Patent Publication Number: US-2007113887-A1

Title: Material system of photovoltaic cell with micro-cavity

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to the material system of a photovoltaic cell, and more especially, to the material system of a solar cell.  
      2. Background of the Related Art  
      Photovoltaic cell, such as solar cell, is familiar with making use of the photovoltaic effect to convert energy from the light into electric energy. Solar radiation is composed of photons, which are particles that have a variable energy depending on the wavelength of the emissions in the solar spectrum. When the photons fall onto the surface of the semiconductor material forming a photovoltaic cell they may either be reflected, absorbed or pass through the cell.  
      There are certain materials that, upon absorbing this type of radiation, generate positive and negative charge couples, i.e. electrons (e−) and holes (h+), which, on being produced, move randomly through the volume of the solid and, if there is no external or internal determining factor, the opposing sign charges recombine and neutralize each other mutually. On the other hand, if a permanent electric field is created in the interior of the material, the positive and negative charges will be separated by this field, which produces a difference of potential between the two areas of material. If these two areas are interconnected by means of an external circuit, at the same time as the solar radiation falls onto the material an electric current will be produced that will run round the external circuit.  
      The most important parts of a solar cell are the intermediate layers made up of semiconductor or organic materials, as it is at the heart of materials of this type where the electron current and proper voltage are created. These semiconductors are specially treated to form two layers in contact with each other, which are doped differently (type p and type n) to form a positive electric field on one side and a negative one on the other side. In addition, solar cells are formed of an upper layer or mesh composed of an electrically-conductive material, which has the function of collecting the photo-generated carrier from the semiconductor and transferring them to the outer circuit and a lower layer or mesh of electrically-conductive material, which has the function of completing the electric circuit. The cells are usually connected to one another, encapsulated and mounted on a structure in the form of a carrier or frame, thereby shaping the solar panel.  
      As an alternative to these conventional solar modules thin-film solar cells have become known on the basis of micrometer film thicknesses. The substantial elements of a thin-film solar cell include a p/n junction structure between the absorber layer and the window layer. Unlike conventional silicon wafer wiring, thin-film cells can have integrated circuitry. Following the individual coating steps on the total surface area, the back conductor, the sandwiched cell, and the front conductor are sliced into longitudinal strips. Staggering these three slices relative to each other forms an electrical connection between cells adjoining the front and back conductor. However, the thickness of thin-film solar cells is incapable of utilizing light beams efficiently.  
     SUMMARY OF THE INVENTION  
      In order to efficiently utilizing incident light beams, a material system for a photovoltaic cell may trap most of the incident light beams within an absorbing structure with two high-reflection layers toward the absorbing structure.  
      In order to enhance the photovoltaic efficiency, a material system may reflect most of light beams back to an absorbing structure.  
      Accordingly, one embodiment of the present invention provides a photovoltaic cell with resonance cavity. A first structure of reflection is attached toward one side of the resonance cavity and configured for reflecting light beams from the resonance cavity. The second structure of reflection is attached toward other side of the resonance cavity and configured for reflecting light beams from exterior and the resonance cavity. Thus, photos will be absorbed efficiently within the resonance cavity and converted into electrons. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional diagram illustrating the structure of material system for a photovoltaic cell in accordance with an embodiment of the present invention; and  
       FIG. 2  is a schematic diagram illustrating the path of the light beams in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  is a cross-sectional diagram illustrating the structure of material system for a photovoltaic cell in accordance with an embodiment of the present invention. A material system of photovoltaic cell  10  includes an absorbing structure  12  between a first layer  14  and a second layer  16 . In one embodiment, the first layer  14  is configured for receiving exterior incident light beams with one side  141  and attached to the absorbing structure  12  with the other side  142 . The absorbing structure  12  is attached to the first layer  14  with one side  121  and the second layer  16  with the other side  122 . The second layer  16  is configured for both reflecting light beams from the absorbing structure  12  and attached to the absorbing structure  12  with one side  161 . The material system of photovoltaic cell  10  may include other structures, for example, a layer of window glass (not shown) is on the first layer  14  and a substrate (not shown) attached to the second layer  16 .  
      In one embodiment, the absorbing structure  12  configured for absorbing photons may include an absorber layer  24  between a layer of front conductor  22  and a layer of back conductor  26 . The layer of front conductor  22  is attached to the first layer  14  with the side  121  and the layer of back conductor  26  is attached to the second layer  16  with the side  122 . The layer of front conductor  22  and the layer of back conductor  26  are made of semiconductor material doped differently, for example, type p and type n, to form a positive electric field on one side and a negative one on the other. On the other hand, the layer of front conductor  22  and the layer of back conductor  26  are placed in contact with each other by way of the absorber layer  24 . The absorber layer  24  is made of, not limited to, single or poly-crystalline or silicon-based material. Alternatively, the absorber layer  24  may be made of semiconductor compound, such as GaAs, CdS or CuInSe 2 , etc.  
      Next, the first layer  14  may have an anti-reflection coating (AR coating) at the side  141  to capable of receiving exterior incident light beams. Furthermore, the surface of the side  141  may be modified, such as roughening, for the help of trapping the light beams. On the other hand, the first layer  14  further have a high-reflection coating (HR coating) (i.e. reflectivity &gt;99.8%) at the side  142 . On the other hand, the first layer  14  may further have a plurality of layers of different refractive index between the side  141  and the side  142 . In one embodiment, the plurality of refractive index increases from the side  141  to the side  142  for sure that exterior incident light beams successfully enter into the absorbing structure  12 .  
      Furthermore, the second layer  16  is configured for reflecting the light beams back to the absorbing structure  12 . In one embodiment, the second layer  16 , one or more layers, may be made of metal or alloy material, such as aluminum (Al) or silver, etc. Alternatively, the second layer  16  may be made of Distributed Bragg Reflector (DBR), such as SiO 2 /TiO 2 , AlAs/GaAs, etc.. Alternatively, the second layer  16  may be made of those materials aforementioned to be formed the structure of hybrid materials. It is noted that the layer of back conductor may be introduced into the second layer  16  on the condition of semiconductor or conductive materials.  
      On application, shown in  FIG. 2 , exterior light beams  30  penetrate into the absorbing structure  12  through the side  141  of the first layer  14 . The portion of incident light beams is absorbed by the absorbing structure  12  and the other portion  32  reach onto the side  161  of the second layer  16 . The second layer  16  according to the spirit of the present invention is advantageous to utilizing of the portion not being absorbed. The second layer  16  may reflect the portion of light beams  34  back to the absorbing structure  12 . On the other hand, the light beams through the absorbing structure  12  are still reflected by the first layer  14  when reaching to the side  121  of the absorbing structure  12 . The first layer  14  provided with the HR coating of the side  142  is also advantageous to reutilizing of the portion of light beams. Accordingly, the association of the first layer  14  and the second layer  16  may trap the light beams  36  into the absorbing structure  12  and cause micro-resonance cavity in the absorbing structure  12 . Thus, photo resonance will be formed within the absorbing structure  12  to enhance photovoltaic efficiency.  
      Accordingly, a material system of photovoltaic cell includes an absorbing structure between a first and a second layers. The absorbing structure is provided with a first side and a second side. The first layer is attached to said first side, configured for accepting light beams from exterior with a third side and reflecting light beams from the absorbing structure with a fourth side. The second layer is attached to the second side and configured for reflecting light beams from the absorbing structure. Photo resonance is implemented within the absorbing structure by the first layer associated with the second layer.  
      Accordingly, a material system of photovoltaic cell includes an absorbing structure between a first and a second layers. The absorbing structure is with a first side and a second side. The first layer is attached to said first side, configured for accepting light beams from exterior with a third side and reflecting light beams from the absorbing structure with a fourth side. The second layer is attached to the second side and configured for reflecting light beams from the absorbing structure. Photo resonance is implemented within the absorbing structure by way of the first layer associated with the second layer.  
      Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that other modifications and variation can be made without departing the spirit and scope of the invention as hereafter claimed.