GB2429837A

GB2429837A - Organic photovoltaic device comprising polycrystalline discotic liquid crystal

Info

Publication number: GB2429837A
Application number: GB0515235A
Authority: GB
Inventors: Paval Ivan Lazarev
Original assignee: Cryscade Solar Ltd; KONTRAKT TECHNOLOGY Ltd
Current assignee: Cryscade Solar Ltd
Priority date: 2005-07-25
Filing date: 2005-07-25
Publication date: 2007-03-07
Also published as: WO2007012835A1; GB2429837A8; GB0515235D0

Abstract

An organic photovoltaic device comprises an anode, a cathode and at least one photovoltaic organic layer. At least one of the anode and cathode are transparent. The photovoltaic organic layer has polycrystalline structure comprising isotropically oriented crystallites. The crystallites contain rod-like molecular aggregates which are composed of planar discotic aromatic cores with intermolecular spacing between cores of 3.4 Ò 0.3Ñ. The organic aggregates are oriented along one optic axis of the crystallites and may be surrounded by an electrically conducting medium of ionic type conductivity. The photovoltaic organic layer is formed by a microdomain crystallization method. The method comprises chemically modifying an organic compound to attach ionogenic groups over the periphery of the discotic aromatic cores, forming a colloidal solution comprising isotropically oriented rod-like aggregates of the discotic cores at a concentration below the lyotropic liquid crystal threshold, applying the colloidal solution to a substrate and drying the colloidal solution.

Description

1 2429837 ORGANIC PHOTOVOLTAIC LAYER, ORGANIC PHOTOVOLTAIC DEVICE AND

METHOD OF

MANUFACTURING THEREOF

The present invention relates generally to photovoltaic devices intended for transforming light and, specifically, for converting solar energy into electric energy.

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 I called short-circuit current (lsc), is produced. When actually used to generate power, a photovoltaic device is connected to a finite resistive load and the output power is given by 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 the 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, if, defined as if = (lmaxVmax)/(IscVoc), where if is always less than unity, as Isc and Voc are never obtained simultaneously in real practice. Nevertheless, as if approaches unity, the device is more eificient.

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 these 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 allowed reaching relatively high conversion efficiencies, on a level of 12-15%. The conversion efficiency of a particular photovoltaic device significantly depends 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 part of electrons to return to the valence band of the semiconductor or to localize on 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.

Only when the amount and influence of the aforementioned defects are small, the electron-hole interactions proceed by mechanism of the socalled radiative recombination.

Possessing a sufficiently large characteristic time, the radiative recombination belongs to "slow" processes. Thus, in the absence of defects, the process of the radiative recombination offers the only channel for decay of the electron-hole pairs. This process involving no local energy levels, the radiative recombination can proceed directly from conduction to valence band. As a result, a high efficiency of converting the solar energy into electricity is an 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 very expensive.

Manufacturing of these devices requires complicated technologies involving high-cost equipment and sophisticated processing methods, which are only capable of providing semiconductor layers and multilayer structures of large area and free of defects.

There were numerous attempts at reducing the cost of production of photosensitive optoelectronic devices, including solar cells. Organic photoconductors and organic semiconductors were 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.

Now we will briefly consider the physical principles underlying operation of photovoltaic devices based on organic semiconductors and define the main terms used in what follows.

When electromagnetic radiation of an appropriate energy is incident upon a semiconducting organic material, for example, an organic molecular crystal, a photon can be absorbed to produce an excited molecular state. This is represented symbolically as SO + hv SO*, where SO and SO* denote ground and excited molecular states, respectively. This energy absorption is associated with the transition of an electron from a bound state in the highest occupied molecular orbital (HOMO), which may be a it bond, to the lowest unoccupied molecular orbital (LUMO), which may be a it bond, or equivalently, the transition of a hole from the LUMO to the HOMO. In organic photoconductor layers, the generated molecular state is generally believed to be an exciton. Exciton is an elementary electrically neutral excitation possessing a quasiparticle character in semiconductors. In organic semiconductors, excitons appears upon the formation of electronhole pairs following the HOMO-LUMO transition. If the photoexcitation energy is smaller than the HOMO-LUMO energy difference, the electron and hole cannot independently move in the semiconductor material and occur in the bound state, representing an electrically neutral quasiparticle (exciton). Travelling in a semiconductor material, excitons can transfer the energy. The excitons can exist for an appreciable time (lifetime) before exhibiting geminate recombination, which refers to the process of the original electron and hole recombination with each other, as opposed to recombination with holes or electrons from other pairs. Thus, the process of photon absorption in organic semiconductors leads to the creation of bound electron-hole pairs (excitons). The excitons can diffuse toward the so-called dissociation centers, where the positive and negative charges can separate. Such dissociation can be realized, for example, at a boundary (interface) of two organic materials, provided that one of these materials has a greater electron affinity (EA) and the other possesses a lower ionization potential (IP). The material of higher EA can accept electrons from the conduction band of the other material and is called electron acceptor. The material possessing a lower ionization potential can accept holes from the valence band of the organic semiconductor in contact, the former material is called the hole acceptor or the electron donor, because it can also donate electrons to an adjacent acceptor. It should be noted that a difference between IP and EA must be sufficiently large so as to overcome the energy of exciton binding (the latter is typically around 0.4 eV). Otherwise excitons do not dissociate (the bound electron-hole pairs do not separate into free charge carriers) and such bound charges eventually recombine at the interface between donor and acceptor materials. Being separated, the charges move toward the corresponding electrodes of the photovoltaic device: holes drifting to the anode and electrons-to the cathode, thus creating the electric current. Therefore, in contrast to inorganic semiconductors, where mobile charge carriers are formed directly upon the absorption of light, the mobile charge carriers in the molecular (organic) semiconductors such as porphyrins, perylenes, and phthalocyanines appear as a result of the decomposition of excitons formed upon light absorption.

The electron-hole pair representing an exciton can be separated in the region of an internal electric field generated in the semiconductor material. To produce such internally generated electric fields occupying a substantial volume, the usual method is to juxtapose two layers of materials with appropriately selected conduction properties, especially with respect to their distribution of molecular quantum energy states. The interface of these two materials is called a photovoltaic heterojunction. In traditional semiconductor theory, materials for forming photovoltaic heterojunctions have been denoted as generally being of either n (donor) or p (acceptor) type. Here, n-type denotes that the majority carrier type is electron. This could be viewed as the type of materials having many electrons in relatively free energy states. The p-type indicates that the majority carrier type is a hole. Such materials have many holes in relatively free energy states. The type of the background (that is, not photogenerated) majority carrier and their concentration depend primarily on the unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the Fermi energy, or the Fermi level position, within the gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) , called the LUMO-HOMO gap. The Fermi energy characterizes the statistical occupation of molecular quantum energy states, representing the value of energy for which the probability of occupation is equal to 0. 5. The Fermi level position near the LUMO energy indicates that electrons are the predominant carrier type. The Fermi energy being close to the HOMO energy indicates that holes are the predominant carriers.

There are the so-called self-assembling solar cells based on a mixture of a crystalline dye and a liquid crystal material. The mixture is capable of self-organizing with the formation of a photovoltaic cell characterized by high photovoltaic conversion efficiency. The liquid crystal component represents an organic compound belonging to hexabenzocoronenes whose discshaped molecules are capable of forming a liquid crystal phase at room temperature. These molecules are aggregated into columns (stacks) effectively conducting at room temperature. The dye component represents a perylene dye. A solution of two components in chloroform is applied onto a solid substrate by centrifuging. Then the solvent is evaporated to leave the substrate covered by a self-organizing layer in which the perylene dye is crystallized. The interface between two organic materials features the light-induced charge separation. The quantum efficiency of photovoltaic devices implementing such organic heterojunctions reaches 34%, which implies that each 100 absorbed photons yield on the average 34 electron-hole pairs.

There is a known photovoltaic converter based on a MEH-PPV copolymer and a perylene derivative (PPEI) (see J. J. Dittmer et al., Synthetic Metals, Vol. 102, 879-880 (1999)).

In this system, MEH-PPV acts as a hole acceptor and PPEI, as the electron acceptor (hole donor). Excitons photogenerated in the organic semiconductor subsequently decay into free charge carriers (electrons and holes) at the interface between the donor and acceptor components. The introduction of PPEI significantly increases the external quantum efficiency of photovoltaic devices employing this system. The PPEI particles are distributed in the MEH-PPV matrix volume over a distance equal to the exciton diffusion length (-9 nm). In presence of PPEI stimulates charge separation in thin-film MEH-PPV structures.

There is a known photovoltaic cell (Klaus Petritsch, PhD Thesis, "Organic Solar Cell Architectures", Cambridge and Graz, July 2000, Chapter 3, Single Layer Devices, p. 31) based on a Schottky barrier containing the active layer of an organic semiconductor, comprising a rectifying junction with electrode. This organic layer is based on undoped poly(acetylene) and has a thickness approximately equal to the depth of a depleted layer. The electrode contains a thin layer of magnesium known to form a rectifying Schottky barrier in contact with poly(acetylene). The magnesium layer is overcoated with a gold film.

Another known photovoltaic cell (Klaus Petritsch, PhD Thesis, "Organic Solar Cell Architectures", Cambridge and Graz, July 2000, Chapter 4, Double Layer Devices, p. 67) comprises the first layer of an organic electron donor material in contact with the second layer made of an organic electron acceptor material. At least one of these materials is capable of absorbing light in a wavelength range from 350 to 1000 nm and the two materials in contact form a rectifying junction. The cell is provided with electrodes forming ohmic contacts at least with a part of the surface of organic layers. A distinctive feature of said photovoltaic cell is that the organic materials employed contain organic compounds with generally planar polycyclic nuclei.

These compounds are capable of forming a layer structure with a total thickness not exceeding 0.5 micron.

A general disadvantage of the organic materials used in the aforementioned photovoltaic devices consists in the fact that the organic layers of these materials do not have crystalline structure. For this reason, the mobility of electrons and holes in these layers is much lower as compared to that in the same bulk crystalline materials. As a result, electrons and holes do not leave the active region of a semiconductor structure during the exciton lifetime and recombine.

Such electron-hole pairs do not contribute to the photocurrent, and the photovoltaic conversion efficiency decreases. In addition, a decrease in the electron and hole mobility leads to an increase in the resistivity of the material and, hence, in the serial resistance of the photovoltaic device. This implies increase of ohmic losses and additional decrease in the photovoltaic conversion efficiency. Another disadvantage of the aforementioned photovoltaic devices employing organic films without crystalline structure is that these materials are characterized by extremely small diffusion length of photogenerated excitons. This necessitates using photovoltaic structures consisting of very thin layers of thicknesses comparable with the exciton diffusion length, which also decreases both external and internal quantum efficiency of such devices.

Various methods have been developed for manufacturing layers capable of forming the structure of organic photosensitive optoelectronic devices.

There is a known method for the epitaxial growth of layers composed of large anisotropic organic molecules on inorganic substrates. According to this, the deposition process or mass transfer is produced by means of vapor phase epitaxy (VPE) in a vacuum chamber. This VPE technique was successfully used for obtaining layers of organic molecules on graphite, alkali halide, and some other suitable materials [see N. Uyeda, T. Kobayashi, E. Suito, Y. Harada, and M. Watanabe, J. AppI. Phys. 43(12), 5181 (1972); M. Ashida, Bull. Chem. Soc. Jpn. 39(12), 2625.-2631, 2632-2638 (1966); H. Saijo, T. Kobayashi, and N. Uyeda, J. Crystal Growth 40, 118-124 (1977); M. Ashida, N. Uyeda and E. Suito, J. of Crystal Growth 8, 45-56(1971); Y. Murata, J. R. Fryer and T. Baird, J. Microsc., 108(3), 261-275 (1976); J. R. Fryer, Acta Cryst.

A35, 327-332 (1979); M. Ashida, N. Uyeda, and E. Suito, Bull. Chem. Soc. Jpn. 39(12), 2616- 2624 (1966); Y. Saito and M. Shiojiri, J. Crystal Growth 67, 91(1984); and Y. Saito, AppI. Surf.

Sci. 22/23, 574-581 (1985)).

Also known are the methods for epitaxial growth and polymerization of synthetic polymers and biopolymers from solutions, melts, and vapor phase on alkali halide substrates. Some other inorganic materials have been also used as substrates [(see. A. Mcpherson and P.J. Schlichto, J. Cryst. Growth 85, 206 (1988)].

There are several disadvantages inherent in inorganic single crystals, which limit the possibilities of using such crystals as substrates for epitaxial growth. In particular, the number of single crystal materials suited for epitaxial growth is rather restricted because the crystal surface can be reactive, and/or covered with oxides, and/or contain adsorbed water molecules. The substrate can be nontransparent, possess undesired electronic and/or thermal properties, and so on.

There is a known method for the formation of bilayer films involving a substrate, at least one surface of which is partly covered with the first layer (called "seed" layer, which will be referred to below as the alignment layer) of a crystalline, uniaxial oriented organic compound, and also bears the second layer of a crystalline uniaxially oriented organic compound formed above the first layer, whereby the second layer is subjected during its growth to the aligning action of the first layer. The second layer will be referred to below as the epitaxial layer.

A serious disadvantage of said known VPE technology 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. Besides, the requirement of matching between crystal lattices of the substrate and the growing film further restricts the list of compounds suitable for deposition.

One of the major general disadvantages of VPE is a strong influence of defects, present on the initial substrate surface, upon the structure of a deposited layer. The deposition of molecules from the vapor phase enhances and/or decorates defects on the substrate surface.

It is possible to use a method of film deposition, in which the film is grown from a solution.

This method is limited to soluble compounds; however, most of solvents are highly hazardous liquids, which make manufacturing difficult and expensive. Also, the deposition process is hindered in cases of low wetting ability of the substrate surface.

There is a known class of organic photosensitive optoelectronic devices (OPOD5) comprising 3,4,9,1 0-perylenetetracarboxylic-bis-benzim idazole (PTCBI) and copper phthalocyanine (CuPc) as photoconductive organic semiconductor materials (see US Patent No. 6,451,415, Forrest et al., "Organic Photosensitive Optoelectronic Devices with an Exciton Blocking Layer"). In such devices, PTCBI is used for obtaining an electron transport layer and CuPc, a hole transport layer. An OPOD typically comprises a cathode made of a suitable electrode material such as silver, an exciton-blocking layer made of a material such as 2,9dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), an electron transport layer such as PTCBI, a hole transport layer such as CuPc, and an anode made of a suitable electrode material such as ITO. In such a device, at least one of the electrodes must be transparent to allow the penetration of electromagnetic radiation. Exemplary embodiments of OPODs were fabricated on pre-cleaned glass substrates coated with a transparent, 1500-nm-thick conducting indium tin oxide (ITO) anode (with a sheet resistance of 40 cl/sq. Prior to deposition, the organic materials were purified via three cycles of thermal gradient sublimation. The films were grown on ITO by means of organic molecular beam deposition in ultrahigh vacuum (1*10b0 Torr) in the following sequence: 30- to 600-nm-thick films of donor-like copper-phthalocyanine (CuPc) was followed by a 30- to 600-nm-thick films of acceptor-like 3,4,9,1 0-perylenetetracarboxylic bisimidazole (PTCBI). Next, a 100- to 200-nm-thick EBL of bathocuproine (BCP) was deposited. Here, BCP with a 3.5 eV energy gap, has previously been shown to be an effective exciton blocker which can easily transport electrons to the top 800-nm- thick Ag cathode (evaporated at 1*1 0 >Torr, through a shadow mask with 1 mm diameter openings, in a separate vacuum chamber after exposing the organics to the atmosphere) from the adjoining PTCBI layer.

There are known organic quasi-epitaxial optoelectronic devices (see US Patent No. 5,315,129, Forrest et al., "Organic Optoelectronic Devices and Methods"). A quasi-epitaxial optoelectronic device comprises a substrate, the first layer deposited on top of said substrate, and the second layer deposited on top of the first layer. Said first layer comprises a planar crystalline organic aromatic semiconductor selected from a group of organic compounds containing polyacenes and porphyrins or their derivatives. Said second layer also comprises a planar crystalline organic aromatic semiconductor. In the general case, the chemical composition of the second layer is different from that of the first layer, although it is typically also selected from a group of organic compounds containing polyacenes and porphyrins or their derivatives. Both the first and second layers have crystalline structures, which are in a certain relationship with respect to each other. In particular, the first and second layers can be independently selected from a group including 3,4,9, 1 0-perylenetetracarboxylic dianhydride, 3,4,7,8- naphthalenetetracarboxylic dianhydride, copper phthalocyanine, 3,4,9,1 Operylenetetracarboxylic-bis-benzimidazole, and oxadiazole derivatives. Organic optoelectronic devices have been grown by organic molecular beam deposition. The organic substances have been deposited in ultrathin layers only 10 Angstrom (A) thick using organic molecular beam deposition methods. The preferred embodiment of the invention utilizes 3,4,9,1 0-perylenetetracarboxylic dianhydride (PTCDA) and 3,4,7,8naphthalenetetracarboxylic dianhydride (NTCDA). The preferred method describes the use of a chamber, containing 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 I 0 Torr. The substrate is spaced from the source of film materials by a minimum distance of 10cm. During deposition, the substrate is kept at a temperature below 150K, while the PTCDA and NTCDA sources are alternatively heated.

Despite all 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 with respect to instability of the technological parameters can be also considered as a disadvantage of said known method, which is especially significant in the case of deposition of relatively thick (1 to 10 pm) epitaxial layers.

Another disadvantage of said method is the need in sophisticated technological equipment. The reactor chamber must hold ultrahigh vacuum (down to I 0 - I 0 Torr) and must withstand considerable temperature gradients between rather 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 technology 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.

In a first aspect of the present invention there is provided an organic photovoltaic device comprising a first and a second electrode and at least one photovoltaic organic layer having a crystalline or polycrystalline structure with textured or isotropically oriented crystallites.

The or each photovoltaic organic layer may have been produced by means of a microdomain crystallization method as described herein below.

Preferably at least one type of said crystallites contains rod-like molecular aggregates, which are composed of planar discotic aromatic coreswith an intermolecular spacing between aromatic cores of 3.4 0.3 A along one optical axis of the crystallite. Said organic molecular aggregates may represent the channels with electron-hole conductivity. The organic molecular aggregates may be surrounded by the electrically conducting medium with the ionic type of conductivity.

In a second aspect of the present invention there is provided a method for manufacturing photovoltaic organic layers comprising the following steps: (I) chemical modification of an organic compound by introducing one or more ionogenic groups which attach over the periphery of discotic aromatic cores of organic molecules; (ii) the formation of a colloidal solution containing isotropically oriented rod-like molecular aggregates and counterions, wherein the solution concentration is below the threshold concentration for forming a lyotropic liquid crystal; (iii) the application of said isotropic colloidal solution onto a substrate; and (iv) drying said isotropic colloidal solution so as to form a crystalline or polycrystalline layer.

This method is called the microdomain crystallization method.

The proposed method is intended for obtaining a photovoltaic layer of organic anions or cations with corresponding counterion. The method involves chemical modification of organic compound by introducing one or more ionogenic (acidic or alkaline) groups attached over the periphery of discotic aromatic cores of organic molecules. The following step is the formation of a colloidal solution containing isotropically oriented rodlike molecular aggregates and counterions, wherein the solution concentration is below the threshold concentration for forming a lyotropic liquid crystal. At this step, the molecules of the chemically modified organic compound form molecular aggregates with an intermolecular spacing between said aromatic cores of 3.4 0.3 A due to intermolecular interaction. The following steps include the application of said isotropic colloidal solution onto a substrate and drying. The duration, temperature, and humidity of the drying stage are selected so as to ensure the formation of a polycrystalline layer. This layer is composed of isotropically oriented crystallites, which contain said molecular aggregates of organic ionic compounds with the peripheral ionogenic groups and counterions. These molecular aggregates may create conducting channels with an electron-hole type of conduction in each crystallite, while the peripheral ionogenic groups and counterions may create conducting ionic media with the ionic type of conduction in the space between said molecular aggregates.

This invention discloses some types of organic photovoltaic devices, including (1) devices converting electromagnetic radiation into electricity, known as photovoltaic devices including solar cells, (2) photoconductor cells, and (3) photodetectors. These three classes of organic photovoltaic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as bias voltage (or simply bias).

Many organic photovoltaic devices contain the so-called barrier contacts (contacts with rectifying Schottky barriers) near which internal electric fields are generated. Such contacts are alternatively called rectifying junctions. A characteristic feature of the barrier contact is the ability to pass electric current under direct (forward) bias and block the current under reverse bias conditions. Examples of rectifying junctions are offered by the contacts between metals and organic or inorganic semiconductors with Schottky barrier formation, contacts between semiconductors possessing different conductivity types with the formation of a p-n junction, and contacts between organic semiconductors of different types, one being electron acceptor and the other, electron donor, with the formation of a photovoltaic heterojunction.

The rectification effect is related to the formation of an internal (built-in) electric field at the interface between two contacting materials. The internal field occupies a certain region of space in the vicinity of the interface, which is frequently called the space charge region or the active region. The thickness of this region depends on the electrical properties of contacting materials, in particular, on the degree of doping and the mutual arrangement of molecular quantum energy levels (i.e., on the energy band diagram). The internal fields play an important role in the operation of some organic photosensitive optoelectronic devices. As was noted above, the dissociation of photogenerated excitons in organic semiconductors leads to the appearance of free mobile charge carriers, electrons and holes. The built-in electric field drives these electrons and holes in the opposite directions, so that the mobile carriers can more rapidly attain the corresponding electrodes and avoid premature recombination. Thus, the higher the built-in field strength, the stronger the photocurrent in an organic photovoltaic device, the lower the probability of electron-hole recombination, the smaller the leak currents in a photovoltaic device, and the higher the photovoltaic conversion efficiency.

In a preferred embodiment, the present invention provides an organic photovoltaic device comprising the first and second electrodes, and at least one photovoltaic organic layer having the front surface and the rear surface. The photovoltaic organic layer is produced by means of microdomain crystallization method and possesses a crystalline or polycrystalline structure having textured or isotropically oriented crystallites. At least one type of said crystallites contains rod-like molecular aggregates which are composed of planar discotic cores with an intermolecular spacing between aromatic cores of 3.4 0.3 A along one optical axis of the crystallite. Said rod-like organic molecular aggregates may represent the channels with electron-hole conductivity. The organic molecular aggregates may be surrounded by the electrically conducting medium with the ionic type of conductivity. Said organic photovoltaic organic layer is also capable of absorbing electromagnetic radiation in a wavelength range from 200 to 3000 nm. At least one of said electrodes is transparent for the incident electromagnetic radiation, to which the photovoltaic organic layer is sensitive.

The cathode materials (Al, Ca, In, Ag) usually employed in organic photovoltaic devices are characterized by low values of the electron work function, while the anode materials (e.g., Au) are characterized by high values of this parameter. In solar cells and photodiodes, one contact (electrode) has to be at least partially transparent to the incident solar radiation. Semitransparent metal electrodes can be obtained when the metal (e.g., Au) film thickness does not exceed 15 to nm, while nontransparent metal contacts are typically 50 tolOO nm thick. The surface resistance of a thin semitransparent layer is higher than that of a thick (50 to 100 nm) film, which increases the serial resistance of a photovoltaic device and decreases the conversion efficiency.

The optical properties of such contacts vary with thickness in the narrow interval from 10 to 20 nm, so that photovoltaic devices with only slightly different metal contact thicknesses may possess incomparable characteristics.

For the above reasons, transparent electrodes in photovoltaic devices are usually made of the so-called conducting glasses. Most widely used is a tin-doped indium oxide (indium tin oxide, ITO) representing a degenerate semiconductor comprising a mixture of In2O3 (90 %) and SnO2 (10 %) with a bandgap width of 3.7 eV and a Fermi level between 4.5 and 4.9 eV. Because of the large bandgap, ITO does not absorb radiation with a wavelength exceeding 350 nm. This material possesses a high electric conductivity, whereby tin acts as a donor impurity rendering the resistivity very low even for ITO layers with thicknesses on the order of 100 nm. Quartz substrates covered with ITO layers are commercially available because such substrates are widely used as conducting screens in liquid crystal displays. The greater the ITO layer thickness, the lower the resistivity of this film. Then typical ITO layer thickness in organic photovoltaic devices is about 100 nm. Substrates with resistivities below 50 Q/sq. (Please see this unit for example in US Patent No. 6,451,415, column 12, line 7.) are commercially available. The ability to transmit radiation does not vary significantly with the ITO layer thickness, since the material virtually does not absorb light in the visible spectral range. However, interference effects may considerably influence the spectral dependence of the optical transmission coefficient. The use of very thick ITO layers (more than several hundred nanometers thick) is problematic, because increasing surface roughness of such thick films may lead to electric shorts in thin organic films. It should be noted that ITO films can be also used as antireflection coatings. Plasma etching can modify the surface of ITO layers. Transparent electrodes can be also made of some other conducting glasses based on tin and indium oxides.

At least one photovoltaic organic layer of the photovoltaic device is manufactured by means of microdomain crystallization method. Said layer possesses crystalline or polycrystalline structure having textured or isotropically oriented crystallites. At least one type of said crystallites contains rod-like molecular aggregates which are composed of planar discotic aromatic cores with intermolecular spacing between aromatic cores of 3.4 0.3 A along one optical axis of the crystallite. Said organic molecular aggregates may represent the channels with electron-hole conductivity. The organic molecular aggregates may be surrounded by a conducting media with the ionic type of conductivity. The photovoltaic organic layer structure is also characterized by insignificant influence of the substrate surface structure. Such a layer can be formed, if required, on a part of the substrate surface or on the entire surface.

A comparison of the physical principles of operation of the organic photovoltaic devices based on inorganic and organic semiconductors leads to a conclusion that the photovoltaic conversion efficiency is generally much higher for the inorganic semiconductors. The main reason is that the mobile charge carriers (electrons and holes) in inorganic semiconductors are generated directly under the action of absorbed electromagnetic radiation. In contrast, the generation of free charge carriers in the organic semiconductors, as considered above, proceeds in several stages.

The bound electron-hole pairs (excitons) produced in the first stage diffuse toward a photovoltaic heterojunction and dissociate with the formation of mobile electrons and holes. Presence of high dielectric spacing between organic core stacks makes dissociation more probable and re- combination of the electro-hole pair less effective. Thus, given the inherently low carrier generation efficiency in the non-ionic organic semiconductors, an important factor in the organic photovoltaic devices is the possibility to optimize the semiconductor material and device structure so as to provide for the maximum possible efficiency.

In particular, the effective operation of an organic photovoltaic device can be achieved only provided when all photovoltaic organic layers possess optimum thicknesses. On the one hand, it is desired that the photovoltaic layer thickness would be comparable with or smaller than the diffusion length of photogenerated excitons. In this case, excitons would dissociate predominantly near the photovoltaic heterojunction. On the other hand, such a small thickness of the photovoltaic layer decreases the fraction of absorbed electromagnetic radiation incident upon the organic photovoltaic device and, hence, reduces the external quantum efficiency of the device. In order to increase the fraction of absorbed electromagnetic radiation, it is desired that the photovoltaic organic layer thickness would be on the order of the effective radiation absorption length 1/c, where ci is the absorption coefficient. In this case, almost all radiation incident on the device will be absorbed within the photovoltaic organic layer and will therefore contribute to the exciton production. However, as soon as the photovoltaic organic layer thickness will exceed that of the active region, excitons will form with increased probability in the electrically neutral region far from the photovoltaic heterojunction. As a result, because of a small diffusion length of excitons, the electron-hole pairs will recombine before such excitons will diffuse to enter the active region. Thus, the conversion efficiency drops with increase in the photovoltaic organic layer thickness. Another adverse effect of increase in the photovoltaic organic layer thickness consists in the related growth of a serial resistance of the organic photovoltaic device, which leads to an increase in the ohmic losses and a decrease in the conversion efficiency. Taking into account all the aforementioned competitive factors related to the characteristic radiation absorption length, the exciton diffusion length, and the resistivity of the photovoltaic material, one may conclude that there is an optimum photovoltaic organic layer thickness ensuring the maximum possible conversion efficiency of each particular organic photovoltaic device. An important factor in reaching the maximum efficiency is the possibility of exactly reproducing the optimum thicknesses of the photovoltaic organic layer. An important advantage of the use of the disclosed photovoltaic organic layer is 1) control over thickness of the layer during deposition from colloidal solutions, 2) control over crystal structure and its growth by control of drying process, and 3) control over dielectric properties of the crystal by optimization of ion-counterion structure in ionic space of the crystal structure which leads to less leakage in recombination and higher carrier's diffusion rate.

The disclosed organic photovoltaic device comprises at least one organic photovoltaic layer having the front surface, which is facing a light source, and the rear surface facing the opposite direction, and two electrodes. In the general case, the two electrodes will be referred to as the first and second electrodes. In some particular cases, the first electrode, which is located between a light source and the front surface of the organic photovoltaic layer and is made transparent to the electromagnetic radiation in the spectral range to which the given organic photovoltaic layer is sensitive, is called front electrode. By the same token, in some particular cases, wherein the second electrode is located next to the rear surface of an organic photovoltaic layer or a structure containing photovoltaic layers, this electrode is called rear electrode.

One of the embodiments of the disclosed photovoltaic organic device comprises a single photovoltaic organic layer.

In one preferred embodiment, the disclosed organic photovoltaic device comprises the front transparent electrode and the rear electrode located next to the rear surface of said organic photovoltaic layer.

The efficiency of an organic photovoltaic device can be increased by allowing the incident electromagnetic radiation to pass two times through the active photovoltaic organic layers of the device structure. For this purpose, in one embodiment of the invention, the front electrode is made transparent while the rear electrode represents a depolarizing mirror with a reflection coefficient of not less than 95% for the electromagnetic radiation penetrating through the device structure.

In addition, since at least one photovoltaic organic layer of said organic photovoltaic device is locally anisotropically absorbing, the electromagnetic radiation transmitted through this layer in one direction will be locally polarized. Being reflected from the reflective electrode, this locally polarized radiation will not be repeatedly absorbed in the anisotropic layer on the second passage. In order to avoid this, it is necessary to rotate the polarization vector by 9O. Therefore, an additional retarder layer has to be introduced into the organic photovoltaic device according to this embodiment, the thickness and optical anisotropy of which are selected so as to ensure a 45 rotation of the polarization vector of the transmitted radiation.

In a further embodiment, the rear electrode is a reflective electrode for the electromagnetic radiation incident upon the device, and the device further comprises an additional retarder layer located between said rear reflective electrode and the rear surface of said photovoltaic layer, wherein the thickness and optical anisotropy of said retarder layer are selected so as to ensure a 45 rotation of the polarization vector of transmitted electromagnetic radiation.

There is another possible embodiment of the disclosed device, wherein a reflection coefficient of the reflective electrode is not less than 95% for the electromagnetic radiation incident upon the device.

In one embodiment, the front electrode serves as the cathode and the rear electrode serves as the anode. In another embodiment the front electrode serves as the anode and the rear electrode serves the cathode. In a further embodiment of the invention, the organic photovoltaic device further comprises at least one electron transport layer situated between said organic photovoltaic layer and the cathode. According to the disclosed invention, the organic photovoltaic device further comprises at least one exciton-blocking layer situated between said organic photovoltaic layer and the electron transport layer.

Another embodiment is possible, whereby the organic photovoltaic device further comprises at least one hole transport layer situated between said organic photovoltaic layer and the anode. Another variant of embodiment of the organic photovoltaic device further comprises at least one excitonblocking layer situated between said organic photovoltaic layer and the hole transport layer.

In another preferred embodiment, the disclosed invention comprises the first electrode, formed on a part of the front surface of the organic photovoltaic layer, and the second electrode formed on another part of the same front surface of said organic photovoltaic layer, wherein the first electrode serves as the cathode and the second electrode serves as the anode. In one embodiment, an organic photovoltaic device further comprises an additional retarder layer which is formed on the rear surface of said organic photovoltaic layer, and an additional reflective layer which is formed on said retarder layer, wherein the thickness and optical anisotropy of said retarder layer are selected so as to ensure a 45 rotation of the polarization vector of the electromagnetic radiation incident upon the device. The reflection coefficient of the reflective layer is not less than 95% for the electromagnetic radiation incident upon the device.

Further embodiment of said organic photovoltaic device is possible, wherein a rectifying Schottky barrier to the front electrode is formed at least on a part of the front surface of the organic photovoltaic layer and an ohmic contact to the rear electrode is formed at least on a part of the rear surface of the organic photovoltaic layer.

The present invention also provides a device comprising two organic photovoltaic layers, which form a double layer structure having the front surface, which is facing a light source, and the rear surface facing the opposite direction, wherein the first layer is an electron donor layer, the second layer is an electron acceptor layer, and these layers are in contact so as to form a photovoltaic heterojunction. The double layer structure is confined between two electrodes. One electrode is situated between a light source and the front surface of the double layer structure.

This electrode is made transparent and is named a front transparent electrode. The other electrode is located next to the rear surface of the double layer structure and is named a rear electrode. In one embodiment, the rear electrode is a reflective depolarizing electrode for electromagnetic radiation incident upon the device. In another embodiment, the rear electrode is a reflective electrode for electromagnetic radiation incident upon the device, and the device further comprises an additional retarder layer which is located between said reflective electrode and said double layer structure, wherein the thickness and optical anisotropy of said retarder layer are selected so as to ensure a 45 rotation of the polarization vector of the incident electromagnetic radiation. The reflection coefficient of the reflective electrode is not less than 95% for the electromagnetic radiation incident upon the device structure. In one embodiment, the front electrode serves as the cathode and the rear electrode serves as the anode. In another embodiment, the front electrode serves as the anode and the rear electrode serves as the cathode. According to the disclosed invention, the organic photovoltaic device may further comprise at least one electron transport layer situated between said double layer structure and the cathode.

In another preferred embodiment, the disclosed invention provides an organic photovoltaic device further comprising at least one excitonblocking layer situated between said double layer structure and the electron transport layer. In one embodiment, the organic photovoltaic device further comprises at least one hole transport layer situated between said double layer structure and the anode. In another preferred embodiment, the organic photovoltaic device further comprises at least one exciton-blocking layer situated between said double layer structure and the hole transport layer.

In one embodiment the device further comprises a protective transparent layer formed on at least one surface of said device. In another embodiment, the device further comprises an additional antireflection coating formed on at least one surface of said device.

Another aspect of the present invention provides a method for manufacturing photovoltaic organic layers, called microdomain crystallization process. The proposed method involves chemical modification of said organic compound by introducing one or more ionogenic groups attached over the periphery of diskotic aromatic cores of organic molecules. The following step is the formation of a colloidal solution containing isotropically oriented rod-like molecular aggregates and counterions, wherein the solution concentration is below the threshold concentration for forming a lyotropic liquid crystal. At this step, the molecules of the chemically modified organic compound form rod- like molecular aggregates with an intermolecular spacing of 3.4 0.3 A due to intermolecular interaction. The following steps include the application of said isotropic solution onto a substrate and drying. The duration, temperature, and humidity of the drying stage are selected so as to ensure the formation of a crystalline or polycrystalline layer. This layer is composed of isotropically oriented crystallites, which contain said rod-like molecular aggregates of organic ionic compounds with the peripheral ionogenic groups and counterions. These molecular aggregates may create conducting channels with an electron-hole type of conduction in each crystallite, while the peripheral ionogenic groups and counterions may create conducting environment with the ionic type of conduction in the space between said molecular aggregates. In one embodiment of the method, the ionogenic groups are acidic or alkaline groups. In another embodiment of the method, the discotic aromatic cores have a planar form.

Chemical modification of an organic compound by introducing one or more ionogenic groups attached at the periphery of a diskotic organic molecule makes this compound soluble in water and stimulates its self-assembly into supramolecules. Being dissolved in water, such an organic compound forms a colloidal system where molecules tend to aggregate into supramolecules, and these supramolecules represent kinetic units of the colloidal system. The spectral characteristics and rheological properties of such systems are indicative of a strong tendency of diskotic molecules to aggregate (or, in other words, to organize molecular aggregates) even in diluted aqueous solutions with the formation of supramolecules possessing a columnar structure. Columnar structure is specific for flat molecules of elliptical shape grouped in "face-to-face" mode, with hydrophobic molecular planar cores of the aromatic conjugated bond system stacked on each other inside the supramolecular core and the hydrophilic peripheral groups exposed to water. The data of X-ray diffraction confirm the fact that supramolecules actually have a rodlike structure. The formation of supramolecules begins at a low concentration of amphiphilic compounds in water. The electronic absorption spectra of aqueous solutions also show that the aggregation of disk-shaped molecules of said compounds starts even in dilute solutions. The formation of molecular aggregates in systems comprising a modified (sulfonated) polycyclic organic compound and water can be studied by methods of polarization optical microscopy, small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS).

According to the disclosed method, the ionogenic groups are sulfonic, sulfate or sulfite groups or other ionogenic (hydrophilic) groups (e.g., COO-, P04-, etc.) used for imparting amphiphilic properties to the initial organic compounds.

The structure of said organic compound and the characteristics of solutions such as the concentration of organic compounds, the ionic strength, the temperature, and the presence of organic resolvents influence the ability of said modified organic compounds to form aggregates in solution.

Despite the fact that the aforementioned organic compounds significantly differ from each other, they have exhibit similar properties in the course of aggregation into supramolecules. In particular, the aggregation can be enhanced by increasing the concentration of the organic compound or the ionic force; on the other hand, the aggregation can be decreased by increasing the temperature and by adding of organic resolvents. The introduction of ionic diluting groups (for example, sulfonic groups) into the organic compound structure decreases aggregation; on the contrary, the addition of alkyl chains increases the aggregation because of enhancement of the hydrophobic interactions in solution.

A solution with isotropically oriented molecular aggregates can be applied onto a substrate by any method known in the art. Each crystallite contains one or several molecular aggregates, which are oriented approximately in one direction. The orientation of a crystallite is determined by the orientation of molecular aggregates contained in this crystallite. Said molecular aggregates form channels with electron-hole conduction, while the peripheral ionogenic groups form a conducting environment with the ionic type of conduction.

The thickness of a photovoltaic organic layer can be determined, once the content of solid matter in the isotropic solution deposited onto the substrate and the volume of the applied solution are known.

The proposed method of fabrication of a photovoltaic organic layer allows using various materials as a substrate, including metals, semiconductors, dielectrics, polycrystals, glasses, polymers, etc. Moreover, the method allows forming the photovoltaic organic layers on various surfaces, including those of complicated shapes (flat, cylindrical, conical, spherical etc.). This property allows using the disclosed photovoltaic organic layers in most complicated constructions of organic photovoltaic devices.

The surfaces, on which photovoltaic organic layers are deposited, can be subjected to additional treatments for providing their homogeneous wetting(i.e., for rendering them hydrophilic). This can be mechanical treatment, annealing, and mechanochemical treatment.

A more complete assessment of the present invention and its advantages will be readily achieved as the same becomes better understood by reference to the following detailed description, considered in connection with the accompanying drawings and detailed specification, all of which forms a part of the disclosure. The subject of the invention is illustrated by the following Figures, of which: Figure 1 is a schematic diagram of an organic photovoltaic device based on a structure with a single photovoltaic organic layer (single-layer structure) with a Schottky junction and an ohmic contact, which are located on the opposite surfaces of the photovoltaic layer.

Figure 2a presents an energy band diagram of the typical Schottky junction involving a photovoltaic layer of the n-type.

Figure 2b presents an energy band diagram of the typical Schottky junction involving a photovoltaic layer of p-type.

Figure 3a schematically depicts the layer structure of an organic photovoltaic device with a Schottky junction, an n-type photovoltaic layer, an electron transport layer, and an ohmic contact.

Figure 3b schematically depicts a layer structure of organic photovoltaic device with a Schottky junction, a p-type photovoltaic layer, a hole transport layer, and an ohmic contact.

Figure 4 is a schematic diagram of an organic photovoltaic device based on a single-layer structure with a Schottky junction and an ohmic contact, which are located on the same surface of the photovoltaic layer.

Figure 5 schematically shows an organic photovoltaic device based on a single-layer structure with a Schottky junction and an ohmic contact, which are located on the same surface of the photovoltaic layer and form an interdigitated system of barrier and ohmic contacts.

Figure 6a schematically depicts the structure of an organic photovoltaic device based on a single photovoltaic layer with a Schottky junction and an ohmic contact located on the same surface, which also contains a reflective depolarizing layer.

Figure 6b schematically depicts the structure of an organic photovoltaic device based on a single photovoltaic layer with a Schottky junction and an ohmic contact located on the same surface, which also contains a phaseshifting layer (retarder) and a reflective layer.

Figure 7a schematically depicts the structure of an organic photovoltaic device based on a single photovoltaic layer with a Schottky junction, which also contains an exciton-blocking layer and a reflective depolarizing electrode (ohmic contact).

Figure 7b schematically depicts the structure of an organic photovoltaic device based on a single photovoltaic layer with a Schottky junction, which also contains an exciton-blocking layer, a phase-shifting layer (retarder), and a reflective layer.

Figure 8a is a schematic diagram of a double-layer organic photovoltaic device based on contacting electron donor and electron acceptor layers forming a photovoltaic heterojunction.

Figure 8b is an energy band diagram of a double-layer organic photovoltaic device depicted in Figure 8a.

Figure 9a is a schematic diagram of an organic photovoltaic device structure comprising a photovoltaic heterojunction, exciton-blocking layers, a hole transport layer, an electron transport layer, and ohmic contacts.

Figure 9b is an energy band diagram of the organic photovoltaic device shown in Figure 9a.

Figure lOa schematically depicts an organic photovoltaic device structure comprising a conducting layer in ohmic contact with one photovoltaic layer, a photovoltaic heterojunction, and a reflective depolarizing electrode (ohmic contact).

Figure lOb schematically depicts an organic photovoltaic device structure comprising a conducting layer in ohmic contact with one photovoltaic layer, a photovoltaic heterojunction, a phase-shifting layer (retarder) and a reflective layer.

Figure 11 schematically shows a photovoltaic organic polycrystalline layer manufactured by means of microdomain crystallization method disclosed in the present invention (Figure 11 a shows a side view, and Figure 11 b shows a top view).

The general description of the present invention having been made, a further understanding can be obtained by reference to the specific preferred embodiments, which are given herein only for the purpose of illustration and are not intended to limit the scope of the appended claims.

Figure 1 presents a schematic diagram of the disclosed organic photovoltaic device, based on photovoltaic organic layer (I) making a Schottky barrier with the front electrode (2) and an ohmic contact with the rear electrode (3). The entire structure is formed on a substrate (5) and the electrodes are connected to a resistive load (4).

Figure 2a presents a schematic energy band diagram of the typical Schottky junction involving an n-type photovoltaic layer in contact with the electrode (metal or conducting glass). As can be seen, there is an active space charge region of thickness d with a built-in field of strength EIN inside. This internal electric field, directed from ohmic contact to rectifying junction, produces bending of the LUMO and HOMO energy levels as depicted in this figure. Figure 2a also indicates the directions of motion of electrons (.) and holes (o) under the action of the built-in electric field in the case when the device is exposed to electromagnetic radiation and connected to a resistive load. In the device under consideration, based on an n-type photovoltaic layer, the ohmic contact is at the cathode and the rectifying junction (Schottky barrier) is at the anode. One of these electrodes is transparent for the electromagnetic radiation in the spectral range to which the given organic photovoltaic device is sensitive. In the case under consideration, either cathode or anode can be transparent: a transparent anode can represent a thin (10- to 20-nm thick) gold film, while a transparent cathode can be made of various metal-like materials such as ITO, gallium indium tin oxide (GITO), zinc indium tin oxide (ZITO), or a polymeric material such as poly(aniline) (PAN I).

Figure 2b presents a schematic energy band diagram of the typical Schottky junction involving a p-type photovoltaic layer in contact with the electrode (metal or conducting glass). In this case, the internal electric field is directed from rectifying junction to ohmic contact, so that the rectifying junction (Schottky barrier) is at the cathode and the ohmic contact is at the anode.

It can be ascertained that, for a photovoltaic organic layer of the ntype, a metal with high value of the work function (e.g., Au) should be used for formation of a contact with the Schottky barrier, while for a ptype photoconductor, the electrodes should be made of a metal (e.g., Al, Mg, or In) with a low electron work function. In this embodiment, the separation of charges (necessary in any photovoltaic device) is due to the dissociation of excitons in the space charge region at the metal/photoconductor interface. The first electrode must form a barrier contact and the second electrode, an ohmic contact. If the two electrodes are made of the same metal (or metal-like material), both contacts will be ohmic or barrier. In case when both contacts are ohmic, no charge regions featuring a built-in electric field are formed in the organic semiconductor. Such structures do not feature the dissociation of excitons and the separation of bound charges. If both contacts are of the barrier type and no external bias voltage is applied, the organic semiconductor contains two identical space charge regions (one at each electrode) in which the built-in electric fields are equal in magnitude and opposite in direction. In this case, said organic photosensitive optoelectronic device generates equal opposite photocurrents compensating one another. In other words, no photocurrent is developed in the absence of external bias voltage. Therefore, in the general case, the electrodes of said organic photosensitive optoelectronic device should be made of different materials. It is recommended that the charge separation would take place at one electrode, while the other would readily transfer the charge carriers. This can be achieved provided that the latter electrode forms no (or very small) potential barrier for the charge carrier transfer (such contact is characterized by very small resistance and is referred to as ohmic).

In one embodiment, the device further comprises a protective transparent layer formed on at least one external surface of said device. In another embodiment, the device further comprises an antireflection coating formed on the external surface of said device.

Figure 3a schematically depicts the layer structure of an organic photosensitive optoelectronic device implementing an n-type photovoltaic layer (1) forming a Schottky junction with electrode (2). This electrode serves as the anode, while electrode (3) on the opposite surface of the organic photovoltaic layer forms an ohmic contact and serves as the cathode. The electron transport layer (6) situated between the photovoltaic layer (1) and the cathode (3) is made of a material possessing high electron mobility and can also play the role of a planarization layer on an ITO electrode. The multilayer structure of the device is based on substrate (5). The cathode representing a thick ITO film has rather a rough surface and sharp protrusions on this surface can damage (perforate) the photovoltaic layer. This will lead to the formation of numerous microscopic conducting channels and a non-uniform current distribution in the junction, which may result in premature failure of the device. Another negative consequence is a decrease in the shunting resistance and, hence, in the conversion efficiency of the organic photovoltaic device. Thus, use of an electron transport layer favours an increase in the photovoltaic conversion efficiency and in the useful yield of device production.

Figure 3b shows another embodiment of the present invention, which is analogous to that shown in Figure 3a but differs from it in implementing a photovoltaic layer of the p-type. This structure contains a hole transport layer (7) between the photovoltaic layer (1) and the anode (3).

The hole transport layer, made of a material possessing a high hole mobility, favours the hole transfer from the photovoltaic layer to the anode and prevents the organic layer from being damaged by a thick electrode. The multilayer structure of the device is located on substrate (5).

Thus, an organic photovoltaic device according to the disclosed invention may contain layers effectively transferring electric charges (electrons and holes), which can be also active photoconducting layers. The terms electron transport layer and hole transport layer refer to the layers which are analogous to electrodes but differ from them in being intended for transferring mobile charge carriers from one to another layer of the given organic photovoltaic device.

Another embodiment of the present invention, illustrated in Figure 4, is based on a single organic photovoltaic layer (1). At least a part of the first surface of said photovoltaic organic layer contacts with the first electrode (2) to form a rectifying Schottky barrier and at least a part of the same surface is in ohmic contact with the second electrode (3); the photovoltaic organic layer (1) is formed on substrate (5) and the electrodes are connected to a resistive load (4).

Figure 5 shows an exemplary embodiment of the organic photovoltaic device with an interdigitated system of electrodes. This device comprises a photovoltaic layer (1) bearing a barrier (2) and ohmic (3) contacts on the first surface. The photovoltaic layer is formed on a substrate (5) and the electrodes are connected to a resistive load (4).

Figure 6a shows another organic photovoltaic device, wherein the first electrode (2) on a part of the first surface of a single photovoltaic organic layer forms a Schottky junction, the second electrode (3) on the same surface forms an ohmic contact, while an additional reflective depolarizing layer (8) with a reflection coefficient of not less than 95% for the incident radiation is formed on the second surface of said photovoltaic organic layer. The reflective depolarizing layer (8) is a diffuse reflector that depolarizes electromagnetic radiation reflected from this layer. The entire multilayer structure is formed on substrate (5) and the electrodes are connected to a resistive load (4). In this structure, the incident electromagnetic radiation passes two times through the active photovoltaic organic layer of the device structure, thus increasing the efficiency of conversion. Since each crystalline particle (grain) of a photovoltaic organic layer acts as a polarizer of light, then the electromagnetic radiation transmitted through this layer will be locally polarized.

According to the present invention, Figure 6b shows the organic photovoltaic device similar to the device depicted in Figure 6a, except for an additional phase-shifting layer (retarder) (9) situated between a photovoltaic organic layer (1) and the reflective layer (80). This device operates as follows. Unpolarized electromagnetic radiation is incident onto polycrystalline layer (1). Being transmitted through this layer, the radiation becomes locally polarized owing to dichroism of the crystallites of the polycrystalline layer (1). Thus, each surface region of a polycrystalline layer containing at least one crystallite operates similarly to a local polarizer in such a way that a fraction of electromagnetic radiation incident upon this part of the layer will be absorbed by said local polarizers, and the other fraction of radiation, whose polarization is parallel to the transmission axis of said local dichroic polarizers, will pass through this layer without absorption. For increasing the conversion efficiency of the disclosed organic device, it is necessary to pass the transmitted radiation through the photovoltaic organic layer once again in the reverse direction. In order to provide that locally polarized radiation reflected from reflective layer (80) would be absorbed by the photovoltaic organic layer (1), it is necessary to rotate its polarization vector by 90 in each region of the polycrystalline layer. To this end, an additional retarder layer (9) is introduced between the photovoltaic layer (1) and the reflective layer (80).

The thickness and optical anisotropy of this retarder are selected so as to ensure a 45'rotation of the polarization vector of the transmitted radiation. Since the electromagnetic radiation doubly passes through this layer, the resulting polarization rotation amounts to 90'. Thus, the combination of retarder and reflective layer provides for a more complete use of the incident electromagnetic radiation and ensures an increase in the photovoltaic conversion efficiency of the photovoltaic device according to this embodiment.

Figure 7a shows one more variant of embodying of the organic photoresponsive optoelectronic device. This organic photosensitive optoelectronic device comprises an organic photovoltaic layer (1) possessing n-type conductivity, forming a rectifying Schottky barrier with a conducting layer (2) situated on the first side of said photovoltaic layer. An exciton-blocking layer (10) formed on the second side of said photovoltaic layer keeps the photogenerated excitons inside the active region of the device. Here, it is necessary to elucidate the term "exciton-blocking layer". The efficiency of a photovoltaic device can be increased by introducing one or several layers restricting the domain of existence of photogenerated excitons to a region in the vicinity of the photovoltaic heterojunction. Such layers hinder the motion of photogenerated excitons toward electrodes where such bound electron-hole pairs can recombine at the interface between the organic semiconductor and electrode material. Thus, the exciton-blocking layer limits the device volume where exciton diffusion is possible. Therefore, this layer (or layers) acts as a diffusion barrier. It should be noted that the exciton-blocking layer should be sufficiently thick to fill small holes in the adjacent photovoltaic layer and exclude the appearance of microscopic conducting channels (microchannels) that might form in the stage of electrode application. Thus, the exciton- blocking layer provides for an additional protection of a brittle organic photovoltaic layer from being damaged in the course of electrode formation. The ability of blocking excitons is related to the fact that the LUMO-HOMO energy difference in the material of this layer is greater than the bandgap width in the adjacent organic semiconductor layers. This implies an energetic prohibition for excitons to enter the blocking layer. While blocking excitons, this layer must allow the motion of electric charges to electrodes. For these reasons, the blocking layer material has to be selected so as to provide for the passage of charge carriers of the corresponding sign. In particular, the exciton-blocking layer on the cathode side must possess a LUMO level close to (or matched with) that of the adjacent electron transport layer, so that the energy barrier for electrons would be minimum. It must be taken into account that the ability of a material to block excitons is not related to the intrinsic properties such as the LUMO-HOMO energy difference. Apparently, the material will block excitons depending on the relative values of LUMO and HOMO energies in the adjacent layers of the organic photovoltaic materials. Therefore, it is impossible to indicate a priori the class of optimum materials for exciton-blocking layers irrespective of the particular function of such materials in a given photovoltaic device. However, once the organic photovoltaic material for the given device is selected, it is always possible to choose an appropriate exciton-blocking layer material as well. In the preferred embodiment of the disclosed invention, an exciton-blocking layer is situated between an electron acceptor layer and the cathode. A recommended material for this layer is 2,0-dimethyl-4,7-diphenyl-1,10-phenanthroline. This exciton-blocking layer simultaneously performs the function of an electron transport layer facilitating the motion of electrons toward a reflective depolarizing electrode (cathode) (85). The reflective depolarizing electrode is a diffuse reflector that depolarizes electromagnetic radiation reflected from this electrode. This electrode acts as ohmic contact. The reflective depolarizing electrode (85)is necessary to provide that the incident radiation would be doubly transmitted through the device structure, thus increasing the conversion efficiency of the device. A resistive load (4) is connected between the barrier contact (2) and the ohmic contact (85). The whole multilayer structure is based on substrate (5). Since each crystalline particle (grain) of a photovoltaic organic layer is a polarizer of a light, then the electromagnetic radiation transmitted through this layer will be locally polarized.

Figure 7b shows an organic photovoltaic device similar to the device depicted in Figure 7a except for an additional phase-shifting layer (retarder) (9) situated between the exciton- blocking layer (10) and the reflective electrode (80). This device operates as follows. The unpolarized electromagnetic is incident onto a polycrystalline layer (I). Being transmitted through this layer, the radiation becomes locally polarized owing to dichroism of the crystallites of the polycrystalline layer (1). Thus, each surface region of a polycrystalline layer containing at least one crystallite operates similarly to a local polarizer in such a way that a fraction of electromagnetic radiation incident upon this part of the layer will be absorbed by said local polarizers, and the other fraction of radiation, whose polarization is parallel to the transmission axis of said local dichroic polarizers, will pass through this layer without absorption. For increasing the conversion efficiency of the disclosed organic device, it is necessary to pass the transmitted radiation through the photovoltaic organic layer once again in the reverse direction. In order to provide that locally polarized radiation reflected from reflective layer (80) would be absorbed by the photovoltaic organic layer (1), it is necessary to rotate its polarization vector by 900 in each region of the polycrystalline layer. To this end, an additional retarder layer (9) is introduced between the photovoltaic layer (1) and the reflective layer (8) . The thickness and optical anisotropy of this retarder are selected so as to ensure a 45rotation of the polarization vector of the transmitted radiation. Since the electromagnetic radiation doubly passes through this layer, the resulting polarization rotation amounts to 90. Thus, the combination of retarder and reflective layer provides for a more complete use of the incident electromagnetic radiation and ensures an increase in the photovoltaic conversion efficiency of the photovoltaic device according to this embodiment.

Another embodiment of the disclosed organic photovoltaic device schematically depicted in Figure 8a represents a two-layer (bilayer) organic photovoltaic cell in which the dissociation of excitons and the separation of bound charges proceed predominantly at the photovoltaic heterojunction. The built-in electric field is determined by the LUMOHOMO energy difference between two materials forming the heterojunction. This embodiment comprises two contacting organic photovoltaic layers-an electron donor layer (11) and an electron acceptor layer (12)- forming ohmic contacts (3) with the adjacent electrodes. The entire multilayer structure is formed on substrate (5). The energy band diagram of this double-layer organic photovoltaic device is presented in Figure 8b. In this structure, bound electron-hole pairs (excitons 13) are generated by the incident electromagnetic radiation in both the electron donor (D) and acceptor (A) layers, with a photovoltaic heterojunction (14) formed at the interface of these layers. This region features dissociation of excitons with the formation of mobile charge carriers, electrons and holes, moving toward the cathode and anode, respectively, under the action of the built-in electric field. These separated electrons and holes move to the corresponding electrodes in different layers, namely electrons drift from the heterojunction to the cathode via the electron acceptor layer, while holes drift from the heterojunction to the anode via the electron donor layer. This property of a double- layer organic photovoltaic structure reduces probability of the electron- hole recombination, thus increasing the photovoltaic conversion efficiency. Another advantage of the double-layer organic photovoltaic device over the single layer counterpart is the basic possibility of using a wider wavelength range of the incident radiation. To this end, the electron donor and acceptor layers have to be made of materials possessing different absorption bands.

An exemplary embodiment of the organic photovoltaic device schematically shown in Fig. 9a represents a modified variant of the device depicted in Fig. 8a. This modified variant comprises an electron donor layer (11) in contact with an electron acceptor layer (12), this contact representing a photovoltaic heterojunction. Excitons (13) (see and Fig. 9b) can be generated by electromagnetic radiation within both electron and donor layers. Said heterojunction (14) serves as the site where excitons exhibit dissociation to yield electrons and holes moving toward the cathode (17) and the anode (18), respectively, under the action of a built-in electric field. An exciton-blocking layer (16) formed between said electron acceptor layer (12) and the cathode (17) limits the region where photogenerated excitons can occur prior to dissociation, while not hindering the drift of electrons toward the cathode. An additional electron transport layer (6) can be formed between the exciton-blocking layer (16) and the cathode (17). In the same way, another excitonblocking layer (15) formed on the other side of said heterojunction between an electron donor layer (11) and the anode (18) also restricts the region where excitons occur in the vicinity of the heterojunciton, while not hindering the drift of holes toward the anode. An additional hole transport layer (7) can be formed between the exciton-blocking layer (15) and the anode (18). The cathode (17) occurs in ohmic contact with the adjacent electron transport layer, while the anode (18) is in ohmic contact with the adjacent hole transport layer. A resistive load (4) is connected between the cathode (17) and the anode (18). The whole multilayer structure is based on substrate (5).

Figure 9b shows an energy band diagram of the device depicted in Fig. 9a. According to this, bound electron-hole pairs (excitons) can be generated under the action of incident electromagnetic radiation in both electron donor and acceptor layer. The boundary between the electron donor and acceptor layers represents a photovoltaic heterojunction (14). The HOMO and LUMO energy levels of the exciton-blocking layer (16) and the adjacent electron acceptor layer (11) are mutually arranged so as to provide for (i) exciton-blocking and (ii) electron passage to the cathode. The photogenerated excitons are blocked because the HOMO-LUMO energy difference in the exciton-blocking layer (16) is greater than the corresponding energy difference in the electron acceptor layer (12). Thus, for energetic reasons, excitons generated in the electron acceptor layer (12) cannot enter the exciton-blocking layer (16) possessing a greater HOMO- LUMO energy difference. As can be seen from Fig. 9b, the LUMO of the exciton-blocking layer (16) lies below the LUMO level of the electron acceptor layer (12) and, hence, electrons can freely move toward the cathode. Analogous considerations are valid for the electron donor layer (11) and the exciton-blocking layer (15); thereby excitons are also blocked while holes can freely drift toward the anode.

Another exemplary embodiment of the organic photosensitive optoelectronic device is schematically depicted in Fig. lOa. This device also comprises an electron donor layer (11) in contact with an electron acceptor layer (12), this contact representing a photovoltaic heterojunction. In order to increase the efficiency of conversion, the device is additionally provided with a reflective depolarizing electrode (85) in ohmic contact with the electron acceptor layer (12).

The reflective depolarizing electrode is a diffuse reflector that depolarizes electromagnetic radiation reflected from this electrode. A resistive load (4) is connected between the ohmic contacts (3) and the reflective depolarizing electrode (85). The whole multilayer structure is based on substrate (5). Since each crystalline particle (grain) of a photovoltaic organic layer is a polarizer of a light, then the electromagnetic radiation elapsing (passing, walking) through this layer will be locally polarized.

There is another embodiment of the disclosed organic photosensitive optoelectronic device, wherein a protective transparent layer is formed on at least one surface of said device.

In still another embodiment of the disclosed organic photosensitive optoelectronic device, an additional antireflection coating is formed on at least one surface of said device.

Figure lOb shows an organic photovoltaic device similar to the device depicted in Figure 1 Oa, except for an additional phase-shifting layer (retarder) (9) situated between electron acceptor layer (12) and the reflective layer (80) located on substrate. This device operates as follows. The unpolarized electromagnetic radiation is incident onto the electron donor layer (II) and then onto the electron acceptor layer (12). Being transmitted through these layers, the radiation becomes locally polarized owing to the dichroism of crystallites in polycrystalline layers (11) and (12). Thus, each surface region of these polycrystalline layerscontaining at least one crystallite operates similarly to a local polarizer in such a way that a fraction of electromagnetic radiation incident upon this part of layer (11) or (12) will be absorbed by said local polarizers, and the other fraction of radiation, whose polarization is parallel to the transmission axis of said local dichroic polarizers, will pass through this layer without absorption. For increasing the conversion efficiency of the disclosed organic device, it is necessary to pass the transmitted radiation through the photovoltaic organic layers (11) and (12) once again in the reverse direction. In order to provide that locally polarized radiation reflected from reflective layer (80) would be absorbed by the photovoltaic organic layers (11) and (12), it is necessary to rotate its polarization vector by 900 in each region of the polycrystalline layers. To this end, an additional retarder layer (9) is introduced between the photovoltaic layers and the reflective layer. The thickness and optical anisotropy of this retarder are selected so as to ensure a 45'rotation of the polarization vector of the transmitted radiation.

Since the electromagnetic radiation doubly passes through this layer, the resulting polarization rotation amounts to 90. Thus, the combination of retarder and reflective layer provides for a more complete use of the incident electromagnetic radiation and ensures an increase in the photovoltaic conversion efficiency of the photovoltaic device according to this embodiment Figure 11 schematically shows the structure of a photovoltaic organic layer made by the microdomain crystallization method proposed in the present invention (Figure 11 a shows a side view, and Figure 11 b shows a top view). This Figure shows a polycrystalline structure of a photovoltaic layer with isotropically oriented crystallites.

Example I

The first example describes the organic photovoltaic device based on a organic photovoltaic layer obtained using a solution with isotropically oriented molecular aggregates formed by molecules of an organic compound possessing a disk shape and containing at least one ionogenic (hydrophilic) group insuring the solubility of the given organic compound in polar solvents for forming molecular aggregates.

A glass plate with a thickness of 0.5mm was used as a substrate. The substrate was covered by a layer of solid solution of Sn02 and 1203 (ITO) by means of a spin coating technique. The ITO layer thickness was typically within 500-800 A. In the example under consideration, the ITO layer was the anode. It was also possible to use other materials for the anode formation. It is important to note that this material should possess a high electron work function. An important property of the ITO layer is transparency to the electromagnetic radiation employed. Therefore, the light incident on the device passes through the transparent ITO layer and the transparent substrate. An isotropic solution was applied onto the ITO layer. In this particular example, a polycrystalline layer with isotropically oriented crystallites was obtained using an isotropic solution based on the organic dye indanthrone sulfonate. This layer was used as the active photovoltaic layer.

A 7.5% aqueous solution of indanthrone sulfonate was used to form an isotropic phase at room temperature. This dye forms isotropically oriented organic molecular aggregates, these aggregates being the basis of the of polycrystalline structure of the photovoltaic layer. After cleaning the ITO surface, the initial solution was applied by method of smearing.

The following operation for the formation of an isotropic polycrystalline layer was drying.

Preferably, the solvent has to be removed slowly so that the polycrystalline structure of the layer formed in the preceding stage would not be disturbed. In the described example, the drying was carried out at room temperature and a relative humidity of 50%.

As a result, a polycrystalline layer with isotropically oriented crystallites and a thickness of 0.3-0.4 microns, possessing highly isotropic properties was obtained with good reproducibility of the characteristics. Perfection of the polycrystalline structure of the resulting isotropic crystal layers was checked by optical methods and X-ray diffractometry. The X -ray diffraction analysis of the isotropic polycrystalline layer showed that, as a result of the technological operations employed, the organic molecular aggregates formed in the layer contained isotropically oriented crystallites with an intermolecular spacing 3.4 0.3A.

An aluminium film applied through a hole in a photoresistive mask was used as the cathode. The thickness of this layer was approximately 100 A. In this case, the thickness of the aluminium film was selected so that the metal coating would serve as a reflective layer with a reflection coefficient of not less than 95 %. Finally, the manufactured multilayer structure was washed in a polar solvent, for example, in water, in order to remove the mask layer (the layer of metal deposited onto the mask layer is removed as well). As the result of the above technological operations, an organic photovoltaic device was formed with the top contact (cathode) made of metal and the bottom contact (anode) made of ITO.

Example 2

A glass plate coated with a solid solution of SnO2 and 1203 (ITO) provided the transparent conducting substrate, on which a layer of copper phthalocyanine about 300 A thick was formed by the conventional vacuum deposition technique. Copper phthalocyanine is thermally very stable, thus allowing the deposition by evaporation in vacuum, which requires a source temperature of about 500 C. During deposition, the substrate was maintained nominally at room temperature. Then, an organic photovoltaic layer with a thickness of about 500 A was deposited above the copper phthalocyanine layer. This layer was obtained by method described in the first example, from an isotropic solution with isotropically oriented molecular aggregates based on an organic dye (indanthrone sulfonate). Finally, an opaque Ag layer was deposited above said organic photovoltaic layer. The area of the Ag electrode (-0.1 cm2) determined the active area of the organic photovoltaic device.

The descriptions of specific embodiments of the invention were presented for the purpose of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications, embodiments, and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

What is claimed is 1. An organic photovoltaic device comprising the first and second electrodes and at least one photovoltaic organic layer having the front surface and the rear surface, which is distinguished by that said photovoltaic organic layer is produced by means of microdomain crystallization method and has a crystalline or polycrystalline structure with textured or isotropically oriented crystallites, wherein at least one type of said crystallites contains rod-like molecular aggregates, which are composed of planar discotic aromatic cores with intermolecular spacing between aromatic cores of 3.4 0.3 A along one optical axis of said crystallites, may operate as channels with the electron-hole type of conductivity, and may be surrounded by an electrically conducting medium with the ionic type of conductivity.
2. The device according to Claim 1, wherein said photovoltaic organic layer is capable of absorbing electromagnetic radiation in a wavelength range from 200 to 3000 nm.
3. The device according to Claim I or Claim 2, wherein at least one of said electrodes is transparent for the incident electromagnetic radiation to which said photovoltaic organic layer is sensitive.
4. The device according to any of Claims I to 3, further comprising a substrate bearing said electrodes and said photovoltaic organic layer.
5. The device according to any of Claims I to 4, further comprising at least one electron transport layer situated between said photovoltaic organic layer and the first electrode that serves as a cathode.
6. The device according to Claim 5, further comprising at least one exciton-blocking layer situated between said photovoltaic organic layer and said electron transport layer.
7. The device according to any of Claims I to 6, further comprising at least one hole transport layer situated between said photovoltaic organic layer and the second electrode that serves as an anode.
8. The device according to Claim 7, further comprising at least one exciton-blocking layer situated between said photovoltaic organic layer and said hole transport layer.
9. The device according to any of Claims 1 to 8, wherein the first electrode is transparent, and the second electrode is a depolarizing mirror having a reflection coefficient of not less than 95% for the electromagnetic radiation transmitted through the device.
10. The device according to any of Claims 1 to 8, wherein the second electrode is transparent, and the first electrode is a depolarizing mirror having a reflection coefficient of not less than 95% for the electromagnetic radiation transmitted through the device.
11. The device according to any of Claims 1 to 4, which contains one said photovoltaic organic layer, having a rectifying Schottky barrier with the first electrode formed at least on a part of the front surface of said layer and an ohmic contact with the second electrode formed at least on a part of the rear surface of said layer.
12. The device according to any of Claims I to 4, which comprises one photovoltaic organic layer having a rectifying Schottky barrier with the first electrode formed on a part of the front surface of said layer and an ohmic contact with the second electrode formed on another part of the front surface of said layer.
13. The device according to Claim 12, further comprising a reflective depolarizing layer situated on the rear surface of said photovoltaic organic layer and having a reflection coefficient not less than 95 % for the incident electromagnetic radiation to which the photovoltaic organic layer is sensitive.
14. The device according to Claim 12, further comprising a retarder layer and a reflective layer situated in series on the rear surface of said photovoltaic organic layer, wherein the reflective layer has a reflection coefficient not less than 95 % for the incident electromagnetic radiation to which the photovoltaic organic layer is sensitive and the thickness and optical anisotropy of said retarder layer are selected so as to ensure a 450 rotation of the polarization vector of the electromagnetic radiation transmitted through said retarder layer in one direction.
15. The device according to any of Claims Ito 4, comprising the first and second organic photovoltaic layers forming a double layer structure with front and rear surfaces, wherein said first organic photovoltaic layer is an electron donor layer, and said second organic photovoltaic layer is an electron acceptor layer and is in contact with the first organic photovoltaic layer so as to form a photovoltaic heterojunction.
16. The device according to Claim 15, comprising at least one electrode transparent for the incident electromagnetic radiation to which said photovoltaic organic layers are sensitive, and another reflective depolarizing electrode intended for reflection of this radiation.
17. The device according to Claim 15, further comprising a retarder layer and a reflective layer which are situated in series on the rear surface of said double layer structure, wherein the reflective layer has a reflection coefficient not less than 95 % for the incident electromagnetic radiation to which the photovoltaic organic layer is sensitive and the thickness and optical anisotropy of said retarder layer are selected so as to ensure a 45 rotation of the polarization vector of the electromagnetic radiation transmitted through said retarder layer in one direction.
18. The device according to any of Claims 16 or 17, wherein said electrodes form ohmic contacts with the adjacent organic layers.
19. The device according to any of Claims I to 18, further comprising a protective transparent layer formed on at least one surface of said device.
20. The device according to any of Claims 1 to 19, further comprising an antireflection coating formed on at least one surface of said device.
21. The organic photovoltaic device substantially as hereinbefore described with reference to and as shown in Figures 1, 3a, 3b, 4,5,6a, 6b, 7a, 7b, 8a, 9a, lOa or lOb of the accompanying drawings.
22. The organic photovoltaic device substantially as hereinbefore described with reference to Example I or Example 2.
23. A method for manufacturing photovoltaic organic layers, called microdomain crystallization process, which comprises the following steps: (I) chemical modification of an organic compound by introducing one or more ionogenic groups attached over the periphery of discotic aromatic cores of organic molecules; (ii) the formation of a colloidal solution containing isotropically oriented rod-like molecular aggregates of the modified organic compound, wherein the solution concentration is below the threshold concentration for forming a lyotropic liquid crystal; (iii) the application of said isotropic colloidal solution onto a substrate; and (iv) drying said isotropic colloidal solution so as to form a crystalline or polycrystalline layer.
24. The method according to Claim 23, wherein said ionogenic groups are acidic or alkaline groups.
25. The method according to any of Claims 23 to 24, wherein said discotic aromatic cores have a planar form.
26. The method according to any of Claims 23 to 25, wherein said rod-like molecular aggregates have the intermolecular spacing between said aromatic cores which is equal to 3.4 0.3 A.
27. The method according to any of Claims 23 to 25, wherein the duration, temperature, and humidity of drying are selected so as to ensure the formation of a crystalline or polycrystalline layer composed of isotropically oriented crystallites containing said rod-like molecular aggregates of organic ionic compounds with the peripheral ionogenic groups and counterions.
28. The method according to any of Claims 23 to 27, wherein said lonogenic groups are selected from the list comprising COO-, P04-, and other ionogenic (hydrophilic) groups imparting amphiphilic properties to organic compounds.
29. A photovoltaic organic layer produced by the method according to any one of Claims 23 to 28.