Patent Publication Number: US-2011048493-A1

Title: Solar cell

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application Nos. 10-2009-0082486, filed on Sep. 2, 2009, and 10-2009-0134517, filed on Dec. 30, 2009, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention disclosed herein relates to a solar cell. 
     A solar cell is a photovoltaic energy conversion system that converts light energy from the sun into electric energy. When the light is incident on the solar cell, electron-hole pairs are generated in a semiconductor. By electric field generated in a P-N junction, the electrons move to an N-type semiconductor and the holes move to a P-type semiconductor, thereby generating electric power. 
     The solar cell generates the electric power using the sun as a light source. Therefore, the solar cell does not generate pollution during the generation of the electric power and thus the solar cell is getting the spotlight as a future-oriented, environment-friendly energy source. However, since the solar cell has relatively lower photovoltaic energy conversion efficiency, it is difficult to put the solar cell to practical use. Accordingly, in order to put the solar cells to the practical use, many researches for improving the photovoltaic energy conversion efficiency have been making much progress. 
     SUMMARY OF THE INVENTION 
     The present invention provides a solar cell having high reliability. 
     The present invention also provides a solar cell having high efficiency. 
     Embodiments of the present invention provide solar cells including a solar cell including: a photovoltaic conversion device including a first surface and a second surface on the opposite side; a first electrode connected to a first surface of the photovoltaic conversiondevice; a second electrode connected to a second surface of the photovoltaic conversiondevice; and an alkaline metal containing layer contacting one of the first and second electrodes. In some embodiments, the alkaline metal containing layer is composed of nanoparticles in which one particle is isolated from each other. In some other embodiments, the alkaline metal containing layer may be provided in the form of a thin film 
     In some embodiments, the alkaline metal containing layer may be provided in the form of a thin film on the second electrode to function as a antireflection layer. At this point, the second electrode is disposed between the alkaline metal containing layer and the photovoltaic conversion layer. 
     In other embodiments, the solar cell may further include a glass substrate disposed on the alkaline metal containing layer; and a antireflection layer disposed on the glass substrate. At this point the alkaline metal containing layer may be disposed between the glass substrate and the second electrode. The glass substrate may be disposed between the antireflection layer and the alkaline metal containing layer. The alkaline metal containing layer may have a greater refractive index than the glass substrate and a less refractive index than the second electrode. 
     In still other embodiments, the first electrode may include a first surface contacting the photovoltaic conversion device and a second surface on the opposite side. At this point, the solar cell may further include a substrate covering the second surface of the first electrode and a metal grid contacting the second electrode through the alkaline metal containing layer. 
     In even other embodiments, the solar cell may further include an antireflection layer on the alkaline metal containing layer. At this point, the metal grid further passes through the antireflection layer. 
     In yet other embodiments, the first electrode may include a first surface contacting the photovoltaic conversion device and a second surface on the opposite side, and the alkaline metal containing layer covers the second surface of the first electrode. 
     In further embodiments, the solar cell may further include a glass substrate on the second electrode; and an antireflection layer on the glass substrate. At this point, the glass substrate may be disposed between the second electrode and the antireflection layer. 
     In still further embodiments, the solar cell may further include a metal grid contacting the first electrode through the alkaline metal containing layer. 
     In even further embodiments, the alkaline metal containing layer may have a higher reflectance for a first wavelength band of incident light than a second wavelength band of the incident light. At this point, the first wavelength band may be different from the second wavelength band. 
     In yet further embodiments, the second wavelength band may include visible light. 
     In still yet other embodiments, the solar cell may further include an additional alkaline metal containing layer on the second electrode. At this point, the second electrode may be disposed between the additional alkaline metal containing layer and the photovoltaic conversion layer. 
     In still further other embodiments, the alkaline metal containing layer may cover a top surface of the photovoltaic conversion device to function as an antireflection layer and the second electrode may be a metal grid connected to the photovoltaic conversion layer through the alkaline metal containing layer. 
     In still yet other embodiments, the first electrode may include a first surface contacting the photovoltaic conversion device and a second surface on the opposite side. At this point, the solar cell further includes an additional alkaline metal containing layer covering the second surface of the first electrode. 
     In still further other embodiments, the alkaline metal containing layer may be disposed between the first surface of the photovoltaic conversion device and the first electrode and electrically may connect the photovoltaic conversion device to the first electrode. 
     In still yet other embodiments, one of the first and second electrodes, which contacts the alkaline metal containing layer, may include a halogen element or a group-VI element. 
     In still further yet other embodiments, the alkaline metal containing layer may include alkaline metal bonded to oxygen, boron, hydrogen, or fluorine. 
     In still yet other embodiments, an amount of alkaline metal contained in the alkaline metal containing layer may be about 5-20 percent by weight. 
     In still further yet other embodiments, the photovoltaic conversion layer may include a plurality of PIN diodes. 
     In still further yet other embodiments, the photovoltaic conversion layer may include a plurality of PN diodes. 
     In still yet other embodiments, the photovoltaic conversion device may include at least one of Si, SiGe, CuInS, CuInSe, CuInGaSe, CuInGaS, CdS, CdTe, ZnO, ZnS, CuZnSnS, CuZnSnSe, Cu 2 O, GaAs, GaInAs, GaInAlAs, and InP. 
     In still further yet other embodiments, some of alkaline metal contained in the alkaline metal containing layer may be diffused to the photovoltaic conversion layer through one of the first and second electrodes, which contacts the alkaline metal containing layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present invention and, together with the description, serve to explain principles of the present invention. In the drawings: 
         FIGS. 1A to 1D  are views illustrating a solar cell according to first embodiment; 
         FIGS. 2A to 2C  are views illustrating a solar cell according to second embodiment; 
         FIGS. 3A to 3C  are views illustrating a solar cell according to third embodiment; 
         FIGS. 4A and 4B  are views illustrating a solar cell according to fourth embodiment; 
         FIGS. 5A and 5B  are views illustrating a photovoltaic conversion layer included in solar cell according to embodiments of the present invention. 
         FIGS. 6A and 6B  are views illustrating a metal grid included in the solar cell according to embodiments of the present invention; and 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. In the drawings, the dimensions of layers and regions are exaggerated for clarity of illustration. It will also be understood that when a layer (or film) is referred to as being ‘on’ another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being ‘under’ another layer, it can be directly under, and one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being ‘between’ two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. Like reference numerals refer to like elements throughout. 
     A solar cell will now be described according to first embodiment of the present invention.  FIG. 1A  is a view of a solar cell according to first embodiment. 
     Referring to  FIG. 1A , a photovoltaic conversion device  120  is provided. The photovoltaic conversion layer  120  is configured to generate carriers (e.g., holes and electrons) by the sunlight incident thereon. The photovoltaic conversion device  120  includes a first surface and a second surface on the opposite side. The photovoltaic conversion device  120  may include a first conductive semiconductor layer, a second conductive semiconductor layer, and an intrinsic semiconductor layer. The first conductive semiconductor layer may be a p-type semiconductor and the second conductive semiconductor layer may be a n-type semiconductor layer. The intrinsic semiconductor layer may be disposed between the first and second conductive semiconductor layers. The first conductive semiconductor layer may be spaced apart from the second conductive semiconductor layer. The photovoltaic conversion device may include a first conductive semiconductor layer and a second conductive semiconductor layer. The first conductive semiconductor layer may be a p-type semiconductor and the second conductive semiconductor layer may be a n-type semiconductor layer. 
     The first and second surfaces of the photovoltaic conversion device  120  may be surfaces included to different types of semiconductor layers. For example, the first surface of the photovoltaic conversion device  120  may be a surface included in an N-type semiconductor layer and the second surface of the photovoltaic conversion device  120  may be a surface included in a P-type semiconductor layer. The photovoltaic conversion device  120  may include at least one of Si, SiGe, CuInS, CuInGaSe, CuInGaS, CdS, CdTe, ZnO, ZnS, CuZnSnS, CuZnSnSe, Cu 2 O, GaAs, GaInAs, GaInAlAs, and InP. The photovoltaic conversion device  120  may has a multi junction structure or a heterojunction with intrinsic thin layer (HIT). 
     The first surface of the photovoltaic conversion device  120  may be connected to a first electrode  110 . The first electrode  110  may cover the first surface of the photovoltaic conversion device  120 . The first electrode  110  may directly contact the first surface of the photovoltaic conversion device  120 . The second surface of the photovoltaic conversion device  120  may be connected to a second electrode  130 . The second electrode  130  may cover the second surface of the photovoltaic conversion device  120 . The second electrode  130  may directly contact the second surface of the photovoltaic conversion device  120 . 
     The first electrode  110  may include metal. For example, the first electrode  110  may include silver (Ag), platinum (Pt), nickel (Ni), chrome (Cr), aluminum (Al), titanium (Ti), molybdenum (Mo), or tungsten (W). Alternatively, the first electrode  110  may include a transparent conductive material. For example, the first electrode  110  may include one of ZnO:Al, ZnO:Ga, ZnO:B, ZnO:Cd, InO, InSnO, SnO 2 , SnO:F, RuO 2 , IrO 2 , and Cu 2 O. 
     The second electrode  130  may include a transparent conductive material. For example, the second electrode  130  may include one of ZnO:Al, ZnO:Ga, ZnO:B, ZnO:Cd, InSnO (ITO), SnO 2 , SnO:F, RuO 2 , IrO 2 , and Cu 2 O. The second electrode  130  may include electric charge compensation material. The electric charge compensation material may be a halogen element or a group-VI element. For example, the second electrode  130  may include one of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), oxygen (O), sulphur (S), selenium (Se), and tellurium (Te). The electric charge compensation material enhances conductivity of the second electrode  130 . 
     The alkaline metal containing layer  140  may be formed on the glass substrate  150  coated with antireflection layer  160  on the opposite side. The second electrode  130  may be disposed between the alkaline metal containing layer  140  and the photovoltaic conversion device  120 . 
     The alkaline metal containing layer  140  may include alkaline metal. For example, the alkaline metal may be sodium (Na). An amount of the alkaline metal compound in the alkaline metal containing layer  140  may be about 5-20% by weight. The alkaline metal containing layer  140  may contain the alkaline metal in a state where it is bonded with oxygen (O), boron (B), hydrogen (H), or fluorine (F). When the alkaline metal is the sodium (Na), the alkaline metal containing layer  140  may exist in the form of at least one of NaF, NaO, NaAlO 2 , Na 2 O-Al 2 O 3 -nSiO 2  (n is integer), NaBO 2 , Na 2 B 4 O 7 , NaBH 4 , Na 2 C 2 , NaBH 4 , Na 2 O 2 , Na 2 Si 2 O 5 , Na 2 SiO 3 , and Na 4 SiO 4  in a thin film including at least one of Al 2 O 3 , TiO 2 , AlTiO, SiO 2 , Si 3 N 4 , SiON, ZnO, ZnS, ZnSe, ZrO 2 , HfO 2 , MO, CuO, and Ta 3 O 5  thin films. Alternatively, the alkaline metal containing layer  140  may be formed of a material containing a precursor including at least one of NaF, NaO, NaAlO 2 , Na 2 O-Al 2 O 3 -nSiO 2  (n is integer), NaBO 2 , Na 2 B 4 O 7 , NaBH 4 , Na 2 C 2 , NaBH 4 , Na 2 O 2 , Na 2 Si 2 O 5 , Na 2 SiO 3 , and Na 4 SiO 4 . Conductivity of the alkaline metal containing layer  140  may be properly adjusted depending on whether there is a need for electrical connection. The alkaline metal containing layer  140  may be formed using a solution precursor through a sol-gel process, a spin-coating process, an imprinting process, a spray process, a dipping process, or screen-printing process. Alternatively, the alkaline metal containing layer  140  may be formed through a sputtering deposition process, an evaporation process, or a chemical vapor deposition process. 
     The alkaline metal containing layer  140  may have a larger refractive index than the glass substrate  150  and have a smaller refractive index than the second electrode  130 . The refractive index of the alkaline metal containing layer  140  may be a square root of a value attained by multiplying the refractive index of the glass substrate  150  by the refractive index of the second electrode  130 . The alkaline metal containing layer  140  may perform as a antireflection layer that reduces the reflection of the incident light. 
     The alkaline metal of the alkaline metal containing layer  140  may be diffused to the photovoltaic conversion device  120  through the second electrode  130 . The alkaline metal diffused to the photovoltaic conversion device  120  removes the defectiveness in the photovoltaic conversion device  120  to improve the photovoltaic conversion efficiency of the photovoltaic conversion device  120 , thereby providing a high efficiency solar cell. The alkaline metal diffused to the photovoltaic conversion device  120  passivates defects in the photovoltaic conversion device  120  to improve the photovoltaic conversion efficiency of the photovoltaic conversion device  120 , thereby providing a high efficiency solar cell. 
     The glass substrate  150  may be a sodalime glass substrate. Alternatively, the glass substrate  150  may be a glass substrate that does not contain sodium (Na). 
     The antireflection layer  160  may include an incident surface on which the light is incident from a light source LS. The light source LS may be the sun. The light introduced through the antireflection layer  160  may be directed to the photovoltaic conversion device  120  through the glass substrate  150 , the alkaline metal containing layer  140 , and the second electrode  130 . The antireflection layer  160  is configured to minimize the reflection of the light incident from the light source LS on a surface of the glass substrate  150 . The antireflection layer  160  may include at least one of aluminum-titanium oxide, silicon-titanium oxide, aluminum-zirconium oxide, zirconium-titanium oxide, hafnium-titanium oxide, zirconium oxide, titanium oxide, magnesium fluoride, magnesium oxide, hafnium oxide, aluminum oxide, silicon oxide, and nitride-silicon oxide. 
     The following will describe modified examples of the photovoltaic conversion device  120  of solar cell according to embodiments of present invention.  FIGS. 5A and 5B  are views illustrating modified examples of the photovoltaic conversion layer of solar cell according to embodiments of present invention. 
     Referring to  FIG. 5A , a photovoltaic conversion device  121  may be a multiple junction structure including first, second, and third PIN diodes  510 ,  520 , and  530 . The first PIN diode  510  may include a first semiconductor layer  512  of a first conductive type, a second semiconductor layer  514  of an intrinsic state on the first semiconductor layer  512 , and a third semiconductor layer  516  of a second conductive type. The second PIN diode  520  may be disposed on the third semiconductor layer  516  of the first PIN diode  510 . The second PIN may include a fourth semiconductor layer  522  of the first conductive type, a fifth semiconductor layer  524  of the intrinsic state, and a sixth semiconductor layer  526  of a second conductive type, which are sequentially stacked on the third semiconductor layer  516 . The third PIN diode  530  may be disposed on the sixth semiconductor layer  526  of the second PIN diode  520 . The third PIN diode  530  may include a seventh semiconductor layer  532  of the first conductive type, an eighth semiconductor layer  534  of the intrinsic state, and a ninth semiconductor layer  536  of the second conductive type, which are consecutively stacked on the sixth semiconductor layer  526 . The  510 ,  520 , and  530  diodes may be composed only two types of semiconductor layers such as the first conductive type semiconductor layer and the second conductive semiconductor layer. That is, the  510 ,  520 , and  530  diodes may be PN diodes. Although three PIN diodes  510 ,  520 , and  530  are illustrated in  FIG. 5A , the present invention is not limited to this. For example, the photovoltaic conversion layer  121  may include two or more than four PIN diodes. 
     Referring to  FIG. 5B , a photovoltaic conversion layer  123  may has a heterojunction with intrinsic thin layer (HIT) structure including a single-crystal layer and amorphous silicon layers. The photovoltaic conversion layer  123  includes a first amorphous silicon layer  612  of a first conductive type, a second amorphous silicon layer  614  of an intrinsic state, a single-crystal silicon layer  620  of the first conductive type, a third amorphous silicon layer  616  of the intrinsic state, and a fourth amorphous silicon layer  618  of a second conductive type. The first conductive type may be an N-type. The amorphous silicon layers  612 ,  614 ,  616 , and  618  may be thinner than the single-crystal silicon layer  620 . 
     The following will describe modified examples of the solar cell according to first embodiment of the present invention. 
       FIG. 1B  is a view of first modified example of the solar cell according to first embodiment of the present invention. 
     Referring to  FIG. 1B , the first and second electrodes  110  and  130  described with reference to  FIG. 1A  may be provided. One of the photovoltaic conversion devices  120 ,  121 , and  123  described with reference to  FIGS. 1A ,  5 A, and  5 B may be provided. The second electrode  130  may be disposed on a glass substrate  150 . The second electrode  130  may be disposed between the glass substrate  150  and the photovoltaic conversion device  120 . An antireflection layer  160  may be disposed on the glass substrate  150 . The glass substrate  150  may be disposed between the second electrode  130  and the antireflection layer  160 . The glass substrate  150  and the antireflection layer  160  may be formed of same materials as the glass substrate  150  and the antireflection layer  160  of  FIG. 1A . 
     The first electrode  110  may include a first surface contacting the photovoltaic conversion device  120  and a second surface opposite to the first surface. An alkaline metal containing layer  140  may be disposed on the second surface of the first electrode  110 . The alkaline metal containing layer  140  may cover the second surface of the first electrode  110 . The first electrode  110  may be disposed between the alkaline metal container layer  140  and the photovoltaic conversion device  120 . The alkaline metal containing layer  140  may be formed of a same material as the alkaline metal containing layer  140  of  FIG. 1A . The alkaline metal contained in the alkaline metal containing layer  140  may be diffused to the photovoltaic conversion device  120  through the first electrode  110 . 
       FIG. 1C  is a view of second modified example of the solar cell according to first embodiment of the present invention. 
     Referring to  FIG. 1C , the second electrode  130 , glass substrate  150 , and antireflection layer  160 , which are described with reference to  FIG. 1A  may be provided. One of the photovoltaic conversion devices  120 ,  121 , and  123  described with reference to  FIGS. 1A ,  5 A, and  5 B may be provided. The photovoltaic conversion device  120  may include a first surface and a second surface on the opposite side. The second surface of the photovoltaic conversion device  120  may contact the second electrode  130 . The first electrode  110  may be disposed on the first surface of the photovoltaic conversion device  120 . The alkaline metal containing layer  140  may be disposed between the first electrode  110  and the photovoltaic conversion device  120 . The alkaline metal containing layer  140  may include a conductive material. The alkaline metal containing layer  140  may electrically interconnect the first surface of the photovoltaic conversion device  120  and the first electrode  110 . The alkaline metal containing layer  140  functions as a reflective layer reflecting the light passing through the photovoltaic conversion device  120 . The light reflected by the alkaline metal containing layer  140  may be re-entered into the photovoltaic conversion device  120 . The alkaline metal containing layer  140  may be formed of a same material as the alkaline metal containing layer  140  described with reference to  FIG. 1   a.    
       FIG. 1D  is a view of third modified example of the solar cell according to first embodiment of the present invention. 
     Referring to  FIG. 1D , the second electrode  130 , alkaline metal containing layer  140 , glass substrate  150 , and antireflection layer  160 , which are described with reference to  FIG. 1A  may be provided. One of the photovoltaic conversion devices  120 ,  121 , and  123  described with reference to  FIGS. 1A ,  5 A, and  5 B may be provided. 
     The photovoltaic conversion device  120  may include a first surface and a second surface on the opposite side. The second surface of the photovoltaic conversion device  120  may contact the second electrode  130 . The first electrode  110  may be disposed on the first surface of the photovoltaic conversion device  120 . A first additional alkaline metal containing layer  142  may be disposed between the first electrode  110  and the photovoltaic conversion device  120 . The first additional alkaline metal containing layer  142  may be the alkaline metal containing layer  140  that is described with reference to  FIG. 1C . 
     The first electrode  110  may include a first surface contacting the first additional alkaline metal containing layer  142  and a second surface on the opposite side. A second additional alkaline metal containing layer  144  may be disposed on the second surface of the first electrode  110 . The first electrode  110  may be disposed between the first and second additional alkaline metal containing layer  142  and  144 . The second additional alkaline metal containing layer  144  may be formed of a same material as the alkaline metal containing layer  140 . 
     A solar cell according to second embodiment will be described hereinafter.  FIG. 2A  is a view of a solar cell according to second embodiment. 
     Referring to  FIG. 2A , a first electrode  210 , a photovoltaic conversion device  220 , and a second electrode  230  are consecutively stacked on a substrate  250 . The photovoltaic conversion device  220  may be one of the photovoltaic conversion devices  120 , 121 , and  123  that are described with reference to  FIGS. 1A ,  5 A, and  5 B. The first and second electrodes  210  and  230  may be same as the first and second electrodes  110  and  130  described with reference to  FIG. 1A . 
     The first electrode  210  may include a first surface contacting the photovoltaic conversion device  220  and a second surface on the opposite side. The substrate  250  may contact the second surface of the first electrode  210 . The substrate  250  may be the glass substrate  150  that is described with reference to  FIG. 1A . Alternatively, the substrate  250  may be an opaque substrate. For example, the substrate  250  may be one of a stainless steel substrate, a copper substrate, a plastic substrate, a ceramic substrate, a flexible polymer substrate, or a flexible metal substrate. 
     An alkaline metal containing layer  240  may be disposed on the second electrode  230 . The alkaline metal containing layer  240  may be formed of a same material as the alkaline metal containing layer  140  of  FIG. 1A . The alkaline metal containing layer  240  may include an incident surface on which the light is incident from a light source LS. The alkaline metal containing layer  240  may have a smaller refractive index than the second electrode  230 . The alkaline metal containing layer  240  is configured to minimize reflection of the light incident from the light source LS. 
     A metal grid may  246  may be disposed to pass through the alkaline metal containing layer  240  and contact the second electrode  230 . The metal grid  246  may protrude from the alkaline metal containing layer  240 . The metal grid  246  may include at least one of silver (Ag), gold (Au), platinum (Pt), nickel (Ni), Copper (Cu), Carbon (C), Chrome (Cr), Aluminum (Al), titanium (Ti), and molybdenum (Mo), and tungsten (W). The metal grid  246  may have a higher conductivity than the second electrode  230 . By the metal grid  246  of smaller resistivity, carriers generated in the photovoltaic conversion device  220  by the light source LS may be collected from the second electrode  230  and delivered to DC or AC load device with smaller loss of carriers. The metal grid  246  may be formed after forming the alkaline metal containing layer  240  on the second electrode  230 . In this case, after the metal grid  246  is formed, the metal in the metal grid  246  diffuses through the alkaline metal containing layer  240  by a heat-treatment process and the metal grid  246  is electrically connected to the second electrode  230 . 
     The following will describe the metal grid.  FIG. 6A  is a top plane view for illustrating the metal grid of  FIG. 2A .  FIG. 2A  is a cross-sectional view taken along line I-I′ of  FIG. 6A . 
     Referring to  FIG. 2A and 6A , the metal grid  246  may extend in a first direction in parallel with the second surface of the photovoltaic conversion layer  220 . The metal grid  246  may extend in a second direction that is in parallel with the second surface of the photovoltaic conversion layer  220  and intersects the first direction. The second direction may intersect the first direction at a right angle. Alternatively, the metal grid  246  may be composed of a plurality of conductive lines extending in the first direction. 
       FIG. 6B  is a perspective view of a modified example of the metal grid of  FIG. 6A . 
     Referring to  FIG. 6B , the metal grid  246  is disposed between the alkaline metal containing layer  240  and the second electrode  230 . The metal grid  246  may extend in a first direction in parallel with the second surface of the photovoltaic conversion device  220 . The metal grid  246  may extend in a second direction that is in parallel with the second surface of the photovoltaic conversion device  220  and intersects the first direction. The second direction may intersect the first direction at a right angle. In this case, the alkaline metal containing layer  240  may be formed after the metal grid  246  is formed on the second electrode  230 . Alternatively, the metal grid  246  may be composed of a plurality of conductive lines extending in the first direction. 
     The following will describe modified examples of the solar cell according to second embodiment of the present invention. 
       FIG. 2B  is a view of first modified example of the solar cell according to second embodiment of the present invention. 
     Referring to  FIG. 2B , the photovoltaic conversion device  220 , second electrode  230 , alkaline metal containing layer  240 , and metal grid  246 , which are described with reference to  FIG. 2A , may be provided. The photovoltaic conversion device  220  may include a first surface and a second surface on the opposite side. 
     The second surface of the photovoltaic conversion device  220  may contact the second electrode  230 . The first electrode  210  may be disposed on the second surface of the photovoltaic conversion device  220 . The first electrode  210  may be formed of a same material as the first electrode  210  of  FIG. 2A . 
     An additional alkaline metal containing layer  242  may be disposed between the first electrode  210  and the first photovoltaic conversion device  220 . The first electrode  210  may include a first surface contacting the additional alkaline metal containing layer  242  and a second surface on the opposite side. The substrate  250  may be disposed on the second surface of the first electrode  210 . The substrate  250  may be the substrate of  FIG. 2A . 
     The additional alkaline metal containing layer  242  may include a conductive material. The additional alkaline metal containing layer  242  may electrically connect the first surface of the photovoltaic conversion device  220  to the first electrode  210 . The additional alkaline metal containing layer  242  may function as a reflective layer reflecting the light passing through the photovoltaic conversion device  220 . The light reflected on the additional alkaline metal containing layer  242  may be re-entered into the photovoltaic conversion device  220 . The additional alkaline metal containing layer  242  may be formed of a same material as the alkaline metal containing layer described with reference to  FIG. 1A . 
       FIG. 2C  is a view of second modified example of the solar cell according to second embodiment of the present invention. 
     Referring to  FIG. 2C , the substrate  250 , first electrode  210 , additional alkaline metal containing layer  242 , photovoltaic conversion device  220 , second electrode  230 , and alkaline metal containing layer  240 , which are described with reference to  FIG. 2B  may be provided. In addition, the metal grid  246  described with reference to  FIGS. 2A and 6A  may be also provided. 
     An antireflection layer  260  may be provided on the alkaline metal containing layer  240 . The antireflection layer  260  may cover the alkaline metal containing layer  240 . The alkaline metal containing layer  240  may be disposed between the antireflection layer  260  and the second electrode  230 . The antireflection layer  260  may be formed of a same material as the antireflection layer  260  described with reference to  FIG. 1A . The antireflection layer  260  may include an incident surface on which the light is incident from the light source LS. The antireflection layer  260  is configured to minimize the reflection of the light incident from the light source LS. 
     The metal grid  246  may contact the second electrode  240  through the alkaline metal containing layer  240  and the antireflection layer  260 . Alternatively, as described with reference to  FIG. 6B , the metal grid  246  may be provided between the second electrode  230  and the alkaline metal containing layer  240 . 
     A solar cell according to third embodiment will be described hereinafter.  FIG. 3A  is a view of a solar cell according to third embodiment. The solar cell of this embodiment may be a transparent solar cell. 
     Referring to  FIG. 3A , a solar cell includes a first electrode  310 , a photovoltaic conversion device  320 , and a second electrode  330 . The first and second electrodes  310  and  330  may be formed of a transparent material. For example, the first and second electrodes  310  and  330  may be formed of at least one of ZnO:Al, ZnO:Ga, ZnO:B, ZnO:Cd, InSnO(ITO), SnO 2 , SnO:F, RuO 2 , IrO 2 , and Gu 2 O. The photovoltaic layer  320  may be one of the photovoltaic layers  120 ,  121 , and  123  described with reference to  FIGS. 1A ,  5 A, and  5 B. 
     An alkaline metal containing layer  340  may be formed on glass substrate  350 , and the glass substrate  350  may be coated with antireflection layer  360  on the opposite side. The alkaline metal containing layer  340  may be disposed between the glass substrate  350  and the second electrode  330 . The glass substrate  350  may be disposed between the antireflection layer  360  and the alkaline metal containing layer  340 . The alkaline metal containing layer  340 , glass substrate  350 , and antireflection layer  360  may be the alkaline metal containing layer  140 , glass substrate  150 , and antireflection layer  160 , which are described with reference to  FIG. 1A . 
     The first electrode  310  may include a first surface contacting the photovoltaic conversion device  320  and a second surface on the opposite side. A metal grid  316  may be disposed on the second surface of the first electrode  310 . The metal grid  316  may protrude from the second surface of the first electrode  310 . Like the metal grid  246  described with reference to  FIG. 6A , the metal grid  316  may extend in a first direction parallel with the second surface of the photovoltaic conversion device  320 , and further extend in a second direction parallel with the second surface of the photovoltaic conversion device  320  and intersect with the first direction. The second direction may interest the first direction at a right angle. Alternatively, the metal grid  316  may be composed of a plurality of conductive lines extending in the first direction. 
     The metal grid  316  may be formed of a same material as the metal grid  246  described with reference to  FIG. 2A . The metal grid  316  may have a higher conductivity than the first electrode  310 . By the metal grid  316  of higher conductivity, carriers generated in the photovoltaic conversion device  320  by the light source LS may be collected from the first electrode  310  and delivered to DC or AC load device with smaller loss of carriers. 
     The following will describe modified examples of the solar cell according to third embodiment of present invention. 
       FIG. 3B  is a view of first modified example of the solar cell according to third embodiment of present invention. 
     Referring to  FIG. 3B , the first electrode  310 , photovoltaic conversion device  320 , second electrode  330 , which are described with reference to  FIG. 3B , may be provided. The first electrode  310  may include electric charge compensation material. The electric charge compensation material may be a halogen element or a group-VI element. For example, the first electrode  310  may include one of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), oxygen (O), sulphur (S), selenium (Se), and tellurium (Te). The electric charge compensation material enhances conductivity of the first electrode  310 . 
     A glass substrate  350  may be disposed on the second electrode  330 . The second electrode  330  may be disposed between the glass substrate  350  and the photovoltaic conversion device  320 . An antireflection layer  360  may be disposed on the glass substrate  350 . The glass substrate  350  may be disposed between the antireflection layer  360  and the second electrode  330 . The glass substrate  350  and the antireflection layer  360  may respectively include same materials as the glass substrate  350  and the antireflection layer  360 , which are described with reference to  FIG. 3A . 
     The first electrode  310  may include a first surface contacting the photovoltaic conversion device  320  and a second surface on the opposite side. An alkaline metal containing layer  342  may be disposed on the second surface of the first electrode  310 . The first electrode  310  may be disposed between the alkaline metal containing layer  342  and the photovoltaic conversion device  320 . The alkaline metal containing layer  342  may be configured such that a reflectance for a first wavelength band of incident light may be higher than that for a second wavelength band of the incident light. The first wavelength band may be different from the second wavelength band. For example, the first wavelength band may include infrared rays or ultraviolet rays. The second wavelength band may include visible rays. The light having the first wavelength band reflected on the alkaline metal containing layer  342  is re-enetered the photovoltaic conversion device  320 , thereby carriers (e.g., holes or electrons) may be generated in the photovoltaic conversion layer  320  due to the re-entered light. 
     The first and second wavelength bands may be adjusted depending on an optical thickness of the alkaline metal containing layer  342 . The optical thickness is a value attained by multiplying a refractive index of a medium by a physical thickness of the medium. The refractive index of the alkaline metal containing layer  342  may be varied depending on a composition ratio of materials of the alkaline metal containing layer  342 . The alkaline metal containing layer  342  may be formed of a same material as the alkaline metal containing layer  140  described with reference to  FIG. 1A . 
     A metal grid passing through the alkaline metal containing layer  342  and contacting the first electrode  310  may be provided. Like the metal grid  246  in  FIG. 6A , the metal gird  316  may extend in a first direction in parallel with the second surface of the photovoltaic conversion layer  320 . The metal grid  316  may extend in a second direction that is in parallel with the second surface of the photovoltaic conversion device  320  and intersects the first direction. The second direction may intersect the first direction at a right angle. Alternatively, the metal grid  316  may be composed of a plurality of conductive lines extending in the first direction. The metal grid  316  may be formed of a same material as the metal grid  316  of  FIG. 3A . The metal grid  316  may protrude from the alkaline metal containing layer  342  and the first electrode  310 . Alternatively, as described with reference to  FIG. 6B , the metal grid  316  may be disposed between the alkaline metal containing layer  342  and the first electrode  310 . 
       FIG. 3C  is a view of second modified example of the solar cell according to third embodiment of present invention. 
     Referring to  FIG. 3C , the first electrode  310 , photovoltaic conversion device  320 , second electrode  330 , alkaline metal containing layer  340 , glass substrate  350 , antireflection layer  360 , and metal grid  316 , which are described with reference to  FIG. 3A , may be provided. The first electrode  310  may include a first surface contacting the photovoltaic conversion device  320  and a second surface on the opposite side. An additional alkaline metal containing layer  342  may be disposed on the second surface of the first electrode  310 . The first electrode  310  may be disposed between the additional alkaline metal containing layer  342  and the photovoltaic conversion device  320 . The additional alkaline metal containing layer  342  may be the alkaline metal containing layer  342  of  FIG. 3B . The metal grid  316  may contact the first electrode  310  through the additional alkaline metal containing layer  342 . 
     A solar cell according to fourth embodiment will now be described.  FIG. 4A  is a view of a solar cell according to fourth embodiment. 
     Referring to  FIG. 4A , a solar cell includes a photovoltaic conversion device  420 . The photovoltaic conversion device  420  is configured to generate carriers (e.g., holes or electrons) by the sunlight incident thereon. The photovoltaic conversion device  420  may include a first surface and a second surface on the opposite side. The photovoltaic conversion device  420  may include a first conductive type semiconductor layer and a second conductive type semiconductor layer, which may contact each other. The first and second conductive types may be different from each other. The first and second surfaces of the photovoltaic conversion device  420  may be surfaces having different types of semiconductor layers. For example, the first surface of the photovoltaic conversion device  420  may be a surface included in an N-type semiconductor layer and the second surface of the photovoltaic conversion layer  420  may be a surface included in a P-type semiconductor layer. The photovoltaic conversion layer  420  may include at least one of Si, SiGe, CuInS, 
     CuInGaSe, CuInGaS, CdS, CdTe, ZnO, ZnS, CuZnSnS, CuZnSnSe, Cu 2 O, GaAs, GaInAs, GaInAlAs, and InP. The photovoltaic conversion device  420  may include an organic semiconductor material. 
     The first surface of the photovoltaic conversion device  420  may be connected to a first electrode  410 . The first electrode  410  may be the first electrode  110  of  FIG. 1A . 
     An alkaline metal containing layer  440  may be disposed on the first surface of the photovoltaic conversion device  420 . The alkaline metal containing layer  440  may directly contact the first surface of the photovoltaic conversion device  420 . The alkaline metal containing layer  440  may function as a antireflection layer for reducing the reflection of light incident from a light source LS. The alkaline metal containing layer  440  may be formed of a same material as the alkaline metal containing layer  140  of  FIG. 1A . The alkaline metal contained in the alkaline metal containing layer  440  may be diffused to the photovoltaic conversion device  420  to improve the photovoltaic conversion efficiency of the photovoltaic conversion device  420 . A portion of the photovoltaic conversion device  420 , which contacts the alkaline metal containing layer  440  may be one of an amorphous, micro-crystal, and polycrystal layers that are formed through a film deposition process. 
     The second surface of the photovoltaic conversion device  420  may be connected to a second electrode  430 . The second electrode  430  may be provided in the form of a metal grid contacting the photovoltaic conversion device  420 . The second electrode  430  may directly contact the photovoltaic conversion device  420  through the alkaline metal containing layer  440 . The second electrode  430  may protrude from the alkaline metal containing layer  440 . Like the metal grid  246  of  FIG. 6A , the second electrode  430  may extend in a first direction in parallel with the second surface of the photovoltaic conversion device  420 . The second electrode  430  may extend in a second direction that is in parallel with the second surface of the photovoltaic conversion layer  420  and intersects the first direction. The second direction may intersect the first direction at a right angle. Alternatively, the second electrode  430  may be composed of a plurality of conductive lines extending in the first direction. The second electrode  430  may be formed of a same material as the metal grid  246  of  FIG. 2A . 
     The following will describe a modified example of the solar cell according to fourth embodiment of present invention.  FIG. 4B  is a view of a modified example of the solar cell according to fourth embodiment of present invention. 
     Referring to  FIG. 4B , the first electrode  410 , photovoltaic conversion device  420 , second electrode  430 , and alkaline metal containing layer  440 , which are described with reference to  FIG. 4A , are provided. The first electrode  410  may include a first surface contacting the photovoltaic conversion device  420  and a second surface on the opposite side. An additional alkaline metal containing layer  442  may be disposed on the second surface of the first electrode  410 . The first electrode  410  may be disposed between the additional alkaline metal containing layer  442  and the photovoltaic conversion device  420 . The additional alkaline metal containing layer  442  may be formed of a same material as the alkaline metal containing layer  140  described with reference to  FIG. 1A . 
     The alkaline metal in the alkaline metal containing layer  440  may be diffused to the photovoltaic conversion device  420 . The alkaline metal in the additional alkaline metal containing layer  442  may be diffused to the photovoltaic conversion device  420  trough the first electrode  410 . 
     The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the true spirit and scope of the present invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.