HIGH LIGHT-TO-ENERGY CONVERSION EFFICIENCY FOR NANOCRYSTALLINE TiO 2 FILMS MADE BY SPUTTER DEPOSITION

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1 HIGH LIGHT-TO-ENERGY CONVERSION EFFICIENCY FOR NANOCRYSTALLINE TiO 2 FILMS MADE BY SPUTTER DEPOSITION Mónica M. Gómez, Jun Lu, Eva Olsson, Claes G. Granqvist Department of Materials Science, The Ångström Laboratory, Uppsala University, P.O. Box 534, SE Uppsala, Sweden, Tel.: , Fax.: , Monica.Gomez@Angstrom.uu.se Anders Hagfeldt Department of Physical Chemistry, Uppsala University, P.O. Box 532, SE Uppsala, Sweden, Tel (0) , Fax: , Anders.Hagfeldt@fki.uu.se Abstract Nanocrystalline solar cells based on TiO 2 were made by incorporation of cis-dithiocyanatobis(2,2 -bipyridyl-4,4 -dicarboxylate) ruthenium (II) into sputter deposited titanium oxide films. After a pyridine treatment, it was possible to achieve a photoelectric conversion efficiency as high as 7% for a solar intensity of 100W/m 2 almost the same as for a Grätzel type cell prepared from colloidal titanium oxide. Structural characterization by X-ray diffraction showed only the presence of a rutile phase. Transmission electron microscopy indicated a porous parallel penniform microstructure. Secondary ion mass spectroscopy showed that the dye incorporation was uniform except in the bottom parts of the sputtered films where a decreased porosity seems to limit the dyeing capability. 1. INTRODUCTION Dye-sensitized nanostructured solar cells based on colloidal titanium oxide have been widely studied during the past decade. 1-4 Several prototypes have been presented, but no large-scale production has yet appeared as a consequence of the many problems that these devices are still facing. It is proposed 5 that the unique properties of the nanostrucutured solar cells stem from the ability to achieve an interaction of the dye and the electrolyte with all interconnected nanoparticles. Recently we 6,7 demonstrated that is possible to achive photoelectric conversion efficiencies up to 4% for nanocrystalline Ti oxide films prepared by DC magnetron sputtering, thereby showing that this technique with particular significance due to its well documented upscaling capability 8 can be employed for the fabrication of dyesensitized nanocrystalline solar cells. In this paper we demonstrate that sputtering can yield solar cells with a conversion efficiency as high as 7%. We also report detailed transmission electron micrographs demonstrating that our films exhibit a well defined parallel penniform structure with high porosity, and we present secondary ion mass spectroscopic evidence for the distribution profile of the dye in our TiO 2 films. 2. EXPERIMENTAL 2.1 Film preparation TiO 2 films were prepared by reactive direct current DC magnetron sputtering in a system based on a Balzers UTT 400 unit. 9 The target was a 5-cm-diameter metallic plate of Ti (99.9%) positioned 13 cm from a substrate holder in a geometry that was described before. 10,11 The chamber was evacuated to 10-7 Torr by turbo molecular pumping. Prior to sputter deposition, Ar (99.998%) and O 2 (99.998%) gas were introduced via separate mass-flowcontrolled inlets. The O 2 /Ar gas flow ratio was maintained at 0.054, and the total sputter gas pressure was 13 mtorr. The deposition took place under oblique angle conditions with 50 o between the substrate s surface normal and the mean direction of the sputtered flux, and the substrate was rotated at 20 rpm. The target current was kept fixed at 980 ma. The films were deposited onto 2-mm-thick Libbey Owens Ford glass substrates precoated with a rough layer of transparent and conducting having a resistance/square of 8 Ω. The film thickness d was measured by surface profilometry. Deposition rate was obtained by dividing d by sputtering time; typically the rate was 0.4 nm/s. During the deposition, the substrate temperature was set at a constant value of 250 o C by a resistive heater. 2.2 X-ray diffraction The crystal structure of the Ti oxide films was characterized by X-ray diffraction (XRD), using a Siemens D5000 diffractrometer operating with CuK α radiation. Data from standards 12 for TiO 2 were used to identify the diffraction peaks. Figure 1 displays XRD data for Ti oxide films deposited with different techniques. The film deposited with sputtering (Fig. 1a) exhibits a pure rutile structure, while the film deposited from colloids has a mixed structure with being anatase the predominant phase. Scherrer s equation 13 was applied and the crystalline size for to the sputtered film was 28 and 21 nm for the rutile (110) and rutile (101) peaks, respectively, while the film

2 prepared from colloids had a crystalline size of 23 and 36 nm for the anatase (101) and rutile (110) peaks, respectively. R(110) (a) Sputtering deposition Thickness 9.7 µm A(101) Intensity (arb. units) R(101) Diffraction angle, 2 θ(deg.) A(101) (b) Colloidal deposition Thickness 10.0 µm Intensity (arb. units) R(110) R(101) Diffraction angle, 2 θ (deg.) Figure 1. X-ray diffractrograms for TiO 2 films on conductive, deposited by different techniques. The thickness of the film is indicated in the figure. The diffraction peaks are assigned to different reflections in the anatase (A) and rutile (R) phases. 2.3 Transmission electron microscopy The cross-sectional morphology was studied by transmission electron microscopy using a Jeol 2000 FX II (200 kv) electron microscope. The specimen was prepared by first joining two identical samples with epoxy glue into a sandwich structure. A low-speed diamond saw was used to cut the sandwich into a slice, which was subsequently polished to a thickness of about 100 µm. The specimen was dimpled in an area with a diameter of 5 to 10 mm and was then milled at low angles (10 o to 4 o ) using a Gatan PIPS system. Figure 2 shows typical data for an 8-µm-thick film. The micrograph gives clear evidence for a parallel penniform structure 11 with contiguity over the full cross section. The close-ups indicate a denser structure near the substrate than at the top of the film. 1 µm Figure 2. Transmission electron micrographs of the crosssection through an 8-µm-thick TiO 2 film, showing the complete cross-section as well as close-ups of the microstructures at the top and the bottom parts of the film.

3 3. SOLAR CELL DATA Dye sensitization of the TiO 2 films was accomplished by immersion in a 5 x 10-4 M solution of cis-dithiocyanatobis(2,2 -bipyridyl-4,4 -dicarboxylate) ruthenium (II) in ethanol. The dye was supplied by Solaronix S.A., Switzerland. Prior to the sensitization, the TiO 2 film was kept in air at 350 o C during 5 minutes to avoid hydration of the film surface from moisture in ambient air; the film was then dipped into the dye solution while still being warm ( 80 o C) and was kept immersed for one day. The surface of the dye-sensitized TiO 2 film was exposed to 4- tert-butyl-pyridine during two minutes and then dried with dehumidified air. This treatment is known to enhance the photoelectric conversion efficiency. 14 Solar cells were constructed as described before; 7 they comprised a dye-sensitized TiO 2 film on a coated substrate, a counter electrode being a similar coated substrate that had been platinized with a 5mM solution of H 2 PtCl 6 in dry isopropanol, and an electrolyte of 0.5M LiI/0.05M I 2 and 0.5M 4-tert-butylpyridine in 3-methoxy-propionitril which had been injected by capillary force into the inter-electrode space. The active cell area was 0.30 cm 2. For comparison, we also prepared a solar cell based on a 10-µm thick colloidal layer and employing a procedure described elsewhere. 15 The cell was irradiated with a Light Drive 1000 lamp (type 1400-E2/1) through an infrared-blocking filter. The current voltage characteristic was recorded by varying an external potential compensating the photovoltage and at the same time measuring the current. The integral photocurrent (short circuit current) was obtained when no external potential was applied. The acquisition of the data employed a computer interface using Labview software. The photoelectric efficiency was calculated with respect to the solar spectrum through a calibration of the lamp with direct sunlight. 7 The power of the light was measured by a pyranometer (Kipp & Zonen CM 11). The overall efficiency η of a photovoltaic cell can be calculated from the expression J scvocff η =, (1) Pin where J sc is the integral photocurrent density (current obtained at short circuit conditions, divided by the area of the cell), V oc is the open circuit voltage, FF is the fill factor (related to the series resistance for a practical solar cell), and P in is the intensity of the incident light. 16 Figure 3 illustrates current-voltage characteristics of solar cells with sputter deposited TiO 2 films having different thicknesses. The efficiency, the fill factor, the open circuit voltage, and the integral photocurrent for the solar cells corresponding to the data in Fig. 3 are displayed in Table I. The photoelectric conversion efficiency increases with film thickness, which is consistent with our earlier studies on similar films. 6,7 The maximum efficiency, reached in the 9.7-µm-thick film, is as high as 6.9 %. For comparison we also present the current-voltage curve for the solar cell based on the colloidally prepared nanoporous TiO 2 film. Photocurrent (ma/cm 2 ) Deposition: d (µm): sputtering sputtering sputtering colloidal Photovoltage (V) Figure 3. Current-voltage characteristics for solar cells based on TiO 2 films made by the shown techniques and having the stated thicknesses d. The irradiation power was 100 W/m 2 Table I also presents information corresponding to the last curve. The most important observations are that the efficiency of a solar cell incorporating a sputter deposited film can be very close to the one obtained from a cell with a colloidally prepared nanoporous film, 1,17 and that the integral photocurrent is higher for the cell with a sputter deposited film than for the cell obtained with a colloidally prepared film. Film dep. d (mm) h (%) FF V oc (V) J sc (ma/cm 2 ) S S S C Table I. Solar cell data for devices incorporating sputter deposited (S) as well as colloidally produced (C) TiO 2 films. The irradiation power was 100 W/m 2.

4 4. DYE INCORPORATION MEASURED BY OPTICAL ABSORPTION The quantitative amount of adsorbed dye was determined by desorbing the dye from films into a 1mM KOH water solution and measuring optical absorption spectra. A calibration curve for the absorbance, and hence for the concentration of the dye, was obtained by preparing a range of reference solutions with different concentrations and relying on Beer-Lambert s law. 18 The volumes of the films were obtained simply by multiplying surface areas by thicknesses. Figure 4 displays absorbance curves for the desorbed dye from the three sputter deposited films, having different thicknesses and from the film prepared from colloids. The most salient observation is that the solution from the 10-µm-thick sputter deposited film displays almost the same absorbance as that from the film prepared from colloids and having about the same thickness. Absorbance Deposition: d (µm): colloidal 10.0 sputtering 9.7 sputtering 7.0 sputtering Wavelength (nm) Figure 4. Absorbance of desorbed dye in a 1mM KOH water solution for three sputter deposited films with different thicknesses and for a film prepared from colloids. Results from calculations of the number of mm of dye vs. the volume of the film are displayed in Table II, showing that the number of mm of dye decreases with the film thickness, thus confirming that the porosity is not uniform in agreement with data in Figure 2. It is also observed that the film prepared from colloids displays a slightly higher amount of dye than the sputter deposited film having a similar thickness. 5. DYE INCORPORATION MEASURED BY SECONDARY ION MASS SPECTROSCOPY Secondary ion mass spectroscopy (SIMS) was applied to the dye-sensitized Ti oxide films in order to study the amount and depth distribution of the dye. To that end we analyzed the ruthenium distribution profiles of 102 Ru and 104 Ru, using 49 Ti as a reference. A CAMECA-IMS 3f instrument was employed. Negative oxygen ions with a current of 200 na and a beam diameter of 50 µm were used for sputtering. The primary ion beam was scanned over an area of 200x200 mm 2, while secondary ions were detected only from the central 50 µm. The depth scale was calibrated by measuring the crater depth with a surface profilometer. Intensities were energy discriminated by an offset of 100 V. The quantitative ruthenium concentration in the oxide was obtained by using several reference specimens with known amounts of ruthenium. Figure 5 shows depth concentrations for four films; three of them were prepared by sputtering to the stated thickness and the fourth one was made from a colloid solution. The 9.7-µm-thick film prepared by sputtering displays a pronounced decrease of the amount of ruthenium after 7 µm of sputtering. The same tendency, but less pronounced, is found in the 7.0-µmthick film, while the 3.0-µm-thick film displays an almost constant concentration of ruthenium. The film prepared from a colloidal solution has a uniform distribution of the dye. Our SIMS data represent a direct measurement and proves assumptions made in previous studies on films made from colloids. 1,17,19 Intensity ratio 102 Ru/ 49 Ti (counts) Deposition: d (µm): sputtering 3.0 sputtering 7.0 sputtering 9.7 colloidal Depth (µm) Figure 5. SIMS depth profiles for the Ru/Ti ratio in Tioxide-based films made by the shown techniques and having the state thicknesses d. The values of the 102 Ru/ 49 Ti intensity ratio were averaged over the full distribution profile during the SIMS analysis, and the mean value was employed for the determination of the overall Ru/Ti ratio (at%) using the calibration data referred to above. The results are displayed in Table II, showing that the amount of dye in the sputter deposited films increases with its thickness. The 9.7-µm-thick sputtered film contains a slightly

5 smaller amount of dye than the 10-µm-thick colloidally prepared film. The decrease of ruthenium in the inner part of the film may be connected to the relatively compact microstructure seen in the transmission electron micrograph taken close to the substrate. Film dep. d (mm) #mm of dye/cm 3 Ru/Ti (at%) S S S C Table II. Dye incorporation calculated from optical absorption (mm of dye/cm 3 ) and secondary ion mass spectroscopy (Ru/Ti in at%). The films were prepared by sputtering (S) and from a colloidal (C) solution. 6. CONCLUSIONS Solar cells were produced from sputter deposited TiO 2 films having a parallel penniform microstructure. Dye sensitization and pyridine treatment yielded a photoelectric conversion efficiency as high as 7% almost the same as for a conventional cell with colloidally prepared Ti oxide. The integral photocurrent was highest for the sputter deposited films, even though it was showm that the amount of dye in the sputter deposited films was sligthly lower than for the colloidally produced films. SIMS data indicated that dye incorporation into the sputter deposited films was rather uniform, except in the bottom parts where a lowered film porosity may prevent the inclusion of the dye. ACKNOWLEDGEMENTS Ulf Södervall is acknowledged for his help with the SIMS measurements and Eva Magnusson for preparing the colloidally based sample. One of us (M.G.) wants to thank the International Science Programs at Uppsala University for a scholarship. The work was carried out under the auspices of the Ångström Solar Center, supported by the Foundation for Strategic Environmental Research (MISTRA) and the Swedish National Energy Administration. REFERENCES 1 O'Reagan B. and Grätzel M. (1991). A low-cost, highefficiency solar cell based on dye-sensitized colloidal TiO 2 films. Nature 353, Bach U., Lupo D., Comte P., Moser J. E., Weissörtel F., Salbeck, Spreitzer H., and Grätzel M. (1998). Solid-state dye-sensitized mesoporous TiO 2 solar cells with high photon-to-electron conversion efficiencies. Nature 395, Pichot F. and Gregg B. A. (2000). The Photovoltage- Determining in Dye-Sensitized Solar Cells. J. Phys. Chem. B. 104, Hagfeldt A. and Grätzel M. (1995). Ligth-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev. 95, Lindström H., Rensmo H., Södergren S., Solbrand A. and Lindquist S.-E. (1996). Electron Transport Properties in Dye-Sensitized Nanoporous-Nanocrystalline TiO 2 Films. J. Phys. Chem. 100, Gómez M., Rodríguez J., Tingry S., Hagfeldt A., Lindquist S.-E. and Granqvist C. G. (1999). Photoelectrochemical effect in dye sensitized, sputter deposited Ti oxide films: The role of thickness-dependent roughness and porosity. Solar Energy Mater. Solar Cells 59, Gómez M., Magnusson E., Olsson E., Hagfeldt A., Lindquist S.-E., and Granqvist C. G. (2000). Nanocrystalline Ti-oxide based solar cells made by sputter deposition and dye sensitization: Efficiency vs. film thickness. Solar Energy Mater. Solar Cells 62, Wasa K. and Hayakawa S. (1992) Handbook of Sputter Deposition: Principles, Technology and Applications Noyes, Park Ridge, USA. 9 Le Bellac D., Niklasson G. A. and Granqvist C. G. (1995). J. Appl. Phys. 77, Kharrazi M., Azens A., Kullman L. and Granqvist C. G. (1997). High-rate dual-target d.c. magnetron sputter deposition of electrochromic MoO 3 films. Thin Solid Films 295, Rodríguez J., Gómez M., Lu J., Olsson E. and Granqvist C. G. (2000). Reactive sputter deposited Ti oxide coatings with parallel penniform microstructure. Adv. Mater. 12, International Center for Diffraction Data, Powder Diffraction Files (1997). 13 Cullity B. D., (1959) Elements of X-ray Diffraction Addison-Wesley, Reading. 14 Huang S. Y., Schlichthörl G., Nozik A. J., Grätzel M. and Frank A. J. (1997). Charge recombination in dyesensitized nanocrystalline TiO 2 Solar Cells. J. Phys. Chem. 101, Nazeeruddin M. K., Kay A., Rodicio I., Humphry- Baker R., Muller E., Liska P., Vlachopoulos N., and Grätzel M. (1993). Convertion of light to electricity by cis-x 2 bis (2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) charge transfer sensitizers (X=Cl -,Br -,I -,CN - and SCN - ) on Nanocrystalline TiO 2 Electrodes J. Am. Chem. Soc. 115, Sze S. M. (1981), Physics of Semiconductor Devices Wiley, New York.

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