Patent Publication Number: US-2010108137-A1

Title: Crystalline solar cell having stacked structure and method of manufacturing the crystalline solar cell

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solar cell, and more particularly, to a crystalline solar cell having a stacked structure with high light absorption efficiency. 
     2. Description of the Related Art 
     In general, it is well known that a solar cell having a stacked structure can absorb light in a very wide wavelength range and has high light absorption efficiency. For this, in the stacked structure of the solar cell, a solar cell layer having a wide band gap is disposed at a front surface on which light is incident and a solar cell layer having a narrow band gap is disposed at a rear surface on which the light incident later. 
       FIG. 1  illustrates a solar cell having a conventional stacked structure. The solar cell  100  having the conventional stacked structure illustrated in  FIG. 1  includes a first solar cell layer  110   a  which has a wide band gap A and is disposed at a front surface in a direction  101  of incident light and a second solar cell layer  110   b  which has a narrow band gap B and is disposed at a rear surface in the direction  101  of incident light. In order to electrically connect the solar cell layers  110   a  and  110   b  having different band gaps, in general, a transparent conductive oxide (TCO) layer  120  which is transparent and conductive is disposed between the solar cell layers  110   a  and  110   b.    
     Light incident on the solar cell  100  is first absorbed by the first solar cell layer  110   a  and light that the first solar cell layer  110   a  cannot absorb is absorbed by the second solar cell layer  110   a  having the narrow band gap B. 
       FIG. 2  illustrates an example of an energy band of the solar cell  100  illustrated in  FIG. 1 . 
     An electron e and a hole h which are generated at first and second solar cell layer  110   a  and  110   b  by incident light, respectively, are separated by potentials, and a recombination  201  of an electron e and a hole h generated at the first and second solar cell layer  110   a  and  110   b,  respectively, occurs at the TCO layer  120 . In addition, due to the electron e and the hole h which are separated from each other to both ends, quasi Fermi levels at the both ends are different from each other, so that a voltage occurs. 
     Therefore, in the solar cell  100  having the stacked structure as illustrated in  FIG. 1 , light absorption occurs in a wider area as compared with a solar cell having a single solar cell layer, so that the solar cell  100  has an advantage of high light absorption efficiency. 
     In order to manufacture the solar cell having the stacked structure as illustrated in  FIG. 1 , stacked solar cell layers having different band gaps and different lattice parameters are needed. 
     However, in a case where crystal growth of materials having different lattice parameters is performed in order to stack the solar cell layers, lattice defects occur due to a difference between the lattice parameters of the two materials for forming two solar cell layers, respectively, at the interface between the solar cell layers, and the generated lattice defects may operate as a recombination center between electron-hole. This results in increase in a recombination rate and decrease in electricity generation efficiency. Therefore, in order to construct a solar cell having high efficiency, a lattice buffer layer for removing the lattice defects that occur between solar cell layers having different lattice parameters is needed. For this, in a conventional method of forming the lattice buffer layer, for example, when a solar cell having a stacked structure including a solar cell layer made of silicon (Si) having a band gap A of about 1.1 eV and a solar cell layer made of germanium (Ge) having a band gap B of about 0.7 eV is to be constructed, a Si 1-x Ge x  (here, 0&lt;x&lt;1) layer having a lattice parameter that is changed according to a ratio of Ge is formed between the Si and Ge layers. Specifically, by changing the value x that is the ratio of Ge of the Si 1-x Ge x  layer serving as the lattice buffer layer between the Si and Ge layers in a range of from 0 to 1, the lattice is controlled. However, the aforementioned method has complex processes and has a disadvantage in that lattice strain cannot be removed. 
     In an alternate method, as illustrated in  FIG. 1 , the solar cell layers  110   a  and  110   b  using amorphous semiconductors are stacked, and the TCO layer  120  is used as an intermediate lattice buffer layer. 
     However, since the TCO layer  120  is formed by an oxide and doped impurities, in a case where the TCO layer  120  is used for a crystalline solar cell having the stacked structure, the doped impurities may contaminate the crystalline solar cell layers in a crystalline growth process performed at a high temperature. Therefore, there is a problem in that the TCO layer  120  cannot be used for the crystalline solar cell. Therefore, although the method using the TCO layer  120  can be used for an amorphous solar cell, the method cannot be applied to the crystalline solar cell. 
     SUMMARY OF THE INVENTION 
     The present invention provides a crystalline solar cell having a stacked structure including a non-conductive lattice buffer layer that can remove lattice defects that may occur at the interface between solar cell layers having different band gaps and different lattice parameters from each other and electrically connect the solar cell layers to each other, thereby increasing light absorption efficiency. 
     The present invention also provides a method of manufacturing a crystalline solar cell having a stacked structure capable of forming a non-conductive lattice buffer layer to increase light absorption efficiency and prevent deterioration in a semiconductor and performing crystal growth of a solar cell layer on an upper portion of the non-conductive lattice buffer layer by using the non-conductive lattice buffer layer as a seed layer to block the inflow of impurities to the solar cell layer due to a high temperature in the crystal growth process by the seed layer and prevent deterioration in the solar cell layer. 
     According to an aspect of the present invention, there is provided a crystalline solar cell having a stacked structure including a non-conductive lattice buffer layer which is made of a non-conductive material and formed between crystalline solar cell layers, wherein the non-conductive lattice buffer layer electrically connects the solar cell layers to each other by a tunneling effect. 
     According to another aspect of the present invention, there is provided a method of manufacturing a crystalline solar cell having a stacked structure, including steps of: forming a crystalline first solar cell layer; forming a non-conductive lattice buffer layer using a non-conductive material on the first solar cell layer; and forming a crystalline second solar cell layer on the non-conductive lattice buffer layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a solar cell having a conventional stacked structure. 
         FIG. 2  illustrates an energy band of the solar cell illustrated in  FIG. 1 . 
         FIG. 3  illustrates a crystalline solar cell having a stacked structure according to an embodiment of the present invention. 
         FIG. 4  illustrates an energy band of the solar cell illustrated in  FIG. 3 . 
         FIG. 5  illustrates a method of manufacturing a crystalline solar cell having a stacked structure according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the attached drawings. 
       FIG. 3  illustrates a crystalline solar cell having a stacked structure according to an embodiment of the present invention. The crystalline solar cell  300  having a stacked structure illustrated in  FIG. 3  includes a first solar cell layer  310   a,  a second solar cell layer  310   b,  and a non-conductive lattice buffer layer  320 . 
     The crystalline first solar cell layer  310   a  is formed at a front surface in a direction  301  of incident light, and the crystalline second solar cell layer  310   b  is formed at a rear surface in the direction  301  of incident light. The non-conductive lattice buffer layer  320  is made of a non-conductive material and formed between the first and second solar cell layers  310   a  and  310   b.    
     The first solar cell layer  310   a  first absorbs the incident light and may have a wide energy band of a relatively wider band gap A, and the second solar cell layer  310   b  absorbs light that passes through the first solar cell layer  310   a  and may have a narrow energy band of a relatively narrower band gap B than the first solar cell layer  310   a.    
     For example, the first solar cell layer  310   a  may be made of silicon (Si) having a band gap A of about 1.1 eV, and the second solar cell layer  310   b  may be made of silicon-germanium (Si-Ge) having a band gap B of about from 0.7 eV to 1.1 eV. As a larger portion of the second solar cell layer  310   b  includes the Ge, the band gap decreases, and as a smaller portion of the second solar cell layer  310   b  includes the Ge, the band gap increases. A ratio of the Ge is determined according to a manufacturing purpose. 
     The non-conductive lattice buffer layer  320  electrically connects the first and second solar cell layers  310   a  and  310   b  to each other. When the non-conductive lattice buffer layer  320  is formed to have a small thickness of about from 1 nm to 20 nm, due to a tunneling effect, the first and second solar cell layers  310   a  and  310   b  can be electrically connected to each other. 
     The non-conductive lattice buffer layer  320  that is made of a non-conductive material may be an oxide or a nitride layer. Examples of the oxide layer include cerium dioxide (CeO 2 ), yttrium oxide (Y 2 O 3 ), aluminum oxide (Al 2 O 3 ), titanium oxide (TiO), strontium titanium oxide (SrTiO), zirconium silicon oxide (ZrSiO 4 ), tantalum oxide (Ta 2 O 3 ), barium titanate (BaTiO 3 ), zirconium dioxide (ZrO 2 ), hafnium dioxide (HfO 2 ), and silicon dioxide (SiO 2 ), and examples of the nitride layer include silicon nitride (SiN), gallium nitride (GaN), titanium nitride (TiN), and aluminum nitride (AlN). And the non-conductive lattice buffer layer  320  has a crystalline structure. 
       FIG. 4  illustrates an energy band of the solar cell  300  illustrated in  FIG. 3  in a case where the Si layer is used as the first solar cell layer  310   a,  the Si-Ge layer is used as the second solar cell layer  310   b,  and the SiN layer is used as the non-conductive lattice buffer layer  320 . 
     Referring to  FIG. 4 , a pair of electron-hole generated at the first solar cell layer  310   a  is separated from each other by a potential, and in this case, the electron e moves to the non-conductive lattice buffer layer  320 , and the hole h moves to a surface of the first solar cell layer  310   a.  In addition, a hole h of a pair of electron-hole generated at the second solar cell layer  310   b  moves to the non-conductive lattice buffer layer  320 , and the electron moves to a surface of the second solar cell layer  310   b.  When the non-conductive lattice buffer layer  320  has a thickness of about from 1 nm to 20 nm, among the electrons e and holes h generated at the solar cell layers  310   a  and  310   b,  the electron e and the hole h moved to the non-conductive lattice buffer layer  320  are recombined (denoted by  401 ) by a tunneling effect through the non-conductive lattice buffer layer  320 . Therefore, the same effect as the case of using the conventional TCO layer (denoted by  120  in  FIG. 1 ) can be achieved. 
       FIG. 5  illustrates a method of manufacturing a crystalline solar cell having a stacked structure according to an embodiment of the present invention. 
     The method  500  of manufacturing a crystalline solar cell having a stacked structure illustrated in  FIG. 5  includes a step S 510  of forming a first solar cell layer, a step S 520  of forming a non-conductive lattice buffer layer, and a step S 530  of forming a second solar cell layer. Hereinafter, for the convenience of description, numerals used to describe the elements in  FIG. 3  are used. 
     In the step S 510  of forming the first solar cell layer, the crystalline first solar cell layer  310   a  is formed. In the step S 520  of forming the non-conductive lattice buffer layer, the non-conductive lattice buffer layer  320  is formed by using a non-conductive material, for example, an oxide layer such as CeO 2 , Y 2 O 3 , Al 2 O 3 , TiO, SrTiO, ZrSiO 4 , Ta 2 O 3 , BaTiO 3 , ZrO 2 , HfO 2 , SiO 2 , and the like or a nitride layer such as SiN, GaN, TiN, AlN, and the like, on the first solar cell layer  310   a.  Here, in the step S 520  of forming the non-conductive lattice buffer layer, the non-conductive lattice buffer layer  320  is formed to have a thickness of about from 1 nm to 20 nm so as to be thinner than the first and second solar cell layers  310   a  and  310   b  so that the first and second solar cell layers  310   a  and  310   b  are electrically connected to each other by the tunneling effect. 
     In the step S 530  of forming the second solar cell layer, the crystalline second solar cell layer  310   b  is formed on the non-conductive lattice buffer layer  320 . 
     The first solar cell layer  310   a  formed in the step S 510  of forming the first solar cell layer and the second solar cell layer  310   b  formed in the step S 530  of forming the second solar cell layer are formed so that the first solar cell layer  310   a  disposed at the front surface in the direction  301  of incident light may have a wider energy band than the second solar cell layer  310   b  disposed at the rear surface in the direction  301  of incident light. 
     The first and second solar cell layers  310   a  and  310   b  have the crystalline structure, and the non-conductive lattice buffer layer  320  formed between the first and second solar cell layers  310   a  and  310   b  has to buffer a lattice difference between the first and second solar cell layers  310   a  and  310   b.  Therefore, an interatomic distance of a material used to form the non-conductive lattice buffer layer  320  in the step S 520  of forming the non-conductive lattice buffer layer may have an intermediate value between interatomic distances of the first and second solar cell layers  310   a  and  310   b.  In addition, in the step S 520  of forming the non-conductive lattice buffer layer, crystal growth of a non-conductive material may be performed to form the non-conductive lattice buffer layer  320 . 
     For example, when the Si and the Ge are used for the first and second solar cell layers  310   a  and  310   b,  respectively, in the step S 520  of forming the non-conductive lattice buffer layer, the SrTiO may be used for the non-conductive lattice buffer layer  320 . Since an interatomic distance of the SrTio has an intermediate value between the Si and Ge, epitaxial growth of the SrTiO may be performed on the crystal-grown Si used as the first solar cell layer  310   a,  and crystal growth of the Ge layer used as the second solar cell layer  310   b  may be performed on the non-conductive lattice buffer layer  320 . In this process, the SrTiO layer used as the non-conductive lattice buffer layer  320  serves as a seed layer for inducing crystal growth of the Ge in the process of crystallizing the Ge. In addition, since the oxide layer is thermally stable at a high temperature, the oxide layer prevents the solar cell layers and impurities from diffusing when the crystal growth of the Si is performed at a high temperature, so that deterioration in a semiconductor can be prevented. 
     The aforementioned stacked structure can be applied to a multi-layer structure using the same structure. Specifically, another non-conductive lattice buffer layer may be formed on the second solar cell layer  110   b,  and another third solar cell layer may be formed thereon, in order to form the multi-layer structure. As described above, the crystalline solar cell having the stacked structure uses the non-conductive lattice buffer layer having a wide band in order to solve defects due to a difference between lattice parameters of light absorption layers having different energy bands and different lattice parameters from each other, so that there are advantages in that lattice defects that may occur at the interface between the solar cell layers can be reduced, the number of recombinations between electrons-holes can be reduced, and light absorption efficiency can be increased. 
     In addition, the crystalline solar cell having the stacked structure does not use a transparent conductive oxide (TCO) layer having impurities, so that deterioration in a semiconductor that may occur due to a diffusion of the impurities at the TCO layer when the semiconductor crystal growth is performed can be avoided. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.