent density on the thickness of the Ru18-n (n ) 3, 6, 9, or 12) films.

Photocurrent Generation in Heterostructured Films Bearing the Ru and Fc Moieties. As mentioned in the Introduction, layer-by-layer deposition is a promising method for the fabrication of heterostructured ultrathin films, leading to directional electron transfer. We investigated the photocurrent response in two kinds of heterostructured films, a Ru18-4/Fc21-4 film (ITO/Ru/Fc) (38) Ikeda, N.; Yoshimura, A.; Tsushima, M.; Ohno, T. J. Phys. Chem. A 2000, 104, 6158-6164. (39) Fleming, C. N.; Maxwell, K. A.; DeSimone, J. M.; Meyer, T. J.; Papanikolas, J. M. J. Am. Chem. Soc. 2001, 123, 10336-10347.

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Figure 7. Photocurrent response in an ITO/Fc/Ru film (solid line) and an ITO/Ru/Fc film (dashed line) in 0.2 M KCl aqueous solution with 40 mM MV2+ as a sacrificial acceptor. Scheme 2. Redox Potentials and Interfacial Electron Transfers in (a) the ITO/Fc/Ru/MV2+ System and (b) the ITO/Ru/Fc/MV2+ System

and a Fc21-4/Ru18-4 film (ITO/Fc/Ru). The thickness per bilayer for the Ru and Fc layers is 5.9 and 1.3 nm, respectively.25,29 The solid and dashed lines in Figure 7 show the photocurrent response in an ITO/Ru/Fc film and an ITO/Fc/Ru film, respectively, in 0.2 M KCl aqueous solution with 40 mM MV2+ as a sacrificial electron acceptor. A steady cathodic photocurrent was observed for the ITO/Fc/Ru/MV2+ system, whereas only a small photocurrent was observed for the ITO/Ru/Fc/MV2+ system except for a spikelike transient current. The difference in the photocurrent generation clearly shows that photocurrent generation depends on the layered structure of the films. The redox potential of Fc/Fc+ in the Fc layer was evaluated to be ∼0.3 V versus Ag/AgCl in our previous study.25 The redox potentials in the ITO/Fc/Ru/MV2+ system and the ITO/Ru/Fc/MV2+ system are shown in parts a and b of Scheme 2, respectively. In the ITO/Fc/Ru/MV2+ system, the potential gradient is designed to generate a cathodic photocurrent: the energy level drops in the order of ITO (0 V) to Fc/Fc+ (0.3 V) to Ru2+/Ru3+ (1.1 V) and also from 3(Ru2+)*/Ru3+ (-1.0 V) to MV+/MV2+ (-0.6 V).18 In accordance with the potential gradient, the photocurrent is mainly generated as follows: (1) a 3(Ru2+)* moiety generated in the Ru layer donates an electron to a MV2+ molecule at the interface of Ru/MV2+ and accepts an electron from a Fc moiety in the Fc layer at the interface of Fc/Ru, (2) the Ru3+ moiety thus formed accepts an electron from a Fc moiety at the interface of Fc/Ru, while the Ru+ moiety thus formed donates an electron to a MV2+ molecule at the interface of Ru/MV2+, and (3) the Fc+ moiety is reduced to a Fc moiety by an electron from the ITO electrode. Here, the inner Fc layer acts as an electronic mediator because of the potential gradient. In the ITO/ Ru/Fc/MV2+ system, on the other hand, there is no

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Figure 8. Photocurrent response in an ITO/Fc/Ru film (dashed line), an ITO/Ru/Fc film (solid line), and an ITO/Ru film (dotted line) in 0.2 M KCl aqueous solution with 1 mM AsA as a sacrificial donor. The pH of the solution was adjusted to 6.4 by adding sodium hydroxide. Scheme 3. Redox Potentials and Interfacial Electron Transfers in (a) the ITO/Fc/Ru/AsA System and (b) the ITO/Ru/Fc/AsA System

potential gradient to generate an anodic or cathodic photocurrent. A 3(Ru2+)* moiety generated in the inner Ru layer cannot encounter a MV2+ molecule outside of the film, owing to the outer Fc layer, which acts as an insulator because the redox potential of Fc/Fc+ is lower than that of MV+/MV2+. Next, we investigated the anodic photocurrent generation in the heterostructured films by using a sacrificial electron donor. The dashed, solid, and dotted lines in Figure 8 show the photocurrent response in an ITO/Ru/ Fc film, an ITO/Fc/Ru film, and a Ru18-4 (ITO/Ru) film, respectively, in a 0.2 M KCl aqueous solution with 1 mM AsA: the Ru layer of the Ru18-4 film has a thickness similar to those of the heterostructured films. The electrolyte solution was adjusted to a pH of 6.4 with sodium hydroxide. At pH 6.4, AsA acts as a sacrificial electron donor for Fc+ because a portion of AsA is oxidized at 0.3 V versus Ag/ AgCl, which is the redox potential of Fc/Fc+. Here, we compare the photocurrent response in the ITO/Ru/Fc and ITO/Fc/Ru films. Only a small anodic photocurrent was observed for the ITO/Fc/Ru/AsA system. On the other hand, a steady anodic photocurrent was observed for the ITO/Ru/Fc/AsA system. This dependence of the anodic photocurrent generation on the layered structures also can be explained by the potential gradient designed in the systems, as shown in Scheme 3. In the ITO/Fc/Ru/ AsA system, as shown in Scheme 3a, there is no potential gradient to generate an anodic photocurrent. A 3(Ru2+)* moiety in the excited state cannot donate an electron to the ITO electrode because the inner Fc layer acts as an insulator. On the other hand, as shown in Scheme 3b, the potential gradient in the ITO/Ru/Fc/AsA system is de-

Photocurrent Generation in Layer-by-Layer Films

signed to generate an anodic photocurrent. A 3(Ru2+)* moiety in the excited state accepts an electron from a Fc moiety at the interface of Ru/Fc to form a Ru+ moiety and a Fc+ moiety. The Ru+ moiety thus formed donates an electron to the ITO electrode. The Fc+ moiety is reduced to the Fc moiety by an AsA molecule. Here, the outer Fc layer acts as an electronic mediator to transport electrons from AsA to the inner Ru layer. Note that the ITO/Fc/Ru film generates only a cathodic photocurrent, whereas the ITO/Ru/Fc film generates only an anodic photocurrent. These results demonstrate that heterostructured films fabricated by the layer-by-layer deposition technique are useful systems for controlling directional electron transfer. Since unidirectional and successive electron transfers along the potential gradients were achieved in the heterostructured films, higher quantum efficiencies are expected in the heterostructured films than in the singlecomponent films. The photocurrent density in the ITO/ Ru/Fc/AsA system shown by the solid line in Figure 8 is almost 2-fold that in the ITO/Ru/AsA system shown by the dotted line in Figure 8. The internal quantum efficiency in the photocurrent generation at 450 nm was 0.13% for the ITO/Ru/Fc/AsA system and 0.07% for the ITO/Ru/ AsA system. This higher efficiency of the ITO/Ru/Fc/AsA system than that of ITO/Ru/AsA system is attributable to the successive electron transfers owing to the potential gradient designed in the system, which suppresses charge recombination. In the ITO/Ru/AsA system, the Ru+ moiety formed by the electron transfer from an AsA molecule is easily affected by the recombination by the back electron transfer. In the ITO/Ru/Fc system, on the other hand, the Ru+ moiety can escape from the back electron transfer with the Fc+ moiety because of the subsequent electron transfer from an AsA molecule to the Fc+ moiety. The fabricated heterostructured film is useful for the spatial

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separation of a pair of hole and electron generated, which suppresses the charge recombination and consequently provides an efficient photocurrent. Conclusions The layer-by-layer deposited films bearing the chromophoric Ru moieties and the electron donating Fc moieties were fabricated onto ITO electrodes. The photocurrent density increased as the thickness of the Ru films increased up to 10 nm, owing to the light-harvesting effect based on the exciton migration. Directional charge transport was facilitated in the heterostructured films. Only a cathodic photocurrent was observed in the ITO/ Fc/Ru film where a gradient of potential for the cathodic photocurrent was built up in the sequence of the Fc and Ru layers. On the other hand, only an anodic photocurrent was observed in the ITO/Ru/Fc film where a gradient of potential for the anodic current was built in the reverse order. The quantum efficiency of the heterostructured film was 2-fold that of the single-component film. This improvement is due to suppression of charge recombination by the unidirectional and successive electron transfers in the heterostructured films. These results demonstrate that heterostructured layer-by-layer films are attractive systems for the unidirectional transfer of charges, which leads to effective photon-current energy conversion. Acknowledgment. This work was partly supported by the Integrative Industry-Academia Partnership (IIAP) including Kyoto University, Nippon Telegraph and Telephone Corporation, Pioneer Corporation, Hitachi, Ltd., Mitsubishi Chemical Corporation, and Rohm Co., Ltd and by a 21st century COE program, COE for a United Approach to New Materials Science. LA047989D