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Nb2O5 Photoelectrodes for Dye-Sensitized Solar Cells: Choice of the Polymorph A. Le Viet,† R. Jose,*,†,‡ M. V. Reddy,† B. V. R. Chowdari,† and S. Ramakrishna† National UniVersity of Singapore, 117576 Singapore, and Faculty of Industrial Sciences and Technology (FIST), UniVersiti Malaysia Pahang, 26300 Kuantan, Malaysia ReceiVed: July 14, 2010; ReVised Manuscript ReceiVed: October 10, 2010
Nanowires of Nb2O5 were developed in three polymorphic forms, namely, pseudo-hexagonal, orthorhombic, and monoclinic, by electrospinning a polymeric solution and by subsequent annealing. The materials were characterized by X-ray and electron diffraction, scanning and transmission electron microscopy, BET surface area measurements, absorption spectrometry, and cyclic voltammometry. These characterizations indicate that the monoclinic phase has a higher conduction band edge compared with the other two and is likely to display higher open circuit voltage in solar cells. Dye-sensitized solar cells were fabricated using the above polymorphs, and the device performances were studied. The pseudo-hexagonal phase showed higher device performance owing to its higher specific surface area compared with the others. However, when normalized, the device performances with respect to the dye-loading, the monoclinic phase gave superior performance. Studies on the charge transport properties using electrochemical impedance spectroscopy and open circuit voltage decay indicate that the monoclinic phase has high resistance against charge recombination and improved electron lifetime compared with the other phases and conventional TiO2 nanostructures. The monoclinic Nb2O5 is likely to be a material of choice as photoelectrode in dye-sensitized solar cells if its mesoporous particles with large surface area could be synthesized. Introduction Dye-sensitized solar cells (DSCs) are a promising alternative to silicon-based solar cells due to their low cost and moderate efficiency.1 The photovoltaic effect in DSC occurs at the interface between a dye-anchored mesoporous semiconductor and a hole conductor. Mesoporous TiO2 is the most successful material so far in DSCs for charge separation and transport due to a number of reasons, such as (i) ease in getting mesoporous particles, (ii) band alignment with various organic and inorganic dyes, and (iii) stability.2,3 However, the charge transport through mesoporous TiO2 is diffusion-driven and is very inefficient. It is generally accepted that photogenerated electrons diffuse through mesoporous particles by a trapping-detrapping process4 in DSCs with a light intensity-dependent diffusion coefficient (Dn) typically ∼10-5 cm2/s at AM1.5 conditions. Even with such inefficient charge transport process, certified photoelectric conversion efficiency (η) as high as 8.4% in submodules (area ∼ 18 cm2) and 11.5% in laboratory scale (area ∼ 0.22 cm2) has been achieved using mesoporous TiO2-based DSCs.5 Structures with one-dimensional (1D) morphology, such as wires, rods, tubes, etc., have been suggested to improve the charge diffusion due to more directed transport.6,7 Therefore, electrospinning of a polymeric solution containing metal ions and their controlled heat treatment to produce 1D morphology of advanced materials have been recently gaining widespread interest due to the scalability, simplicity, and versatility of the process.8 Recently, suitability of other metal oxides for DSCs was reviewed.2 It was identified that Nb2O5, an n-type transition metal oxide semiconductor with an oxygen stoichiometry* Corresponding authors. E-mails: (R.J.)
[email protected], (S.R.)
[email protected]. † National University of Singapore. ‡ Universiti Malaysia Pahang.
dependent bandgap ranging between 3.2 and 4 eV, has a higher conduction band edge than TiO2, and therefore, it is possible to attain higher open circuit voltage and photo- conversion efficiency (η). The Nb2O5 has different polymorphs: H-Nb2O5 (pseudohexagonal), O-Nb2O5 (orthorhombic), T-Nb2O5 (tetragonal), and M-Nb2O5 (monoclinic) are the most common. Among these phases, the M phase is thermodynamically more stable, whereas the H phase is the least stable one and can be readily transformed into the M phase by appropriate heat treatment.9 Several groups used Nb2O5 as photoelectrodes in DSSC as nanoparticles,10-15 nanobelts,16 and as TiO2-Nb2O5 bilayers.17,18 Nb2O5 has also been used as blocking layers that prevents electron back-transfer.19,20 In many of the above reports, the crystal structure of the Nb2O5 polymorph used for the device fabrication was not considered. Notable exceptions include (i) Aegerter et al.,15 who used nanoparticles of the H-Nb2O5 and reported a photoelectric conversion efficiency (η) of ∼4%; and (ii) Wei et al.,16 who reported an η of 1.42% for orthorhombic Nb2O5 nanobelts. A systematic study is therefore required to identify the most promising polymorph for DSC application. We have now synthesized the three most common polymorphs of Nb2O5: H-Nb2O5, O-Nb2O5, and M-Nb2O5 as 1D nanostructures and systematically compared their photoelectric performances. Various polymorphs were obtained by annealing electrospun polymeric fibers embedded with Nb ions at predetermined temperatures. Their crystal structures were determined by powder X-ray diffraction and electron diffraction techniques. Morphologies, surface, bandgaps, and band edges of all three polymorphs were systematically studied. Our studies show that the H phase has relatively higher photovoltaic performances due to its higher specific surface area achievable from its lower processing temperature. However, the M phase, with the lowest surface area reported here, has attractive charge transport properties. Our results are reported herein.
10.1021/jp106515k 2010 American Chemical Society Published on Web 11/11/2010
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Figure 1. SEM images of the (A) as-spun, (B) as-spun fibers annealed at 500 °C (H-Nb2O5), (C) 800 °C (O-Nb2O5), and (D) 1100 °C (M-Nb2O5). The insets show their respective surfaces indicated by a bright field TEM image.
Experimental Details In the present work, 1D nanostructures of Nb2O5 were prepared by electrospinning a polymeric solution prepared from polyvinylpyrrolidone (PVP; mol wt ) 1 300 000, SigmaAldrich) and niobium ethoxide (NbEt; 99.95% trace metals basis, Sigma Aldrich) in ethanol.21 The solution was hydrolyzed using acetic acid. The electrospinning of Nb2O5 was performed in a commercial electrospinning instrument (NANON, MECC, Japan) with an electric field of 3 × 105 V m-1 and at a flow rate of 2 mL h-1. The composite fibers containing Nb5+ ions in PVP were collected on a grounded collector wrapped with aluminum foil. The composite fibers were annealed at 500, 800, and 1100 °C for 1 h in air in a carbolyte box furnace based on thermal analysis results.21 Crystal structures of the annealed Nb2O5 fibers were studied by X-ray and electron diffraction techniques. The X-ray diffraction (XRD) patterns were recorded by X-ray diffractometer (Philips, X’PERT MPD, CuKR radiation). Lattice parameters were calculated using TOPAS software by fitting the observed XRD patterns to the respective crystal structure. Morphologies of the Nb2O5 nanofibers were examined by field emission scanning electron microscope (FE-SEM, JEOL JSM5600LV). Surface, morphology, and crystal structure were also studied by transmission electron microscopy (JEOL JEM 3010) operating at 300 kV. The Brunauer-Emmett-Teller (BET) surface area of the fibers was measured by a surface area analyzer (Micromeritics Tristar 3000). The annealed fibers were ultrasonically dispersed in acetic acid to develop high-aspect-ratio NWs.7 Polyethylene oxide and ethyl cellulose were added to the above dispersion, and the resulting paste was coated on FTO glass substrates (1.5 cm × 1 cm; 25 Ω/0, Asahi Glass Co. Ltd., Japan), duly spin-coated with a thin layer (∼100 nm) of Nb2O5, by the doctor blade technique. The films were then annealed at 500 °C for 1 h. The DSCs were prepared by soaking the sintered electrodes of area
0.28 cm2 in a dye solution containing N322,23 in a 1:1 volume mixture of acetonitrile and tert-butyl alcohol for 24 h at room temperature. The dye-sensitized samples were then washed in ethanol to remove unanchored dye and dried in a vacuum desiccator. Samples were sealed using a 50 µm spacer. Aetonitrile containing 0.1 M lithium iodide, 0.03 M iodine, 0.5 M 4-tert-butylpyridine, and 0.6 M 1-propyl-2,3-dimethyl imidazolium iodide was used as the electrolyte. A Pt-sputtered FTO glass was used as the counter electrode. Photocurrent measurements of the assembled DSCs were performed using a solar simulator (San Ei, Japan) under 1 sun and at AM1.5G condition. The J-V curves were obtained using a potentiostat (Autolab PGSTAT30, Eco Chemie B.V., The Netherlands). The AC responses of the cells were studied by electrochemical impedance spectroscopy using the Autolab PGSTAT30. Results and Discussion Figure 1 shows the morphology and surface of the as-prepared as well as the sintered nanofibers at the three temperatures. The H-Nb2O5 was obtained by heating the as-spun polymeric fibers at 500 °C for 1 h. The fibers had a diameter of ∼150 nm, a smooth surface (Figure 1B) composed of spheroidal particles of size ∼10-15 nm, and a BET surface area of 55 m2/g. The O-Nb2O5 was obtained by increasing the annealing temperature to ∼800 °C. Grain coarsening and change from spheroidal particles to cylindrical with large aspect ratio while maintaining the continuous fibrous morphology of the electrospun structure was observed (Figure 1C). However, there was reduction in the surface area. The BET surface area of the O-Nb2O5 was 8 m2/ g, which is less than one-sixth that of the H-Nb2O5 fibers. Finally, the M-Nb2O5 was obtained by heating the fibers at 1100 °C (Figure 1D). The fiber morphology was lost due to further grain growth with temperature increase, resulting in nugget morphology. It can be noticed that each fiber did not break down into several nuggets; hence, the large length of
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Figure 2. Electron diffraction and HREM lattice images showing difference in crystallinity and phase purity of the Nb2O5 samples.
Figure 3. XRD patterns of the Nb2O5 polymorphs.
TABLE 1: Lattice Parameters of the Electrospun Nb2O5 Polymorphsa crystal structure
space group
a (Å)
b (Å)
c (Å)
pseudohexagonal orthorhombic monoclinic
P6/mmm Pbam P12/m1
3.600 6.144 21.172
3.600 29.194 3.828
3.919 3.940 19.374
a
The angle for the monoclinic phase was β ) 119.92°.
each nugget was maintained. The M-Nb2O5 had a BET surface area of ∼1.3 m2/g. Figure 2 shows the electron diffraction and high-resolution lattice images of the polymorphs. All the fibers showed a single crystalline, spotty diffraction pattern; however, the degree of crystallinity, which was examined from the discreteness of the diffraction pattern, increased from the H phase to the M phase owing to the higher processing temperatures needed. Figure 3 shows the XRD patterns of the polymorphs. The patterns displayed single phase materials. For convenience, the lattice parameters of these polymorphs determined from Rietveld analyses of the XRD patterns are summarized in Table 1. The band edges of the materials, especially the conduction band edge, play significant role in the open circuit voltage (VOC) of DSCs. The conduction band potential of mesoporous electrode material is measured by direct methods, such as spectro electrochemistry, which follows the measurement of electron absorption in the material.24 In the present study, the band edges of the polymorphs were determined by combining cyclic
Figure 4. The Tauc plots derived from the absorption spectra of the polymorphs.
TABLE 2: Bandgap and Band Edges of the Polymorphs with Respect to Vacuum Level Determined in Comparison with That of the TiO2 material
bandgap (eV)
V. B edge (eV)
C. B. (eV)
TiO2 H-Nb2O5 O-Nb2O5 M-Nb2O5
3.2 3.85 3.77 3.79
-7.4 -7.23 -7.19 -7.12
-4.2 -3.38 -3.42 -3.33
voltammograms (CVs) and absorption spectra. The bandgap of the materials were determined from the experimental UV-vis absorption spectra of the polymorphs using the Tauc procedure; that is, by extrapolating the linear portion of (Rhν)1/2 vs hν plots, where R is the absorption coefficient. Figure 4 shows the (Rhν)1/2 vs hν plots determined from the absorption spectra of the Nb2O5 polymorphs. The bandgap of the materials determined from the above plots are summarized in Table 2. H-Nb2O5 and O-Nb2O5 showed the largest and smallest bandgaps for these polymorphs, respectively. The band edges of Nb2O5 polymorphs were determined by comparing their CV with that of TiO2, which has well established band edges. Our recent study showed that the TiO2 band edges are similar for nanoparticles and nanofibers.25 Figure 5 shows the CVs of the Nb2O5 and TiO2 nanofibers developed by electrospinning. In the present procedure, the valence band edges of the materials were determined from the peak corre-
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Figure 5. Cyclic voltammograms of (A) H-Nb2O5, (B) O-Nb2O5, (C) M-Nb2O5, and (D) TiO2. All the materials were developed by electrospinning.
sponding to the oxidation event in the CVs. Assuming the conduction band edge of TiO2 at -4.2 eV with respect to a vacuum and bandgap energy of 3.2 eV, the valence band edge of TiO2, ETiO2, is taken as -7.4 eV. Knowing the oxidation peak value in TiO2 and Nb2O5 (VTiO2 and VNb2O5), the valence band energy of Nb2O5, ENb2O5, was calculated as
ENb2O5 ) ETiO2 + VTiO2 - VNb2O5
(1)
The band edges of the polymorphs, determined using the above procedure, are also summarized in Table 2. Obviously, the M-Nb2O5 has a higher conduction band edge with respect to vacuum than the other polymorphs and is expected to exhibit high VOC in DSCs. The surfaces of the Nb2O5 films developed onto the FTO substrates were smooth; therefore, any variation in the photovoltaic properties is not arising from the surface nonhomogeneity. The dye-anchored films showed a uniform pink color. The normalized dye-loading to the electrode surface measured using desorption test for the polymorphs were 3.3 × 10-7, 9.6 × 10-8, and 6.6 × 10-8 mol/cm2 for H-Nb2O5, O-Nb2O5, and M-Nb2O5 polymorphs, respectively. If we assume the dye loading to be uniform within the bulk of the films, which was ascertained by keeping the films for more than 24 h in the dyesolution, the bulk dye loading was 1.1 × 10-4, 3.8 × 10-5, and 2.6 × 10-5 mol/cm3, respectively. The above observation shows that dye-loading of the H-phase was only four times higher than that of the M-phase, despite the large difference in their BET surface areas. Figure 6 shows the J-V characteristics of the DSCs fabricated using the electrospun Nb2O5 polymorphs. Table 3 summarizes the photovoltaic properties of the polymorphs. The H-Nb2O5 and O-Nb2O5 displayed similar VOC’s (∼ 0.77 V). The monoclinic analogue has a higher VOC (∼0.81 V), which is attributed to its higher conduction band edge. The JSC and η apparently decreased in the order H-Nb2O5, O-Nb2O5, and M-Nb2O5, respectively. Higher JSC (∼6.7 mA/cm2) observed for the H phase is partially due to the higher dye loading. However, the devices used here were slightly different in terms of their electrode thickness, and a direct comparison of the results is meaningless. To compare their device performance, we normalized the JSC with respect to the absolute value of dyeloading in the units of moles. One major source of error in this procedure is differences in the diffusion lengths of the three polymorphs, which is expected to be much higher than the
Figure 6. I-V characteristic of cells based on the three Nb2O5 crystal structures under standard illumination condition.
TABLE 3: Photovoltaic Parameters of the Three Nb2O5 Polymorphs under Standard Illumination Conditions Nb2O5 type H-Nb2O5 O-Nb2O5 M-Nb2O5
thickness BET durface Jsc (µm) area (m2/g) (mA/cm2) 30 25 25
55 8 1.3
6.68 5.99 4.24
Voc (V)
FF (%)
η (%)
0.77 59.06 3.05 0.77 54.70 2.53 0.81 56.07 1.92
thickness of the present devices. Thus, normalized JSC with respect to the dye loading was ∼2, ∼62, and ∼64 × 106 mA/ mol for the H-, O-, M-Nb2O5, respectively. Interestingly, the normalized JSC of the M phase is notably higher than the H phase, despite its apparently lower device performance. The differences in electrical properties of these polymorphs were determined from electrochemical impedance spectral (EIS) measurements of the DSCs in the dark under forward bias of 0.61 V (Figure 7). A detailed characterization of the charge transport parameters through the Nb2O5 polymorphs will be published elsewhere. The Nyquist plots (Figure 7A) show that the radius of the right semicircle increases from the H phase to the M phase through the O phase. The apparently larger semicircle indicates increase in the charge transfer resistance and, therefore, an increased electron lifetime.26 The lifetime τn can be calculated from the Bode plots (Figure 7B) using the equation τn ) 1/(2πfc), where fc is the peak frequency which for H-, O-, and M-Nb2O5 were 3.22, 3.46, and 2.46 Hz at 0.61 V, respectively. These frequencies correspond to a τn of 49.5, 43.1, and 64.7 ms, respectively; that is, the τn of M-Nb2O5 is apparently longer than the other phases. The longer τn of the device fabricated using the M-Nb2O5 indicates more effective suppression of the back reaction between electrons in its
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Figure 8. Open circuit voltage decay of the Nb2O5 polymorphs.
conduction. The OCVD displayed notably higher τn (∼3500 ms) for the M phase compared with the other two phases. A similar behavior was also observed in the τn determined from the Bode plots. In general, the electron τn determined from the OCVD for electrospun Nb2O5 nanostructures was much higher compared with that of the electrospun TiO2.7,30 The enhanced lifetime and, therefore, slower recombination measured for the M-phase could result from the higher conduction band potential which defines lower chemical capacitance at similar voltages.31 Detailed charge transport studies are currently underway to determine the exact mechanistic details and will be reported elsewhere. Conclusions
Figure 7. Impedance spectra of the polymorphs (A) Nyquist and (B) Bode plots.
conduction band and ions in the electrolyte. The increased τn is expected to be the source of high normalized JSC and η of the device fabricated using M-Nb2O5. The difference in charge decay behavior of the devices fabricated using the different polymorphs was examined by open circuit voltage decay (OCVD) measurements. The OCVD measures the τn of the electrons as a function of VOC. The OCVD curves were recorded by turning off the illumination in a steady state and monitoring the subsequent photovoltage decay. Because the measurement is performed in the dark, recombination with the oxidized dye molecule does not take place, but this is acceptable because the electrolyte accounts for the majority of the recombination, even under illumination.27 Thus, assuming a first-order recombination reaction, the electron lifetime is given by τn ) -(kT/e)(dVOC/dt)-1,28,29 where kT is the thermal energy, e is the positive elementary charge and dVoc/ dt is the first-order time derivative of the VOC. Figure 8 shows the electron lifetime as a function of VOC for the polymorphs considered here. The M and H phases showed an exponential dependence on quasi-Fermi level in the 0.35-0.7 V region, which is typical of the trap-limited diffusion process in mesoporous particles of high surface area. A lowering of τn was observed in the OCVD of the H phase in the 0.2-0.3 V region, confirming the trap-limited diffusion process, which was not observed in OCVD curves of the other two phases, possibly due to their enhanced crystallinity. The OCVD displayed similar τn (∼430 ms) for H and O phases, except in the 0.2-0.3 V region. The τn determined from the OCVD was an order of magnitude higher than that determined from the Bode plots; the source of this discrepancy is not clear to us. The M phase showed faster exponential decay, indicating the band-type
In conclusion, an investigation using electrospun 1D nanostructures of the three polymorphs of Nb2O5, namely, its pseudohexagonal, orthorhombic, and monoclinic analogues, indicates that M-Nb2O5 has a higher conduction band edge and improved charge transport characteristics than the other phases. Owing to the higher conduction band edge, DSCs fabricated using them gave the highest VOC among all the phases. However, the M-Nb2O5 for the present study was obtained by involving a higher processing temperature, which reduced the specific surface area, thereby leading to inferior device performance. The H phase gave the highest photoelectric conversion efficiency owing to its order of magnitude higher surface area compared with the others. The present study indicates that M-Nb2O5 could be an acceptable photoelectrode in DSCs if its mesoporous particles with higher surface area could be developed. Efforts such as doping and partial substitution to minimize the nucleation temperature are currently under investigation. We note that although the M-Nb2O5 has improved charge transport parameters, normalization with respect to the higher crystallinity is still lacking in this study. High processing temperature of M-Nb2O5 imposes severe limitations to synthesize them as mesoporous particles with large surface areas. Acknowledgment. This work is supported in part by the Clean Energy Program Office of the National Research Foundation, Singapore, and the Universiti Malaysia Pahang (RDU 100337). References and Notes (1) Gra¨tzel, M. Nature 2003, 414, 338. (2) Jose, R.; Thavasi, V.; Ramakrishna, S. J. Am. Ceram. Soc. 2009, 92, 289. (3) Thavasi, V.; Renugopalakrishnan, V.; Jose, R.; Ramakrishna, S. Mater. Sci. Eng., R 2009, 63, 81. (4) Bisquert, J.; Fabregat-Santiago, F.; Mora-Sero´, I.; Garcia-Belmonte, G.; Barea, E. M.; Palomares, E. Inorg. Chim. Acta 2008, 361, 684. (5) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W. Prog. PhotoVoltaics 2009, 17, 320.
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Viet et al. (20) Xia, J.; Masaki, N.; Jiang, K.; Yanagida, S. J. Photochem. Photobiol., A 2007, 188, 120. (21) Viet, A. L.; Reddy, M. V.; Jose, R.; Chowdari, B. V. R.; Ramakrishna, S. J. Phys. Chem. C 2010, 114, 664. (22) Jose, R.; Kumar, A.; Thavasi, V.; Fujihara, K.; Uchida, S.; Ramakrishna, S. Appl. Phys. Lett. 2008, 93, 023125. (23) Jose, R.; Kumar, A.; Thavasi, V.; Ramakrishna, S. Nanotechnology 2008, 19, 424004. (24) Diamant, Y.; Chen, S. G.; Melamed, O.; Zaban, A. J. Phys. Chem. B 2003, 107, 1977. (25) Reddy, M. V.; Jose, R.; Teng, T. H.; Chowdari, B. V. R.; Ramakrishna, S. Electrochim. Acta 2010, 55, 3109. (26) Kern, R.; Sastrawan, R.; Ferber, J.; Stangl, R.; Luther, J. Electrochim. Acta 2002, 47, 4213. (27) Ferber, J.; Stangl, R.; Luther, J. Sol. Energy Mater. Sol. Cells 1998, 53, 29. (28) Zaban, A.; Greenshtein, M.; Bisquert, J. ChemPhysChem 2003, 4, 859. (29) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Sero, I. J. Am. Chem. Soc. 2004, 126, 13550. (30) Mukherjee, K.; Teng, T. H.; Jose, R.; Ramakrishna, S. Appl. Phys. Lett. 2009, 95, 012101. (31) Bisquert, J. Phys. Chem. Chem. Phys. 2008, 10, 49.
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