Patent Publication Number: US-2011048523-A1

Title: Dye-sensitized solar cell

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2009-0079315, filed Aug. 26, 2009 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     One or more embodiments of the present invention relate to a dye-sensitized solar cell, and more particularly, to a dye-sensitized solar cell which includes a collector electrode. 
     2. Description of the Related Art 
     Recently, in order to solve energy-related problems, various studies have been conducted to substitute existing fossil fuels. In particular, in order to substitute oil resources that are to be exhausted within decades, a wide range of studies have been conducted on how to use natural energies. Examples include the use of wind power, atomic power, solar power, or the like. In this regard, a solar cell using solar energy has been highlighted in recently since, unlike other energy sources, the source of solar energy is unlimited and solar energy is environmentally friendly. 
     From among various types of solar cells, a silicon solar cell is in highlight. Since the silicon solar cell is very expensive due to high manufacturing costs, it is difficult to commercialize the silicon solar cell and to improve its efficiency. In order to overcome the aforementioned problems, the silicon solar cell has been positively examined to develop a dye-sensitized solar cell that has considerably lower manufacturing costs. 
     Unlike the silicon solar cell, the dye-sensitized solar cell is a photo-electro-chemical solar cell. The dye-sensitized solar cell is formed of photosensitive dyes capable of absorbing visible light and generating electron-hole pairs, and a transition metal oxide for delivering the generated electron. This dye-sensitized solar cell has drawn great attention in that the photo-electro-chemical solar cell can substitute a silicon solar cell since the dye-sensitized solar cell can be manufactured at lower costs per unit of power, as compared with the silicon solar cell. 
     SUMMARY 
     According to one or more embodiments of the present invention, a dye-sensitized solar cell includes a first substrate and a second substrate facing each other; a first electrode unit and a second electrode unit disposed between the first substrate and the second substrate and respectively including at least one or more grid electrodes; an electrolyte filled in the first electrode unit and the second electrode unit; a sealing material for sealing the electrolyte between the first substrate and the second substrate; a collector electrode unit including a first collector electrode and a second collector electrode electrically connected to the first electrode unit and the second electrode unit, respectively; and a protruding terminal unit including a first protruding terminal and a second protruding terminal electrically connected to the first collector electrode and the second collector electrode, respectively, wherein at least a portion of at least one of the collector electrodes is disposed in an internal area sealed by the sealing material, and the first electrode unit comprises an oxide layer comprising dye molecules. 
     According to an aspect of the invention, the protruding terminal unit may protrude in a direction orthogonal to a direction in which the collector electrode unit extends. 
     According to an aspect of the invention, protruding terminal unit that is connected to the collector electrode unit disposed in the internal area sealed by the sealing material may cross the sealing material and is electrically connected to an outside. 
     According to an aspect of the invention, at least one selected from among the group consisting of the first electrode unit, the second electrode unit, the collector electrode unit, and the protruding terminal unit may further include a protective layer. 
     According to an aspect of the invention, at least one of the grid electrodes may have a wider line width than that of the other ones. 
     According to an aspect of the invention, the grid electrode having the wider line width may be electrically connected to the collector electrode unit so as to correspond to the protruding terminal unit. 
     According to an aspect of the invention, the number of the protruding terminals of the protruding terminal unit may be less than the number of the grid electrodes. 
     According to an aspect of the invention, a line width of the first protruding terminal or the second protruding terminal of the protruding terminal unit may be wider than that of the collector electrode unit. 
     According to an aspect of the invention, a line width of the first collector electrode or the second collector electrode of the collector electrode unit may be wider than that of the grid electrodes. 
     According to an aspect of the invention, the protruding terminal unit may be enabled to be directly connected to another protruding terminal unit of another dye-sensitized solar cell. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the exemplary embodiments. 
    
    
     
       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 diagram for describing an operating principle of a general dye-sensitized solar cell; 
         FIG. 2  is a perspective view of a basic structure of the general dye-sensitized solar cell of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the general dye-sensitized solar cell of  FIG. 1 , taken along a line III-III in  FIG. 2 ; 
         FIG. 4  is an exploded perspective view of a structure of a dye-sensitized solar cell with a protruding terminal formed at one side of the dye-sensitized solar cell, according to an embodiment of the present invention; 
         FIG. 5  is a top-plan view of the dye-sensitized solar cell in  FIG. 4 ; 
         FIG. 6  is a cross-sectional view of the dye-sensitized solar cell, taken along a line VI-VI in  FIG. 5 ; 
         FIG. 7  is a cross-sectional view of the dye-sensitized solar cell, taken along a line VII-VII in  FIG. 5 ; 
         FIG. 8  is a cross-sectional view of the dye-sensitized solar cell, taken along a line VIII-VIII in  FIG. 5 ; 
         FIG. 9  is a plan view of a structure of a dye-sensitized solar cell with protruding terminal units formed at both sides of the dye-sensitized solar cell, according to an embodiment of the present invention; 
         FIG. 10  is a plan view of a dye-sensitized solar cell having a similar structure to the structure in  FIG. 9 , according to an embodiment of the present invention, in which one or more grid electrodes have line widths that are not equal to each other; 
         FIG. 11  is a plan view of a structure of a dye-sensitized solar cell formed by serially connecting dye-sensitized solar cells as the one in  FIG. 10 ; 
         FIG. 12  is a top-plan view of a structure of a dye-sensitized solar cell in which a portion of a collector electrode is disposed through a sealing material, according to an embodiment of the present invention; and 
         FIG. 13  is a top-plan view of a structure of a dye-sensitized solar cell having third protruding terminals with a modified shape, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. 
       FIG. 1  is a diagram for describing an operating principle of a general dye-sensitized solar cell. The general dye-sensitized solar cell operates in the following manner. When solar rays are absorbed in dye-sensitized solar cell, photons with sufficient energy hit dye molecules (not shown) on a surface of an oxide layer  131 . The dye molecules turn to an exited state and emit electrons e−. The electrons e− emitted from the dye molecules generate electricity while moving according to a chemical diffusion gradient. The dye molecules are photosensitive dye molecules capable of absorbing visible light and generating electron-hole pairs. In addition, each of the dye molecules is very small, and thus, in order to contain a large number of dye molecules, the oxide layer  131  is used as a scaffold for the dye molecules. 
     Referring to  FIG. 1 , the dye molecules turn to the excited state (S+/S*) from a ground state (S+/S), and the excited dye molecules are oxidized by emitting the electrons e−. The oxidized dye molecules are reduced by receiving electrons e− from iodine ions in an oxidation-reduction electrolyte (I−/I 3−) disposed between a semiconductor electrode  130  and a counter electrode  120  formed of platinum, and the iodine ions are oxidized. Meanwhile, the excited electrons e− are injected into a conduction band of the oxide layer  131 , and transferred to the counter electrode  120  via the semiconductor electrode  130  and an external circuit (not shown). The electrons e− that reach the counter electrode  120  reduce the oxidized iodine ions. In this manner, by absorbing the solar rays, the general dye-sensitized solar cell induces a transfer of the electrons e−. That is, the general dye-sensitized solar cell induces a flow of current, thereby functioning as a solar cell. 
     The general dye-sensitized solar cell is sealed with a sealing material so an electrolyte does not leak. In this regard, since the semiconductor electrode  130  and the counter electrode  120  are connected to the outside via the sealing material, there is a chance that the electrolyte may leak at an intersection portion between the sealing material and the semiconductor electrodes  130  or between the sealing material and the counter electrode  120 . 
       FIG. 2  is a perspective view of a basic structure of the general dye-sensitized solar cell of  FIG. 1 .  FIG. 3  is a cross-sectional view of the general dye-sensitized solar cell of  FIG. 1 , taken along a line III-III in  FIG. 2 . Referring to  FIG. 2  and  FIG. 3 , the general dye-sensitized solar cell includes a first substrate  100 , a second substrate  110 , the counter electrode  120 , the semiconductor electrode  130 , the oxide layer  131 , an electrolyte  140 , and first and second sealing materials  150  and  150 ′. The counter electrode  120  may be formed with a platinum thin film that is adhered thereto as a catalyst for the electrolyte  140  so as to boost reduction and to improve reflectivity of solar rays. The semiconductor electrode  130  functions to enable absorption of solar energy and to transfer electrons to an external circuit (not shown). Dye molecules (not shown) absorb the solar energy of visible light and generate electron-hole pairs. The oxide layer  131  absorbs the dye molecules so as to deliver the electrons that are generated in the dye molecules. The electrolyte  140  functions to reduce the oxidized dye molecules. The first sealing material  150  seals the first and second substrates  100 ,  110  to prevent the electrolyte  140  between the first substrate  100  and the second substrate  110  from leaking. Here, the counter electrode  120  and/or the semiconductor electrode  130  is connected to the outside via the first sealing material  150 . In this regard, if a plurality of the counter electrodes  120  and/or a plurality of the semiconductor electrodes  130  pass through the first sealing material  150  at one time, the first sealing material  150  does not evenly seal the plurality of counter electrodes  120  and/or the plurality of the semiconductor electrodes  130 . Thus there is a high chance that the electrolyte  140  may leak past the material  150  at the intersection with the electrodes  120 ,  130 . In addition, the second sealing material  150 ′ seals in a longitudinal direction of the counter electrode  120  and/or the semiconductor electrode  130 , so as to prevent the electrolyte  140  between the first substrate  100  and the second substrate  110  from leaking. 
       FIG. 4  is an exploded perspective view of a structure of a dye-sensitized solar cell with a protruding terminal  230  formed at one side of the dye-sensitized solar cell so as to decrease a leak possibility of the electrolyte  140 , according to an embodiment of the present invention.  FIG. 5  is a top-plan view of the dye-sensitized solar cell in  FIG. 4 .  FIG. 6  is a cross-sectional view of the dye-sensitized solar cell, taken along a line VI-VI′ in  FIG. 5 .  FIG. 7  is a cross-sectional view of the dye-sensitized solar cell, taken along a line VII-VII′ in  FIG. 5 .  FIG. 8  is a cross-sectional view of the dye-sensitized solar cell, taken along a line VIII-VIII′ in  FIG. 5 . 
     Referring to  FIGS. 4 through 8 , the dye-sensitized solar cell includes the first substrate  100 , the second substrate  110 , a first oxide layer  311 , dye molecules (not shown), the electrolyte  140 , the first and second sealing materials  150  and  150 ′, a protective layer  160 , a first counter electrode  240 , and a first semiconductor electrode  340 . As shown, the first substrate  100 , the second substrate  110 , the first oxide layer  311 , and the electrolyte  140  have generally the same functions and operate in the same manner as the first substrate  100 , the second substrate  110 , the oxide layer  131 , and the electrolyte layer  140  described with reference to  FIG. 2  and  FIG. 3 . Referring to  FIG. 4 , the first substrate  100  is disposed in a direction C of an incident light relative to the second substrate  110  so as to be closest to the incident light. 
     The first counter electrode  240  includes a counter grid electrode unit  210 , a counter collector electrode  220 , and a counter protruding terminal unit  230 . The counter grid electrode unit  210  comprises a plurality of counter grid electrodes  201 ,  202 ,  203 . The first semiconductor electrode  340  includes a semiconductor grid electrode unit  310 , a semiconductor collector electrode  320 , and a semiconductor protruding terminal unit  330 . The semiconductor grid electrode unit  310  comprises a plurality of semiconductor grid electrodes  301 ,  302 ,  303 . While shown with two counter terminal units  230  and two semiconductor terminal units  330 , it is understood that other numbers can be used. 
     Although the first semiconductor electrode  340  further includes the first oxide layer  311 , compared with the first counter electrode  240 , a structural connection of the semiconductor grid electrodes unit  310 , the semiconductor collector electrode  320 , and the semiconductor protruding terminal unit  330  may be similar to that of the counter grid electrode unit  210 , the counter collector electrode  220 , and the counter protruding terminal unit  230  of the first counter electrode  240 . Thus, for convenience of description, the structure of the first counter electrode  240  will be mainly described with reference to  FIGS. 5 through 8 . Referring to  FIG. 5 , the first counter electrode  240  includes the counter collector electrode  220  disposed inside the first and second sealing materials  150  and  150 ′ and to which the counter grid electrodes  201 ,  202 ,  203  are commonly connected. Referring to  FIG. 6 , the counter grid electrode unit  210  is disposed between the first substrate  100  and the second substrate  110  and connects to the first substrate  100 . Such counter grid electrodes  201 ,  202 ,  203  extend in a longitudinal direction and are all connected to the counter collector electrode  220 . 
       FIG. 7  illustrates a cross-section of the counter collector electrode  220 . Referring to  FIG. 7 , the counter collector electrode  220  is disposed inside the first and second sealing materials  150  and  150 ′ while the counter grid electrodes  201 ,  202 ,  203  are commonly connected thereto. Since the counter collector electrode  220  is disposed inside the first and second sealing materials  150  and  150 ′, the dye-sensitized solar cell further includes the counter protruding terminal unit  230  for electrical connection with the outside, such as a load. 
     The first and second sealing materials  150  and  150 ′ may include a thermoplastic polymer material such as Surlyn  1702  available from DuPont Company. 
     The counter protruding terminal unit  230  extends through the first sealing material  150  and has a side connected to the counter collector electrode  220 , and another side electrically connected to the outside as illustrated in  FIG. 5 . The shown number of counter protruding terminal units  230  is less than that of the counter grid electrodes  201 ,  202 ,  203  of the counter grid electrode unit  210 . In this manner, by disposing the counter collector electrode  220  in the first and second sealing materials  150  and  150 ′, a large number of counter grid electrodes  201 ,  202 ,  203  are not connected to the outside by extending through the first sealing material  150 , but the counter protruding terminal units  230  that are less in number than the number of counter grid electrodes  201 ,  202 ,  203  are electrically connected to the outside and extends through the first sealing material  150 . Thus, a total number of elements passing through the first sealing material  150  is decreased, which decreases a possibility that the electrolyte  140  may leak via the first sealing material  150  due to a defective seal. 
     While not required in all aspects, the first counter electrode  240  and the first semiconductor electrode  340  include the protective layer  160 . The protective layer  160  prevents the electrode  240 ,  340  from being corroded by the electrolyte  140 . The protective layer  160  may be formed as a dielectric layer formed of a glass material. Also, the protective layer  160  may be formed of Surlyn. In addition, the protective layer  160  is formed on the first counter electrode  240  or the first semiconductor electrode  340  disposed inside the first and second sealing materials  150  and  150 ′. Thus, the layer  160  may prevent the counter collector electrode  220  or the semiconductor collector electrode  320  from being damaged when the electrolyte  140  is injected. The protective layer  160  may cross the first and second sealing materials  150  and  150 ′ so as to be formed on the counter protruding terminal unit  230  and the semiconductor protruding terminal unit  330  that are connected to the outside as shown in  FIG. 7 . 
     While not required in all aspects, referring to  FIG. 5 , a line width D 2  of the counter collector electrode  220  is formed to be wider than a line width D 1  of each counter grid electrode  201 ,  202 ,  203  of the counter grid electrode unit  210 . Having such wider line widths facilitates a flow of current generated in the counter grid electrodes  201 ,  202 ,  203 . That is, the larger the cross-sectional area of an electrode is, the smaller the electrical resistance is, whereby the flow of the current may be facilitated. 
     While not required in all aspects, a line width D 3  of the counter protruding terminal unit  230  is formed to be greater than the line width D 2  of the counter collector electrode  220 . By doing so, an amount of current that flows in the counter collector electrode  220  may increase by enlarging the line width D 3  of the counter protruding terminal unit  230 . 
     The dye-sensitized solar cell may be embedded in various structures; however, in the case where the dye-sensitized solar cell is attached to a glass window and used, the counter protruding terminal unit  230  disposed at the edges of the dye-sensitized solar cell may be disposed to correspond to a frame of the glass window to less affect an aperture ratio. 
     Here, the line width D 2  of the counter collector electrode  220  may be from about 500 μm to about 6 mm, and the line width D 3  of the counter protruding terminal unit  230  may be from about 0.5 mm to about 5 mm. Also, a length L 3  from the first sealing material  150  to an end of the counter protruding terminal unit  230  may be from about 0.5 mm to about 1.5 mm. However, the line width D 2  of the counter collector electrode  220 , the line width D 3  of the counter protruding terminal unit  230 , and the length L 3  of the counter protruding terminal unit  230  are not limited thereto, and thus may vary. 
     Here, the counter protruding terminal units  230  may have structures to be interconnected. Thus, the counter protruding terminal units  230  facing each other may be electrically interconnected or may be structurally combined with each other. 
     The first semiconductor electrode  340  may be formed in a similar manner to the first counter electrode  240 . That is, the first semiconductor electrode  340  includes the plurality of semiconductor grid electrodes  301 ,  302 ,  303  forming semiconductor grid electrode unit  310 ; the semiconductor collector electrode  320 ; and the semiconductor protruding terminal unit  330 . Referring to  FIGS. 4 through 8 , the semiconductor collector electrode  320  is disposed inside the first and second sealing materials  150  and  150 ′. Since the semiconductor collector electrode  320  is disposed inside the first sealing material  150 , the dye-sensitized solar cell further includes the semiconductor protruding terminal unit  330  for electrical connection with the outside. The number of the semiconductor protruding terminal unit  330  is less than that of the semiconductor grid electrodes  301 ,  302 ,  303  of the semiconductor grid electrode unit  310 . Since the number of the semiconductor protruding terminal unit  330  is less than that of the semiconductor grid electrodes  301 ,  302 ,  303 , the number of elements passing through the first sealing material  150  decreases, which decreases a possibility that the electrolyte  140  may leak via the first sealing material  150  due to a defective seal. Line widths and lengths of the semiconductor grid electrodes  301 ,  302 ,  303 , the semiconductor collector electrode  320 , and the semiconductor protruding terminal unit  330  may be substantially the same as those of the counter grid electrodes  201 ,  202 ,  203 , the counter collector electrode  220 , and the counter protruding terminal unit  230 , respectively. However, the invention is not limited thereto. 
     As described above, the counter collector electrode  220  of the first counter electrode  240  is disposed inside the first and second sealing materials  150  and  150 ′, and the semiconductor collector electrode  320  of the first semiconductor electrode  340  is disposed inside the first and second sealing materials  150  and  150 ′. However, the present embodiment may not be limited thereto. For example, the counter collector electrode  220  of the first counter electrode  240  or the semiconductor collector electrode  320  of the first semiconductor electrode  340  may be disposed inside the first and second sealing materials  150  and  150 ′. 
       FIG. 9  is a plan view of a structure of a dye-sensitized solar cell with protruding terminal units  230  and  330  formed at both sides of the dye-sensitized solar cell, according to an embodiment of the present invention.  FIG. 10  is a plan view of a dye-sensitized solar cell having a similar structure to the structure in  FIG. 9 , according to an embodiment of the present invention, in which one or more grid electrodes  201 ,  202 ,  203 ,  301 ,  302 ,  303  have line widths that are not equal to each other.  FIG. 11  is a plan view of a structure of a dye-sensitized solar cell formed by serially connecting dye-sensitized solar cells as the one in  FIG. 10 . 
     Referring to  FIG. 9 , the dye-sensitized solar cell includes the protruding terminal units  230  and  330  that are formed at both sides of the dye-sensitized solar cell. Other than these elements, the rest of the elements are the same as those of the dye-sensitized solar cell of  FIG. 4 . That is, the first counter electrode  240  includes the counter grid electrode unit  210 , the counter collector electrode  220 , and the counter protruding terminal unit  230 , and the first semiconductor electrode  340  includes the semiconductor grid electrode unit  310 , the semiconductor collector electrode  320 , and the semiconductor protruding terminal unit  330 . In the shown embodiment, since the protruding terminal units  230  and  330  are connected to both sides of the dye-sensitized solar cell, it is possible to electrically connect the protruding terminal units  230  and  330  and to facilitate the structural connection between dye-sensitized solar cells. That is, when a large number of the dye-sensitized solar cells are connected, the dye-sensitized solar cells may be electrically connected in series without using a separate connecting component therebetween. 
     Also, as illustrated in  FIG. 10 , it is possible to form the dye-sensitized solar cell including one or more electrodes  200 L 1 ,  300 L 1  and  200 L 2 ,  300 L 2  having a line width greater than that of other grid electrodes  201 ,  301 ,  202 ,  302 .  FIG. 11  illustrates the structure of the dye-sensitized solar cell formed by serially connecting dye-sensitized solar cells each as the one in  FIG. 10 , and in this regard, as the number of dye-sensitized solar cells increases, an amount of generated current increases and also, a loss of the current increases in proportion to the square of the current according to Equation 1. 
       p=I 2 R (where, p indicates a power loss, I indicates current, and R indicates resistance)  Equation 1
 
     In order to decrease the power loss, one or more grid electrodes  200 L 1 ,  300 L 1  and  200 L 2 ,  300 L 2  may have a line width greater than that of other grid electrodes  201 ,  301 ,  202 ,  302 . By having the greater line width, a cross-sectional area whereon the current flows increases so that an electrical resistance may decrease. In other words, when the amount of current flowing in the dye-sensitized solar cells increases by connecting the dye-sensitized solar cells, it is possible to offset the power loss by allowing one or more grid electrodes  200 L 1 ,  300 L 1  and  200 L 2 ,  300 L 2  to have the line width greater than that of other grid electrodes  201 ,  301 ,  202 ,  302 . 
     As shown, the grid electrodes  200 L 1 ,  300 L 1  and  200 L 2 , are electrically connected to the counter collector electrode  220  and the semiconductor collector electrode  320 , respectively, so as to face the counter protruding terminal unit  230  and the semiconductor protruding terminal unit  330 . 
       FIG. 12  is a top-plan view of a structure of a dye-sensitized solar cell in which a portion of the collector electrode  221 ,  231  is disposed extending through the sealing material  150 , according to an embodiment of the present invention. Referring to  FIG. 12 , the dye-sensitized solar cell includes counter and semiconductor grid electrode units  210  and  310 , second collector electrodes  221  and  321 , and protruding terminals  231  and  331 . Here, a structure of the counter and semiconductor grid electrode units  210  and  310  is the same as that of the counter and semiconductor grid electrode units  210  and  310  of the dye-sensitized solar cell of  FIGS. 4 through 6 . According to the present embodiment of  FIG. 12 , portions of the collector electrode  221 ,  231  are disposed inside the first and second sealing materials  150  and  150 ′, and simultaneously, other portions of the collector electrode  221 ,  231  are electrically connected to the outside by passing through the first sealing material  150 . The protruding terminals  231  and  331  are connected to the portion of the collector electrode  221 ,  231  extending outside of the first sealing material  150 , while the counter and semiconductor grid electrode units  210  and  310  are connected to the portion of the collector electrode  221 ,  231  inside the first sealing material  150 . 
       FIG. 13  is a top-plan view of a structure of a dye-sensitized solar cell having third protruding terminals  232  and  332  with a modified shape, according to an embodiment of the present invention. Referring to  FIG. 13 , the dye-sensitized solar cell includes grid electrode units  210  and  310 , third collector electrodes  222  and  322 , and the third protruding terminals  232  and  332 . A structure of the grid electrode units  210  and  310  is the same as that of the counter and semiconductor grid electrode units  210  and  310  of the dye-sensitized solar cell of  FIGS. 4 through 6 . According to the present embodiment of  FIG. 13 , the third collector electrodes  222  and  322  are disposed inside the first and second sealing materials  150  and  150 ′, and the third protruding terminals  232  and  332  extend through the first sealing material  150  and are connected to the outside. As illustrated in  FIG. 13 , the third protruding terminals  232  and  332  has a T shape in which a bottom portion of the T shape extends through the sealing material  150  and connects to the third collector electrodes  222 ,  322 , and the top portion of the T shape is disposed outside of the sealing material  150  so as to be directly connected to another dye-sensitized solar cell in another module. However, while described in terms of a T shape, it is understood that other shapes can be used and varied according to the external device to which the cell is being connected. 
     It should be understood that the exemplary embodiments described therein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. Further, 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.