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Mesostructured self-assembled titania films for photovoltaic applications
Microporous and Mesoporous Materials 88 (2006) 304–311 www.elsevier.com/locate/micromeso
Mesostructured self-assembled titania films for photovoltaic applications Luca Malfatti a, Paolo Falcaro b, Heinz Amenitsch c, Stefano Caramori d, Roberto Argazzi e, Carlo Alberto Bignozzi d, Stefano Enzo f, Michele Maggini g, Plinio Innocenzi a,* a
Laboratorio di Scienza dei Materiali e Nanotecnologie, Dipartimento di Architettura e Pianificazione, Universita` di Sassari and Nanoworld Institute, Palazzo Pou Salid, Piazza Duomo 6, 07041 Alghero (SS), Italy b Dipartimento di Ingegneria Meccanica, Settore Materiali, Universita` di Padova, Via Marzolo 9, 35131 Padova, Italy c Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Schmiedelstrasse, A-8042, Graz, Austria d Dipartimento di Chimica, Universita` di Ferrara, Via Luigi Borsari 46, 44100 Ferrara, Italy e Istituto per la Sintesi Organica e la Fotoreattivita`, C.N.R. (sezione di Ferrara), c/o Dipartimento di Chimica, Universita` di Ferrara, Via Luigi Corsari 46, 44100 Ferrara, Italy f Dipartimento di Chimica, Universita` di Sassari, Via Vienna, Sassari, Italy g Dipartimento di Scienze Chimiche and ITM-CNR, Universita´ di Padova, Via Marzolo 1, 35131 Padova, Italy Received 25 September 2004; accepted 30 September 2005 Available online 16 November 2005
Abstract Mesostructured titania thick films were tested as photovoltaic materials to be used for the fabrication of Gra¨tzel-type dye-sensitized solar cells. The titania films, prepared by evaporation-induced self-assembly, showed a 3D orthorhombic porous mesostructure obtained using non-ionic tri-block copolymers as templating agents and controlled conditions of processing. Thick films (up to 1 lm) were synthesized via repetitive dip-coating. Grazing incidence small angle X-ray scattering and X-ray diffraction analysis showed that, after calcination at temperatures higher than 350 C, anatase crystallites were formed in the titania pore walls without loss of organization. The block copolymers were removed after thermal calcination at 350 C, as shown by infrared spectroscopy. Photoaction spectra of 1 lm thick films, treated at 350 C, exhibited an incident photon-to-current efficiency above 40% at k = 380 nm. 2005 Elsevier Inc. All rights reserved. Keywords: Self-assembly; Titania; Solar cells; Films
1. Introduction Mesoporous titania materials have important applications in photocatalysis, electrochemical sensors and photovoltaic devices [1–4]. For thin film synthesis, a selfassembling process using surfactant templates, which is driven by the fast evaporation of the solvent, evaporation-induced self-assembly (EISA), allows the preparation of highly organized and porous layers [5,6]. These films are not only mesoporous but also mesostructured, because
*
Corresponding author. Tel.: +39 07998630; fax: +39 0799720420. E-mail address: [email protected] (P. Innocenzi).
1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.09.027
a monodispersion size of pores is coupled with long range ordered voids. By using amphiphilic macromolecules as templating agents it is possible to prepare a TiO2 matrix that shows several mesostructures depending by chemical and processing parameters [7–12]. Some configurations give the structural stability necessary to fabricate thin or even thick films; in all of these systems a change in the pore shape or in the periodicity is observed. This fact is generally caused by thermal treatment and leads to a loss of symmetry operations: for example the 2D hexagonal structure (P6m symmetry group) becomes face-centred cubic (C2m) [12]. The use of block copolymers, specifically poly(ethylene oxide)-co-poly(propylene oxide), as templating agents allows a complete removal of the non-ionic
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organic macromolecules at low temperature (350 C) without the extraction problems that can be observed in systems based on ionic surfactants. The principal goal in EISA synthesis is to obtain a wellordered film with high crystallinity and large pore walls. The latter characteristic gives high thermal and mechanical stability [10]; the former, associated with an anatase titania phase, gives the best photoaction efficiency in Gra¨tzel-type photovoltaic devices [13]. Mesoporous films prepared by wide-band gap semiconductor oxides, such titania, niobia or tantalia and formed by interconnected nanocrystalline particles allow, in fact, an efficient charge carrier transport. The interface between the organic dye and the surface of the titania mesoporous walls forms a heterojunction where photo-induced charge transfer separation is observed. By filling the mesopores with a proper liquid hole conductor (usually a solution containing the redox couple I =I 3 ), a heterojunction with a very large contact area will be formed. If the formation of an interconnected network of oxide nanocrystals is crucial for electronic conduction, oxide porous films with a large surface area and controlled pore organization are expected to add a significant improvement to the overall performances of a dye-sensitized solar cell (DSSC) device. The presence of an organized porosity within the films can, in fact, allow the formation of highly controlled morphology that facilitates the electronic conduction. Mesoporous channels organized with a preferential orientation that is normal to the substrate will give, therefore, one of the best configurations for DSSC materials. Nowadays two fundamental synthesis routes to prepare TiO2 mesostructured films seem to be the most promising. The first route uses a titanium alkoxide (titanium isopropoxide) in acidic aqueous solution to nucleate TiO2 nanoparticles. After a suitable aging time, the structuring agent (a tri-block non-ionic copolymer) is added to the initial solution. Starting from this sol, thin layers deposited by dip-coating show a high crystallinity degree even at low temperatures (200–300 C) [14,15]. The second route uses an alcoholic solution of TiCl4 to moderate the high reactivity of the titanium moieties (acidic hindrance) [6]: the films obtained by this solution, after a very controlled aging at high relative humidity (RH 60%), show high organization and high thermal stability [10,11]. The possibility to control a priori the induced mesophases into the hybrid films, is quite controversial: Alberius et al. [9] predict the mesostructure of silica and titania films considering the volume fraction of block copolymer in the non-volatile components of the starting solution; Crepaldi et al. [10] observed that this approach does not take into account the changes of non-volatile components with humidity of the atmosphere during the aging. In this paper, mesostructured titania films have been used for a photovoltaic application. Since the discovery of organic-sensitized photovoltaic devices, the utilization of nanocrystalline anatase electrodes is a standard in the
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Gra¨tzel-type solar cells [16–20]. The DSSC combines the high light absorption of a specifically designed organic dye with the electronic conductivity of a nanocrystalline anatase network. Several papers have reported systematic studies about the optimal conditions to prepare thick titania layers by screen-printed (or doctor-bladed) colloidal solutions [21,22]; using this technique the typical thickness of nanocrystalline anatase electrode is 1–10 lm [4]. As reported in literature, a crystallized and shrinked cubic titania mesostructure can evolve to form a channel array that connects the glass substrate to the external surface of the anatase film [23]. This configuration is close to the ideal one, previously described, for oxide semiconductor materials to be used in DSSC devices. With the aim to improve the control of morphology of TiO2 electrodes for DSSC applications, we have used EISA technique to prepare mesostructured titania films with partially crystallized anatase that forms a continuous TiO2 network of organized porosity. We have prepared TiO2 thick films, impregnated with a ruthenium dye, to fabricate a DSSC device whose photoactivity has been tested. 2. Experimental 2.1. Preparation of the precursor solution TiCl4 (99.9%, Aldrich) was used as the inorganic source. Tri-block copolymer (PEO–PPO–PEO) Pluronic P105 (BASF) was used as templating agent. The precursor solution was obtained by slow addition of TiCl4 into a mixture of EtOH and surfactant; water was added, drop by drop, after 5 min of stirring. The molar ratio of the components was TiCl4:EtOH:H2O = 1:40:12 and the TiCl4/Pluronic ratio, s, was varied in the range 0.006 6 s 6 0.012. 2.2. Film preparation Films were deposited at room temperature by dip-coating using silicon wafers or SnO2:F coated (FTO) glass slides as substrates. The withdrawal speed was 16 cm min1 (2.7 mm s1). The relative humidity (RH) inside the deposition chamber was carefully maintained between 27% and 30%. After the drying process, the films were aged, at RH 70% and 20–25 C, between 24 h and 1 week. For single layer coatings a gradual heating was applied to increase the inorganic polycondensation and stabilize the mesophase, typically 60, 100, and 130 C in air (24 h at each temperature); to obtain multilayer coatings, after each deposition, a stabilization thermal treatment at 90 C for 24 h was necessary. Finally, the template was removed by calcination in air at 350 C (1 C min1 heating rate). A TiO2 reference photoanode was prepared using a nanoparticle colloidal suspension. 125 ml of Ti-isopropoxide (97% Aldrich) was added, drop-wise and at room temperature, to 750 ml of 0.1 M HNO3 acid solution under stirring. Immediately after the hydrolysis, the slurry obtained was heated to 80 C and stirred for 8 h. Water
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was added to adjust the final solid concentration to 5 wt%. The growth of these particles was achieved under hydrothermal conditions in autoclave for 12 h at 200– 250 C. After two cycles of sonication, the colloidal suspension was evaporated to reach a final concentration of 11% wt. The colloidal suspension was deposited by a doctor blade technique [4], then dried and fired in air at 450 C for 30 min. 2.3. Characterization The mesostructure, in films deposited on silicon substrates, was investigate using high flux grazing incidence small angle X-ray scattering (GISAXS) apparatus at the Austrian beamline (2 GeV electron storage ring) of Elettra Synchrotron (Trieste, Italy) [24] taking for each image the average of 5 single acquisitions and 1 s of exposition time. The instrumental grazing angle was set-up maintaining an ˚ ) smaller than 3. incident X-ray beam (wavelength 1.54 A From the recording of the CCD detector (1024 · 1024 pixels, Oxford instruments) the out of plane diffraction maxima were observed. Low-angle X-ray diffraction (XRD) spectra were collected in Seeman–Bohlin geometry using a Philips goniometer PW 1710 with Cu Ka as radiation source, an angular range from 23 to 50 with 0.005 step and 10 s of exposition time per step. The template removal in titania films was followed using a Perkin–Elmer 2000 Fourier transform infrared (FTIR) spectrometer: the spectra are the average of 100 scans with 2 cm1 of resolution. For photoaction measurements, the N3 complex [RuII(dcH2bpy)2(NCS)2], (dcH2bpy = 4,4 0 -dicarboxy-2,2 0 bipyridine), was prepared and purified according to a literature procedure [25]. Electrodes 2.5 cm · 2 cm were cut from fluorine doped tin dioxide (FTO) glass sheets (Nippon Sheet Glass) having a surface resistivity of 10 X/cm2. Sensitized photoanodes were prepared by dipping for 24 h at room temperature the TiO2 coated electrodes in a 5 · 103 M solution of [RuII(dcH2bpy)2(NCS)2] dissolved in absolute ethanol. UV–Vis spectra of the dye-adsorbed TiO2 films were recorded with a Perkin Elmer k 40 using air as reference. Photocurrent measurements were performed on sandwich type cells made by clipping together a photoanode and a counter electrode of platinized conducting glass, with an electrolytic solution in between containing LiI 0.3 M and I2 0.03 M dissolved in acetonitrile. The apparatus for photocurrent measurements was comprised of a water cooled 150 W xenon arc lamp (Osram XBO) whose output was focused onto the entrance slit of an Applied Photophysics high radiance monochromator. The monochromatic light, with a spectral bandwidth of 10 nm, was collected by a quartz lens and focused onto the working plane to produce a rectangular uniformly illuminated area of (0.5 · 1) cm2. The current was measured with a Kontron 4021 digital multimeter and the radiant power density was measured with a 100 mm2 active area
silicon photodiode (Centronic OSD100–7Q) whose responsivity was known. 3. Results and discussion 3.1. FTIR study of template removal with thermal treatment The non-ionic block copolymers removal from the films was achieved by thermal calcination: to impregnate by N3 the mesostructured materials, it is, in fact, necessary to empty the pores. In Fig. 1 the FTIR absorption spectra in the range 1000–1200 cm1 of titania films, as a function of the thermal treatment temperature, are shown. The absorption bands in the 1050–1175 cm1 interval, which are the most intense modes observed in a pure Pluronic P105 infrared spectrum [26], were used to monitor the block copolymer removal during thermal calcination. In mesostructured silica based films the detailed stages of this process are generally difficult to follow because of the full overlapping of the intense mas (Si–O–Si) band around 1100 cm1 with those of Pluronic in this region. The main bands observed in Fig. 1 are attributed to C–O–C stretching (1110 cm1), C–C stretching (1150 cm1) and CH2 rocking (1060 cm1), and show a significant change even at low temperatures of calcination. The organic template is, in fact, very sensitive to the temperatures and shows a significant degradation from 100 C, where the bands are no longer sharp and resolved. The absorption bands with the increasing of the temperature become broader as a consequence of the higher mobility of shorter chains produced by the thermal fragmentation of the block copolymer. At thermal treatments around 350 C, the organic template resulted completely removed. 3.2. Mesophase characterization by SAXS The study of the mesophase in the films was done by GISAXS using synchrotron radiation. The diffraction patterns of titania films on thick silicon substrates (0.5 mm 0.16 0.14
Absorbance
306
0.12 0.10
60 ˚C
0.08
100 ˚C
0.06
130 ˚C
0.04 270 ˚C
0.02 0.00 1000
350 ˚C
1050
1100
1150
1200
Wavenumber/cm-1 Fig. 1. FTIR absorption spectra of mesostructured titania films prepared using Pluronic P105 as templating agent and thermally calcined at different temperatures. The F105 spectrum is reported for reference (dot line).
L. Malfatti et al. / Microporous and Mesoporous Materials 88 (2006) 304–311
thickness), allowed to obtain the Bragg and Laue reflections: a ‘‘distorted Im3m’’ mesophase symmetry was detected. The formation of a distorted cubic mesostructure was already described in mesostructured films. It has been associated to the uniaxial contraction of the films in a direction normal to the substrate [10,27]. This contraction is generally related to a substantial loss of symmetry. Some authors have described the distorted Im3m structure as triclinic without however a specific identification of the structure; others, with a more detailed experimental support, have described the mesophase as an orthorhombic Fmmm structure [28]. Fig. 2 shows the GISAXS patterns, attributed to an orthorhombic mesostructure, in titania films calcinated at 350 C and obtained varying the (surfactant/Ti) ratio, s. By enhancing the Pluronic P105 concentration in a limited range, it is possible to modify the cell parameter without loss of symmetry. Fig. 2 clearly shows that, for s between 6 · 103 and 11 · 103, the diffraction pattern retains the orthorhombic configuration, with the presence of three spots not symmetry-equivalents. Using a high surfactant concentration, s = 12 · 103, the mesostructure becomes more disordered, according with the vanishing of (1 1 1) and ð 1 1 1Þ spots and with the qualitative increase in intensity of the ring due to a presence of poly-oriented domains.
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The calculation of cell parameters was achieved by integration of the maximum intensity regions in the 2D diffraction patterns using Fit2D programme [29]. As shown in Table 1, all the interplanar distances, d200, d111 and d020, from which the a, b, c parameters are calculated, become larger with the increase of the ratio s. In particular, the b parameter, for 7 · 103 6 s 6 11 · 103, shows an almost linear behaviour that the other two parameters do not present. This fact can be explained considering that the percentage uniaxial contraction, in this range of surfactant concentration, is very similar whereas the micelle dimension changes in accordance with the amount of templating agent.
Table 1 Changes of the interplanar distances (d200 and d111) and correlated cell parameters (a, b and c) as a function of (Pluronic P105/Ti) ratio, s s 3
7 · 10 8 · 103 9 · 103 10 · 103 11 · 103
d200/nm
d111/nm
d020/nm
a/nm
b/nm
c/nm
9.6 9.9 10.1 10.1 10.2
6.6 6.8 7.0 7.2 7.4
4.0 4.1 4.1 4.2 4.2
19.2 19.8 20.2 20.3 20.4
8.1 8.2 8.3 8.4 8.5
14.7 15.5 18.0 19.6 24.0
Fig. 2. SAXS spectra of mesostructured titania films calcined at 350 C and prepared using different (Pluronic P105/Ti) ratios, s. The spots have been marked (continuous black line) for sake of clarity.
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3.3. Crystallites evolution A remarkable property of this titania mesostructure is the channel formation, because of the crystallization and thermal shrinkage that are induced by calcination at temperatures higher than 400 C [23]. Fig. 3 shows how by of the biggest crystallites, the crystallographic direction h1 1 1i, which is the most pore-rich direction in a periodic cubic distorted array, becomes a site of channel formation by pores merging. Using the more correct attribution to a Fmmm orthorhombic symmetry group, the crystallographic direction of channel formation becomes h1 1 0i. A fully-crystallized film, in spite of loosing the symmetry operations that characterize the Fmmm group, shows a mesoporous structure containing channels that form a 45 angle with the normal to the substrate plane. Such an ordered matrix should allow an improvement in the redox reaction (or hole conduction) that takes place at the ‘‘electrolytic mediator/electrode’’ interface, thus enhancing the DSSC efficiency [2,17,18,30]. Mesostructured films calcined at 350 C exhibit a large surface area (150–300 m2 g1 [11,31]): an enhanced dye chemical adsorption is expected and thus a quantitative enhancement of the photovoltaic effect in the titania electrode. The diffraction spectra of films treated at different temperatures (Fig. 4) show a progressive growth of crystalline domains between 350 and 400 C. Using the Maud programme [32] (Fig. 5), the crystalline phase was univocally identified as anatase. A sudden enhancement of ordering within the anatase phase (in crystallinity) characterizes the 450–500 C thermal step: comparing the (1 0 1) area peaks for samples calcined at 450 C and at 500 C, a 45–
50% signal increase was calculated; moreover the spectra collected at 500 C reach about 60% of the (1 0 1) peak intensity with respect to the spectra collected at 600 C. The refinement procedure using MAUD accounts for the instrument broadening and, within the sample broadening, separates the microstrain from reduced crystallite size effects. In principle, this conducts to ‘‘rigorous’’ growth determination of the nanocrystalline dimension, d, in the electrode. Table 2 shows the average crystallite size, d, as a function of the maximum calcination temperature. This determination stands from the assumptions which are done in the evaluation of broadening and is supposed to be associated with an error bar of 15%. The main assumptions concern the isotropicity of crystallite size and microstrain across all the X-ray peaks and the ability of Voigt functions to describe correctly the peak shape. This performance can be evaluated from the analysis of residuals, i.e., the scatter between calculated and experimental data points across the peak signals. It should also be noted that the X-ray crystallite size worked out by the MAUD procedure is a volume weighted average of the coherent domain of diffraction and is generally different from the particle surface weighted mean, which is obtained by transmission electron microscopy observations. The average d value obtained after a thermal treatment at 600 C is similar to the typical nanocrystalline dimensions obtained in classical synthesis of DSSC electrodes [2]. 3.4. Photoaction measurements To synthesize titania thick films as DSSC electrode via EISA, a repetitive coating procedure was necessary. Four
Fig. 3. Schematic drawing of the uniaxial shrinkage that conduces from an as-deposited cubic mesostructure to a thermally-contracted orthorhombic array. Further increasing in thermal treatment leads to a partially collapsed mesostructure with formation of channels in the h1 1 0i crystallographic direction.
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309
1200 1000 800
Counts
600 ˚C 550 ˚C 500 ˚C 450 ˚C 400 ˚C
600 ˚C 550 ˚C 500 ˚C 450 ˚C 400 ˚C
(101)
600 400
(103) (004) (112)
(200)
200 0 24 26 28 30 32 34 36 38 40 42 44 46 48 50
2θ / °
(a)
23
(b)
24
25
26
27
28
2θ/ °
Fig. 4. (a) XRD diffraction patterns of titania films prepared using Pluronic P105 as a function of the thermal treatment and (b) particular of the (1 0 1) peak.
Fig. 5. Fitting of X-ray diffraction pattern of a titania film (350 C calcination). The peaks not directly attributed to anatase phase are due to the silicon substrate. The fitting was made by MAUD software.
Table 2 Anatase crystallite size ‘‘d’’, calculated by Maud software, as a function of the calcination temperature Temperature/C
d/nm
400 450 500 550 600
2.1 ± 0.5 4.2. ± 0.6 17.4 ± 2 21.7 ± 2 24.9 ± 3
sequential layers (each layer followed by an accurate processing treatment) were deposited to obtain a thick titania film (1 lm). After impregnation of the multilayer coating, the sample was impregnated with the ruthenium complex, [RuII(dcH2bpy)2(NCS)2] that is generally indicated as N3 [25] (Scheme 1). This dye is the most popular heteroge-
neous charge transfer sensitizer for mesoporous solar cells and, even if is challenged in terms of photovoltaic performances by the ‘‘black-dye’’ tri(cyanato)-2,2 0 ,200 -terpyridyl4,4 0 ,400 -tri(carboxylate)Ru(II), we have preferred to use N3 because of the lower cost and the large availability of reference literature data. The mechanism of generation of electronic charge is activated by the excitation of the dye that causes the transfer of an electron from the metal to the carboxylate bipyridyl ligand. Because the chemical bonding of the ligands groups to the mesostructured nanocrystalline film, the electron is very quickly released (50 fs) into the titania conducting band with a unit quantum yield (Fig. 6). The UV–Vis spectra of the films impregnated by N3 showed the presence of an intense absorption band peaking at 515 nm, attributed to the dp ! p* (RuII ! dcH2bpy)
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L. Malfatti et al. / Microporous and Mesoporous Materials 88 (2006) 304–311 COOH
COOH
HOOC N N
N Ru N C
Absorbance
0.6
0.5
0.4
0.3 N
N
0.2 400
COOH
S
600
700
Wavelength/nm
C S
500
Fig. 7. Absorption spectra of 1 lm thick mesostructured titania film sensitized by N3 (continuous line) and N3 in CH3CN (dashed line).
Scheme 1. Chemical structure of [RuII(dcH2bpy)2(NCS)2] N3 Dye. 100
Standard TiO2 (≈ 7 µm) Mesop. TiO2 (≈ 1 µm)
IPCE %
80 60 40 20 0 400
500
600
700
800
Wavelength/nm Fig. 8. Photoaction spectra of a 1 lm thick mesostructured titania film (calcined at 350 C) compared with a 7 lm thick TiO2 reference electrode. The incident photon current conversion efficiency is shown as a function of the wavelength. The two samples are both sensitized by N3.
IPCE%ðkÞ ¼ 1:24 103 ðV nmÞ
Fig. 6. Schematic representation of dye-sensitized heterojunction photovoltaic cell fabricated by mesostructured titania film and N3 dye.
metal-to-ligand charge transfer (MLCT) transition (Fig. 7), the spectrum of the Ru complex in acetonitrile is also reported for comparison. The UV–Vis spectrum gives a direct indication of the successfully impregnation of the dye within the film. The photoaction spectra of the sample, prepared with Pluronic P105 and calcined at 350 C, were collected reporting the incident-photon-to-current-efficiency (IPCE) as a function of the incident radiation wavelength, k, (Fig. 8). The IPCE index is defined as ‘‘number of collected electrons/number of incident photons’’. Incident photon to current conversion efficiencies were calculated according to the formula [33]:
J ðl A cm2 Þ k ðnmÞ P ðW m2 Þ
where J is the monochromatic photocurrent density and P is the radiant power density. The IPCE decreased at higher excitation wavelengths, with a similar trend in the standard and in the mesoporous TiO2 electrodes. A peak in IPCE around 550 nm, with a photocurrent onset close to 700 nm, was observed in both the samples. The mesostructured titania films sensitized with N3 showed that are suitable electrodes for Gra¨tzel-type solar cells because of the photocurrent observed in the samples even if the IPCE resulted lower than the TiO2 standard electrode. This behaviour, however, cannot be directly attributed to an effective lower efficiency of mesostructured titania. The difference of thickness can, in fact, explain the measured values [34]. On the other hand, a direct comparison was difficult to do because the standard electrode was deposited by doctor blade that did not allow obtaining films of lower thickness. Thicker samples of mesostructured titania films must be, therefore, produced to achieve photovoltaic electrodes that can represent a challenging alternative material in dye-sensitized solar cells applications.
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4. Conclusions Evaporation-induced self-assembly is a suitable route to obtain titania mesostructured films. The control of the synthesis parameters allows obtaining thick films via repetitive dip-coating, this process does not disrupt the organization of the pores in the single layers and the structure of the porous phase is maintained even after removal of the organic template. Controlled crystallization of the amorphous titania pore walls into crystalline anatase, whose crystallite dimensions can be adjusted within a certain degree as a function of the processing conditions, can be reached through thermal calcination. Thermal treatments at temperatures between 400 and 600 C give a high surface area mesoporous nano-anatase coating presenting channel-voids very interesting for dye-sensitized solar cells. The measurement of the incident-photon-to-current-efficiency of prototyped solar cell based on titania mesostructured films showed that this mesostructured material can be efficiently used, upon optimization of the thickness and processing conditions, as electrode in dye-sensitized solar cells. Acknowledgment FIRB Italian projects are acknowledged for financial support (FIRB contract no. RBNE01P4JF). References [1] [2] [3] [4]
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Mesostructured self-assembled titania films for photovoltaic applications Luca Malfatti a, Paolo Falcaro b, Heinz Amenitsch c, Stefano Caramori d, Roberto Argazzi e, Carlo Alberto Bignozzi d, Stefano Enzo f, Michele Maggini g, Plinio Innocenzi a,* a
Laboratorio di Scienza dei Materiali e Nanotecnologie, Dipartimento di Architettura e Pianificazione, Universita` di Sassari and Nanoworld Institute, Palazzo Pou Salid, Piazza Duomo 6, 07041 Alghero (SS), Italy b Dipartimento di Ingegneria Meccanica, Settore Materiali, Universita` di Padova, Via Marzolo 9, 35131 Padova, Italy c Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Schmiedelstrasse, A-8042, Graz, Austria d Dipartimento di Chimica, Universita` di Ferrara, Via Luigi Borsari 46, 44100 Ferrara, Italy e Istituto per la Sintesi Organica e la Fotoreattivita`, C.N.R. (sezione di Ferrara), c/o Dipartimento di Chimica, Universita` di Ferrara, Via Luigi Corsari 46, 44100 Ferrara, Italy f Dipartimento di Chimica, Universita` di Sassari, Via Vienna, Sassari, Italy g Dipartimento di Scienze Chimiche and ITM-CNR, Universita´ di Padova, Via Marzolo 1, 35131 Padova, Italy Received 25 September 2004; accepted 30 September 2005 Available online 16 November 2005
Abstract Mesostructured titania thick films were tested as photovoltaic materials to be used for the fabrication of Gra¨tzel-type dye-sensitized solar cells. The titania films, prepared by evaporation-induced self-assembly, showed a 3D orthorhombic porous mesostructure obtained using non-ionic tri-block copolymers as templating agents and controlled conditions of processing. Thick films (up to 1 lm) were synthesized via repetitive dip-coating. Grazing incidence small angle X-ray scattering and X-ray diffraction analysis showed that, after calcination at temperatures higher than 350 C, anatase crystallites were formed in the titania pore walls without loss of organization. The block copolymers were removed after thermal calcination at 350 C, as shown by infrared spectroscopy. Photoaction spectra of 1 lm thick films, treated at 350 C, exhibited an incident photon-to-current efficiency above 40% at k = 380 nm. 2005 Elsevier Inc. All rights reserved. Keywords: Self-assembly; Titania; Solar cells; Films
1. Introduction Mesoporous titania materials have important applications in photocatalysis, electrochemical sensors and photovoltaic devices [1–4]. For thin film synthesis, a selfassembling process using surfactant templates, which is driven by the fast evaporation of the solvent, evaporation-induced self-assembly (EISA), allows the preparation of highly organized and porous layers [5,6]. These films are not only mesoporous but also mesostructured, because
*
Corresponding author. Tel.: +39 07998630; fax: +39 0799720420. E-mail address: [email protected] (P. Innocenzi).
1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.09.027
a monodispersion size of pores is coupled with long range ordered voids. By using amphiphilic macromolecules as templating agents it is possible to prepare a TiO2 matrix that shows several mesostructures depending by chemical and processing parameters [7–12]. Some configurations give the structural stability necessary to fabricate thin or even thick films; in all of these systems a change in the pore shape or in the periodicity is observed. This fact is generally caused by thermal treatment and leads to a loss of symmetry operations: for example the 2D hexagonal structure (P6m symmetry group) becomes face-centred cubic (C2m) [12]. The use of block copolymers, specifically poly(ethylene oxide)-co-poly(propylene oxide), as templating agents allows a complete removal of the non-ionic
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organic macromolecules at low temperature (350 C) without the extraction problems that can be observed in systems based on ionic surfactants. The principal goal in EISA synthesis is to obtain a wellordered film with high crystallinity and large pore walls. The latter characteristic gives high thermal and mechanical stability [10]; the former, associated with an anatase titania phase, gives the best photoaction efficiency in Gra¨tzel-type photovoltaic devices [13]. Mesoporous films prepared by wide-band gap semiconductor oxides, such titania, niobia or tantalia and formed by interconnected nanocrystalline particles allow, in fact, an efficient charge carrier transport. The interface between the organic dye and the surface of the titania mesoporous walls forms a heterojunction where photo-induced charge transfer separation is observed. By filling the mesopores with a proper liquid hole conductor (usually a solution containing the redox couple I =I 3 ), a heterojunction with a very large contact area will be formed. If the formation of an interconnected network of oxide nanocrystals is crucial for electronic conduction, oxide porous films with a large surface area and controlled pore organization are expected to add a significant improvement to the overall performances of a dye-sensitized solar cell (DSSC) device. The presence of an organized porosity within the films can, in fact, allow the formation of highly controlled morphology that facilitates the electronic conduction. Mesoporous channels organized with a preferential orientation that is normal to the substrate will give, therefore, one of the best configurations for DSSC materials. Nowadays two fundamental synthesis routes to prepare TiO2 mesostructured films seem to be the most promising. The first route uses a titanium alkoxide (titanium isopropoxide) in acidic aqueous solution to nucleate TiO2 nanoparticles. After a suitable aging time, the structuring agent (a tri-block non-ionic copolymer) is added to the initial solution. Starting from this sol, thin layers deposited by dip-coating show a high crystallinity degree even at low temperatures (200–300 C) [14,15]. The second route uses an alcoholic solution of TiCl4 to moderate the high reactivity of the titanium moieties (acidic hindrance) [6]: the films obtained by this solution, after a very controlled aging at high relative humidity (RH 60%), show high organization and high thermal stability [10,11]. The possibility to control a priori the induced mesophases into the hybrid films, is quite controversial: Alberius et al. [9] predict the mesostructure of silica and titania films considering the volume fraction of block copolymer in the non-volatile components of the starting solution; Crepaldi et al. [10] observed that this approach does not take into account the changes of non-volatile components with humidity of the atmosphere during the aging. In this paper, mesostructured titania films have been used for a photovoltaic application. Since the discovery of organic-sensitized photovoltaic devices, the utilization of nanocrystalline anatase electrodes is a standard in the
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Gra¨tzel-type solar cells [16–20]. The DSSC combines the high light absorption of a specifically designed organic dye with the electronic conductivity of a nanocrystalline anatase network. Several papers have reported systematic studies about the optimal conditions to prepare thick titania layers by screen-printed (or doctor-bladed) colloidal solutions [21,22]; using this technique the typical thickness of nanocrystalline anatase electrode is 1–10 lm [4]. As reported in literature, a crystallized and shrinked cubic titania mesostructure can evolve to form a channel array that connects the glass substrate to the external surface of the anatase film [23]. This configuration is close to the ideal one, previously described, for oxide semiconductor materials to be used in DSSC devices. With the aim to improve the control of morphology of TiO2 electrodes for DSSC applications, we have used EISA technique to prepare mesostructured titania films with partially crystallized anatase that forms a continuous TiO2 network of organized porosity. We have prepared TiO2 thick films, impregnated with a ruthenium dye, to fabricate a DSSC device whose photoactivity has been tested. 2. Experimental 2.1. Preparation of the precursor solution TiCl4 (99.9%, Aldrich) was used as the inorganic source. Tri-block copolymer (PEO–PPO–PEO) Pluronic P105 (BASF) was used as templating agent. The precursor solution was obtained by slow addition of TiCl4 into a mixture of EtOH and surfactant; water was added, drop by drop, after 5 min of stirring. The molar ratio of the components was TiCl4:EtOH:H2O = 1:40:12 and the TiCl4/Pluronic ratio, s, was varied in the range 0.006 6 s 6 0.012. 2.2. Film preparation Films were deposited at room temperature by dip-coating using silicon wafers or SnO2:F coated (FTO) glass slides as substrates. The withdrawal speed was 16 cm min1 (2.7 mm s1). The relative humidity (RH) inside the deposition chamber was carefully maintained between 27% and 30%. After the drying process, the films were aged, at RH 70% and 20–25 C, between 24 h and 1 week. For single layer coatings a gradual heating was applied to increase the inorganic polycondensation and stabilize the mesophase, typically 60, 100, and 130 C in air (24 h at each temperature); to obtain multilayer coatings, after each deposition, a stabilization thermal treatment at 90 C for 24 h was necessary. Finally, the template was removed by calcination in air at 350 C (1 C min1 heating rate). A TiO2 reference photoanode was prepared using a nanoparticle colloidal suspension. 125 ml of Ti-isopropoxide (97% Aldrich) was added, drop-wise and at room temperature, to 750 ml of 0.1 M HNO3 acid solution under stirring. Immediately after the hydrolysis, the slurry obtained was heated to 80 C and stirred for 8 h. Water
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was added to adjust the final solid concentration to 5 wt%. The growth of these particles was achieved under hydrothermal conditions in autoclave for 12 h at 200– 250 C. After two cycles of sonication, the colloidal suspension was evaporated to reach a final concentration of 11% wt. The colloidal suspension was deposited by a doctor blade technique [4], then dried and fired in air at 450 C for 30 min. 2.3. Characterization The mesostructure, in films deposited on silicon substrates, was investigate using high flux grazing incidence small angle X-ray scattering (GISAXS) apparatus at the Austrian beamline (2 GeV electron storage ring) of Elettra Synchrotron (Trieste, Italy) [24] taking for each image the average of 5 single acquisitions and 1 s of exposition time. The instrumental grazing angle was set-up maintaining an ˚ ) smaller than 3. incident X-ray beam (wavelength 1.54 A From the recording of the CCD detector (1024 · 1024 pixels, Oxford instruments) the out of plane diffraction maxima were observed. Low-angle X-ray diffraction (XRD) spectra were collected in Seeman–Bohlin geometry using a Philips goniometer PW 1710 with Cu Ka as radiation source, an angular range from 23 to 50 with 0.005 step and 10 s of exposition time per step. The template removal in titania films was followed using a Perkin–Elmer 2000 Fourier transform infrared (FTIR) spectrometer: the spectra are the average of 100 scans with 2 cm1 of resolution. For photoaction measurements, the N3 complex [RuII(dcH2bpy)2(NCS)2], (dcH2bpy = 4,4 0 -dicarboxy-2,2 0 bipyridine), was prepared and purified according to a literature procedure [25]. Electrodes 2.5 cm · 2 cm were cut from fluorine doped tin dioxide (FTO) glass sheets (Nippon Sheet Glass) having a surface resistivity of 10 X/cm2. Sensitized photoanodes were prepared by dipping for 24 h at room temperature the TiO2 coated electrodes in a 5 · 103 M solution of [RuII(dcH2bpy)2(NCS)2] dissolved in absolute ethanol. UV–Vis spectra of the dye-adsorbed TiO2 films were recorded with a Perkin Elmer k 40 using air as reference. Photocurrent measurements were performed on sandwich type cells made by clipping together a photoanode and a counter electrode of platinized conducting glass, with an electrolytic solution in between containing LiI 0.3 M and I2 0.03 M dissolved in acetonitrile. The apparatus for photocurrent measurements was comprised of a water cooled 150 W xenon arc lamp (Osram XBO) whose output was focused onto the entrance slit of an Applied Photophysics high radiance monochromator. The monochromatic light, with a spectral bandwidth of 10 nm, was collected by a quartz lens and focused onto the working plane to produce a rectangular uniformly illuminated area of (0.5 · 1) cm2. The current was measured with a Kontron 4021 digital multimeter and the radiant power density was measured with a 100 mm2 active area
silicon photodiode (Centronic OSD100–7Q) whose responsivity was known. 3. Results and discussion 3.1. FTIR study of template removal with thermal treatment The non-ionic block copolymers removal from the films was achieved by thermal calcination: to impregnate by N3 the mesostructured materials, it is, in fact, necessary to empty the pores. In Fig. 1 the FTIR absorption spectra in the range 1000–1200 cm1 of titania films, as a function of the thermal treatment temperature, are shown. The absorption bands in the 1050–1175 cm1 interval, which are the most intense modes observed in a pure Pluronic P105 infrared spectrum [26], were used to monitor the block copolymer removal during thermal calcination. In mesostructured silica based films the detailed stages of this process are generally difficult to follow because of the full overlapping of the intense mas (Si–O–Si) band around 1100 cm1 with those of Pluronic in this region. The main bands observed in Fig. 1 are attributed to C–O–C stretching (1110 cm1), C–C stretching (1150 cm1) and CH2 rocking (1060 cm1), and show a significant change even at low temperatures of calcination. The organic template is, in fact, very sensitive to the temperatures and shows a significant degradation from 100 C, where the bands are no longer sharp and resolved. The absorption bands with the increasing of the temperature become broader as a consequence of the higher mobility of shorter chains produced by the thermal fragmentation of the block copolymer. At thermal treatments around 350 C, the organic template resulted completely removed. 3.2. Mesophase characterization by SAXS The study of the mesophase in the films was done by GISAXS using synchrotron radiation. The diffraction patterns of titania films on thick silicon substrates (0.5 mm 0.16 0.14
Absorbance
306
0.12 0.10
60 ˚C
0.08
100 ˚C
0.06
130 ˚C
0.04 270 ˚C
0.02 0.00 1000
350 ˚C
1050
1100
1150
1200
Wavenumber/cm-1 Fig. 1. FTIR absorption spectra of mesostructured titania films prepared using Pluronic P105 as templating agent and thermally calcined at different temperatures. The F105 spectrum is reported for reference (dot line).
L. Malfatti et al. / Microporous and Mesoporous Materials 88 (2006) 304–311
thickness), allowed to obtain the Bragg and Laue reflections: a ‘‘distorted Im3m’’ mesophase symmetry was detected. The formation of a distorted cubic mesostructure was already described in mesostructured films. It has been associated to the uniaxial contraction of the films in a direction normal to the substrate [10,27]. This contraction is generally related to a substantial loss of symmetry. Some authors have described the distorted Im3m structure as triclinic without however a specific identification of the structure; others, with a more detailed experimental support, have described the mesophase as an orthorhombic Fmmm structure [28]. Fig. 2 shows the GISAXS patterns, attributed to an orthorhombic mesostructure, in titania films calcinated at 350 C and obtained varying the (surfactant/Ti) ratio, s. By enhancing the Pluronic P105 concentration in a limited range, it is possible to modify the cell parameter without loss of symmetry. Fig. 2 clearly shows that, for s between 6 · 103 and 11 · 103, the diffraction pattern retains the orthorhombic configuration, with the presence of three spots not symmetry-equivalents. Using a high surfactant concentration, s = 12 · 103, the mesostructure becomes more disordered, according with the vanishing of (1 1 1) and ð 1 1 1Þ spots and with the qualitative increase in intensity of the ring due to a presence of poly-oriented domains.
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The calculation of cell parameters was achieved by integration of the maximum intensity regions in the 2D diffraction patterns using Fit2D programme [29]. As shown in Table 1, all the interplanar distances, d200, d111 and d020, from which the a, b, c parameters are calculated, become larger with the increase of the ratio s. In particular, the b parameter, for 7 · 103 6 s 6 11 · 103, shows an almost linear behaviour that the other two parameters do not present. This fact can be explained considering that the percentage uniaxial contraction, in this range of surfactant concentration, is very similar whereas the micelle dimension changes in accordance with the amount of templating agent.
Table 1 Changes of the interplanar distances (d200 and d111) and correlated cell parameters (a, b and c) as a function of (Pluronic P105/Ti) ratio, s s 3
7 · 10 8 · 103 9 · 103 10 · 103 11 · 103
d200/nm
d111/nm
d020/nm
a/nm
b/nm
c/nm
9.6 9.9 10.1 10.1 10.2
6.6 6.8 7.0 7.2 7.4
4.0 4.1 4.1 4.2 4.2
19.2 19.8 20.2 20.3 20.4
8.1 8.2 8.3 8.4 8.5
14.7 15.5 18.0 19.6 24.0
Fig. 2. SAXS spectra of mesostructured titania films calcined at 350 C and prepared using different (Pluronic P105/Ti) ratios, s. The spots have been marked (continuous black line) for sake of clarity.
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3.3. Crystallites evolution A remarkable property of this titania mesostructure is the channel formation, because of the crystallization and thermal shrinkage that are induced by calcination at temperatures higher than 400 C [23]. Fig. 3 shows how by of the biggest crystallites, the crystallographic direction h1 1 1i, which is the most pore-rich direction in a periodic cubic distorted array, becomes a site of channel formation by pores merging. Using the more correct attribution to a Fmmm orthorhombic symmetry group, the crystallographic direction of channel formation becomes h1 1 0i. A fully-crystallized film, in spite of loosing the symmetry operations that characterize the Fmmm group, shows a mesoporous structure containing channels that form a 45 angle with the normal to the substrate plane. Such an ordered matrix should allow an improvement in the redox reaction (or hole conduction) that takes place at the ‘‘electrolytic mediator/electrode’’ interface, thus enhancing the DSSC efficiency [2,17,18,30]. Mesostructured films calcined at 350 C exhibit a large surface area (150–300 m2 g1 [11,31]): an enhanced dye chemical adsorption is expected and thus a quantitative enhancement of the photovoltaic effect in the titania electrode. The diffraction spectra of films treated at different temperatures (Fig. 4) show a progressive growth of crystalline domains between 350 and 400 C. Using the Maud programme [32] (Fig. 5), the crystalline phase was univocally identified as anatase. A sudden enhancement of ordering within the anatase phase (in crystallinity) characterizes the 450–500 C thermal step: comparing the (1 0 1) area peaks for samples calcined at 450 C and at 500 C, a 45–
50% signal increase was calculated; moreover the spectra collected at 500 C reach about 60% of the (1 0 1) peak intensity with respect to the spectra collected at 600 C. The refinement procedure using MAUD accounts for the instrument broadening and, within the sample broadening, separates the microstrain from reduced crystallite size effects. In principle, this conducts to ‘‘rigorous’’ growth determination of the nanocrystalline dimension, d, in the electrode. Table 2 shows the average crystallite size, d, as a function of the maximum calcination temperature. This determination stands from the assumptions which are done in the evaluation of broadening and is supposed to be associated with an error bar of 15%. The main assumptions concern the isotropicity of crystallite size and microstrain across all the X-ray peaks and the ability of Voigt functions to describe correctly the peak shape. This performance can be evaluated from the analysis of residuals, i.e., the scatter between calculated and experimental data points across the peak signals. It should also be noted that the X-ray crystallite size worked out by the MAUD procedure is a volume weighted average of the coherent domain of diffraction and is generally different from the particle surface weighted mean, which is obtained by transmission electron microscopy observations. The average d value obtained after a thermal treatment at 600 C is similar to the typical nanocrystalline dimensions obtained in classical synthesis of DSSC electrodes [2]. 3.4. Photoaction measurements To synthesize titania thick films as DSSC electrode via EISA, a repetitive coating procedure was necessary. Four
Fig. 3. Schematic drawing of the uniaxial shrinkage that conduces from an as-deposited cubic mesostructure to a thermally-contracted orthorhombic array. Further increasing in thermal treatment leads to a partially collapsed mesostructure with formation of channels in the h1 1 0i crystallographic direction.
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309
1200 1000 800
Counts
600 ˚C 550 ˚C 500 ˚C 450 ˚C 400 ˚C
600 ˚C 550 ˚C 500 ˚C 450 ˚C 400 ˚C
(101)
600 400
(103) (004) (112)
(200)
200 0 24 26 28 30 32 34 36 38 40 42 44 46 48 50
2θ / °
(a)
23
(b)
24
25
26
27
28
2θ/ °
Fig. 4. (a) XRD diffraction patterns of titania films prepared using Pluronic P105 as a function of the thermal treatment and (b) particular of the (1 0 1) peak.
Fig. 5. Fitting of X-ray diffraction pattern of a titania film (350 C calcination). The peaks not directly attributed to anatase phase are due to the silicon substrate. The fitting was made by MAUD software.
Table 2 Anatase crystallite size ‘‘d’’, calculated by Maud software, as a function of the calcination temperature Temperature/C
d/nm
400 450 500 550 600
2.1 ± 0.5 4.2. ± 0.6 17.4 ± 2 21.7 ± 2 24.9 ± 3
sequential layers (each layer followed by an accurate processing treatment) were deposited to obtain a thick titania film (1 lm). After impregnation of the multilayer coating, the sample was impregnated with the ruthenium complex, [RuII(dcH2bpy)2(NCS)2] that is generally indicated as N3 [25] (Scheme 1). This dye is the most popular heteroge-
neous charge transfer sensitizer for mesoporous solar cells and, even if is challenged in terms of photovoltaic performances by the ‘‘black-dye’’ tri(cyanato)-2,2 0 ,200 -terpyridyl4,4 0 ,400 -tri(carboxylate)Ru(II), we have preferred to use N3 because of the lower cost and the large availability of reference literature data. The mechanism of generation of electronic charge is activated by the excitation of the dye that causes the transfer of an electron from the metal to the carboxylate bipyridyl ligand. Because the chemical bonding of the ligands groups to the mesostructured nanocrystalline film, the electron is very quickly released (50 fs) into the titania conducting band with a unit quantum yield (Fig. 6). The UV–Vis spectra of the films impregnated by N3 showed the presence of an intense absorption band peaking at 515 nm, attributed to the dp ! p* (RuII ! dcH2bpy)
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L. Malfatti et al. / Microporous and Mesoporous Materials 88 (2006) 304–311 COOH
COOH
HOOC N N
N Ru N C
Absorbance
0.6
0.5
0.4
0.3 N
N
0.2 400
COOH
S
600
700
Wavelength/nm
C S
500
Fig. 7. Absorption spectra of 1 lm thick mesostructured titania film sensitized by N3 (continuous line) and N3 in CH3CN (dashed line).
Scheme 1. Chemical structure of [RuII(dcH2bpy)2(NCS)2] N3 Dye. 100
Standard TiO2 (≈ 7 µm) Mesop. TiO2 (≈ 1 µm)
IPCE %
80 60 40 20 0 400
500
600
700
800
Wavelength/nm Fig. 8. Photoaction spectra of a 1 lm thick mesostructured titania film (calcined at 350 C) compared with a 7 lm thick TiO2 reference electrode. The incident photon current conversion efficiency is shown as a function of the wavelength. The two samples are both sensitized by N3.
IPCE%ðkÞ ¼ 1:24 103 ðV nmÞ
Fig. 6. Schematic representation of dye-sensitized heterojunction photovoltaic cell fabricated by mesostructured titania film and N3 dye.
metal-to-ligand charge transfer (MLCT) transition (Fig. 7), the spectrum of the Ru complex in acetonitrile is also reported for comparison. The UV–Vis spectrum gives a direct indication of the successfully impregnation of the dye within the film. The photoaction spectra of the sample, prepared with Pluronic P105 and calcined at 350 C, were collected reporting the incident-photon-to-current-efficiency (IPCE) as a function of the incident radiation wavelength, k, (Fig. 8). The IPCE index is defined as ‘‘number of collected electrons/number of incident photons’’. Incident photon to current conversion efficiencies were calculated according to the formula [33]:
J ðl A cm2 Þ k ðnmÞ P ðW m2 Þ
where J is the monochromatic photocurrent density and P is the radiant power density. The IPCE decreased at higher excitation wavelengths, with a similar trend in the standard and in the mesoporous TiO2 electrodes. A peak in IPCE around 550 nm, with a photocurrent onset close to 700 nm, was observed in both the samples. The mesostructured titania films sensitized with N3 showed that are suitable electrodes for Gra¨tzel-type solar cells because of the photocurrent observed in the samples even if the IPCE resulted lower than the TiO2 standard electrode. This behaviour, however, cannot be directly attributed to an effective lower efficiency of mesostructured titania. The difference of thickness can, in fact, explain the measured values [34]. On the other hand, a direct comparison was difficult to do because the standard electrode was deposited by doctor blade that did not allow obtaining films of lower thickness. Thicker samples of mesostructured titania films must be, therefore, produced to achieve photovoltaic electrodes that can represent a challenging alternative material in dye-sensitized solar cells applications.
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4. Conclusions Evaporation-induced self-assembly is a suitable route to obtain titania mesostructured films. The control of the synthesis parameters allows obtaining thick films via repetitive dip-coating, this process does not disrupt the organization of the pores in the single layers and the structure of the porous phase is maintained even after removal of the organic template. Controlled crystallization of the amorphous titania pore walls into crystalline anatase, whose crystallite dimensions can be adjusted within a certain degree as a function of the processing conditions, can be reached through thermal calcination. Thermal treatments at temperatures between 400 and 600 C give a high surface area mesoporous nano-anatase coating presenting channel-voids very interesting for dye-sensitized solar cells. The measurement of the incident-photon-to-current-efficiency of prototyped solar cell based on titania mesostructured films showed that this mesostructured material can be efficiently used, upon optimization of the thickness and processing conditions, as electrode in dye-sensitized solar cells. Acknowledgment FIRB Italian projects are acknowledged for financial support (FIRB contract no. RBNE01P4JF). References [1] [2] [3] [4]
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