The scientific background of the invention belongs to the discipline of Physics and Chemistry while its technological applications belong to the Energy Sector and to the Electronics Sector, since a PV device is an optoelectronic sensor of light.

There are already known versions of PECSC, as those published in international journals (cf. O'Reagan, B. ; Graetzel, M. Nature, 1991,353, 737 Ka Nazeeruddin, M. K. ; Kay, A.; Rodicio, I. ; Humphry-Baker, R. ; Mueller, E. ; Liska, P. ; Vlachopoulos, N. ; Graetzel, M. ; JAm. Chem. Soc. 1993,115, 6382). These above works refer to a liquid cell with solid electrodes, where the synthesis methods and the type of materials used are different from those of the present invention.

The present invention refers to a totally solid cell deposited in the form of a multilayer film. It is also an evolution from a cell that has been published in international journals (cf. E. Stathatos, P. Lianos, U. Lavrencic-Stangar, B. Orel, Adv. Mater., 2002, 14, No5, 354) and is protected by a former Greek patent (OBI, No. 1003816). The present invention uses new improved materials made by different processes from those of the above-mentioned patent. Specifically, in the present invention a different process is used for the synthesis and deposition of titanium dioxide. In the present invention, the active surface of TiOa is increased, accordingly increasing the quantity of the adsorbed organic photosensitizer and the overall efficiency of the cell.

Corresponding efficiency increase is achieved also by the use of a solid gel electrolyte where solvents are incorporated in the structure of the electrolyte that enhance electric conductivity.

Explanation of the drawings and short description of the cell.

Drawing 1 shows a crossectional view of the proposed PECSC: (1) Negative electrode made of transparent electroconductive glass; (2) Film of mesoporous titania with adsorbed dye; (3) Solid gel containing redox couple; (4) Positive electrode made of transparent electroconductive glass with deposited thin platinum layer.

Drawing 2 shows flat and three-dimensional AFM image of a titania film.

Drawing 3 shows adsorption spectrum of a titania film without (1) and with (2) adsorbed dye (its structure appears in the insert), and Drawing 4 shows an I-V characteristic curve of the PECSC.

Follows a short description of the proposed solar cell. The cell consists of the following parts, which appear in the crossectional drawing #1 : (1) A glass plate with deposited thin transparent film of Tin Dioxide doped with fluorine (Sn02 : F), which gives glass surface electroconductive properties and which is commercially available, or a glass plate with deposited thin transparent film of Indium Oxide doped with Tin (ITO), which is commercially available, or any other type of transparent electroconductive plate which is commercially available and which provides electric conductivity with surface resistance <100 Ohm, preferably <20 Ohm ; (2) A layer of titanium dioxide (TiO2) of mesoporous structure, made of nanocrystalls of anatase or mixture of anatase and rutile, in the form of thin transparent film of controlled thickness, which is synthesized and deposited by chemical processes, as described below. On this titania layer, a commercially available organometallic ruthenium complex, cis-bis (isothiocyanato) bis (2,2'-bipyridyl-4, 4'-dicarboxylato)-ruthenium (II) (cf. insert of drawing #3), which acts as a photosensitizer of Ti02, is adsorbed, by dipping in a solution of the complex ; (3) a layer of solid gel electrolyte, made by the sol-gel route as described below ; and (4) a second Sn02 : F plate or ITO or any other transparent electroconductive plate, same as that of component #1, which makes the second electrode that completes the cell. Alternatively, on this second electrode, a thin layer of platinum (Pt) can be deposited by thermal evaporation under vacuum, which acts as a catalyst increasing cell efficiency.

Detailed description of each part, chemical syntheses and construction of the cell Preparation of the electroconductive transparent plates that are used as electrodes.

The transparent conductive glass plates, which are used as substrates in the construction of the PECSC, are cut into the desired dimensions from a commercially available larger sample. Their cleaning is made in an ultrasonic bath, usually of alcohol. Cleaning process lasts about 30 min. Then the glasses are dried by blowing dry clean air or dry clean inert gas. Two such glass plates are used as substrate positive and negative electrodes.

Preparation of the positive electrode. One of the two clean transparent conductive electrodes will be used as positive electrode or, alternatively, will be covered by a thin platinum layer, which is deposited by thermal evaporation under vacuum (appr. 10-6 Torr). The Pt layer can be very thin so as the cell to be semi-transparent and thus to be used in PV windows. It can also be deposited as a thick opaque reflective layer, so as to increase the probability of photon absorption by the photosensitizer. In that case, the cell is opaque and acts exclusively as PV cell.

Deposition of mesoporous TiO_ film. Deposition of thin Titania (TiO2) films on the transparent conductive glass electrode is made by purely chemical processes by employing a colloidal solution where controlled solvolysis and polymerization of titanium isopropoxide takes place. Specifically, in a premeasured volume of ethanol, we add a premeasured quantity of a surfactant by the commercial name Triton X-100 [polyoxyethylene-(l0) isooctylphenyl ether], or other surfactant of the Triton family, or any other surfactant of any other category, preferably non-ionic, at a weight percentage that varies according to the chosen composition. Then we add an excess of acetic acid (AcOH) and, fmally, a premeasured volume of titanium isopropoxide, under vigorous stirring. All above reagents are commercial. The evolution of the above mixture is conversion into a gel (sol-gel process) through chemical reactions that lead to solvolysis and inorganic polymerization of titanium isopropoxide, that is, formation of-O-Ti-O-networks. Before completion of this procedure and while formation of TiO2 oligomers is advanced, the conductive glass plate is dipped into the above colloidal solution and withdrawn at constant and controlled speed, resulting in

formation of a homogeneous film made of nanocomposite organic-inorganic material.

Alternatively, the same material can be deposited by centrifugation or by simple casting. The film is left to dry under ambient conditions and then it is introduced into a warm oven, where it is calcined at 550°C for 10 min. Heating at such high temperature results in burning all organic content so that the remaining film consists only of Ti02nanoparticles. The process of dipping and calcination is repeated a few more times, producing successive titanium dioxide layers, till a satisfactory thickness is achieved. Thin films are completely transparent while thick films might become opaque, due to extensive scattering of light. Films made by the above procedure consist of TiO2 nanoparticles of 10-30 nm average diameter. The characterization was made by microscopy methods, such as Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM), already mentioned above. Such an AFM image is attached (Drawing #2). Application of the film is made only on the one (conductive) side of the glass plate. For this reason, in case of dipping, the other side is temporarily covered by a protective tape.

Presentation and deposition of the organic photosensitizer. TiO2 nanocrystallites absorb light only in the Near W, therefore it is necessary to photosensitize it in the visible, in order to exploit visible light. For this reason we use a commercially available organometallic dye which has been proven to have a satisfactory capacity of injecting, when excited, electrons into the conduction band of TiO2. We propose a ruthenium complex with the chemical structure cis-bis (isothiocyanato) bis (2,2'- bipyridyl-4, 4'-dicarboxylato)-ruthenium (II) (cf. insert of drawing #3). Attachment of the dye on the Ti02 surface is made by chemical bonding by means of the carboxylate groups and is achieved after adsorption on titania nanocrystallites, for example, by dipping in an ethanolic solution of the dye. Adsorption is verified by absorption spectrophotometry. Under the above conditions, maximum optical density of the TiO2/photosensitizer system reached with transparent titania films is 0.80, that corresponds to 84% absorption of incident light, at the absorption maximum (cf.

Drawing #3). This percentage can be increased or decreased by controlling thickness of TiOa films. At any rate, this percentage is the maximum internationally achieved for transparent titania films and it owes to the synthesis and deposition method used, as described above. This method endows titania films with extensive porous structure

and active surface towards adsorption and bonding of the photosensitizer molecules.

The thus prepared electrode makes the negative electrode of the Solar Cell.

Presentation of the nanocomposite organic-inorganic gel. Synthesis and deposition of the gel electrolyte. The electrolyte we propose to intervene between the two electrodes already described, in order to close the circuit and complete the cell is the following : we must prepare a colloidal solution which contains a silicon alkoxide, or a titanium alkoxide or an alkoxide of another metal, which in the presence of AcOH and ambient humidity is polymerized yielding a-O-M-O-network, where M is a metal or Si. Gel formation is due to (inorganic) polymerization-O-M-O-. In the colloidal solution we add an organic material which is incorporated in the gel and forms an organic subphase, which provides ionic conductivity. Such substances are either surfactants or ethyleneglycol oligomers or polymers, incorporated either by simple mixture or by chemical bonding with the-O-M-O-network. In addition, we add an organic solvent, which is also incorporated in the gel, takes part in the formation of the organic subphase and allows increase of ionic conductivity. Finally, a redox couple is added to the colloidal solution, l3'/T by preference. This couple is produced in the presence of I2 and of an iodide salt XI, where X+ is an elemental or an organic cation.

The colloidal solution slowly gels after AcOH addition. AcOH acts as a gel-control factor through ester formation M-O-Ac (cf. U. Lavrencic-Stangar, B. Orel, Adv. Mater., 2002,14, No5, 354; E. Stathatos, P. Lianos, B. Orel, A. Surca Vuk and R. Jesse, Langmuir, 2003,19, 7587) or through slow water production by interaction between AcOH and alcohol.

Completion of the Cell. When gelling of the above solution is sufficiently advanced but while it is still a fluid, one drop is cast on the negative electrode (i. e. the glass plate that bears the titania and the adsorbed dye). Then the two electrodes are brought in contact by squeezing them together. The material is spread over the whole active surface of the electrodes. As gelling is completed, the two electrodes are strongly held together and they are not detached even under stress. Attachment is obtained by-0- M-O-bonds. Electric contacts with the two electrodes are obtained by electroconductive paste, or by epoxy paste enriched with silver grains or by copper adhesive tape, all commercially available.

Examples of PECSC's Example 1. As a substrate for deposition of titania film we used a glass plate bearing a Snoba : F layer (negative electrode). As a substrate for deposition of a thin layer of platinum we used a glass plate bearing a SnO2 : F layer (positive electrode). On the positive electrode we deposit by thermal evaporation under vacuum a semi- transparent Pt layer of a thickness of about 200nm. On the negative electrode we deposit the colloidal solution from which the titania film will be produced after calcination. The colloidal solution is made as follows: 3g EtOH are mixed with 0. 71g Triton X-100. Then we add 0. 64g AcOH and 0. 36g Titanium Isopropoxide under vigorous stirring and ambient conditions. After 30 min stirring a drop of this colloidal solution is placed on the negative electrode and it is stretched over the film by using a glass blade. After drying for five minutes, it is introduced in a preheated oven and it is calcined at 550°C for ten minutes. Then we take it out from the oven and we let it cool at ambient conditions. This procedure is repeated ten times. In this way we obtain a thin transparent film of about 1-2pm thick. The titania film thus obtained is mesoporous and it has the structure seen in the attached AFM image (drawing #2).

Then the film is dipped into an ethanol solution of cis-bis (isothiocyanato) bis (2,2'- bipyridyl-4, 4'-dicarboxylato)-ruthenium (II) at concentration SxlO-SM. The dye is adsorbed and attached on the titania mesoporous film which becomes colored. The related absorption spectrum is presented in drawing #3. Maximum absorbance in the visible is 0.80 (84%). On this electrode we then place one drop of the fluid gel that bears the redox couple. This sol is prepared under ambient conditions as follows: 1. 5ml propylene carbonate are mixed with 1 ml Triton X-100. Then we add 0. 35g Tetramethoxysilane [abbreviated TMOS, i. e. Si (OCH3) 4] and 0. 65ml AcOH under vigorous stirring. Last, we add 0. 05M 12 and 0.5M KI and the mixture is continuously stirred for 12 hours. Then it is ready to be applied. The PECSC is completed with the attachment of the positive electrode which is simply done by pressing by hand the two electrodes against each other, sandwiching between them the above mixture.

Electric conducts are made using silver paste. For this reason, a small part of the negative electrode is protected against Ti02 deposition so as to make contact which underlying the Snoba : F layer. When the above cell is illuminated by simulated solar radiation of an intensity of 100 mW/cm, it produces a short circuit current of

11. 8mA/cm2 an open circuit voltage of 0.60volts, with a fill factor of 0.69 and overall efficiency 4.9%.

Example 2 A PECSC with the same components, as that of Example 1, the same proportions of the employed reagents and the same methods of preparation but propylene carbonate been substituted by a 1 : 1 mixture of propylene carbonate and ethylene carbonate, under illumination by simulated Solar Radiation of 100 mW/cm, produces 11.6 mA/cm2 short circuit current, 0.62 volts open circuit voltage, fill factor 0. 69 and overall efficiency 5.0%.

Example 3 A PECSC with the same components, the same proportion of used reagents and the same preparation procedures as those used in Example 1 but with propylene carbonate been substituted by poly (ethyleneglycol)-200, when illuminated by simulated Solar Radiation of 100 mW/cm2, produces 12.4 mA/cm2 short circuit current, 0.61 volts open circuit voltage, fill factor 0.7 and overall efficiency 5.3%.

Example 4 A PECSC with the same components, the same proportion of used reagents and the same preparation procedures as those used in Example 1, but propylene carbonate been substituted by propylene carbonate containing a few drops of pyridine, when illuminated by simulated Solar Radiation of 100 mW/cm2, produces 8.4 mA/cm2 short circuit current, 0.69 volts open circuit voltage, fill factor 0.68 and overall efficiency 3.9%.

Example 5 A PECSC with the same components, the same proportion of used reagents and the same preparation procedures as those used in Example 1, but KI been substituted by 1-methyl-3-propylimidazolium iodide, when illuminated by simulated Solar Radiation of 100 mW/cm2, produces 12, 9 mA/cm2 short circuit current, 0.65 volts open circuit voltage, fill factor 0.66 and overall efficiency 5.4%.

Example 6 The components of the cell, the proportion of the employed reagents and the preparation procedures are the same as for Example 1 but the sol which contains the redox couple is made under the following procedure: 0.75g Ureasil 230, a bis- triethoxysilane precursor by the chemical formula is mixed with 1, 75g sulfolan.

Then we add 0.7g AcOH and 0. 05M 12 + 0. 5M KI under vigorous stirring. After 24 hours stirring the colloidal solution is ready for application. The obtained cell, when illuminated by simulated Solar Radiation of 100 mW/cm, produces 13,9 mA/cm2 short circuit current, 0.64 volts open circuit voltage, fill factor 0.70 and overall efficiency 5.3%. The corresponding I-V curve is shown in drawing #4.

Example 7. In the examples 1-6, the SnO2 : F glasses are substituted by ITO glasses.

The obtained cells have an overall efficiency of about 20% less than those made of Sn02 : F glasses.

Example 8. In the examples 1-6, we change the procedure of deposition of TiO2 films by modifying the Triton X-100 content in the original sol. The mesoporous structure of nanocrystalline titania is affected and this affects adsorption capacity towards the dye photosensitizer. Optimum results are obtained with the surfactant content employed in Examples 1-6 Applications The above PECSC can be used as an independent energy source for supplying isolated devices or by connection to the Electricity Network. Low energy consumption apparatus, such as quartz watches or small calculators can be powered by a combination of small size cells. The above PECSC can be also used as light sensor where the presence of light is signaled by an electric signal. The semi-transparency of the cell allows it to be applied as photovoltaic window.