Abstract:
A dye-sensitized solar cell (DSSC) comprising nanoparticles formed on a surface of a nanowire formed on a substrate and a method of fabricating the same is disclosed. The dye-sensitized solar cell comprises a first substrate. A nanowire is formed on the first substrate. A plurality of nanoparticles is then contacted with a surface of the nanowire. The dye-sensitized solar cell further comprises a dye adsorbed onto a surface of the nanoparticles. A second substrate is corresponded to the first substrate. Finally, an electrolyte is filled between the first substrate and the second substrate, and in contact with the dye and nanoparticles. The nanoparticles are bonded to the surface of nanowire to extend and increase surface contact with the dye for promoting cell efficiency (η) of the dye-sensitized solar cell.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to dye-sensitized solar cells and a method for fabricating same and more particularly to a dye-sensitized solar cell comprising nanoparticles formed on a surface of a nanowire and a method for fabricating same. 
         [0003]    2. Description of the Related Art 
         [0004]    Low or non-polluting power sources have become a subject of great interest due to global warming, the increasing scarcity of raw materials, environmental conditions and other concerns. Solar cells, which capture solar energy, are a popular alternative as they emit relatively little or no pollution, and have a long productive life. 
         [0005]    Conventional solar cells can be divided into. Semiconductor solar cells, such as photovoltaic, and photo electrochemical solar cells, such as, dye-sensitized solar cells (DSSC).  FIG. 1A  shows a cross section of conventional dye-sensitized solar cells. A plurality of nanoparticles  14  is formed on a substrate  10 . A dye  18  is then formed on the substrate  10  and in contact with nanoparticles  14 . The nanoparticles  14  are arranged randomly, so that the nanoparticles  14  become a thin film. While a surface area of the nanopartitcles  14  is increasing, the thin film is densified, thus, the surface in contact with dye  18  is reduced. The recombination effect of electrons, for example electrons captured by positive charge of nanoparticles, is generated since defects of the densified nanoparticles, dye  18  thus does not effectively function, resulting in exciting and passing electrons to the conductive band of nanoparticles  14 . Accordingly, cell efficiency (η) of the dye-sensitized solar cell suffers. 
         [0006]    In  FIG. 1B , a dye-sensitized solar cell comprising nanowires, as disclosed in patent cooperation treaty publication number WO2005/017957, is depicted. A nanowire  15  is formed on a substrate  10 . Dye  18  is then adsorbed on a surface of the nanowire  15 . While the nanowire  15  is a formation of single crystal, the nanowire  15  has a specific growth direction. And after thermal process, active bond on the surface of the nanowire  15  is not enough to form chemical bonding to dye  18 , result in adsorption efficiency between the dye  18  and nanowire  15  is decrease. The contacting surface between nanowire  15  and dye  18  is decrease, since the dye  18  does not effective adsorb on the surface of the nanowire  15 . So that, cell efficiency of the dye-sensitized solar cell does not effectively promote. 
         [0007]    A dye-sensitized solar cell comprising an increased surface contacted with dye is needed to promote cell efficiency. 
       BRIEF SUMMARY OF INVENTION 
       [0008]    Accordingly, an object of the invention is to provide a method of fabricating a dye-sensitized solar cell. The method includes providing a first substrate and forming a nanowire thereon. A plurality of nanoparticles is formed on the surface of the nanowire. The method further includes providing a dye on the first substrate and contacting with the nanoparticles. A second substrate is then provided and corresponding to the first substrate. An electrolyte is filled between the first substrate and the second substrate, wherein the electrolyte contacts the nanowire and the nanoparticles. The nanoparticles are linearly arranged on the surface of the nanowire. The nanowire has a large surface area, high volume ratio, and aspect ratio. A surface contacted with the dye is increasing, while nanoparticles formed on the surface of the nanowire. According that, the cell efficiency of the dye-sensitized solar cell is promoted. 
         [0009]    Another object of the invention is to provide a dye-sensitized solar cell. The dye-sensitized solar cell comprises a first substrate. A nanowire is formed on the first substrate, and a plurality of nanoparticles is then in contact with a surface of the nanowire. The dye-sensitized solar cell further comprises a dye adsorbed on a surface of the nanoparticles, a second substrate corresponding to the first substrate. An electrolyte is between the first substrate and the second substrate and in contact with the nanoparticles and the dye. The nanowire has a large surface area, high volume ratio, and aspect ratio. A surface contacted with the dye is increasing, while nanoparticles formed on the surface of the nanowire. According that, the cell efficiency of the dye-sensitized solar cell is promoted. 
         [0010]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0012]      FIG. 1A to 1B  show cross-sections of conductive substrate of a conventional dye-sensitized solar cell; 
           [0013]      FIG. 2A to 2F  show cross-sections of fabricating a dye-sensitized solar cell according to the embodiment of the invention; 
           [0014]      FIG. 3  shows a cross-section of a dye-sensitized solar cell according to the embodiment of the invention; 
           [0015]      FIG. 4A to 4D  show graphs of current density vs. bias voltage of dye-sensitized solar cell comprising different arrangement of nanoparticles formed on the surface of the nanowire; and 
           [0016]      FIG. 5  shows a flow chart of fabricating a dye-sensitized solar cell according to the embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0017]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0018]    Referring to  FIG. 2A , a first substrate  20  is provided. The first substrate  20  may comprises any suitable material. For example the material may be rigid, flexible, transparent, semitransparent, metal or semiconductor comprising silicon or gallium arsenide. Preferably, the first substrate  20  may be glass or polymer comprising plastic. 
         [0019]    In  FIG. 2A , a conductive layer  22  is formed on the first substrate  20  to provide a path for electron flow. As shown in  FIG. 2B , a nanowire  24  is formed over the first substrate  20  to increase a contact surface conductive layer  22  and subsequent dye. The nanowire  24  may also be referred to as a nanorod. Preferably, the nanowire  24  and conductive layer  22  are formed by an in situ process, for example thermal evaporation, sputtering or applicable process well-known in the art. The conductive layer  22  and the nanowire  24  are preferably, for example, indium tin oxide (ITO), aluminum doped zinc oxide (AZO), antimony doped tin dioxide (ATO), fluorine doped tin dioxide (FTO), conductive impurity doped titanium oxide (TiO 2 ) or other semiconductor oxide having a preferable matching potential with the dye. 
         [0020]    The nanowire  24  is conductive and combines with the conductive layer  22  to increase the contact surface between the conductive layer  22 , and the nanowire  24  with the dye, and to provide a varied path for flow of electricity. 
         [0021]    Preferably, the conductive layer  22  of indium tin oxide, for example, is formed on the first substrate  20 , and then stacked and saturated in a vapor of indium tin oxide by thermal evaporation to form the nanowire  24 . The conductive layer  22  and the nanowire  24  are formed at a temperature between 400° C. and 950° C., for 5 mins to 60 mins. A length of the nanowire  24  may be hundreds of micrometers, for example between 5 μm to 500 μm, and the nanowire  24  has a preferable diameter between 5 nm and 60 nm. Note that the conductive layer  22  are formed to provide electric flow path and to facilitate formation of the subsequent nanowire  24 . Therefore, a thickness of the conductive layer  22  is adequate to fulfill the described purposes. 
         [0022]    As shown in  FIG. 2C , a plurality of nanoparticles  26  is formed on a surface of the nanowire  24 , to increase surface contact with the subsequently formed dye. Preferably, a metal oxide layer is formed on the first substrate  20  (not shown) by, for example, dip coating or sputtering. The metal oxide layer is preferably titanium dioxide (TiO 2 ), zinc oxide (ZnO), silicon dioxide (SiO 2 ) or stannum dioxide (SnO 2 ). The metal oxide is then sintered at preferable temperature between 400° C. and 550° C. for 30 mins to 60 mins, to form the nanoparticles  26  on the surface of the nanowire  24 . Preferably, the nanoparticles  26  have a diameter between 5 nm and 20 nm. 
         [0023]    The preparation of the metal oxide may be Sol-Gel method. In one embodiment, a precursor comprising titanium alkoxides or titanium slats is provided. The precursor is processed by hydrolysis and condensation to form a nano titanium dioxide. 
         [0024]    Preferably, the nanoparticles  26  are linearly or randomly arranged, and combined to the surface of the nanowire  24  for increasing the surface contact with subsequently formed dye. Note that the subsequent dye may be adsorbed on the surface of the nanowire  24  and between the nanoparticles  26  arranged in random. The nanoparticles  26  are formed on the surface of the nanowire  24  by chemical bond. 
         [0025]    In  FIG. 2D , a dye  28 , also referred to as dye-sensitized dye, is provided on the first substrate  20  and adsorbed on the surface of the nanoparticles  26  to transform form solar energy to electric energy. In some embodiments, the dye  28  may be an organic metal complex dye comprising porphyrin or Ru-bipyridine (N3), or an organic dye comprising counmarin, indoline, cyanine, or rhodamine B. In some embodiments, the dye  28  is formed on the first substrate  20  by, for example, spin coating, and dip coating or filing recycle. Note that the dye  28  used is related to the material of nanoparticles  26 , such as the adsorbability or oxidation reduction potential between the dye  28  and nanoparticles  26 . Thus, the material of the dye  28  is an example for description of the embodiment, but is not limited to this. 
         [0026]    Preferably, dye  28  adsorbed on the surface of the nanoparticles  26  by dipping nanoparticles  26  formed on the first substrate  20  to a dye solution between 0.2 mM and 1 mM for 18 hrs to 24 hrs. 
         [0027]    Referring to  FIG. 2E , a second substrate  40  comprising a conductive layer  42  is provided, and correspondingly to the first substrate  20 . The conductive layer  42  is formed on the second substrate  40  by evaporation, sputtering, electroplating, deposition, or applicable process well-known in the art. The material of the second substrate  40  is the same as previously described. The conductive layer  42  may be metal comprising copper, platinum or silver, or any conductive material. 
         [0028]    In  FIG. 2F , an electrolyte  30  is filled between the first substrate  20  and the second substrate  40 , to provide electron to dye  28  for reduction of dye  28 . Preferably, the electrolyte  30  may be a solution comprising iodine ion and iodine complex. 
         [0029]      FIG. 3  shows a dye-sensitized solar cell  50  according to an embodiment of the invention. The dye  28  becomes excited and passes electrons to nanoparticles  26 , while dye  28  absorbs solar energy. As shown, an electric flow path  32  in  FIG. 3 , electrons along the nanoparticles  26  pass through nanowire  24 , the first substrate  20  (also called lower electrode) to the second substrate  40  (also called upper electrode) to generate current. Thereafter, electrons from electrolyte  30  are provided to dye  28  for reduction of oxidized dye  28 . The above oxidization and reduction of dye  28  is repeatedly performed to generate current continually. 
         [0030]    Note that the electron may pass to the first substrate  20  by adjacent nanoparticles  26 . 
         [0031]      FIG. 4A  shows a dye-sensitized solar cell according to another embodiment of the invention. A plurality of nanoparticles  26  is formed a surface of a nanowire  24 , and arranged in random. The arrangement may be, for example, nanoparticles  26  separated by a distance by dye  28 , or in contact with each other. 
         [0032]    Thereafter,  FIG. 4B  shows a graph of current density (mA cm −2 ) vs. bias voltage (V) according to the dye-sensitized solar cell in  4 A. Curve a depicts a dye-sensitized solar cell comprising the nanoparticles. Curve b depicts a dye-sensitized solar cell comprising the nanowire. Curve c depicts a dye-sensitized solar cell comprising nanoparticles formed on the surface of the nanowire. It is found that curve c, namely a dye-sensitized solar cell comprising nanoparticles formed on the surface of the nanowire, shows the product of current multiplied voltage is higher than curves a and b. Cell efficiency (η) of dye-sensitized solar cell has a positive relative to the product of current and voltage. Accordingly, the dye-sensitized solar cell of the invention has greater cell efficiency the dye-sensitized solar cell comprising a single nanowire or nanoparticles. 
         [0033]      FIG. 4C  shows the nanoparticles  26  formed on the surface of the nanowire  24  of the first substrate  20  and arranged linearly. The arrangement may be, for example, the nanoparticles  26  contacting each other without a gap. In some embodiments, dye (not shown) may be adsorbed on the surface of the nanoparticles  26 , or adjacent to nanoparticles  26 . 
         [0034]      FIG. 4D  shows a graph of current density (mA cm −2 ) vs. bias voltage (V) according to dye-sensitized solar cell in  FIG. 4C . Curve a depicts a dye-sensitized solar cell comprising nanoparticles. Curve b depicts a dye-sensitized solar cell comprising nanowire. Curve c depicts a dye-sensitized solar cell comprising nanoparticles formed on the surface of the nanowire. It is found that curve c, namely a dye-sensitized solar cell comprising nanoparticles formed on the surface of the nanowire, shows the product of current multiplied voltage is higher than curves a and b. Cell efficiency (η) of dye-sensitized solar cell has a positive relation relate to product of current and voltage. Accordingly, the dye-sensitized solar cell of the invention has better cell efficiency than the dye-sensitized solar cell comprising a single nanowire or nanoparticles. 
         [0035]    It&#39;s found that the cell efficiency of the dye-sensitized solar cell comprising nanoparticles formed on the surface of the nanowire is greater than the dye-sensitized solar cell comprising a single nanowire or nanoparticles, in  FIG. 4A to 4D . Comparing the arrangement of nanoparticles in  FIG. 4A  with  4 B shows that the cell efficiency of the dye-sensitized solar cell comprising nanoparticles formed linearly on the surface of the nanowire is greater than the dye-sensitized solar cell comprising nanoparticles formed randomly on the surface of the nanowire. 
         [0036]      FIG. 5  shows a flow chart of fabricating a dye-sensitized solar cell according to an embodiment of the invention. A first substrate is provided, as step  100 . A nanowire is then formed on the first substrate, as step  102 . A conductive layer is formed on the first substrate, before the nanowire is formed. A plurality of nanoparticles is formed on the surface of the nanowire, as step  104 . The nanoparticles may be arranged linearly and combined with the nanowire in chemical bond. A dye is then formed on the first substrate by dip coating, as step  106 . Thereafter, a second substrate is provided and corresponding to the first substrate, as step  108 . As shown in step  110 , an electrolyte is filled between the substrates to yield a dye-sensitized solar cell. 
         [0037]    A conductive substrate of the invention comprises a plurality of nanoparticles formed on a surface of a nanowire. A sheet resistance of the conductive substrate is measured by 4 point probe, wherein the sheet resistance is about 0.7 Ω/cm 2 . A conventional conductive substrate, for example, FTO used in dye-sensitized solar cell has a sheet resistance between 5 Ω/cm 2  and 7 Ω/cm 2 . Thus, the conductive substrate of the invention has better conductivity than the conventional. That is, while electrons pass from the dye to the conductive substrate, the conductive substrate of the invention has a lower resistance, cell efficiency is thus improved. 
         [0038]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.