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2009, 113, 6910–6912 Published on Web 04/03/2009
Structural Changes of Electrodeposited ZnO Matrices for Dye-Sensitized Solar Cells During Preparation Harald Graaf,*,† Carsten Maedler,† Mirko Kehr,† and Torsten Oekermann*,‡ Institute of Physics, Chemnitz UniVersity of Technology, 09107 Chemnitz, Germany, and Institute of Physical Chemistry and Electrochemistry, Leibniz UniVersity HannoVer, 30167 HannoVer, Germany ReceiVed: March 11, 2009
Nanoporous zinc oxide films can be prepared by electrochemical codeposition with the dye eosin Y (EY) as a template and its subsequent desorption with aqueous KOH. In this contribution, the partial dissolving and reorganization of the zinc oxide film during the desorption step was studied in detail using X-ray diffraction, Kelvin probe force microscopy, and atomic force microscopy. It was found that the reorganization leads to an enhancement of crystal orientation and a reduction of the oxygen defect concentration at the surface of the zinc oxide, which is supposed to be a reason for suppressed recombination of electrons in these films. Aside from TiO2, ZnO is one of the most promising materials for porous electron transport layers in dye-sensitized solar cells (DSSCs). Both materials exhibit very similar band gaps (ZnO, 3.2 eV; TiO2, 3.0 eV) and conduction band edge positions (ZnO, -4.3 eV; TiO2, -4.5 eV).1 An advantage of ZnO is the direct electrodeposition of fully crystalline and highly porous ZnO films from aqueous solution without the need of high temperature treatment.2,3 The pores are introduced by codeposition of structure-directing additives and their subsequent removal. The ZnO films showing the highest porosity to date were prepared by codeposition with water-soluble dye molecules. Especially ZnO/eosin Y films electrodeposited from O2-saturated ZnCl2 aqueous solution with concentrations of the disodium salt of eosin Y in the order of some tens of µM have been investigated in detail in recent years.4,5 An especially high dye content could be achieved at deposition potentials of < -0.9 V vs SCE, leading to highly porous ZnO films after removal of the dye by extraction with aqueous KOH.6 In this potential region, the eosin Y dianion EY2- is reduced to EY4-, which forms a strong complex with Zn2+,7 meaning that it can also strongly interact with Zn-terminated surfaces of the growing ZnO films. In addition, EY4- and Zn2+ can form a polymeric structure, which acts as a template for the pores.8 After desorption of the eosin Y agglomerates and the formation of a porous ZnO film, eosin Y or another dye can be readsorbed, typically from 0.5 mM ethanolic dye solution, in order to form a dye monolayer suitable for DSSCs on the ZnO surface.6 Efficiencies of up to 5.6% have been achieved with such cells using the indoline dye D149 as a sensitizer, which is the highest efficiency achieved with ZnO-based DSSCs so far.9 However, many details of the ZnO/eosin Y hybrid film electrodeposition and the influence of the further preparation steps on the film properties, e.g., crystallographic orientation and surface topography, are still unknown. In this contribution, * Corresponding authors. E-mail:
[email protected] (H.G.);
[email protected] (T.O.). † Chemnitz University of Technology. ‡ Leibniz University Hannover.
10.1021/jp902190t CCC: $40.75
we present recent results on changes within the ZnO matrix occurring especially during desorption of the dye. In order to understand the underlying processes, we investigated ZnO/eosin Y samples directly after their electrodeposition (“as deposited”) and after extraction of the dye (“desorbed”) by means of X-ray diffraction (XRD) and atomic force microscopy (AFM). For comparison, a zinc oxide film electrodeposited without additive (“pure ZnO”) under otherwise identical conditions was also investigated. ZnO/EY hybrid films were electrodeposited from O2-saturated 0.1 M KCl/5 mM ZnCl2 aqueous solution onto glass substrates coated by F-doped tin oxide (FTO, 10 Ω/square, Asahi Glass) in a rotating disk setup (300 rpm) with an active area of ca. 2 cm2. The substrates were cleaned with acetone, ethanol, and distilled water and etched in 2 M HNO3 for 2 min before the deposition. A Zn wire was used as a counter electrode, while a Ag/AgCl electrode served as a reference electrode. The deposition was carried out for 30 min at -0.91 V vs Ag/AgCl () -1.0 V vs SCE), leading to a film thickness of 2.5 µm. The deposited films were briefly rinsed with distilled water and immediately dried in a stream of air in order to avoid any changes of the film after the electrodeposition process. Desorption of the dye was performed by immersing the as-deposited ZnO/EY films into an aqueous KOH solution for 24 h. The X-ray diffraction patterns were measured with a HZG4 diffractometer of the company “Freiberger Pra¨zisionsmechanik” in a Bragg-Brentano geometry. Apertures of 0.35 mm for the beam as well as in front of the detector were used to minimize stray light. A copper anode with 40 kV and 30 mA was used. The signal was detected by a scintillation detector. An Anfatec Level atomic force microscope was used to simultaneously image the topography and the work function of the samples by Kelvin probe force microscopy (KPFM).10 The same conductive cantilever was used throughout each series of investigations, which consisted of the calibration of the cantilever by measuring the work function of a previously cleaned piece of platinum and the subsequent investigation of the three different samples. The actual work functions of the material on the samples and 2009 American Chemical Society
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Figure 1. X-ray diffraction patterns of pure electrodeposited ZnO (“pure ZnO”) and electrodeposited ZnO/EY films before (“as deposited”) and after (“desorbed”) desorption of the dye by aqueous KOH. Diffractions of the substrate (F-doped SnO2) are marked with an asterisk. Part a shows the pattern between 25 and 40°, and part b, the ZnO (100) and (101) peaks in detail.
their statistical error were determined by fitting the graphs in the histogram of the bias values obtained from the 256 × 256 pixel scans with Gaussian functions. XRD patterns of all samples show reflexes attributable to the wurtzite structure, however, with strong (002) but weak (100) and (101) reflexes (Figure 1). This has been reported before for ZnO films electrodeposited even without additive, meaning that the ZnO crystals grow preferentially with their c-axis perpendicular to the substrate. The reason is an intrinsic anisotropy in the growth rate of ZnO crystals due to differences in the dissolution rates of different ZnO surfaces.11,12 The polar Zn-terminated (001) face of ZnO is known to dissolve more slowly than the nonpolar surfaces perpendicular to this face. It has also been observed before that this crystallographic orientation of the ZnO can be further enhanced in electrodeposited ZnO/eosin Y films, probably caused by preferential adsorption of eosin Y on the (001) face, since eosin Y is known to catalyze the electrodeposition of ZnO.5 However, this effect seems to be rather weak, since we found only slightly smaller intensities of the (100) and (101) peaks for the “as-deposited” ZnO/eosin Y film compared to the “pure ZnO” film (Figure 1, middle and bottom XRD). It has not been observed before, though, that a much more drastic enhancement in the crystallographic orientation occurs during the dye desorption step in aqueous KOH. This is suggested by the virtual disappearance of the (100) and (101) peaks in the “desorbed” film (Figure 1, top XRD). Obviously, the KOH solution not only desorbs the dye from the ZnO surface, but it also partly dissolves the ZnO surface, enabling its rearrangement. It may actually be expected that, due to the higher dissolution rates of the nonpolar ZnO faces, especially the crystals with (100) and (101) orientation will be dissolved in this process. However, due to the high local Zn2+ and OH- concentrations within the pores of the film during the dye desorption process, most of the dissolved ZnO will be recrystallized, and this recrystallization will almost exclusively take place at the surface of ZnO crystals with (001) orientation, since they already dominate the films before the dye desorption process and are dissolved slower. The result is a rearrangement of ZnO originally electrodeposited in (100) and (101) orientation to the (001) orientation. The crystallographic orientation of the films is also seen in the work functions of the samples (Figure 2). The pure ZnO films (4.1 ( 0.3 eV) as well as the as-deposited ZnO/eosin Y films (4.25 ( 0.25 eV) exhibit similar energetic positions as
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Figure 2. Histograms of the work functions of as-deposited and desorbed ZnO/EY films, a pure ZnO film, and a platinum foil as a reference. The maximum of the platinum foil histogram was set to the literature value of 5.3 eV for the work function of platinum,15 and all other values are normalized with respect to that value.
those reported earlier for the Zn-terminated (001) face of wurzite ZnO (4.25 eV13), but they clearly differ from the value reported for the O-terminated face (4.95 eV14). This result confirms that the net growth rate of ZnO is fastest on the Zn-terminated (001) face. The work function of the dye cannot be seen by this method, as the electronic properties are dominated by the inorganic semiconductor. Beside the absolute values of the work functions, a clear increase in the fwhm (full width at half-maximum) value from the first measured “as-deposited” film to the two other samples as well as the last measured platinum foil is observed, which can be partly attributed to changes in the cantilever tip as well as a variation in the work function distribution of the different samples. A more detailed analysis shows that the histogram for the “desorbed” film exhibits a slight deviation from a symmetric distribution. This deviation indicates changes in a very small fraction of the zinc oxide, leading to a smaller work function. The Gaussian fits in Figure 2 (red lines) show that the asymmetric histogram of the “desorbed” film (thin red line) contains two symmetric histograms with a difference between the maxima of about 0.2 eV (main peak at 4.25 eV, small peak at 4.02 eV). This is an indication for the existence of two phases with different work functions. Such a decrease in the work function of ZnO was reported before and discussed to be due to changes in the defect concentration in zinc oxide caused by oxygen diffusion and therefore a new equilibrium of defect concentrations at the surface as well as in the bulk.14 However, the additional peak is quite small compared to the main peak, indicating that there is only a small fraction of the second phase, and the peak is only shifted by about 0.2 eV which is less than reported for bulk zinc oxide, where a shift of about 0.5 eV was found. All together, this suggests that the changes are limited to changes in the defect concentrations at the surface caused by the partial surface dissolution during the dye desorption process. Mapping of the surface shows neither specific sites on the surface nor accumulation of this additional phase, which means that the phase is distributed statistically on the surface. It can be assumed that the reorganization of the surface during the dye desorption process leads to a curing of defects, i.e., to a lower concentration of defect states and therefore to a decrease in the doping level. This may be one reason for the high efficiency of DSSC based on these films even compared to
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Figure 3. AFM topography images of (a) an as-deposited ZnO/eosin Y film, (b) a ZnO/eosin Y film after desorption of the dye, and (c) a pure electrodeposited ZnO film.
sintered nanoparticulate ZnO films, since defects as well as a high doping level of the ZnO at the surface would lead to enhanced back reaction of photogenerated electrons to the redox electrolyte. After desorption of the template molecules, the modified ZnO surface layer may act as a blocking layer in dyesensitized solar cells, which tends to keep the photogenerated electrons inside the porous ZnO network. On the other hand, the fast electron transport to the back contact is preserved, as seen by intensity modulated photocurrent spectroscopy (IMPS).16 This indicates that the defects within the ZnO bulk are conserved, which are needed for the n-type character of the ZnO. At the same time, intensity modulated photocurrent spectroscopy (IMVS) showed relatively long electron lifetimes, confirming the relatively low tendency of the ZnO surface in these films toward back reaction of photogenerated electrons to the redox electrolyte. The topography of the samples (Figure 3) allows further conclusions. The “pure ZnO” sample (Figure 3c) is composed of large crystals (about 500 nm to 1 µm in diameter), suggesting a very high crystallinity of the film. The “desorbed” sample (Figure 3b) shows domains of the same size, which are made up of a lot of small features with a size of a few tens of nanometers. These domains are the porous ZnO crystals observed before in scanning electron microscropy (SEM) and transmission electron microscropy (TEM) investigations.5 The pores are not detectable in our topography measurements due to the large curvature radii of the used tips (about 30 nm). Nevertheless, the AFM image of the “as-deposited” film shown in Figure 3a reveals a totally different topography for this sample compared to the other images. The film does not show much structure in its topography and no individual crystals are visible, giving the impression of an amorphous material. This indicates that the surface of this film consists mainly of eosin Y, meaning that the dye adsorption is faster than the ZnO formation. For the growth mechanism of the ZnO/eosin Y hybrid films it can therefore be concluded that the EY4-/Zn2+ agglomerates are formed first and are then surrounded by electrodeposited ZnO. In conclusion, we propose that the dye desorption step in the preparation of DSSC based on electrodeposited dye-templated
ZnO films plays a crucial role in their performance. During the desorption step, the ZnO film is partly dissolved and reorganized, leading to an enhanced crystallographic orientation, a reduced oxygen defect concentration at the surface, and therefore a reduced recombination of photogenerated electrons. In general, the results show that “surface engineering” may be a valuable method to further improve the efficiency of DSSC. Acknowledgment. Financial support of the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie is gratefully acknowledged. References and Notes (1) Memming, R. Topics in Current Chemistry; Springer Verlag: Berlin, 1994; Vol. 169. (2) Izaki, M.; Omi, T. J. Electrochem. Soc. 1996, 143, L53-L55. (3) Peulon, S.; Lincot, D. AdV. Mater. 1996, 8, 166–170. (4) Yoshida, T.; Terada, K.; Schlettwein, D.; Oekermann, T.; Sugiura, T.; Minoura, H. AdV. Mater. 2000, 12, 1214–1217. (5) Yoshida, T.; Pauporte´, T.; Lincot, D.; Oekermann, T.; Minoura, H. J. Electrochem. Soc. 2003, 150, C608-C615. (6) Yoshida, T.; Iwaya, M.; Ando, H.; Oekermann, T.; Nonomura, K.; Schlettwein, D.; Wo¨hrle, D.; Minoura, H. Chem. Commun. 2004, 400–401. (7) Goux, A.; Pauporte, T.; Lincot, D.; Dunsch, L. ChemPhysChem 2007, 8, 926–931. (8) Goux, A.; Pauporte, T.; Yoshida, T.; Lincot, D. Langmuir 2006, 22, 10545–10553. (9) Yoshida, T.; Zhang, J.; Komatsu, D.; Sawatani, S.; Minoura, H.; Pauporte, T.; Lincot, D.; Oekermann, T.; Schlettwein, D.; Tada, H.; Wo¨hrle, D.; Funabiki, K.; Matsui, M.; Miura, H.; Yanagi, H. AdV. Funct. Mater. 2009, 19, 17–43. (10) Nonnenmacher, M.; O’Boyle, M. P.; Wickramasinghe, H. K. Appl. Phys. Lett. 1991, 58, 2921–2923. (11) Gerischer, H.; Sorg, N. Electrochim. Acta 1992, 37, 827–835. (12) Choi, J. H.; Jang, E. S.; Won, J. H.; Chung, J. H.; Jang, D. J.; Kim, Y. W. AdV. Mater. 2003, 15, 1911–1914. (13) Moormann, H.; Kohl, D.; Heiland, G. Surf. Sci. 1979, 80, 261– 264. (14) Jacobi, K.; Zwinger, G.; Gutmann, A. Surf. Sci. 1984, 141, 109– 125. (15) Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1989. (16) Oekermann, T.; Yoshida, T.; Minoura, H.; Wijayantha, K. G. U.; Peter, L. M. J. Phys. Chem. B 2004, 108, 8364–8370.
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