Semiconductor CdO as a Blocking Layer Material on DSSC Electrode

Sep 8, 2009 - We fabricated dye-sensitized solar cells (DSSCs) with SnO2 nanoparticles coated with thin layers of CdO on the surfaces into a core-shel...
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J. Phys. Chem. C 2009, 113, 17176–17182

Semiconductor CdO as a Blocking Layer Material on DSSC Electrode: Mechanism and Application Min-Hye Kim and Young-Uk Kwon* Department of Chemistry, BK-21 School of Chemical Materials Science, SKKU AdVanced Institute of Nanotechnology, Sungkyunkwan UniVersity, Suwon, 440-746 Korea ReceiVed: May 6, 2009; ReVised Manuscript ReceiVed: August 3, 2009

We fabricated dye-sensitized solar cells (DSSCs) with SnO2 nanoparticles coated with thin layers of CdO on the surfaces into a core-shell-type structure with various Cd/Sn ratios, and studied their effects on the DSSC performance as a function of the CdO shell thickness. CdO is an n-type semiconductor with a conduction band edge lower than that of SnO2 and, thus, has not been considered as a blocking layer material in the previous studies. The overall conversion efficiency of the DSSC was increased until the Cd/Sn ratio was 0.10, but was dramatically decreased when Cd/Sn was 0.20, compared with that of pure SnO2. The DSSC with Cd/Sn ) 0.05 showed an efficiency of 3.25%, which is improved by 59% from that of pure SnO2. The influences of the CdO shells were studied by X-ray diffraction, nitrogen adsorption, dye adsorption, scanning and transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, and electrochemical measurements. When the Cd-content is low, the flat band potential of the electrode, determined by the Mott-Schottky analyses, was negatively shifted, and the electron density accumulated into the electrode, determined by analyzing the chronoamperometry data, was decreased from those of pure SnO2. In addition, the amount of adsorbed dye molecules on the SnO2 surface was increased. Since the CdO shell thickness is very small, the semiconductor properties of CdO do not appear in these samples. These changes of the physicochemical properties of SnO2 by the thin CdO shells improved the conversion efficiency and the effects of the CdO shells could be explained with the high basicity of CdO. However, when the Cd/Sn ratio was 0.20, the CdO shell became thick enough to show a narrower band gap and a lower conduction band edge than those SnO2, whose negative effects were more than offset those from the basicity of the CdO shell, and the conversion efficiency was greatly suppressed. 1. Introduction Dye-sensitized solar cells (DSSCs) have been a focus of intensive research activities as a promising platform of photovoltaics to produce high-performance solar cells at low costs. Numerous works have been performed to improve the performance of DSSCs since the first report of this concept.1 The various processes at the anode-electrolyte interface are the important factors that govern the performances of a DSSC. The anode of a DSSC is a porous layer of a semiconductor oxide with a wide band gap, typically TiO2 or SnO2, on which dye molecules are adsorbed. The dye molecules absorb photons and inject the excited electrons to the conduction band of the oxide layer. The passage of the electrons through the external circuit generates electricity.2-10 One of the problems with this process is the recombination of the injected electrons with the red-ox species in the electrolyte.11-17 So, many works have been done to suppress the recombination process. One of the successful approaches is the formation of a blocking layer on the surface of the oxide electrode material.12,15,17 The major function of the blocking layer is to form a potential barrier to the recombination process, although in some cases, the blocking layer can increase the amount of adsorbed dye molecules as an additional effect.15,17 According to this view, the materials for the blocking layer are restricted to insulators. Indeed, all of the literature data on the blocking layers are on insulating oxides such as Al2O3, NiO, * Corresponding author. [email protected].

SiO2, ZnO, ZrO2, Nb2O5, and rare earth oxides, but there has been no report with a narrow band gap semiconductor such as CdO. On the other hand, it is well-known that the electronic properties of a semiconductor can be tuned as a function of the size, namely, the quantum size effect, sometimes extending to the insulator regime. Because the blocking layer is bound to be thin, it seems possible that some semiconductors can be used as the blocking layer material even if they may be ohmic conductors and/or have intense colors in the bulk states. The use of blocking layers made of narrow band gap semiconductors may bring some advantages because it widens the range of usable metals rather drastically and, hence, provides the means to further control the surface properties of the oxide electrodes. In order to verify this idea, we chose to study CdO as a blocking layer material on SnO2. CdO is a well-known n-type semiconductor with a band gap (Eg) of 2.2-2.9 eV.18-20 It has a high electrical conductivity, by which it is considered a material for transparent conducting oxide electrodes.18,19 Moreover, the conduction band edge (Ecb) of CdO is lower than that of SnO2 .20,21 According to the accepted theories on the function of the blocking layer, these properties of CdO are very disadvantageous. However, for the purpose of the present study, which is to see whether a thin semiconductor layer can be used as a blocking layer, these characteristics are ideal. In the present study, we synthesized SnO2 nanocrystals with CdO shells in various thicknesses and studied the physicochemical properties and their performances as oxide electrodes in

10.1021/jp904206a CCC: $40.75  2009 American Chemical Society Published on Web 09/08/2009

CdO as a Blocking Material on DSSC Electrode DSSCs. As expected, the properties of the SnO2-CdO core-shell structure and the performance of the resultant DSSCs are strongly dependent on the CdO shell thickness; a small amount of CdO increases the DSSC performance by as much as 59%. The various effects of the CdO layers as a function of the Cd/ Sn ratio are reported. 2. Experimental Section 2.1. Materials. All the reagents were purchased and used without further purification: N3 dye (Ru(bpy)2(NCS)2H4, Solaronix); ethanol (Merck, anhydrous); Na2SO4 (Samchun, anhydrous); acetonitrile (Aldrich, anhydrous); 1,2-dimethyl-3propyl imidazolium iodide (Solaronix); I2 (Aldrich, 99.9%); LiI (Aldrich, anhydrous); tert-butyl pyridine (TBP) (Aldrich, 99.8%); LiClO4 (Aldrich); tetrabutylammonium perchlorate (Aldrich, electrochemical grade); 1-propanol (Aldrich, anhydrous); H2PtCl6 (Aldrich); hydroxypropylcellulose (HPC) (Aldrich, MW 85 000); NaOH (Aldrich); cadmium acetate (Aldrich, dihydrate); glacial acetic acid (Samchun); fluorine doped tin oxide substrate (FTO) (Pilkington, 8Ω/square); 15% SnO2 colloid solution (Alfa-Aesar); sealant (SX1170-60 µm, Solaronix). 2.2. Preparation of SnO2 Film. 1 g of the SnO2 colloid solution and 0.09 g of HPC were ground together in an agate mortar until HPC dissolved completely. Then, 0.1 mL of acetic acid was added and ground further. For the surface modification of SnO2 colloid particles, cadmium acetate was added before the addition of acetic acid to make the Cd/Sn molar ratio 5, 10, or 20. The viscous paste was spread on a FTO substrate (1.5 × 1.5 cm2) by the doctor-blade method with a 3 M Magic adhesive tape as a spacer and dried at the ambient conditions. The film was calcined at 450 °C for 30 min. The area covered by the SnO2 film was 0.28 cm2, and the thickness was controlled to be 5 µm. 2.3. Fabrication of DSSC. Solar cells were fabricated with dye adsorbed SnO2 films, platinized FTO, a sealant, and an electrolyte through the following procedure: after the calcination, SnO2 films were cooled to 70 °C and immersed in a 0.3 mM N3 ethanol solution for 16 h. The films were washed with ethanol and naturally dried at the ambient condition. For the platinized FTO electrode, 12 µL of a H2PtCl6 solution (5 mM in 1-propanol) was spread on a clean FTO (1.5 × 1.5 cm2, with two holes drilled previously) and dried naturally, followed by heating at 380 °C for 30 min. The fabrication of DSSC was done by sandwiching the SnO2 covered FTO glass and the platinized FTO glass with strips of sealant. An iodide/triiodide electrolyte composed of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.1 M LiI, 0.005 M I2, and 0.5 M TBP in anhydrous acetonitrile was prepared and used. 2.4. Analyses and Characterization. The surface morphology and the thickness of the SnO2 films were analyzed with a field-emission scanning electron microscope (FE-SEM; JEOL JSM-7401F). The transmission electron microscopic (TEM) images were obtained with a HR-TEM (JEOL JEM-3011). X-ray diffraction patterns (XRD) were recorded on a Rigaku DC/Max 2000 diffractometer. The Raman spectra of SnO2 films were recorded on a micro-Raman setup (Renishaw, Inc., New Mills, UK). The specific surface areas were obtained by the Brunauer-Emmett-Teller (BET) method on the N2-adsortiondesorption isotherm data recorded on a BEL-mini (JAPAN INC.) at 77 K. The films were scraped off to obtain powder samples for the measurements. UV-vis diffuse reflectance (UVDRS) spectra of SnO2 films were obtained with a UV-vis-NIR spectrometer (Shimazu 3600) using BaSO4 as a reference. The amounts of the N3 dye molecules adsorbed on the SnO2 films were calculated from the UV-absorption spectra of the dye

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17177 TABLE 1: Photovoltaic Performances of DSSCs Constructed of CdO-Modified SnO2 sample

Jsc (mA/cm2)

Voc (V)

FF

efficiency (%)

CS0 CS5 CS10 CS20

8.57 10.13 8.58 1.97

0.45 0.54 0.62 0.58

0.53 0.59 0.58 0.42

2.04 3.23 3.09 0.48

desorbed into 1 mL of 0.1 M NaOH solutions. (Scinco S-3100 spectrophotometer). All of the electrochemical analyses were performed using a potentiastat (Ivium CompactStat). Electrochemical impedance spectra (EIS) were obtained in the range 0.05-105 Hz at the open-circuit condition with an amplitude of 5 mV under a 100 mW/cm2 light illumination condition. Capacitance data were obtained in the range -1.0-1.0 V at 500 Hz on films soaked in a 0.5 M Na2SO4 aqueous solution.22,23 The flat-band potentials (Vfb) of the films were determined by extrapolating the Mott-Schottky plots obtained from the capacitance data.22-24 Chronoamperometric data were obtained in a three-electrode system using a SnO2 film, a coiled Pt wire, and a Ag/AgCl electrode as the working electrode, the counter electrode, and the reference electrode, respectively. The electrolyte was composed of 0.1 M tetrabutyl ammonium perchlorate and 0.1 M lithium perchlorate in acetonitrile. The measurements were made in the potential range of -425-0 mV with a 100 mV interval. The SnO2 films were biased at a given voltage for 10 min to make the equilibrium. A biased step of -25 mV was applied and the current was monitored as a function of time. 3. Results and Discussion In this study, we synthesized surface-modified SnO2 nanoparticles by adding cadmium acetate into a SnO2 colloidal solution, followed by fabrication into thin film and calcination at 450 °C. The Cd content was adjusted to Cd/Sn ) 0%, 5%, 10%, and 20% in molar ratio. The corresponding Cd-modified SnO2 samples and devices will be denoted as CS0, CS5, CS10, and CS20, respectively. 3.1. Photovoltaic Performances of DSSC. DSSCs were assembled by using the Cd-modified SnO2 nanoparticles as the cathode materials. In Table 1, the solar cell parameters obtained under a 100 mW/cm2 illumination are summarized. The Cd content in the cathode material is reflected in these parameters in diverse ways. The short-circuit current (Jsc) shows the most dramatic effect. CS5 shows an increased Jsc from that of CS0. However, further increase of the Cd-content decreases Jsc; CS10 shows practically the same Jsc as CS0 and CS20 shows a considerable decrease of Jsc from the other three electrodes. The open-circuit voltage (Voc) is gradually increased with the Cd content up to CS10 and, then, is slightly decreased for CS20. The fill factor is also increased initially and decreased upon further addition of Cd. The overall conversion efficiency is the highest with CS5 that is 59% higher than that of CS0, comparable to the record of 35% in the literature on the modification of the SnO2 electrode with ZnO.15 As will be seen later, the Cd atoms in CS10 and CS20 exist as surface-coating CdO layers on SnO2 cores. Therefore, one may be tempted to explain the above observations with the blocking layer effect. However, while the literature examples of blocking layers are made of insulating oxides, CdO is a semiconductor with the conduction band edge lower than that of SnO2. According to the explanations presently accepted for the role of blocking layer, the surface modification with CdO

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Figure 1. XRD patterns of CdO-modified SnO2 films.

should reduce Jsc and Voc and, hence, the conversion efficiency. Thus, the improvement of the conversion efficiency by modification with CdO is rather surprising. CS10 also shows a better efficiency than CS0, but the performance of CS20 is very poor because its Jsc is very small. 3.2. Structural Analyses of Modified and Nonmodified SnO2. Figure 1 shows the XRD patterns of the CdO-modified and nonmodified SnO2 films. These patterns agree well with that of cassiterite (SnO2; JCPDS-71-0652). The peaks are broad, which is indicative of a nanocrystalline state. By using the Scherrer equation, the particle sizes were estimated to be around 6 nm for all the samples. It is noteworthy that neither the peak width nor the peak position is influenced by the Cd content. The Raman spectroscopic data (not shown) of these samples show only one peak at 630 nm and do not show any change with the Cd content. These results imply that there is no or very little Cd doping in the SnO2 lattice. Many previous works indicate that doping of SnO2 with Cd2+ is limited under the present synthesis condition.25,26 In the literature on sensors based on Cd-modified SnO2 by using solution synthesis methods, CdO is reported to be segregated at the surface of SnO2. This was attributed to the different crystal structures between CdO and SnO2 and the different electronegativities and ionic radii between Cd2+ and Sn4+.25 The solid-state reaction between CdO and SnO2 to produce CdSnO3 is reported to occur at 600-650 °C,26 higher than the calcination temperature in the present study. The absence of any Raman signal attributable to Cd-containing phases suggests that the amount of Raman-active phase such as CdSnO3 and Cd2SnO4 is very small if there is any.27 This leaves CdO as the only possibility, which is reported to be hard to detect by Raman spectroscopy.28 Therefore, we conclude that the Cd atoms in the present study form CdO on the SnO2 surfaces. That the XRD patterns show no peaks which can be associated with any Cd-containing crystalline phase suggests that the segregated CdO is amorphous and dispersed on the SnO2 surfaces. The electron microscopy data corroborate with the expectation based on the XRD data. The SEM images in Figure 2 show that all the samples are composed of nanoparticles and the Cd content does not influence the morphology. The TEM images in Figure 3 reveal that all the samples are composed of SnO2 nanocrystals with well-developed lattices. They are ∼5 nm in size, in agreement with that estimated from the XRD data. The images of CS10 and CS20 further show that there are amorphous shells surrounding the SnO2 nanocrystals and that the shell thickness increases with the Cd content. CS5 does not show any indication of the shell formation in its TEM image probably because the shell is very thin. However, the EDX spectra (with a beam size 20 nm) on several spots invariably showed both Cd and Sn with the Cd/Sn ratio close to the loaded composition

Figure 2. SEM images of CS0 (a), CS5 (b), CS10 (c), and CS20 (d) (scale bar is 100 nm).

Figure 3. TEM images of CS0 (a), CS5 (b), CS10 (c), and CS20 (d) (scale bar is 5 nm). Insets are enlarged images to show the lattice fringes of the SnO2 nanoparticle cores.

of 5%, indicating that Cd atoms are evenly distributed on the SnO2 surfaces. These observations lead us to conclude that our materials are composed of core-shell-type nanoparticles with crystalline SnO2 cores and amorphous CdO shells. However, if one assumes that each SnO2 nanoparticle is surrounded by a CdO shell, the shell thickness is calculated to be very thin (i.e., 1.7 Å for CS20). Therefore, we believe that the distribution of CdO on the SnO2 nanoparticles is not homogeneous and that some of the SnO2 nanoparticles are aggregated to have direct contacts among themselves. 3.3. Diffuse-Reflectance UV-vis Spectroscopy. Figure 4 shows the UV-DRS spectra and the corresponding KubelkaMunk plots of the SnO2 films with varied Cd content. CS0 is transparent to the visible light. Its Eg from the Kubelka-Munk plot, 3.5 eV, agrees with that on SnO2 in the literature.29 The CdO shell shifts the absorption edge toward longer wavelengths. The Eg’s of CS5, CS10, and CS20 are 3.3, 3.2, and 2.8 eV, respectively. As the thickness of the CdO shell increases, the Eg gradually approaches that of bulk CdO, 2.2-2.9 eV.18-20 Consistent with these spectra, CS20 is yellow in appearance, while the other films are white or colorless. The gradual increase of Eg with the decrease of the shell thickness can be explained with the quantum size effect. Electrode materials with Eg’s smaller than 3.0 eV are disadvantageous because they screen the photons to be absorbed by the dye molecules on the SnO2 surface. This can be one of the reasons for the abruptly decreased

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Figure 5. (A) Proposed model for the structure of CdO-modified SnO2 nanoparticles and (B) their electronic properties. The various electronic processes are shown in the case of CS20. a,b: injection of electrons from excited dye molecules. c: trapping of conduction electron by the CdO shell. d: recombination.

Figure 4. Diffuse-reflectance UV-vis spectra (top) and Kubelka-Munk plots (bottom) of CdO-modified SnO2 films.

TABLE 2: Specific Surface Areas and Pore Volumes of CdO-Modified SnO2 sample

surface area (m2/g)

total pore volume (cm3/g)

dye amount (mol/cm2)

CS0 CS5 CS10 CS20

82 93 88 81

0.25 0.26 0.23 0.21

6.3 × 10-8 1.1 × 10-7 1.5 × 10-7 2.2 × 10-7

Jsc of the CS20 cell. The other CS materials have Eg’s larger than 3.0 eV and are not expected to influence the photon absorption by the dye molecules. 3.4. Effects of CdO-Shell on the Pore and Surface Properties. The film materials were scraped off into powder samples, and their pore properties were studied by N2 adsorption measurements. The results are summarized in Table 2. As can be seen in this table, the influence of the CdO shell formation on the surface area is not large with the variation within 13%. The slight increases of the surface areas of CS5 and CS10 from CS0 are mainly due to the increases of the micropores with pores smaller than 1 nm (data not shown). The pore volume is unchanged when the Cd content is small (CS5) and is decreased slightly when the Cd content is large (CS10 onward); CdO probably has a higher density than SnO2. To the sharp contrast to the more or less constant pore properties by N2 adsorption data, the CdO shell increased the amount of dye adsorption by 74% in CS5, 147% in CS10, and 247% in CS20 from that of CS0. Apparently, the CdO-modified surface has a stronger affinity toward the dye molecules than the SnO2 surface, probably because the Cd2+-O2- bond is easier to break than the Sn4+-O2- bond for the dye molecules to bind to the metal centers and/or the higher basicity of CdO than SnO2 makes CdO a better H+ acceptor to form stronger H-bonds with the dye molecules. Added to these, the larger Cd2+ ion can expand its coordination sphere to accept the dye ligand, which

is not easy for the smaller Sn4+ ion.30 The increased dye adsorption in CS5 can serve as a support of our explanation that the CdO in this sample is located on the surface of SnO2, although they are not seen in the TEM image. On the basis of the data discussed thus far, we propose a model for the structures of our materials as shown in Figure 5A. We believe that during the mixing of the SnO2 colloid solution with cadmium acetate each SnO2 particle is covered by a shell of cadmium acetate, which converts into a CdO shell during the calcination step. The shell thickness increases with the Cd content. Therefore, the SnO2 particles are joined to the next one through a CdO wall. The electronic structures of these materials can be deduced as shown in Figure 5B. The UV-DRS data indicate that the band structure of the CdO shell varies with the thickness. When a thick CdO shell is formed as in CS20, the band structure of CdO is a step closer to that of bulk CdO.31 That is, its Eg is smaller than that of SnO2 and its Ecb lies below that of SnO2. As the shell thickness decreases, the Eg widens and the Ecb rises. While the UV-DRS data on CS5 and CS10 can give direct information on their Eg’s, it is not clear where the Ecb’s are located. Nevertheless, one can safely expect them to be closer to that of SnO2 than the Ecb in CS20. In the case of CS5, the Ecb of the CdO shell may be even higher than that of SnO2, forming a potential barrier between SnO2 nanoparticles. However, since the shell is very thin, the location of the shell Ecb may not be critical for the electronic conduction. As the shell thickness increases, the CdO shell will form a potential well between the SnO2 nanoparticles because its Ecb is lower than the Ecb of SnO2. The well is deep and wide when the shell is thick (CS20) and shallow and narrow when the shell is thin (CS10). 3.5. Determination of Flat-Band Potential. The MottSchottky plots of the electrodes were obtained in a 0.5 M Na2SO4 aqueous electrolyte solution to determine their flat-band potentials (Vfb). The Vfb of the electrode material in a DSSC is very important because the Voc is determined as the difference between the red-ox potential in the electrolyte and the quasi Fermi level, which increases linearly with Vfb.22,32 A negative shift of Vfb in this plot means a shift away from the red-ox potential, thereby an increase of Voc. Figure 6 shows that Vfb is negatively shifted for CS5 and CS10 and is positively shifted for CS20 with respect to CS0. While the positive shift in CS20 can be understood by the fact that the Ecb of the CdO shell is lower than that of SnO2,20,21 the negative shifts in the lower Cd

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Figure 6. Mott-Schottky plots of CdO-modified SnO2 electrodes.

content films are counterintuitive. The concept of blocking layer widely adopted in the DSSC literature cannot be applied to these observations. On the basis of the proposed electronic structures in Figure 5B, the different behaviors of Vfb between CS20 and the other materials suggest that the Ecb’s of CS5 and CS10 are close to that of SnO2 or higher. However, the fact that CS10 with a thicker CdO shell than CS5 shows a more negative Vfb value than CS5 cannot be explained with this model alone. Bandara et al. reported that surface modification of SnO2 with MgO resulted in a negative shift of Vfb by the basic properties of the MgO surface.22 They explained that the basic character of MgO with an isoelectric point (IEP) of 12 produced an interfacial dipole, deprotonating the SnO2 surface, and hence, a negative shift of Vfb. Because CdO is basic with an IEP of 10, their explanation can be applied to the present system. This means that the basic property of the CdO shell has a mechanism to negatively shift Vfb of SnO2, which counteracts the effect of lower-lying Ecb of CdO to positively shift Vfb. In the present system, both effects increase with the shell thickness. As seen in the UV-DRS data, the electronic effect is negligible when the shell thickess is very small, so the basicity effect dominates in CS5, explaining the negative shift of Vfb of this sample. The larger negative shift in Vfb of CS10 than CS5 suggests that the chemical effect grows with the shell thickness; the Ecb of the shell may be lowered but still remains close to that of SnO2. The negatively shifted Vfb’s of CS5 and CS10 contribute to the increased Voc’s of the corresponding DSSCs seen in Table 1. Note that CS10 has a larger Voc than CS5, which is consistent with their Vfb data. Although the Voc of CS20 is slightly lower than that of CS10, it is still much larger than that of CS0, which may imply that the basicity effect is stronger than the effect of the lower-lying Ecb in CS20. That CS20 shows a much larger Voc than CS0 despite CS20 having a more positive Vfb than CS0 means that there is an additional contribution to the Voc of CS20, which we believe is due to the much larger amount of adsorbed dye molecules in CS20 than in CS0. 3.6. Electrochemical Impedance Spectroscopy. Figure 7 shows the Nyquist plots of the DSSCs made of the CdO modified SnO2 electrodes measured under 100 mW/cm2 illumination at an open-circuit condition. Each Nyquist plot is composed of three semicircles, which correspond to (1) the electrochemical reaction at the Pt counter electrode, (2) the charge transfer resistance at electrolyte/dye/metal oxide interface, and (3) the charge transport in the metal oxide layer and Warburg diffusion process of I-/I3-, respectively, from the left to the right of the plots.33-36 The diameters of the semicircles corresponding to the mechanism (2) of CS5 and CS10 are smaller than that of CS0. This means that the charge transfer

Kim and Kwon

Figure 7. Electrochemical impedance spectra of DSSCs based on SnO2 and Cd-modified SnO2.

Figure 8. Plots of electron densities of CdO-modified SnO2 films as functions of bias potential.

resistances of CS5 and CS10 are smaller than that of CS0. Probably, the greatly increased amount of adsorbed dye molecules and the higher Vfb’s of CS5 and CS10 are the major factors for this behavior. The reason CS10 has a larger resistance than CS5 may be related to the different thicknesses of the CdO shells in these materials. Although the shell thickness of CS10 is too small to show the bulk Eg, the Ecb of the CdO layer lies below that of SnO2 and functions as a potential well, increasing the resistance between the SnO2 nanoparticles. In the case of CS5, the CdO shell is very thin allowing tunneling through it even if its Ecb may be located above that of SnO2. The Nyquist plot of CS20 shows anomalously large semicircles and is hard to explain on the basis of currently available theories on DSSCs. However, it is clear that it has the largest charge transfer resistance among the four electrodes studied. The deep and wide potential well produced by the thick CdO shell appears to be the main reason for the large resistance of CS20. 3.7. Chronoamperometric Measurement. Figure 8 shows the accumulated electron densities of the SnO2 electrodes as a function of the applied bias voltages obtained from the chronoamperometric measurement data. The electron densities accumulated to SnO2 electrodes were calculated by integrating the capacitance curves obtained from the chronoamperometric data. The electron density in this plot may be approximated to be the trap site density.13,37 As can be seen in this figure, the electron densities of CS5 is lower than that of CS0, and that of CS10 is similar to CS0, while CS20 has a higher electron density at a given bias voltage. This trend is consistent with the abovementioned models of the electronic structures. In CS5, the CdO shell has a higher energy level than that of SnO2 but is thin

CdO as a Blocking Material on DSSC Electrode enough not to influence the flow of electrons. As the thickness of the CdO shell increases, it starts to create energy levels below the Ecb of SnO2, which can be the trap sites for electrons. The density of such a site increases with the thickness and the depth of the trap energy increase. Willis et al. have suggested that the electron density has an inverse relationship with the half-time of the charge recombination dynamics.13 That is, a high electron density means a rapid electron recombination process. Although we did not measure the half-life of the recombination dynamics, the electron density data indicate that the recombination dynamics are influenced by the Cd content. On the basis of our data, one can expect that the half-time for the charge recombination is shortened for CS20 and lengthened for CS5 and CS10 from that of CS0. 3.8. Explanation of Photovoltaic Performances. The effects of the CdO shell on SnO2 nanoparticles as the electrode material in a DSSC can be explained with the scheme shown in Figure 5B. The major effects of the CdO shell can be explained in terms of the electronic effects due to the relative positions of the conduction band edges of CdO and SnO2 and the chemical effect arising from the higher basicity of CdO than that of SnO2. As have seen already, both of these effects strongly depend on the shell thickness. The position of Ecb of the CdO shell is raised as the shell thickness decreases, and the basicity of the surface increases with the thickness of the CdO shell. The increase of the surface basicity increases the amount of adsorbed dye molecules, up to 3.5 times in the case of CS20, which can increase Jsc, and increases Voc through the increase of the Vfb of SnO2. These positive effects increase with the shell thickness to some extent. The electronic effect of a semiconductor shell is mainly negative for the solar cell performance. The lowerlying Ecb of CdO than that of SnO2 causes two negative effects. At the surface, it provides easy paths for the injected electrons to backflow to the electrolyte, facilitating the recombination process. Inside the electrode material, the CdO shells between the SnO2 nanoparticles create trap sites for the electrons, exerting resistance to the electron diffusion to the FTO electrode. The negative electronic effect is almost negligible when the shell is very thin as in CS5. Therefore, a thin CdO shell can improve the solar cell efficiency, which is the case for CS5. However, as soon as the shell thickness is larger than a critical value which appears to be around CS10, the electronic effect starts to show up and becomes dominating. Therefore, in case of CS10, although the Vfb is further raised and the amount of adsorbed dye molecules is further increased from those of CS5, they are counterbalanced by the negative effects by the increased recombination rate and the increased resistance. As the CdO shell thickness further increased to CS20, the negative effects dominate the situation despite the fact that the chemical effects are increased as seen in the high Vfb and the largest amount of adsorbed dye molecules. The lower-lying Ecb of the CdO shell not only increases the recombination process, but also exerts a large resistance. The strong absorption at the short-wavelength region hampers the situation further. 4. Conclusion The present study was performed to verify a hypothesis that some semiconductors in the bulk states may be used as the blocking layer materials in DSSCs. The major role of a blocking layer is to form an energy barrier to keep injected electrons from undergoing recombination processes and, thus, implies properties of an insulator. Probably, because of this understanding, the materials for the blocking layers have been restricted to insulating oxides. We hypothesized that, because the proper-

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17181 ties of a semiconductor could be controlled by the particle size, a thin layer of a semiconductor could function as a blocking layer. Our experimental results proved that to be the case, although the details are more complicated than for an insulator. With the present results, we can conclude that there are many more candidates for the blocking layer materials. Some of them may bring in advantages that are not possible with the limited number of insulating oxides. For example, the greatly increased amount of adsorbed dye molecules can be an advantage of using a large transition metal ion. In addition, some materials with a better efficiency for the electron injection rate from the dye molecules and the other effects may be harvested. Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2008-005-J00701). We also thank SAINT and CNNC for the financial supports and CCRF for TEM data. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 335, 737. (2) Law, M.; Greene, L. E.; Johson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (3) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Seiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402. (4) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (5) Sapp, S. A.; Elliott, C. M.; Contado, C.; Caramori, S.; Bignozzi, C. A. J. Am. Chem. Soc. 2002, 124, 11215. (6) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; HumphryBaker, R.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 14336. (7) Usui, H.; Matsui, H.; Tanabe, N.; Yanagida, S. J. Photochem. Photobiol. A 2004, 164, 97. (8) He, J.; Lindstro¨m, H.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. B 1999, 103, 8940. (9) Wang, X.; Zhi, L.; Mu¨llen, K. Nano Lett. 2008, 8, 323. (10) Chen, D.; Huang, F.; Cheng, Y.-B.; Caruso, R. A. AdV. Mater. 2009, 21, 1. (11) Huang, S. Y.; Schlichtho¨rl, G.; Nozik, A. J.; Gra¨tzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576. (12) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475. (13) Willis, R. L.; Olson, C.; O’Regan, B.; Lutz, T.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B. 2002, 106, 7605. (14) Green, A. N. M.; Palomares, E.; Haque, S. A.; Kroon, J. M.; Durrant, J. R. J. Phys. Chem. B 2005, 109, 12525. (15) Park, N.-G.; Kang, M. G.; Kim, K. M.; Ryu, K. S.; Chang, S. H. Langmuir 2004, 20, 4246. (16) Gregg, B. A.; Pichot, F.; Ferrere, S.; Fields, C. L. J. Phys. Chem. B 2001, 105, 1422. (17) Kay, A.; Gra¨tzel, M. Chem. Mater. 2002, 14, 2930. (18) Jager, S.; Szyszka, B.; Sczyrbowski, J.; Brauer, G. Surf. Coat. Technol. 1998, 98, 1304. (19) Deokate, R. J.; Pawar, S. M.; Moholkar, A. V.; Sawant, V. S.; Pawar, C. A.; Bhosale, C. H.; Rajpure, K. Y. Appl. Surf. Sci. 2008, 254, 2187. (20) Zhu, Y. Z.; Chen, G. D.; Ye, H. Phys. ReV. B 2008, 77, 245209. (21) Park, N.-G.; Kim, K. Phys. Status Solidi 2008, 8, 1895. (22) Bandara, J.; Pradeep, U. W. Thin Solid Films 2008, 517, 952. (23) Bandara, J.; Divarathne, C. M.; Nanayakkara, S. D. Sol. Energy Mater. Sol. Cells. 2004, 81, 429. (24) Radecka, M.; Rekas, M.; Trenczek-Zajac, A.; Zakrzewska, K. J. Power Sources 2008, 181, 46. (25) Castro, R. H. R.; Hidalgo, P.; Perez, H. E. M.; Ramirez-Fernandez, F. J.; Gouveˆa, D. Sens. Actuators, B 2008, 133, 263. (26) Tianshu, Z.; Hing, P.; Li, Y.; Jiancheng, Z. Sens. Actuators, B 1999, 60, 208. (27) Pis Diez, R.; Baran, E. J.; Lavat, A. E.; Grasselli, M. C. J. Phys. Chem. Solids 1995, 56, 135. (28) Maye´n-Herna´ndez, S. A.; Torres-Delgado, G.; Castanedo-Pe´rez, R.; Mendoza-Alvarez, J. G.; Zelaya-Angel, O. Mater. Chem. Phys. 2009, 115, 530. (29) Szczuko, D.; Werner, J.; Oswald, S.; Behr, G.; Wetzig, K. Appl. Surf. Sci. 2008, 179, 301. (30) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic chemistry: Principle of structure and reactiVity, 4th ed.; HarperCollins: New York, 1993; Chapter 4.

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