Patent Publication Number: US-2007095390-A1

Title: Solar cell and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims priority to and the benefit of Korean Patent Application No. 2005-104883 filed in the Korean Intellectual Property Office on Nov. 3, 2005, the entire disclosure of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      Aspects of the present invention relate to a solar cell, and in particular, to a high efficiency solar cell and a method of manufacturing the same.  
      2. Description of the Related Art  
      Generally, a solar cell generates electrical energy using solar energy, thereby supplying environmentally-friendly energy from an unlimited energy source with a long-term life span. Types of solar cells include silicon solar cells and dye-sensitized solar cells.  
      The dye-sensitized solar cell has better photoelectric conversion efficiency, lower production cost, and more flexible processing compared to the silicon solar cell. Furthermore, since the dye-sensitized solar cell has transparent electrodes, it may be used in constructing outer walls for buildings or greenhouses.  
      However, since the photoelectric conversion efficiency of solar cells is not high, solar cells are not yet in widespread use. Many studies have been carried out in order to enhance the photoelectric conversion efficiency, but most of the studies have been limited to the field of development of new dyes. In this connection, it is desirable that a new technology for enhancing the photoelectric conversion efficiency of the solar cell be developed.  
     SUMMARY OF THE INVENTION  
      A solar cell with enhanced photoelectric conversion efficiency by improving the collection of electrons, and a method of manufacturing the solar cell are provided.  
      According to one aspect of the present invention, the solar cell may include an electrode and a first porous film formed on the electrode with pores and containing metallic oxide particles. The metallic oxide particles may have a mean particle diameter of 5 nm-14 nm. The pores of the first porous film may have a mean pore size of 7.5 nm-15 nm.  
      According to an aspect of the present invention, the first porous film may have a thickness of 10 nm-3000 nm, preferably of 10 nm-1000 nm.  
      According to an aspect of the present invention, an additional porous film containing metallic oxide particles may be formed on the first porous film. The metallic oxide particles of the additional porous film may have a mean particle diameter greater than that of the metallic oxide particles of the first porous film.  
      According to an aspect of the present invention, the mean particle diameter of the metallic oxide particles of the additional porous film may be in the range of 15 nm-50 nm.  
      According to an aspect of the present invention, the additional porous film may be thicker than the first porous film. The additional porous film may have a thickness of 5 μm-40 μm.  
      According to another aspect of the present invention, a method of manufacturing a solar cell may include forming a first porous film through self-assembling. Forming a first porous film may include preparing a self-assembling composition, coating the self-assembling composition onto an electrode, and heat-treating the coated composition.  
      According to an aspect of the present invention, the self-assembling composition may contain a solvent, a block copolymer, and a metallic oxide precursor. The solvent may be selected from acetyl acetone, alcohol, and a combination thereof. The block copolymer may contain polyethylene oxide and polypropylene oxide. The metallic oxide precursor may be selected from alkoxide, chloride, and a combination thereof.  
      According to an aspect of the present invention, the self-assembling composition may be coated by dip coating, spin coating, or electrochemical coating.  
      According to an aspect of the present invention, forming an additional porous film by coating a paste in which metallic oxide particles are diffused may be conducted after heat-treating the coated composition.  
      Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a sectional view of a solar cell according to an embodiment of the present invention;  
       FIG. 2  is a flow diagram of the steps of producing the solar cell shown in  FIG. 1 ;  
       FIG. 3  is a sectional view of a solar cell according to another embodiment of the present invention;  
       FIG. 4  is an image of a surface of a porous film formed through self-assembling of a solar cell according to Example 1;  
       FIG. 5A  is an image of a section of a first electrode, a porous film, and an additional porous film of the solar cell according to Example 1;  FIG. 5B  is an enlargement of the indicated area of  5 B;  
       FIG. 6A  is an image of a section of a first electrode and an additional porous film of a solar cell according to Comparative Example 1;  FIG. 6B  is an enlargement of the indicated area of  6 B;  
       FIG. 7  is a graph illustrating the electric currents as functions of voltages of the solar cells according to Example 1 and Comparative Example 1;  
       FIG. 8  is a image of a section of a first electrode, a porous film, and an additional porous film of a solar cell according to Example 10;  
       FIG. 9  is a image of a section of a first electrode, a porous film, and an additional porous film of a solar cell according to Example 11; and  
       FIG. 10  is a image of a section of a first electrode and a porous film of a solar cell according to Example 12.  
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
      Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain aspects of the present invention by referring to the figures.  
       FIG. 1  is a sectional view of a solar cell according to an embodiment of the present invention.  
      As shown in  FIG. 1 , the solar cell according to the present embodiment includes a first substrate  10  and a second substrate  20  facing the first substrate  10 . The first substrate  10  is provided with a first electrode  11 , a first porous film  13 , and a dye  15  adsorbed on the first porous film  13 . (In the solar cell according to the embodiment of  FIG. 1 , the first porous film  13  is the only porous film formed on the first electrode  11 .) The second substrate  20  is provided with a second electrode  21 . An electrolyte  30  is disposed between the first and second electrodes  11  and  21 . A separate case (not shown) may be placed external to the first and second substrates  10  and  20 . This structure will now be explained more specifically.  
      In this embodiment, the first substrate  10  supports the first electrode  11 , and is formed of a transparent material to allow external light to pass therethrough. The first substrate  10  may be formed of transparent glass or plastic. The plastic may be selected from polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polypropylene (PP), polyimide (PI), and triacetyl cellulose (TAC). The first substrate  10  is not limited to these materials, and other materials are possible.  
      The first electrode  11 , provided on the first substrate  10 , may be formed of a transparent material, such as indium tin oxide (ITO), fluorine tin oxide (FTO), antimony tin oxide (ATO), zinc oxide, tin oxide, ZnO—Ga 2 O 3 , and ZnO—Al 2 O 3 . The first electrode  11  is not limited to these materials, and other materials are possible. The first electrode  11  may be formed as a transparent material-based single layer structure or laminated layer structure.  
      A first porous film  13  is formed on the first electrode  11 . The first porous film  13  contains extremely fine metallic oxide particles  131  with even or uniform particle diameters and is formed through self-assembling. The metallic oxide particles  131  may be joined by interparticle necking. Furthermore, the first porous film  13  has very fine pores that also have even or uniform sizes. Hence, the first porous film  13  has a nanoporous characteristic.  
      The mean particle diameter of the metallic oxide particles  131  of the first porous film  13  may be in the range of 5 nm-14 nm. The mean particle diameter of the metallic oxide particles  131  and the extent of interparticle necking may be controlled by varying the firing or heat treating temperature required for the formation of the first porous film  13 .  
      If the mean particle diameter of the metallic oxide particles  131  is less than 5 nm, the temperature of the firing process required to form such particles is too low to form the first porous film  13  in a stable manner. By contrast, if the mean particle diameter of the metallic oxide particles  131  exceeds 14 nm, the temperature of the firing process required to form such particles is so high that the first substrate  10  provided with the first porous film  13  may be damaged or bent. Furthermore, necking of the metallic oxide particles  131  is inhibited so that the amount the dye  15  that can be adsorbed on the first porous film  13  may be reduced. Accordingly, the efficiency of a solar cell containing the first porous film  13  may be lowered.  
      That is, in this embodiment, a first porous film  13  containing fine metallic oxide particles  131  with even particle diameters is formed on the first electrode  11 , and necking of the metallic oxide particles  131  is improved so that the interface contact characteristic between the metallic oxide particles  131  and the first electrode  11  can be enhanced. Consequently, the interface contact characteristic between the first porous film  13  and the first electrode  11  is enhanced.  
      In this embodiment, the mean pore size of the pores of the first porous film  13  is in the range of 7.5 nm-15 nm. If the mean pore size of the first porous film  13  is less than 7.5 nm, it is difficult for the electrolyte  30  to pass through the first porous film  13 . By contrast, if the mean pore size exceeds 15 nm, necking of the metallic oxide particles  131  is inhibited. When the first porous film  13  has a suitable mean pore size, the electrolyte  30  may easily pass therethrough, and necking of the metallic oxide particles  131  may be improved.  
      In this embodiment, the first porous film  13  may have a thickness of 10 nm-3000 nm. If the thickness of the first porous film  13  is less than 10 nm, it is difficult for the first porous film  13  to function satisfactorily. By contrast, if the thickness of the first porous film  13  exceeds 3000 nm, the relevant processing operations need to be repeated to form the first porous film  13 , thereby complicating the overall process. Therefore, in order to stably form the first porous film  13  by a one-time process, the thickness of the first porous film  13  may be in the range of 10 nm-1000 nm. However, the thickness of the first porous film  13  is not limited thereto, and the thickness may be varied pursuant to technological development.  
      The metallic oxide particles  131  may be formed with titanium oxide, zinc oxide, tin oxide, strontium oxide, indium oxide, iridium oxide, lanthanum oxide, vanadium oxide, molybdenum oxide, tungsten oxide, niobium oxide, magnesium oxide, aluminum oxide, yttrium oxide, scandium oxide, samarium oxide, gallium oxide, or strontium titanium oxide. It is preferable that the metallic oxide particles  131  are formed with titanium oxide of TiO 2 , tin oxide of SnO 2 , tungsten oxide of WO 3 , zinc oxide of ZnO, or a combination thereof. The metallic oxide particles  131  are not limited to these materials, and other materials are possible.  
      A dye  15  is adsorbed onto the surface of the first porous film  13  to absorb external light and generate excited electrons. The dye  15  may be formed with a metal complex containing aluminum (Al), platinum (Pt), palladium (Pd), europium (Eu), lead (Pb), iridium (Ir), or ruthenium (Ru). Since ruthenium, belonging to the platinum group, is capable of forming many organic metal complexes, a ruthenium-containing dye is commonly used. The metal complex is not limited to these materials, and other materials are possible. Furthermore, an organic dye may be also used, selected from coumarin, porphyrin, xanthene, riboflavin, and triphenylmethan. The organic dye is not limited to these materials, and other materials are possible.  
      The second substrate  20 , which faces the first substrate  10  in the assembled solar cell, supports the second electrode  21 , and is formed of a transparent material. As with the first substrate  10 , the second substrate  20  may be formed of glass or plastic. The plastic may be selected from polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polypropylene, polyimide, and triacetyl cellulose. The second substrate  21  is not limited to these materials, and other materials are possible.  
      The second electrode  21  formed on the second substrate  20  faces the first electrode  11 , and includes a transparent electrode  21   a  and a catalyst electrode  21   b . The transparent electrode  21   a  may be formed of a transparent material such as indium tin oxide, fluorine tin oxide, antimony tin oxide, zinc oxide, tin oxide, ZnO—Ga 2 O 3 , and ZnO—Al 2 O 3 . The transparent electrode  21   a  is not limited to these materials, and other materials are possible. The transparent electrode  21   a  may be formed as a single layer structure based on the transparent material or as a laminated layer structure. The catalyst electrode  21   b  activates a redox couple, and may be formed of platinum, ruthenium, palladium, iridium, rhodium Rh, osmium Os, carbon C, WO 3 , or TiO 2 . The catalyst electrode  21   b  is not limited to these materials, and other materials are possible.  
      The first and second substrates  10  and  20  are attached to each other using an adhesive  41 , and the electrolyte  30  is injected into the interior between the first and second electrodes  11  and  21  through holes  25   a  formed in the second substrate  20  and the second electrode  21 . The electrolyte  30  is uniformly diffused into the first porous film  13 . The electrolyte  30  receives and transfers electrons from the second electrode  21  to the dye  15  through reduction and oxidation. As a non-limiting example, the electrolyte may be an iodide-containing electrolyte. As a further non-limiting example, the electrolyte may comprise tetrapropylammonium iodide and iodine (I 2 ) in a solvent mixture of ethylene carbonate and acetonitrile. The holes  25   a  formed in the second substrate  20  and the second electrode  21  are sealed by an adhesive  42  and a cover glass  43 . The electrolyte  30  is not limited to a liquid electrolyte as described herein. For example, the electrolyte  30  may be a gel or solid electrolyte.  
      When an external light, such as sunlight, hits the interior of the solar cell, photons are absorbed into the dye so that the dye is shifted from a ground state to an excited state, thereby generating excited electrons. The excited electrons migrate into the conduction bands of the metallic oxide particles  131  of the first porous film  13 , and flow to an external circuit (not shown) through the first electrode  11 . Thereafter, the electrons are transferred to the second electrode  21 . Meanwhile, the iodide within the electrolyte  30  is oxidized into triiodide, and dye  15  that was oxidized by the transfer of electrons in response to the external light is reduced to its original state. The triiodide is reacts with the electrons transferred to the second electrode  21  and is thereby reduced to iodide. The solar cell thus operates due to the migration of the electrons.  
      Unlike other types of solar cells, such as a silicon solar cell, the dye-sensitized solar cell operates through an interface reaction at the interface between the first porous film  13  and the first electrode  11 . Therefore, it is beneficial to improve the contact characteristics of the interface. In this embodiment, a first porous film  13  containing fine, uniform metallic oxide particles  131  is formed on the first electrode  11  to improve the interface contact characteristics between the first electrode  11  and the first porous film  13 . Consequently, current collection is maximized to thereby enhance the photoelectric conversion efficiency.  
      Furthermore, in this embodiment, the mean pore size of the first porous film  13  is optimized so that the electrolyte  30  can easily pass through the first porous film  13  and necking of the metallic oxide particles  131  is improved. As a result, the photoelectric conversion efficiency can be further enhanced.  
      A method of manufacturing the solar cell will be specifically explained with reference to  FIGS. 1 and 2 .  FIG. 2  is a flow diagram of the operations of processing the solar cell shown in  FIG. 1 . Detailed explanation of the materials for the above-identified structural components of the solar cell will be omitted.  
      A first electrode  11  is formed on a first substrate  10  (ST 11 ). The first electrode  11  may be formed through sputtering, chemical vapor deposition, or spray pyrolysis deposition. The formation of the first electrode  11  is not limited to these methods, and other methods may be used.  
      Thereafter, a first porous film  13  is formed on the first electrode  11  through a self-assembly process (ST 13 ). For this purpose, a self-assembling composition is first prepared and then coated onto the first electrode  11 . The coated composition is heat-treated through a firing process. This will now be explained in more detail.  
      The self-assembling composition may contain a solvent, a block copolymer, and a precursor of metallic oxide. Various materials such as, for example, acetyl acetone and alcohol may be used as the solvent. The block copolymer may include, for example, polyethylene oxide and polypropylene oxide. The metallic oxide precursor is capable of forming a predetermined metallic oxide, and may include, for example, alkoxide and chloride. As a non-limiting example, the solvent may comprise about 60-90wt. % of the self-assembling composition.  
      The self-assembling composition is coated onto the first electrode  11  through dip coating, spin coating, or electrochemical coating or other methods of coating. In consideration of the components and characteristics of the self-assembling composition, the coated composition is heat-treated so that the block copolymer is removed, and metallic oxide particles are formed from the metallic oxide precursor.  
      With the self-assembling composition for forming a first porous film  13 , the metallic oxide precursor is uniformly diffused, thereby preventing the formation of metallic oxide particles with large particle diameters due to accumulation of the metallic oxide. Consequently, a first porous film  13  containing fine metallic oxide particles with uniform particle diameters and pores with a suitable mean pore size is formed. As the metallic oxide precursor on the first electrode  11  is converted into metallic oxide particles, the interface contact characteristics between the first porous film  13  and the first electrode  11  are further enhanced.  
      The first substrate  10 , with the first porous film  13  and the first electrode  11  formed thereon, is dipped in a dye-dissolved alcoholic solution for a predetermined period of time, thereby adsorbing the dye  15  onto the first porous film  13  (ST 17 ). However, the adsorbing of the dye  15  is not limited thereto and may be accomplished in various ways.  
      Meanwhile, a transparent electrode  21   a  and a catalyst electrode  21   b  are sequentially formed on a second substrate  20  to thereby form a second electrode  21  (ST 21 ).  
      The transparent electrode  21   a  may be formed through sputtering, chemical vapor deposition, or spray pyrolysis deposition. The formation of the transparent electrode  21  a is not limited to these methods, and other methods may be used.  
      The catalyst electrode  21   b  may be formed through a physical vapor deposition method, such as, for example, electrolyte plating, sputtering, or electron beam deposition, or a wet coating method, such as, for example, spin coating, dip coating, or flow coating. If the catalyst electrode  21   b  is formed of platinum, a solution of H 2 PtCl 6  dissolved in an organic solvent, such as methanol, ethanol, or isopropyl alcohol (IPA), may be coated onto the transparent electrode  21   a  through wet coating, and heat-treated at 400° C. under an air or oxygen atmosphere. However, the formation of the catalyst electrode is not limited thereto, and the process may be conducted in various ways.  
      Holes  25   a  are formed in the second substrate  20  and the second electrode  21 .  
      The first and second substrates  10  and  20  are arranged such that the first porous film  13  faces the second electrode  21 , and are attached to each other using an adhesive  41  (ST 30 ). A thermoplastic polymer film (such as, for example, a resin provided by DuPont under the registered trademark SURLYN™), an epoxy resin, or an ultraviolet hardener may be used as the adhesive  41 . If the adhesive  41  is formed of a thermoplastic polymer film, the thermoplastic polymer film may be disposed between the first and second substrates  10  and  20  and thermally pressed to thereby attach the first and second substrates  10  and  20  to each other.  
      An electrolyte  30  is injected into the interior between the first and second substrates  10  and  20  through the holes  25   a  formed in the second substrate  20  and the second electrode  21  (ST 40 ). The present description is for when the electrolyte  30  is in a liquid phase. If the electrolyte is in a solid or gel phase, other techniques may be used to provide the electrolyte  30  between the first and second substrates, Solar cells having solid or gel electrolytes are also within the scope of the present invention.  
      The holes  25   a  are sealed using an adhesive  42  and a cover glass  43  (ST 50 ), thereby completing a solar cell. A separate case (not shown) may be provided external to the first and second substrates  10  and  20 .  
      A solar cell according to another embodiment of the present invention and a method of manufacturing such will now be specifically explained with reference to  FIG. 3 . As the basic structural components of the solar cell according to the present embodiment are the same or similar to those described with reference to  FIG. 1 , explanation thereof will be omitted, and only the differences will be explained in detail. Like reference numerals will be used to refer to similar structural components shown in the drawings.  
       FIG. 3  is a sectional view of a solar cell according to another embodiment of the present invention.  
      In this embodiment, an additional porous film  53  containing metallic oxide particles  531  is formed on the first porous film  13 . The mean particle diameter of the metallic oxide particles  531  is in the range of 15 nm-50 nm, which is greater than the mean particle diameter of the metallic oxide particles  131  for the first porous film  13 . As a non-limiting example, the mean particle diameter of the metallic oxide particles  531  may be in the range of 15 nm-40 nm.  
      The additional porous film  53  may be formed through coating a paste in which nanometer-sized metallic oxide particles are diffused. The coating may be carried out by, for example, using a doctor blade, screen printing, spin coating, spraying, or wet coating, and then heat-treating the coated paste in a suitable manner.  
      The additional porous film  53  may be formed with a large thickness through a one-time process, and the additional porous film  53  may be thicker than the first porous film  13 . The additional porous film  53  may have a thickness of 5 μm-40 μm, and, as a non-limiting example, may have a thickness of 10 μm -30 μm. However, the thickness of the additional porous film  53  is not limited thereto.  
      As with the first porous film  13 , the additional porous film  53  may be formed of titanium oxide, zinc oxide, tin oxide, strontium oxide, indium oxide, iridium oxide, lanthanum oxide, vanadium oxide, molybdenum oxide, tungsten oxide, niobium oxide, magnesium oxide, aluminum oxide, yttrium oxide, scandium oxide, samarium oxide, gallium oxide, or strontium titanium oxide. It is preferable, but not necessary, that the metallic oxide particles  531  of the additional porous film  53  are formed of titanium oxide of TiO 2 , tin oxide of SnO 2 , tungsten oxide of WO 3 , zinc oxide of ZnO, or a combination thereof. The metallic oxide particles  531  of the additional porous film  53  are not limited to these materials, and other materials are possible.  
      In order to enhance the performance characteristics of the additional porous film  53 , conductive micro particles and light scattering particles may be added to the additional porous film  53 . The conductive micro particles added to the additional porous film  53  may enhance the mobility of the excited electrons. For instance, the conductive micro particles may be based on indium tin oxide. The light scattering particles added to the additional porous film  53  extend the optical path within the solar cell to thereby enhance the photoelectric conversion efficiency thereof. The light scattering particles may be formed of the same material as the additional porous film  53 . As a non-limiting example, the light scattering particles may have a mean particle diameter of 100 nm or more to effectively scatter the light.  
      As described above, in this embodiment, the first porous film  13  and the additional porous film  53 , each containing metallic oxide particles differentiated from each other in mean particle diameter, are sequentially formed so as to each thereby exert the advantageous effects thereof. The first porous film  13  serves to enhance the interface contact characteristics and the additional porous film  53  serves to increase the porous film thickness without increasing the number of processing steps.  
      However, aspects of the present invention are not limited thereto, and only the first porous film  13  may be formed, or additional separate porous films may be formed. Of course, a plurality of porous films may be formed with various arrangements.  
      The method of manufacturing the solar cell according to an aspect of the present embodiment further includes forming an additional porous film  53  after forming the first porous film  13 . Other processing steps are the same as those related to  FIG. 2  and the relevant description, and hence, explanation thereof will be omitted.  
      During the formation of the additional porous film  53 , polyethylene oxide, polyethylene glycol, polyvinyl alcohol (PVA), or polyvinyl pyrrolidone may be added to the paste to enhance the porosity of the additional porous film  53  and the film formation adhesiveness thereof by increasing the diffusivity and viscosity of the paste. The polymer may be removed after heat treatment.  
      If a binder is present, the heat treatment may be carried out at 450-600° C. for 30 minutes, while if no binder is present, the heat treatment may be carried out at 200° C. or less. However, the heat treatment may be altered depending upon the composition of the paste, and the heat treatment is not limited to these temperatures.  
      In the method of manufacturing the solar cell according to an aspect of the present invention, a first porous film  13  with an excellent interface contact characteristic and an additional porous film  53  with a large thickness are sequentially formed to thereby enhance the efficiency and the process characteristics.  
      A solar cell according to an aspect of the present invention will now be specifically explained by way of Examples. The Examples are given only to illustrate aspects of the present invention, and are not intended to limit the scope of the present invention. Particularly, the Examples exemplify the formation of an additional porous film to specify the characteristics of the porous film formed through the self-assembly process, but the solar cell is not limited thereto.  
     EXAMPLE 1  
      A first electrode was formed of tin oxide on a first 1 cm×1 cm glass substrate, such that the first electrode had a surface resistivity of 10 Ω.  
      A self-assembling composition was prepared by adding a block copolymer of P123 (BASF) (15 wt. %) and a TiO 2  precursor of Ti 4-isopropoxide (TTIP) (21 wt. %) to a solvent of acetyl acetone (64 wt. %) to thereby form a self-assembling compound. The self-assembling composition was coated on the first electrode through dip coating with a speed of 5 mm/min. The coated composition was aged at ambient temperature for one hour, and heat-treated at 450° C. for 30 minutes during the firing process, thereby forming a TiO 2 -containing porous film. The thickness of the porous film was 150 nm.  
      A solution in which TiO 2  particles with a mean particle diameter of 15 nm were diffused was coated onto the porous film through doctor blade method, thereby forming an additional TiO 2 -containing porous film.  
      The first substrate with the first electrode, the porous film, and the additional porous film was dipped in 0.3 mM of a solution of ruthenium(4,4-dicarboxy-2,2′-bipyridine) 2 (NCS) 2  for 24 hours, thereby adsorbing the dye into the porous film. The dye-adsorbed porous film was cleaned using ethanol.  
      A tin oxide-based transparent electrode with a surface resistivity of 10 Ω, and a platinum-based catalyst electrode with a surface resistivity of 0.5 Ω were formed on a second 1 cm×1 cm glass substrate, thereby forming a second electrode. Holes were formed in the second substrate and the second electrode using a drill with a diameter of 0.75 m.  
      The first and second substrates were arranged such that the porous film formed on the first substrate faced the second electrode. A thermoplastic polymer film with a thickness of 60 μm was placed between the first and second electrodes, and thermally pressed at 100° C. for 9 seconds to thereby attach the first and second substrates to each other.  
      An electrolyte was injected into the interior between the first and second substrates through the holes in the second substrate and the second electrode, and the holes were sealed using a thermoplastic polymer film and a cover glass, thereby completing a solar cell. The electrolyte was a solution formed by dissolving 21.928 g of tetrapropylammonium iodide and 1.931 g of iodine (I 2 ) in 100 ml of a mixture solvent of 80 vol. % of ethylene carbonate and 20 vol. % of acetonitrile.  
     COMPARATIVE EXAMPLE 1  
      A solar cell was manufactured in the same way as in Example 1 except that only a porous film corresponding to the additional porous film of Example 1 was formed on the electrode. In other words, in Comparative Example 1, only a porous film having TiO 2  particles with a mean particle diameter of 15 nm was formed, and a porous film having a smaller mean particle diameter and formed through a self-assembly process was not formed on the electrode.  
      An image of a surface of the porous film formed through the self-assembly process of the solar cell according to Example 1 is shown in  FIG. 4 . Images of a section of the first electrode, the porous film, and the additional porous film of the solar cell according to Example 1 are shown in  FIGS. 5A and 5B . Images of a section of the first electrode and the additional porous film of the solar cell according to Comparative Example 1 are shown in  FIGS. 6A and 6B . Furthermore, the electric currents as functions of voltages of the solar cells according to Example 1 and Comparative Example 1 were measured using a light source comprising a xenon (Xe) lamp of 100 mW/cm 2  and a filter of AM1.5. The measurement results are shown in  FIG. 7 . The data from  FIG. 7  are summarized and listed in Table 1.  
                                   TABLE 1                                   Short circuit   Open circuit                   current density   voltage   Fill factor   Efficiency                                                        Ex. 1   26.88   0.72   0.60   11.64       Com. Ex. 1   19.09   0.68   0.62   8.04                  
 
      As shown in  FIG. 4 , the surface of the porous film formed through the self-assembly process of the solar cell according to Example 1 was very smooth without generating any cracks and was provided with extremely fine and even pores. The metallic oxide particles of the porous film were also formed with very fine and even particle diameters. Since such a porous film contacted the first electrode, the interface contact characteristic between the porous film and the first electrode was excellent.  
      Comparing  FIGS. 5A and 5B  with  FIGS. 6A and 6B , it can be clearly seen that the interface contact characteristic between the surface of the porous film and the first electrode of the solar cell according to Example 1 is excellent (see  FIG. 5B ), while non-contact portions exist at the interface between the additional porous film and the first electrode of the solar cell according to Comparative Example 1 (see  FIG. 6B ). The metallic oxide particles were aggregated due to the heat treatment made during the formation of the additional porous film of the solar cell according to Comparative Example 1 and so aggregates with particle diameters of 200 nm-1000 nm were generated, and the interface contact characteristic was therefore deteriorated due to the aggregates.  
      Referring to  FIG. 7  and Table 1, the short circuit current density of the solar cell according to Example 1 is very high compared to that of the solar cell according to Comparative Example 1. This is because the interface contact characteristic between the porous film and the first electrode is enhanced in the solar cell according to Example 1 so that the excited electrons generated and transferred from the dye to the porous film can migrate easily to the first electrode.  
      Furthermore, it is shown that the solar cell according to Example 1 has a significantly higher efficiency than the solar cell according to Comparative Example 1, which may be due to the enhanced short circuit current density.  
     EXAMPLE 2  
      A solar cell was manufactured in the same way as in Example 1 except that in the formation of the porous film, the self-assembling compound was prepared by adding 4.5 wt. % of P123 and 6.3 wt. % of titanium 4-isopropoxide to 89.2 wt. % of acetyl acetone, and heat-treated at 350° C. during the firing process.  
     EXAMPLE 3  
      A solar cell was manufactured in the same way as in Example 2 except that in the formation of the porous film, the firing process was carried out at 400° C.  
     EXAMPLE 4  
      A solar cell was manufactured in the same way as in Example 2 except that in the formation of the porous film, the firing process was carried out at 450° C.  
     EXAMPLE 5  
      A solar cell was manufactured in the same way as in Example 2 except that in the formation of the porous film, the firing process was carried out at 500° C.  
     COMPARATIVE EXAMPLE 2  
      A solar cell was manufactured in the same way as in Example  2  except that in the formation of the porous film, the firing process was carried out at 300° C.  
     COMPARATIVE EXAMPLE 3  
      A solar cell was manufactured in the same way as in Example  2  except that in the formation of the porous film, the firing process carried out at 550° C.  
      With the solar cells according to Examples 2 to 5 and Comparative Examples 2 and 3, the mean particle diameter of the metallic oxide particles forming the porous film was measured. Furthermore, the electric currents as functions of voltages of the solar cells according to the Examples and Comparative Examples were measured using a light source of a xenon Xe lamp of 100 mW/cm 2  and a filter of AM1.5. The measurement results are evaluated and listed in Table 2.  
                                       TABLE 2                                   Mean   Short circuit   Open                   particle   current   circuit   Fill           diameter   density   voltage   factor   Efficiency                                                            Ex. 2   5   21.6   0.67   0.64   9.26       Ex. 3   8   23.1   0.69   0.64   10.2       Ex. 4   10   24.14   0.69   0.66   10.91       Ex. 5   14   22.3   0.68   0.67   10.16       Com.   —   —   —   —   —       Ex. 2       Com.   20   20.3   0.69   0.65   9.1       Ex. 3                  
 
      Table 2 does not contain data related to Comparative Example 2, since when the firing process was carried out at 300° C., the polymer material did not flow due to the lower temperature so that porous films were not formed. That is, considering that the mean particle diameter of the metallic oxide particles forming the porous film is smaller when the firing temperature is lower, it is difficult in practice to form metallic oxide particles with a mean particle diameter of less than 5 nm when the firing temperature is lower than a predetermined degree.  
      As shown in Table 2, the solar cells according to Examples 2 to 5 had a short circuit current density significantly higher than that of the solar cell according to Comparative Example 3. This is because with the solar cells according to Examples 2 to 5, the porous film has even particle diameters, and the interface contact characteristic between the porous film and the first electrode is enhanced so that the excited electrons generated and transferred from the dye to the porous film can migrate easily to the first electrode. Furthermore, when that the mean particle diameter of the metallic oxide particles is large, as in the solar cell according to the Comparative Example 3, the amount of dye adsorption is reduced, thereby deteriorating the efficiency.  
      For this reason, the solar cells according to Examples 2 to 5 had efficiencies significantly higher than that of the solar cell according to Comparative Example 3. That is, when the mean particle diameter of the metallic oxide particles was in the range of 5 nm-14 nm, the solar cell had a high efficiency.  
     EXAMPLE 6  
      A solar cell was manufactured in the same way as in Example 4 except that in the formation of the porous film, the self-assembling compound was prepared by adding 4.5 wt. % of P123 and 6.3 wt. % of titanium 4-isopropoxide to 89.2 wt. % of acetyl acetone.  
     EXAMPLE 7  
      A solar cell was manufactured in the same way as in Example 4 except that in the formation of the porous film, the self-assembling compound was prepared by adding 11.083 wt. % of P123 and 15.517 wt. % of titanium 4-isopropoxide to 73.4 wt. % of acetyl acetone.  
     EXAMPLE 8  
      A solar cell was manufactured in the same way as in Example 4 except that in the formation of the porous film, the self-assembling compound was prepared by adding 12.5 wt. % of P123 and 17.5 wt. % of titanium 4-isopropoxide to 70 wt. % of acetyl acetone.  
     EXAMPLE 9  
      A solar cell was manufactured in the same way as in Example 4 except that in the formation of the porous film, the self-assembling compound was prepared by adding 16.677 wt. % of P123 and 23.355 wt. % of titanium 4-isopropoxide to 60 wt. % of acetyl acetone.  
     COMPARATIVE EXAMPLE 4  
      A solar cell was manufactured in the same way as in Example 4 except that in the formation of the porous film, the self-assembling compound was prepared by adding 2.5 wt. % of P123 and 3.5 wt. % of titanium 4-isopropoxide to 94 wt. % of acetyl acetone.  
     COMPARATIVE EXAMPLE 5  
      A solar cell was manufactured in the same way as in Example 4 except that in the formation of the porous film, the self-assembling compound was prepared by adding 20 wt. % of P123 and 28 wt. % of titanium 4-isopropoxide to 52 wt. % of acetyl acetone.  
      With the solar cells according to Examples 4 and 6 to 9 and Comparative Examples 4 and 5, even though the weight percent of the acetyl acetone was varied, the weight percent ratio of P123 to titanium 4-isopropoxide had the same value of 5/7.  
      With the solar cells according to Examples 4 and 6 to 9 and Comparative Examples 4 and 5, the mean particle diameter of the metallic oxide particles forming the porous film was measured. Furthermore, the electric currents as functions of voltages of the solar cells according to Examples 4 and 6 to 9 and Comparative Examples 4 and 5 were measured using a light source of a xenon Xe lamp of 100 W/cm 2  and a filter of AM1.5. The measurement results were evaluated and are listed in Table 3. In Table 3, the Examples and the Comparative Examples are listed in descending order according to the weight percent of the solvent. In other words, Comparative Example 4 has the highest weight percent of solvent (94 wt. %) and Comparative Example 5 has the lowest weight percent of solvent (52 wt. %), with Examples 4 and 6 to 9 falling in between.  
                                       TABLE 3                                   Mean   Short circuit   Open                   pore   current   circuit   Fill           size   density   voltage   factor   Efficiency                                                            Com.   4   20.33   0.68   0.64   8.82       Ex. 4       Ex. 4   10   24.14   0.69   0.66   9.90       Ex. 6   7.5   21.58   0.70   0.66   9.90       Ex. 7   11   26.09   0.70   0.62   11.34       Ex. 8   12   25.19   0.71   0.66   11.77       Ex. 9   13   24.75   0.69   0.60   10.20       Com.   25   20.12   0.68   0.64   8.73       Ex. 5                  
 
      It can be determined from Table 3 that the mean pore size of the porous film is gradually enlarged as the weight percent of the solvent is increased. This is assumed to be because the mean pore size of the porous film is influenced by the viscosity of the self-assembling compound.  
      As shown in Table 3, the solar cells according to Examples 4 and 6 to 9 were significantly enhanced in short circuit current density and efficiency compared to those according to Comparative Examples 4 and 5. The reason is that when the mean pore size is too small, as in the solar cell according to Comparative Example 4, it is difficult for the electrolyte to pass through the porous film, while when the mean pore size is too large, as in the solar cell according to Comparative Example 5, necking of the metallic oxide particles forming the porous film is inhibited.  
     EXAMPLE 10  
      A solar cell was manufactured in the same way as in Example 1 except that the porous film formed through the self-assembly process had a thickness of 750 nm.  
     EXAMPLE 11  
      A solar cell was manufactured in the same way as in Example 1 except that the porous film formed through the self-assembly process had a thickness of 1070 nm.  
     EXAMPLE 12  
      A solar cell was manufactured in the same way as in Example 1 except that the porous film formed through the self-assembly process had a thickness of 1320 nm.  
      Images of sections of the first electrode, the porous film, and the additional porous film of the solar cells according to Examples 10 and 11 are shown in  FIGS. 8 and 9 , and an image of a same section of the solar cell according to Example 12 is shown in  FIG. 10 . The porous films of the solar cells according to Examples 10 to 12 were formed with very fine metallic oxide particles and even-sized pores. The porous film of the solar cell according to Example 10 with a thickness of less than 1000 nm turned out to be more structurally stable compared to the porous films of the solar cells according to Examples 11 and 12.  
      As described above, a porous film formed through the self-assembly process according to an aspect of the present invention has even and fine metallic oxide particles and even and fine-sized pores. When the porous film contacts the electrode the interface contact characteristics between the electrode and the porous film can be enhanced. Furthermore, the ionic conductivity is enhanced due to the fine and even-sized pores. Consequently, the excited electrons from the porous film are easily collected at the first electrode, and hence, the photoelectric conversion efficiency is enhanced.  
      Furthermore, the additional porous film may be formed using a paste in which the metallic oxide particles are diffused so that it can have a sufficient thickness without increasing the number of processing steps. That is, a plurality of porous films with different characteristics can be formed to thereby enhance the characteristics of each film simultaneously.  
      Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.