Abstract:
An organic photovoltaic (“PV”) device comprises a plurality of organic PV cells connected in series to cover a large area. The organic PV device optionally has an electrical circuit element connected in parallel to each organic PV cell. The organic PV device allows for continued operation even when short circuits develop or electrical interruption occurs in one of the cells. The device is conveniently manufactured using a shadow mask, which allows for the formation of several consecutive layers in one apparatus.

Description:
BACKGROUND OF INVENTION  
         [0001]    The present invention relates to optically absorptive photonic devices. In particular, the present invention relates to photovoltaic (“PV”) devices having large areas and methods of making the same.  
           [0002]    Semiconductive PV devices are based on the separation of electron-hole pairs formed following the absorption of a photon. An electric field is generally required for the separation of the charges. The electric field may arise from a Schottky contact where a built-in potential exists at a metal-semiconductor interface or from a p-n junction between p-type and n-type semiconducting materials. Such devices are commonly made from inorganic semiconductors, especially silicon, which can have monocrystalline, polycrystalline, or amorphous structure. Silicon is normally chosen because of its relatively high photon conversion efficiency. However, silicon technology has associated high costs and complex manufacturing processes, resulting in devices that are expensive in relation to the power they produce.  
           [0003]    Organic PV devices, which are based on active semiconducting organic materials, have recently attracted more interest as a result of advances made in organic semiconducting materials. These materials offer a promise of better efficiency that had not been achieved with earlier organic PV devices. Typically, the active component of an organic PV device comprises at least two layers of organic semiconducting materials disposed in contact with one another. The first organic semiconducting material is an electron acceptor, and the second an electron donor. An electron acceptor is a material that is capable of accepting electrons from another adjacent material due to a higher electron affinity of the electron acceptor. An electron donor is a material that is capable of accepting holes from an adjacent material due to a lower ionization potential of the electron donor. The absorption of photons in an organic photoconductive material results in the creation of bound electron-hole pairs, which must be dissociated before charge collection can take place. The separated electrons and holes travel through their respective acceptor (semiconducting material) to be collected at opposite electrodes.  
           [0004]    In order to have a practical energy source from PV devices, large-area devices are needed to capture a large amount of sunlight. However, the manufacture of large-area defect-free PV devices is a challenge. Typically, a defect in the fabrication of a device, such as one that allows a short circuit, would render the whole device inoperative and useless.  
           [0005]    Therefore, it is very desirable to provide PV devices that cover a large area, but are more tolerant to fabrication defects. It is also very desirable to provide large-area PV devices that remain operative and produce electrical energy even when there are microscopic short circuits in the originally made devices.  
         SUMMARY OF INVENTION  
         [0006]    According to one aspect of the present invention, an organic PV cell comprises at least one organic electron acceptor and at least one organic electron donor. The organic electron acceptor and the electron donor are disposed adjacent to one another to form a junction, and together are sandwiched between a pair of electrodes: a cathode and an anode. The cathode of one organic PV cell is electrically connected to the anode of an adjacent organic PV cell.  
           [0007]    According to another aspect of the present invention, an electrical circuit element that is capable of providing a path for an electrical by-pass is connected in parallel to each of the organic PV cells.  
           [0008]    According to another aspect of the present invention, a method is provided for making a large-area PV device. The method comprises: (a) forming a plurality of organic PV cells on a substrate, each cell comprising at least two organic semiconducting materials disposed between a pair of first and second electrodes; and (b) forming an electrical contact between the first electrode of one cell and the second electrode of an adjacent cell. The step of forming a plurality of organic PV cell comprises: (1) providing a plurality of distinct first electrodes on a substrate; (2) disposing a first layer of a first organic semiconducting material on each of the first electrodes, each of the first layers being separated from other first layers; (3) disposing a second layer of a second organic semiconducting material on each of the first layers, the first and second organic semiconducting materials forming a junction of an electron acceptor and an electron donor; and (4) disposing a second electrode on each of the layers of second organic semiconducting material.  
           [0009]    According to still another aspect of the present invention, the method for making a large-area PV device comprises: (a) forming a plurality of separate organic PV cells, each cell comprising at least two organic semiconducting materials disposed between a pair of first and second electrodes; (b) disposing the plurality of the separate organic PV cells on a substrate; and (c) forming an electrical contact between the first electrode of one cell and the second electrode of another adjacent cell. The step of forming a separate organic PV cell comprises: (1) providing a first electrode layer; (2) disposing a first organic semiconducting material on the first electrode layer; (3) disposing a second organic semiconducting material on the first organic semiconducting material; and (4) disposing a second electrode layer on the second organic semiconducting material.  
           [0010]    Other features and advantages of the present invention will be apparent from a perusal of the following detailed description of the invention and the accompanying drawings in which the same numerals refer to like elements. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]    [0011]FIG. 1 shows schematically a PV device comprising several PV cells connected in series.  
         [0012]    [0012]FIG. 2 shows a side view of an embodiment of a PV device comprising several PV cells connected in series.  
         [0013]    [0013]FIG. 3 shows a side view of a different embodiment of a PV device comprising several PV cells connected in series.  
         [0014]    [0014]FIG. 4 shows schematically a PV device comprising several PV cells connected in series wherein a circuit element is connected in parallel to each PV cell.  
         [0015]    [0015]FIG. 5 shows the steps of a method of making a PV device comprising several PV cells connected in series. 
     
    
     DETAILED DESCRIPTION  
       [0016]    [0016]FIG. 1 illustrates a PV device according to a first embodiment of the present invention. It should be understood that the elements shown in the drawings are not drawn to scale. The PV device  10  of FIG. 1 includes a plurality of organic PV cells  12 , which are connected in series and arranged to cover a large area. The term “large area” means an area greater than about 100 cm 2 . For example, FIG. 1 illustrates six organic PV cells  12 . However, the number of organic PV cells can be chosen as desired to cover an available area provided all cells are connected in series. The number of organic PV cells also can be chosen to provide a desired output potential V.  
         [0017]    Each of the individual organic PV cells  12  has an anode  14  and a cathode  16 . The organic PV cells  12  are electrically connected in a series arrangement; e.g., anode  14  to cathode  16 , as shown in FIG. 1. In this regard, the respective anodes and cathodes may be electrically connected via interconnect wiring  18  as shown in FIG. 1. Each organic PV cell  12  is capable of absorbing photon energy and generating an electrical potential between its anode  14  and cathode  16 . An output potential V from the plurality organic PV cells  12  is available at  20  between conducting line  22  connected to anode  14  of the first cell, and conducting line  14  connected to cathode  16  of the last cell in the series. Output potential V is the combined potential generated by all of the individual cells  12 .  
         [0018]    Furthermore, several groups of PV cells, each group comprising a plurality of PV cells connected in-series, can be connected together in any desired arrangement, such as in series or in parallel or a combination thereof, to provide an overall working PV device having a desired electrical potential.  
         [0019]    [0019]FIG. 2 shows a side view of a plurality of organic PV cells  12  connected in series and disposed on a substrate  150 . Substrate  150  can be any electrically non-conducting material, such as glass, ceramic, wood, paper, or polymeric materials. Polymeric materials, such as polyesters, polycarbonates, poly(ethylene terephthalate) (“PET”), polyimides, polyetherimides, or silicones, are suitable. Cathodes  16  are provided on substrate  150 , each cathode being separated from the other cathodes. A layer  15  of an organic semiconducting electron acceptor material is disposed on cathode  16 , leaving a portion of cathode  16  uncovered for subsequent electrical connection. A layer  17  of an organic semiconducting electron donor material is disposed on layer  15 . An anode layer  14  is disposed on layer  17 . An electrical connection  18  comprising a high-conductivity material is formed to connect cathode  16  of one organic PV cell  12  to anode  14  of another adjacent organic PV cell. Alternatively, separate electrical connections  18  may be eliminated by extending anode  14  of a PV cell  12  to a cathode  16  of an adjacent PV cell, as illustrated in FIG. 3. It should be understood that the roles of the electrodes  14  and  16  can be reversed. In other words, electrode  14  can be an anode, and electrode  16  can be a cathode. In this case, layer  15  is an electron acceptor layer, and layer  17  is an electron donor layer. The group of PV cells  12  can further be protected by a substantially transparent protective barrier coating. The term “substantially transparent” means allowing at least 80 percent, preferably at least 90 percent, and more preferably at least 95 percent, of incident electromagnetic (“EM”) radiation to pass through a film having a thickness of about 0.5 micron at an incident angle less than about 10 degrees. The term “electromagnetic radiation” means electromagnetic radiation having wavelength in the range from ultraviolet (“UV”) to infrared (“IR”), such as from about 100 nm to about 1 mm. The organic semiconducting materials preferably absorb strongly in the wavelength range of sunlight. Suitable materials for each of the elements of the PV device are disclosed below.  
         [0020]    Photons absorbed in organic semiconducting layers  15  and  17  produce excited electron-hole pairs (or excitons) that migrate to the junction between layers  15  and  17  where they dissociate into free electrons and holes, which migrate to the respective electrodes to be collected. The life time and diffusion length of excitons depend upon the nature of the organic semiconducting materials, but are typically very short. Exciton diffusion length has been estimated to be on the order of about 10 nm. The thicknesses of layers  15  and  17  ideally should not be much greater than the diffusion length, preferably smaller than about 100 nm. However, as the thicknesses of layers  15  and  17  decrease, the probability for short circuits through defects in the organic semiconducting layers increases. In addition, as the surface area of a cell increases, the probability for introducing defects into the cell also increases. Such defects can be in the form of, for example, pin holes, scratches, tears, conducting impurities, etc. When such a defect exists in such thin organic layers, a short circuit between electrodes  14  and  16  through the defect can easily occur. Such a short circuit renders a cell  12  inoperative because the charges will flow preferentially through the defect, and a charge separation will not result. Therefore, if a PV device consisting of only one large PV cell such that its surface area satisfies the energy requirement has a defect, the whole device will not produce energy. On the contrary, a PV device of the present invention comprising a plurality of PV cells connected in series avoids such a result. Even if one or more PV cells have short circuits, the remaining cells still are operative and produce electrical energy.  
         [0021]    Alternatively, electrode  16  can be the anode, and electrode  14  can be the cathode. In this case, layer  17  comprises an electron acceptor material, and layer  15  comprises an electron donor material.  
         [0022]    In another embodiment of the present invention, each organic PV cell further comprises one or more layers that enhance the transport of charges to the electrodes. For example, a layer of electron transport can be disposed between the cathode and the layer of electron acceptor material. Suitable materials for electron transport are metal organic complexes of 8-hydroxyquinoline, such as tris(8-quinolinolato) aluminum; stilbene derivatives; anthracene derivatives; perylene derivatives; metal thioxinoid compounds; oxadiazole derivatives and metal chelates; pyridine derivatives; pyrimidine derivatives; quinoline derivatives; quinoxaline derivatives; diphenylquinone derivatives; nitro-substituted fluorine derivatives; and triazines. A layer of hole transport material can be disposed between the anode and the electron donor layer. Suitable materials for hole transport are triaryidiamine, tetraphenyldiamine, aromatic tertiary amines, hydrazone derivatives, carbazole derivatives, triazole derivatives, imidazole derivatives, oxadiazole derivatives having an amino group, and polythiophene. The electron and hole transport materials may be deposited on the underlying layer by a method selected from the group consisting of physical vapor deposition, chemical vapor deposition, spin coating, and spraying, using a mask.  
         [0023]    Another embodiment of the present invention is illustrated in FIG. 3. PV device  10  comprises a plurality of organic PV cells  12  connected in series. Each organic PV cell  12  comprises the elements disclosed above. In addition, a circuit element  30  is connected in parallel with an organic PV cell  12 . Circuit element  30  provides an electrical by-pass to the associated organic PV cell when there is an interruption of charge flow to either the anode or the cathode of the organic PV cell through the organic semiconducting layers. Such an interruption can occur, for example, when there is a separation between two adjacent layers in the PV cell, such as between the organic semiconducting layers, or between an electrode and an adjacent organic semiconducting layer. Such a separation may be a defect resulting, for example, from the manufacturing, or from a long-term use of the organic PV cell. Circuit elements  30  are selected from the group consisting of resistors, diodes, varistors, and combinations thereof.  
         [0024]    Modules, each comprising a plurality of organic PV cells connected in series, can be arranged to cover a desired large area to collect photon energy from sunlight, and generate electrical energy. It is desirable to mount the organic PV cells on flexible substrates, such as a polymeric film comprising one of the polymers disclosed above. Then the modules can be installed on surfaces of any curvature. In one embodiment, the modules can be installed on rooftops or outside walls of buildings.  
         [0025]    Generally, the electrodes are made of materials having different work functions in order to induce an electric field across the PV cell. Cathode  16  is typically made of a metal having a low work function, such as one selected from the group consisting of K, Li, Na, Mg, La, Ce, Ca, Sr, Ba, Al, Ag, In, Sn, Zn, Zr, Sm, Eu, mixtures thereof, and alloys thereof. The cathode material can be deposited on substrate  150  to form separated cathodes  16  by physical vapor deposition, chemical vapor deposition, electron beam evaporation, sputtering, or electroplating, using a mask. Alternatively, a metal film can be deposited on the entire surface of substrate  150 , and then is selectively etched to leave behind a pattern of cathodes  16 . As another alternative, a negative pattern is formed on the substrate (for example, using photolithography), and the resultant pattern is subject to a plating treatment to produce the pattern of cathodes  16 . Typically, the thickness of cathode  16  is in the range from about 10 nm to about 1000 nm.  
         [0026]    Anode  16  is typically made of an electrically conducting material having a higher work function. In an embodiment in which incident EM radiation impinges on the anode side, anode  16  is made of a substantially transparent material, such as one selected from the group consisting of indium tin oxide (“ITO”), tin oxide, indium oxide, zinc oxide, indium zinc oxide, zinc indium tin oxide, antimony oxide, and mixtures thereof. Anode  16  can be deposited on the underlying layer by a method selected from the group consisting of physical vapor deposition, chemical vapor deposition, electron beam evaporation, sputtering, and electroplating, using a mask. Alternatively, a negative pattern is formed on the substrate (for example, using photolithography), and the resultant pattern is subject to a plating treatment to produce the pattern of anodes  14 . A thin, substantially transparent layer of a metal is also suitable. Such a metal may be selected from the group consisting of Au, Co, Ni, Pt, mixtures thereof, and alloys thereof. The thickness of anode  14  is typically in the range from about 50 nm to about 400 nm, preferably from about 50 nm to about 200 nm.  
         [0027]    Suitable electron acceptor materials for layer  15  are perylene tetracarboxidiimide, perylene tetracarboxidiimidazole, anthtraquinone acridone pigment, polycyclic quinone, naphthalene tetracarboxidiimidazole, CN- or CF 3 -substituted poly (phenylene vinylene), and Buckminsterfullerene (C 60 ).  
         [0028]    Suitable electron donor materials for layer  17  are metal-free phthalocyanine; phthalocyanine pigments containing copper, zinc, nickel, platinum, magnesium, lead, iron, aluminum, indium, titanium, scandium, yttrium, cerium, praseodymium, lanthanum, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium; quinacridone pigment; indigo and thioindigo pigments; merocyanine compounds; cyanine compounds; squarylium compounds; hydrazone; pyrazoline; triphenylmethane; triphenylamine; conjugated electroconductive polymers, such as polypyrrole, polyaniline, polythiophene, polyphenylene, poly(phenylene vinylene), poly(thienylene vinylene), poly (isothianaphthalene); and poly(silane).  
         [0029]    The thickness of layer  15  or  17  is typically in the range from about 5 nm to about 300 nm, preferably from about 10 nm to about 100 nm. The organic semiconducting material is typically deposited on the underlying layer by a method selected from the group consisting of vacuum deposition, spin coating, spraying, and ink-jet printing. The methods of vacuum deposition, spin coating, and spraying are conveniently carried out using a mask. The ink-jet printing can be carried out using a computer-aided design or computer-aided manufacturing software to control the locations where the material is laid down. Alternatively, a film of a an organic semiconducting material is deposited on the entire surface area, and then is patterned using a laser ablation method to leave behind material at desired locations. When the desired material is a polymer, its monomer can be deposited first, and then polymerized.  
         [0030]    In another embodiment of the present invention, a group of organic PV cells connected in series can be protected from attack by reactive species in the environment, or from physical damage by providing a protective barrier coating disposed on the entire group. Such a protective barrier can advantageously comprise a plurality of alternating layers of at least an organic material and an inorganic material. For example, a layer of a polymer selected from the group consisting of polyacrylates, epoxy, silicone, silicone-functionalized epoxy, polycarbonates, and polyesters is first deposited on the entire group. The polymer can be deposited by a method selected from the group consisting of vacuum deposition, physical vapor deposition, chemical deposition, casting, spin coating, dip coating, and spraying. Then a layer of an inorganic material is deposited on the polymer layer by a method selected from the group consisting of physical vapor deposition, chemical vapor deposition, sputtering, electron beam deposition, and electroplating. Suitable inorganic materials for this layer are metals, metal nitrides, metal carbides, metal borides, metal oxides, and mixtures thereof. Alternatively, a protective barrier can comprise a polymer having low diffusion coefficients of reactive gases, such as oxidizing species and water vapor.  
         [0031]    In one embodiment of the method of making a plurality of PV cells, successive layers  16 ,  15 ,  17 , and  14  can be formed by a deposition method through a series of masks applied successively, each providing an appropriate pattern for the specific layer. Non-limiting examples of suitable deposition methods are physical vapor deposition, chemical vapor deposition, spin coating, spray coating, casting, sputtering, and electron beam vaporization. The method is selected to be compatible with the material deposited. Alternatively, the layers of PV cells can be formed by a combination of applying masks and selective patterning by, for example, cutting, etching, or ablating.  
         [0032]    In another embodiment of the method of making a plurality of PV cells, layers  15  and  17  of organic semiconducting materials, and anode layer  14  are formed successively using a shadow mask. FIG. 4 shows the steps of such a method. First, a substrate  150  comprising one of the substrate materials disclosed above is provided in step (a). Substrate  150  has a plurality of distinct and separate first electrodes  16  formed thereon, such as by physical vapor deposition, chemical vapor deposition, sputtering, or electron beam deposition, using a mask. A layer of first electrode material may be deposited on the entire surface of substrate  150 ; then the layer is etched to form the first electrode pattern. In step (b), a plurality of walls  50  is formed on and near the edge of electrodes  16 . One wall  50  is disposed on each electrode  16 . Walls  50  can be formed from a negative-working photoresist composition, for example, by spin-coating and patterning by a photolithographic processing step. Walls  50  provide a shadow for the deposition of subsequent layers. In step (c), a first semiconducting material is deposited on electrodes  16  at an angle θ 1  with respect to a normal to the surface of substrate  150  to form a layer  15 . For example, if first electrode  16  is a cathode, layer  15  comprises an electron acceptor. If first electrode  16  is an anode, layer  15  comprises an electron donor. In step (d), a second semiconducting material is deposited on layer  15  at an angle θ 2  to form layer  17 . If layer  15  comprises an electron acceptor, layer  17  comprises an electron donor. If layer  15  comprises an electron donor, layer  17  comprises an electron acceptor. Angle θ 2  can be the same or different than angle θ 1 . In step (e), a second electrode material is deposited on layer  17  at an angle θ 3  to form second electrode  14 . Deposition using a show mask effect of walls  50 , as disclosed here, reduces the effort in forming layers  15 ,  17 , and  14 . The work piece remains in place, and only the source of the deposited material and the deposition angle need be changed. Subsequently, walls  50  can be optionally removed by, for example, laser ablation or etching. In step (f), interconnects  18  are formed, each connecting first electrode  16  of a PV cell to second electrode  14  of an adjacent PV cell. Interconnects  18  can be formed by any suitable method, such as physical vapor deposition, chemical vapor deposition, sputtering, or electron beam deposition, using a mask.  
         [0033]    Each of electrodes  16 , walls  50 , layers  15 ,  17 , and  14 , and interconnects  18  can be formed in strips extending across a first dimension of substrate  150 . After all of the deposition steps are complete, the substrate having the layers formed thereon can be cut in the second dimension of substrate  150  to provide groups of PV cells connected in series.  
         [0034]    While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations, equivalents, or improvements therein may be made by those skilled in the art, and are still within the scope of the invention as defined in the appended claims.