Abstract:
A high-concentration multi-junction solar cell and method for fabricating same is provided. The high-concentration multi-junction solar cell comprises a top cell, an intermediate cell, a bottom cell and two tunneling junctions connecting the top cell and intermediate cell and the intermediate cell and bottom cell. The emitter layers of the top and intermediate cells both employ the graded doping concentrations and have high open circuit voltage and short circuit current. The top cell emitter layer is over several hundred nanometers thicker than that of the traditional multi-junction cell so as to decrease the whole series resistance of the multi-junction cell, improve the fill factor, and gain higher photoelectric conversion efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT/CN2012/075134 filed on May 7, 2012 and published on Dec. 27, 2012 as publication WO 2012/174952, which claims priority to Chinese Patent Application No. 201110168522.9 entitled “High-concentration multi-junction solar cell and method for fabricating same”, filed on Jun. 22, 2011, the contents of which are incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention pertains to the field of compound semiconductor solar cell, and specifically relates to a high-concentration multi-junction solar cell and a method for fabricating the same. 
       BACKGROUND OF THE INVENTION 
       [0003]    The photovoltaic power generation technology, after the development of the first-generation crystalline silicon cells and the second-generation thin film photovoltaic cells, is now entering upon an age of the second-generation concentrated photovoltaic (CPV) technology. III-V concentrating multi-junction solar cell serves as the core of CPV technology. Compared with other types of solar cells, III-V concentrating multi-junction solar cell features high photoelectric conversion efficiency, excellent temperature characteristics, short energy recovery period and the like. It allows maximized use of solar energy resources and reduces harm to the environment due to construction of power plants. 
         [0004]    The multi-junction solar cell is made by the connection through tunneling junction of several semiconductor cells with different band gaps. Different sub-cells absorb the solar spectrum of different wavelengths and further convert the solar energy as much as possible into the electric energy. With the unique design idea and high photoelectric conversion efficiency, the multi-junction solar cell has become the basic cell structure employed presently for the research of solar cell by research institutes and enterprises of the photovoltaic field around the world. Spire Corporation declared in October 2010 that it had developed a triple junction solar cell. With an area of 0.97 cm 2  under the test conditions of 406 times of concentration of solar radiation, AM1.5 atmospheric optical quality and the temperature of 25° C., the triple junction solar cell has the conversion efficiency of up to 42.3%. The InGaP/(In)GaAs/Ge triple junction solar cell manufactured by Emcore, a major player of CPV in the world, has the conversion efficiency of 39% under 500 times of solar concentration and of 36.3% under 1150 times of solar concentration. Along with the advancing CPV technology industrialization, high-concentration (−1000×) solar cells have become the key product in the CPV industry due to its outstanding cost effectiveness. This kind of cell can concentrate, through a condensing lens, solar energy hundreds of thousands of times on a very small cell chip for power generation so as to greatly reduce the number of solar cell chip needed. Along with the comparatively high open circuit voltage and short-circuit current under high concentration (−1000×), the cell can also produce greater series resistance, which seriously affects the fill factor of cell and decreases the conversion efficiency. 
       SUMMARY 
       [0005]    The object of this invention is to provide a novel high-concentration multi-junction solar cell, which not only has high open circuit voltage and short circuit current, but also keeps high fill factor, i.e., to keep high photoelectric conversion efficiency under high concentration condition. 
         [0006]    In accordance with an aspect of the invention, a high-concentration multi-junction solar cell is provided, comprising: a top cell, an intermediate cell, a bottom cell, and two tunneling junctions. The emitter layers of the top cell and the intermediate cell both feature graded doping concentrations, and the top cell emitter layer is over 100 nm thicker than that of the traditional multi-junction cell. 
         [0007]    As is well known in the art of solar cells, an emitter layer forms a p-n junction with an underlying layer, typically having the same conductivity type as the substrate. Emitter layers are typically n-type and the substrate is a p-type. By a built-in potential difference generated due to the p-n junction, a plurality of electron-hole pairs, which are generated by incident light into the emitter layer, are separated into electrons and holes, and the separated electrons move toward the n-type semiconductor and the separated holes move toward the p-type semiconductor. Thus, when the substrate is of the p-type and the emitter layer is of the n-type, the separated holes move toward the substrate and the separated electrons move toward the emitter layer. Accordingly, the holes become major carriers in the substrate and the electrons become major carriers in the emitter layer. By providing a plurality of stacked p-n junctions (i.e., multiple emitter layers), there is more of a likelihood that a photon from the sunlight entering the solar cell will create an electron-hole pair near a pn junction, to effectively convert the photon to current. 
         [0008]    Preferably, the top cell emitter layer has a thickness of 0.05-0.5 microns. 
         [0009]    Preferably, the top cell emitter layer has a thickness of 0.3 microns. 
         [0010]    Preferably, in the top cell and intermediate cell, the emitter layer close to the base area is a low doping concentration area, with a doping concentration of 1×10 17 /cm 3 -1×10 18 /cm 3  and the emitter layer far from the base area is a high doping concentration, with a doping concentration of 1×10 18 /cm 3 -1×10 19 /cm 3 . 
         [0011]    Preferably, the doping concentration of the top cell emitter layer is graded from 5×10 17 /cm 3  to 5×10 18 /cm 3 . 
         [0012]    Preferably, the doping concentration of the intermediate cell emitter layer is graded from 5×10 17 /cm 3  to 5×10 18 /cm 3 . 
         [0013]    In accordance with an aspect of the invention, a method for fabricating the high-concentration multi-junction solar cell is provided, comprising the following steps: by the epitaxial method including metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) or ultrahigh vacuum chemical vapor deposition (UHVCVD), a Ge bottom cell grows epitaxially on a selected Ge substrate; a GaAs tunneling junction grows epitaxially on the Ge substrate; a base area of a (In) GaAs intermediate cell grows epitaxially on the GaAs tunneling junction; an (In) GaAs intermediate cell emitter layer grows epitaxially on the base area of the (In) GaAs intermediate cell, forming the (In) GaAs intermediate cell; an AlGaAs tunneling junction grows epitaxially on the (In) GaAs intermediate cell; an InGaP top cell base area grows epitaxially on the AlGaAs tunneling junction; a thick and graded doping InGaP top cell emitter layer grows epitaxially on the InGaP top cell base area, forming the InGaP top cell. 
         [0014]    Preferably, the doping concentration of the intermediate cell emitter layer is graded, including step grading and continuous grading; the emitter layer close to the base area is a low doping concentration area, with a doping concentration of 1×10 17 /cm 3 -1×10 18 /cm 3 , and that far from the base area is a high doping concentration area, with a doping concentration of 1×10 18 /cm 3 -1×10 19 /cm 3 . 
         [0015]    Preferably, the doping concentration of the top cell emitter layer is graded, including step grading and continuous grading; the emitter layer close to the base area is a low doping concentration area, with a doping concentration of 1×10 17 /cm 3 -1×10 18 /cm 3 , and that far from the base area is a high doping concentration area, with a doping concentration of 1×10 18 /cm 3 -1×10 19 /cm 3 . 
         [0016]    Preferably, the doping concentration of the said top cell and intermediate cell emitter layers is graded from 5×10 17 /cm 3  to 5×10 18 /cm 3 . 
         [0017]    Preferably, the thickness of the whole top cell emitter layer is 0.05-0.5 micron. 
         [0018]    Each sub-cell emitter layer of the traditional multi-junction solar cell is uniformly doped, and the thinner the emitter layer is, the higher the photoelectric conversion efficiency of the cell is. However, under the high-concentration condition, the thinner top cell emitter layer produces greater series resistance, which decreases the fill factor of the cell and finally affects the conversion efficiency. The invention relates to a high-concentration multi-junction solar cell. The emitter layers of the top and intermediate cells both employ the graded doping concentration and have high open circuit voltage and short circuit current; meanwhile, under the high concentration condition, the top cell emitter layer is allowed to have greater thickness compared with the traditional multi-junction cell so as to decrease the total series resistance of the multi-junction cell, improve the fill factor and finally gain higher photoelectric conversion efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The attached drawings help further understand this invention and constitute a part of the instructions. Together with the embodiments of the invention, these drawings are used for explaining the invention, but do not constitute a limitation to the invention. In addition, figures on the attached drawings are just to describe an outline of the invention rather than being drawn in proportion. 
           [0020]      FIG. 1  is the cross sectional view of a high-concentration multi-junction solar cell of the invention. 
       
    
    
       [0021]    In the figure,
         100 : p-type Ge substrate;     101 : n-type Ga 0.5 In 0.5 P window layer     200 : GaAs tunneling junction;     300 : (In)GaAs intermediate cell back surface field layer;     301 : (In) GaAs intermediate cell base area;     302 : (In) GaAs intermediate cell emitter layer;     303 : (In) GaAs intermediate cell window layer;     400 : AlGaAs tunneling junction;     500 : GaInP top cell back surface field layer;     501 : GaInP top cell base area;     502 : GaInP top cell emitter layer;     503 : GaInP top cell window layer   A: Ge bottom cell;   B: intermediate cell;   C: top cell.       
 
       DETAILED DESCRIPTION 
       [0037]    Detailed explanation will be given to the invention by combining the attached drawings and the embodiments. It should be noted that in case of no discrepancies, the embodiments of the invention and each feature of the embodiment can be combined with each other and those are all within the protection scope of the invention. 
       Embodiment One 
       [0038]    As illustrated in  FIG. 1 , a high-concentration multi-junction solar cell comprises a Ge bottom cell A, an intermediate cell B, a top cell C and two tunneling junctions  200  and  400  connecting the cells. 
         [0039]    More specifically, the figure shows: one P-type Ge substrate  100  and one n-type Ga 0.5 In 0.5 P window layer  101  deposited on the substrate, which form a Ge bottom cell A. 
         [0040]    A series of highly doped P-type and n-type layers are deposited on the top of the Ge bottom cell A, forming a GaAs tunneling junction  200  and used for connecting the Ge bottom cell A to the intermediate cell B. 
         [0041]    An intermediate cell back surface field layer  300  is deposited on the top of the formed GaAs tunneling junction  200  and used for reducing recombination loss. The layer is preferably formed by P-type AlGaAs. 
         [0042]    An intermediate cell base area  301  and an intermediate cell emitter layer  302  are deposited on an intermediate cell back surface field layer  300 . In the preferred embodiment, the intermediate cell base area  301  is formed by P-type (In) GaAs with a thickness of 3.5 micron; the intermediate cell emitter layer  302  is formed by n-type (In) GaAs with a thickness of 0.1 micron, and the n-type doping is gradually increased with the thickness and the doping concentration is continuously graded from 5×10 17 /cm 3  to 5×10 18 /cm 3 . An intermediate cell window layer  303  formed by n-type AlInP is deposited on the intermediate cell emitter layer  302 , forming the intermediate cell B. 
         [0043]    A tunneling junction  400  preferably formed by AlGaAs is deposited on the top of the intermediate cell B and used for connecting the intermediate cell B to the top cell C. 
         [0044]    A top cell back surface field layer  500  preferably formed by P-type AlInGaP is deposited on the top of the tunneling junction  400 . 
         [0045]    A top cell base area  501  and a top cell emitter layer  502  are deposited on the top of the top cell back surface field layer  500 . In the preferred embodiment, the top cell base area  501  is formed by a 0.8 micron thick P-type GaInP; the top cell emitter layer  502  is formed by 0.3 micron thick n-type GaInP, and the doping concentration is continuously graded from 5×10 17 /cm 3  to 5×10 18 /cm 3  as the n-type doping is gradually increased with the thickness. A top cell window layer  503  formed by n-type AlInP is deposited on the top cell emitter layer  502 , forming the top cell C. Therefore, the emitter layers of the top cell and the intermediate cell both contain a graded doping concentration, and the thickness of the top cell emitter layer is 0.3-0.5 micron to decrease the total series resistance of the multi-junction solar cell. 
       Embodiment Two 
       [0046]    The embodiment is a fabricating process of the high-concentration multi-junction solar cell in Embodiment One, comprising the process of the sub-cells A, B and C and each layer between the sub-cells. In the course of MOCVD epitaxial growth, by controlling and adjusting the flow ratio of n-type dopant source in the reaction source, the grading of the doping concentration of the emitter layer can be realized. 
         [0047]    The specific fabrication process comprises the following steps: 
         [0048]    The p-type doped Ge substrate  100  has a thickness of 150 micron and functions as the Ge bottom cell base area. 
         [0049]    The P-type Ge substrate  100  is well cleaned and placed in a MOCVD reaction chamber; first the P-type Ge substrate  100  is baked for ten minutes at the temperature of 750° C. and then decreased to a temperature of 600° C. n-type Ga 0.5 In 0.5 P window layer  101  grows epitaxially to form the Ge bottom cell A. 
         [0050]    The GaAs tunneling junction  200  connecting the bottom and intermediate cells grows epitaxially on the Ge bottom cell. 
         [0051]    The back surface field layer  300  of the intermediate cell B grows to prevent the photo-generated electron of the intermediate cell base area from spreading to the bottom cell. The specific method is as follows: the temperature of the MOCVD reaction chamber is controlled to be 620° C. and V/III source molar flow ratio to be  120 ; and a layer of P-type Al 0.2 Ga 0.8 As grows epitaxially on the GaAs tunneling junction  200  and functions as the back surface field layer of the intermediate cell B. 
         [0052]    The base area  301  and emitter layer  302  of the intermediate cell B grow epitaxially on the back surface field layer of the intermediate cell B. After the V/III source molar flow ratio in the MOCVD reaction chamber is regulated to be  40 , a layer of P-type In 0.01 Ga 0.99 As grows epitaxially on the back surface field layer  300  of the intermediate cell B and functions as the base area  301  of the intermediate cell B, with a thickness of 3.5 micron. And the emitter layer  302  grows epitaxially on the intermediate cell base area  301 . In the course of MOCVD epitaxial growth, a low n-type dopant flow is used in the initial stage of the growth and the dopant flow is increased with the thickness of the emitter layer and finally the doping concentration is continuously graded from 5×10 17 /cm 3  to 5×10 18 /cm 3 , namely the n-type In 0.01 Ga 0.99 As intermediate cell emitter layer  302 , with a thickness of 0.1 micron. 
         [0053]    A layer of n-type AlInP grows epitaxially on the emitter layer  302  of the intermediate cell B and functions as the window layer  303  of the intermediate cell B, forming the In 0.01 Ga 0.99 As intermediate cell B. 
         [0054]    The AlGaAs tunneling junction  400  grows epitaxially on the In 0.01 Ga 0.99 As intermediate cell B. 
         [0055]    The back surface field layer  500  of the top cell C grows to prevent the photo-generated electron of the top cell base area from spreading to the intermediate cell. The specific method is as follows: the temperature of the MOCVD reaction chamber is controlled to be 650° C. and the V/III source molar flow ratio to be  200 ; and a layer of p-type AlInGaP grows epitaxially on the AlGaAs tunneling junction  400  and functions as the back surface field layer  500  of the top cell C. 
         [0056]    The base area  501  and emitter layer  502  forming the top cell C grow epitaxially on the back surface field layer  500  of the top cell C. After the V/III source molar flow ratio is regulated to be  180 , a layer of P-type Ga 0.5 In 0.5 P grows epitaxially on the back surface field layer  500  of the top cell B and functions as the base area  501  of the top cell B, with a thickness of 0.8 micron. And the top cell emitter layer  502  grows epitaxially on the top cell base area  501 . In the course of MOCVD epitaxial growth, a low n-type dopant flow is used in the initial stage of the growth and the dopant flow is increased with the thickness of the emitter layer and finally the doping concentration is continuously graded from 5×10 17 /cm 3  to 5×10 18 /cm 3 , namely the n-type Ga 0.5 In 0.5 P top cell emitter layer  502 , with a thickness of 0.3 micron. 
         [0057]    A layer of n-type AlInP grows epitaxially on the emitter layer  502  of the top cell B and functions as the window layer  503  of the top cell B, forming the Ga 0.5 In 0.5 P top cell C. 
         [0058]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications that are within the true spirit and scope of this invention.