Article pubs.acs.org/JPCC
X‑ray Characterization of Dye Adsorption in Coadsorbed DyeSensitized Solar Cells Mitsunori Honda,*,† Masatoshi Yanagida,†,‡ Liyuan Han,‡ and Kenjiro Miyano† †
Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN) and ‡Photovoltaic Materials Unit, National Institute of Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ABSTRACT: We performed X-ray measurements to elucidate the adsorption mode of N719 dye molecules on nanoporous TiO2 with and without coadsorption of D131 dye. Two techniques, X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy, were employed to obtain depth profile information about the substrate. In both cases, we found that the isothiocyanate groups of N719 strongly interact with TiO2 via S atoms when the dye is adsorbed from a single-component solution. In contrast, S-substrate interaction is strongly suppressed when D131 is coadsorbed with N719, indicating that the presence of D131 changes the adsorption mode of N719. On the basis of this finding, we designed a procedure to promote the preferential adsorption of D131, by which we successfully improved the short-circuit current and conversion efficiency.
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INTRODUCTION Various mechanisms that lead to the improvement of the current generation efficiency of dye-sensitized solar cells (DSCs) have been devised.1,2 An essential prerequisite for such methods is understanding the adsorption mode of dyes onto nanoporous TiO2 electrodes and its influence on the current generation mechanism of the cell. Recently, a conversion efficiency in excess of 12% was reported.3 We also achieved high photoconversion efficiency in DSCs by coadsorption of dyes whereby the light absorption spectrum was expanded and the reverse electron transfer at the surface was reduced.4 At present, however, we still lack an in-depth understanding of the interaction between dye molecules and the electrode surface, and such information is even more difficult to obtain when multiple dyes are involved, as in the case in a coadsorption system. An example of these difficulties is the modifications, if any, of the adsorption structure of the primary dye induced by the presence of the molecules of a coadsorbent secondary dye. Thus far, dyes adsorbed onto TiO2 single crystal surfaces have been investigated by various means, such as scanning tunneling microscopy,5 X-ray photoelectron spectroscopy (XPS),6,7 and theoretical computations8,9 to clarify the nature of dye adsorption.10−12 However, it is not obvious whether the behavior of the dye molecules on a single crystal surface is faithfully reproduced in the case of dye molecules on nanoporous TiO2, which is used as electrode material in photovoltaic cells. Unfortunately, the complex structure of nanoporous TiO2 makes the direct study of the dye−TiO2 interface extremely difficult. Therefore, we need to develop a method that allows for observation of microscopic structures on the surfaces of porous materials with high resolution and specificity toward different dyes. © 2013 American Chemical Society
In the present study, we observed the adsorption of dyes onto the surface of nanoporous TiO2 by X-ray absorption spectroscopy and XPS. Samples were prepared in parallel under the same device fabrication conditions, which allowed us to make a straightforward connection between microscopic structure and macroscopic cell performance. Although it was previously reported that the dye is anchored on TiO2 via carboxylate groups, the adsorption structure and the number of anchors are still under debate.13−15 Here, to gain a deeper insight into the adsorption mode of dyes onto nanoporous TiO2, we performed measurements with the following distinct features: (1) To selectively observe one type of coadsorbent dye, we focused on the S atom in the isothiocyanate [−NC S] functional group in N719, which is known to be a highly effective sensitizing dye. The coadsorbent dye D13116 does not contain S atoms (Figure 1). (2) To facilitate the interpretation of the measurement results, we used the inner shell (1s) with the simplest spectral feature and high-intensity soft X-rays. (3) To investigate the adsorption depth profile, we compared the results of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and XPS. We studied the difference in the binding mode of N719 onto the TiO2 surface between the cases of single-dye adsorption and coadsorption systems (N719 and D131) of varying mixing ratios. We also investigated the influence of different absorption methods and observed an improvement in solar cell performance. Received: May 8, 2013 Revised: July 11, 2013 Published: July 11, 2013 17033
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TiO 2. In XPS measurements, the mean free path of photoelectrons with the kinetic energy of hundreds of eV is rather short (4−6 nm).20 Because nanoporous TiO2 has a sintered structure consisting of nanoparticles of tens of nanometers in diameter, the photoelectrons emitted from the molecules bound only within the first several nanometers of the outermost TiO2 surface layer contribute to the XPS signal, where any electrons emitted from molecules located deeper inside the pores are blocked because they are in the shadow of other nanoparticles (Figure 3c). A peak is observed at 2481.6 eV in Figure 1A (a) (N719 dye alone) and at 2472.8 eV in Figures1A (b) and (c) (N719 and D131 coadsorption). A hint at the higher energy peak is seen in (b) and (c), indicating a slight admixture of the chemical state in (a). Previous XPS S 2p measurements of dyes containing −NCS functional groups on TiO2 reported11 an energy shift of ∼1 eV as a result of S-TiO2 interaction.21,22 Information on the chemical shift of S 1s is scarce, and information on −NCS is not available. Although it is reasonable to expect that the effect of chemical modification is stronger on 1s spectra than on 2p spectra,23 the present results, together with the complete absence of the lower energy peak in (a), indicate much stronger S-TiO2 interaction than previously reported. On the basis of the above discussion, we assume that the S atom in −NCS is free from strong interaction when N719 is coadsorbed with D131, while the binding of S atoms in N719 to TiO2 is rather strong in the single-component system. This is illustrated in the right panel of Figure 2. The mixing ratios of N719 and D131 do not affect the results significantly. D131 coadsorbent appears to inhibit the S-mediated interaction between N719 and TiO2. Some degree of S-substrate interaction has been already observed for N719/nanostructured TiO2 systems.11,21 However, nearly all S atoms are interacting with the substrate in Figure 1A (a). Considering that there are two −NCS groups in a N719 molecule, this is rather surprising. The large quantitative discrepancy between the present data and those in the literature may be due to the subtle difference in the preparation procedure, although we cannot currently provide a plausible explanation. The strong S-substrate interaction is corroborated by the substrate spectra as well. Figure 1B shows the Ti 2p XPS spectra recorded with TiO2 substrates carrying (d) N719 alone and (e) a N719 + D131 coadsorption system. In (e), the spin− orbit (2p3/2 and 2p1/2) split peaks located at 459.5 and 465.2 eV, respectively, correspond to Ti (IV) in a tetragonal structure. On close inspection, however, a small overall shift of the doublet from (e) to (d) is discernible. An upward shift of the doublet binding energies by ∼1.5 eV has been reported when Ti−S linkage is formed.24 Although the chemical shift of ∼0.5 eV observed in (d) is certainly less significant, this is a clear indication of strong S-substrate interaction, which is effective only when N719 is adsorbed alone. The generally accepted view is that dye molecules bind to the substrate via carboxylate groups. However, our results unambiguously suggest that the −NCS groups in N719 can also interact strongly with the TiO2 surface under some circumstances.9,22 These two cases can be reconciled by considering the molecular arrangements shown in the right top panel in Figure 2. This is only one possible arrangement, and our intention is merely to point out that simultaneous interaction via carboxylate and isothiocyanate groups is allowed from a structural point of view. Naturally, this has profound
Figure 1. (A) S 1s XPS spectra for N719 alone and N719 + D131 coadsorption systems on a nanoporous TiO2 surface measured at hν = 3000 eV. Spectra (a), (b), and (c) correspond to samples infused with N719 alone, a 2:1 mixture of N719 and D131, and a 1:1 mixture of N719 and D131, respectively. The S atoms belong to the −NCS groups in N719, while D131 does not contain S atoms. (B) Ti 2p XPS spectra for N719 alone and N719 + D131 coadsorption systems on a nanoporous TiO2 surface. Spectra (d) and (e) correspond to samples infused with N719 alone and a 1:1 mixture of N719 and D131, respectively.
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EXPERIMENTAL SECTION Nanoparticle TiO2 (JGC Catalysts and Chemicals, 18NR) was sintered at 500 °C and subsequently cooled to 100 °C, after which the dye was adsorbed onto the nanoporous substrate from a 0.1 M ethanol solution by immersion. The total immersion time was 24 h. We evaluated in detail the K-edge effect by NEXAFS spectroscopy and the 1s orbital by XPS method using soft X-rays. NEXAFS measurements were conducted at the BL27A soft X-ray beamline at the Photon Factory, Japan High Energy Accelerator Research Organization (KEK-PF). The X-ray beam was linearly polarized in horizontal direction. The total electron yield method was used. XPS measurements were conducted at a photon energy of 3000 eV with an HAC5000 spectrometer (VSW).17,18 The energy shift due to electrostatic charging was determined using the C 1s main peak at 284.5 eV as reference. The monochromatic incident photon-to-current conversion efficiency (IPCE) was recorded using a computerized setup consisting of a current meter (Bunko Keiki, CED99-W) and a xenon arc lamp (Bunko Keiki, SX150C).19
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RESULTS AND DISCUSSION Figure 1A shows the S 1s XPS results for a single dye (N719) and mixtures of dyes (N719 and D131) with different mixing ratios (2:1 and 1:1) adsorbed onto the surface of nanoporous 17034
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Figure 2. Polarized S K-edge NEXAFS spectra for N719 (blue line) and a N719/D131 coadsorption system (green line) on TiO2 and difference spectra (red line) at an incidence angle of (a) 90, (b) 50, and (c) 15°, respectively. The inset shows the definition of polarization and incidence angles. Peaks A−C are identified as π*(SC), σ*(S−C), and oxidized sulfur, respectively. The right panel illustrates possible molecular arrangements with (top) and without (bottom) strong S-substrate interaction. The orientations of the respective transition dipole moments for peaks A−C are also shown.
consequences in terms of photovoltaic performance, as we discuss below. We performed NEXAFS measurements to investigate the microstructure in the vicinity of S atoms in more detail. Figure 2 shows the NEXAFS spectra from the TiO2 substrate covered with N719 dye alone (blue line) and that covered with a 2:1 mixture of N719 and D131 (green line) at an incidence angle of (a) 90, (b) 50, and (c) 15°. In this energy region, two peaks in the S 1s NEXAFS spectra of CS groups (located at 2474 and 2478 eV, denoted as peaks A and B, respectively) are known to correspond to π*(SC) and σ*(S−C) bonds.25 Peak C at 2483 eV can be attributed to the oxidized sulfur,26 which we interpret as a consequence of S-TiO2 interactions. It is likely that when the S atom in an −NCS group strongly interacts with the substrate the CS bond valence is strongly affected so that peaks A and B at the respective energy values shown in Figure 2 vanish. Thus, peaks A and B and peak C are mutually exclusive for each −NCS group. The blue and green lines in Figure 2 were normalized in the high-energy region around 2500 eV, where the signal intensity should be insensitive to the chemical state of the S atom. The difference spectra were calculated and are shown in red in
Figure 2. In accordance with our expectation that peaks A and B and peak C are mutually exclusive, the difference spectra show a peak at C and dips at A and B. However, the separation into two types of adsorption modes is not complete. There is large contribution from peaks A and B even in the sample with N719 alone, in stark contrast with Figure 1A (a), in which all S atoms interact with the substrate. This is due to the difference in probing depth in XPS and NEXAFS spectroscopy. This point is revisited below. In general, the orientation of molecules adsorbed on a flat surface can be obtained from angle-dependent polarized NEXAFS spectroscopy.27 However, because the surface of the nanoporous TiO2 plate used in this study exhibits morphological features on the order of 20 nm,28 it is difficult to extract orientational information directly. Nonetheless, we performed angle-dependent measurements in an attempt to determine the geometrical signature associated with the three peaks, as illustrated in the right panel in Figure 2; that is, peak C should be substantially polarized compared with peaks A and B. The azimuth or zenith angles of the −NCS group are distributed over a wide range when the group is not bound to the substrate; therefore, the orientations of the transition 17035
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Figure 3. Morphology of nanoporous TiO2: (a) AFM image (0.5 × 0.5 μm). (b) Cross-sectional profile corresponding to red line in panel a. (c) Ranges of depth in XPS and angle-dependent NEXAFS measurements, respectively.
adsorption of N719 alone. It is rather difficult to obtain a depth profile of the adsorption mode from these observations alone. However, a semiquantitative argument can be made as follows. The peak and dip areas in the difference spectra (red) in Figure 2 are similar, even though the noise is substantial. Under the assumption that the joint density of states is conserved within this energy region, we may argue that the ratio of the area under peaks A and B to the area under peak C (provided that a step-function-like background at the edge is properly subtracted) in the blue curve is the ratio of N719 molecules with no S-substrate interaction to that with strong S-substrate interaction averaged over a depth of 70 nm. A rough estimate is that 10% of N719 molecules interact strongly with the substrate via S atoms. Clearly there is a steep gradation in the probability of N719 molecules interacting strongly, starting at 100% at the surface and rapidly diminishing to rather low values in the interior. This suggests that the adsorption of the dye is not uniform across the cross section of the nanoporous film. For example, one can easily imagine that the initial dye concentration decreases substantially as the solution penetrates deeper into the substrate. This is a dynamic effect because the adsorption/dissolution reaction eventually reaches equilibrium. On the basis of this finding, we developed several different protocols for sample fabrication. In particular, we compared a method in which D131 was adsorbed first, followed by infusion with a dye mixture (method I), and a method in which N719 was adsorbed first, followed by infusion with a dye mixture (method II). In this way, we compared the dynamic aspect of dye adsorption and derived its relation to cell performance. In actual DSCs, the isothiocyanate group accepts an electron from I− under photoexcitation.30,31 Strong S-substrate interaction is obviously detrimental to the charge transfer and should be avoided. Thus, we fabricated a number of DSCs with a clear strategy to enhance the effect of D131 in terms of suppressing S-substrate interaction.
dipole moments corresponding to peaks A and B should be random. The intensity of peak C indeed seems to increase steadily as the incidence angle becomes shallower, which should be the case if the transition dipole moment of peak C is predominantly parallel to the film normal, but the quality of data does not allow us to perform further quantitative analysis. The surface morphology of nanoporous TiO2 was observed by atomic force microscopy (AFM) (Hitachi High-Tech, Nanocute SII). Figure 3a shows a typical AFM image corresponding to a scanned area with a size of 0.5 × 0.5 μm. Although it is difficult to obtain quantitative measurements of the rough surface with AFM, the cross-sectional profile along the red line in Figure 3a (shown in (b)) provides a qualitative picture of our sample. A cross-sectional model of the nanoporous TiO2 based on AFM observations is illustrated in Figure 3c. The results of XPS and NEXAFS spectroscopy are discussed below. As previously noted, the mean free path of electrons in XPS measurements is short. Thus, XPS photocurrent is generated only by N719 molecules located on the outer surface of the outermost nanoparticles because the particle size exceeds the mean free path length. Figure 1 indicates that on these surfaces TiO2 interacts with nearly all S atoms in the case of N719 alone and none of the S atoms in the case of the dye mixtures. In contrast, in NEXAFS spectroscopy, the escape depth of electrons that contribute to the total electron yield is estimated to be ∼70 nm.29 Because the penetration of 3 keV incident Xrays is rather deep, the entire depth of our sample absorbs Xrays regardless of the incidence angle. Therefore, all molecules within a depth of 70 nm (and perhaps even deeper considering the porous structure of the substrate) from the surface contribute to the signal. The fact that peak A in Figure 2 is strong despite the absence of a corresponding peak in Figure 1A (a) implies that a considerable amount of dye molecules in the subsurface region (between a few nanometers and 70 nm) do not interact with TiO2 via S atoms, even in the case of 17036
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Table 1. Photocurrent−Voltage Characteristics of DSCs Sensitized with N719 Alone and a Mixture of N719 and a Coadsorbenta device 1 2 3 4 a
adsorbents N719 N719 N719 D131
alone + D131 (1:1) → N719 + D131 (II) → N719 + D131 (I)
Voc [V]
Jsc [mA cm‑2]
fill factor [%]
efficiency [%]
0.740 0.740 0.720 0.728
12.61 12.93 12.92 13.41
0.758 0.752 0.747 0.753
7.07 7.20 6.95 7.34
For each procedure, we fabricated several cells whose efficiencies agreed within 0.05%.
multiple dyes can be optimized to improve the efficiency of DSCs. In future work, we will investigate structures yielding even higher Jsc by performing various structural optimizations of the coadsorbed dyes.
The results of cell performance evaluation are given in Table 1. Four types of DSCs were tested: Device 1 (a cell with N719 dye alone), Device 2 (a cell infused with a 1:1 mixture of N719 and D131), Device 3 (a cell prepared by method II), and Device 4 (a cell prepared by method I). Figure 4 shows the measured photocurrent action spectra. Compared with N719 alone, N719 coadsorbed with D131
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CONCLUSIONS We conducted X-ray spectroscopy using soft X-rays to clarify the dye adsorption structure on the surface of nanoporous TiO2 electrodes used in actual DSCs. Regarding the adsorption structures of N719 dye alone and mixtures of N719 and D131 dyes, the addition of D131 dye suppresses the interaction between N719 and TiO2 via S atoms. On the basis this finding, we focused on the initial adsorption process. In the case of initial D131 adsorption followed by infusion with a dye mixture, both Jsc and the conversion efficiency were higher than the case of initial N719 adsorption followed by infusion with a dye mixture. This is a clear demonstration that efficiency can be increased further in comparison with conventional dye mixture infusion.
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Figure 4. IPCE spectra of DSCs with D131 coadsorbent (green, blue, and red lines) and without coadsorbent (black line) (Table 1).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
increased IPCE at a wavelength of around 450 nm due to the presence of D131, which is reasonable. Unexpectedly, in Device 4, IPCE increased not only around 450 nm but also around 550 nm. The presence of D131 not only widens the absorption spectrum toward shorter wavelengths but also increases the efficiency of N719. These results provide a clear illustration of the mechanism of the synergetic effect, namely, that D131 reduces the number of N719 molecules in an unfavorable adsorption configuration. The efficiency trend 1 ∼ 3 < 2 < 4 (Table 1) reflects the effect of D131 in terms of blocking strong S-substrate interaction. This effect is minimal when the substrate surface is covered with N719 first and most pronounced when the substrate surface is covered with D131 first. Naturally, we should be cautious in interpreting our X-ray spectroscopy results. In actual working devices, dye molecules interact with solvents and various anions (such as I−) rather than existing in a vacuum. One can easily imagine that in actual cells the sharp distinction between the two types of adsorption suggested by Figure 1 is much more moderate. However, looking at Devices 3 and 4, it should be noted that the fraction of bound S atoms released by D131 treatment (∼10%) is not far from the increase in JSC (∼4%). There have been a number of attempts at structural optimization of dye molecules in terms of the relative locations of the anchoring moiety and HOMO.30 The results of this experiment suggest that a similar strategy can be employed in dye coadsorption systems as well. Not only the type of the coadsorbed secondary dye but also the order of adsorption of
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the staff of KEK-PF for assistance in experiments using synchrotron radiation. This work was conducted with the approval of KEK-PF (proposal no. 2012G113) and partially supported under the Precursory Research for Embryonic Science and Technology (PRESTO) program of the Japan Science and Technology Agency.
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