Patent Publication Number: US-2011056548-A1

Title: Wafer-Based Solar Cell with Deeply Etched Structure

Description:
FIELD OF THE INVENTION  
     The present invention relates to a solar cell; more particularly, relates to providing a solar cell made of a semiconductor wafer having a deeply-etched structure for enhencing opto-electric conversion efficiency without using buried contact. 
     DESCRIPTION OF THE RELATED ART  
     In  FIG. 4 , a prior art of a solar cell is made of a semiconductor wafer  7  having an N-type semiconductor layer  71  and a P-type semiconductor layer  72  to form a P-N junction  73 . The prior art further has a front contact  74  and a rear contact  75 . When sun rays are shined on a front-side surface of the solar cell  76 , the inlet light panatrates through the front-side surface of the solar cell  76  and is absorbed near the surface with carriers of electrons and holes produced. Then the electrons and the holes are gathered at the front contact  74  and the rear contact  75  to form a terminal voltage difference. But, a part of the inlet light at the front-side surface of the solar cell  76  will be reflected so that energy generation efficiency of the solar cell becomes worse. Hence, generally, the front-side surface of the solar cell  76  is a textured surface; or, an anti-reflective film is coated on the front-side surface of the solar cell  76 ; or, the above two solutions are both used for anti-reflection. In  FIG. 5 , when sun rays  8   a  are shined on a textured surface  77 , most part of energy enters into the solar cell and a small part of the sun rays  8   a  are reflected and then re-inlet into the solar cell to be absorbed for generating energy, where there is a back surface field (BSF)  78  at a rear side for improving efficiency of the solar cell. 
     However, thickness of the P-type semiconductor layer  72  is almost as thick as that of the whole wafer while the N-type semiconductor layer  71  is far thinner. When sun shines, a few carriers do not have suitable diffusion distance and so some electrons and holes may recombine, which is not good for the electrons and the holes to be gathered at the front contact  74  and the rear contact  75 . As a result, opto-electric conversion efficiency of the solar cell is limited with power not so well generated. 
     A general wafer-based solar cell is made of single-crystalline poly-crystalline or amorphous silicon. Concerning opto-electric conversion efficiency, single-crystalline silicon is better than polycrystalline silicon and amorphous silicon. To solve the problem of the duffusion distance for carriers, the solar cell is usually made into a silicon thin film. Yet, the solar cell of the silicon thin film does not fully absorb and use the sun rays and so the opto-electric conversion efficiency is not good. Furthermore, a single-crystalline structure or a polycrystalline structure is not easily fabricated by a non-silicon semiconductor substrate with the silicon thin film; and so the opto-electric conversion efficiency is not good either. Hence, the prior art does not fulfill all users&#39; requests on actual use. 
     SUMMARY OF THE INVENTION  
     The main purpose of the present invention is to provide a solar cell made of a semiconductor wafer having a deeply-etched structure where recombination of carriers are reduced for enhencing opto-electric conversion efficiency without using buried contact. 
     To achieve the above purpose, the present invention is a wafer-based solar cell with a deeply etched structure, comprising a semiconductor wafer, a semiconductor layer and a P-N junction, where the semiconductor wafer has a deeply-etched structure while an etched region and a flat region are thus formed; the semiconductor layer is deposed on a surface of the semiconductor wafer; the P-N junction is formed between the semiconductor wafer and the semiconductor layer; and the etched region has a depth larger than one fourth thickness of the semiconductor wafer or the etched region has a bottom within 50 micrometers to an opposite side of the semiconductor wafer. Accordingly, a novel wafer-based solar cell with a deeply etched structure is obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       The present invention will be better understood from the following detailed descriptions of the preferred embodiments according to the present invention, taken in conjunction with the accompanying drawings, in which 
         FIG. 1  is the sectional view showing the first preferred embodiment according to the present invention; 
         FIG. 2A  is the view showing the groove array; 
         FIG. 2B  is the view showing the independent holes; 
         FIG. 3  is the sectional view showing the second preferred embodiment; 
         FIG. 4  is the view of the prior art; and 
         FIG. 5  is the view of the textured surface of the prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     The following descriptions of the preferred embodiments are provided to understand the features and the structures of the present invention. 
     Please refer to  FIG. 1 , which is a sectional view showing a first preferred embodiment according to the present invention. As shown in the figure, the present invention is a wafer-based solar cell with a deeply etched structure. The solar cell is made of a single-crystalline, polycrystalline or amorphous semiconductor material; and has a deeply-etched structure obtained through wet etching, dry etching, laser scribing or mechanical scribing to form an etched region and a flat region. The etched region has a depth larger than one fourth thickness of the semiconductor wafer; or has a bottom within 50 micrometers to an opposite side of the semiconductor wafer. 
     A wafer-based solar cell  1  is made of a semiconductor wafer, where the semiconductor wafer is a P-type semiconductor layer  11  made of a group IV element, like silicon or germanium, or its alloy; or a group III or group V element or its alloy. The wafer-based solar cell  1  has a groove array or a plurality of independent holes  12  at a front side to form the etched region and the flat region. A P-N junction  14  is formed by the P-type semiconductor layer  11  together with an N-type semiconductor layer  13  which is formed at the front side. A front contact  15  and a rear contact  16  are respectively coated at the front side and the rear side to direct electric power to an external circuit. Therein, the front contact  15  at the front side is coated on a surface in the etched region  17  or on a surface of the flat region  18 ; and a back surface field (BSF)  19  is obtained at the rear side of the wafer-based solar cell  1 . The side having the etched region and the flat region is further covered with an anti-reflective film; has a textured anti-reflective surface; or, has a textured surface with an anti-reflective film. With the above structure, a novel wafer-based solar cell with a deeply etched structure is obtained. 
     On using the present invention, if sun rays  2  are shined on the surface of the flat region  18 , the sun rays  2  are absorbed near surface with carriers of electrons and holes produced. If the sun rays  3  are shined on the surface in the etched holes  12 , the electrons and holes are produced at a thinner place of the semiconductor wafer where diffusion distance of the carriers is shortened to reduce the possibility of recombination of the carriers. Thus, more electric charge is accumulated at the front contact  15  and the rear contact  16  with more power produced. Furthermore, since the etched region  12  has a big opening down to a small bottom, a small opening down to a big bottom or an opening down to a bottom of the same size, illuminated area on a surface of the solar cell is increased with increased area as well for producing electrons and holes. Hence, power generated is increased with enhanced opto-electric conversion efficiency while no buried contact is used. 
     Please further refer to  FIG. 2A  and  FIG. 2B , which are views showing a groove array and independent holes. As shown in the figures, a deeply-etched structure of a wafer-based solar cell according to the present invention comprises a groove array  121  or independent holes  122 , where the groove array  121  has straight grooves or curve grooves and the independent holes have geographic openings. 
     Please refer to  FIG. 3 , which is a sectional view showing a second preferred embodiment. As shown in the figure, a wafer-based solar cell  5  is made of a semiconductor wafer, where the semiconductor wafer is a P-type semiconductor layer  51  made of a group IV element, like silicon or germanium, or its alloy; or a group III or group V element or its alloy. The wafer-based solar cell  5  has a groove array or a plurality of independent holes  52  at a rear side to form an etched region and a flat region. A P-N junction  54  is formed by the P-type semiconductor layer  51  together with a N-type semiconductor layer  53  which is formed at the front side. A front contact  55  and a rear contact  56  are respectively coated at the front side and the rear side to direct electric power to an external circuit. Therein, the rear contact  56  at the rear side is coated on a surface in the etched region  57 , on a surface of the flat region  58  or on a whole surface at the rear side; and a BSF  19  is obtained at the rear side of the semiconductor wafer  5 . The front side is further covered with an anti-reflective film; has a textured anti-reflective surface; or, has a textured surface with an anti-reflective film. 
     On using the present invention, if sun rays  6  are shined on a front-side surface  59  at the illuminated side, the rear contact  56  is very close to the N-type semiconductor layer  53  due to the deeply-etched structure  52  for reducing the possibility of recombination of electrons and holes. Thus, electrons and holes are separately accumulated at the front contact  55  and the rear contact  56  with more power produced and opto-electric conversion efficiency enhenced. Furthermore, since the etched region  52  has a big opening down to a small bottom, a small opening down to a big bottom or an opening down to a bottom of the same size, illuminated area on a surface of the solar cell  5  is increased while areas for producing electrons and holes are increased as well. Hence, power generated is increased without using buried contact. 
     Although the above semiconductor wafers are made of P-type semiconductors with deeply-etched structures used to improve opto-electric conversion efficiency, the deeply-etched structure can also be applied to semiconductor wafers made of N-type semiconductors having the like structures as shown in  FIG. 1  and  FIG. 3 . 
     To sum up, the present invention is a wafer-based solar cell with a deeply etched structure, where a solar cell is made of a semiconductor wafer having a deeply-etched structure; and recombination of carriers are reduced by the deeply-etched structure for enhencing opto-electric conversion efficiency without using buried contact. 
     The preferred embodiments herein disclosed are not intended to unnecessarily limit the scope of the invention. Therefore simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention.