Quantum-Dot-Sensitized Solar Cell Using a Photoanode Prepared by

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J. Phys. Chem. C 2010, 114, 16837–16842

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Quantum-Dot-Sensitized Solar Cell Using a Photoanode Prepared by in Situ Photodeposition of CdS on Nanocrystalline TiO2 Films Yasuaki Jin-nouchi,† Shin-ichi Naya,‡ and Hiroaki Tada*,†,‡ Department of Applied Chemistry, School of Science and Engineering and EnVironmental Research Laboratory, Kinki UniVersity, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan ReceiVed: July 6, 2010; ReVised Manuscript ReceiVed: August 16, 2010

CdS quantum dots (QDs) have been incorporated into mesoporous TiO2 nanocrystalline films by a photodeposition (PD) technique we have recently developed [CdS(PD)/mp-TiO2], and for comparison, the conventional successive ionic layer adsorption and reaction (SILAR) and self-assembled monolayer (SAM) methods have also been used for preparing the coupling system. The most important characterstic of the PD technique is that the efficicent interfacial charge transfer between the semiconductors is guaranteed because the photocatalytic redox property of TiO2 is taken advatage of to form the heteronanojunction. The N2 adsorption-desorption data analysis by the Barret-Joyner-Halenda method and the elemental depth profile by electron probe microanalysis showed that CdS QDs are distributed in the mesopores of the film without pore-blocking in the PD sample and with partial pore-blocking in the SILAR sample, whereas only the upper part of the film is covered with CdS QDs in the SAM sample. The PD technique enables one to control the loading amount and particle size of CdS QDs by UV-light irradiation time (λ > 320 nm) with excellent reproducibility. Owing to these unique features, sandwich-type solar cells using the CdS(PD)/mpTiO2(photoanode showed a power conversion efficiency (η) under simulated sunlight (AM 1.5, 100 mW cm-2) of up to 2.51% more than those for the cells employing CdS(SILAR)/mp-TiO2 (η ) 1.21%) and CdS(SAM)/mp-TiO2 (η ) 0.14%). I. Introduction

SCHEME 1: Essential Action Principle of QD-SSC

For about 2 decades, cost-performance compatible dyesensitized solar cells (DSSCs) have attracted a great deal of attention as a sustainable energy source for the next generation.1 Energy conversion efficiencies over 11% have so far been achieved for a DSSC using a Ru(II) complex monolayer coated mesoporous TiO2 nanocrystalline film photoanode (mp-TiO2).2 Also, the development of quantum-dot-sensitized solar cells (QD-SSCs) using narrow gap semiconductor QDs in the place of organic dyes as a photosensitizer is rapidly in progress.3-5 The action principle of the QD-SSC is depicted in Scheme 1 involving key processes p1-p7: the increase and decrease in the efficiency of each process are denoted by the up (v) and down (V) arrows, respectively. The most important advantage of inorganic semiconductor QDs over organic dyes is that the band gap can be tuned only by changing the particle size because of size quantization enabling one to optimize the electron- and hole-injection efficiencies (p3v and p4v, respectively).6 In addition, QDs have possibilities to generate multiple excitons with a single photon by the impact ionization effect7,8 and to enhance light absorption (p1v) and charge separation (p2V) resulting from the large dipole moment.9-13 Recently, the performance and stability of CdS QD-SSC have been greatly improved by using a polysulfide redox pair (Sx2-/S2-), because of its excellent holecapturing ability (p4v),14 and further fluorine-doped SnO2 (FTO) with Au nanoparticles (NPs) as a counter electrode, owing to the high electrocatalytic activity of Au NPs for the sulfur reduction (p7v).15 The coming major challenges of QD-SSC are * To whom correspondence should be addressed. Phone: +81-6-67212332. Fax: +81-6-6727-2024. E-mail: [email protected]. † Department of Applied Chemistry. ‡ Environmental Research Laboratory.

increasing the loading amount of QD on mp-TiO2 to increase the light absorption (p1v) and improving the electron-collecting efficiency (p6v) by enhancing the visible light-induced interfacial electron transfer (IET) from QD (p3v) and suppressing the back electron transfer from TiO2 to the oxidant in the electrolyte solution (p5V). A self-assembled monolayer (SAM) technique using bifunctional coupling molecules such as mercaptoacetic acid is frequently used for the preparation of metal chalcogenide QD-loaded mp-TiO2 [QD(SAM)/mp-TiO2].16,17 Although this method allows us to precisely control the particle size, the loading amount of QDs is limited below monolayer coverage,

10.1021/jp1062226  2010 American Chemical Society Published on Web 09/01/2010

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and thus, the light absorption efficiency becomes low (p1V) and the back electron transfer is apt to occur (p5v). Also, the photocurrent of TiO2/FTO electrodes was reported to decrease significantly as a result of the ultrathin SiOx film coating on the TiO2 surface.18 This finding suggests that the insulating molecules intervening between QD and TiO2 at the junction formed by the SAM method retard the interfacial electron transfer (p3V).19 At present, successive ionic layer adsorption and reaction (SILAR) technique is believed to be the best way for growing QDs in the pores of mp-TiO2 to yield the direct coupling system [QD(SILAR)/mp-TiO2].10 However, the cycle of metal ion adsorption-rinse/drying and the subsequent anion adsorption-rinse/drying must be repeated several times to endow mpTiO2 with sufficient visible light absorption, and the rigorous control of each experimental procedure is indispensable for obtaining samples with reproducibility. On the other hand, we have recently found that the CdS-TiO2 heterojunction is formed by the photodeposition (PD) of CdS on TiO2 by UV-light irradiation to ethanol TiO2 suspensions containing Cd2+ ions and S8.20 This PD technique is unique in that the efficient IET between semiconductors is inherently guaranteed because the photocatalytic redox property of TiO2 is taken advantage of to form direct coupling systems (p3v) (feature I).21-28 Thus, the resulting CdS(PD)/mp-TiO2 is highly expected as the photoanode for the QD-SSC. Here we show the amount- and size-controlled incorporation of CdS QDs into the pores of mp-TiO2 by the PD technique. Sandwich-type QD-SSCs consisting of CdS(PD)/mp-TiO2 (photoelectrode)|electrolyte solution containing I3-/I- redox pairs|Au film (counter electrode) were constructed, and the cell performances were evaluated as functions of the amount and size of CdS QDs. The performances were compared with those for the cells employing CdS(SILAR)/mp-TiO2 and CdS(SAM)/ mp-TiO2 as photoanodes under optimum conditions. II. Experimental Section A. Preparation of mp-TiO2 Films. A paste containing anatase TiO2 particles with a mean size of 20 nm (PST-18NR, Nikki Syokubai Kasei) or 400 nm (PST-400C, Nikki Syokubai Kasei) was coated on FTO electrodes (12 Ω/0) by a squeegee method, and the sample was heated in air at 773 K for 1 h to form mp-TiO2 (20 nm)/FTO or mp-TiO2 (400 nm)/FTO electrode, which are designated as mp-TiO2 and mp-TiO2-L, respectively. Unless otherwise noted, mp-TiO2 was used as the substrate. B. Preparation of CdS(PD)/mp-TiO2. CdS QDs were deposited according to the PD technique previously reported.20 mp-TiO2/FTO electrode was immersed in ethanol solution (250 mL) containing S8 (1.72 × 10-4 mol dm-3) and Cd(ClO4)2 (2.76 × 10-4 to 1.38 × 10-2 mol dm-3), and the solution had been bubbled with argon for 0.5 h in the dark. Irradiation was carried out for a given period with a high-pressure mercury lamp at 298 K; the light intensity integrated from 320 to 400 nm (I320-400nm) was 3.7 mW cm-2. After irradiation, the resulting substrates were washed with ethanol three times to be dried. C. Preparation of CdS(SILAR)/mp-TiO2. An advanced SILAR method using ethanol as a solvent was used to prepare the direct coupling system.29 mp-TiO2/FTO electrode was immersed in a solution of Cd(ClO4)2 (5.0 × 10-2 mol dm-3) in ethanol (20 mL) at room temperature for 1 min, and then the electrode was washed with pure ethanol and dried in air. Subsequently, the electrode was immersed in a solution of Na2S (5.0 × 10-2 mol dm-3) in ethanol (20 mL) at room temperature for 1 min, and then the electrode was washed with pure ethanol

Jin-nouchi et al. and dried in air. Such an immersion cycle was repeated N times (N ) SILAR cycle number). D. Preparation of CdS(SAM)/mp-TiO2. The indirect coupling system was prepared according to the procedures reported by Hirai et al.30 To a solution of Na2S (1.00 × 10-5 mol dm-3) and mercaptoacetic acid (1.00 × 10-5 mol dm-3) in H2O (150 mL) was added a solution of Cd(ClO4)2 (3.46 × 10-3 mol dm-3) in H2O (150 mL) dropwise slowly, and the mixture stirred at room temperature for 20 min. A mp-TiO2/FTO electrode was immersed in the resulted solution (30 mL) for a given period. After adsorption, the resulting substrates were washed with distilled water three times to be dried. E. Characterization of the Photoanodes. The sample (2 cm2) was immersed in concentrated HCl (10 mL), and the deposits were thoroughly dissolved into the solution by stirring for 0.5 h. The solution was diluted three times in volume with water, and then the Cd concentration was determined by inductively coupled plasma (ICP) spectroscopy (ICPS-7500, Shimadzu). Electron probe microanalyses (EPMA) were performed using an EPMA-1610 (Shimadzu). Analyses were performed at 15 kV and 30 nA using the pure elements as standards. The specific surface area and the porosity were determined by nitrogen adsorption-desorption isotherms at 77 K with a micromeritics automatic surface area and porosimetry analyzer (TriStar 3000, Shimadzu). Prior to N2-sorption, all samples were degassed at 423 K for 1 h under vacuum. Pore size distributions were determined from the isotherms according to the Barret-Joyner-Halenda (BJH) methods. Diffuse reflectance UV-vis spectra of the resulting sample were recorded on a Hitachi U-4000 spectrometer mounted with an integrating sphere at room temperature. The reflectance (R∞) was recorded with respect to a reference of BaSO4, and the Kubelka-Munk function [F(R∞)] expressing the relative absorption coefficient was calculated by the equation F(R∞) ) (1 - R∞)2/2R∞. F. Incident Photon to Current Conversion Efficiency (IPCE) Measurements. The IPCE was measured for sandwichtype photoelectrochemical solar cells (CdS/mp-TiO2|SO32-/ S2-|Au film) fabricated as follows. Au thin films with a thickness of ca. 100 nm were formed on 20 nm Cr-undercoated nonalkaline glass plates (NA35, Nippon Sheet Glass) by vacuum deposition. The cell gap was controlled at ca. 60 µm and the active area of the cell was 1.76 cm2. The aqueous electrolyte solution of Na2S (0.1 mol dm-3), Na2SO3 (5.4 × 10-3 mol dm-3), and NaClO4 (0.1 mol dm-3) was used after argon bubbling to remove the oxygen present in the solution. The short-circuit current (Jsc/A cm-2) was measured using a potentio/ galvanostat (HZ-5000, Hokuto Denko) as a function of excitation wavelength (λ/m), and the IPCE was calculated using eq 1.

IPCE (%) ) (JscNAhc/IFλ) × 100

(1)

where NA is Avogadro constant, I (W cm-2) is light intensity, F is Faraday constant, h is Planck constant, and c is the speed of light. G. Power Conversion Efficiency Measurements. Photocurrent-voltage (J-V) curves were measured under illumination by a solar simulator (PEC-L10, Peccell technologies, Inc.) at one sun (AM 1.5, 100 mW cm-2) for the sandwich-type photoelectrochemical solar cells (photoanodes|3-methoxypropionitrile solution containing 0.1 mol dm-3 LiI, 0.05 mol dm-3 I2, 0.6 mol dm-3 1-propyl-2,3-dimethylimidazolium iodide, and 0.5 mol dm-3 4-tert-butylpyridine |Pt). CdS(PD)/mp-TiO2, CdS(SILAR)/mp-TiO2, and CdS(SAM)/mp-TiO2-L were used

Quantum-Dot-Sensitized Solar Cell

Figure 1. (A) Time courses for the CdS photodeposition on mp-TiO2 (a) ([Cd2+]0 ) 1.38 × 10-2 mol dm-3, [S8]0 ) 1.72 × 10-4 mol dm-3) and the adsorption of CdS QDs in the SAM process on mp-TiO2 (b) and mp-TiO2-L (c). (B) Plots of CdS loading amount on mp-TiO2 vs SILAR cycle number (N).

as the photoanodes. Prior to use, ZnS thin films were coated on the photoanodes by the following procedure:31 the electrode was immersed in a solution of Zn(ClO4)2 (5.0 × 10-2 mol dm-3) in water (20 mL) at room temperature for 1 min, and then the electrode was washed with water and dried in air. Subsequently, the electrode was immersed in a solution of Na2S (5.0 × 10-2 mol dm-3) in water (20 mL) at room temperature for 1 min, and then the electrode was washed with water and dried in air. The active area of the cell was 0.16 cm2. The potentio/ galvanostat (HZ-5000, Hokuto Denko) was used to record the J-V characteristics. III. Results and Discussion UV light irradiation of TiO2 in an ethanol solution containing Cd2+ ions and S8 leads to the photodeposition of CdS QDs on TiO2.20 The amounts of CdS deposited per unit apparent area of mp-TiO2 (x/µg cm-2) were determined by ICP spectroscopy as functions of irradiation time (tp) for the PD sample and adsorption time (ta) for the SAM sample. Figure 1A shows time courses for the CdS deposition on mp-TiO2 in the PD and SAM processes. In the PD process (a), the x almost linearly increases with increasing tp to reach 134.5 µg cm-2 at tp ) 3 h. In the SAM process (b), the x value is much smaller than that in the PD process. The diffusion of CdS QDs into the mesopores of mp-TiO2 would be difficult because the mean particle size of the CdS QDs (d ) 6.5 nm) is comparable to the pore size of mp-TiO2 (peak pore size ≈ 20 nm). The use of mp-TiO2-L as a substrate increases x; however, even in this case, it is only 19.2 µg cm-2 at ta ) 3 h (c). The CdS-PD was carried out under various [Cd2+]0 ranging from 2.76 × 10-4 to 1.38 × 10-2 mol dm-3 with [S8]0 maintained at 1.72 × 10-4 mol dm-3. In spite of the ratio being 50 times different, the deposition rate hardly changed. This fact indicates that the reaction rate is scarcely affected by small errors of [S8]0 and [Cd2+]0, and the good reproducibility in the PD technique is actually shown in Figure 1A(a). Figure 1B shows plots of x vs SILAR cycle number (N). The x increases with increasing N, reaching a constant value of ca. 150 µg cm-2 at N > 7. Although a large amount of CdS can be loaded on mp-TiO2 by the SILAR method, the x value at the same N is significantly scattering, as shown by the error bars. Evidently, the PD technique has the feature that a large CdS amount comparable with that in the SILAR method can be deposited on mp-TiO2 with excellent reproducibility (feature II). Pore size distributions were determined by BJH analyses for the N2 adsorption-desorption on/from mp-TiO2, CdS(PD)/mpTiO2, CdS(SILAR)/mp-TiO2, and CdS(SAM)/mp-TiO2. The pristine mp-TiO2 possessed a pore volume (Vp) of 0.72 cm3 g-1. Interestingly, in CdS(PD)/mp-TiO2, the Vp was almost main-

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16839 tained (∼0.72 cm3 g-1), whereas the values of CdS(SILAR)/ mp-TiO2 and CdS(SAM)/mp-TiO2 decreased to 0.20 and 0.25 cm3 g-1, respectively. Further, Cd- and S-elemental depth profilesofCdS(PD)/mp-TiO2,CdS(SILAR)/mp-TiO2,andCdS(SAM)/ mp-TiO2 were measured by EPMA (Figure 2). The crosssectional scanning electron microscopic image (left, upper) and the Si distribution (left, lower) show the mp-TiO2/FTO bilayer structure and the boundary between the FTO film and glass substrate, respectively. In CdS(PD)/mp-TiO2 and CdS(SILAR)/ mp-TiO2, both Cd and S are distributed widely toward the inner part of the mp-TiO2 film (a, b). On the other hand, in the CdS(SAM)/mp-TiO2, the CdS QD deposition on mp-TiO2 occurs only at the thin upper part of the film (c). In the QD-SSCs based on mp-TiO2, due to the low mobility of electrons injected from QDs,32 the back reaction of the electrons with the redox pair on the TiO2 surface lowers the efficiency.33 To achieve both the higher level of QD loading (p1v) and the shorter electrontransporting distance from TiO2 to FTO (p5V), the deposition of QDs in the inner surfaces of the mp-TiO2 film is necessary. Additionally, the permeability of the electrolyte solution into the mesopores is crucial for the QDs to be reduced back by the reductant in it (p4v). Consequently, an additional feature of the PD technique that CdS QDs can be deposited on not only the external surfaces but also the inner ones of mp-TiO2 without pore-blocking (feature III) should have a great impact on the performances of QD-SSCs. Figure 3 shows plots of the apparent water contact angle (θ) for the mp-TiO2 surface vs tp [λ > 300 nm, I320-400nm ) 2.0 mW cm-2 (open circle), 4.0 mW cm-2 (solid circle)]. The θ steeply decreases with increasing tp to reach 0 at tp > 5 min for I320-400nm ) 2.0 mW cm-2 and tp > 3 min for I320-400nm ) 4.0 mW cm-2. UV light irradition (I320-400nm ) 6.0 mW cm-2) of mp-TiO2 for 0.5 h prior to the CdS-PD increased the x value by ca. 30%. This increment would be caused by the photoinduced surface superhydrophilicity34 enhancing the penetration of the reaction solution into the mesopores. Thus, feature III can be attributed to the two photoinduced properties of TiO2, i.e., photocatalysis and surface superhydrophilicity. The CdS particle-size dependence of the optical properties of CdS/mp-TiO2 was studied by UV-vis spectroscopy. Figure 4A shows the absorption spectra of CdS(PD)/mp-TiO2 prepared by varying tp: F(R∞) expresses the Kubelka-Munk function. As tp increases, new absorption due to the CdS interband transition grows in the visible region at λ < 520 nm, and the blue-shift of the absorption edge is observed at tp < 3 h. A simiar trend is observed for CdS(SILAR)/mp-TiO2 (Figure 4B); i.e., the absorption edge red-shifts as N increases from 1 to 10. On the other hand, in the spectra of CdS(SAM)/mp-TiO2-L (Figure 4C), increasing ta intensifies the absorption in the visible region, while the absorption edge is invariant. When compared under the conditions examined (tp < 3 h, N < 10, and ta < 3 h), the order of the visible light absorption intensity is CdS(SAM)/ mp-TiO2-L , CdS(PD)/mp-TiO2 < CdS(SILAR)/mp-TiO2. The band gap (Eg) and mean size (d) of CdS were calculated by the Tauc plot35 and from the Brus equation (eq 2),36 respectively.

∆Eg ) (π2p2 /2R2)(me*-1 + mh*-1) - 1.8e2 /4πε0εR

(2) where ∆Eg is a shift with respect to the bulk Eg, R is the radius of CdS particle, me* ()0.19me0; me0 is electron mass) and mh* ()0.80me0) are the effective masses of electron and hole in CdS, respectively, ε0 is vacuum permittivity, and ε is the relative permittivity of CdS (5.7).37

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Jin-nouchi et al.

Figure 2. Elemental depth profiles by EPMA for CdS(PD)/mp-TiO2 with tp ) 3 h (a), CdS(SILAR)/mp-TiO2 with N ) 7 (b), and CdS(SAM)/ mp-TiO2 with ta ) 24 h (c).

Figure 3. Apparent water contact angle (θ) for the mp-TiO2 surface as a function of tp at I320-400nm ) 2.0 mW cm-2 (solid circle) and 4.0 mW cm-2 (open circle).

Figure 4D shows the Eg values for CdS(PD)/mp-TiO2 (a), CdS(SILAR)/mp-TiO2 (b), and CdS(MAA)/mp-TiO2-L (c). In CdS(PD)/mp-TiO2 and CdS(SILAR)/mp-TiO2, the increases in Eg from the bulk value (2.42 eV)38 with decreasing tp and N, respectively, provides evidence for the CdS quantum size effect at d < 7 nm. In CdS(SAM)/mp-TiO2-L, the Eg is almost constant at 2.7 ( 0.1 eV (d ≈ 5 nm) irrespective of ta, indicating no aggregation of prepared CdS QDs due to the function of MAA as a protective reagent. The d value is slightly smaller than that of the prepared CdS QD colloid (6.5 nm), suggesting the preferential penetration of smaller CdS QD into the mesopores. Clearly, the band energies of CdS QDs are widely tunable by tp in the PD technique (feature IV). To study the influence of the CdS-TiO2 heterojunction state on the cell performance, IPCEs of the photoanodes|0.1 mol dm-3 Na2S + 5.4 × 10-3 mol dm-3 Na2SO3 + 0.1 mol dm-3 NaClO4 (solvent ) water)|Au film type QD-SSCs were measured at λex ) 420 nm. Figure 5A shows IPCE (λex ) 420 nm) as a function of x: photoanode ) CdS(PD)/mp-TiO2 (a), CdS(SILAR)/mpTiO2 (b), and CdS(SAM)/mp-TiO2-L (c). The maximum IPCE is in the order of CdS(PD)/mp-TiO2 (85% at x ) 40 µg cm-2) > CdS(SILAR)/mp-TiO2 (73% at x ) 117 µg cm-2) . CdS(SAM)/mp-TiO2-L (6.3% at x ) 19 µg cm-2). At the same

Figure 4. UV-vis electronic absorption spectra of CdS(PD)/mp-TiO2 (A, tp ) 10 - 180 min), CdS(SILAR)/mp-TiO2 (B, N ) 1, 3, 5, 7, 10), and CdS(SAM)/mp-TiO2-L (C, ta ) 10 - 180 min): F(R∞) expresses the Kubelka-Munk function. (D) Eg values for CdS(PD)/mp-TiO2 (a), CdS(SILAR)/mp-TiO2 (b), and CdS(SAM)/mp-TiO2-L (c) as functions of tp, N, and ta, respectively.

x, the IPCE for CdS(SAM)/mp-TiO2-L is much smaller than those for CdS(PD)/mp-TiO2 and CdS(SILAR)/mp-TiO2. This difference between the indirect and direct coupling systems can be explained in terms of a barrier for the IET due to the insulating MAA molecules at the interface between CdS QD and TiO2 (p3V).18 The comparison between the direct coupling systems indicates that the IPCE of CdS(PD)/mp-TiO2 is greater than that of CdS(SILAR)/mp-TiO2 in spite of the smaller x. The absorbance of CdS/mp-TiO2 and thus the absorbed photon-tocurrent conversion efficiency (APCE) could not be determined because of the scattering character. However, since the Kubleka-Munk function that is proportional to the absorbance

Quantum-Dot-Sensitized Solar Cell

Figure 5. (A) Plots of IPCE (λex ) 420 nm) vs CdS loading amount for the cells using the following photoanodes: CdS(PD)/mp-TiO2 (a), CdS(SILAR)/mp-TiO2 (b), and CdS(SAM)/mp-TiO2-L (c). (B) Plots of IPCE (λex ) 420 nm) vs CdS mean particle size (d): CdS(PD)/mp-TiO2 (a) and CdS(SILAR)/mp-TiO2 (b). The inset shows the cb- and vb-edge positions with respect to the standard hydrogen electrode potential (SHE) calculated by the Brus equation (eq 2) as a function of d.

for CdS(SILAR)/mp-TiO2 is significantly greater than that for CdS(PD)/mp-TiO2 (Figure 4A,B), the APCE value for the latter must exceed that for the former. This superior performance of CdS(PD)/mp-TiO2 over CdS(SILAR)/mp-TiO2 probably results from the large contact area between CdS and the electrolyte solution owing to its nonpore-blocking character (p4v) (vide infra). The lowering in IPCE for CdS(PD)/mp-TiO2 at x < 40 µg and CdS(SILAR)/mp-TiO2 at x < 117 µg cm-2 can be attributed to the reduction in the light absorption due to the decrease in x and the blue-shift of the absorption edge (p1V). In the PD and SILAR processes, the values of d and x change at the same time as functions of tp and N, respectively. Figure 5B shows plots of IPCE (λex ) 420 nm) vs d: photoanode ) CdS(PD)/mp-TiO2 (a) and CdS(SILAR)/mp-TiO2 (b). In curves a and b, the IPCEs decrease at d > 4 and 7 nm, respectively. The inset shows the conduction band (cb)- and valence band (vb)-edge positions of CdS particle with respect to the standard hydrogen electrode potential (SHE) calculated by the Brus equation (eq 2) as a function of d ()2R).36 In these calculations, a value of -1.05 V (vs SHE) was used as the flatband potential for the bulk CdS.39 For comparison, the cb edge of bulk TiO2 at pH 13 (the pH of the electrolyte solution used) is also shown.40 On decreasing d, the cb(CdS) rises in the range of d below 7 nm with a steep increase at d j 4 nm, whereas the vb(CdS) change is much smaller. The cb-offset between CdS and TiO2 provides the driving force for the visible-light-induced IET from CdS to TiO2. Accordingly, the reductions in IPCE at d > 4 nm in curve a and d > 7 nm in curve b in Figure 5B can be caused by the decrease in the driving force for the IET (p3V). In a CdSe QD-SSC, a similar argument was made on the basis of the IET rate (ket) determined by photoluminescence lifetime measurements [ket (d ) 2.3 nm) ) 2.5 × 109 s-1, ket (d ) 3.7 nm) ) 0.63 × 109 s-1] for the particle size-dependence of IPCE, although the particle size range examined is narrow.41 Thus, the balance between the light absorption intensity and the driving force for the IET determines the optimum x or d values. Power conversions were evaluated for the sandwich-type QDSSCs consisting of the photoanodes|0.1 mol dm-3 LiI + 0.05 mol dm-3 I2 + 0.6 mol dm-3 1-propyl-2,3-dimethylimidazolium iodide +0.5 mol dm-3 4-tert-butylpyridine (solvent ) 3-methoxypropionitrile)|42Au film counter electrode. The open circuit voltage (Voc) is essentially determined by the difference between the cb-edge (TiO2) and the redox potential of the redox pairs in the electrolyte (∆). In order to improve the cell performances through the increase in Voc, the redox pair was changed from the sulfur system (∆ ≈ 0.44 V in alkaline solution at pH 13)40,43 in the IPCE measurements to the I3-/I- system

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Figure 6. Jph-V curves for the solar cells using CdS(PD)/mp-TiO2 with tp ) 3 h (a), CdS(SILAR)/mp-TiO2 with N ) 7 (b), and CdS(SAM)/mp-TiO2-L with ta ) 24 h (c) as the photoanodes.

TABLE 1: Cell Performances Obtained from the Jph-V Measurements architecture CdS(PD)/mp-TiO2 (tp ) 3 h) CdS(SILAR)/mp-TiO2 (N ) 7) CdS(SAM)/mp-TiO2-L (ta ) 24 h)

Jsc/mA cm-2

Voc/V

ff

η/%

6.53

0.69

0.56

2.51

2.73

0.70

0.64

1.21

0.49

0.64

0.46

0.14

(∆ ≈ 0.74 V in organic media)44,45 in this photovoltaic cell. Figure 6 compares the photocurrent-voltage (Jph-V) curves for the cells using CdS(PD)/mp-TiO2 (a), CdS(SILAR)/mp-TiO2 (b), and CdS(SAM)/mp-TiO2-L (c) as the photoanodes under illumination of one sun (AM 1.5, 100 mW cm-2). Note that the cell performances are compared under each optimum condition. The Voc, short circuit current (Jsc), fill factor (ff), and the power conversion efficiency (η) for the cells are summarized in Table 1. The order of the cell performances is in agreement with that of IPCE. Noticeably, a conversion efficiency as high as 2.51% is achived for the cell employing CdS(PD)/mp-TiO2 as a photoanode. The Voc value is comparable for each cell (0.64-0.70 V), which is near to the expected ∆ value. The Jsc for the CdS(SAM)/mp-TiO2-L photoanode is very low (0.49 mA cm-2), and the Jsc (2.73 mA cm-2) for the CdS(SILAR)/mp-TiO2 photoanode is below half of that for the CdS(PD)/mp-TiO2 photoanode (6.53 mA cm-2). The same discussion on the difference in the IPCEs between the photoanodes should be valid also in this case.45 The nonblocking property of CdS(PD)/mp-TiO2 deserves to be discussed in more detail. The CdS-PD proceeds in situ on the inner surfaces of mp-TiO2 via the preferential reduction of adsorbed Cd2+ ions (Cd2+ad) followed by the reaction of Cd0 with S (eq 3),20 where e-cb denotes the excited electrons in the cb(TiO2).

mp-TiO2, hν (λ > 320nm) 2+

Cd

ad

+ S + 2e-cb f Cd0 + S f CdS

(3)

First, TiO2 exhibits strong adsorptivity for Cd2+ ions, of which saturated adsorption amount reaches 2.4 ions nm-2.20 Second, it is apparent from Figure 4D that the rate of particle growth becomes small at tp > 1 h. Once CdS is formed on TiO2, both of them are photoexcited at the same time under the present conditions. The initial rapid growth of CdS may be ascribable for the slow IET from CdS to TiO2 in the Marcus inverted regime due to the small reorganization energy of such semiconductor heteronanostructures.46 When the CdS grows (d < ca. 7 nm), the IET from CdS to TiO2 should be very fast [ket ) (6 ( 4) × 1010 s-1]47 to cause the formation of new nuclei on

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the TiO2 surface rather than the particle growth. This event has recently been demonstrated to be enhanced by the addition of mercaptoacetic acid as a surface modifier in the PD of PbS QDs.28 Consequently, the pore-blocking would be suppressed in CdS(PD)/mp-TiO2. In recent years, highly ordered anodic TiO2 nanotube arrays (NTAs), in comparison to nanoparticulate systems, have been revealed in DSSCs to have superior photoelectric properties and light-harvesting efficiency.48-50 The application of the present PD technique to TiO2 NTA-based QDSSCs should further improve the conversion efficiency. IV. Conclusions The application of the CdS photodeposition technique to mesoporous TiO2 nanocrystalline films [CdS(PD)/mp-TiO2] allows us to obtain the following unique characteristics: (I) efficient visible-light-induced IET between CdS and TiO2 is inherently guaranteed, (II) a large amount of CdS can be loaded on mp-TiO2 during a fairly short period with excellent reproducibility, (III) CdS QDs can be deposited on not only the external surfaces but also the inner ones of mp-TiO2 without poreblocking, and (IV) the band energies of CdS QDs are widely tunable by irradiation time. Owing to these features, a power conversion of 2.51% has been achieved for a sandwich-type solar cell using the CdS(PD)/mp-TiO2 photoanode under illumination of one sun, the value of which is much greater than those for the cells using the CdS/mp-TiO2 photoanodes prepared by the conventional SILAR and SAM methods. This PD technique is highly expected for the preparation of various QDsensitized mp-TiO2 photoanodes in the photoelectrochemical cells for converting solar energy to electric and chemical energy. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (B) No. 20350097 from the Ministry of Education, Science, Sport, and Culture, Japan. The authors acknowledge T. Hattori and Y. Sumida (Nippon Shokubai Co.) for the EPMA measurements. References and Notes (1) Gra¨tzel, M. Nature 2001, 414, 338. (2) Nazeeruddin, M. K.; Angelis, F. D.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Takeru, B.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (3) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737. (4) Hodes, G. J. Phys. Chem. C 2008, 112, 17778. (5) Lee, H.; Leventis, H. C.; Moon, S.-J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nu¨esch, F.; Geiger, T.; Zakeeruddin, S. M.; Gra¨tzel, M.; Nazeeruddin, M. K. AdV. Funct. Mater. 2009, 19, 2735. (6) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (7) Schaller, R. D.; Klimov, V. I. Phys. ReV. Lett. 2004, 92, 186601. (8) Guyot-Sionnest, P. Nat. Mater. 2005, 4, 653. (9) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241. (10) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (11) Peter, L. M.; Riley, D. J.; Tull, E. J.; Wijayantha, K. G. U. Chem. Commun. 2002, 1030. (12) Plass, R.; Pelet, S.; Krueger, D. J.; Gra¨tzel, M.; Bach, U. J. Phys. Chem. B 2002, 106, 7578.

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