ARTICLE pubs.acs.org/JPCC
Charge Recombination and Band-Edge Shift in the Dye-Sensitized Mg2+-Doped TiO2 Solar Cells Changneng Zhang, Shuanghong Chen, Li’e Mo, Yang Huang, Huajun Tian, Linhua Hu, Zhipeng Huo, Songyuan Dai,* Fantai Kong, and Xu Pan Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P. O. Box 1126, Hefei, Anhui 230031, People's Republic of China
bS Supporting Information ABSTRACT: The effect of Mg2+ ions substituted into the anatase lattice on the charge recombination and band-edge movement in dye-sensitized solar cells was investigated in this study. The HRTEM results indicated that Mg2+ ions incorporation into the TiO2 lattice led to the increased lattice spacing of the (101) plane of the anatase phase. Mg2+doped TiO2 could produce a blue shift in the optical absorption edge compared with that of the untreated samples. Detailed analysis of the open-circuit photovoltage (Voc) under different surface charge densities showed that the Mg2+-doped TiO2 samples resulted in the negative shift of the TiO2 conduction band about 70 mV in comparison with the untreated samples. From Raman spectra and light intensity-dependent variation of the short-circuit current density (Jsc) of the solar cells, it could be concluded that the decreased efficiency of electron injection for DSCs with Mg2+-doped TiO2 was attributed to the negative shift of the band edge in the Mg2+-doped TiO2 electrode to obtain a decreased Jsc. The electron diffusion coefficient in Mg2+-doped TiO2 was found to be higher than that in TiO2 at the same photoelectron density. We present evidence that the increase of trap states in Mg2+-doped TiO2 as recombination channels to decrease the electron lifetime could compensate for the effect of band-gap widening to obtain a slightly increased Voc for DSCs with Mg2+-doped TiO2. It is suggested that the recombination channels should be suppressed to enhance the performance of dye-sensitized Mg2+-doped TiO2 solar cells.
1. INTRODUCTION Dye-sensitized solar cells (DSCs) are considered to be a promising renewable source of nonpolluting energy16 because of their low-cost manufacturing process and high-power conversion efficiency. In this system, the nanocrystalline TiO2 is an important thin film semiconductor material available for dye adsorption and effective electron transport.5,6 The photoelectron transport process in the nanostructure TiO2 film would compete with charge recombination of the oxidized dye molecules and electron-accepting species in the electrolyte (i.e., I3). Because of the rapid regeneration of the oxide dye molecules by I ions, the recombination reaction occurring from the TiO2 conduction band and surface states with I3 is generally considered to be the disappearance of photoelectrons in the nanoporous TiO2 film.7 The charge recombination of the injected electron reaction with oxidized dye or I3 results in the positive shift of the conduction band to decrease the open-circuit voltage Voc and solar cell efficiency. It is suggested that an increase in open-circuit photovoltage (Voc) could provide a great opportunity to improve the solar conversion efficiency.8 Experimental evidence showed the improved Voc of DSCs was attributed to the upward shift of conduction band edge of TiO2 and lower dark current at the TiO2/electrolyte interface.818 Recently, the Mg2+-doped nanostructure TiO2 electrodes with a higher conduction band position have been investigated.19 r 2011 American Chemical Society
The results show that the obtained solar cells could produce a high Voc together with an organic dye with the higher lowest unoccupied molecular orbital. The enhanced Voc was ascribed to the Mg2+ ions incorporated in the anatase lattice, which resulted in the Mg2+-doped TiO2 electrode with a higher conduction band. However, there is no detailed study about the mechanism of the band-edge movement and charge recombination for the improved Voc in dye-sensitized Mg2+-doped TiO2 solar cells. In this work, we reported the effect of the substituted Mg2+ ions into the anatase lattice on the band-edge movement and charge recombination to improve the Voc of DSCs. The electron lifetime and electron transport in the Mg2+-doped TiO2 samples were also investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. 1-Methylbenzimidazole (MBI) and LiI were obtained from Aldrich. 3-Methoxypropionitrile (MePN) and iodine were purchased from Fluka. 1-Propyl-3-methylimidazolium iodide (DMPII) was synthesized by the quaternization of 1-methylimidazole and 1-iodopropane, and its purity has been Received: March 15, 2011 Revised: June 24, 2011 Published: July 04, 2011 16418
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The Journal of Physical Chemistry C confirmed by 1H NMR.13 Electrolyte composition was as follows: 0.1 M LiI, 0.1 M I2, 0.45 M MBI, and 0.6 M DMPII in the solvent of MePN. 2.2. DSC Assembly. The Mg2+-doped TiO2 colloidal nanoparticles containing 1 mol % of the dopant in this study was prepared by the thermal reaction of titanium tetraisopropoxide and magnesium acetate tetrahydrate in deionized water, as described elsewhere.19 The 0.215 g of magnesium acetate tetrahydrate (98%, Sinopharm Chemical Reagent Co. Ltd.) was added in 30 mL of titanium isopropoxide (97%, fluka) under constant stirring. The prepared solution containing magnesium acetate tetrahydrate was then added dropwise to the deionized water (50 mL) under vigorous stirring. A white precipitate was produced immediately due to hydrolysis of titanium isopropoxide. The formed precipitate solution was kept under vigorous stirring for 23 h and heated to 80 °C for 815 h to form a transparent sol. The solution was then autoclaved at 200 °C for 12 h to obtain a gelatin. The remaining procedures were the same as described in ref 20. The TiO2 and Mg2+-doped TiO2 paste were printed on the transparent conducting glass sheets (TEC-8, LOF) by using a screen-printing technique and were sintered in air at 450 oC for 30 min to form a nanostructured TiO2 and Mg2+-doped TiO2 electrodes. The film thickness was about 12 μm, which was determined by a profilometer (XP-2, AMBIOS Technology, Inc., USA). After cooling to 80 oC, the films were immersed in an acetonitrile solution (0.3 mM) of dye N719 [NaRu(4,40 -bis(5-(hexylthio)thiophen-2-yl)-2,20 -bipyridine)(4-carboxylic acid40 -carboxylate-2,20 -bipyridine) (NCS)2] overnight. The excess of N719 dye in the TiO2 and Mg2+-doped TiO2 film was rinsed off with anhydrous ethanol before assembly. The counter electrode was platinized by spraying H2PtCl6 solution onto the FTO glass and firing in air at 410 oC for 20 min. It was then placed directly on the top of the dye-sensitized TiO2 and Mg2+-doped TiO2 films. The gap between the two electrodes was sealed by thermal adhesive films (Surlyn, Dupont). The electrolyte was filled from a hole made on the counter electrode, which was later sealed by a cover glass and thermal adhesive films. The total active electrode area of DSCs was 0.8 cm2. 2.3. Methods. The particle morphology of the TiO2 and Mg2+TiO2 electrode microstructure was investigated using a field emission scanning electron microscope (FE-SEM, sirion200, FEI Corp., Holland). Powder XRD was used to study the crystalline phase of the TiO2 and Mg2+TiO2, which was carried out on a Philips X’pert X-ray diffractometer measurement with Cu KR (λ = 0.15406 nm). The crystallite size D was calculated by the Sherrer equation. Crystallite sizes, phases, and shapes of the TiO2 and Mg2+TiO2 powders were investigated with transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) (TEM, JEOL, JEM-2010, Japan). The XPS analysis of TiO2 and Mg2+-doped TiO2 powders were performed at ambient temperature on an ESCALAB 250 XPS (Thermo Electron Corp., U.S.) with a Mg KR X-ray source, and the binding energies were referred to the C 1s neutral carbon peak at 284.6 eV.12 The optical properties of TiO2 and Mg2+-doped TiO2 films were evaluated using the TU1901UVvis spectrophotometer (PGeneral Instrument Inc., China). The pore size distributions of TiO2 and Mg2+-doped TiO2 powders were determined by using a Beckman Coulter nitrogen adsorption/desorption apparatus. The desorption data were analyzed using the BJH (Barrett, Joyner, and Halenda)
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Figure 1. XRD for TiO2 and 1% Mg2+-doped TiO2 powders.The characteristic XRD patterns of anatase [fcc lattice units with space group I41/amd, cell constant a = 0.3785 nm] were drawn in vertical lines using the data in JCPDS card no. 21-1272.
method for cumulative pore volume and pore volume distribution. IMPS/IMVS measurements were carried out on an IM6ex (Germany, Zahner Company) using light-emitting diodes (λ = 610 nm) driven by Expot (Germany, Zahner Company). The LED provided both dc and ac components of the illumination. A small ac component was 10% or less than that of the dc component. The frequency range is 3 kHz to 0.1 Hz. Raman spectra of DSC were recorded on a laser micro-Raman spectrometer (LABRAM-HR, JY., France) equipped with a 514 nm argon laser. The laser spot size was approximately 45 μm2, and the power of the laser beam on the samples was estimated to be 1 mW. The Raman spectrometer was calibrated using a Si single crystal. Raman lines from DSC in situ at the open circuit were measured to a resolution of 0.5 cm1. The short-circuit photocurrent density and open-circuit photovoltage of the DSC dependence of the irradiation intensity were derived with a Keithley 2420 digital source meter (Keithley, USA) under a 450 W xenon lamp (Oriel Sol3A Solar Simulator 94043A, Newport Stratford Inc., USA).
3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology of the TiO2 and Mg2+ Ion-Doped TiO2 Samples. The XRD patterns of TiO2 and
Mg2+-doped TiO2 by the hydrothermal process (Figure 1) showed the anatase crystal structures of the samples after annealing at 450 °C. The XRD pattern did not show a distinct compound of a magnesium oxide phase crystallized as part as TiO2. The lattice cell parameters calculated for the TiO2 sample are a = 3.7845 Å and c = 9.4902 Å, which was in agreement with the literature.18 The Mg2+ insertion into the TiO2 lattice could induce a slight expansion of the lattice in both directions. The cell parameters calculated for the Mg2+-doped TiO2 sample were a = 3.7895 Å and c = 9.4960 Å, which could be attributed to the larger ionic radius of the Mg2+ ion (0.072 nm) than that of Ti4+ (0.0605 nm).21 The average crystallite sizes calculated from the broadening of the (101) XRD peak of the anatase phase were about 14.1 and 12.2 nm for TiO2 and Mg2+-doped TiO2 samples, respectively. It is concluded that the presence of Mg2+ ions introduced into the TiO2 crystal lattice could influence the lattice cell parameters of TiO2 particles.
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Figure 2. (ac) FESEM image of the TiO2 films deposited on an FTO substrate and TEM and high-resolution TEM images of TiO2 powders. (df) FESEM image of the Mg2+-doped TiO2 films deposited on an FTO substrate and TEM and high-resolution TEM images of Mg2+ ion-doped TiO2 powders.
The particle size and morphology of TiO2 and Mg2+-doped TiO2 samples were investigated by SEM, TEM, and HRTEM, as shown in Figure 2. The average particle size of Mg2+-doped TiO2 nanoparticles was about 14 nm, whereas the average particle size of TiO2 nanoparticles was about 17 nm. The SEM and TEM results indicated that the average crystallite sizes of Mg2+-doped TiO2 could be controlled by the modified solgel method. The Mg2+-doped TiO2 nanoparticles as MgO can be deduced from the peak of Mg 1s spectra of the Mg2+-doped TiO2 powders and the decreased Ti 2p3/2 peak intensity for Mg2+-doped TiO2 compared to the TiO2 nanoparticles (Figure S1, Supporting Information). The observed spacing between the lattice planes of the Mg2+doped TiO2 was obtained as 0.385 nm for the (101) plane of the anatase crystal, which was higher than the lattice fringes with a distance of about 0.359 nm for TiO2. It is known that metal ions could be conveniently substituted into the TiO2 lattice if their ionic radii were comparable to that of the Ti4+ cations.21,22 The Mg2+ ionic radius is slightly larger than the Ti4+ ionic radius.21 This clearly indicated that the Mg2+ introduced into TiO2 lattice as a substitutional dopant could increase the observed spacing between the (101) lattice planes of the anatase phase.
3.2. Effect of Mg2+ Ion Doping on the Conduction Band Edge Shift. As TiO2 is an indirect transition band-gap semi-
conductor, we have plotted UV/vis absorption spectra as a function of the excitation energy for the TiO2 and Mg2+ iondoped TiO2 films.18 The TiO2 pore size distribution of the nanoporous film was not obviously different between TiO2 and Mg2+-doped TiO2 powder (Figure S2, Supporting Information), and the absorption edges of the thin films are shown in Figure 3. It is apparent that the optical band gaps exhibited about 3.20 and 3.27 eV for the TiO2 and Mg2+-doped TiO2 sample, respectively. The Mg2+ ion-doped TiO2 led to the blue shift of the optical absorption edge in comparison to that of the TiO2 film, which was in accordance with the literature.19 The supplementary charges incorporated in the anatase lattice by doping with Mg2+ ions could be compensated by the creation of Ti vacancies, which was similar with Nb-doped TiO2 samples.18 The position of the conduction band edge of the samples (Ec) was considered to be determined by the band gap (Eg) of the semiconductor as expressed by Ec = 2.94 Eg.19,23 The negative conduction band edge shift of about 70 mV corresponded to the increased bandgap energy for the Mg2+ ion-doped TiO2 samples. 16420
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Figure 3. Determination of the band-gap energy for a TiO2 film and a Mg2+-doped TiO2 film from the plot of (Rhγ)0.5 versus the excitation energy, hγ (R is the absorption coefficient, h is Planck’s constant, and γ is the photon frequency).
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Figure 5. Schematic band structure of TiO2 and Mg2+-doped TiO2 samples.
Figure 6. Dependence of the short-circuit current density on illumination intensity for DSCs. Figure 4. Effect of Mg2+-doped TiO2 on Voc at ln(Q) for DSCs (Q is the surface charge density for the working electrodes).
The change of Voc under different surface charge densities (Q) is attributed to the band-edge shift for DSCs with TiO2 and Mg2+-doped TiO2 samples. The relationship between Voc and ln(Q) can be expressed as the following linear equation: Voc = Vc + mcln(Q),24,25where Vc is the vertical intercept and mc is the slope rate. Example data from Figure 4 indicated that values of mc ranged from 42.3 to 36.7 mV for all samples and deviated. Because the values of mc were significantly larger than 26 mV, it could be deduced that recombination occurs principally via surface states rather than via the conduction band.24 The change of Vc for the Mg2+ ions doped TiO2 samples found that the band edge of the Mg2+-doped TiO2 electrode shifted negatively by about 71 mV in comparison with the untreated samples. This may be interpreted as ionic and covalent bonding. When the bonding between Ti4+OTi4+ and Mg2+OTi4+ is compared, the TiO bond in the latter would have more covalent character than that in the former. This was expected to decrease the bond distance between Ti and O, which resulted in the negative shift of the conduction band edge of the Mg2+-doped TiO2. The band structure of TiO2 and Mg2+-doped TiO2 can be described schematically in Figure 5. This result was in accord with the above experimental observation, suggesting that the intercalated Mg2+ into the TiO2 lattice resulted in a negative shift in the conduction band edge of the nanostructure electrodes. 3.3. Effect of Mg2+ Ion-Doping on Electron Injection Efficiency. Figure 6 shows that the Jsc of DSCs with and without Mg2+-doped TiO2 varied in direct proportion to the illumination
intensity (I) (AM 1.5, 23.2100 mW cm2). The value of Jsc for DSCs with Mg2+-doped TiO2 was obviously lower than that of DSCs without Mg2+-doped TiO2 at all illumination levels. The slopes of the Jsc versus irradiation intensity I decreased from 141 to 72 mV1, when Mg2+ ions were added to TiO2. The lower slope for DSCs was the indication of the lower efficiency of electron injection (Φinj). The decreased rate of electron injection for DSCs was determined by the decreased energetic overlap between the sensitizer excited state distribution function and the density of semiconductor acceptor states.2628 The negative shift in the band edge of the TiO2 electrode would decrease the energetic overlap for DSCs with Mg2+-doped TiO2. Our results indicated that the addition of Mg2+ ions in the TiO2 could decrease the Φinj, resulting in a decreased Jsc. To get information of Mg2+-doped TiO2 on the electron injection, Raman spectroscopy measurements were made on the dyed TiO2/electrolyte interfacial properties for DSCs with TiO2 and Mg2+-doped TiO2 samples. Figure 7 shows the characteristic Raman spectra from the solar cell constituents. The peak at 143 cm1 is ascribed to the vibration modes of the anatase phase corresponding predominantly to OTiO bending.29,30 The peak at 110 cm1 is attributable to the symmetric stretching vibration of the triiodide peak,31 and the peak at 167 cm1 is attributed to the symmetric stretching vibration of I3 in the stable [D+]/I3complex (the oxidized form of the dye expressed as D+).13,32,33 The intensities of the Raman lines at 110 cm1 in DSCs with and without Mg2+-doped TiO2 showed almost no apparent differences, indicating the same concentration of I3 in the electrolyte. Comparison of the Raman peak at 143 cm1 for DSCs with and without Mg2+-doped TiO2 indicated that the 16421
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Figure 7. Raman spectrum of the peaks at 110 cm1 (I3), 167 cm1 ([D+]I3), and 143 cm1 (TiO2) in DSCs with and without Mg2+doped TiO2.
TiO2 (143) peak was damped after Mg2+ was doped in the TiO2 film. The decrease of the band intensity of the TiO2 (143) peak may be due to that the nanostructured Mg2+-doped TiO2 electrode decreased the charge transfer resistance at the TiO2/ electrolyte interface.13 The intensity at 167 cm1 for DSCs with Mg2+-doped TiO2 was obviously higher than that for DSCs without Mg2+-doped TiO2. The vibration bands at 167 cm1 were sensitive to the applied bias potentials. Stergiopoulos et al. reported that the intensity at 167 cm1 for DSCs was decreased with the applied bias potential from positive to negative values investigated by situ Raman spectroscopy.33 The negative applied bias potentials can increase the concentration of electrons in the TiO2 conduction band. After the conduction band of TiO2 is filled with electrons due to negative applied bias potentials, electron injection from the dye would be inhibited to obtain the lower concentration of the oxide dye (D+).34 Thus, we can conclude that the negative shift in the band edge of the Mg2+-doped TiO2 electrode could decrease the concentration of oxide dye (D+) and the efficiency of electron injection for DSCs. 3.4. Electron Transport and Charge Recombination in Mg2+-Doped DSCs. The Jsc dependence of the electron diffusion coefficients and electron transport time τd of DSCs with and without Mg2+-doped TiO2 is shown in Figure 8. The value of the electron diffusion coefficient (Figure8a) depended on the illumination intensity and was observed to increase with the photoelectron density. It is interesting to find that the electron diffusion coefficient in Mg2+-doped TiO2 was higher than that in TiO2 at the same photoelectron density. As shown in Figure8b, the electron transit time τd was associated with electron transport from the injection sites to the substrate, which can be obtained directly from IMPS. The data of double logarithmic representation allowed the Jsc dependence of τd to be expressed in the form of the power law, τd µ Jscβ1,24,25 where β was related to the steepness of the exponential trap state distribution. The τd for DSCs without Mg2+-doped TiO2 was longer than that for DSCs with Mg2+doped TiO2, which indicated that electrons spend much more time on the transport process. The power exponent β 1 for with and without Mg2+-doped TiO2 is 0.51 and 0.59, respectively, suggesting that the substitution of Mg2+ ions for Ti4+ ions in the titania lattice increases the trap state distribution. Thus, we understand that the addition of Mg2+ ions in the TiO2 could increase the electron diffusion coefficient and the trap state
Figure 8. Electron diffusion coefficient (a) and electron transit time τd (b) as a function of short-circuit current density (Jsc) for DSCs with and without Mg2+-doped TiO2.
Figure 9. Electron lifetimes (τn) as a function of open-circuit potential for DSCs with and without Mg2+-doped TiO2.
distribution of Mg2+-doped TiO2 in the nanostructure TiO2 electrodes. The IMVS mainly studies the process of electron transfer at the dyed TiO2/electrolyte interface.24,35 Figure 9 shows the lifetime of the electrons investigated at open-circuit conditions using IMVS as a function of open-circuit potential for DSCs with TiO2 and Mg2+doped TiO2 samples. A strong correlation between the electron lifetime τn and open-circuit potentials for DSCs indicated that the electron lifetime decreases with increasing open-circuit potentials, due to the increase of the trap state distribution in the Mg2+ ions 16422
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samples. Our results indicated that the decreased recombination channels should be needed for the improved performance of dyesensitized Mg2+-doped TiO2 solar cells.
’ ASSOCIATED CONTENT
bS
Supporting Information. XPS spectra and the pore size distribution of the TiO2 and Mg2+-doped TiO2 powders. This material is available free of charge via the Internet at http://pubs. acs.org.
Figure 10. Dependence of Voc in TiO2 electrodes on the light intensity for dye-sensitized solar cells.
substituted into the anatase lattice. The coated MgO layer onto the TiO2 semiconductor electrode was applied to effectively improve the Voc for DSCs.36,37 The increase in Voc could be due to the increased electron density caused by the increase in electron lifetime at open-circuit conductions and/or a negative shift in the TiO2 band edge potential. The experimental results suggested that the cathodic deposition reaction on the surface of the TiO2 electrode did not create new electron traps.37 Thus, it can be concluded that the increase of the trap state of Mg2+-doped TiO2 as recombination channels yielded the smaller electron lifetime for DSCs at a given voltage. Figure 10 shows that the Voc varied linearly with ln Iinj up to a radiant power of 100 mW/cm2 for DSCs with TiO2 and Mg2+doped TiO2 samples. The Voc values were negatively shifted with the increase of illumination intensity. The slope of the Voc versus ln Iinj plots for solar cells with TiO2 and Mg2+-doped TiO2 samples was about 68 mV for a 10-fold increase in light intensity. An increase of Voc by up to 15 mV for DSCs with Mg2+-doped TiO2 when compared to pure TiO2 was observed (Figure 10) at all illumination levels. Indeed, the Mg2+-doped TiO2 samples caused the TiO2 conduction band to shift negatively by about 70 mV in comparison to the untreated samples, which sets the upper limit for the change in Voc. This suggested that the increased trap state distribution as an additional recombination channel to decrease the electron lifetime could compensate for the effect of band-gap widening to influence the observed Voc for DSCs.
4. CONCLUSIONS In conclusion, we have studied the effect of the substituted Mg2+ ions into the anatase lattice on the band-edge movement and charge recombination. The presence of Mg2+ ions introduced into the TiO2 crystal lattice could influence the lattice cell parameters of TiO2 particles and obviously increase the observed spacing between the lattice planes. Experimental results showed that the Mg2+-doped TiO2 sample led to the TiO2 conduction band to shift negatively by about 70 mV in comparison with the untreated samples. We have found that the electron diffusion coefficient in Mg2+-doped TiO2 was higher than that in TiO2 at the same photoelectron density. The supplementary charge incorporated in the anatase lattice by doping with Mg ions was found to increase the trap states in Mg2+-doped TiO2 as recombination channels and decrease the electron lifetime at the interface of the Mg2+-doped TiO2 electrode/electrolyte. Both the negative shift of the conduction band edges and the faster surface recombination produce an increase of Voc by up to 15 mV for DSCs with Mg2+-doped TiO2 in comparison with the pure TiO2
’ AUTHOR INFORMATION Corresponding Author
*Phone: +86-551-5591377. Fax: +86-551-5591377. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (Grant No. 2011CBA00700), National High Technology Research and Development Program of China (Grant No. 2009AA050603), Foundation of the Chinese Academy of Sciences (Grant No. KGCX2-YW-326), National Natural Science Foundation of China (Grant No. 21003130), and Natural Science Foundation of Anhui Province, China (Grant No. 090414174). ’ REFERENCES (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (2) Nazeeruddin, M. K.; Angelis, F. D.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gr€atzel, M. J. Am. Chem. Soc. 2005, 127, 16835–16847. (3) Wang, Z. S.; Yanagida, M.; Sayama, K.; Sugihara, H. Chem. Mater. 2006, 18, 2912–2916. (4) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gr€atzel, M. J. Am. Chem. Soc. 2008, 130, 10720–10728. (5) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595–6663. (6) Barbe, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gratzel, M. J. Am. Ceram. Soc. 1997, 80, 3157–3171. (7) Gratzel, M. J. Photochem. Photobiol., A 2004, 164, 3–14. (8) Kopidakis, N.; Neale, N. R.; Frank, A. J. J. Phys. Chem. B 2006, 110, 12485–12489. (9) Kusama, H.; Orita, H.; Sugihara, H. Langmuir 2008, 24, 4411–4419. (10) Kusama, H.; Konishi, Y.; Sugihara, H.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2003, 80, 167–179. (11) Boschloo, G.; Haggman, L.; Hagfeldt, A. J. Phys. Chem. B 2006, 110, 13144–13150. (12) Zhang, C.; Dai, J.; Huo, Z.; Pan, X.; Hu, L.; Kong, F.; Huang, Y.; Sui, Y.; Fang, X.; Wang, K.; Dai, S. Electrochim. Acta 2008, 53, 5503–5508. (13) Zhang, C.; Huo, Z.; Huang, Y.; Guo, L.; Sui, Y.; Hu, L.; Kong, F.; Pan, X.; Dai, S.; Wang, K. J. Electroanal. Chem. 2009, 632, 133–138. (14) Zhang, Z.; Zakeeruddin, S. M.; O’Regan, B. C.; HumphryBaker, R.; Gratzel, M. J. Phys. Chem. B 2005, 109, 21818–21824. (15) Zhang, Z.; Evans, N.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra1tzel, M. J. Phys. Chem. C 2007, 111, 398–403. (16) Chappel, S.; Chen, S. G.; Zaban, A. Langmuir 2002, 18, 3336– 3342. (17) Wang, K.; Teng, H. Phys. Chem. Chem. Phys. 2009, 11, 9489– 9496. 16423
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