Dye Regeneration by Spiro-MeOTAD in Solid State Dye-Sensitized

Mar 19, 2009 - Photoinduced absorption (PIA) spectroscopy is presented as a tool for the systematic study of dye regeneration and pore filling in soli...
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J. Phys. Chem. C 2009, 113, 6275–6281

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Dye Regeneration by Spiro-MeOTAD in Solid State Dye-Sensitized Solar Cells Studied by Photoinduced Absorption Spectroscopy and Spectroelectrochemistry Ute B. Cappel,† Elizabeth A. Gibson,† Anders Hagfeldt,†,‡ and Gerrit Boschloo*,† Department of Physical and Analytical Chemistry, Ångstro¨m Laboratory, Uppsala UniVersity, Box 259, 751 05 Uppsala, Sweden ReceiVed: December 18, 2008; ReVised Manuscript ReceiVed: February 13, 2009

Photoinduced absorption (PIA) spectroscopy is presented as a tool for the systematic study of dye regeneration and pore filling in solid state dye-sensitized solar cells (DSC). Oxidation potentials and extinction coefficients for oxidized species of the perylene dye, ID28, on TiO2 and of the hole conductor, 2,2′7,7′-tetrakis-(N,Ndi-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD), were determined by spectroelectrochemistry. The onset of oxidation of a solid film of spiro-MeOTAD was found to be 0.15 V versus Fc/Fc+ and extinction coefficients of spiro-MeOTAD+ were found to be 33 000 M-1 cm-1 at 507 nm and 8500 M-1 cm-1 at 690 nm. Electrons in TiO2 films were shown to alter the ground-state absorption spectra of ID28 attached to TiO2. PIA measurements indicated a good contact between ID28 and spiro-MeOTAD for different spiroMeOTAD concentrations for both 2- and 6-µm thick TiO2 films. We discuss the possibility of estimating the quality of pore filling from the positions of absorption peaks. Results suggested that with a spiro-MeOTAD concentration of 300 mg mL-1 in chlorobenzene, a uniform distribution of spiro-MeOTAD in the pores of the 6-µm thick TiO2 film could be achieved. Introduction Since their discovery in 1991,1 much research has been carried out in the field of dye-sensitized solar cells (DSC).2-4 For several years now, efforts have been made to replace the liquid electrolyte by a solid state hole conducting material. The organic hole conductor, 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD), is one promising candidate.5-7 Devices incorporating this material have reached efficiencies up to 5%,8 which is about half-that of the best cells with iodide/iodine liquid electrolyte. The optimal mesoporous TiO2 film thickness for these cells was found to be about 2 µm. This is much thinner than in the liquid cell equivalents and limits the light harvesting efficiencies of the solar cells. One reason for this limitation has been identified in the shorter electron lifetime of the cells due to the faster recombination of electrons with holes in spiro-MeOTAD than with tri-iodide.9 Another reason, which has been cited as inhibiting the use of thicker films, is poor pore penetration of spiro-MeOTAD which is usually applied by spin coating from solution.10 In an unfilled pore, the dye molecules can still inject electrons into the TiO2 but they are not regenerated and the oxidized dye molecules recombine with electrons from the TiO2. Alternatively, if the hole conductor is not interconnected everywhere, then all dye molecules can be regenerated but the positive charge cannot be transported out of the cell. While recombination times of electrons in TiO2 with holes in spiro-MeOTAD have been measured by intensity modulated photovoltage spectroscopy (IMVS),9,11 small perturbation opencircuit voltage decay measurements8 and impedance spectroscopy,12 the main method for studying pore filling has been * To whom correspondence should be addressed. E-mail: [email protected]. † Uppsala University. ‡ Center of Molecular Devices, Department of Chemistry, KTH Chemical Science and Engineering, 10044 Stockholm, Sweden.

scanning electron microscope (SEM) cross-sections.10 However, these give only a qualitative result and though “empty pores” can be seen, nothing is said about the contact between dye and hole conductor, and whether the pores are completely unfilled or whether the hole conductor only coats the TiO2 surface without filling the pore completely. Kroeze et al.13 have used transient absorption spectroscopy at selected wavelengths to determine hole transfer yields from different hole conductors to the ruthenium-based dye, N719. Recently, Snaith et al.14 have estimated the pore filling fractions from overstanding layer thicknesses of spiro-MeOTAD over nanoporous TiO2 films compared to the overstanding layer thickness over compact TiO2 films. In their study, they also compared the photoinduced absorption (PIA) spectra for TiO2 films of different thicknesses and for different spiro-MeOTAD concentrations to understand the pore filling, but without analyzing these results in detail. Photoinduced absorption spectroscopy is a popular tool for studying photoactive polymers15,16 and organic solar cells17,18 and was previously presented by our group as a method to study injection and regeneration processes in DSCs.19,20 In the absence of a redox mediator, the absorption spectra of electrons in TiO2 and of oxidized dye can be measured. In the presence of redox mediator, the spectrum of the oxidized redox mediator is seen instead of the spectrum of the oxidized dye. If the redox mediator is not able to regenerate the dye or not in contact with every dye molecule, then the spectrum of the oxidized dye would still be observed in presence of the mediator. Therefore, if under certain conditions, pores remain unfilled by spiro-MeOTAD, signals for the oxidized dye should be observed in the PIA spectrum. Here, we have used two different TiO2 film thicknesses and a variety of spiro-MeOTAD concentrations for the spin-coating solutions to systematically study the effect on dye regeneration. To obtain a basic understanding of the coupling between dye and hole conductor, the additives to the hole conducting matrix employed for the best performing cells, Li(CF3SO2)2N and tert-

10.1021/jp811196h CCC: $40.75  2009 American Chemical Society Published on Web 03/19/2009

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Figure 1. Chemical structure of the perylene dye, ID28, and of the hole conductor, spiro-MeOTAD.

butyl pyridine, were not used. The dye, ID28, is a perylene dye (Figure 1), which performed well in the liquid cell.21 A derivative of this dye has recently achieved an efficiency of 1.8% in the solid state.22 Perylenes are promising dyes for DSC applications due to their stability but in order to achieve high efficiencies in the solid state, thicker films are needed for better light harvesting. ID28 binds to TiO2 through ring opening of the anhydride group and has a bis((tert-octyl)-phenyl)amine group at the other end which is somewhat similar to the methoxyphenyl amine groups on spiro-MeOTAD (Figure 1). Therefore, a favorable contact between ID28 and spiroMeOTAD is expected, which should lead to good wetting of the dye molecules by the hole conductor. In addition to measuring PIA spectra of the films, we used spectroelectrochemistry to determine the spectral responses of the different transient species and their extinction coefficients. In analyzing the PIA spectra, we consider the effect the groundstate and transient species can have on each others absorption spectra in addition to the separate transient spectra. This way, qualitative judgements about the pore wetting as well as the pore filling can be made. The results are supported by comparison to SEM cross-sections for selected samples. Experimental Methods Sample Preparation. Nanoporous TiO2 films were prepared by spincoating of a colloidal TiO2 paste containing nanoparticles of 18 nm in diameter. Different film thicknesses were obtained by diluting the paste with different amounts of terpineol. For spectroelectrochemical measurements, the paste was directly spincoated onto fluorine-doped tin oxide (FTO) glass. PIA substrates were prepared using substrates with compact layers of TiO2 prepared by spray pyrolysis. All substrates were sintered for 1 h at 450 °C in an oven. The resulting film thicknesses for substrates used in spectroelectrochemistry was 2 µm, while the film thicknesses for PIA samples were either 2 or 6 µm. After cooling to about 60 °C, the films were immersed in a dye bath containing 0.5 mM ID28 in dichloromethane. Where applicable, spiro-MeOTAD (purchased from Merck) was applied to the films by leaving the solution of spiro-MeOTAD to soak the substrates for 1 min and then spin coating with 2000 rpm for 30 s. The spiro-MeOTAD concentrations used for PIA samples were 50 mg, 100 mg, 200 mg and 300 mg spiro-MeOTAD per 1 mL chlorobenzene. Spiro-MeOTAD solutions were heated at 90 °C for 20 min to aid dissolution. For electrochemical measurements, films prepared from spiro-MeOTAD contentrations of 200 mg mL-1 were used.

Cappel et al. Spectroelectrochemistry. Electrochemical measurements were performed on a CH Instruments 660 potentiostat with a 3-electrode setup. At the same time as carrying out the electrical measurements, UV-visible spectra were recorded in the range from 300 to 1000 nm on an HR-2000 Ocean Optics fiber optics spectrophotometer. For measurements of spiro-MeOTAD, FTO substrates with spiro-MeOTAD spincoated directly onto the substrate, or FTO substrates with spiro-MeOTAD spincoated into the pores of nanoporous TiO2 films (both dyed and not dyed) were used as working electrodes. The ionic liquid, 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide (purchased from Ionic Liquids Technologies), was used as supporting electrolyte to minimize the dissolution of spiro-MeOTAD. A platinum wire was used as the counter electrode and an Ag/ AgCl electrode in LiCl saturated ethanol as reference. For measurements of ID28, a dye-coated nanoporous TiO2 film on an FTO substrate was used as the working electrode, a platinum wire was used as the counter electrode and an Ag electrode as pseudo reference electrode. The electrolyte solution was 0.1 M tetrabutylammonium perchlorate in acetonitrile. Both systems were internally calibrated with ferrocene/ferrocenium (Fc/Fc+) using a platinum working electrode. To determine the oxidation potentials, cyclic voltammetry was performed with a scan rate of 1 mV s-1 for spiro-MeOTAD and a scan rate of 5 mV s-1 for ID28, first scanning toward positive potential. To determine the absorption spectra and extinction coefficients of the oxidized species, potential step experiments were performed, where the potential was stepped to a potential above the onset of oxidation, while the current was being recorded. Data analysis was carried out in MATLAB. UV-visible spectra were analyzed by Principal Component Analysis (PCA)23 to determine the difference in the spectra of oxidized and ground-state species. PIA Measurements. For the PIA measurements, the same setup as described by us previously20 was used. White probe light provided by a 20 W tungsten-halogen lamp was superimposed with a square-wave modulated (on/off), blue LED (Luxeon Star 1W, Royal Blue, 460 nm) used for excitation. Excitation of spiro-MeOTAD at this wavelength should be negligible compared to excitation of the dye, and PIA spectra in the region of 500 to 1000 nm could be obtained. The transmitted probe light was focused onto a monochromator (Acton Research Corporation SP-150) and detected by a UV enhanced silicon photodiode connected to a current amplifier and lock-in amplifier (Stanford Research Systems models SR570 and SR830, respectively). The intensity of the probe light on the sample was ∼1/10th of a sun. The intensity of the excitation LED could be varied between 1 mW cm-2 and 53 mW cm-2. For the spectral resolved PIA measurements presented here, an intensity of 8.3 mW cm-2 and a modulation frequency of 9.3 Hz were used. Scanning Electron Microscope Images. SEM images were obtained on a Zeiss LEO1550 high resolution SEM at 5.00 kV acceleration voltage and working distances between 12 and 26 mm. For the images of the 2-µm thick TiO2 films, a magnification of 60 000× was used, and for the images of the 6-µm thick TiO2 films, a magnification of 30 000× was used. Results and Discussions Spectroelectrochemistry. Cyclic voltammograms for spiroMeOTAD in solution as well as on ITO substrates have been previously described by Garcı´a-Can˜adas et al.24 In their experiments, they used 0.1 M KPF6 in propylene carbonate as the supporting electrolyte for measurements on solid spiro-MeOTAD films. In our experiments, we found there was a tendency

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Figure 2. Cyclic voltammetry of a spiro-MeOTAD film on FTO with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide as electrolyte and a scan rate of 1 mV s-1. Inset: zoom of the onset of the first oxidation of spiro-MeOTAD.

Figure 3. Extinction coefficients of oxidized spiro-MeOTAD minus extinction coefficients of ground-state spiro-MeOTAD (black); extinction coefficients of oxidized ID28 dye minus extinction coefficients of ground-state ID28 dye on TiO2 (gray).

for spiro-MeOTAD to dissolve when using propylene carbonate as a solvent for the electrolyte. We resolved this by employing the ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, as electrolyte. The cyclic voltammogram for a spiro-MeOTAD film on FTO is shown in Figure 2. In the forward direction, we observe two distinct onsets for oxidation, the second one leading to a peak at 0.5 V vs Fc/Fc+. However, we do not observe the two separate peaks for the formation of spiro-MeOTAD+ and spiro-MeOTAD2+ reported by Garcı´aCan˜adas et al.24 In the reverse direction, the voltammogram is very complex, showing several reduction peaks, and it is beyond the scope of this work to discuss their meaning in detail. As the positions of oxidation and reduction peaks are influenced by charge transport and nonreversibility of some of the processes, it is not possible to determine formal oxidation potentials from the cyclic voltammetry. Instead, we estimate the onset of the oxidation processes from the intersection of the tangents of the rising oxidation current and the background current as described by Micaroni et al.25 The onset of the first oxidation of spiro-MeOTAD was found to be 0.15 V vs Fc/ Fc+. This potential indicates the position of the valence band edge of spiro-MeOTAD and is quite similar to the first oxidation potential of spiro-MeOTAD in dichloromethane, 0.12 V vs Fc/ Fc+.26 Above this potential, an absorption band at about 500 nm and a smaller one at 700 nm began to form simultaneously. These features have been assigned to the formation of spiroMeOTAD+ and spiro-MeOTAD2+, which have the same spectral signals.26 The onset of the formation of the next oxidized species was observed at 0.36 V versus Fc/Fc+ by the emergence of a strong absorption band at around 900 nm, which has been attributed to the formation of spiro-MeOTAD4 +.26 This oxidation was not reversible and if the scan was reversed before reaching this second onset, then only one reduction peak was found at 0.12 V vs Fc/Fc+ (data shown in Supporting Information). To determine the extinction coefficients of the absorption by spiro-MeOTAD+, we stepped to a potential of 0.25 V versus Fc/Fc+ and recorded current and absorption spectra simultaneously. The difference in coloration efficiency, ∆CE (cm2 C-1), between the oxidized species and the ground-state species at a particular wavelength, λ, is given by,

spectroelectrochemical experiments if all of the measured current is going into the oxidation reaction and can be converted into an extinction coefficient using the relation,

∆CE(λ) ) ∆A(λ)/∆Q

(1)

where ∆A(λ) is the change in absorbance at the wavelength and ∆Q (C cm-2) is the injected charge causing the oxidation. The difference in coloration efficiency can be determined from the

∆ε(λ) ) CE(λ) · F/1000 ) 96.5 · CE(λ)

(2)

where ∆ε(λ) is the difference in extinction coefficient between the oxidized and the ground-state species in M-1 cm-1 and F the Faraday constant () 96.5 × 103 C mol-1). We performed PCA on the measured UV-visible absorption spectra and determined that there were two components in the spectra as expected: one due to the ground-state absorption signals of spiroMeOTAD, and the other due to the absorption signals of spiroMeOTAD+. Plotting the absorbance against charge over time once the potential had been stepped to 0.25 V vs Fc/Fc+ gave a straight line at each wavelength (data shown in Supporting Information). The slope of this line is then proportional to ∆ε, which is shown in Figure 3. The maximum of the oxidized spiroMeOTAD absorption lies at 507 nm with an extinction coefficient of 33 000 M-1 cm-1. The second band has its maximum 690 nm with an extinction coefficient of 8500 M-1 cm-1. When performing the same experiment on a nanoporous TiO2 film infiltrated with spiro-MeOTAD, the absorbance did not change linearly with charge. Initially, the change in absorbance was larger and then flattened off. When analyzing the UV-visible spectra by PCA small changes in the spectra in addition to the change from ground-state to oxidized, spiro-MeOTAD were observed. A similar observation was made when repeating the measurement on a nanoporous TiO2 film dyed with ID28 and infiltrated with spiro-MeOTAD. Comparing the initial absorption spectrum of oxidized spiro-MeOTAD infiltrating dyed and nondyed TiO2 to the absorption spectrum of spiro-MeOTAD on FTO, small changes in shape and position of the 700 nm band were observed (Figure 4). The peak was broadened at the red absorption edge for oxidized spiro-MeOTAD in the pores of TiO2, and in the presence of ID28, the maximum of the band was red-shifted to 700 nm. It therefore seems that the presence of TiO2 and dye can have an effect on the absorption spectrum of oxidized spiro-MeOTAD, which will have to be considered when analyzing PIA spectra. The oxidation of ID28 on TiO2 occurs through a hole hopping mechanism along the dye molecules.27 The onset of oxidation was at 0.22 V vs Fc/Fc+. The reaction was not fully reversible, and the cyclic voltammogram showed an oxidation peak at 0.5 V vs Fc/Fc+ and two smaller reduction peaks at 0.36 and -0.05 V vs Fc/Fc+ (Figure S3 in Supporting Information). The formal

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Figure 4. Absorption spectra of oxidized spiro-MeOTAD normalized to 1: (1) absorption spectrum of spiro-MeOTAD+ on FTO (black); (2) initial absorption spectrum of spiro-MeOTAD+ in nanoporous TiO2 (blue); and (3) initial absorption spectrum of spiro-MeOTAD+ in nanoporous TiO2 dyed with ID28 (red).

Figure 6. (a) PIA spectra of ID28 on 2-µm thick TiO2 film (black) and on 6-µm thick TiO2 film (gray, dashed); (b) (1) PIA spectrum of ID28 on 2-µm thick TiO2 (black); (2) extinction coefficients of oxidized ID28 dye minus extinction coefficients of ground-state ID28 dye scaled by 2.2 × 10-8 and shifted by 15 nm (red); (3) absorption spectrum of ID28 on TiO2 at -1.16 V vs Fc/Fc+ minus absorption spectrum of ID28 on TiO2 at 0 V scaled by 2.8 × 10-3 and shifted by 15 nm (blue), (4) fit of PIA spectrum by linear addition of (2) and (3) (green).

Figure 5. (a) Absorption spectrum of ID28 on TiO2: at 0 V (black); at -1.16 V vs Fc/Fc+ (gray, dashed); (b) absorption spectrum at -1.16 V minus absorption spectrum at 0 V.

oxidation potential, indicating the HOMO level of ID28, was estimated from the oxidation peak and from the reduction peak at 0.36 V vs Fc/Fc+ to lie at about 0.43 V vs Fc/Fc+. Using a conversion factor of 0.63 V for Fc/Fc+ vs NHE,28 this places the oxidation potential at 1.06 V vs NHE, which is very similar to the formal oxidation potential determined from electrochemistry in solution (1.1 V vs NHE).21 This is 0.3 V positive of the onset of the spiro-MeOTAD oxidation and should give a sufficient driving force for the regeneration of the dye. Therefore, signals corresponding to oxidized ID28 observed in the PIA spectra are expected to come from dye molecules not in contact with spiro-MeOTAD. To determine the absorption spectrum and extinction coefficient of the oxidized dye, the current was recorded after stepping the potential from -0.14 V vs Fc/Fc+ to 0.46 V vs

Fc/Fc+. The spectral data were analyzed in the same manner as described above for spiro-MeOTAD. The absorption did not change linearly with charge, and upon stepping back to the initial potential the absorption spectrum did not revert to the original ground-state absorption. We attribute this to the instability of the oxidized dye on TiO2 in the electrolyte used. We therefore used the initial slope of A vs Q to estimate the shape of absorption spectrum of oxidized ID28 and the difference in extinction coefficient between oxidized and ground-state dye (data in Supporting Information). Up to 530 nm, the difference in extinction coefficient is dominated by a bleach signal from ground-state ID28 and between 530 and 850 nm by an absorption signal from oxidized ID28, which peaks at 600 nm and at 770 nm (Figure 3). In addition to oxidizing the dye, potential step measurements were performed where the potential was stepped to -1.16 V vs Fc/Fc+. At this potential, current flows due to the injection of electrons into the conduction band of TiO2 but it is positive of the reduction potential of ID28 on TiO2 (- 1.1 V vs NHE21). A reversible change in the absorption spectrum of ID28 was observed when stepping to this potential (Figure 5a). We attribute the observed effect to the proximity of the orbitals making up the LUMO of the dye to the TiO2 surface. Increasing electron density in the conduction band of TiO2 seems to affect the LUMO, which causes a change in the absorption spectrum of the dye. If this is the case, then the difference between the spectra (Figure 5b) should be observed in PIA when there are electrons in the conduction band. As the ground-state spectrum becomes bleached at its red edge between 500 and 600 nm,

Dye Regeneration by Spiro-MeOTAD

Figure 7. (a) PIA spectra of dyed 2-µm thick TiO2 films infiltrated with different spiro-MeOTAD concentrations; (b) PIA spectra of dyed 6-µm thick TiO2 films infiltrated with different spiro-MeOTAD concentrations. PIA spectra were recorded by illumination from the glass side.

this signal will overlap with the absorption signal of the holes in spiro-MeOTAD at 510 nm. A similar spectral shift due to charge density changes in the vicinity of dye molecules was recently observed by Staniszewski et al.29 for a ruthenium sensitizer. In their case, a blue-shift in the absorption spectrum of the dye was observed after regeneration of the oxidized sensitizer by iodide in an environment depleted of Li+. PIA Measurements. PIA spectra for two dyed TiO2 films (2 and 6 µm thick) without spiro-MeOTAD are shown in Figure 6a. The PIA spectrum of the thicker film is about 1.5 times as intense as the spectrum of the thinner film. This corresponds well with an increase of the light harvesting efficiency (LHE) at 460 nm for the thicker film from 61% to 93%, measured by UV-visible spectroscopy. The 770 nm peak in the spectrum of oxidized ID28 measured by spectroelectrochemistry appears red-shifted by about 15 nm in the PIA spectra. This can be attributed to the different surrounding of the dye: for spectroelectrochemistry measurements, the dye molecules were surrounded by supporting electrolyte, for the PIA measurement, they were surrounded by air. The latter is the relevant environment for measuring the pore filling if we assume that any unfilled pores are filled with air. The bleach is much more redshifted for the PIA spectrum by comparison and the first oxidized peak (at 600 nm in the spectroelectrochemical spectrum) is less intense than the second peak for the oxidized dye. This can be rationalized by the effect of electrons in TiO2 on the ground-state dye spectra measured, which we measured by spectroelectrochemistry at negative potentials. As only a small fraction of the dye molecules are in the oxidized state during PIA measurements, the electrons in TiO2 can have an effect on the many dye molecules remaining in the ground-state. Indeed,

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Figure 8. PIA spectra of 6-µm thick TiO2 films infiltrated with spiroMeOTAD with illumination from glass side (black) or illumination from spiro-MeOTAD side (gray, dashed). Spiro-MeOTAD concentration: (a) 50 mg mL-1; (b) 200 mg mL-1; and (c) 300 mg mL-1.

the PIA spectra can be fitted well (Figure 6b) by a linear addition of the spectrum of oxidized ID28 (Figure 3) and the difference in ground-state dye spectrum, caused by electrons in TiO2 (Figure 5b). Both spectra were shifted by 15 nm for this calculation to account for the different environment during the spectroelectrochemical and PIA measurements (electrolyte and air). PIA spectra for 2-µm thick, dye-sensitized TiO2 films infiltrated with spiro-MeOTAD from solutions of different concentrations are shown in Figure 7a. The spectra show a bleach centered at 550 nm and a positive absorption feature with a maximum between 730 and 750 nm, depending on the sample. This corresponds neither to the oxidized dye peak at 790 nm nor to the oxidized spiro-MeOTAD peak at 700 nm and can not be fitted by a linear addition of the two independent oxidized spectra. No additional side shoulders corresponding to either peak are observed for any of the used spiro-MeOTAD concentrations. The intensities of the peaks are lower than the intensity of the absorption features of oxidized ID28 on TiO2. As dye molecules remaining in their oxidized state would be in the same environment as before, i.e., not in contact with spiroMeOTAD molecules, the absorption spectrum of the oxidized dye should not change and this indicates that the absorption spectrum of oxidized ID28 is not hidden in this new absorption feature. We therefore assign this new signal to oxidized spiroMeOTAD. There are several possible reasons why we were not able to measure this exact spectrum in the spectroelectrochemical experiments on spiro-MeOTAD, although we tried to include ID28 and TiO2 in these measurements, we do not know where exactly within the film the oxidation of spiro-MeOTAD is taking place, and therefore, we do not know where the holes are relative to the TiO2 /ID28/spiro-MeOTAD interface. We found that there was a slight red-shift of the 700 nm peak in the presence of

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Figure 9. SEM cross-sections of (a) 2-µm thick dyed TiO2 films and (b) 6-µm thick dyed TiO2 films (i) without spiro-MeOTAD, (ii) infiltrated with 50 mg mL-1 of spiro-MeOTAD, and (iii) infiltrated with 200 mg mL-1 of spiro-MeOTAD.

ID28, although it is not as pronounced as that seen in PIA. Additionally, in the PIA measurements, there are electrons in the TiO2, which can cause localized electric fields affecting the absorption spectra of spiro-MeOTAD+. The peak intensities and positions follow the order: 50 mg mL-1 > 100 mg mL-1 > 300 mg mL-1 > 200 mg mL-1, as indicated by the gray line in Figure 7a. For the latter two concentrations, we would assume good pore filling as these are concentrations typically used in fabricating complete solid state DSCs. Assuming that the spectral changes, compared to the absorption spectra of oxidized spiro-MeOTAD measured in spectroelectrochemistry, are most pronounced close to the TiO2 /ID28/spiro-MeOTAD interface, having a red-shifted and more intense absorption feature would indicate that the holes measured are relatively closer to that interface. This would then mean that for the samples with lower spiro-MeOTAD concentrations, we maintain a good contact between spiro-MeOTAD and ID28 and are able to regenerate a large fraction of oxidized dye molecules but the pores are relatively less filled than for the higher spiro-MeOTAD concentrations. The signal between 500 and 600 nm matches a linear addition of the oxidized spiroMeOTAD spectrum together and the difference in ground-state ID28 absorbance caused by electrons in TiO2. This further supports our assumption that the observed signal mainly comes from oxidized spiro-MeOTAD and not from oxidized ID28. The PIA spectra of the 6-µm thick, dye-sensitized TiO2 films infiltrated with spiro-MeOTAD, where illumination occurred from the glass side (the working electrode side), are shown in Figure 7b. Surprisingly, for these thicker films, there is still no indication for the presence of oxidized dye molecules observed in the PIA spectra. The peak intensities and positions for the 730 to 750 nm peak now follow the order: 50 mg mL-1 > 100 mg mL-1 > 200 mg mL-1 = 300 mg mL-1. Applying the same rationalization as above, this means that, again, most dye molecules are in good contact with spiro-MeOTAD and that the best pore filling occurs for the two samples with the highest spiro-MeOTAD concentration used. The penetration depth of light, which is the reciprocal value of the absorption coefficient R, can be estimated using eq 3, in which d is the film thickness.

LHE(λ) ) 1 - e-Rd

(3)

For TiO2 films dyed with ID28, the blue excitation LED used in our experiments has a penetration depth of about 2 µm.

Therefore, for the 6-µm thick films, different parts of the film can be probed depending on the illumination direction, and we performed measurements where the samples were illuminated from the spiro-MeOTAD side (the counter electrode side). For the lower spiro-MeOTAD concentration (50 mg mL-1 (Figure 8a), 100 mg mL-1 and 200 mg mL-1 (Figure 8b), the 730 nm peak increases in intensity and is broadened to the red when illuminated from the spiro-MeOTAD side. This indicates that overall, there are differences in the pore filling between the top and bottom of the sample. If our above assumption is true (i.e., a blue shift of the absorption feature at 730 nm corresponds to the holes in spiro-MeOTAD being localized further away from the TiO2 /dye/spiro-MeOTAD interface), then the pore penetration and filling of spiro-MeOTAD is better closer to the substrate. For a concentration of 300 mg mL-1 almost no difference between PIA spectra from the different sides of illumination can be seen (Figure 8c). This indicates that, for this high concentration, the pore filling becomes uniform throughout the thickness of the film. SEM Cross Sections. SEM images of films without spiroMeOTAD (i), with 50 mg mL-1 (ii), and 200 mg mL-1 (iii), spiro-MeOTAD applied to the 2-µm thick films and 6-µm thick films are shown in Figure 9, parts a and b, respectively. Above the thinner TiO2 films, a small overstanding layer of spiroMeOTAD is observed with the low concentration (a,i) and a thicker overstanding layer (about 250 nm) for the higher spiroMeOTAD concentration (a,ii). For the thicker TiO2 films, no overstanding spiro-MeOTAD layers can be seen (b,ii,iii). With increasing spiro-MeOTAD concentrations (a,iii and b,iii), the images appear somewhat smoother. This is particularly marked in the thinner films and indicates a higher amount of spiroMeOTAD present in the pores. That no overstanding layer forms above the thicker samples could indeed mean that the pore filling is better in the deeper pores of the sample, as seen in the PIA measurements. In these cases, insufficient hole conducting material is present to fill all pores. This means that for thicker films, more spiro-MeOTAD is located at the bottom of the film than at the surface. We propose that wetting interaction between spiro-MeOTAD and ID28 accounts for the good pore penetration of the hole conductor. Whether such a process occurs will, of course, be strongly dependent on the dye used. Additives to spiroMeOTAD solution are also likely to alter the wetting behavior of spiro-MeOTAD.

Dye Regeneration by Spiro-MeOTAD Conclusions We have demonstrated that photoinduced absorption spectroscopy can be used as a tool to study dye regeneration by solid state hole conductors in dye-sensitized solar cells. Assignment of the complex PIA spectra can be made by comparison with the transient absorption spectra of the component species, measured under the same conditions. For organic dyes, care must be taken to consider how the coupling of the assembled components and the photoinjected electrons in the TiO2 may alter the spectra of the dye and the hole conductor. Once the spectral responses for a particular dye/hole conductor system are understood, the method becomes easy to use, as it is fast and nondestructive. For our system of ID28 and spiroMeOTAD, we found that for a variety of spiro-MeOTAD concentrations at two different film thicknesses, there is good pore wetting of spiro-MeOTAD resulting in efficient dye regeneration. No long-lived oxidized dye could be observed and it was not possible to determine the pore filling directly by PIA as had been previously proposed. There is thus still need for a direct and reliable method to quantify the amount of spiroMeOTAD or other hole conductors in the pores of TiO2. For our system, however, we believe that information on the quality of pore filling could be deduced from the shape of the PIA spectra. For lower spiro-MeOTAD concentrations, it does not seem that all pores are completely filled even though there is good contact between the dye molecules and spiro-MeOTAD. With a concentration of 300 mg mL-1, the PIA response was independent of the illumination side indicating uniform pore filling for 6 µm thick TiO2 films. We now plan to use PIA to study the effect of additives in the spiro-MeOTAD and of parameter variation in the sample preparation on dye regeneration and are looking for other methods to quantify the pore filling. In addition to the PIA results, we have been able to measure the oxidation potential and the extinction coefficients of spiroMeOTAD+ in a solid film of the material by spectroelectrochemistry with an ionic liquid electrolyte. For TiO2 films dyed in ID28, we showed that accumulation of electrons in the TiO2 blue-shifts the absorption spectra of attached dyes. Acknowledgment. We thank the BASF SE for supply of ID28 and would like to acknowledge the Bundesministerium fu¨r Bildung and Forschung (BMBF) and the BASF SE for financial support. Supporting Information Available: Additional cyclic voltammograms and spectroelectrochemical results of a spiro-

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