Organic/inorganic nanocomposite gels employed as electrolyte supports in Dye-sensitized Photoelectrochemical cells

INTRODUCTION

Dye sensitized photoelectrochemical cells (DSPEC) are studied with increasing interest by several groups around the world [1–11], ever since the original work of Graetzel [1], where it was announced that visible light can be efficiently converted into electricity using mesoporous titania films and a tris(2,2'-bipyridine) ruthenium derivative as photosensitizer. The original DSPEC's employed a liquid electrolyte containing the I3−/I− redox couple, which is a standard choice [1, 12– 15] when combined with TiO2 and Ru-bipyridyl photosensitizers. Most recent efforts are directed towards the choice of a solid electrolyte [5] since liquid electrolytes present many practical disadvantages. On the contrary, a solid electrolyte allows for a sandwich thin film configuration, which is ideal for most applications. In the present work we present a sol-gel procedure for the deposition of a nanocomposite inorganic/organic thin film, combining silica and poly(ethylene glycol)- 200 (SiO2/PEG-200), enriched with I3−/I−, which supports a solid sandwich thin film DSPEC. The sol-gel procedure is ideal for this application, since in the original sol (liquid) phase, the material can enter into the pores of the mesoporous semiconductor and greatly increase the interface between the semiconductor and the electrolyte.

2. EXPERIMENTAL
Titanium (IV) isopropoxide, tetramethoxysilane (TMOS),polyoxyethylene(10) isooctylphenyl ether (Triton X-100), poly(ethylene glycol)-200 (PEG-200), were purchased from Aldrich and used as received. Cis-bis(isothiocyanato) bis(2,2'-bipyridyl-4,4' -dicarboxylato)-ruthenium (II) [16] (RuL2(NCS)2) was provided by Solaronix SA (rue de l'Ouriette 129, 1170 Aubonne VD, Switzerland). The rest of the reagents were from Merck, while Millipore water was used in all experiments. Optically transparent electrodes (OTE) were cut from an indium-tin-oxide (ITO) coated glass (<>

The diameter of the nanoparticles employed in the present work, as estimated by using AFM images, was around 30 nm. X-ray diffusion study of TiO2 powder made with the above procedure showed that films consist of anatase nanocrystals. When the TiO2 film was taken out of the oven and while it was still hot, it was dipped into an 1mM ethanolic solution of (RuL2(NCS)2) and was left there for about 24 hours. Then it was copiously washed with ethanol, dried in a stream of N2 and studied by absorption spectrophotometry. The dye is steadily attached on the TiO2 film, obviously, by means of its carboxylate groups. Figure 1 shows the absorption spectra of the TiO2 film with and without the adsorbed dye. The absorption of visible light by the film is, obviously, possible only through the dye. The oscillating part in the TiO2 absorption spectrum is due to interference fringes. 2.2. Synthesis of the nanocomposite SiO2/PEG-200 film containing electrolyte. On the top of the TiO2/dye layer, a thin composite organic/inorganic film containing I3−/I− has been deposited under the following procedure, which was carried out at ambient conditions. TMOS was partially hydrolyzed by mixing with acidified water (HCl, pH 3.0) at a molar ratio TMOS:water = 1:2. The mixture was stirred for one hour. It was originally turbid but it became clear in the course of proceeding hydrolysis. Then to 1ml of this sol, we added 5 g of an aqueous PEG-200 solution containing the redox-couple. In particular, I2 was diluted in pure PEG-200 while KI was diluted in water. Then the two solvents were mixed and produced a transparent solution. Only if I2 is first diluted in PEG-200, it can finally produce a transparent solution, since it is not directly soluble in water. Different PEG-200/water ratios have been obtained for the purpose of the present work, while the overall concentration was 0.03M for I2 and 0.3M for KI. After mixing with prehydrolyzed TMOS, the solution was stirred for 4 hours, when it was judged ready for application. During that time, a condensation procedure goes on by −Si − O−Si−Polymerization, slowly producing a gel. 4 hours of waiting time under stirring still leaves the solution at an early stage of gelation. A thin film of this composite material was deposited by dip-coating. As before, the back inactive side of the glass support was covered with a tape before dipping and was pealed off afterwards. 2.3. Application of the counter-electrode that ends the fabrication of the cell. On the top of the SiO2/PEG-200/electrolyte layer, while the electrolyte layer was still in the fluid phase, we placed an ITO electrode covered with a thin Pt film and pressed against the underlying support. −Si − O− bridges help binding the counter electrode so that the composite SiO2/PEG- 200 material additionally acts in holding the parts of the cell together in a stable thin sandwich configuration.

Pt was applied prior to cell binding by vacuum evaporation on the ITO slide. Its presence is necessary to improve cell performance. A schematic diagram of the cell cross section is shown in Figure 2. Absorption measurements were made with a Cary 1E spectrophotometer. Incident Photon to Current Efficiency (IPCE %) values [21] have been measured by illumination of the samples with a 250 Watt Phillips tungsten halogen lamp through a filter monochromator (Oriel-7155). The lamp spectrum satisfactorily simulates solar radiation at the surface of the earth. The number of incident photons was calculated by employing a radiant power/energy meter (Oriel-70260).

Asignatura: CRF

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