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J. Phys. Chem. B 2000, 104, 8712-8718
Dye-Sensitized Nanocrystalline Titanium-Oxide-Based Solar Cells Prepared by Sputtering: Influence of the Substrate Temperature During Deposition M. M. Go´ mez,† J. Lu,† J. L. Solis,† E. Olsson,† A. Hagfeldt,‡ and C. G. Granqvist*,† Department of Materials Science, The Ångstro¨ m Laboratory, Uppsala UniVersity, P.O. Box 534, SE-751 21 Uppsala, Sweden, and Department of Physical Chemistry, Uppsala UniVersity, P.O. Box 532, SE-751 21 Uppsala, Sweden ReceiVed: April 25, 2000; In Final Form: July 11, 2000
Nanocrystalline titanium oxide films were prepared by DC magnetron sputtering onto SnO2:F-coated glass substrates kept at temperatures in the 50 < τs < 300 °C range. Dye sensitization in cis-dithiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II) yielded solar cells with a conversion efficiency η. The dye incorporation was dependent upon τs, and an optimum value of η ) 1.7% was found with ∼0.8-µm-thick titanium oxide films prepared at 250 °C. The microstructure then displayed a well-defined parallel penniform pattern, and the luminous transmittance was 42%. The crystallite size was substantially enlarged at τs > 250 °C, and η showed an ensuing decrease.
I. Introduction Dye-sensitized nanocrystalline oxide materials can show pronounced photoelectric effects and are of interest for novel solar cells.1-4 Conventional technology employs colloidal films of titanium oxide to which a ruthenium-containing dye is attached. It is our contention that substantial progress for solar cell applications is dependent on materials preparation techniques that allow critical parameters to be changed systematically in order to facilitate performance optimization. Sputter deposition is an interesting technique with a large degree of versatility,5,6 and the microstructure of the deposit can be varied so that nanocrystalline and porous, inclined columnar,7 and penniform8 configurations can be achieved. Thus, a very large internal surface can be obtained without sacrificing electrical contiguity over the full cross section of the film. The present work expands our initial study of nanocrystalline titanium-oxide-based solar cells made by sputter deposition and dye sensitization9 by considering, in detail, the role of the substrate temperature during film preparation. Under optimized conditions, we obtained a photoelectric conversion efficiency of ∼1.7% with films as thin as ∼0.8 µm. These films have a luminous transmittance of 42% and are of interest for electrochromic smart windows10,11 with “self-powering”,12 as well as for many other applications. II. Film Deposition Titanium oxide films were prepared by reactive DC magnetron sputtering in a versatile deposition system based on a Balzers UTT 400 unit. The target was a 5-cm-diameter metallic Ti (99.9%) plate positioned 13 cm from a substrate holder in a geometry that was described elsewhere.13,14 The chamber was evacuated to ∼10-7 Torr by turbomolecular pumping, and sputtering took place in an atmosphere of Ar (99.998% pure) and O2 (99.998% pure). The O2/Ar gas-flow ratio was maintained at 0.053 by mass-flow-controlled regulators, and the total * Author to whom correspondence should be addressed. † Department of Materials Science. ‡ Department of Physical Chemistry.
sputter gas pressure was ∼15.2 mTorr. The deposition took place under oblique angle conditions with 50° 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 glass substrates precoated with a layer of transparent and conducting SnO2:F having a resistance/square of 8 Ω. The film thickness d was 780 ( 100 nm, as measured by surface profilometry. The 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 in the 50 < τs < 300 °C range by a resistive heater. III. Analysis of As-Deposited Films A. Crystallinity Studied by X-ray Diffraction. The crystal structure of the titanium oxide films was characterized by X-ray diffraction (XRD), using a Siemens D5000 diffractrometer operating with Cu KR radiation and equipped with a Go¨bel mirror and a parallel-plate collimator. Data from standards15 for TiO2 were used to identify the diffraction peaks. The mean grain size D was determined from Scherrer’s equation16
D)
Kλx β cos θ
(1)
where K is a dimensionless constant, 2θ is the diffraction angle, λx is the wavelength of the X-ray radiation, and β is the full width at half-maximum of the diffraction peak. Figure 1 displays XRD data for titanium oxide films deposited at six different τs values. No diffraction peaks were apparent for samples prepared at τs < 100 °C, which indicates that these films had a grain size smaller than the detection limit of the instrument. However, an increase of τs to 150 °C led to the appearance of a prominent broad peak assigned to an anatase (101) reflection and to very small features ascribed to anatase as well as rutile. When the substrate temperature was set at values higher than 200 °C, the XRD features became more distinct, thereby indicating, as expected, that the crystallites grew larger in size. Similar results were obtained before.17 Scherrer’s
10.1021/jp001566c CCC: $19.00 © 2000 American Chemical Society Published on Web 08/17/2000
Dye-Sensitized Nanocrystalline TiO2-Based Solar Cells
Figure 1. X-ray diffractograms for titanium oxide films on SnO2:Fcoated glass substrates. The substrate was kept at six different temperatures, denoted τs, during the deposition. The diffraction peaks are assigned to different reflections in the anatase (A) and rutile (R) phases.
TABLE 1. Titanium Oxide Crystallite Size as Calculated from Scherrer’s Formula Applied to Diffraction Peaks Corresponding to the Anatase (A) and Rutile (R) Structures diffraction peak
τs ) 150 °C
A(101) R(110) R(101)
11 -
a
crystallite size (nm) τs ) 200 °C τs ) 250 °C 17 12 16
27 20 26
τs ) 300 °C 49 37 48
τs denotes substrate temperature during sputter deposition.
equation was applied to the anatase (101), rutile (110), and rutile (101) peaks and provided the results shown in Table 1. It appears that, roughly, D grows from 10 nm at τs ) 150 °C to a value between 40 and 50 nm at τs ) 300 °C. The strong growth between 250 and 300 °C should be noted. B. Infrared Absorption Studied by Spectrophotometry. Infrared absorption spectroscopy was applied to our set of titanium oxide films deposited at different τs values. The measurements were taken in the wavenumber range 400-4000 cm-1 using a Perkin-Elmer 983 double-beam infrared spectrophotometer equipped with an air drier. Reflectance was recorded with p-polarized light at a 60° angle of incidence. An aluminum mirror was used as a reference. Figure 2 displays infrared reflectance spectra for the titanium oxide films. Dips in the reflectance curves correspond to absorption maxima. The clear features at ∼3400 cm-1 (showing a strong absorption) correspond to O-H stretching modes, whereas the features at 1630 cm-1 (showing relatively welldefined but weak absorption) correspond to O-H bending modes due to water.18 These absorption features exhibit distinct differences depending on the magnitude of τs; they are especially prominent for films deposited at τs < 150 °C, whereas they are almost undiscernible for films deposited at τs ) 300 °C. Sharp absorption peaks, assigned to longitudinal optical (LO) modes of Ti-O bonds, were centered in the wavenumber range
J. Phys. Chem. B, Vol. 104, No. 36, 2000 8713
Figure 2. Spectral reflectance of p-polarized light at 60° incidence angle for titanium oxide films made by sputtering. The curves are vertically displaced. The bar indicates the reflectance scale. Deposition took place at the temperatures τs, as indicated. Arrows point at characteristic wavenumbers for the indicated species. The inset depicts atomic displacements when viewed along the c axis of the tetragonal rutile structure for the Eu optical mode at zero vector.
820-880 cm-1. There is a small shift of this absorption toward lower wavenumbers when τs is increased, and the spectral dip appears at 880 cm-1 for films deposited at τs < 100 °C, whereas this feature is present at 822 cm-1 for films deposited at τs > 200 °C. In addition to the shift in wavenumber for the LO phonon absorption, the absorption band also becomes significantly narrower for films deposited at high τs values. This narrowing is associated with the increment of the grain size with increasing substrate temperature. Our observations are consistent with data reported for LO modes of the Eu-type vibrations in the space group D14 4h of the rutile phase of TiO2, which yield an absorption centered at 831 cm-1.19 This type of mode is visualized in the inset in Figure 2, showing the symmetry of the optical mode at zero wave vector.20 Some small features between 350 and 500 cm-1 can be associated with another type of Eu infrared-active mode expected in that range;20 this effect is weak, most likely as a consequence of the small film thickness. C. Surface Topography Studied by Atomic Force Microscopy. Surface topography was probed by atomic force microscopy (AFM) using a NanoScope II instrument with an etched silicon cantilever having a tip radius of 10 nm. Data were collected in ambient air with a contact force of about 10-7 N. Scans were extended over areas of 5 × 5 µm2. Figure 3 displays typical AFM micrographs obtained for films deposited at different values of τs. The surfaces of the films prepared at τs < 100 °C are represented by domains that are about 500 nm in linear extent. These features are assigned to the SnO2:F-coated substrate. As the value of τs increases, it is obvious that the films develop surface roughness on a finer scale.
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Go´mez et al.
Figure 3. Atomic force micrographs taken on titanium oxide films deposited at the indicated temperatures.
[ ]
The root-mean-square (RMS) roughness RAFM is defined by N
RAFM )
M
∑ ∑ z2nm n)1m)1 NM
1/2
(2)
where znm is the distance difference from the average height level for the point whose coordinates are given by the numbers n and m. The data were obtained by employing software supplied with the instrument.21 Quantitative measurements of RAFM indicated a value of ∼30 nm for films deposited at temperatures below 100 °C and a value of ∼50 nm for films deposited at 300 °C. The magnitude of RAFM varied rather linearly with τs for intermediate substrate temperatures. D. Cross-Sectional Morphology Studied by Transmission Electron Microscopy. The morphology of the titanium oxide films was studied by transmission electron microscopy (TEM) using a JEOL 2000 FX II (200 kV) instrument. Cross sections
of the films were prepared by joining two pieces of the samples with epoxy glue into a sandwich structure. The sandwich was then cut with a low-speed diamond saw, and each slice was subsequently polished to a thickness of 100 µm. Finally, the specimen was dimpled to around 5-10 µm and ion milled at low angles (10°-4°) using a Gatan PIPS system. Figure 4 shows images as well as electron diffraction patterns of films deposited at four different τs values. The sequence of images illustrates the transition from a fine-scale columnar structure at τs values of 50 and 150 °C, through a clear-cut penniform structure at τs ) 250 °C, to a large-scale structure signifying substantial grain growth at 300 °C. The electron diffractograms are consistent with a concomitant change from amorphous into crystalline atomic arrangement. These latter data are in excellent agreement with our results from XRD studies. IV. Analysis of Dye-Sensitized Films Dye sensitization of the titanium oxide films was accomplished by first keeping the films at 350 °C in air for five
Dye-Sensitized Nanocrystalline TiO2-Based Solar Cells
J. Phys. Chem. B, Vol. 104, No. 36, 2000 8715
Figure 4. Transmission electron micrographs of cross sections through titanium oxide films deposited at the indicated values of substrate temperature τs. Small inset figures depict electron diffractrograms of the films.
Figure 5. Amount of dye in titanium oxide films deposited at different temperatures. Dots denote data, and the curve was drawn for convenience.
minutes. The samples were then immersed in a dye solution of 0.5 mM cis-dithiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II) in ethanol when their temperature had dropped to ∼80 °C. Immersion took place for 12 h, and excess dye was removed by rinsing with ethanol. A. Determination of the Amount of Dye. The quantitative amount of adsorbed dye was determined by desorbing the dye from films into a 1 mM 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.22 The volumes of the films were obtained simply by multiplying surface areas by thicknesses. Figure 5 shows the amount of dye in films made at different
τs values. It is clear that the dye incorporation goes up with increasing temperature until τs reaches 250 °C, where the quantity is ∼0.145 mM/cm3. A further increase of τs to 300 °C leads to a precipitous drop of the amount of dye. B. Optical Transmittance. Optical transmittance through our films was obtained for wavelengths between 350 and 800 nm, using a Beckman 5240 spectrophotometer with an integrating sphere. Total and diffuse spectra were recorded, and the measurements were corrected for geometrical errors according to ref 23. Figure 6 shows the total and diffuse transmittance for asdeposited and dye-sensitized titanium oxide films deposited at τs ) 250 °C. The transmittance data for the films are similar except for the broad absorption band centered at 520 nm, which is due to the dye. The diffuse transmittance is most prominent at short wavelengths; it is probably dominated by scattering from the rough surface of the film. We introduce a luminous transmittance defined by
Tlum )
∫T(λ) Γe(λ) dλ ∫Γe(λ) dλ
(3)
where Γe is a weighting function for the response of the human eye.24 Figure 7 displays Tlum of titanium oxide films deposited at six different τs values. The as-deposited films yield Tlum ≈ 0.7 for 50 < τs < 250 °C, whereas the film deposited at τs ) 300 °C gives Tlum ≈ 0.55. As expected, the luminous transmittance is lower for the corresponding dye-sensitized films. It is noteworthy that all of the dye-sensitized films have 0.62 < Tlum
