Patent Publication Number: US-2006008580-A1

Title: Hybrid solar cells with thermal deposited semiconductive oxide layer

Description:
This application is a continuation-in-part of application Ser. No. 10/799,257, filed Mar. 12, 2004, now pending, which application is a continuation of application Ser. No. 09/989,848, filed Nov. 21, 2001, now issued U.S. Pat. No. 6,706,962, both applications being incorporated herein by reference. 
    
    
     DESCRIPTION  
      The present invention is related to the manufacture of organic hybrid solar cells in which the semiconductive oxide layer of the organic hybrid cell is vapor deposited.  
      Among chief materials used in the past for solar cells have been inorganic semiconductors made from, for example, silicon. However, such devices have proven to be very expensive to construct, due to the melt and other processing techniques necessary to fabricate the semiconductor layer.  
      In an effort to reduce the cost of solar cells, organic photoconductors and semiconductors have been considered, due to their inexpensive formation by, e.g. thermal evaporation, spin coating, self-assembly, screen printing, spray pyrolysis, lamination and solvent coating. The most often followed strategies in this field can be summarized as follows:  
      All-organic solar cells produced by vapor deposition are known in the literature. For example, Tang (Tang, Two-layer organic photovoltaic cell, Appl. Phys. Lett. 48(2) (1986) 183-5) reported about organic thin two layer solar cells showing the following structure: 
 
Substrate+ITO/CuPc (30 nm)/ST2 (50 nm)/Ag 
 
 in which ITO is indium tin oxide, CuPc is copper-phtalocyanine, ST2 is a dye and in which all organic layers were deposited by evaporation. The deposition by evaporation required source temperatures of about 500 and 600° C., respectively, which the substrate was maintained nominally at room temperature during deposition. The resulting cell is herein designated as “Tang cell.” The Tang cell does not include an additional dense semiconducting oxide layer (SOL) and has an efficiency of 0.96%. 
 
      Similarly, Wöhrle et al. and Takahashi et al. reported organic two and three layer solar cells which were prepared by vapor deposition and/or spin-coating (Wöhrle D., Tennigkeit B., Elbe J., Kreienhoop L., Schnurpfeil G.: Various Porphyrins and Aromatic Terracarbxcylic Acid Diimides in Thin Film p/n-Solar cells, Molecular Crystals and Liquid Crystals 230 (1993B) 221-226 Takahashi, K.; Kuraya, N.; Yamaguchi, T.; Komura, T.; Murata, K. Three-layer organic solar cell with high-power conversion efficiency of 3.5%, Solar Energy Materials &amp; Solar Cells 61 (2000) 403-416). These all-organic cells do not contain a dense SOL layer.  
      Petrisch and co-workers (Petrisch et al. Dye-based donor/acceptor solar cells, Solar Energy Materials &amp; Solar Cells 61 (2000) 63-72) reported organic solar cells consisting of three dyes, in particular a perylene-tetracarboxylic acid-bisimide with aliphatic side chains (perylene), a metal-free phtalocyanine with aliphatic side chains (HPc). The materials are soluble, which allowed cell performance other than vapor deposition (Yu G., Gao J., Hummelen J. C., Wudl F., Heeger A. J.: Polymer Photovoltaic Cells: Enhanced Efficiencies via a network of Internal Donor-Acceptor Heterojunctions, Science 270 (1995) 1789-1791.).  
      Further, laminated cells or cells containing mixtures of donor and acceptor materials (polymers) were also reported by Friend et al. (Friend et al., Nature 397 (1999) 121; Granstrom et al., Nature 395 (1998) 257-260) and Sariciftici et al. (Sariciftici et al. Science 258 (1992) 1474). Schön et al. (Schön et al. Nature 403 (2000) 408-410) reported on the use of single crystals of organic material as doped pentacene having an efficiency of up to 2.4%. Most of the organic solar cells showing a higher efficiency use I 2 /I 3   −  as a doping system, which is unstable with time.  
      The use of porous nanocrystalline TiO 2  layers in solar cells is further known from WO 91/16719, EP-A-0 333 641 and WO 98/48433 as well as from other publications of Gratzel et al. (Bach U., Lupo D., Comte P., Moser J. E., Weissörtel F., Solbeck J., Spreitzer H., Grätzel M.: Solid state dye-sensitized porous nanocrystalline TiO 2  solar cells with high photon-to-electron conversion efficiencies, Nature 395 (1998) 583-585. Bach U., Gratzel M., Salbeck J., Weissortel F., Lupo D.: Photovoltaic Cell, Brian O&#39;Regan and Michael Grätzel: A low cost, high-efficiency solar cell based on de-sensitized colloidal TiO 2  films, Nature 353, (1991) 737-740.) These cells have efficiencies between 0.74% (for the solid state solar cells) and 7.1% (for the liquid hybrid solar cell). Nevertheless, as pointed out by the authors themselves, the liquid cells described in these publications are difficult to produce and have a reduced long-term stability, whilst the solid cells described have a low efficiency. Furthermore, the porous nanocrystalline TiO 2  layer preparation requires high temperature sintering with temperatures of 450° C.  
      U.S. Pat. No. 3,927,228 to Pulker describes a method of depositing titanium dioxide layers by evaporation of a molten titanium-oxygen phase. The method of producing TiO 2  layers comprises evaporating a molten titanium-oxygen having a composition corresponding to a proportion of the number of oxygen atoms to the number of titanium atoms of from 1.6 to 1.8, and condensing the vapor on a layer support in the presence of oxygen. The use of this method for the production of solar cells is not disclosed or proposed.  
      Therefore, most organic/hybrid solar cells known so far show either a low efficiency, a small long-term stability, or they are not suitable to be transferred on flexible substrates. Further, it is still difficult to produce hybrid organic solar cells on large sized carrier substrates.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the present invention to provide a solar cell which is both inexpensive to produce and sufficiently efficient as to be useful in terrestrial applications.  
      It is a related object of the invention to provide a method for the production of a thin, high efficient hybrid solar cell, which can be produced on flexible substrates.  
      This problem is solved by a method for the production of a hybrid organic solar cell in which the semiconducting oxide layer (SOL) is introduced by thermal deposition. Preferably, the SOL layer is vapor deposited. Also, the SOL is preferably a dense SOL.  
      The term “dense” SOL in the context of the present invention means an SOL that substantially consists of an amorphous, crystalline and/or polycrystalline layer of the semiconductive oxide material. The dense SOL layer of the present invention is applied to the device by thermal evaporation. The thermal evaporation allows for a much more stringent control of the applied thickness, and leads to a tight and amorphous, crystalline and/or polycrystalline packaging of the SOL material in contrast to the commonly applied sintering of e.g. nanoparticles of a diameter of between about 8 and 20 nm, leading to a porous layer with larger variations in the porosity consisting of sintered nanoparticles, and having a more irregular thickness. A dense SOL layer according to the present invention can, for example, be controlled to exhibit a thickness of between about 15±0.5 nm to 35±0.5 nm by an evaporation rate of between, for example, 0.11 to 0.5 nm/s.  
      The addition of the dense SOL layer can be used to improve the efficiency of known organic solar cells, e.g. the ones as reported by Friend et al. (Friend et al., Nature 397 (1999) 121; Granström et al., Nature 395 (1998) 257-260).  
      The problem of the invention is further solved by a method for the production of a hybrid organic solar cell having the general structure: 
 
Substrate+EM/HTM/dye/SOL/EM, or 
 
Substrate+EM/SOL/dye/ HTM/EM, or 
 
Substrate+EM/HTM/SOL/EM, and 
          wherein the EM is selected from a group consisting of a transparent conductive oxide (TCO), a transparent conductive polymer or a transparent organic material, and a metal, with at least one of the EM layer(s) of the hybrid solar cell being a TCO, and     wherein the SOL comprises a dense semiconductive oxide layer.        

      The additional layer of dense SOL enhances the electron transport to the anode and therefore increases the efficiency of the hybrid organic solar cell according to the invention in comparison with all-organic thin layer solar cells, like the above-mentioned “Tang cell.” The method according to the invention provides a solar cell which is both inexpensive to produce and sufficiently efficient as to be promising in view of future terrestrial applications.  
      The problem of the invention is further solved by a method for the production of a hybrid organic solar cell having the general structure: 
 
Substrate+EM/HTM/dye/SOL/EM, or 
 
Substrate+EMISOL/dye/HTM/EM, or 
 
Substrate+EM/HTM/SOL/EM, and 
          wherein the EM is selected from a group consisting of a transparent conductive oxide (TCO), a transparent conductive polymer or a transparent organic material, and a metal, with at least one of the EM layer(s) of the hybrid solar cell being a TCO, and     wherein the SOL comprises a dense semiconductive oxide layer.        

      The multilayer strategy of the present invention is a promising alternative to the expensive production of solar cells based on single crystal and polycrystalline materials, and a new alternative to the known strategies in the field of organic solar cells and hybrid solar cells. All other layers of the hybrid organic solar cell can be applied by conventional techniques, e.g. thermal evaporation, spin coating, self-assembly, screen printing, spray pyrolysis, lamination, solvent coating, LB technique, sputtering and others.  
      In a preferred method of the invention, an additional layer of lithium fluoride can be deposited and/or vapor deposited close to the EM interfaces either on one side or both sides. The additional layer of lithium fluoride can have a thickness of between about 0.1 Å to about 50 Å.  
      In a further preferred method of the invention, the surfaces of the interfaces of the layers are increased. In general, interfaces can be increased by the following approaches, namely use of structured ITO or other EM, co-evaporation of HTM and dye and/or dye/TiO 2  (also in addition to layers of the bare materials) and co-evaporation of HTM and dopant (e − -acceptor, e.g. fullerene).  
      In a further preferred method of the invention, the substrate is selected from glass, coated glass, polymeric foils, like foils made from PET, PEN or PI (polyimide), norbornene-based foils, SnO 2 -coated metal foils, e.g. stainless steel foils. Preferably, the substrate is flexible.  
      In a further preferred method of the invention, the EM is selected from a group consisting of indium tin oxide (ITO), fluorine doped tin oxide (FTO), zinc oxide (ZnO), doped zinc oxide, tin oxide (SnO 2 ), highly doped poly(3,4-ethylenedioxythiophene) (PEDOT) or combination thereof, and metals, such as Au, Al, Ca or Mg or combinations of metals such as Al/Li, Mg/Ag.  
      PEDOT is discussed, for example, in the reference: Lambertus Groenendaal, Friedrich Jonas, Dieter Freitag, Harald Pielartzik, and John R. Reynolds “poly(3,4-ethylenedioxythiophene) and its Derivatives: Past, Present, and Future,” Adv. Matter. 2000, 12, No. 7.  
      In a further preferred method according to the invention EM is selected from the group of indium tin oxide, fluorine doped tin oxide, zinc oxide or doped zinc oxide. Further, in case of a metal, the EM layer can be selected from Au, Al, Ca or Mg or combinations of metals like Al/Li, Mg/Ag and the like. In order to allow a proper function of the solar cell according to the invention, at least one of the EM-layer(s) has to be transparent.  
      Most preferred is a method, in which the EM is indium tin oxide.  
      According to the invention, HTM can be selected from the group of phthalocyanine and derivatives thereof (with or without a central atom or group of atoms), metal-free and metal containing porphyrins and derivatives thereof, TPD derivatives, triphenylamine and derivatives thereof, (including different ground structure as TDATAs, TTABs, TDABs, and cyclic variations like N-carbazoles and derivatives thereof), thiophenes, polythiophenes and derivatives thereof, polyanilines and derivatives thereof and hexabenzocoronene and derivatives thereof, triphenyldiamine derivatives, aromatic diamine compounds having connected tertiary aromatic amine units of 1,-bis(4-(di-p-tolylamino)phenyl)-cyclohexane, aromatic diamines containing two or more tertiary amines and having two or more fused aromatic rings substituted on the nitrogen atoms as typified by 4,4-bis[(N- 1-naphthyl)-N-phenylamino]-biphenyl, aromatic trimers having a starburst structure derived from triphenylbenzene, aromatic diamines such as N,N′-diphenyl-N,N′-bis(3-methyphenyl)-(1,1′-biphenyl)-4,4′diamine, α,α,α′,α′-tetramethyl-α,α′-bis(4-di-p-tolylaminophenyl)-p-xylene, triphenylamine derivatives whose molecule is sterically asymmetric as a whole, compounds having a plurality of aromatic diamino groups substituted on a pyrenyl group, aromatic diamines having tertiary amine units connected through an ethylene group, aromatic diamines having a styryl structure, starburst type aromatic triamines, benzyl-phenyl compounds, compounds having tertiary amine units connected through a fluorene group, triamine compounds, bisdipyridylaminobiophenyl compounds, N,N,N-triphenylamine derivatives, aromatic diamines having a phenoxazine structure, diaminophenylanthridine, and other carbazole derivatives, hydrazoen compounds, silazane compounds, silanamine derivatives, phosphamine derivatives, quinacridone compounds, stilbene compounds such as 4-di-p-tolylamino-stilbene and 4-(di-p-tolylamino)-4′-[4-di-p-tolylamino)-styryl]stilbene, triazole derivatives, oxadiazole derivatives, imidazole derivatives, polyarylalkane derivatives, pyrazoline derivatives, pyrazolone derivatives, oxadiazole derivatives, imidazole derivatives, polyarylalkane derivatives, pyrazoline derivatives, amino-substituted chalcone derivatives, oxazole derivatives, styrylanthracene derivatives, fluorenone derivatives and polysilane derivatives, all compounds alone or in admixture of two or more, polymers, like polyvinyl carbazole and polysilanes, polyphosphazenes, polyamides, polyvinyl triphenylamine, polymers having a triphenylamine skeleton, polymers having triphenylamine units connected through a methylene group and polymethacrylates containing aromatic amine, preferably having an average molecular weight of at least 1,000, more preferably at least −5,000. In general, all kinds of hole transport materials known to the person skilled in the art.  
      In an even more preferred method according to the invention, HTM is copper-phthalocyanine (CuPc).  
      The substance of the dense SOL layer can be selected from the group of all kinds of semiconducting oxides, like TiO 2 , SnO 2 , ZnO, Sb 2 O 3 , PbO, Nb 2 O 5 , ZrO 3 , CeO 2 , WO 3 , SiO 2 , Al 2 O 3 , CuAlO 2 , SrTiO 3 , SrCu 2 O 2  or a complex oxide containing several of these oxides.  
      Most preferred the dense SOL layer is TiO 2 .  
      In a further preferred method of the invention, the dye is selected from the group of di- or mono-substituted perylenes with all possible substituents, e.g. perylene anhydrid, perylene dianhydrides, perylene imides, perylene diimides, perylene imidazoles, perylene diimidazoles and derivatives thereof, terrylene, quinacridone, anthraquinone, nealred, titanylphthalocyanine, porphines and porphyrines and derivatives thereof, polyfluorenes and derivatives thereof and azo-dyes. In general, all suited and commercially available dyes, which are known to the person skilled in the art, can be used.  
      Preferably, the dye layer is deposited in a thickness of about 5 to about 65 nm and the dense SOL layer is deposited in a thickness of about 5 to about 50 nm. More than one dye can be used in one cell, which can be either applied by a layer-by-layer technique or a co-evaporation of different dyes. The application of several dyes leads to an advantageous broadening of the spectral region in which the absorption takes places.  
      In a further preferred method according to the invention, the substance of the HTM is doped. Possible approaches for doping can be employed by mixing the material of the HTM layer prior to application with, for example, Tris(4-bromophenyl)ammoniumyl hexachloroantimonate (N(PhBr) 3 SbCl 6 ), 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ) or Nitrosonium-tetrafluorborat (BF 4 NO). Co-evaporation of dopants like 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ) or other electron acceptors is also possible. The dopants can be used in combination with Li-salts, e.g. Bistrifluoromethane sulfonimide Li-salt (Li((CF 3 SO 2 ) 2 N)). The dopants can be used in both fully or non-fully evaporated cells.  
      The problem of the present invention is further solved by a hybrid solar cell, which is obtainable, by the inventive method, mentioned above.  
      Preferably the thickness of the complete cell is about 100 nm, and the hybrid solar cell has an efficiency of about 0.7 to about 1.3% at 60 mW/cm 2 .  
      Most preferred is a hybrid solar cell according to the invention, which is flexible.  
      The term “hybrid” is generally known in chemistry as combination of organic and inorganic materials forming any kind of new structure. In the case of photovoltaic cells this term has been introduced for the first time by Hagen et al. (J. Hagen, W. Schaffrath, P. Otschik, R. Fink, A. Bacher, H.-W. Schmidt, L D. Haarer, Synth. Met., 89, 215 (1997).) for describing a dye sensitized solid state solar cell using nanocrystalline TiO 2  and TPD derivatives as HTM. Within the invention, the term “thin film hybrid photovoltaic cell” may be used in order to indicate the difference between dye-sensitized solar cells with porous nanocrystalline TiO 2  and the inventive solar cells with a vapor deposited TiO 2  layer.  
      In general, the inventors developed a new concept to fabricate hybrid organic solar cells. The new idea in this concept is that a dense SOL layer was introduced into the cell by vapor deposition. All other layers of the hybrid organic solar cell can be applied by conventional techniques, e.g. thermal evaporation, spin coating, self-assembly, screen printing, spray pyrolysis, lamination, solvent coating, LB technique, sputtering and others.  
      In one embodiment of the invention, all layers, or all layers except one or both EM layer(s) are vapor deposited. Compared to the fully organic solar cells consisting of derivatives of phthalocyanin and perylene, an additional layer of TiO 2  was introduced. This surprisingly enhances the electron transport to the anode. The resulting cell shows, in one example a configuration of the following structure: 
 
Substrate+ ITO/HTM /dye/TiO 2 /Al, with a total thickness&lt;100 nm. 
 
      The advantages of this concept are the low costs, use of purchasable materials with high thermal and electrochemical stability. Furthermore the possibility to prepare large area photovoltaic cells is given and no further temperature treatment is needed, which enables the application to flexible substrates.  
      To determine the best cell performance, a combinatorial method for vapor deposition was utilized by varying the TiO 2  layer thickness, the CuPc layer thickness, and the thickness of the dye derivatives. Cells were characterized by measuring I-V characteristics of solar cells. The use of TiO 2  improved the energy conversion efficiency of solar cells from 0.7 to 1.3 % (light of 60 mW/cm 2 ). Further, co-vapor deposition of TiO 2  and dye or a modified device structure ITO/TiO 2 /dye/HTM/Au are possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The method according to the present invention shall now be further explained with reference to the figures in which  
       FIG. 1  shows a schematic outline of the composition of different hybrid solar cell prepared according to the invention,  
       FIG. 2  shows a schematic outline of the preparation of a layer thickness gradient,  
       FIG. 3  shows a schematic outline of the preparation of a hybrid solar cell prepared according to the invention, in which a cell was produced having a TiO 2  gradient, in which the substrate/EM carrier is schematically shown at ( 1 ), the HTM layer at ( 2 ), the substrate/EM/HTM carrier at ( 3 ), the dye layer at ( 4 ), the substrate/EM/HTM/dye carrier at ( 5 ), the mask at ( 6 ) and the TiO 2  gradient layer at ( 7 ),  
       FIG. 4  shows an SEM of a cross section of a cell produced according to the invention showing the structure Substrate ( 1 )+EM ( 2 ) /HTM ( 3 )+dye ( 4 ) /SOL ( 5 ) /EM ( 6 ),  
       FIG. 5  shows the current-voltage curves of hybrid organic solar cells with different TiO 2  layer thickness, and  
       FIG. 6  shows current-voltage curves showing the light intensity dependence of I sc  and V oc  of a hybrid organic solar cell according to the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Example 1  
     Preparation of a Cell According to the Invention  
      In a first step according to  FIG. 3 , commercially available EM coated glass (1) is used as starting material. Suited EM materials are all materials, which can be used to create transparent electrodes, like indium tin oxide, fluorine doped tin oxide, zinc oxide or doped zinc oxide. Further, an evaporated metal electrode (EM layer) selected from metals, like Au, Al, Ca or Mg, or combinations of metals like Al/Li, Mg/Ag, and the like can be used. In order to allow a proper function of the solar cell according to the invention, at least one of the EM-layers should be a TCO.  
      The starting material is then coated with a constant thickness layer of HTM ( 2 ), which can be applied by vapor deposition, resulting in a HTM-coated device substrate/TCO/HTM ( 3 ). Suitable HTM materials can be selected from the group of phthalocyanine and derivatives thereof (with or without a central atom or group of atoms), metal-free and metal containing porphyrins and derivatives thereof, TPD derivatives, triphenylamine and derivatives thereof, (including different ground structure as TDATAs, TTABs, TDABs, and cyclic variations like N-carbazoles and derivatives thereof), thiophenes, polythiophenes and derivatives thereof, polyanilines and derivatives thereof and hexabenzocoronene and derivatives thereof. In general, all kinds of hole transport materials known to the person skilled in the art can be used.  
      For example, CuPc or its derivatives can be used having the following formula:  
                 
 
      Further suited hole transporting agents are in principle disclosed in European patent application EP 00 111 493.3 and EP 0 901 175 A2, whose disclosures are incorporated herein by reference.  
      More particularly, EP 0 901 175 A2 discloses suited organic hole conducting agents which include aromatic diamine compounds having connected tertiary aromatic amine units of 1,-bis(4-(di-p-tolylamino)phenyl)-cyclohexane as described in JP-A 194393/1984, aromatic diamines containing two or more tertiary amines and having two or more fused aromatic rings substituted on the nitrogen atoms as typified by 4,4-bis[(N-1-naphthyl)-N-phenylamino]-biphenyl as described in JP-A 234681/1983, aromatic trimers having a starburst structure derived from triphenylbenzene as described in U.S. Pat. No. 4,923,774, aromatic diamines such as N,N′-diphenyl-N,N′-bis(3-methyphenyl)-(1,1′-biphenyl)-4,4′diamine as described in U.S. Pat. No. 4,764,625, α,α,α′,α′-tetramethyl-α,α′-bis(4-di-p-tolylaminophenyl)-p-xylene as described in JP-A269084/1991, triphenylamine derivatives whose molecule is sterically asymmetric as a whole as described in JP-A 129271/1992, compounds having a plurality of aromatic diamino groups substituted on a pyrenyl group as described in JP-A 175395/1992, aromatic diamines having tertiary amine units connected through an ethylene group as described in JP-A 264189/1992, aromatic diamines having a styryl structure as described in JP-A 290851/1992, starburst type aromatic triamines as described in JP-A 308688/1992, benzyl-phenyl compounds as described in JP-A 364153/1992, compounds having tertiary amine units connected through a fluorene group as described in JP-A 25473/1993, triamine compounds as described in JP-A 239455/1993, bis-dipyridylaminobiophenyl compounds as described in JP-A 320634/1993, N,N,N-triphenylamine derivatives as described in JP-A 1972/1994, aromatic diamines having a phenoxazine structure as described in JP-A 290728/1993, diaminophenylanthridine derivatives as described in JP-A 45669/1994, and other carbazole derivatives.  
      Other hole transporting agents which are disclosed in EP 0 901 175 and which may be used in the present invention, include hydrazoen compounds (JP-A 311591/1990), silazane compounds (U.S. Pat. No. 4,950,950), silanamine derivatives (JP-A 49079/1994), phosphamine derivatives (JP-A 25659/1994), quinacridone compounds, stilbene compounds such as 4-di-p-tolylamino-stilbene and 4-(di-p-tolylamino)-4′-[4-di-p-tolylamino)-styryl]stilbene, triazole derivatives, oxadiazole derivatives, imidazole derivatives, polyarylalkane derivatives, pyrazoline derivatives, pyrazolone derivatives, oxadiazole derivatives, imidazole derivatives, polyarylalkane derivatives, pyrazoline derivatives, amino-substituted chalcone derivatives, oxazole derivatives, styrylanthracene derivatives, fluorenone derivatives and polysilane derivatives. These compounds may be used alone or in admixture of two or more. The same applies also to the other compounds disclosed herein, including those incorporated herein by reference.  
      In addition to the aforementioned compounds, polymers can be used as the hole-transporting agent. Suitable polymers include polyvinyl carbazole and polysilanes (Appl. Phys. Lett., vol. 59, 2760, 1991), polyphosphazenes (JP-A 310949/1993), polyamides (JP-A 10949/1993), polyvinyl triphenylamine (Japanese Patent Application No. 133065(1993), polymers having a triphenylamine skeleton (JP-A 133065/1992), polymers having triphenylamine units connected through a methylene group (Synthetic Metals, vol 55-57, 4163, 1993) and polymethacrylates containing aromatic amine (J. Polym. Sci., Polym. Chem. Ed., vol 21, 969, 1983). When polymers or mixtures thereof are used as the hole transporting agent, they preferably have a number average molecular weight of at least 1,000, more preferably at least 5,000.  
      In a second step according to  FIG. 3 , constant thickness layers of a dye (4) are vapor deposited on the substrate/EM/HTM carrier (3), in this case dyes named ST 1/1 (N,N′-dimethyl-3,4:9,10-perylene-bis(carboximid) or ST2 (Bisbenzimidazol[2,2-a: 1′,2′-b′]anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-6,11-dione) (both commercially available from Syntec Company) are used. In our experiments, we tested the different dyes in a layer thickness in the range from 5 to 50 nm. In general, suited dyes are, for example, di- or mono-substituted perylenes with all possible substituents. For the structure of dyes, all types of perylene diimides with different amino residues and all types of perylene benzimidazoles with different diamine components can be added. Moreover, the perylene rest can be differently substituted.  
      The general structure of perylene diimidazole is shown in the following formula  
                 
 
 in which R 1 , R 2 , R 3  are alkyl, aryl alkoxyl or halogen, etc. 
 
      The general structure of perylene diimide is shown in the following formula  
                 
 
 in which R 1 , R 2 , R 3  are alkyl, aryl, alkoxyl, or halogen, etc. 
 
      The inorganic oxide layer or electrode transport layer can consist of all kinds of semi-conducting oxides as TiO 2 , SnO 2 , ZnO, Sb 2 O 3 , PbO, etc.  
      In a third step according to  FIG. 3 , in this embodiment a step gradient of titanium dioxide ( 7 ) is evaporated on the substrate/EM/HTM/dye carrier ( 5 ) using a mask ( 6 ). In addition to the variation of the titanium dioxide layer described for this embodiment, similar strategies can be used in order to produce other embodiments with gradients for all materials possible to be used as EM, HTM, dye or a dense SOL. Titanium dioxide was evaporated from a tantalum crucible starting with Ti 3 O 5  powder (pellets commercially available from Balzers); the vacuum chamber was evacuated to a pressure of 2.0×10 −5  mbar followed by feeding 02 into the vacuum deposition chamber via a needle valve resulting in an O 2 partial pressure of 2.5×10 −4  mbar (possible partial pressures are in the range of 2.0×10 −5  to 3.0×10 −4  mbar). The crucible-substrate distance was 36 cm. The evaporation rates were controlled to be in the range between 0.11 to 0.5 nm/s. The evaporation rates were detected with oscillating quartz crystals placed inside the evaporation chamber.  
      The production of the gradient is also schematically depicted in  FIG. 2 .  
      Furthermore, the term “dense” SOL in the context of the present invention means an SOL that substantially consists of an amorphous, crystalline and/or polycrystalline layer of the semiconductive oxide material. The dense SOL layer of the present invention is applied to the device by thermal evaporation. The thermal evaporation allows for a much more stringent control of the applied thickness, and leads to a tight and amorphous, crystalline and/or polycrystalline packaging of the SOL material in contrast to the commonly applied sintering of e.g. nanoparticles of a diameter of between about 8 and 20 nm, leading to a porous layer with larger variations in the porosity consisting of sintered nanoparticles, and having a more irregular thickness. A dense SOL layer according to the present invention can, for example, be controlled to exhibit a thickness of between about 15±0.5 nm to 35±0.5 nm by an evaporation rate of between, for example, 0.11 to 0.5 nm/s.  
      Finally, 24 Al stripes were applied as electrodes on top of the resulting hybrid organic solar cells having (in this embodiment) the structure: 
 
Substrate+ITO/CuPc (35 nm)/ST2 (25 mn)/TiO 2 (0, 15 or 35 mn)/Al 
 
       FIG. 4  shows an SEM of a cross section of the cell produced according to the invention in which the substrate is glass ( 1 ) on which a layer of ITO ( 2 ), a layer of both CuPc (35 nm, ( 3 )) and ST2 (25 nm ( 4 )), a layer of TiO 2  ( 5 ) and a layer of Al ( 6 ) is applied.  
     Example 2  
     Characteristics of a Cell Prepared According to the Invention  
      The hybrid organic solar cell produced according to example 1 was tested for its current-voltage characteristics and light intensity dependence of I sc  and V oc . An Oriel 75W xenon short arc lamp with a water filter, a 345 nm sharp edge filter, a mirror and a PP diffuser was used as the light source. Current-voltage characteristics were measured with an SMU Keithley 2400, an IEEE-card together with a self-developed Labview measuring program (I [A], V [V], I dens  [mA/cm 2 ], FF [%], P max  [mW/cm 2 ], η [%]. Standard measurement parameters were: ambient conditions, 60 mW/cm 2  to 100 mW/cm 2 , a cycle of 0 V to −0.1 V to +1.0 V (illuminated and dark), a 5 mA step size and 3 seconds delay time. The results of the measurements are graphically depicted in  FIGS. 5 and 6  and listed in Tables 1 and 2, below.  
               TABLE 1                          Current-voltage characteristics                                 Device   I sc  [mA/cm 2 ]   V oc  [mV]   FF [%]   η [%]               A (0 nm TiO 2 )   1.861   420   57.6   0.75       B (15 nm TiO 2 )   2.816   440   42.5   0.87       C (35 nm TiO 2 )   3.326   440   42.4   1.04                  
 
      The results show, that the introduction of a vapor-deposited layer of TiO 2  clearly increases the efficiency of the hybrid solar cell up to a value of η&gt;1%.  
               TABLE 2                          Light intensity dependence       of I sc  and V oc  for device C (35 nm TiO 2 )                                 Light Int.                       [mW/cm 2 ]   I sc  [mA/cm 2 ]   V oc  [mV]   FF [%]   η [%]                                         7   0.280   375   52.3   0.79       11   0.438   385   53.4   0.82       22   1.101   415   51.7   1.07       39   2.008   435   49.9   1.12       60   3.572   445   49.1   1.30       82   5.319   455   47.3   1.40                  
 
      The results show a value of η=1.30% at 60 mW/cm 2 , indicating the good efficiency of the inventive hybrid organic solar cell.