Influence of Water on the Electronic and Molecular Surface Structures

ARTICLE pubs.acs.org/JPCC

Influence of Water on the Electronic and Molecular Surface Structures of Ru-Dyes at Nanostructured TiO2 Maria Hahlin,*,† Erik M. J. Johansson,‡ Rebecka Sch€olin,† Hans Siegbahn,† and Hakan Rensmo*,† †

Molecular and Condensed Matter Physics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden ‡ Department of Physical and Analytical Chemistry, Uppsala University, Box 259, 751 05 Uppsala, Sweden ABSTRACT:

The influence of water on the surface electronic and molecular properties of three Ru-dyes adsorbed at nanostructured TiO2 was investigated using photoelectron spectroscopy (PES). The dyes investigated were the Ru(dcbpy)2(NCS)2 in its acid (N3) and in its 2-fold deprotonated form (N719) as well as a similar dye (Z-907) containing the hydrophobic ligand 4,40 -dinonyl-2,20 -bipyridine. Trends in surface structures depending on water exposure were followed for the three dyes. The results showed that the hydrophobic chains of the Z-907 dye effectively inhibit surface reorganization while large changes in surface electronic and molecular structure were observed for the N3 and N719 molecular layers. Specifically, large effects involving the thiocyanate ligands were detected, and the S2p and N1s core level spectra indicate that the changes involve mixing of only two dominating surface configurations. Moreover, the PES results also showed water-induced changes in the energy level matching between the dye and the TiO2, and water induced desorption of the TBAþ counterion. Basic photoelectrochemical trends depending on water exposure to dye-sensitized solar cell systems were also verified.

’ INTRODUCTION Dye-sensitized solar cells (DSCs) have received widespread interest as a promising alternative to conventional solar cells.15 In the DSC the dye absorbs light through the excitation of an electron. The dyes are adsorbed on a nanostructured TiO2 network, and the light absorption is followed by a fast injection of the excited electron into the conduction band of the TiO2 network and subsequently transported to the back contact. The oxidized dye is reduced by an electron transfer from a redox mediator. The most efficient DSCs, with efficiencies reaching over 10%, use metal complexes, such as ruthenium polypyridines (N3, N719, and black dye), as light harvesting material, and a liquid based electrolyte containing the redox couple I/I 3 as redox mediator.68 The preparation of the DSC is generally performed under normal atmospheric conditions, and in this environment, the materials are exposed to water present in the air. Under these circumstances it is inevitable that all DSCs will contain certain amounts of water molecules. The permeation of water at the sealing of the DSC will over time also increase the water content r 2011 American Chemical Society

in the electrolyte.9 Several groups have deliberately added water to the DSC, either by addition of water in the solvents used for assembly of DSC or by pretreatment of the TiO2 with water, and by doing so improved the efficiency of the DSC.1016 An increase in the Voc and a decrease in Jsc have generally been observed. The improved results have been explained by a decrease in the rate of the back reaction possibly due to blocking of the TiO2 surface by adsorbed H2O and/or by a reduced amount of I 3 on the TiO2 surface due to higher solubility in the electrolyte.1012 The decrease in Jsc is explained by the detachment of the adsorbed dye or by a weakening of the dye/TiO2 interaction.12 Effects from a depletion of the thiocyanate groups due to the presence of water have also been discussed; however, the water induced changes in the working electrode have been found reversible thus suggesting little depletion of the NCS groups.10 In systems using molten salts as electrolytes the increase in efficiency is to some Received: August 13, 2010 Revised: April 19, 2011 Published: May 26, 2011 11996

dx.doi.org/10.1021/jp1076609 | J. Phys. Chem. C 2011, 115, 11996–12004

The Journal of Physical Chemistry C

ARTICLE

Although the focus of the work is the PES study, we also include a basic series of photoelectrochemical measurements for samples exposed to water under the same experimental conditions.

’ EXPERIMENTAL SECTION

Figure 1. Molecular structure of the dyes N3, N719, and Z-907, used for sensitizing TiO2 samples.

extent explained by an increase of the conductivity of the electrolyte. However, increasing the amount of added water above 10 wt % resulted in a decrease of efficiency even though the conductivity was increased.14 For smaller amounts of water an increase of the injection rate is also observed in these systems, and the effect was correlated to the increase in Voc.11 Systematic studies have shown that the presence of water in the DSC speeds up the degradation process.17 Modification of the N3 dye by addition of hydrophobic chains to one of the bipyridine ligands effectively increased the long-term stability of the DSC.17 The improved function from the use of hydrophobic chains was explained by the protection of the dye-sensitized surface against desorption of the dye. Attempts to construct water-based DSCs by using water as a solvent for the electrolyte were made in the early history of DSCs by Gr€atzel et al.1820 Since then a number of groups have followed this attempt, and efficiencies of up to 2.4% have been reached using a hydrophobic dye for sensitization and 100% water content in the electrolyte.2123 Long-term studies on this system have also given promising results concerning developments of water-based DSC.23 Generally, the effect of water on the dye-sensitized surface is not fully understood. The purpose of this contribution is to demonstrate the effects of water on the electronic and molecular surface structures of a Ru complex containing thiocyanate ligands. In focus is a series of Ru-complexes, N3, N719, and Z-907, see Figure 1, also allowing us to investigate the effect from counterions and hydrophobic chains. The surface characterization will be accomplished by PES (photoelectron spectroscopy). PES is a technique that can be used to gain element specific information on the electronic structure and the molecular surface structure of the dye-sensitized TiO2 surface.2437 The probing depth in a PES experiment is only a few nanometers into the material, and thus, this technique is very sensitive to the interfacial region between the materials used.

Sample Preparation. The samples were prepared as follows. A colloidal TiO2 solution was spread out onto conducting SnO2: F glass pieces (for preparation procedure of TiO2 solution, see ref 38). The electrodes were heated to 450 °C for 30 min. The electrodes were allowed to cool before being immersed in the dye solutions for 12 h. The dye solutions were 0.5 mM cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylato)ruthenium(II) (N3) dissolved in ethanol, 0.5 mM cis-diisothiocyanato-bis(2, 20 -bipyridyl-4,40 -dicarboxylato)ruthenium(II) bis(tetrabutylammonium) (N719) dissolved in ethanol, and 0.3 mM cis-disothiocyanato(2,20 -bipyridyl-4,40 -dicarboxylic acid)-(2,20 -bipyridyl-4,40 -dinonyl)ruthenium(II) (Z-907) dissolved in acetonitrile and tert-butanol with volume ratio 1:1. After sensitization the electrodes were quickly rinsed in respective solvent. The dyesensitized samples deliberately exposed to water were put into a solution containing 30% water and 70% ethanol for 20 min. The samples not subjected to water were instead put into ethanol (99.97%) for equal amounts of time, and are hereafter referred to as reference samples. These electrodes were used for PES, and identical electrodes were prepared and used for both absorption measurements and solar cell assembly. The solar cells consisted of dye-sensitized working electrode, 50 μm Surlyn frame used as hot melt sealant, a liquid electrolyte, and a counter electrode of platinized SnO2:F glass. The electrolyte consisted of 0.6 M tetrabutylammonium iodide (TBAI), 0.1 M lithium iodide (LiI), 0.05 M iodine (I2), 0.05 M guanidium thiocyanate (GuSCN), and 0.5 M 4-tertbutylpyridine (4-TBP) in acetonitrile. The area of the film was 0.32 cm2, and the thickness of the films was approximately 5 μm. A complementary study of lower concentrations is also shortly included in this investigation. The sensitization of the TiO2 electrodes is in this case performed similarly as above with the exception of introducing water in the sensitization process by using different mixtures of water/ethanol as solvents. The concentrations of water were in these cases 0%, 2%, 10%, and 30%. Furthermore, it is noted that the samples exposed to 99.97% ethanol solution may also contain effects from some water. Still, however, they are referred to as reference samples, and their PES spectra are in agreement with previous measurements.26 Measurements. Photovoltaic measurements were made with a xenon lamp with a Schott Tempax 113 filter used to simulate sunlight (AM 1.5) and calibrated to 1000 W m2 with a reference silicon diode, and the DSCs were masked to 0.16 cm2. The measurements were performed by illumination from the side of the dye-sensitized electrode. Monochromatic incident photon to current conversion efficiency (IPCE) was recorded using a computerized setup consisting of Keithley 2400 source/meter as a current meter and a xenon arc lamp (300 W Cermax, ILC Technology), followed by a 1/8 m monochromator (CVI Digikrom CM 110) as a light source. For the IPCE measurements, the DSCs were masked to 0.25 cm2. Absorbance spectra were recorded in the range from 300 to 1000 nm on an HR-2000 Ocean Optics fiber optics spectrophotometer. The absorption spectrum was corrected for reflection, which is assumed to be constant over the visible region. 11997

dx.doi.org/10.1021/jp1076609 |J. Phys. Chem. C 2011, 115, 11996–12004


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Photoelectron spectroscopy (PES) and X-ray absorption spectroscopy (XAS) measurements were performed using synchrotron radiation at BL I411 at the Swedish national laboratory MAX-lab in Lund. The electron take off angle was 70°, and the electron take off direction was collinear with the e-vector of the incident photon beam. The kinetic energy of the photoelectrons was measured using a Scienta R4000 WAL analyzer. N1s XAS spectra were recorded by detection of secondary electrons in partial yield mode and intensity normalized versus the number of incident photons. The PES spectra were energy calibrated by setting the Ti2p substrate signal to 458.56 eV and intensity normalized using the Ru3d5/ 26 The N1s XAS spectra were energy calibrated 2 signal. through measurement of the Ti3p peak using first and second order light.35 Quantitative characterization comparing the amount of the different elements at the surface was performed with a Scienta ESCA 300 instrument, using monochromated Al KR radiation (1486.7 eV) and calibrated for cross-section and analyzer transmission. The electron take off angle was 90°.

’ RESULTS AND DISCUSSION Before reporting in detail the experiments from the PES surface characterization, we briefly summarize photoelectrochemical and surface coverage effects from water on the DSC systems studied here. Effects on DCS Function. The electrodes used in the DSC photoelectrochemical characterization were treated in the same way as the materials investigated by PES, and the purpose of the photoelectrochemical investigation is mainly to demonstrate functional effects from the method of water exposure to the surfaces studied here. N3, N719, and Z-907 sensitized TiO2 electrodes deliberately exposed to water as well as TiO2 electrodes sensitized with N719 from different water/ethanol mixtures were used for DSC assembly, and the IV, IPCE, and absorption characteristics were compared to that of the reference samples. The IV curves and IPCE spectra of the different DSC conditions are shown in Figures 2 and 3 and summarized in Table 1 along with the absorption characteristics. It is observed that the exposure to water leads to a decrease in short-circuit current (Jsc) by over 20% for the N3 and N719 based systems, and that the open circuit voltages (Voc) decrease for the N719 based system while constant for the N3 based system. For Z-907 sensitized electrodes, the IV characteristics are very similar for the sample exposed to H2O and the reference sample. For the TiO2 electrodes sensitized with N719 using solutions with different H2O/EtOH ratios there is a continuous decrease in current and voltage. The IPCE, see Figure 3, clearly decreases for the systems based on the N3 and N719 dye when the materials were exposed to water and for the Z-907 the variation in IPCE from water exposure is smaller. For all samples the absorption decreases when exposed to water. For each dye both the absorption and the IPCE decrease with similar quantities, and thus the IPCE decrease may, to a large extent, be explained by a decrease in dye surface concentration reducing the light absorption. Interestingly, a 20 nm shift of the IPCE maximum is also observed for the N3 and N719 sensitized TiO2 electrodes after exposure to water clearly indicating changes in the electronic structure of adsorbed molecules. As expected, no such shift is observed for the Z-907 dye.

Figure 2. IV curves for (a) N3, (b) Z-907, and (c) N719 samples, with and without H2O. (d) The IV curves for N719 with different percentage of H2O in the dye solution.

In summary, clear effects from water exposure are measured although they vary with the dye used, and the results observed for these DSCs are similar to previously observed data.1016 These results demonstrated similar effects as have previously been observed and justified the preparation procedure of the model system studied here. A general explanation for these findings is complex and most probably involves mechanisms linked to the molecular as well as the mesoscopic structure. The present work only focuses on the information obtained from PES experiments for a better understanding of the surface molecular and electronic structure. Photoelectronspectroscopy. Amount of Dye at the TiO2 Surface. Changes in the amount of surface adsorbed N3, N719, and Z-907 molecules can be determined rather accurately by measuring the photoemission intensity from a spectroscopically well-defined core level, such as Ru3d5/2 relative to the Ti2p3/2 substrate signal. This procedure is most accurate for samples measured with high photon energy and for which the binding distance of the atoms in the molecule with respect to the substrate is short compared to the electron mean free path and where the molecular structure is similar. In Table 2 the Ru/Ti intensity 11998



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Table 2. Ratio between the Ru3d5/2 and the Ti2p3/2 Intensities Measured Using Photon Energy 758 eV and the Ratio between the S2p and Ru3d5/2 Intensities Measured Using Photon Energy 1487 eV (Bulk Sensitive) 0% H2O (Ru)/(Ti)

(S)/(Ru)

Figure 3. IPCE spectra for (a) N3, (b) Z-907, and (c) N719, all with and without exposure to H2O. (d) The IPCE spectra of N719 with different percentages of H2O in the dye bath.

Table 1. IV Characteristics of the N719, N3, and Z-907 DyeSensitized Solar Cells along with IPCE and Absorption (Abs) Values at Maximum IPCE Position (λmax) sample

η/% Voc/V Jsc/A/cm2

ff

IPCE/% λmax/nm abs/%

N719 H2O

1.5 0.675

3.25

0.68

49.5

510

51

N719 no H2O

2.1 0.715

4.04

0.72

52.8

530

60

N3 H2O N3 no H2O

1.5 0.72 2.0 0.71

2.98 3.89

0.71 0.74

50.2 55.3

510 530

57 65

Z-907 H2O

2.2 0.72

4.41

0.69

64.5

530

66

Z-907 no H2O

2.5 0.74

4.46

0.75

66.8

530

69

N719 2% H2O

2.5 0.76

4.47

0.75

60.1

530

68

N719 10% H2O 1.8 0.720

3.45

0.73

50.1

530

63

N719 30% H2O 1.6 0.675

3.36

0.71

50.5

520

54

ratio is shown for the three dyes, for both the samples exposed to water and the reference samples. It is observed that for the reference samples the amount of dye on the surface is rather similar for all three dyes. There are, however, small differences, and the results show that the largest amount of dye on the TiO2 surfaces is observed on the Z-907 sample, while there is about 10% less dye on the N3 sample and 20% less dye on the N719 sample. The influence of water on the amount of dye on the sensitized surfaces can similarly be observed when comparing the Ru/Ti intensity ratio for the samples exposed to water to that of a reference sample, see Table 2. For Z-907 the Ru/Ti ratios show that the amount of dye is very similar in both samples. However, the Ru/Ti ratios for the N3 and N719 samples show that the amount of dye decreased with the exposure to water. For N3 the Ru/Ti ratio decreased by about 80%, and for N719 the Ru/Ti ratio decreased by approximately 50%. It can thus be concluded

30% H2O

N719

6.6

3.6

N3

7.2

1.1

Z-907

8.1

8.4

N719

2.0

1.6

N3

1.63

1.62

Z-907

1.75

1.77

that the hydrophobic chains of the Z-907 effectively protect the surface against water induced dye desorption, and also that effects from TBAþ counterion in the formation of the molecular layer (N719) to some extent also limit permanent dye desorption due to the presence of water. It is noted that the effect of water on the amount of adsorbed dyes as measured in the UVvis spectrum is not as dramatic as for the PES measurements. The difference between these measurements arises from the fact that absorption measurements probe an average of the entire nanostructure while in the PES measurements only the outermost surface of the electrode is investigated. Thus, together the results show that the procedure for water exposure discussed here gives rise to some variations in dye surface concentration throughout the nanoporous network, and that the magnitude of the change in the outermost surface observed by PES may be higher than the average change within the nanostructure. The limited time that the nanostructured electrodes were exposed to water may result in systems that have not reached steady state surface concentrations throughout the film. That is, even if the penetration of the ethanol/water solution into the interior of the network is almost instantaneous, the diffusion of the detached dye molecules throughout the network may require longer times.39 Moreover, inefficient permeation of aqueous electrolytes into the TiO2 network has previously been discussed in the literature.21 Table 2 also reports on the relative amount of S and Ru at the outermost part of the electrode. These measurements were obtained using a high photon energy (Al KR), where the electron mean free path was substantially larger than the size of the molecule. By a comparison of the S/Ru ratio it is concluded that the exposure to water will not enforce any substantial permanent chemical changes to the molecule such as ligand exchange. These results are important for more surface sensitive measurements in the following discussion. It allows us to interpret the changes observed in the surface sensitive spectra of the dye molecule as arising from changes in surface molecular orientations. Water Enhanced Core Levels States. Detailed core level PES investigations give further information about the molecular and electronic surface structure of the dyes at the TiO2 surface. In the following we will discuss the S2p and N1s core level peaks. The S2p spectra are shown in Figure 4. A S2p spectrum is deconvolved into one spinorbit split doublet for each chemical state, i.e., one doublet, S2p1/2 and S2p3/2, with intensity ratio 1:2 and peak split of 1.18 eV. Previously, the S2p spectra of multilayer of N3 and N719 have been deconvolved into one spin orbit split doublet, showing the presence of only one sulfur state in the respective dye multilayer.26 For all monolayer reference samples measured here at least two spinorbit split doublets are 11999



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Table 3. Ratio between the S2p and the Ru3d5/2 Intensities Measured Using a Photon Energy of 454 eV (Surface Sensitive) and the Ratio between the N1s and the Ru3d5/2 Intensities Measured Using a Photon Energy of 758 eV A

tot

(S )/(S )

(Stot)/(Ru)

Figure 4. S2p spectra of reference samples of N719, N3, and Z-907 and samples exposed to H2O, measured using a photon energy of 454 eV. The increase of the second S2p doublet can be related to the amount of H2O.

needed to deconvolve the S2p spectra, implying that there are at least two different states for the sulfur atoms at the TiO2 surfaces. This implies that upon adsorption to the TiO2 surface some of the dye molecules are involved in interactions using the thiocyanate ligand. In the S2p spectra, the spinorbit split doublet located at lower binding energy is hereafter referred to as S2pA and the spinorbit split doublet at higher binding energy is referred to as S2pB. The ratio between the S2pA peak relative to the total S2p intensity (S2ptot), measured using a photon energy of 454 eV, is shown in Table 3. For the reference samples this percentage is rather similar for both the N719 and Z-907 dyes. The surface adsorbed N3 dye has a higher ratio compared to that of the other two dyes, implying a more pronounced mix of states for the adsorbed N3. Effects from exposure to water are clearly observed in the S2p spectra of N3 and N719 while no difference can be observed in the S2p spectrum of the Z-907 samples. Thus, the hydrophobic chains effectively protect the surface from molecular changes induced by water. In the spectra of both N719 and N3 large differences are observed in the shape of the spectra between the

0% H2O

30% H2O

N719

0.90

0.59

N3

0.74

0.67

Z-907

0.87

0.87

N719

50

20

N3

48

34

Z-907

50

55

(NNCS)/(Ru)

N719 N3

68 68

66 58

Z-907

60

61

(Nbpy)/(N1sNCS)

N719

2.1

2.7

N3

2.3

2.6

Z-907

2.6

2.6

samples exposed to water and the reference samples. The S2pA doublet has decreased in intensity relative the total S2p doublet, see Figure 4. At the same time the intensity of the S2pB relative Ru3d5/2 increased in the N719 sample but decreased slightly in the N3 sample, indicating that the relative mixture of states is influenced by the presence of water. In Table 3 it is observed that the increase in SA/Stot ratio is largest for the N719 dye. This suggests that the N719 molecular dye layer in this respect is more sensitive to water compared to the N3 dye layer. The origin of the S2pB state is not fully understood. Previously, interactions between the thiocyanate ligand and the TiO2 surface or adsorption induced intermolecular interactions involving the thiocyanate ligands have been suggested; however, other dyedye interactions involving the sulfur atom cannot be excluded.26 Interestingly, from Figure 4 it is observed that the binding energy difference between the two S2p states is the same for the samples exposed to water and the reference samples, strongly indicating that the surface does not hold a multitude of states but only involves two dominating configurations. It may therefore be suggested that the S2pB states observed in the reference samples are also linked to the presence of water when using such an ex-situ preparation procedure. A complementary study on the effects from water on the N719 surface configuration was also performed by using different water/ethanol solvent mixtures in the dye solution. Such addition of water in the preparation of N719 sensitized surfaces also gives rise to drastic changes in the S2p PES characterization, see Figure 5. It is observed that the amount of sulfur atoms in the chemical state observed at higher binding energy (S2pB) increases with the concentration of water, and that the effect is observed already at low concentrations. Changes in the S2p spectrum are related to the thiocyanate ligand. Since the thiocyanate ligands also contain nitrogen atoms, changes in the N1s spectra are also expected. The N1s core level spectra of the N3, N719, and Z-907 samples exposed to water and the reference samples are displayed in Figure 6. Two N1s peaks are visible in the spectra of N3 and Z-907. Previously for N3 the peak located at a binding energy of 398 eV has been assigned to the N atoms in the thiocyanate ligand and the peak located at a binding energy of 400 eV assigned to the N atoms 12000



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Figure 5. S2p spectrum of N719 sensitized electrodes using different mixtures of H2O/EtOH as solvent. The spectra were measured with a photon energy of 454 eV.

in the bpy ligands:26 these are hereafter referred to as N1sNCS and N1sbpy. The two peaks in the Z-907 N1s spectrum are assigned to the same atoms as for N3, due to the similar molecular structures. The N1sNCS and N1sbpy peaks are also visible in the N719 spectrum, and in addition to this, a third peak located at binding energy of 402 eV is present in the N719 sample. The third peak has previously been assigned to originate from the TBAþ counterion and is referred to as N1sTBA.26 It is observed, like in the S2p spectra, that water affects the N719 and the N3 dye but that the Z-907 dye is almost unaffected. The N1sNCS peak of the N719 dye clearly experiences a broadening when exposed to water. The N1sNCS peaks can be deconvolved into two peaks which have the same relative ratio as between the two S2p doublets, see Figure 6. The changes of the N1s spectra thus follow effects observed in the S2p spectra. In the case of N3, the N1sNCS peak clearly decreases relative the N1sbpy peak, and this will be further discussed in the next section. In the N1s spectrum of N719 large changes are also observed for the N1sTBA peak. From the molecular structure, see Figure 1, the intensity ratio between the N1sbpy and N1sTBA for N719 should be 2:1. As shown previously,26 the intensity ratio between the N1sbpy/N1sTBA in N719 is measured to be 3.5:1 in the reference sample and thus far from 2:1, indicating that some TBAþ do not coadsorb on the TiO2 surface. For the N719 sample exposed to water it is observed that the N1sTBA peak

Figure 6. N1s spectra of reference samples of N3, N719, and Z-907 and samples exposed to H2O, measured using a photon energy of 758 eV.

substantially decreases, showing that TBAþ counterions desorb from the dye-sensitized surface when exposed to H2O. Water Induced Changes in the Adsorption Configuration. In PES, information on the surface molecular structure can be obtained by comparing the core level intensities of different elements in the molecular layer. For organized surface layers, differences in position of the atom in the molecular layer can affect scattering and thereby attenuation and the measured intensity. From the molecular structure, see Figure 1, the intensity ratio N1sbpy/N1sNCS should be 2:1. In the experimental data of the reference samples the N1sbpy/N1sNCS ratios are 2.1:1 for N719 and 2.3:1 for N3 and 2.6:1 for Z-907, which is slightly more than expected from the molecular formula. The higher ratios observed may be explained by a larger attenuation of the N1sNCS compared to N1sbpy. Specifically, the values for Z-907 indicate that on average the NNCS atoms are forced into the molecular layer and toward the TiO2 surface by the presence of the long alkane chains and by the fact that it at maximum can bind with two carboxylic groups. In Table 3 the N1sbpy/N1sNCS ratios are shown for both the samples exposed to water and the reference samples. It is observed that the N1sbpy/N1sNCS ratio increases to 2.7:1 for 12001



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Figure 7. Valence band spectra of (—) the reference samples of N3, N719, and Z-907 and (---) the samples exposed to H2O, measured using photon energy 454 eV.

N719 and 2.6:1 for N3 when exposed to water, indicating a rearrangement of these dyes at the TiO2 surface. Furthermore, the S2p measurements obtained at 454 eV photon energy show that the total intensity of the sulfur peak in the N3 and N719 samples decreases with respect to the Ru3d5/ 2 peak when the samples are exposed to water, see Stot/Ru ratio in Table 3. In this context it is important to note that the molecules do not decompose as was shown by the more bulk sensitive measurements using higher photon energies (1487 eV, see above) for which we only observed differences in S to Ru ratio in the range of approximately 1% for N3 and Z-907. The difference in Stot/Ru when using different photon energies is therefore explained by an increased surface sensitivity when using a photon energy of 454 eV. As discussed in the previous section the S2p results indicate that there are only two configurations that dominate the surface molecular structure. Although variations in the S2p (or N1s) intensity alone cannot give their exact molecular orientation, we note one important finding. When water exposure changes the adsorption configuration the position of the Ru and S atoms may change with respect to the surface (also implying a change of the N1sNCS and N1sbpy positions). For surface sensitive measurements (454 eV photon energy) such reorientation results in a change of the attenuation for the Ru3d5/2 and S2p signals, respectively. The observed decrease in Stot/Ru ratio together with the observed increase of the N1sbpy/ N1sNCS ratio therefore both indicate that the NCS ligand is forced toward the TiO2 surface by the presence of water. The N3, N719, and Z-907 dyes are generally believed to adsorb to the TiO2 surface through two or three carboxylic groups on the bpy ligands.4043 Although this limits the freedom for the surface molecular structure, it may still allow for interaction of the thiocyanate ligands with the TiO2 surface or with neighboring molecules.26 The HOMO of the Ru based dyes is partly located on the thiocyanate ligands, which in the function of a solar cell favor

Figure 8. N1s-XAS spectra of N719, N3, and Z-907 reference samples (—) and water exposed samples (---). The main peak originates from resonances in both bipyridine ligands and thiocyanate ligands. The contribution from the bipyridine ligands dominates on the low photon energy side of the main peak, and the contribution from the thiocyanate ligands dominates on the high photon energy side of the main peak.

reduction of the oxidized dye via the thiocyanate ligand. Thus, the thiocyanate ligand preferably should be located toward electrolyte and away from the TiO2 surface to suppress recombination. Variations in surface structure such as that discussed above may explain observation of more than one injection component.44 Water Induced Changes in Energy Level Matching. The frontier electronic structure is important for the charge transfer in the solar cell. The injection of electrons from the dye into the TiO2 will depend on the energy level matching between the excited dye molecules and the conduction band edge. Also, the reduction of the dye in a working solar cell involves an electron transfer from a redox couple to the oxidized dye. The influence of water on the energy levels directly involved in the photoconversion is studied in detail using PES and XAS. The valence spectra of the N3, N719, and Z-907 sensitized TiO2 surfaces displayed in Figure 7 are intensity normalized by the peak located at approximately 8 eV. In the spectra, at a binding energy of approximately 1.8 eV, the HOMO levels originating from the dye molecules are clearly visible above the valence band edge of the TiO2. The positions of the HOMO levels in the reference samples relative to the TiO2 are similar for all dyes, with variations smaller than 0.1 eV. As expected from the core level measurements the valence spectra for the reference Z-907 sample and the water exposed 12002



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Table 4. Binding Energy of the N1sbpy Peak, N1s-XAS Resonance Photon Energy, the Binding Energy Position of the LUMO Level, the Binding Energy Position of the HOMO Level, and the Binding Energy Difference between the HOMOLUMOa N1sbpy/eV

a

N1s-XAS/eV

LUMO/eV

HOMO/eV

HOMOLUMO/eV

ref

H2O

ref

H2O

ref

H2O

ref

H2O

ref

H2O

N719

399.9

399.9

399.4

399.4

0.5

0.5

1.86

2.12

1.36

1.62

N3

399.8

399.9

399.4

399.4

0.4

0.5

1.85

2.02

1.45

1.52

Z-907

399.8

399.8

399.4

399.4

0.4

0.4

1.81

1.77

1.41

1.37

These were measured for the reference samples (ref) and the samples exposed to water (H2O).

Z-907 sample are very similar. Specifically the energy level position of the HOMO level is the same in both samples. However, for the N3 and N719 larger changes are observed over the entire valence band spectra when comparing the samples exposed to water and the reference samples. Specifically, clear shifts of the HOMO levels toward higher binding energies are also observed for both dyes. The magnitude of the shift is 0.17 eV for N3 and 0.26 eV for N719. The unoccupied energy levels are probed using XAS, and the N1s-XAS spectra from the N3, N719, and Z-907 sensitized TiO2 surfaces are displayed in Figure 8. The spectra are normalized at the maximum intensity. All spectra show a large resonance feature located around 400 eV, and some smaller structures above 402 eV. Theoretical calculations indicate that the large resonance structure contains a contribution from both NNCS and Nbpy atoms.35 On the lower photon energy side of this resonance the N1sbpy contribution largely dominates the spectra, and on the higher photon energy side the N1sNCS contribution is dominating. The XAS and valence PES energies are summarized in Table 4, together with the binding energy of the N1sbpy peak. It is observed that the position of the N1s peak and the photon energy of the XAS resonance are similar in all three reference samples, differing at most by 0.1 eV. The positions of the LUMO levels for these molecules in the presence of a N1s core hole can be estimated by subtracting the resonance photon energy from the binding energy of respective core level peak, and are also included in Table 4. A similar binding energy position of the LUMO level is observed, and these results indicate a similar energy level matching between the dye LUMO and the TiO2 in all reference samples. The general structure of the N1s-XAS spectra for N3 and Z-907 show no significant change when exposed to water. However, the N719 spectrum show a slight decrease of the intensity on the higher photon energy side of the most intense resonance. The position of the LUMO level was determined according to the procedure described above, and are displayed in Table 4. From such estimation we observe only minor changes in the position of LUMO for all three dyes as a result of water exposure. Summarizing the effect from water exposure on the energy level matching, we observe the most pronounced changes in the HOMO position of N3 and N719, and only minor changes in the LUMO position. This implies an increased gap between the HOMO and LUMO energy levels, and subsequently a blue shift of the absorption spectrum is expected. The changes of the HOMOLUMO gap observed in the PES measurements are supported by the observed blue shift of the absorption maximum and in IPCE. The estimation of the increased gap is somewhat larger from PES measurements compared to the absorption measurements. This may, similar

to the differences observed for the amount of dye on the samples, be explained by the fact that PES probes the outermost surface whereas the absorption measurements probe the entire nanostructure as was explained above. The quantitative differences in the matching between the water induced shift in the absorption peak and in changes in the HOMOLUMO energy difference may partly be explained by the differences in relaxation in measurements of core excitation and valence excitation.24

’ CONCLUSIONS TiO2 electrodes, sensitized using the N3, N719, and Z-907 dyes, were exposed to water, and the solar cell performance was studied. Clear effects from water exposure were observed although they varied with the dye used. A clear shift in the IPCE was observed and only small changes in the Voc. Interestingly, although some decrease in the photocurrent was observed the IPCE and absorption results show that almost all molecules contribute to current generation. These results demonstrated similar effects as have previously been observed. PES measurements were performed on N3, N719, and Z-907 sensitized TiO2 electrodes deliberately exposed to water and compared to reference samples. It was found that the hydrophobic chains of the Z-907 dye effectively protect the surface against water. No observable difference was found between the Z-907 sample exposed to water and the reference sample. For the N3 and N719 dyes, however, large differences were observed. Specifically, water exposure substantially reduced the amount of N3 and N719 molecules at the TiO2 surface, but there was no depletion of thiocyanate ligands. Moreover, changes in the relative core level intensities when measuring with high surface sensitivity showed a change in the dye orientation at the surface. Specifically, it was proposed that water exposure rearranges the thiocyanate ligand toward the TiO2 surface. Changes observed in the S2p spectra of N3 and N719 show an increased number of dye molecules with sulfur atoms in another chemical surrounding, i.e., having a different bonding configuration after exposure to water. Interestingly, the binding energy position of the two S2p states is the same for the samples exposed to water and in the reference samples indicating that one of the S2p doublets observed in the reference samples may also be linked to the presence of water when using ex situ preparation procedure. For N719 it was also observed that the counterion TBAþ is not present on the dye-sensitized surface after the sample has been exposed to water. The frontier electronic structures are important for the energy conversion in the DSC. A shift of the HOMO level of the N3 and of the N719 dyes toward higher binding energies was observed after exposure to water. At the same time no change was observed in the position of the LUMO levels, implying no change in the 12003


The Journal of Physical Chemistry C energy level matching between the LUMO of the dye relative the conduction band of the TiO2. In the DSC the increased HOMOLUMO gap explains the blue shift of the absorption spectra of the water exposed N3 and N719 samples.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT The experimental work was supported by the Swedish Research Council (VR), the G€oran Gustafsson Foundation, the Carl Trygger Foundation, the Knut and Alice Wallenberg foundation, and the Swedish Energy Agency. We thank the staff at MAX-lab for competent and friendly assistance, Dr. Leif H€aggman for the supplying of the TiO2 working electrodes, and Brian O’Regan for sharing his view on the subject. ’ REFERENCES (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737. (2) Murakoshi, K.; Kano, G.; Wada, Y.; Yanagida, S.; Miyazaki, H.; Matsumoto, M.; Murasawa, S. J. Electroanal. Chem. 1995, 396, 27–34. (3) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Gratzel, M. Nature 1998, 395, 583. (4) Hagfeldt, A.; Didriksson, B.; Palmquist, T.; Lindstr€ om, H.; S€odergren, S.; Rensmo, H.; Lindquist, S. Sol. Energy Mater. Sol. Cells 1994, 31, 481–488. (5) Boschloo, G.; Lindstrom, J.; Magnusson, E.; Holmberg, A.; Hagfeldt, A. J. Photochem. Photobiol., A 2002, 148, 11–15; 1st International Conference on Semiconductor Photochemistry (SP-1), Glasgow, Scotland, July 2325, 2001. (6) Nazeeruddin, M.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gr€atzel, M. J. Am. Chem. Soc. 1993, 115, 6382–6390. (7) Nazeeruddin, M.; Pechy, P.; Renouard, T.; Zakeeruddin, S.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G.; Bignozzi, C.; Gratzel, M. J. Am. Chem. Soc. 2001, 123, 1613–1624. (8) Nazeeruddin, M.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gratzel, M. J. Am. Chem. Soc. 2005, 127, 16835–16847. (9) Tropsha, Y. G.; Harvey, N. G. J. Phys. Chem. B 1997, 101, 2259–2266. (10) Liu, Y.; Hagfeldt, A.; Xiao, X.-R.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 1998, 55, 267–281. (11) Hui, Z.; Xiong, Y.; Heng, L.; Yuan, L.; Yu-Xiang, W. Chin. Phys. Lett. 2007, 24, 3272. (12) Lu, H.-L.; Lee, Y.-H.; Huang, S.-T.; Su, C.; Yang, T. C.-K. Sol. Energy Mater. Sol. Cells 2011, 95, 158–162. (13) Jung, Y.-S.; Yoo, B.; Lim, M. K.; Lee, S. Y.; Kim, K.-J. Electrochim. Acta 2009, 54, 6286–6291. (14) Mikoshiba, S.; Murai, S.; Sumino, H.; Hayase, S. Chem. Lett. 2002, 1156–1157. (15) Mikoshiba, S.; Murai, S.; Sumino, H.; Kado, T.; Kosugi, D.; Hayase, S. Curr. Appl. Phys. 2005, 5, 152–158;Indo-Japan Workshop on Advanced Molecular Electronics and Bionics. (16) Weidmann, J.; Dittrich, T.; Konstantinova, E.; Lauermann, I.; Uhlendorf, I.; Koch, F. Sol. Energy Mater. Sol. Cells 1998, 56, 153–165. (17) Zakeeruddin, S.; Nazeeruddin, M.; Humphry-Baker, R.; Pechy, P.; Quagliotto, P.; Barolo, C.; Viscardi, G.; Gratzel, M. Langmuir 2002, 18, 952–954. (18) Liska, P.; Vlachopoulos, N.; Nazeeruddin, M.; Comte, P.; Gratzel, M. J. Am. Chem. Soc. 1988, 110, 3686–3687. (19) Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gratzel, M. J. Am. Chem. Soc. 1988, 110, 1216–1220.

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Influence of Water on the Electronic and Molecular Surface Structures

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