Patent Publication Number: US-2006016473-A1

Title: Dye-sensitized solar cell employing photoelectric transformation electrode and a method of manufacturing thereof

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims the priority of Korean Patent Application No. 0.10-2004-0049728, filed on Jun. 29, 2004, in the Korean Intellectual Property Office, which is hereby incorporated by reference for all purposes as if fully set forth herein.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a solar cell, and more particularly, to a dye-sensitized solar cell including a transition metal oxide nanoparticle semiconductor electrode. Specifically, the present invention relates to a dye-sensitized solar cell including a transition metal oxide nanoparticle semiconductor electrode, in which metal wires are provided in spaces between unit cells constituting a module, which increases the transfer rate of excited electrons into the semiconductor electrode and significantly decreases a reduction in photoelectric transformation efficiency that may be caused during fabrication of a large-scale module, thereby increasing photocurrent.  
      2. Description of the Related Art  
      Currently available dye-sensitized solar cells, commonly called “Graetzel cells,” are photoelectrochemical solar cells using a photosensitive dye molecule and an oxide semiconductor made of titanium oxide nanoparticles. The dye-sensitized solar cells have a lower manufacturing cost relative to a conventional silicon-based solar cells, and include a transparent electrode that enables them to be used in windows installed in external walls of buildings, glasshouses, etc. Therefore, a lot of research has been conducted relating to dye-sensitized solar cells.  
       FIG. 1  is a schematic view illustrating a conventional dye-sensitized solar cell. Referring to  FIG. 1 , a conventional dye-sensitized solar cell includes a first electrode  1  and a second electrode  2 . A porous film  3 , in which a dye  5  is adsorbed, and an electrolyte  4  are provided between the first electrode  1  and the second electrode  2 . When sunlight is incident in the dye-sensitized solar cell, photons are absorbed in the dye  5 . Electrons are excited in the dye  5  and injected into a conduction band of transition metal oxide constituting the porous film  3 . After the injection, the electrons are transported or attracted to the first electrode  1  and then the electrons transfer electric energy to an external circuit. The electrons, which have fallen to a lower energy level by the energy transfer, are subsequently sent to the second electrode  2 . The dye  5  is returned to an original state after the number of electrons corresponding to the number of the electrons injected into the conduction band of the transition metal oxide of the porous film  3  are supplied from the electrolyte  4 . The electrolyte  4  receives electrons from the second electrode  2  using an oxidation and reduction, i.e., redox, reaction and then supplies the electrons to the dye  5 .  
      Conventional solar cells as described above have a low manufacturing cost and are environmental friendly. However, energy transition efficiency may be lowered by recombination of electrons and holes at an interface between a first electrode coated with a porous film and an electrolyte, which restricts practical application. In view of this problem, a dye-sensitized solar cell with the structure shown in  FIG. 2  has been proposed.  
      Referring to  FIG. 2 , a solar cell has a sandwich structure in which two plate electrodes, i.e., a first electrode  10  and a second electrode  20  face with each other. A porous film  30  made of nanoparticles is coated on or directly on a surface of the first electrode  10 . A photosensitive dye, in which electrons are excited by absorbing visible light, is attached to surfaces of the nanoparticles of the porous film  30 . The first electrode  10  and the second electrode  20  are bonded and fixed by a support  60  and a space defined between the first electrode  10  and the second electrode  20  is filled with a redox electrolyte  40 .  
      As the first electrode  10 , there is used a transparent plastic substrate or a glass substrate  11  coated with a conductive film  12  made of indium tin oxide, etc. A buffer layer  50  made of at least two layers is formed on a surface of the conductive film  12  of the first electrode  10 . The buffer layer  50  includes a first layer  51  made of a material with a conduction band energy level lower than a conduction band energy level of the porous film  30  and a second layer  52  made of a material with a conduction band energy level higher than the conduction band energy level of the first layer  51 . The first layer  51  and the second layer  52  are made of a material with a particle size smaller than the nanoparticles constituting the porous film  30 , and thus, have a dense structure. The first layer  51  serves to improve interface characteristics between the first electrode  10  and the electrolyte  40 , and thus, to reduce hole-electron recombination at the interface between the first electrode  10  and the electrolyte  40 , thereby enhancing electron trapping or collection characteristics.  
      In the above-described dye-sensitized solar cells, the photoelectric transformation efficiency of the solar cells is proportional to the amount of electrons generated by sunlight absorption. In this regard, to increase the photoelectric transformation efficiency, the following methods have been proposed: methods of increasing the reflectivity of a platinum electrode, increasing sunlight absorption using a plurality of micrometer-sized semiconductor oxide photo-scattering particles, or increasing the absorption of photons into a dye, to increase the amount of electrons; a method of preventing annihilation of excited electrons by electron-hole recombination; a method of improving sheet resistance of an interface and an electrode to increase the transfer rate of excited electrons, etc. However, photoelectric transformation efficiency may be lowered during fabrication of large-scale solar cells or modules, which restricts practical applications and renders large-scale solar cell fabrication difficult.  
     SUMMARY OF THE INVENTION  
      The present invention provides a dye-sensitized solar cell in which metal wires are provided on a transparent electrode that is used as an oxide semiconductor electrode to increase a transfer rate of an electron and thus improve reduction in photoelectric transformation efficiency.  
      In particular, the present invention discloses a dye-sensitized solar cell in which metal wires are provided in a spacer to prevent direct contact of the metal wires with unit cells constituting a module. Therefore, a short circuit by direct contact of the metal wires with an electrolyte solution or an oxide semiconductor layer and corrosion of the metal wires by the electrolyte solution is prevented. Further, there is no need to add a blocking layer, which is required for a common metal wire layer. The present invention discloses a dye-sensitized solar cell using a photoelectric transformation electrode, the solar cell includes a semiconductor electrode, a counter electrode provided opposite to the semiconductor electrode, an oxide semiconductor layer provided between the semiconductor electrode and the counter electrode and having a dye adsorbed thereon, an electrolyte solution provided between the semiconductor electrode and the counter electrode, a spacer partitioning a space defined between the semiconductor electrode and the counter electrode to form at least one unit cell, and a metal wire at least partially patterned in spaces defined between the at least one unit cell.  
      The present invention discloses a method of manufacturing a dye-sensitized solar cell that uses a photoelectric transformation electrode, the method including preparing a semiconductor substrate having a conductive layer, determining a position on the conductive layer for a spacer to be provided to define a unit cell, forming a metal wire in the determined position on the conductive layer of the semiconductor substrate, forming the spacer over the metal wire, and filling the unit cell with an electrolyte solution, wherein the spacer insulates the metal wire from the electrolyte solution.  
      It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
       FIG. 1  is a schematic view illustrating a conventional dye-sensitized solar cells.  
       FIG. 2  is a schematic sectional view illustrating dye-sensitized solar cells.  
       FIG. 3  is a schematic sectional view illustrating a dye-sensitized solar cell according to an embodiment of the invention.  
       FIG. 4  is a perspective view of the solar cell of  FIG. 3 . 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION  
      Dye-sensitized solar cells according to embodiments of the invention are described below with reference to the accompanying drawings.  
       FIG. 3  is a schematic sectional view illustrating a dye-sensitized solar cell according to an embodiment of the invention and  FIG. 4  is a schematic perspective view of the solar cell of  FIG. 3 .  
      Referring to  FIG. 3  and  FIG. 4 , a dye-sensitized solar cell using a photoelectric transformation electrode has a sandwich like structure in which two plate electrodes, i.e., a semiconductor electrode  110  and a counter electrode  120 , face with each other. For example, the two electrodes may be substantially parallel with each other. An oxide semiconductor layer  130  having a dye adsorbed therein, is provided between the semiconductor electrode  110  and the counter electrode  120 . In particular, the oxide semiconductor layer  130  is formed on a surface of the semiconductor electrode  110 .  
      A redox electrolyte solution  140  is provided or filled in a space between the semiconductor electrode  110  and the counter electrode  120 . A spacer  160 , e.g., support, serving as a partition wall is provided in the space between the semiconductor electrode  110  and the counter electrode  120 , so that the space defined between the semiconductor electrode  110  and the counter electrode  120  is partitioned to form unit cells  142  separated from each other by a predetermined distance. Metal wires  150  are patterned between the unit cells  142 , i.e., in spaces defined between the unit cells  142 . Thus, the number of unit cells  142  is determined by the number of partitioned spaces provided between the semiconductor electrode  110  and the counter electrode  120 .  
      The semiconductor electrode  110  includes a semiconductor electrode substrate  111  and a transparent conductive film  112  for a semiconductor electrode formed on a surface of the semiconductor electrode substrate  111 . The semiconductor electrode substrate  111  may be made of a transparent material, for example, a glass, polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN), or polycarbonate (PC), and may be used as a cathode of a solar cell. The transparent conductive film  112  coated on the surface of the semiconductor electrode substrate  111  may be made of a transparent conductive material, such as indium tin oxide (ITO) or fluorine tin oxide (FTO). Therefore, sunlight can be incident in and transmitted through the transparent semiconductor electrode  110  having the structure discussed above.  
      Meanwhile, the counter electrode  120  positioned opposite to the semiconductor electrode  110  includes a counter electrode substrate  121 , a transparent conductive film  122  for a counter electrode formed on a surface of the counter electrode substrate  121 , and a conductive film  123  formed on a surface of the transparent conductive film  122 , wherein the conductive filing includes platinum or a noble metal.  
      The counter electrode substrate  121  may be is a transparent plastic substrate, including a glass substrate or one of PET, PEN, PC, polypropylene (PP), polyimide (PI), and tri-acetyl-cellulose (TAC). The transparent conductive film  122  for a counter electrode may be a transparent and conductive film made of ITO or FTO.  
      The conductive film  123  formed on the surface of the transparent conductive film  122  may be a conductive film made of platinum that is obtained by wet coating of a solution of H 2 PtCl 6  in an organic solvent (methanol, ethanol, isopropylalcohol, etc.) on the transparent conductive film  122 . The wet coating is followed by high-temperature treatment at 400° C., e.g., heat treating, or more in air or an oxygen atmosphere or by electroplating or physical vapor deposition (PVD) (techniques such as sputtering, e-beam evaporation, etc.). Here, the wet coating may be performed by spin coating, dip coating, or flow coating.  
      Thus, in a nonlimiting embodiment of the invention, a solar cell includes the semiconductor electrode  110  on which photosensitive dye molecules are adsorbed, the counter electrode  120  in which the conductive film  123  containing platinum is coated, and the redox electrolyte solution  140  filled between the semiconductor electrode  110  and the counter electrode  120 . The semiconductor electrode  110  includes the semiconductor electrode substrate  111  which may be a transparent conductive glass substrate coated with ITO or FTO. The metal wires  150  are arranged in the spacer  160 . The spacer  160  provides a support structure, e.g., side support, for the oxide semiconductor layer  130  formed on the semiconductor electrode substrate  111  coated with the transparent conductive film  112 . The spacer  160  is provided or installed to partition the space between the semiconductor electrode  110  and the counter electrode  120  and form unit cells  142  to be filled with the electrolyte solution  140 , according to either a dry or wet method.  
      The semiconductor electrode  110  and the counter electrode  120  are attached by arranging the conductive film  123  including at least platinum and the oxide semiconductor layer  130  to face with each other and providing a polymer layer made of SURLYN (trade name of Dupont™) used as the spacer  160  on the metal wires  150 , and then pressing the polymer layer used as the spacer  160  on the metal wires  150  when the same is at a temperature of approximately 100° C. The spacer  160  may be formed by various other techniques, such as by a dispenser method using one of various polymer adhesives, in addition to SURLYN.  
      For example, the redox electrolyte solution  140  is prepared by dissolving an iodine (I) source, i.e., 0.5M tetrapropylammonium iodide or 0.8M lithium iodide (LiI) and 0.05M iodine (I 2 ) in acetonitrille.  
      The metal wires  150  are isolated or separated from the electrolyte solution  140  filled in the unit cells  142  by the spacer  160 . The metal wires  150  may be made of Au, Ag, Al, Pt, Cu, Fe, Ni, Ti, Zr, or an alloy of two or more of the foregoing metals.  
      According to an embodiment of the invention, the metal wires  150  may be formed by patterning a metal paste of one or more elements selected from Au, Ag, Al, Pt, Cu, Fe, Ni, Ti, and Zr by a known patterning method, such as a screen printing method, a printing method, or a dispenser method. Alternatively, the metal wires  150  may be formed by patterning a colloidal solution of one or more selected from Au, Ag, Al, Pt, Cu, Fe, Ni, Ti, and Zr by a screen printing method, a printing method, or a dispenser method.  
      In addition, the metal wires  150  may be formed by etching a metal film made of one or more elements selected from Au, Ag, Al, Pt, Cu, Fe, Ni, Ti, and Zr using a combination of a lithography process with one of chemical deposition, sputtering, and electrodeposition.  
      Thus, in a non-limiting example, to isolate the metal wires  150  from the electrolyte solution  140 , the metal wires  150  are formed narrower than the spacer  160 , e.g., metal wire is fully contained in the spacer  160 . In other words, the metal wires  150  are formed such that they are buried in the spacer  160 .  
      As shown in  FIG. 4 , the metal wires  150  surround the unit cells  142  since they are positioned in spaces that are formed to operate as boundaries to partition the unit cells. Since the metal wires  150  are formed to be buried in the spacer  160 , either subsequently or during formation of the metal wires  150 , even though the number of the unit cells  142  increases for fabrication of large-scale solar cells or modules, the metal wires  150  can be appropriately installed. With such an arrangement, the transfer rate of excited electrons into the semiconductor electrode  110  increases, thereby preventing a reduction in photoelectric transformation efficiency.  
      The metal wires  150  have a thickness or diameter of approximately 0.1 to 30 μm. The unit cells  142  are generally formed in a square or rectangular shape, however the invention is not limited thereto. The unit cells  142  may be formed having any predetermined shape. However, the metal wires  150  should be buried in the spacer  160  according to the shapes of the unit cells  142  partitioned by the spacer  160  so that the metal wires  150  are not exposed to the counter electrode  120  or the electrolyte solution  140 .  
      For example, when the unit cells  142  are formed in a square shape, then a side length of each unit cell is in the range of approximately 0.1 to 30 mm. It is understood that there may be more than one unit cell  142 . As shown in  FIG. 3  and  FIG. 4 , the metal wires  150  are formed on the semiconductor electrode  110  along the spacer  160  partitioning the unit cells  142  in such a way to be buried in the spacer  160 .  
      A method of manufacturing a dye-sensitized solar cell including a semiconductor electrode on which metal wires are formed according to the embodiment of the invention is described.  
      A semiconductor electrode substrate  111  is prepared. For example, the semiconductor electrode substrate  111  may be a substrate having a good transparency that is and capable of being used as a cathode of a solar cell, for example, a glass substrate, a PET substrate, a PEN substrate, or a PC substrate, or a substrate coated with a transparent conductive material such as ITO or FTO.  
      A position intended for a spacer  160 , e.g. predetermined position, is determined on a conductive layer of the semiconductor electrode substrate that defines a position intended for an oxide semiconductor layer  130 . The spacer  160  also prevents an electrode short circuit between unit cells  142  during fabrication of modules. The spacer  160  also defines a space between the semiconductor electrode  110  and the counter electrode  120  to be filled with an electrolyte solution. When the position intended for the spacer is determined, metal wires are formed having the same pattern or space as the spacer will be formed. The metal wires should be narrower than the spacer in order to prevent direct contact of the metal wires with the electrolyte solution by exposure of the metal wires.  
      The metal wires may be patterned according various methods and patterning techniques. According to an embodiment of the invention, the patterning is performed directly on a surface of the semiconductor electrode substrate. For example, the patterning may be performed using a paste or a colloidal solution of highly conductive metal particles selected from gold, silver, platinum, and an alloy thereof and applied to the surface using one of the following techniques, a screen printing technique, a printing technique, and a dispenser technique. Further, patterning may also be performed by combining a lithography process with a deposition process, such as chemical vapor deposition (CVD) or sputtering. Patterning may additionally be performed by etching a metal film formed by electrodeposition or electroplating.  
      When the metal wires  150  are patterned by any one of the above-described methods, an oxide semiconductor paste is coated or applied to a surface of the semiconductor electrode  110  between the metal wires and heated to form necking between oxide particles. A photosensitive dye is then absorbed into the resultant semiconductor electrode substrate structure, which completes the formation of the oxide semiconductor electrode  110 . For example, the photosensitive dye may be selected from one of the following a complex compound of a metal such as Al, Pt, Pd, Eu, Pb, or Ir, wherein the photosensitive dye is preferably formed of a ruthenium dye (Ru-dye) molecule.  
      A counter electrode  120  is additionally prepared. The counter electrode  120  is formed by a wet coating process, e.g., spin coating, dip coating, or flow coating, of a transparent or glass substrate that coated with ITO or FTO or a transparent conductive polymer film having a solution of H 2 PtCl 6  in an organic solvent, such as methanol, ethanol, isopropylalcohol, etc. followed by high temperature treatment at 400° C. or more in air or an oxygen atmosphere, or by coating a conductive film made of platinum on the glass substrate using electroplating or PVD, such as sputtering or e-beam evaporation.  
      The semiconductor electrode and the counter electrode are then attached or coupled by arranging or positioning the conductive film and the oxide semiconductor layer to face each other, e.g., parallel with each other, and a polymer layer, e.g, SURLYN, to form a spacer on the metal wires  150 . about the polymer layer is then pressed on the metal wires  150  when the same is at a temperature of approximately 100° C. Alternatively, the spacer  160  may also be formed by a dispenser method using one of various polymer adhesives, in addition to SURLYN.  
      A redox electrolyte solution  140  is then supplied or filled in unit cells  142 . The redox electrolyte solution  140  may be prepared by dissolving an iodine source, such as 0.5M tetrapropylammonium iodide or 0.8M LiI, and 0.05M I 2 , in acetonitrile. The electrolyte solution thus prepared is then injected or supplied the unit cells  142  via an inlet  136  that is formed through the counter electrode. After the electrolyte solution  140  is supplied, the inlet is sealed or covered by a sealing portion  134 . The sealing portion  134  may be made of an epoxy resin or SURLYN. A glass (see  132  of  FIG. 3 ) for sealing the inlet is disposed on the sealing portion to thereby complete a solar cell. It is understood that multiple such inlets  136  may be formed into the counter electrode  120  to provide for the electrolyte solution  140 .  
      According to a dye-sensitized solar cell described in at least the embodiments of the present invention discussed above, metal wires are arranged on a transparent electrode used as an oxide semiconductor electrode. Therefore, the transfer rate of excited electrons in a dye into the oxide semiconductor electrode can be increased, and thus, reduction in photoelectric transformation efficiency that may occur in fabrication of large-scale dye-sensitized solar cells or modules can be improved.  
      A dye-sensitized solar cell of the present invention exhibits approximately a 35% increase in photoelectric transformation efficiency as compared to a common solar cell without metal wires. Further, the metal wires are provided in a spacer, which prevents a short circuit from occurring between the oxide semiconductor electrode and a counter electrode and defines a space to be filled with an electrolyte. Therefore, there is no need to form a separate coating layer, often referred to as either a protective layer or blocking layer to prevent a short circuit from occurring between the metal wires and an electrolyte solution or an oxide layer and corrosion of the metal wires by the electrolyte solution.  
      It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.