ARTICLE pubs.acs.org/JPCC
Analysis of Charge Carrier Kinetics in Nanoporous Systems by Time Resolved Photoconductance Measurements Dennis Friedrich* and Marinus Kunst Helmholtz-Zentrum Berlin f€ur Materialien und Energie, Institute Solar Fuels, Division Solar Energy Research, Hahn-Meitner Platz 1, D-14109 Berlin, Germany ABSTRACT: A quasi-solid-state dye-sensitized solar cell is presented, where an electrolyte film replaces the conventional liquid electrolyte, reaching a solar light-to-current conversion efficiency of 3%. This proves the importance of surface conduction of the positive charge, e.g., via the Grotthuss mechanism. Contactless transient photoconductance measurements were performed on operating cells, revealing decay behavior of photoinduced charge carriers, dependent on external applied potential conditions. The measurements show that the decay is controlled by the injection of electrons into the front contact, hindered or enhanced by the field in the space charge region. Furthermore, the influence of redox molecules on the decay of photogenerated charge carriers in sensitized TiO2 films was analyzed and found to be dependent on the redox pair concentration. We ascribe this effect to the regeneration of the oxidized dye by iodide, in this way screening the positive charge from possible recombination with injected electrons and leading to a prolonged decay.
’ INTRODUCTION Dye sensitized solar cells are based on a nontoxic oxide semiconductor that is sensitized with an organic dye molecule. Since the demonstration of this cell concept as a bioanalogue model for primary photosynthesis in 1972,1 efficient dye sensitized solar cells have been developed.2 Efficient cells with η ≈ 10% have been presented recently.3 However, a main drawback on the road to industrial implementation is the encapsulation of the liquid electrolyte. Nowadays polymer foils are used for the sealing of the cell, which are extremely vulnerable for leakage when being exposed to day/night cycles with changing temperatures. In contrast to conventional silicon solar cells, little effort to purify the starting material is necessary, resulting in potentially very cheap cell production. We present here a solar cell, which avoids some of the disadvantages of the conventional liquid dye-sensitized solar cell (DSSC).2 This cell consists also of sensitized TiO2 nanoparticles, but the electrolyte is reduced to a stable film on the surface of the nanoparticles. It still includes a redox pair (Iodine-Iodide), specific chemicals, and also strongly bound H2O molecules. This cell still has a modest efficiency, but it promises a significant potential for further optimization. Fortunately, the absence of a liquid electrolyte in this cell allows the application of highly performing transient techniques to the complete cell such as transient photoconductivity in the microwave frequency range (TRMC). Knowledge of kinetic charge transfer behavior is important not only for the improvement of this cell but also for the understanding of charge transport in nanosystems in general. Previously, TRMC studies were performed on bare and r 2011 American Chemical Society
sensitized TiO2 powders and films,4,5 for different excitation intensities and bias illumination6 as well as particle size.7 In contrast to conventional DSSC, the presented solar cell works without any organic solvent. The redox pair (Triiodide/ iodide in a supersaturated state) equilibrates with ambient humidity to form a quasi-solid state gel. The group of quasisolid-state electrolytes refers to both organic solvents and ionic liquids that can be gelated, polymerized, or dispersed with polymeric materials. Both types of liquids have been used as starting materials, and the inclusion of gelating or polymeric agents transforms the electrolyte into a quasi-solid electrolyte.8,9 Carbon particles are directly placed by spraying with an airbrush on the front electrode to catalyze the reduction of triiodide. The functional principle of the dye-sensitized solar cell is the following: Upon photoexcitation, the dye molecules inject an electron into the conduction band of TiO2, leaving the dye in its oxidized state (D+, also referred to as dye cation). The dye is restored to its ground state by electron transfer from the redox pair. The regeneration of the sensitizer by iodide intercepts the recombination of the conduction band electron with the oxidized dye. The I3 ions formed by oxidation of I diffuse to the cathode where the regenerative cycle is completed by electron transfer to reduce I3 to I. Received: January 24, 2011 Revised: July 12, 2011 Published: July 21, 2011 16657
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The proposed pathway for the reduction of the oxidized dye by iodide is given by the following reactions:1012 D f Dþ þ eTiO2
ð1Þ
Dþ þ I f ðD 3 3 3 IÞ
ð2Þ
ðD 3 3 3 IÞ þ I f D þ I2 •
ð3Þ
2I2 • f I3 þ I
ð4Þ
After electron injection from the excited dye (D*), the oxidized dye D+ is reduced by iodide, under the formation of a complex (D 3 3 3 I).11 This complex dissociates with a second iodide ion leading to the dye in the ground state D and the formation of the diiodide radical I2•. The diiodide radicals react to form triiodide and iodide. Previously, the formation of I• 2 was observed by transient absorption (TA) spectroscopy.1315 However, recently transient absorbance (TA) measurements on working cells under operational conditions showed no evidence of such a long-lived radical.16 The performance of a solar cell depends strongly on the efficiency of charge carrier separation. For a DSSC it is convenient to discuss the relevant dynamic processes in two time ranges: i The initial time range: Here the electron injection time must be much smaller than the dye excited-state decay time. ii The longer time range: The injected electron is moving by diffusion to the electrode. Here the time needed for this diffusion must be much smaller than the relevant charge carrier recombination time. We present in this study both the analysis of charge carrier kinetics in an operating quasi-solid-state dye-sensitized solar cell under the influence of externally applied potentials as well as measurements on sensitized TiO2 layers on glass substrates with varied concentrations of redox species applied to these device subsets.
’ EXPERIMENTAL DETAILS Materials. All solvents and reagents were used as received. Transparent conductive glass, TEC 7 (FTO, sheet resistance: 7 Ω/sq, thickness: 2.3 mm) was purchased from Pilkington. TiO2 paste DSL18NR-AO (containing both ∼20 nm and ∼350450 nm average particle size, opaque), DSL18NR-T (∼20 nm average particle size, transparent), and Dye N719 from Dyesol were used as received. Ethanol, acetonitrile (99.8%, anhydrous), 2-propanol (g98.5%), titanium(IV) chloride tetrahydrofuran complex (1: 2) (TiCl4*2THF, 97%), 4-t-butylpyridine (99%), and iodine (99.99%, metals basis) were provided by Sigma-Aldrich. Lithium iodide (anhydrous) and t-butanol (g98.5%) were purchased from Fluka. Lithium perchlorate (99.5%, anhydrous) was purchased from Ventron-Alfa. Printex XE2 carbon black was provided by Degussa-Evonik. RBS 50 concentrate (Carl Roth) was diluted to 3% v/v in water. Milli-Q (Millipore) grade water was used in all experiments. Device Fabrication. The front electrode was prepared by masking a 1 1 cm area plus the contact area on the FTO glass with a polyimide (PI) tape and by covering the rest of the FTO substrate with zinc powder. The surrounding FTO layer was removed by adding drops of 2 M hydrochloric acid solution.
Then the PI-tape was removed and the substrates were sonicated for 15 min in RBS solution, followed by 10 min in water, acetone and ethanol and dried in a stream of N2. The TiO2 layers were deposited on the FTO substrates via screen printing (SEFAR 6164W screen mesh) the titanium dioxide paste (DSL18NRAO) in two subsequent cycles, resulting in ∼12 μm thick layer of TiO2 particles with a surface area of 1 cm2. The substrates were sintered at 520 °C for 30 min, cooled, followed by a posttreatment in a 50 mM TiCl4 solution for 30 min at 70 °C, flushed with water and again sintered for 30 min at 450 °C.17 After the second heat treatment the substrates were cooled to 60 °C, immersed into the dye solution (1 mM of N719 in a 1:1 ratio of acetonitrile/t-butanol), and stored at room temperature for ∼16 h. After sensitization, the area surrounding the sensitized TiO2 was masked with adhesive PTFE tape (Chem-Tec) and 3 μL of the electrolyte solution (5 M lithium iodide/0.05 M iodine/0.05 M 4-t-butylpyridine in ethanol) were deposited on the electrode. After 24 h, a layer of carbon nanoparticles was deposited on the substrate by spraying a carbon suspension (Printex XE2 in 2-propanol) with an airbrush, acting as catalyst for the reduction of iodine and as the electric contact to the counter electrode. The electrodes were stored for another 24 h to remove volatile compounds from the carbon layer. Finally, the PTFE tape was removed and a FTO glass was pressed on top of the front electrode, fixed with epoxy resin, and the cell was contacted. TiO2 layers from titanium dioxide paste DSL18NR-T were deposited on quartz glass and sensitized with N719 in the same way as for the solar cells. No TiCl4 post-treatment was performed for these device subsets. The constitutents and deposition procedure of the electrolyte film for the working devices and the sensitized TiO2 films were identical. IV Measurements. The IV characteristics of the solar cells were determined by using a solar simulator (VOSS Electronic GmbH, WXS-140S-Super). AM1.5G (equivalent to an irradiance of 100 mW cm2) illumination conditions were simulated by combined use of a 100 W xenon lamp and a 120 W halogen lamp. Measurements were performed both at irradiation intensity of 100 mW cm2 and 10 mW cm2, while the latter was realized by the use of a neutral density filter. The system was periodically calibrated with a Si-reference solar cell. The currentvoltage characteristics were obtained by applying an external potential bias to the cell and by measuring the generated photocurrent with a measuring unit (Keithley SMU 238). The temperature of the sample holder was set constant by a thermostat at a given value, normally set at 23 °C for standard measurements. Time Resolved Microwave Conductivity Measurements (TRMC). Transient photoconductance measurements in the microwave frequency range were performed by the time resolved microwave conductivity (TRMC) technique in a Ka-band (28.540 GHz) apparatus as described previously.18,19 The excitation occurred by 10 ns (fwhm) pulses of a Nd:YAG laser at a wavelength of 532 nm with a diameter of about 3 mm. The excitation intensity was adjusted by the use of calibrated filters. External potentials were applied by connecting the front contact of the cell with the working electrode, and the back contact to the combined counter- and reference electrode of the potentiostat (Wenking POS 73). The potentials were set at 0.1 V steps for the regime from +0.6 to 0.6 V. The TRMC signal, ΔP(t)/P, is the relative change of the microwave power reflected by the sample induced by a photogenerated change of the conductance (ΔS(t)).18 In general the TRMC signal is determined by all mobile excess species but for 16658
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Figure 1. PhotoIV characteristic of a quasi-solid-state dye-sensitized solar cell under AM1.5G illumination conditions (100 mW/cm2, solid line) and 10 mW/cm2 (dashed line).
Figure 3. Lifetime τ of the photogenerated charge carriers as a function of the applied external potential. Solid diamonds (open diamonds) represent the lifetime from the exponential fit of the TRMC signal for the time regime of 200 ns to 8 μs (20 to 75 μs).
well as recombination processes. Also TRMC measurements under external applied potential conditions will be presented. It is important to note that the TRMC signal reflects the kinetics of excess electrons.5 The kinetics of the corresponding positive charges is only observed insofar this kinetics influences electron kinetics by recombination.
’ RESULTS
Figure 2. Semilogarithmic plot of the TRMC signal of a quasi-solidstate dye-sensitized solar cell induced by 532 nm light pulses at an excitation density of 20 mJ/cm2. Signals are plotted for an applied external potential to the cell as indicated. Included are the exponential fits of the decay times for time intervals 200 ns to 8 μs (solid lines) and 20 to 75 μs (dashed lines). The insert shows the signals in a doublelogarithmic representation.
the case of TiO2 it will be assumed that only electrons at the bottom of the conduction band with mobility μe contribute to the photoconductance: ΔPðtÞ ¼ AΔSðtÞ ¼ AeΔNðtÞμe P
ð5Þ
A is a sensitivity factor depending on the experimental configuration and the electrical parameters of the sample.19 The total number of excess electrons with mobility μe at time t, ΔN(t), refers to an integration of the excess electron concentration, Δn(t), only over the thickness d of the sample. Any process that decreases the number of excess electrons in the conduction band leads to a decay of the photoconductivity. Photoconductance measurements by the TRMC technique allow the characterization of nanoporous systems such as DSSC, providing information on charge carrier kinetics for injection as
A. Operating Solar Cell. The IV curve of our quasi-solidstate dye-sensitized solar cell is shown in Figure 1. Reasonable values for open-circuit potential and fill factors could be obtained, whereas the rather small current seems to be the limiting parameter. The cell has a better performance for reduced light intensities. This indicates a higher carrier loss with increasing excess charge concentration. The most probable reason for this effect is a saturation of the transport of the positive charge from the excited dye to the counter electrode. Therefore, the recombination reaction of injected electrons, e TiO2, with oxidized redox species R+, as well as the recombination of electrons with the oxidized dye may limit the short-circuit photocurrent density. The overall smaller photocurrent, compared to conventional liquid iodine/iodide dye cells, may be due to an incomplete covering of the sensitized TiO2 with the electrolyte film, preventing an efficient regeneration of the excited dye. The results of the TRMC measurements for different applied external potentials are presented in Figure 2. The TRMC signals show a potential dependence. The decay is rather slow in the short time range, up to a few microseconds. Much slower than the previously reported power-law decay behavior of excess charge carriers in dye sensitized TiO2 films, usually ascribed to dispersive transport and recombination of electrons in the TiO2 film.4 An appreciable decay of the TRMC signal is observed after about 5 μs and this decay depends on the applied potential. The TRMC signals are presented in Figure 2 together with the results of the exponential fits for the time interval between 20 and 75 μs showing an increase of the decay time with decreasing external potential; reaching the maximum at 0.6 V and the minimum at 0.6 V. 16659
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Figure 4. Normalized TRMC signal for sensitized TiO2 film on quartzglass with/without redox pair. The insert shows the signals in a semilogarithmic representation on a shorter time scale. Laser intensity: 1.3 mJ/cm2.
Figure 3 shows the lifetimes for the two time intervals, which were chosen for a convenient description from 200 ns to 8 μs and from 20 to 75 μs as a function of the applied external potential. The lifetime of the photogenerated charge carriers decreases with increasing applied potential. There are in principle three decay channels for the electron injected from the dye into the TiO2: (1) The injected electron can recombine with the oxidized dye, (2) with the oxidized redox species in the electrolyte, and (3) as well be injected into the front contact. The potential dependence of the decay suggests that the decay is mainly due to the injection of charges into the front contact because the predominant influence of an applied external potential is at the FTO/TiO2 interface. In the short time range, a change in decay time from ∼20 μs at 0.6 V to ∼80 μs at 0.6 V is observed, while in the long time range it increases from ∼75 μs at 0.6 V to ∼450 μs at 0.6 V. The lifetime reaches its maximum at an external potential of 0.6 V. This potential lies near the open-circuit potential of the cell. The photocurrent density in this potential regime is minimal, i.e. the conclusion can be drawn that the potential dependent decay observed is limited by the injection of electrons into the front contact. The field in the space charge region at the FTO/TiO2 interface controls this injection. The onset of the decay at about 5 μs can be interpreted as the time required for the excess charge carriers to reach the front contact and to enable electron injection into the FTO contact. Haque et al. observed a drastic increase of the decay time of the cation state with increasing applied potential.20 However, their samples did not contain the I/I3 redox couple. They attributed this effect to a decrease of the electron-cation recombination rate. This suggests that the presence of a high concentration of redox molecules in our samples quenches the recombination by fast transfer between dye cation and the redox couple. This effect will be investigated in the next section. Furthermore, no appreciable effect of the applied voltage on the amplitude of the signal is observed in our studies. This is probably at least partially due to the high excitation density used. The results of this section show that the quasi solid-state DSSC presented in this work functions as a standard DSSC albeit with lower efficiency. However, the absence of liquid in this cell makes a better investigation of the (opto) electronic properties of
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Figure 5. Normalized TRMC signal for sensitized TiO2 films with different electrolyte concentrations. The insert shows the signals in the first 500 ns and the exponential fits for the fast initial decay in the time regime up to 60 ns. Laser intensity: 1.3 mJ/cm2.
DSSC possible. For a higher sensitivity, the samples used for the analysis will be on a quartz substrate but the configuration is still very near to the device described here to warrant the value of this analysis for the functioning of DSSC. B. Sensitized TiO2 Films. In this section the TRMC signals of different subsets of the device presented in the previous section will be compared in order to investigate the influence of the different components on the electron transport. In Figure 4 the TRMC signals of a sensitized TiO2 film on quartz glass are compared to that of a sensitized TiO2 layer with an electrolyte film as used in the device. The deposition of the electrolyte film with a high iodine/iodide concentration leads to a much slower decay when compared to the untreated sample. The decay of the signals cannot be described by only one exponential function with a single time constant. Therefore, the signals are separated into time regimes characterized by their respective, exponential decay constants. The signals show a fast initial decay with time constants (determined by an exponential fit in the time regime up to 60 ns) τ1 ≈ 160 ns for the “bare” TiO2/N719 sample and τ2 ≈ 390 ns for the TiO2/N719/electrolyte sample. In the time regime from 10 to 75 μs the signal of both the untreated TiO2/ N719 and TiO2/N719/electrolyte sample decays rather slowly with τ3 ≈ 160 μs and τ4 ≈ 600 μs, respectively. The fast initial decay suggests a fast recombination of photogenerated charge carriers, especially in view of the high excitation densities. Most probably electrons injected from the dye into TiO2 recombine with the oxidized dye or via surface states. In the presence of the redox pair the oxidized dye can be reduced by iodide, thus quenching this decay channel. Besides, the dye adsorbed at the surface may also passivate surface states responsible for the dispersive transport. To further evaluate the decay behavior in the presence of the redox pair, the concentration of the electrolyte constituents was varied (Figure 5). The decay of the TRMC signal shows a dependence on the electrolyte concentration, having the fastest decay for the lowest concentration of iodine/iodide. The decay time in the range up to 60 ns decreases with decreasing electrolyte concentration. For a 10-fold-reduced concentration of the electrolyte solution the lifetime is nearly halved. However, for the lifetime in the time region from 10 to 75 μs there is only a minor diminution for the lowest concentration. In addition, the 16660
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Figure 6. Influence of electrolyte constituents ethanol and 4-t-butylpyridine (TBP) on the TRMC signal. Laser intensity: 1.3 mJ/cm2.
halved and 5-fold-reduced concentrations appear to induce a saturated condition, as there is no notable difference in the decay behavior. Since the electrolyte does not only consist of the redox pair but also 4-t-butylpyridine (TBP) as an additive and ethanol is used as a solvent, the possible influence of these components on the charge carrier decay was analyzed. The addition of ethanol to the sensitized TiO2 layer causes an increase of charge carrier lifetime as can be seen in Figure 6. It was previously reported that adsorption of 2-propanol on TiO2 powder leads to a decrease of the electron decay rate after band-to-band excitation and this was attributed to a decrease of the electron recombination rate by the trapping of excess holes in surface states induced by 2-propanol with a low recombination probability.5 Similar conclusions are drawn here for the case of ethanol acting as a hole scavenger in TiO2 films.21,22 In general the decrease of the decay rate under influence of ethanol (Figure 6) is explained by a decrease of the recombination rate as described above, a decrease of the electron trapping rate by suppression of active electron traps, or both. Adding TBP leads to a further prolonged decay. One effect of TBP in the redox electrolyte of dye-sensitized TiO2 solar cells was observed to change the surface charge of TiO2 by decreasing the amount of adsorbed protons, Li+ ions, or both.23 For the case of ethanol/TBP, the slower decay may be attributed to a screening of positive charges on the TiO2 surface, therefore hindering the recombination of electrons. The adsorption of cations like Li+ on the TiO2 surface was proposed to accelerate the regeneration of oxidized dye by reversing the particle surface charge from negative to positive, which causes I to adsorb electrostatically onto the nanoparticles.10 The formation of (I,I) ion pairs on the surface of TiO2 was suggested to allow the more energetically favorable and faster regeneration reaction involving oxidation of I to I2• to take place.10,11 To distinguish the influence of adsorbed Li+ from the effect of the redox couple on the signal decay, TiO2 films were treated with electrolyte solutions containing high amounts of lithium perchlorate (Figure 7). The decay of the sample treated with LiClO4 in ethanol shows only a slightly slower decay, comparable to the effect of ethanol on the sensitized TiO2 films (Figure 6). The samples corresponding to the two upper curves in Figure 7 were treated with the same concentration of LiI/I2, but only one sample received an additional treatment with
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Figure 7. Influence of cation concentration on the TRMC signal. Laser intensity: 1.3 mJ/cm2.
LiClO4. The sample treated with the high LiClO4 concentration in addition to the standard LiI/I2 composition shows a slightly accelerated decay. The presence of the LiClO4 might induce a competition for adsorption sites of the two species, thereby partially hindering the regeneration of oxidized dye by I. As there is no significantly prolonged decay for the sample treated only with LiClO4 it can be concluded that the slower decay is due to the presence of the Iodine/iodide redox pair in the electrolyte solution. In general, trapping or recombination may occur during the excitation pulse. In particular, recombination during the pulse is probable at the high excitation densities used in our experiments. However, an analysis of such effects is beyond the scope of this paper. Of interest for the present work is that an increase of the effective lifetime under surface modification (Figures 47) coincides with higher amplitude of the TRMC signal. This indicates that fast decay processes during the excitation pulse are also quenched by the surface modification. So although there is an effect of this modification on the processes during the excitation, this does not influence at all the discussion given above.
’ DISCUSSION Figure 1 shows that the DSSC proposed in this work functions as a conventional DSSC albeit with lower efficiency. This implies that not only electron transport to the front electrode takes place (as it is expected because this is not much different from a conventional DSSC) but also hole (positive charge) transport to the back electrode. This is more astonishing because no more solvent is present. This implies that diffusive ion transport in a solvent can be excluded as a mechanism. Only some kind of surface transport is possible as for example transfer of the charge by a iodide network, e.g., via the Grotthuss mechanism,24 which has been proposed as an alternative type of charge transport in devices where diffusion limitations might occur, such as iodide based ionic-liquids.25,8,26 Here, charge transport occurs via the rearrangement of chemical bonds in a network of polyiodide species25 I3 þ I f I
I2 3 3 3 I f I 3 3 3 I2
I f I þ I 3
The formation of polyiodide species was detected in spectroscopic measurements, giving additional support for this kind of 16661
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The Journal of Physical Chemistry C enhanced charge transport.27,28 The high iodide concentration of our quasi solid-state DSSC supports such an explanation. This work is an unambiguous proof that surface transport of the positive charge occurs in our quasi-solid-state DSSC. For conventional DSSC at least a part of the transport may occur via this surface transport. The TRMC signals observed are not monoexponential and are characterized by a decay rate constantly decreasing with time as it can be observed in the Figures 27. This is expected because of the heterogeneity of samples: a distribution of electron traps and recombination centers will show this behavior if also emission from these states play a role. Previous studies employing transient absorbance (TA) techniques revealed an increase in the rate of dye cation decay with increasing iodide concentration.13,15 The observed decay was separated into two components: (1) a fast decay channel involving quenching of the cation signal and assigned to the reduction of the oxidized dye by iodide leading to the formation of I2• species and (2) a slower channel assigned to long-lived I2• and e TiO2, with the decay of this signal being assigned to the I2• dismutation reaction (eq 4) and interfacial charge recombi11,15 In agreement with nation of e TiO2 with the redox couple. these findings, we attribute the observed slower decay of the TRMC signal for high concentrations of the redox pair (depending on the concentration of the redox pair) to the regeneration of the oxidized dye by iodide, therefore screening the positive charge from possible recombination with injected electrons e TiO2. The recombination of electrons with I3 does not seem to be rate limiting the fast decay behavior as the decay constants decrease with reduced electrolyte concentration, i.e., reduced I3 ion concentration. Furthermore, the main decay channel for the sensitized TiO2 films in absence of any redox species is ascribed to the recombination of injected electrons with the oxidized dye D+. Also experiments can be cited on nonsensitized TiO2 reporting a decrease of the electron lifetime by adding a redox couple.29 This provides additional support for the explanation given above: the increase of the electron lifetime observed here is intimately connected to the presence of the positive counter charge in the form of the dye cation. The interaction of this dye cation with the redox couple decreases the recombination probability of the electrons. The high excess charge concentration used in this work can be compared to the excess charge concentrations in working DSSC.30 This suggests that our qualitative conclusions are applicable to the function of DSSC. It is clear that information on fast electron trapping6,7 cannot be obtained from our measurements.
’ CONCLUSIONS A quasi solid-state dye sensitized solar cell is presented with an efficiency of 3%. Excess charge carrier decay was shown to be dependent on external applied potential conditions. Electron injection into the front contact limits this decay and is controlled by the space charge field. The important role of surface transport of the positive counter charge was shown. Contactless transient photoconductance measurements on sensitized TiO2 layers revealed charge carrier kinetics strongly dependent on the concentration of redox species in the electrolyte film. In the presence of the redox pair the prolonged decay is attributed to the regeneration of the oxidized dye by iodide. Also the
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adsorption of cations such as Li+ leads to a (relatively weak) increase of the electron lifetime, although in combination with a redox couple these effects are less clear. The adsorption of cations such as Li+ was not identified to be the predominant process.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank the Deutsche Bundesstiftung Umwelt (DBU) for financial support. ’ REFERENCES (1) Tributsch, H. Photochem. Photobiol. 1972, 16, 261–269. (2) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (3) Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Gr€atzel, C.; Nazeeruddin, M. K.; Gr€atzel, M. Thin Solid Films 2008, 516, 4613–4619. (4) Kunst, M.; Goubard, F.; Colbeau-Justin, C.; W€unsch, F. Mater. Sci. Eng., C 2007, 27, 1061–1064. (5) Schindler, K. M.; Kunst, M. J. Phys. Chem. 1990, 94, 8222–8226. (6) Kroeze, J. E.; Savenije, T. J.; Warman, J. M. J. Am. Chem. Soc. 2004, 126, 7608–7618. (7) Katoh, R.; Huijser, A.; Hara, K.; Savenije, T. J.; Siebbeles, L. D. A. J. Phys. Chem. C 2007, 111, 10741–10746. (8) Kubo, W.; Murakoshi, K.; Kitamura, T.; Yoshida, S.; Haruki, M.; Hanabusa, K.; Shirai, H.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2001, 105, 12809–12815. (9) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gr€atzel, M. J. Am. Chem. Soc. 2003, 125, 1166–1167. (10) Pelet, S.; Moser, J.-E.; Gr€atzel, M. J. Phys. Chem. B 2000, 104, 1791–1795. (11) Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Gr€atzel, M.; Durrant, J. R. J. Phys. Chem. C 2007, 111, 6561–6567. (12) Boschloo, G.; Hagfeldt, A. Acc. Chem. Res. 2009, 42, 1819–1826. (13) Nogueira, A. F.; De Paoli, M.-A.; Montanari, I.; Monkhouse, R.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2001, 105, 7517–7524. (14) Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. J. Phys. Chem. B 2002, 106, 12693–12704. (15) Montanari, I.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2002, 106, 12203–12210. (16) Anderson, A. Y.; Barnes, P. R. F.; Durrant, J. R.; O’Regan, B. C. J. Phys. Chem. C 2010, 114, 1953–1958. (17) Sommeling, P. M.; O’Regan, B. C.; Haswell, R. R.; Smit, H. J. P.; Bakker, N. J.; Smits, J. J. T.; Kroon, J. M.; van Roosmalen, J. A. M. J. Phys. Chem. B 2006, 110, 19191–19197. (18) Kunst, M.; Beck, G. J. Appl. Phys. 1986, 60, 3558–3566. (19) Swiatkowski, C.; Sanders, A.; Buhre, K.-D.; Kunst, M. J. Appl. Phys. 1995, 78, 1763–1775. (20) Haque, S. A.; Tachibana, Y.; Willis, R. L.; Moser, J. E.; Gr€atzel, M.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. B 2000, 104, 538–547. (21) Peiro, A. M.; Colombo, C.; Doyle, G.; Nelson, J.; Mills, A.; Durrant, J. R. J. Phys. Chem. B 2006, 110, 23255–23263. (22) Tamaki, Y.; Furube, A.; Murai, M.; Hara, K.; Katoh, R.; Tachiya, M. J. Am. Chem. Soc. 2006, 128, 416–417. (23) Boschloo, G.; H€aggman, L.; Hagfeldt, A. J. Phys. Chem. B 2006, 110, 13144–13150. (24) Agmon, N. Chem. Phys. Lett. 1995, 244, 456–462. (25) Papageorgiou, N.; Athanassov, Y.; Armand, M.; Bonhote, P.; Pettersson, H.; Azam, A.; Gratzel, M. J. Electrochem. Soc. 1996, 143, 3099–3108. (26) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gr€atzel, M. J. Am. Chem. Soc. 2003, 125, 1166–1167. 16662
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