ARTICLE pubs.acs.org/JPCC
Influence of Coumarin 343 Monomer Codeposition on the Structure and Electronic Properties of Electrodeposited ZnO Harald Graaf,*,†,‡ Franziska L€uttich,† Christian Dunkel,§ Michael Wark,§,|| and Torsten Oekermann*,§,^ †
Institute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany Institute of Chemistry, University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany § Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, 30167 Hannover, Germany Laboratory of Industrial Chemistry, Ruhr University Bochum, 44801 Bochum, Germany ^ Friemann & Wolf Batterietechnik GmbH, 63654 B€udingen, Germany
)
‡
ABSTRACT: Codeposition of the dye coumarin 343 (C343) in its monomeric form strongly influences the crystallographic orientation and electronic properties of electrodeposited ZnO films. This is opposed to the codeposition of Eosin Y (EY), which forms aggregates in ZnO/dye films, leading to much less influence on crystallographic orientation and electronic properties. Highly porous ZnO films are formed upon dye extraction in both cases, which is due to the action of the dye aggregates as pore templates in the case of EY and due to a blocking effect on ZnO electrodeposition, as opposed to the catalytic effect of EY, in the case of C343.
’ INTRODUCTION While titanium dioxide (TiO2) is generally recognized as the current standard material for dye-sensitized solar cells (DSCs), zinc oxide (ZnO) has recently gained increased interest due to very similar bandgaps (3.2 eV for ZnO versus 3.0 eV for TiO2) and band positions (conduction band of ZnO at 4.3 eV on the vacuum level versus 4.5 eV for TiO2).1 In addition, it was found to have superior electrical properties compared to TiO2 due to its higher electron mobility.2 Another 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.3 Porous films can be easily obtained by codeposition of structure-directing organic molecules and their subsequent removal. Certain water-soluble dye molecules proved to be most suitable for this purpose to date since they form pores in a size range most useful for DSCs (520 nm) and can be easily removed from the deposited ZnO/dye hybrid films by extraction with aqueous KOH (pH = 10.5).4 The most prominent and best investigated example to date is ZnO codeposited with the disodium salt of eosin Y (EY),5,6 which achieved efficiencies of up to 5.6% after removal of the EY and adsorption of the indoline dye D149 as an efficient sensitizer with a broad absorption band.7 One of the most interesting features of the ZnO/EY hybrid films is the preferred crystallographic orientation induced by the dye molecules. Compared to ZnO films electrodeposited without an additive, an enhanced (001) orientation was found for these films.6 In earlier studies, we had investigated the further r 2011 American Chemical Society
preparation steps of ZnO/EY films toward DSCs, namely, the desorption of the structure-directing EY and its readsorption, in much detail and found that especially the dye desorption step leads to a partial reorganization of the ZnO accompanied by a further intensified crystallographic orientation and the healing of surface states.8,9 ZnO films with a very similar porous structure but with a totally different crystallographic orientation can be electrodeposited by using the coumarin dye C343 instead of EY as the structure-directing additive. In this case, the c-axis was found to be preferably oriented parallel to the substrate.10 Since ZnO is known to have its highest electron mobility perpendicular to the c-axis,11 these films led to improved electron transport in DSC compared to EY-templated films.12 In the present study, we have investigated these films and the arrangement of the dye molecules in the inorganic matrix in more detail using scanning electron microscopy (SEM), X-ray diffraction (XRD), optical measurements in diffuse reflectance, atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM). As in our earlier study on ZnO/EY films, ZnO/C343 hybrid films were investigated before (“as-deposited”) and after (“desorbed”) removal of the dye, while a ZnO film electrodeposited under otherwise identical conditions without dye (“pure ZnO”) was Received: October 5, 2011 Revised: December 14, 2011 Published: December 14, 2011 5610
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used as a standard. Comparison to the results obtained at ZnO/EY films reveals interesting differences concerning the dye adsorption on the ZnO and its influence on the film properties.
’ EXPERIMENTAL SECTION Film Preparation. ZnO/C343 hybrid films were electrodeposited from O2-saturated 0.1 M KCl/5 mM ZnCl2/80 μM C343 aqueous solution onto glass substrates coated by F-doped tin oxide (FTO, 10 Ω/square, Asahi Glass). The substrates were vertically inserted into the deposition solution, which was stirred with 200 rpm; the deposited films have an active area of 2 cm 3.5 cm. The substrates were etched in 2 M HNO3 for 2 min before use. 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 by a potentiostat (Amel Instruments model 7050), leading to a film thickness of 1.5 μm (compared to 2.5 μm for ZnO/EY films8). The deposited films were briefly rinsed with distilled water and immediately dried in a stream of air to avoid any changes of the film after the electrodeposition process. Desorption of the dye was performed by immersing the as-deposited ZnO/C343 films into an aqueous KOH solution (pH = 10.5) for 24 h. X-ray Diffraction. A HZG4 instrument (Freiberger Pr€azisionsmechanik) with a Cu anode at 50 kV and 30 mA was used to obtain X-ray reflections. Atomic Force Microscopy. Topographies and the work functions of the samples by Kelvin probe force microscopy (KPFM) were taken simultaneously by AFM (Level AFM, Anfatec AG).13 The same conductive cantilever (Mikromasch NSC18/TiPt) 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 (the work function here is set to 5.3 eV) and the subsequent investigation of the three different samples. The work function of each sample and its related statistical error was determined by fitting the histogram of the bias values with Gaussian functions. To avoid an influence of adsorbed water, the samples were stored under vacuum, and the measurements were obtained at relative humidities of about 10%. UVvis Reflection Spectroscopy. UVvis measurements of the films were performed with a Varian Cary 4000 spectrophotometer in diffuse reflection. The reflection R∞ was then converted into the KubelkaMunk-function f(R∞). According to Beranek and Kisch,14 the bandgap energies Eg were calculated by an approach using the equation
a¼A
ðhν Eg Þn hν
In this equation, α is the optical absorption coefficient; A is a constant; hν is the energy of the illuminated light; and n is a constant describing the nature of the electron transition. Under the assumption that α is proportional to f(R∞) and that ZnO has a direct bandgap (n = 1/2), we receive the equation (f(R∞) 3 hν)2 = A(hν Eg). If we plot (f(R∞) 3 hν)2 against hν and extrapolate the linear part of the curve to the x-axis, the point of intersection provides Eg. Optical Absorption Spectroscopy. Optical absorption spectra of C343 aqueous solutions were taken at two different concentrations (104 and 105 mol/L). The spectra were performed on a Varian Cary 100 spectrometer.
Figure 1. X-ray diffraction patterns of pure electrodeposited zinc oxide (“pure ZnO”) and electrodeposited ZnO/C343 films before (“as-deposited”) and after (“desorbed”) desorption of the coumarin 343 dye by aqueous KOH. The relevant peaks of the ZnO are indicated by their crystal orientations. Reflections of the substrate (F-doped SnO2) are marked with asterisks.
’ RESULTS AND DISCUSSION XRD patterns of ZnO/C343 films show reflections attributable to the wurtzite structure (Figure 1). They exhibit strong (100) and (101) but no (002) reflexes as reported earlier.10 We confirmed this finding for as-deposited as well as desorbed films, which is opposed to our earlier findings at ZnO/EY films, where (100) and (101) reflexes were still present in the as-deposited film and the dye desorption process led to a further enhancement of the (001) orientation. The topography and SEM images (Figure 2) of ZnO/C343 films are dominated by disk-like structures (some tens of nanometers wide and a few tens of nanometers thick) standing perpendicular on the substrate. While the images of the desorbed ZnO/C343 film (Figure 2c and d) resemble the SEM images of this kind of film reported earlier by Yoshida et al.,10,15 exhibiting the porous structure of the electrodeposited ZnO, the pores are much less visible in the images of the as-deposited films (Figure 2a and b). This is due to the pore filling and surface coverage by C343 in the case of the asdeposited film. A similar observation had been made for ZnO/EY films, however, with a much stronger difference in the appearances of as-deposited and desorbed films, which was attributed to a higher amount of dye due to the formation of larger dye aggregates.9 In the present case, the dye layer on top of the ZnO and in the pores seems to be much thinner, indicating a much lower tendency toward aggregation in the case of C343 compared to EY. The results for the work functions of the samples (Table 1) are consistent with the observed crystallographic orientations. The experimental value of 4.1 ( 0.3 eV for the pure electrodeposited ZnO film is in accordance with the value of 4.25 eV reported for the Zn-terminated (001) face of wurtzite.16 A slightly lower work function of electrodeposited ZnO is actually expected due to its usually higher doping level17 and the resulting higher Fermi level. The as-deposited ZnO/C343 film exhibits a clearly higher work function of 4.95 eV, while the desorbed film is characterized by a work function of 4.65 eV (Figure 3). The latter value is in very good agreement with the 4.64 eV reported for the nonpolar (100) facet of ZnO.16 Since the crystal structure does not change during desorption of the C343, it can be concluded that the ZnO near the surface of the as-deposited sample exhibits (100) facets as well. Therefore, the higher work function of the as-deposited sample must be attributed to the dye layer observed on top of the ZnO as discussed in conjunction with Figure 1. 5611
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Figure 2. Typical SEM and AFM topography images of a ZnO/C343 film directly after deposition (“as-deposited film”/a and b) and a ZnO film after desorption of the C343 (“desorbed film”/c and d). The bars indicate 500 nm.
Table 1. Work Functions and Band Gaps of Electrodeposited ZnO Films Measured by KPFM and Optical Measurements sample ZnO/C343 as deposited ZnO/C343 desorbed
work function/eV 4.95 ( 0.05 4.65 ( 0.05
band gap/eV 3.395 3.425
Pure ZnO
4.1 ( 0.3 [ref 8]
3.418
ZnO/EY desorbed
4.2 ( 0.2 [ref 8]
-
Figure 3. Left side: Schematic molecular orientation of a C343 molecule on the ZnO surface. The permanent dipole is indicated by the arrow. Right side: Energetic shift of the measured work function during desorption of C343.
Figure 4. (a) Diffuse reflection spectra (or more exactly Tauc plots) of an as-deposited and a desorbed ZnO/C343 film. The linear regressions indicate the band gap of the zinc oxide. (b) Optical absorption spectra in transmission of C343 in aqueous solution taken at two different dye concentrations.
The significant influence of the dye in the case of the ZnO/ C343 system suggests an ordered assembly of the C343 molecules on the ZnO surface in an upright position (Figure 3, left side). Since C343 molecules possess strong permanent dipole moments within their molecular planes (AM1 calculations gave a value of 10.41 D),18 this would lead to an orientation of the dipole moments more or less perpendicular to the ZnO surface, turning the nonpolar ZnO surface into a polar one and thereby changing its work function. Examples of the influence of electrical dipoles in adsorbed molecules on the work function of a surface have been reported before for metals covered with oligomers.19 However, the shift in the work function in those studies was about 180 meV smaller than in the present case due to lower dipole moments of the oligomers and different orientations with respect to the surface. In contrast to the present results for ZnO/C343 films, no influence of the dye on the work function had been found for the (001) oriented ZnO films codeposited with EY, where work functions of 4.25 eV had been measured for as-deposited as well as desorbed films.8 Since the permanent dipole moment of EY (16.93 D)2,20 is considerably higher than that of C343, this
observation indicates a different arrangement of the molecules on the surface. One possibility is that the EY molecules are orientated with their dipole moments more or less parallel to the surface. In such a configuration, the dipole moments can not be detected. A second possible explanation takes into account that EY strongly tends to form aggregates, especially in conjunction with Zn2+ ions.9 Within such aggregates, the molecules may be arranged in random orientations, or in the case of an ordered arrangement of the molecules within the aggregates, the latter may be randomly oriented on the ZnO surface, so that all the dipole moments of the molecules would compensate each other. In addition to the change in the work function, C343 is also found to alter the band gap of the zinc oxide in ZnO/C343 hybrid films (Table 1, Figure 4). Reflection measurements at the films show a shift of 30 meV from 3.395 eV for the as-deposited film to 3.425 eV for the desorbed film. This indicates that the strong dipolar character of the adsorbed dye molecules also influences the electronic bulk properties of the ZnO. Furthermore, the band gap of 3.425 eV observed for the desorbed film is slightly higher than the 3.35 eV typical for pure bulk ZnO1 but rather similar to that 5612
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The Journal of Physical Chemistry C of pure electrodeposited ZnO, for which we found a band gap of 3.418 eV. The difference between the experimental work function of electrodeposited ZnO and the literature value for bulk ZnO confirms the higher doping level of electrodeposited ZnO. While optical measurements of as-deposited films show no clear distinction between the ZnO and the incorporated C343 if measured in transmission,21 the reflection data allow a better analysis of light absorption by C343 in the ZnO/dye hybrid structures. The C343 exhibits a maximum at 386 nm and a shoulder at 440 nm (Figure 4a). A similar but red-shifted combination with a maximum at 415 nm and a shoulder at 448 nm has been found for aqueous solutions of C343 at low concentration, while only one peak at 446 nm was found for a higher concentration (Figure 4b). For the higher concentration the formation of dye aggregates can be assumed. It follows that the absorption maximum at lower wavelength can be attributed to C343 monomers, meaning that most C343 molecules in the as-deposited ZnO/C343 films are not aggregated but adsorbed on ZnO surfaces as monomers. At the same time, the obviously rather strong adsorption explains the strong blue shift of the absorption maximum compared to the C343 monomer in solution. These observations are totally different from the result for as-deposited ZnO/EY films, where the absorption spectrum proves the dominance of dye aggregates,9 which again proves the much lower tendency of C343 toward aggregation compared to EY. The fact that, nevertheless, the ZnO in the ZnO/C343 films is porous and all C343 can be extracted to form pure ZnO films indicates that the formation of the dye aggregates may not be as important for the formation of the pores as it has been thought before. Obviously, pores in the ZnO can also be formed just with growth-blocking C343 monomers adsorbed to the ZnO films, without the need to fill the whole pore volume with dye molecules. The different mechanisms by which the dye molecules influence the growth of the ZnO may, however, be a reason for the observed difference between the BET surface areas of the resulting films, which (relative to the film thickness) were found by us as well as by Nonomura et al.12 to be about half the value of that of desorbed ZnO/EY films in the case of desorbed ZnO/C343 films. When used in DSCs, this lower BET surface area of the ZnO/C343 film actually leads to conversion efficiencies η comparable to those achieved with ZnO/EY films (e.g., η = 1.3% with eosin Y as readsorbed sensitizer for both films),12 meaning that the more efficient electron transport and resulting higher electron collection efficiency are offset by a lower concentration of sensitizer molecules. To achieve higher efficiencies in DSCs, the porous structure of the ZnO/C343 films therefore still has to be optimized.
’ CONCLUSIONS In conclusion, we have demonstrated that the adsorption of single coumarin 343 dye molecules, as opposed to dye aggregates in the case of eosin Y, strongly influences the structure, crystallographic orientation, and electronic properties of electrodeposited ZnO films. This is proven by the absorption spectrum of the dye in the ZnO/ C343 hybrid films as well as by changes in the work function and the band gap of the ZnO, which can only be caused by the adsorption of dye monomers with their dipole moments perpendicular to the ZnO surface. Nevertheless, highly porous ZnO films are formed upon extraction of the dye from electrodeposited ZnO/C343 hybrid films despite the absence of dye aggregates as pore templates. In this case, the pore formation is possibly explained by a blocking effect of the C343 on ZnO electrodeposition as opposed to a catalytic effect in the case of eosin Y.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]; torsten.oekermann@ friwo-batterien.de. Tel.: ++4937153134807. Fax: ++49 371531834807.
’ ACKNOWLEDGMENT We thank A.A. Auer and W. Schmidt for calculations of the orientation of the permanent dipoles in EY and C343 and T. Baumg€artel for the visualization of the molecule orientation on the ZnO surface. Special thanks go to S. Schulze for scanning electron microscopy images and M. Kehr for XRD investigations. Financial support by Deutsche Forschungsgemeinschaft (DFG) (OE 420/5-1 and WA 1116/15) is gratefully acknowledged. ’ REFERENCES (1) Memming, R. Semiconductor Electrochemistry; Wiley-VCH: Weinheim, 2002. (2) Bellingeri, E.; Marre, D.; Pellegrino, L.; Pallecchi, I.; Canu, G.; Vignolo, M.; Bernini, C.; Siri, A. S. Superlattices Microstruct. 2005, 38, 446–454. (3) (a) Izaki, M.; Omi, T. J. Electrochem. Soc. 1996, 143, L53–L55. (b) Peulon, S.; Lincot, D. Adv. Mater. 1996, 8, 166–170. (4) Yoshida, T.; Iwaya, M.; Ando, H.; Oekermann, T.; Nonomura, K.; Schlettwein, D.; W€ohrle, D.; Minoura, H. Chem. Commun. 2004, 400–401. (5) Yoshida, T.; Terada, K.; Schlettwein, D.; Oekermann, T.; Sugiura, T.; Minoura, H. Adv. Mater. 2000, 12, 1214–1217. (6) Yoshida, T.; Pauporte, T.; Lincot, D.; Oekermann, T.; Minoura, H. J. Electrochem. Soc. 2003, 150, C608–C615. (7) Minoura, H.; Yoshida, T. Electrochemistry 2008, 76, 109–117. (8) Graaf, H.; M€adler, C.; Kehr, M.; Oekermann, T. J. Phys. Chem. C 2009, 113, 6910–6912. (9) Graaf, H.; Maedler, C.; Kehr, M.; Baumg€artel, T.; Oekermann, T. Phys. Status Solidi A 2009, 206, 2709–2714. (10) Yoshida, T.; Zhang, J.; Komatsu, D.; Sawatani, S.; Minoura, H.; Pauporte, T.; Lincot, D.; Oekermann, T.; Schlettwein, D.; Tada, H.; W€ohrle, D.; Funabiki, K.; Matsui, M.; Miura, H.; Yanagi, H. Adv. Funct. Mater 2009, 19, 17–43. (11) Wanger, P.; Helbig, R. J. Phys. Chem. Solids 1974, 35, 327–335. (12) Nonomura, K.; Komatsu, D.; Yoshida, T.; Minoura, H.; Schlettwein, D. Phys. Chem. Chem. Phys. 2007, 9, 1843–1849. (13) Nonnenmacher, M.; O’Boyle, M. P.; Wickramasinghe, H. K. Appl. Phys. Lett. 1991, 58, 2921–2923. (14) Beranek, R.; Kisch, H. Photochem. Photobiol. Sci. 2008, 7, 40–748. (15) Oekermann, T.; Yoshida, T.; Schlettwein, D.; Sugiura, T.; Minoura, H. Phys. Chem. Chem. Phys. 2001, 3, 3387. (16) Moormann, H.; Kohl, D.; Heiland, G. Surf. Sci. 1979, 80, 261–264. (17) Anthony, S. P.; Lee, J. I.; Kim, J. K. Appl. Phys. Lett. 2007, 90, 103107. (18) McCarthy, P. K.; Blanchard, G. J. J. Phys. Chem. 1993, 97, 12205–12209. (19) Umeda, K.; Kobayashi, K.; Ishida, K.; Hotta, S.; Yamada, H.; Matsushige, K. Jpn. J. Appl. Phys. 2001, 40, 4381. (20) Bandyopadhyay, A.; Amlan, J. P. J. Phys. Chem. B 2005, 109, 6084–6088. (21) Oekermann, T.; Yoshida, T.; Tada, H.; Minoura, H. Thin Solid Films 2006, 511512, 354–357.
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