WO2007066098A1

WO2007066098A1 - Organic solar cell

Info

Publication number: WO2007066098A1
Application number: PCT/GB2006/004547
Authority: WO
Inventors: Pavel Ivan Lazarev; Elena N. Sidorenko
Original assignee: Cryscade Solar Limited
Priority date: 2005-12-05
Filing date: 2006-12-05
Publication date: 2007-06-14
Also published as: GB0524799D0; GB2433071A

Abstract

The present invention relates to an organic solar cell based on an organic ionic-crystalline photoelectric layer. In a preferred embodiment, the present invention provides an organic acid of general structure: Formula (I) wherein said acid absorbs electromagnetic radiation in at least one predetermined spectral subrange within a wavelength range from 400 to 3000 nm, and aqueous solution of its water- soluble salt is capable of forming a photoelectric layer of rodlike supramolecules on a substrate. In another preferred aspect, the present invention provides an organic solar cell comprising two electrodes facing each other, an organic ionic-crystalline photoelectric layer situated between two electrodes and contacting with the first electrode and an electrolyte which is situated between the organic ionic-crystalline photoelectric layer and the second electrode and impregnates the organic ionic-crystalline photoelectric layer. The organic ionic-crystalline photoelectric layer is capable of absorbing electromagnetic radiation in at least one predetermined spectral subrange within a wavelength range from 400 to 3000 nm. This layer consists of rodlike supramolecules, which are comprised of the molecules of at least one organic compound of the general structural Formula (II).

Description

ORGANIC SOLAR CELL

The present invention relates to an organic solar cell based on an organic ionic-crystalline photoelectric layer.

Photovoltaic devices are intended for converting electromagnetic radiation into electricity. Such devices are used to drive power consuming loads so as to provide, for example, lighting or heating, or to operate electronic equipment. Thereby, an electronic device (e.g., a computer monitor, display, exposure meter, etc.) connected as the external resistive load to a photovoltaic source can operate using converted solar energy. Such power generation applications often involve the charging of batteries or other energy storage devices, so that equipment operation may continue when direct illumination from the sun or other ambient light source is no longer available. As used herein, the term "resistive load" refers to any power consuming or storing device, equipment, or system.

Photovoltaic devices produce a photogenerated built-in voltage when they are connected to a resistive load and are irradiated by light. When irradiated without any external resistive load, a photovoltaic device generates its maximum possible built-in voltage V called open-circuit voltage (Voc). If a photovoltaic device is irradiated with its electrical contacts shorted, a maximum current, the short-circuit current (Isc), is produced. When actually used to generate power, a photovoltaic device is connected to a finite resistive load and the output power is given as the product of the current and voltage, I x V. The maximum total power generated by a photovoltaic device is inherently incapable of exceeding the product Isc x Voc. When a load value is optimized for maximum power extraction, the current and voltage have values Imax and Vmax, respectively.

The estimation of conversion efficiency of a photovoltaic device is the fill factor, ff, defined as ff = (Imax- Vmax)/(lsc- Voc), where ff is always less than unity, as Isc and Voc are never obtained simultaneously in real practice. Nevertheless, as ff approaches unity, the device is more efficient. Other criteria of the efficiency of a photovoltaic device can be used as well. In particular, the external quantum efficiency characterizes the number of electrons generated per one incident radiation quantum (photon) and the internal quantum efficiency is the number of electrons produced per one photon absorbed by the given photovoltaic device.

It is similarly possible to give definitions of efficiency for other photosensitive optoelectronic devices.

There are photosensitive optoelectronic devices of various types (solar cells,

photodetectors, photoresistors, etc.) based on inorganic semiconductors (see, e.g., S. M. Sze, Physics of Semiconductor Devices, Wiley-lnterscience, New York, 1981 ). Previously, inorganic semiconductors (such as crystalline, polycrystalline, and amorphous silicon, gallium arsenide, and cadmium telluride) were the main materials used for the development of solar cells. The term "semiconductor" refers to a material capable of conducting electric current, in which the free carriers of the electric charge (electrons and holes) are generated by means of thermal or electromagnetic excitation.

Conventional photovoltaic devices or photovoltaic elements typically comprise a p— n junction formed in a single crystal semiconductor (e.g., silicon) substrate. Typically, an n-type surface region is diffused into a p-type silicon substrate and ohmic contacts are applied. When the device is exposed to solar radiation, photons incident upon the n-type surface travel to the junction and the p-type region where they are absorbed in the production of electron— hole pairs.

The conversion efficiency of conventional photovoltaic devices, however, is limited by a number of factors. First, the built-in voltage is limited by a relatively narrow bandgap of silicon and by the limited extent to which both p- and n-type layers of silicon can be doped. While the built-in voltage of the device can be increased through increased doping of both layers forming the junction, such excess doping tends to reduce the conversion efficiency by reducing the lifetime of charge carriers and thereby the collection efficiency of the device. As a consequence, the open- circuit voltage of a typical silicon photovoltaic device is only about 50% of the silicon bandgap value. Second, silicon tends to absorb high-energy photons, that is, blue and ultraviolet light, very close to the surface (typically within a micron thick layer). As a consequence, many of the high- energy photons are absorbed near the surface of the n-type region, causing charge carriers generated by such absorption to recombine at the surface and be lost as mediators of photocurrent. Still a third limiting factor resides in the fact that photons of lower energy, representing red light and near infrared radiation, tend to penetrate deep into silicon before they are absorbed. While minority carriers created by deep-layer absorption can contribute to the photocurrent, provided that minority carrier lifetimes are sufficient to permit them to drift into the junction region, the high-temperature diffusion step required to form the n-type region significantly reduces the minority carrier lifetime in p-type silicon substrates. As a consequence, many charge carriers created by deep-layer absorption are also lost as mediators of photocurrent.

As noted above, photovoltaic devices (including solar cells) are characterized by the efficiency of converting solar energy into useful electricity. Silicon-based photovoltaic devices reach relatively high conversion efficiencies, on a level of 12— 15%. The conversion efficiency of a particular photovoltaic device depends significantly on the quality of materials employed. For example, important limiting factor in real devices are leak currents caused by the recombination of photoproduced charge carriers. In other words, undesired electron— hole interactions cause a portion of electrons to return to the valence band of the semiconductor or to localize in allowed energy levels in the forbidden band of the semiconductor. The leak currents are usually caused by the presence of so-called point defects and/or other deviations from the ideal crystalline structure of a semiconductor, which lead to the appearance of such allowed energy states in the forbidden band.

When the amount and influence of the aforementioned defects are small, the electron— hole interactions proceed by a mechanism called 'radiative recombination'. As it possesses a sufficiently large characteristic time, the radiative recombination is a "slow" process. In the absence of defects, the process of the radiative recombination offers the only channel for decay of the electron— hole pairs. This process involves no local energy levels and the radiative recombination can proceed directly from a conduction to a valence band. As a result, a high efficiency of converting the solar energy into electricity is indirect evidence of the absence of more rapid (i.e., more effective) channels of the nonradiative recombination in a given material.

There are some other disadvantages of photovoltaic devices based on inorganic semiconductors, besides those mentioned above. In particular, such devices are expensive. Manufacturing of these devices requires complicated technology involving high-cost equipment and sophisticated processing methods, to provide semiconductor layers and multilayer structures of large area and free of defects.

There have been numerous attempts at reducing the cost of production of photosensitive optoelectronic devices, including solar cells. Organic photoconductors and organic semiconductors have also considered as candidate materials because of the option to produce organic films by deposition from solutions or by other low-cost techniques. However, the conversion efficiency of solar cells employing such organic materials was always less than the conversion efficiency of conventional solar cells based on inorganic materials. Practical on-ground applications require greater values of the photovoltaic conversion efficiency.

Experimental data has shown that the efficiency of solar cell and other organic-based optoelectronic devices increases when the molecular planes in molecular crystals forming organic semiconductor films in such devices are oriented predominantly parallel to the substrate surface and electrodes (see P. Fenter et. a/., Layer-by-Layer Quasi-Epitaxial Growth of a Crystalline Organic Thin Film", J. Crystal Growth, 152, 65-72 (1995); S. R. Forrest and P. E. Burrows, Growth Modes of Organic Semiconductor Thin Film Using Organic Molecular Beam Deposition: Epitaxy, van der Waals Epitaxy, and Quasi-Epitaxy", Supramol. ScL, 4, 127-139 (1997)). This fact implies the existence of channels for the transfer of electrons and holes along the axis of the π-π interaction in rodlike stacks inside crystal particles, which provides high mobility of charges (facilitating the transport of electrons and holes). The short vertical way between electrodes also reduces the probability of undesired recombination of charge carriers.

Materials with molecular stacks oriented perpendicularly to the substrate surface are obtained by epitaxy of planar polycyclic molecules.

There is a known organic quasi-epitaxial method intended for the formation of

optoelectronic devices (see US Patent No. 5,315,129, Forrest et al., Organic Optoelectronic Devices and Methods). According to this method, the planes of organic molecules are oriented parallel to the substrate surface. A quasi-epitaxial optoelectronic device structure comprises a substrate, the first layer deposited on said substrate, and the second layer deposited above the first layer. Said first layer represents a planar crystalline film of an organic aromatic semiconductor compound and is selected from a group of organic compounds including polyacenes, porphyrins, and their derivatives. Said second layer also represents a planar crystalline film of an organic aromatic semiconductor, whose chemical composition (generally, different from that of the first layer) is also selected from a group of organic compounds including polyacenes, prophyrins, and their derivatives. The first and second layers have crystalline structures, which are in a certain relationship with each other. In particular, the first and second layers can be independently selected from a group including 3,4,9, 10-perylenetetracarboxylic dianhydride (PTCDA), 3,4,7,8- naphthalenetetracarboxylic dianhydride (NTCDA), copper phthalocyanine, 3,4,9,10- perylenetetracarboxylic acid bis-benzimidazole, and -oxadiazole derivatives. Organic

optoelectronic devices have been grown by organic molecular beam deposition. The organic substances have been deposited as ultrathin layers only 10 Angstrom (A) thick using organic molecular beam deposition methods. PTCDA and NTCDA have been identified as excellent materials for the manufacture of organic optoelectronic IC devices, but any planar organic aromatic semiconductor capable of readily forming a crystalline structure may be used. The preferred method of the prior art employs a chamber, comprising an inorganic substrate made of an appropriate material for making electrical contact to the organic structures, and sources of PTCDA and NTCDA. The chamber is maintained at a pressure generally below 10^'6 Torr. The substrate is spaced from the source of film materials by a minimum distance of 10 cm. During deposition, the substrate is kept at a temperature below 150K, while the PTCDA and NTCDA sources are alternatively heated.

Despite all the advantages of said quasi-epitaxial growth method (see US Patent Nos. 6,451 ,415 and 5,315,129), it is not free of drawbacks. According to said known method, a constant temperature regime and vacuum level have to be maintained in the chamber throughout the epitaxial growth process. Any breakdowns in the temperature and vacuum regime lead to the appearance of defects in the growing layer, whereby both crystallographic parameters and the orientation of molecular layer exhibit changes. This sensitivity of the process is a disadvantage of said known method, which is especially significant in the case of deposition of relatively thick (1 to 10 μm) epitaxial layers.

Another disadvantage of said method is the need in sophisticated technological equipment. The reactor chamber must hold an ultrahigh vacuum (down to 10^"6 - 10^~10 Torr) and must withstand considerable temperature gradients between closely spaced zones. The equipment must include the means of heating sources and cooling substrates, a complicated pumping stage, and facilities for gas admission, temperature and pressure monitoring, and technological process control. The high vacuum requirements make the process expensive and limit the substrate dimensions.

One more disadvantage of said known method is limitation on the substrate materials: only substances retaining their physical, mechanical, optical, and other properties under the conditions of large pressure differences, high vacuum, and considerable temperature gradients can be employed.

The production of a two-dimensional bimolecular surface structure using weak noncovalent interactions has been demonstrated and characterized by scanning tunneling microscopy (see L. Scudiero et a/., "A Self-Organized Two-Dimensional Bimolecular Structure", J. Phys. Chem. B, 107, 2903-2909 (2003)). This work follows closely the ideas of three-dimensional crystal engineering and applies the concepts of supramolecular reactants (synthons) to molecular systems constrained to two dimensions by physical adsorption (physisorption) on a conducting surface. A well-ordered planar structure that self-assembles through the influence of fluorine-phenyl interactions has been demonstrated. This study provides an example of the systematic design of self-organized layers. Fully fluorinated cobalt phthalocyanine (F16CoPc) films thermally deposited onto gold were characterized by reflection-absorption infrared spectroscopy (RAIRS), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), and scanning tunnelling microscopy (STM). The UPS spectra of thin films of CoPc, F16CoPc, and nickel tetraphenylporphyrin (NiTPP) on gold were measured and their relative surface charges were compared. STM images of single molecular layers of F16CoPc, NiTPP, and NiTPP-FI 6CoPc and NiTPP-CoPc mixtures were obtained. It was found that, while NiTPP-FI 6CoPc spontaneously formed a well-ordered 1 :1 structure, NiTPP-CoPc formed a two-dimensional solid solution. Ultrathin films prepared from inorganic and organic materials are of increasing interest as hybrid nanocomposite materials. The formation of nanostructured ultrathin films of montmorillonite clay (MONT) and a bicationic sexithiophene derivative (6TN) was investigated using the layer-by- layer self-assembly approach (see X. Fan, J. Locklin, J. Ho Youk, et al., Nanostructured

Sexithiophene/Clay Hybrid Mutilayers: A Comparative Structural and Morphological

Characterization, Chem. Mater., 14, 2184-2191 (2002)). The main goal was to investigate the structure and layer ordering in those films suitable for future applications in organic semiconductor devices. The structure and morphology of 6TN/MONT multilayer films prepared from pure water and 0.1 M NaCI systems have been compared. The 6TN amphiphile showed unique aggregation behaviour both in solution and on the surface, which changed in the presence of salts and THF as a cosolvent. On clay surfaces, the 6TN aggregates deposited from saline solutions exhibited more uniform size distribution and surface coverage as compared to those obtained from a pure water system. This was verified by UV-VIS spectra, X-ray diffraction (XRD), and atomic force microscopy (AFM). The idea of incorporating more 6TN species adsorbed on the surface so as to obtain a smoother surface morphology can be of great significance in semiconductor device fabrication.

The available literature presents no examples of the films with the vertical orientation of stacks prepared by a low-cost and effective way of solution application on the substrate. The films with the horizontal orientation of stacks are usually obtained using the lyotropic liquid crystal (LLC) solutions of sulfoderivatives (see: U.S. Patent Nos. 5,739,296 and 6,049,428 and the following publications: P. Lazarev et al., X-ray Diffraction by Large Area Organic Crystalline Nanofilms, Molecular Materials, 14(4), 303-311 (2001), and Y. Bobrov, Spectral Properties of Thin Crystal Film Polarizers, Molecular Materials, 14(3), 191-203 (2001 )).

On the other hand, it is known from the literature that some molecules are capable of forming regularly arranged planar fragments (supramolecules) on a substrate surface, being deposited from solutions in water and various organic solvents, and that hydrogen bonding (H- bonding) is the driving force for the formation of such planar supramolecules. This phenomenon was observed for heterocyclic amines, amides, and carboxylic acids. The type of the obtained monolayer structure depends on the molecular structure, the solvent, and the surface activity. The layer structures of various types - stable and unstable, dense and loose - can be obtained using different molecular structures and conditions.

There are many novel adsorbate-substrate systems that are known to exhibit a high degree of large-scale ordering. The method of scanning tunnelling microscopy (STM) has proved to be capable of studying the electronic properties of such systems and their structures on a submolecular resolution level. It was established that, in some systems, H-bonding is the predominant interaction between molecules and governs the molecular self-assembly process.

Selective noncovalent interactions have been widely used in solution chemistry to direct the assembly of molecules into nanometer-sized functional structures such as capsules, switches and prototype nanomachines. The concepts of supramolecular organization have also been applied to two-dimensional (2D) assemblies on surfaces stabilized by means of H-bonding, dipolar coupling, or metal coordination. Another approach to controlling surface structures uses adsorbed molecular monolayers to create preferential binding sites that accommodate individual target molecules. James A. Theobald et al. (Controlling Molecular Deposition and Layer Structure with Supramolecular Surface Assemblies, Nature, 424, 1029-1031 (2003)) combined these approaches by using H-bonding to guide the assembly of two types of molecules into a 2D open honeycomb network. This network controls and templates new surface phases formed by subsequently deposited fullerene molecules. It was found that the open network acts as a 2D array of large pores of sufficient capacity to accommodate several large guest molecules and serves as a template for the formation of an ordered fullerene layer.

Self-assembly of a 2D loosely packed H-bonded network of trimesic acid (TMA) at the liquid-solid interface has been observed using STM (see Lackinger et al., Langmuir, 21 , 4984- 4988 (2005)). Two crystallographically different 2D phases of TMA were identified and selected by varying the solvent. In this paper, some models of various crystallographic structures with the corresponding H-bonding modes were introduced: (a) chickenwire structure, a = b = 1.7 nm, angle A = 60°, area = 2.5 nm², 2 molecules per unit cell; (b) flower structure, a = b = 2.5 nm, angle A = 60°, area = 5.4 nm², 6 molecules per unit cell; (c) "super flower" structure, representing more densely packed 2D TMA polymorph based entirely on 3-fold H-bonding. It was suggested that the denser "flower" structure (b) is likely to be the most thermodynamically stable of the two observed monolayer polymorphs. Studies of these adsorbed polymorph structures for TMA dissolved in a series of acid solvents [CH₃(CH₂)_nCOOH with n = 2-7] showed that the flower structure was favored for the shorter-chain solvents, which also corresponded to those in which TMA had the maximum solubility. It should be noted that an even more densely packed TMA structure could theoretically be formed with a purely 3-fold H-bonded structure ("super flower" structure), but this TMA form was not observed. A possible explanation for this behaviour is the stabilization, in short- chain solvents, of a TMA trimer [(TMA)₃] solution phase nucleation species, which is a likely precursor to the flower form of TMA; however, an explanation based on differential solvent stabilization of the surface monolayer of flower and chickenwire structures cannot be ruled out.

The crystal packing of some fluorinated azobenzenecarboxylic acids was studied by R.

Centore and A. Tuzi (Crystal Eng., 6, 87-97 (2003)). The X-ray crystal structures of

C₆H₅COOH₁C₆F₅COOH (1), C₆H₅CONH₂₁C₆F₅CONH₂ (2), and C₆H₅CONH₂₁C₆F₅COOH (3) were analyzed in order to elucidate the role of Ph-PhF synthon in directing self-assembly and H-bonding in these cocrystals (see Reddy et ah, Crystal Growth & Design, 4, 89-94 (2004)). The strong H- bond donor acidity of C₆F₅COOH and C₆F₅CONH₂ together with mixed stacks of phenyl and perfluorophenyl rings steer acid-acid and amide-amide H-bonding in cocrystals 1 and 2. The acid- amide H-bonding is sufficiently strengthened by donor acidity and acceptor basicity in 3, so that the role of the Ph-PhF synthon is weaker because the aromatic rings stack with lateral offset. The complex C₆H₅COOH₁C₆F₅CONH₂ (4) could not be obtained under similar crystallization conditions. The crystal structure of C₆F₅CONH₂ was also determined to compare the molecular conformation and H-bonding with motifs in the cocrystals.

It has been found that 4-hydroxybenzoic acid (1) crystallizes into three crystalline forms: (i) monoclinic from a DMSO solution (1A), (ii) triclinic from a solution in 1 :1 DMSO/hot ethyl acetate (1 B), and (iii) triclinic from a pyridine solution (1 C) (see Jayaraman et al., Crystal Growth & Design, 4, 1403-1409 (2004)). The formation of these pseudopolymorphs and the structural similarity of their packing motifs can be rationalized in terms of few-multipoint solute-solvent interactions. In all three structures, the crystallographic aspects pertaining to the influence of solvent molecules towards the formation of H-bonded network structures are described. In addition to the strong H- bonds, intermolecular C-H--O, C-H-π, and π-π interactions were found to stabilize the crystal structures.

A series of 4,4-dipyridyl (4,4-DP) derivatives have been prepared and studied using single- crystal X-ray diffraction techniques (see D. E. Lynch et ah, Crystal Eng., 2, 137-144 (1999)). The structures had increasing degree of complexity in the overall H-bonded network. The structure of 1 comprises polymeric H-bonded chains of associated 4,4-DP and ICA molecules that propagate through complementary sites on the ICA molecules. The structure of 2 consisted of two parallel polymeric H-bonded chains, each involving associated 4,4-DP and 3-ABA molecules cross-linked through complementary 3-ABA sites. The structure of 3 was an extensive 3-dimensional H-bonded network involving all H-bonded donor and acceptor sites on the constituent molecules. In each case, the positions and directions of the N-H groups were important in determining the final lattice network.

Conventionally, crystalline silicon solar cells have been well known and widely utilized as devices for directly converting light into electric energy in the field of weak power consumption and as independent power sources. Crystalline silicon solar cells are made predominantly of single- crystalline or amorphous silicon. The production of silicon single crystals and amorphous silicon, however, requires enormous amounts of energy and, in order to recover energy consumed for manufacturing silicon-based cells, electric power generation needs to be carried out continuously for nearly a ten-year long period. Under these circumstances, solar cells utilizing dye sensitizers have received much attention (see B. O'Regan and M. Gratzel, A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO₂ Film, Nature, 353, 737 - 740 (1991); M. Gratzel, Perspective for Dye-Sensitized Nanocrystalline Solar Cell, Prog. Photovolt. Res. Appl., 8, 171 - 185 (2000); M. Gratzel, Photoelectrochemical Cell, Nature, 414, 338 - 344 (2001)).

The present invention will now be described solely by way of examples and with reference to the accompanying drawings in which:

Figure 1 shows the cross section of a prior art dye-sensitized solar cell; and

Figure 2 shows the cross section of a solar cell according to one embodiment of the present invention.

Figure 3 shows the cross section of a solar cell according to another embodiment of the present invention.

Dye-sensitized solar cells differ from the conventional semiconductor devices in that they separate the functions of light absorption and charge carrier transport. In the case of n-type semiconductor materials such as TiO₂, current is generated when the absorption of photons by dye molecules leads to the injection of electrons into the conduction band of the semiconductor (see Figure 1). In order to close the circuit, the dye must be regenerated at the expense of the electron transfer from redox species in solution, which are subsequently reduced at the counter electrode. In the simplest variant, a dye-sensitized solar cell consists of two transparent substrates 1 and 2 (e.g., glass plates) with transparent conducting layers 3 and 4 (e.g., tin oxide (SnO₂) films). Said substrates and conducting layers form the first electrode (photoelectrode) 5 and the second electrode (counter electrode) 6. One side of the photoelectrode is coated with a porous layer 7 of a wide-bandgap semiconductor, usually TiO₂, which is sensitized for visible light by an adsorbed dye film. The semiconductor layer 7 is deposited onto conducting layer 3 from a colloidal solution and has a large specific surface. This semiconductor layer, typically with a thickness of 10 μm and a porosity of about 50%, has a surface area (available for dye chemisorption) more than a thousand times that of a flat layer of the same size. Short-term sintering at 45O⁰C leads to the formation of electrical contacts between particles in the semiconductor layer, which are depicted by open circles 8 in Figure 1. The dimensions of particles and pores making up the porous semiconductor layer 7 are determined by the size of particles in the colloidal solution. The internal surface area of the porous semiconductor layer 7 depends on the particle size and the layer thickness. These parameters should be optimized so as to provide for the effective light collection, while maintaining the pore size large enough to allow the redox electrolyte 9 to diffuse easily. Alternative wide- bandgap oxides such as ZnO and Nb₂O₅ can be used as well. The space between the two electrodes is filled with electrolyte 9, which contains redox couples such as iodide (Q/triiodide (I₃ ^"). The dye molecules 10, which are depicted in Figure 1 by dark contours surrounding the white circles, are adsorbed on TiO₂ particles. Upon the absorption of incident photons 11 , the dye molecules inject electrons into TiO₂ particles on which they are adsorbed. The electrons are transported via TiO₂ particles (which are sintered together at their contact points) until they reach the conducting layer 3. The oxidized dye species resulting from the photoinduced electron transfer are reduced by iodide (l~) species in the electrolyte that fills pores in the semiconductor layer 7. The remaining triiodide (I₃ ^") species diffuse to the counter electrode, where they are converted back to iodide (f) by electrons arriving via an external chain with load 12.

Dye-sensitized solar cells are expected to serve as solar cells for the next generation because of simplicity and efficiency of fabrication technology, reduced material costs, and the like. One approach to manufacturing dye-sensitized solar cells has been described in J. Am Ceram. Soc, 80(12), 3157-3171 (1997), according to which a dye sensitizer (such as a transition metal complex) is adsorbed on the surface of a titanium oxide layer representing a porous

semiconductor. In this technology, a dye-sensitized solar cell is manufactured as follows: a transparent substrate, on which a transparent conducting layer and a semiconductor layer of titanium oxide are formed, is immersed in a solution containing a dye sensitizer, so that the dye sensitizer is adsorbed on the semiconductor surface. Then, an electrolyte solution containing redox species is applied dropwise onto the semiconductor layer. Finally, a counter electrode is stacked above the resulting semiconductor layer. When the solar cell thus obtained is irradiated with visible light from the side of the semiconductor layer, the dye sensitizer supported on the semiconductor layer absorbs the light so that electrons in the dye sensitizer molecules are excited. As a result of this excitation, electrons are injected into the semiconductor layer, transferred to the transparent electrode, and transported via the external electric chain to the counter electrode. Then, electrons enter the electrolyte layer and are carried, together with holes or ions, through the electrolyte layer and returned to the semiconductor layer. Electric energy is generated by repetition of this process.

However, in order to obtain dye-sensitized solar cells applicable in practice, it is necessary to provide for a further increase in the photovoltaic conversion efficiency. For this purpose, solar cells have to be optimized so as to increase the generated current (short-circuit current), the open- circuit voltage, and the durability. In order to increase the open-circuit voltage, it is necessary to decrease a reverse current passing from the semiconductor layer to the dye sensitizer and/or to the electrolyte layer.

The dyes used as sensitizers in prior art possessed high resistivities. Accordingly, these dyes were used as thin monolayers adsorbed on the surface of the semiconductor layer. However, a thin dye layer does not provide for the absorption of all or a substantial part of the incident light. Therefore, it is necessary to increase the thickness of the dye layer so as to increase the absorbed light fraction. Unfortunately, thick sensitizer layers lead to a considerable increase in the serial resistance of a solar cell, which, in turn, results in a decrease in the photovoltaic conversion efficiency of the cell.

One possible method of decreasing said resistance and increasing the absorbed fraction of incident light is to provide a considerable increase in the specific surface area of the semiconductor layer. Accordingly, porous semiconductor layers have been used in prior art. A porous

semiconductor layer, typically having a thickness of 10 μm and a porosity of about 50%, has a surface area (available for dye adsorption) more than a thousand times that of a flat, simple nonporous electrode of the same size. Then, even if the dye is adsorbed as a monomolecular layer, a sufficient amount of it can be retained on a given electrode area so as to provide the absorption of almost all of the incident light.

The charge transport in porous semiconductor layers known in prior art can be described in terms of a diffusion model for random displacements of charge carriers in disordered solids. For this reason, the mobility of electrons in these layers is much lower than in the same ordered bulk materials. In addition, a decrease in the electron mobility leads to an increase in the resistivity of the material and, hence, in the serial resistance of the photovoltaic device. This implies an increase in the ohmic losses and an additional decrease in the photovoltaic conversion efficiency.

Another drawback of the dye-sensitised solar cell known in prior art is that the dye may completely block pores in the semiconductor layer, thus hindering the penetration of electrolyte into these pores.

According to one aspect, the present invention provides an organic acid of general structure

where Het is a planar conjugated heterocyclic molecular system,

X is an acid group,

m is 1 , 2, 3, 4, 5, 6, 7 or 8,

Y is an amide of acid group,

n is 1 , 2, 3 or 4,

R is a substituent selected from the list comprising -CH₃, -C₂H₅, -NO₂, -Cl, -Br, -F, -

CF₃, -CN, -OH, -OCH₃, -OC₂H₅, -OCOCH₃, -OCN, -SCN, -NH₂, -NHCOCH₃, z is 0, 1 , 2, 3 or 4,

wherein said acid absorbs electromagnetic radiation in at least one predetermined spectral subrange within a wavelength range from 400 to 3000 nm, and aqueous solution of its water- soluble salt is capable of forming a photoelectric layer of rodlike supramolecules on a substrate. According to one aspect, the present invention provides an organic solar cell comprising: a first electrode and a second electrode spaced from each other; an organic ionic-crystalline photoelectric layer situated between the first electrode and the second electrode and contacting with the first electrode; and an electrolyte which is situated between the organic ionic-crystalline photoelectric layer and the second electrode and impregnates the organic ionic-crystalline photoelectric layer.

The organic ionic-crystalline photoelectric layer is capable of absorbing electromagnetic radiation in at least one predetermined spectral subrange within a wavelength range from 400 to 3000 nm. This layer consists of rodlike supramolecules, which are comprised of the molecules of at least one organic compound of the general structural formula

where Het is a planar conjugated heterocyclic system; X is an acid group; m is 1 , 2, 3, 4, 5, 6, 7 or 8; Y is an acid amide group; n is 0, 1 , 2, 3 or 4; R is a substituent selected from the list including - CH3, -C2H5, -NO2, -Cl, -Br, -F, -CF3, -CN, -OH, OCH3, -OC2H5, -OCH3, -OCN, -SCN -NH2, - NHCOCH3, and -CONH2; z is 0, 1 ,2, 3 or 4, K is a counterion selected from the list comprising H⁺, NH⁺ ₄, Na⁺, K⁺, Li⁺, Ba⁺⁺, Ca⁺⁺, Mg⁺⁺, Sr⁺⁺, Zn⁺⁺; and p is the number of counterions providing neutral state of the molecule.

The present invention provides an organic solar cell based on a new organic ionic- crystalline photoelectric layer with a resistivity approximately equal to the resistivity of crystalline or amorphous silicon (see Figure 2 and Figure 3). Therefore, the disclosed solar cell may contain a thick photoelectric ionic-crystalline layer 13 having at least one thousand monolayers. The use of thick photosensitive layers considerably increases the fraction of absorbed incident light at an insignificant increase in the serial resistance of the cell. Thus, the disclosed organic solar cell employs the ionic-crystalline layer as an active photosensitive element. The organic ionic- crystalline photoelectric layer is situated between the first electrode 5 and the second electrode 6 and contacts with the first electrode and the electrolyte. The organic ionic-crystalline photoelectric layer 13 consists of rodlike supramolecules 14, which are oriented both predominantly

perpendicular (as shown in Figure 2) and predominantly parallel (as shown in Figure 3) to the surface of the first electrode. The rodlike supramolecules are good conductors for charge carriers (electrons or holes). The organic ionic-crystalline photoelectric layer is impregnated with electrolyte 9 containing redox couples. This electrolyte provides electric contact with the second electrode 6.

The disclosed organic solar cell operates as follows. The thick organic ionic-crystalline photoelectric layer, which is made of disclosed organic dye compound, is formed on the surface of the first electrode. Photoexcitation of the dye results in the injection and passage of electrons or holes along the rodlike supramolecules and their subsequent injection into the first electrode. The initial state of the dye is subsequently restored by electron (or hole) donation from the electrolyte that permeates into the organic ionic-crystalline photoelectric layer. The electrolyte is usually an organic solvent containing a redox system such as the iodide/triiodide couple. Regeneration of the organic dye compound by iodide eliminates recapture of the conduction band electrons by the oxidized dye. The iodide species are regenerated, in turn, by the reduction of triiodide at the second electrode, the circuit being closed due to electron migration via an external chain with load.

In one embodiment of the organic solar cell, the organic ionic-crystalline photoelectric layer is made of a conducting organic compound. In another embodiment of the organic solar cell, at least one of said electrodes is transparent. In one variant of the disclosed solar cell, at least one acid group is carboxy group (COOH). In another variant of the disclosed solar cell, at least one acid group is sulfonic group (SO3H). In still another variant of the disclosed solar cell, at least one acid amide group is an amide of carboxylic acid (CONH2). In a possible variant of the disclosed solar cell, at least one acid amide group is an amide of sulfonic acid (SO2NH2). In a possible embodiment of the disclosed solar cell, the organic compound comprises carboxylic group COOH and group of carboxylic acid amide CONH2 and the rodlike supramolecules are oriented predominantly perpendicularly to the surface of the first electrode. In one variant of the disclosed solar cell, the organic compound comprises sulfonic group SO3H and the rodlike supramolecules are oriented predominantly parallel to the surface of the first electrode.

In one possible variant of the disclosed organic solar cell, the planar conjugated heterocyclic system is a vat dye comprising anthraquinone fragments. Table 1 shows some examples of vat dyes comprising anthraquinone fragments of the general structural formula corresponding to structures 1-11.

Table 1. Examples of vat dyes comprising anthraquinone fragments

In one variant of the disclosed invention, the planar conjugated heterocyclic system is a vat dye comprising perylene fragments. Table 2 shows some examples of vat dyes comprising perylene fragments of the general structural formula corresponding to structures 12-41

Table 2. Examples of vat dyes comprising perylene fragments

In another variant of the disclosed invention, the planar conjugated heterocyclic system is a vat dye comprising anthanthrone fragments. Table 3 shows some examples of vat dyes comprising such anthanthrone fragments of the general structural formula corresponding to structures 42 and 43.

Table 3. Examples of vat dyes comprising anthanthrone fragments

(42)

In another variant of the disclosed invention, the planar conjugated heterocyclic system is a dye comprising quinoxaline fragments. Table 4 shows some examples of dyes comprising such quinoxaline fragments of the general structural formula corresponding to structures 44-55.

Table 4. Examples of dyes comprising planar conjugated quinoxaline fragments

In still another variant of the disclosed invention, the planar conjugated heterocyclic system comprises a dioxazine fragment. Table 5 shows some examples of dyes comprising such dioxazine fragments of the general structural formula corresponding to structures 56-57.

Table 5. Examples of dyes comprising planar conjugated dioxazine fragments

In still another variant of the disclosed invention, the planar conjugated heterocyclic system comprises a quinacridone fragment. Table 6 shows some examples of dyes comprising such quinacridone fragments of the general structural formula corresponding to structures 58-59. Table 6. Examples of dyes comprising planar conjugated quinacridone fragments

In still another variant of the disclosed invention, the planar conjugated heterocyclic system comprises naphthoylenebenzimidazole fragments. Table 7 shows some examples of such systems of the general structural formula corresponding to structures 60-61. Table 7. Examples of dyes comprising planar conjugated

naphthoylenebenzimidazole fragments

In still another variant of the disclosed invention, the planar conjugated heterocyclic system comprises phthalocyanine of the general structural formula 62:

where M is Cu, Zn, Fe, Co, Mn, Al, or a vacancy. In one variant of the disclosed organic solar cell, the organic ionic-crystalline photoelectric layer is substantially insoluble in the electrolyte. In some embodiments of the disclosed organic solar cell, the first electrode comprises a transparent substrate and a conducting film that is formed on the surface of the transparent substrate and is in contact with the organic ionic-crystalline photoelectric layer. In a possible variant of the disclosed organic solar cell, the transparent substrate is a polymer film. In another embodiment of the organic solar cell, said polymer film is made of a material selected from the group comprising poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), polycarbonate (PC), polypropylene (PP), polyimide (Pl), and triacetate cellulose (TAC). In one possible variant of the organic solar cell, said conducting film is made of a material selected from the group comprising indium tin oxide (ITO), fluorine tin oxide (FTO), ZnO-Ga₂O₃, ZnO-AI₂O₃, and SnO₂-Sb₂O₃. The second electrode transfers electrons arriving from the external chain back to the redox electrolyte. This electrode also carries the photocurrent over the width of each solar cell. Hence, the second electrode must be well conducting and possessing low overvoltage for the reduction of redox couples. Accordingly, in another variant of the organic solar cell, the second electrode comprises a transparent substrate and a two-layer conducting film, with the first and second conducting layers formed on the inner surface of the substrate facing electrolyte. In still another variant of the organic solar cell, the transparent substrate of the second electrode is a polymer film. In yet another variant of the organic solar cell, said polymer film is made of a material selected from the group comprising polyethylene terephthalate (PET), polyethylene naphthalate) (PEN), polycarbonate (PC), polypropylene (PP), polyimide (Pl), and triacetate cellulose (TAC). In one variant of the disclosed organic solar cell, the first conducting layer of the two-layer conducting film is made of a material selected from the group comprising indium tin oxide (ITO), fluorine tin oxide (FTO), ZnO-Ga₂O₃, ZnO-AI₂O₃ and SnO₂- Sb₂O₃. In another possible variant of the organic solar cell, the second conducting layer of the two- layer conducting film is made of a precious metal. In another variant of the disclosed organic solar cell, the second conducting layer is made of a porous material. Any convenient conducting material can be used in the second conducting layer. The use of a porous conducting layer substantially increases conversion efficiency of a solar cell due to extension of the effective interaction area of said porous conducting layer with the electrolyte.

In another embodiment of the disclosed invention, the organic solar cell further comprises an anti-reflection film formed on the surface of the first electrode opposite to the surface facing the second electrode. In still another variant of the disclosed invention, the organic solar cell further comprises an ultraviolet absorbing film formed on the surface of the first electrode opposite to the surface facing the second electrode. In a possible variant of the disclosed organic solar cell, the ultraviolet absorbing film is made of a polymer. In a possible embodiment of the disclosed organic solar cell, the organic ionic-crystalline photoelectric layer comprises light-scattering particles. In another embodiment of the organic solar cell, the surface of the second electrode facing the first electrode is further coated with a thin layer of an electrocatalyst facing the electrolyte. In one embodiment of the organic solar cell, the electrocatalyst is platinum. Platinum is the preferred material for the use as electro-catalyst, since it is an excellent catalyst for triiodide reduction.

The liquid electrolyte leakage and the possible corrosion of the platinum second electrode as a result of its interaction with the triiodide/iodide couple may be critical factors limiting the long- term performance of a dye-sensitized solar cell, especially at elevated temperatures. For this reason, in some embodiments of the disclosed invention, the liquid electrolyte is replaced by a polymer gel electrolyte. In one possible variant of the organic solar cell, said electrolyte is a gel electrolyte containing a redox system. In another variant of the organic solar cell, said gel electrolyte is made of 3-methoxypropionitrile (MPN)-based liquid electrolyte solidified by poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) copolymer. In still another variant of the solar cell, said electrolyte is a liquid electrolyte selected from the group of electrolytes comprising the redox couple of cerium(lll) sulfate and cerium(IV), redox couple of iodide (Q and triiodide (I₃ ^"), redox couple of sodium bromide and bromine, and redox couple of lithium iodide and iodine in solution in one or more solvents selected from the group including water, N-methyloxazolidinone, nitromethane, propylene carbonate, ethylene carbonate, butyrolactone, dimethyl imidazolidine, N- methylpyrrolidine, and mixtures of said solvents. In one embodiment of the disclosed invention, the organic solar cell further comprises an insulating porous layer situated between the organic ionic- crystalline photoelectric layer and the second electrode, wherein the electrolyte fills the pores of said insulating porous layer. Said insulating porous layer is required to prevent short circuit between the organic ionic-crystalline photoelectric layer and the second electrode. The insulating layer may, at the same time, act as a diffuse reflector that reflects light that has not yet been absorbed back into the organic ionic-crystalline photoelectric layer.

The present invention provides an inexpensive and relatively simple fabrication technology of organic ionic-crystalline photoelectric layers. In order that the invention may be more readily understood, reference is made to the following examples, which are intended to be illustrative of the invention, but are not intended to be limiting in scope.

Example 1

This example describes the preparation of an organic ionic-crystalline photoelectric layer from a solution of 5a,6,13,13a-tetrahydroquinoxalino[2,3-b]phenazine-2 carboxylic acid (carboxylic acid of base structure 48 in Table 4)

A. Synthesis of 5a,6,13,13a-tetrahydroquinoxalino[2,3-b]phenazine-2 carboxylic acid

Synthesis is performed via condensation of 2,5-dihydroxy-p-benzoquinone with o- phenylenediamine and 3,4-diaminobenzoic acid. A mixture of 1.40 g (0.01 mole) of 2,5-dihydroxy-p- benzoquinone and 1.08 g (0.01 mole) of o-phenylenediamine in 50 ml of acetic acid is stirred on boiling for 30 min. Then 1.52 g (0.01 mole) of 3,4-diaminobenzoic acid is added to the boiling mixture during 16 hours. The precipitate of 5a,6,13,13a-tetrahydroquinoxalino[2,3-b]phenazine-2 carboxylic acid is separated by filtration and washed with acetic acid. The isolated product is purified by crystallization from a mixture of acetic acid and DMF. The final product yield is 2.54 g.

B. Preparation of a photoelectric layer

A solution of 1.0 g of 5a,6,13,13a-tetrahydroquinoxalino[2,3-b]phenazine-2 carboxylic acid in 35.0 g of deionized water is stirred for 5 min at a temperature of 2O⁰C. Then, 1.3 ml of a 10% aqueous ammonia solution is added and the mixture is stirred until complete dissolution. An appropriate substrate is sequentially washed with surfactants and with deionized water and then dried with airflow from a compressor. The pretreated substrate is coated with the above solution using Mayer rod #2.5 moved at a linear rate of 15 mm/s, a temperature of 20⁰C, and a relative humidity of 65%.

Finally, the film is dried at the same humidity and temperature. The duration, humidity, and temperature of drying are selected so as to ensure 95% removal of solvent from the solution while retaining the porous structure of the dye-sensitizer layer after termination of the drying. The obtained film is characterized by measuring the specific resistance in the direction of rodlike supramolecules, which are approximately perpendicular to the substrate surface. The measured value of said specific resistance is in the range from 190 to 200 ohm-cm.

Example 2

The example describes the preparation of an organic ionic-crystalline photoelectric layer, which is similar to the procedure described in Example 1 B. In contrast to Example 1 , the drying stage is replaced by an annealing. A solution of 1.0 g of 5a,6,13,13a-tetrahydroquinoxalino[2,3- b]phenazine-2 carboxylic acid in 35.0 g of deionized water is stirred for 5 min at a temperature of 20⁰C. Then, 1.3 ml of a 10% aqueous ammonia solution is added and the mixture is stirred until complete dissolution. An appropriate substrate is sequentially washed with surfactants and with deionized water and then dried with airflow from a compressor. The pretreated substrate is coated with the above solution using Mayer rod #2.5 moved at a linear rate of 15 mm/s, a temperature of 20⁰C, and a relative humidity of 65%.

Finally, the film is annealed at a temperature up to 370 ⁰C. The duration, humidity, and temperature of annealing are selected so as to ensure 95% removal of carboxylic groups from the organic ionic-crystalline photoelectric layer, while retaining the porous structure after termination of the annealing. The obtained film is characterized by measuring the specific resistance in the direction of rodlike supramolecules, which are approximately perpendicular to the substrate surface. The measured value of said specific resistance is in the range from 190 to 200 ohm-cm.

Claims

W 22 CLAIMS What is claimed is

1. An organic acid of general structure

where Het is a planar conjugated heterocyclic molecular system, X is an acid group,

m is 1, 2, 3, 4, 5, 6, 7 or 8,

Y is an amide of acid group,

n is 1 , 2, 3 or 4,

R is a substituent selected from the list comprising -CH₃, -C₂H₅, -NO₂, -Cl, -Br, -F, - CF₃, -CN, -OH, -OCH₃, -OC₂H₅, -OCOCH₃, -OCN, -SCN, -NH₂, -NHCOCH₃, z is 0, 1 , 2, 3 or 4,

wherein said acid absorbs electromagnetic radiation in at least one predetermined spectral subrange within a wavelength range from 400 to 3000 nm, and

aqueous solution of its water-soluble salt is capable of forming a photoelectric layer of rodlike supramolecules on a substrate.

2. The organic acid according to Claim 1 , wherein at least one acid group is carboxylic group

COOH.

3. The organic acid according to Claim 1 , wherein at least one acid group is sulfonic group

SO₃H.

4. The organic acid according to any of Claims 1 to 3, wherein at least one amide group is an amide of carboxylic acid CONH₂.

5. The organic acid according to any of Claims 1 to 3, wherein at least one amide group is an amide of sulfonic acid SO₂NH₂.

6. An organic solar cell comprising:

a first electrode and a second electrode spaced from each other,

an organic ionic-crystalline photoelectric layer located between the first electrode and the second electrode and contacting with the first electrode, and

an electrolyte located between the organic ionic-crystalline photoelectric layer and the second electrode and impregnating the organic ionic-crystalline photoelectric layer, wherein the organic ionic-crystalline photoelectric layer absorbs electromagnetic radiation in at least one predetermined spectral subrange within a wavelength range from 400 to 3000 nm and consists of rodlike supramolecules, which comprise at least one organic compound of the general structural formula Il

where Het is a planar conjugated heterocyclic molecular system,

X is an acid group,

m is 1 , 2, 3, 4, 5, 6, 7 or 8,

Y is an amide of acid group,

n is O, 1 , 2, 3 or 4,

R is a substituent selected from the list comprising -CH₃, -C₂H₅, -NO₂, -Ci, -Br, -F, - CF₃, -CN, -OH, -OCH₃, -OC₂H₅, -OCOCH₃, -OCN, -SCN, -NH₂, -NHCOCH₃, z is O, 1 , 2, 3 or 4,

K is a counterion selected from the list comprising H⁺, NH⁺ ₄, Na⁺, K⁺, Li⁺, Ba⁺⁺, Ca⁺⁺, Mg⁺⁺, Sr⁺⁺, Zn⁺⁺, and

p is the number of counterions providing neutral state of the molecule.

7. The solar cell according to Claim 6, wherein the organic ionic-crystalline photoelectric layer is made of a conducting organic compound.

8. The solar cell according to any of Claims 6 to 7, wherein at least one of said electrodes is transparent.

9. The solar cell according to any of Claims 6 to 8, wherein at least one acid group is

carboxylic group COOH.

10. The solar cell according to any of Claims 6 to 8, wherein at least one acid group is sulfonic group SO₃H.

11. The solar cell according to any of Claims 6 to 8, wherein at least one acid amide group is an amide of carboxylic acid CONH₂.

12. The solar cell according to any of Claims 6 to 8, wherein at least one acid amide group is an amide of sulfonic acid SO₂NH₂.

13. The solar cell according to Claim 9, wherein at least one amide group is an amide of

carboxylic acid CONH_2ι and the rodlike supramolecules are oriented predominantly perpendicular to the surface of the first electrode.

14. The solar cell according to Claim 10, wherein the rodlike supramolecules are oriented predominantly parallel to the surface of the first electrode.

15. The solar cell according to any of Claims 1 to 14, wherein said planar conjugated

heterocyclic molecular system is a vat dye comprising anthraquinone fragments.

16. The solar cell according to Claim 15, wherein said vat dye has the general structural formula from the group comprising structures 1-11 : )

17. The solar cell according to any of Claims 1 to 14, wherein said planar conjugated heterocyclic system is a vat dye comprising perylene fragments.

18. The solar cell according to Claim 17, wherein said vat dye has the general structural formula from the group comprising structures 12-41 :

19. The solar cell according to any of Claims 1 to 14, wherein said planar conjugated heterocyclic system is a vat dye comprising anthanthrone fragments.

20. The solar cell according to Claim 19, wherein said vat dye has the general structural formula from the group comprising structures 42-43:

21. The solar cell according to any of Claims 1 to 14, wherein said planar conjugated heterocyclic system comprises a quinoxaline fragment.

22. The solar cell according to Claim 21 , wherein said said planar conjugated heterocyclic system has the general structural formula from the group comprising structures 44-55:

23. The solar ceil according to any of Claims 1 to 14, wherein said planar conjugated

heterocyclic system comprises a dioxazine fragment.

24. The solar cell according to Claim 23, wherein said planar conjugated heterocyclic system has the general structural formula from the group comprising structures 56-57:

25. The solar cell according to any of Claims 1 to 13, wherein said planar conjugated

heterocyclic system comprises a quinacridone fragment.

26. The solar cell according to Claim 24, wherein said planar conjugated heterocyclic system has the general structural formula from the group comprising structures 58-59:

27. The solar cell according to any of Claims 1 to 14, wherein said planar conjugated heterocyclic system is naphthoylenebenzimidazole of the general structural formula from the group comprising structures 60-61 :

28. The solar cell according to any of Claims 1 to 14, wherein said planar conjugated

heterocyclic system comprises phthalocyanine of the general structural formula 62:

where M is Cu, Zn, Fe, Co, Mn, Al, or vacancy.

29. The solar cell according to any of Claims 1 to 28, wherein said organic ionic-crystalline photoelectric layer is substantially insoluble in the electrolyte.

30. The solar cell according to any of Claims 1 to 29, wherein the first electrode comprises a transparent substrate and a conducting film formed on the surface of the transparent substrate in contact with the organic ionic-crystalline photoelectric layer.

31. The solar cell according to Claim 30, wherein the transparent substrate is a polymer film.

32. The solar cell according to Claim 31, wherein the polymer film is made of a material selected from the group comprising polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polypropylene (PP), polyimide (Pl), and triacetate cellulose (TAC).

33. The solar cell according to any of Claims 29 to 31 , wherein the conducting film is made of a material selected from the group comprising indium tin oxide (ITO), fluorine tin oxide

(FTO), ZnO-Ga₂O₃, ZnO-AI₂O₃, and SnO₂-Sb₂O₃.

34. The solar cell according to any of Claims 1 to 33, wherein the second electrode comprises a transparent substrate and a two-layer conducting film comprising the first and second conducting layers formed on the substrate surface facing the electrolyte.

35. The solar cell according to Claim 34, wherein the transparent substrate is a polymer film.

36. The solar cell according to Claim 35, wherein the polymer film is made of a material selected from the group comprising polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polypropylene (PP), polyimide (Pl), and triacetate cellulose (TAG).

37. The solar cell according to any of Claims 34 to 36, wherein the first conducting layer is made of a material selected from the group comprising indium tin oxide (ITO), fluorine tin oxide (FTO), ZnO-Ga₂O₃, ZnO-AI₂O₃ and SnO₂-Sb₂O₃.

38. The solar cell according to any of Claims 34 to 37, wherein the second conducting layer is made of a precious metal.

39. The solar cell according to any of Claims 35 to 38, wherein the second conducting layer is made of a porous material.

40. The solar cell according to any of Claims 1 to 39, further comprising an antireflection film formed on a surface of the first electrode facing to the surface facing the second electrode.

41. The solar cell according to any of Claims 1 to 39, further comprising an ultraviolet

absorbing film formed on the surface of the first electrode opposite to the surface facing the second electrode.

42. The solar cell according to Claim 41 , wherein the ultraviolet absorbing film made of a

polymer.

43. The solar cell according to any of Claims 1 to 42, wherein the organic ionic-crystalline photoelectric layer comprises light-scattering particles.

44. The solar cell according to any of Claims 1 to 43, wherein a surface of the second

electrode facing the first electrode is further coated with a thin layer of an electrocatalyst facing the electrolyte.

45. The solar cell according to Claim 44, wherein the electrocatalyst is platinum.

46. The solar cell according to any of Claims 1 to 45, wherein said electrolyte is a gel

electrolyte containing a redox couple.

47. The solar cell according to Claim 46, wherein said gel electrolyte is made of 3- methoxypropionitrile (MPN)-based liquid electrolyte solidified by poly(vinylidenefluoride- hexafluoropropylene (PVDF-HFP) copolymer.

48. The solar cell according to any of Claims 1 to 45, wherein the electrolyte is a liquid

electrolyte selected from the group of electrolytes comprising the a redox couple of cerium(lll) sulfate and cerium(IV), redox couple of iodide (Qand triiodide (I₃ ^"), redox couple of sodium bromide and bromine, and redox couple of lithium iodide and iodine in solution in one or more solvents selected from the group including water, N- methyloxazolidinone, nitromethane, propylene carbonate, ethylene carbonate, butyrolactone, dimethyl imidazolidine, N-methylpyrrolidine, and a mixture of said solvents.

49. The solar cell according to any of Claims 1 to 48, further comprising an insulating porous layer situated between the organic ionic-crystalline photoelectric layer and the second electrode, wherein the electrolyte fills the pores of said insulating porous layer.