Patent Publication Number: US-8993873-B2

Title: Solar cell with a backside via to contact the emitter layer

Description:
RELATED APPLICATION 
     This application claims priority to, and the benefit of, U.S. provisional patent application No. 61/118,296 filed on Nov. 26, 2008, the content of which is herein incorporated by reference in its entirety. 
    
    
     This invention was made with Government support under FA9453-09-C-0365 awarded by Air Force Research Laboratory. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     The present application concerns photovoltaic devices, such as solar cell devices. More specifically, the present application concerns photovoltaic devices with backside contact to a buried semiconductor layer, such as an emitter layer. 
     A photovoltaic device converts light energy into electricity. Although the term “solar cell device” may sometimes be used to refer to a device that captures energy from sunlight, the terms “solar cell device” and “photovoltaic device” are interchangeably used in the present application regardless of the light source. 
       FIG. 1  is a cross-sectional view of a conventional multi-junction solar cell device  100 . The multi-junction solar cell device  100  may include multiple junction regions, such as the first p-n junction region  110 , the second p-n junction region  120 , and the third p-n junction region  130 , which are connected in series. Each p-n junction region may contain multiple layers including an emitter layer and a base layer (not shown). 
     The multi-junction solar cell device  100  may receive light from the front or top (illuminated) side of the device. The multi-junction solar cell device  100  may include emitter “grid” contacts  140  on the front or top side of the device  100 , which may contact the emitter layer of the first junction region  110 . The multi-junction solar cell device  100  may also include base contacts  150  on the backside or bottom (non-illuminated) surface of the device  100 , which may contact the base layer of the third junction region  130 . 
     Electrical connection to the device  100  may be made through the emitter “grid” contacts  140  and the base contacts  150 . The base contacts  150  may cover the entire backside surface of the device  100 , while the emitter “grid” contacts  140  may consist of an array of fingers or buses that collect current from the top or front side of the device  100 . An antireflection coating  160  may be deposited over the exposed top surface of the emitter layer of the first junction region  110  to minimize losses due to reflections. 
     In the conventional multi-junction solar cell device  100 , the area shadowed by the top grid contact  140  may cause a significant loss in device efficiency, because no light is absorbed in the shadowed area. The metal fingers of the top grid contact  140  may cover at least 5% of the top surface of the device  100 , which is significant. Conventional methods for minimizing the shadowed area are extremely limited due to the performance trade-off between the shadowed area and associated resistive losses. In other words, reducing the area of grid metal covering the surface of the solar cell device  100  may decrease the shadow losses, but it increases the series resistance in the device  100 . Particularly in high current applications, such as optical concentrator systems, this series resistance becomes a key factor that limits overall device efficiency. 
     Accordingly, a new solar cell structure is needed for reducing shadow losses without unnecessarily increasing the series resistance of the solar cell device. 
     SUMMARY 
     An embodiment of the present application may provide a solar cell structure for reducing shadow losses without increasing the series resistance in the solar cell device. The solar cell device includes a contact to a buried solar cell layer, such as an emitter layer, from the backside of the solar cell device. The backside of the solar cell device refers to the non-illuminated side of the solar cell device in the description of the present application. With this structure, the embodiment of the present application may significantly reduce emitter contact shadow losses while simultaneously and advantageously reducing the device series resistance. 
     In accordance with one embodiment, a solar cell device includes at least one junction region having an emitter region and a base region. The solar cell device may also include a via hole extending axially from the bottom surface of the device into the junction region and terminating in the junction region without extending completely through the junction region. The solar cell device may further include an insulating layer formed on an inner surface of the via hole, and a conductive layer formed on the insulating layer. 
     In accordance with another embodiment of the present application, a solar cell device includes a first junction region having an emitter region and a base region, and a second junction region having an emitter region and a base region. The solar cell also includes a first via hole extending axially from the base region of the second junction region into emitter region of the first junction region and terminating in the emitter region of the first junction region, a first insulating layer formed on an inner surface of the first via hole, and a first conductive layer formed on the first insulating layer on the inner surface of the first via hole to electrically couple the emitter region of the first junction region and the base region of the second junction region through an external load. The solar cell further includes, according to certain embodiments, a second via hole extending axially from the base region of the second junction region into the emitter region of the second junction region and terminating in the emitter region of the second junction region, a second insulating layer formed on an inner surface of the second via hole, and a second conductive layer formed on the second insulating layer of the second via hole to electrically couple the emitter region of the second junction region and the base region of the second junction region through an external load. 
     In accordance with another embodiment of the present application, a method is provided for forming a solar cell device with an emitter layer, a base layer and a junction or interface between the emitter layer and the base layer. A via hole is formed so as to extend axially from the bottom surface of the device into the junction region and terminate in the junction region. An insulating layer is formed on an inner surface of the via hole, and a conductive layer on the insulating layer. 
     In accordance with still another embodiment of the present application, a method is provided for forming a solar cell device with a first junction region having an emitter region and a base region, and a second junction region having an emitter region and a base region. A first via hole extends axially from the base region of the second junction region into emitter region of the first junction region and terminates in the emitter region of the first junction region. A first insulating layer is formed on an inner surface of the first via hole, and a first conductive layer on the first insulating layer on the inner surface of the first via hole to electrically couple the emitter region of the first junction region and the base region of the second junction region. A second via hole is formed to extend axially from the base region of the second junction region into the emitter region of the second junction region and terminate in the emitter region of the second junction region. A second insulating layer is formed on an inner surface of the second via hole, and a second conductive layer on the second insulating layer of the second via hole to electrically couple the emitter region of the second junction region and the base region of the second junction region. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       These and other characteristics of the present application will be more fully understood by reference to the following detailed description in conjunction with the attached drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of a conventional multi-junction solar cell device; 
         FIG. 2  is a schematic cross-sectional view of a multi-junction solar cell structure with a backside emitter contact according to one embodiment of the present application; 
         FIG. 3  is a schematic cross-sectional view of an exemplary solar cell device with an array of backside emitter contacts according to the teachings of the present invention; 
         FIG. 4  is a cross-sectional view of the backside contact layer as seen from the  4 - 4  line depicted in  FIG. 3 ; 
         FIG. 5  is a schematic cross-sectional view of a multi-junction solar cell device employing multiple via holes with different depths according to the teachings of the present application; 
         FIG. 6  is a schematic flow chart diagram depicting the steps for fabricating a single or multi-junction solar cell device employing a backside via hole according to the teachings of the present application; 
         FIG. 7  is a schematic flow chart diagram depicting the steps for fabricating a single or multi-junction solar cell device employing multiple backside via holes according to the teachings of the present application; and 
         FIGS. 8A and 8B  are schematic cross-sectional diagrams illustrating the possible various layers of exemplary solar cell devices suitable for use with the present invention. 
     
    
    
     DESCRIPTION 
     Embodiments of the present application provide solar cell device structures and methods for forming the solar cell device structures. In the solar cell devices, an electrical contact to a solar cell emitter layer is provided from the backside (non-illuminated side) of the solar cell device. These structures and methods reduce emitter contact shadow losses while concomitantly reducing the device series resistance. 
       FIG. 2  is a schematic cross-sectional diagram of an exemplary multi-junction solar cell device  200  suitable for use with the backside via holes of the present application. The multi-junction solar cell device  200  may include a first or top junction region  210 , a second or middle junction region  220 , and a third or bottom junction region  230 . Those of ordinary skill in the art will appreciate that the multi-junction solar cell device  200  is exemplary and that any number of junctions can be employed in the illustrated solar cell device. For example, the illustrated solar cell device can include a single-junction or more, such as two or three junctions. Those of ordinary skill will also readily understand the various layers that comprise each junction of the solar cell device  200 . 
     Each junction region  210 ,  220 ,  230  of the illustrated solar cell device can be formed of one or more III-V compounds, such as Gallium Arsenide (GaAs), Gallium Indium Phosphide (GaInP), Gallium Indium Arsenide (GaInAs), Gallium Iridium Arsenide Phosphide (GaInAsP), or any other suitable III-V compound. Each junction region may contain an emitter region, a base region, and a junction between the emitter region and the base region. The emitter region may include an emitter layer formed of an n-type III-V compound and the base region may include a base layer formed of a p-type III-V compound. The emitter layer and the base layer may be formed of a p-type III-V compound and an n-type III-V compound, respectively, in other embodiments. Those of ordinary skill will readily recognize that each junction region can have a certain band gap energy characteristic, which uses a certain portion of the solar spectrum to generate electricity. The junction regions in the multi-junction solar cell device  200  may be formed of different doping and material so that the multiple junction regions may have different band-gaps to absorb different wavelengths of the solar spectrum. For example, the first junction region  210 , the second junction region  220  and the third junction region  230  may be formed of GaAs, GaInP and GaInAs, respectively. Therefore, the first junction region  210 , the second junction region  220  and the third junction region  230  may absorb different wavelengths of the solar spectrum. 
     The junction regions may be formed on a substrate and lifted off the substrate using epitaxial lift-off (ELO) methodologies. The ELO methodologies are described in detail in United State Patent Application Publication No. 2009/0038678 A1, the contents of which are herein incorporated by reference. The junction regions may also be formed using other well-known methodologies. For example, the junction regions may be epitaxially grown on substrates and remain affixed thereto throughout fabrication and deployment as a solar cell. 
     In the illustrative embodiment, the solar cell device  200  includes a backside via hole  290  formed so as to contact a selected junction region, for example, the first junction region  210 . Further, the via hole  290  can be formed so as to terminate at or within a selected region or layer of the junction region.  FIGS. 8A and 8B  illustrate various layers that can comprise the solar cell device. The illustrated layers are simplified for purposes of illustration and discussion.  FIGS. 8A and 8B  will be described in further detail below. With reference to  FIGS. 2 ,  8 A and  8 B, the via hole  290  can terminate at or within the emitter region of the first or top junction region  210  furthest from the base region of the third or bottom junction region  230 . Those of ordinary skill in the art will appreciate that the structure of the via hole  290  is exemplary and that the via hole may terminate at or within any of the other junction or regions or, layers or regions of the junction regions, such as at or within the emitter region of the second or third junction region. Furthermore, those of ordinary skill in the art will appreciate that the via hole  290  may terminate at or within any of the layers in the emitter region, for example, at or within the emitter contact layer. Those of ordinary skill in the art will also appreciate that a plurality of via holes at various locations may be formed in the solar cell device  200 , as described below with reference to  FIGS. 3-5 . 
     The illustrative embodiment of the present application provides a contact  240  to the emitter region or layer by forming the emitter contact via hole  290  from the backside of the solar cell device  200 . The emitter contact via hole  290  may be formed by well-know etching methodologies, such as wet-etching or dry-etching methodologies. According to the teachings of the present invention, the via hole  290  may be formed only partially through the solar cell device  200  and terminates or stops at or within the selected emitter region. Although the etched via hole  290  extends from the backside of the device  200  into the emitter region and terminates at or within the emitter region, the via hole  290  does not extend completely through to the front or top (illuminated) side of the device  200 . 
     An insulating layer  270  is formed on the sidewalls of the via hole  290  and on at least a portion of a bottom surface of the base region in the third junction region. The insulating layer  270  may be formed of SiO 2  or other suitable insulating material. One of ordinary skill in the art will appreciate that the insulating layer  270  may be formed using well-known techniques, such as plasma-enhanced chemical vapor deposition (PECVD). The insulating layer  270  insulates the conductive layer  280  from the sidewall of the via hole  290  and the bottom surface of the third junction region  230 . 
     The conductive layer  280  is formed on the insulating layer  270 . The contact  240  is formed between the conductive layer  280  and the emitter region or layer of the first junction region  210  so that the conductive layer  280  is electrically connected to the emitter layer of the first junction region  210 . Another conductive layer (backside contact  250 ) is formed on the bottom surface of the third junction region  230 . The backside contact  250  contacts the base region of the third junction region  230  so that the backside contact  250  is electrically connected to the base layer of the third junction region  230 . One of ordinary skill in the art will appreciate that the conductive layers may be formed using well-known techniques, such as the well-known plating techniques. The contacts  250  and  240  can be electrically coupled through an external load (not shown). 
     An antireflection coating  260  may be deposited over the top surface of the solar cell device  200 , for example, over the emitter region of the first junction region to minimize losses due to reflections. The antireflection coating  260  may be a zinc sulfide/magnesium fluoride coating. One of ordinary skill in the art will appreciate that the antireflection coating  260  can be implemented using other suitable antireflection coatings or combinations of coatings or materials. 
     With the above structure, since the contact on the illuminated side of the solar cell device is removed or eliminated, the illustrative embodiment provides the advantages of reducing or eliminating the emitter contact shadow losses. The illustrative embodiment also provides the advantages of reducing the device series resistance by providing an emitter contact from the non-illuminated side of the solar cell device, which does not limit the size of the emitter contact. 
     In operation, the multiple junctions are connected in series and receive light from the top or front side of the solar cell device  200 . An electric potential is generated between the emitter layer of the first junction region  210  and the base layer of the third junction region  230 . This potential allows electric current to flow through the conductive layer  280  when an electric circuit is formed between the conductive layer  280  and the base contact  250 . 
     In  FIG. 2 , the multi-junction solar cell device  200  is depicted in simplified form. However, those skilled in the art will appreciate that the solar cell device  200  may contain additional layers, for example, tunnel junctions, backside field (BSF) layers and window layers, such as illustrated in  FIGS. 8A and 8B . A substrate may also be provided beneath the third junction region. 
       FIG. 3  is a cross-sectional view of an exemplary solar cell device  300  employing an array of backside emitter contact via holes  390 . An array of backside emitter contact via holes  390  may be implemented for both space and terrestrial applications. For example, an array of backside emitter contact via holes  390  may be implemented in a solar cell device for high concentration applications. The structure of each via hole has substantially the same structure as depicted in  FIG. 2 . Therefore, the insulation layer formed on the sidewall of the via hole are not shown in  FIG. 3 . Those of ordinary skill will readily recognize that the hole can be arranged in any suitable pattern and number depending upon the particular application. 
     In the exemplary solar cell device  300 , the via holes  390  may be formed in the junction region  310  in a grid pattern. The junction region  310  may include a single junction. In a different embodiment, the junction region  310  may include multiple junctions. The depth of the via holes  390  may be the same or different. An embodiment with different depth via holes will be described below in more detail with reference to  FIG. 5 . 
     A backside contact layer  320  is provided on the bottom surface of the junction region  310 . The backside contact layer  320  includes a base contact  350  that contacts the base region of the junction region  310  so that the base contact  350  is electrically connected to the base region of the junction region  110 . The backside contact layer  320  also includes a emitter contact grid  380  that contacts the conductive layers formed on the sidewall of the via hole holes  390 . The emitter contact grid  380  is insulated from the base contact  350  by the insulating material  370 . The emitter contact grid  380  is interconnected by a contact pad, which will be described below in more detail with reference to  FIG. 4 . 
     An antireflection coating  360  may be deposited over the top surface of the solar cell device  300  to minimize losses due to reflections. The antireflection coating  360  may be a zinc sulfide/magnesium fluoride coating or other suitable antireflection coating. 
     In operation, the junction region  310  receives light from the top or front side of the solar cell device  300 . Electric potential is generated between the emitter layer and the base layer of the junction region. This potential allows electric current to flow through the emitter contact grid  380  when an electric circuit (external load) is formed between the emitter contact grid  380  and the base contact  350 . 
       FIG. 4  depicts a cross-sectional view of the backside contact layer  320  as seen from the  4 - 4  line depicted in  FIG. 3 . In the exemplary backside contact layer  320 , the emitter contact grid  380  is interconnected by the large, horizontal contact pad  410  at the top edge of the device (“emitter contact pad”). The base contact  350  also includes a large, horizontal contact pad  420  at the bottom edge of the device (“base contact pad”). The emitter contact grid  380  is insulated from the base contact  350  by the insulating material  370  and provided between the base contacts  350  in an alternating manner. 
     In the illustrative embodiment of the present application, the emitter contact grid  380  can be made as wide and thick as necessary because there are no shadowing constraints on the backside of the solar cell. The emitter contact grid  380  is interconnected using a low-resistance backside contact pad  410 . With the emitter contact grid  380  having 10-μm diameters and regular spacing on a 100-μm grid, it is calculated that the solar cell suffers a shadow loss of approximately 1%. 
     In particular, external connection to the emitter contact pad  410  and the base contact pad  420  is possible from only the backside of the device. Therefore, the backside contact layer of the present application provides efficient device packaging configurations in both space and terrestrial applications. 
       FIG. 5  is an exemplary solar cell device  500  including multiple via holes with different depths in another illustrative embodiment of the present application. The solar cell device  500  may include the first junction region  510 , the second junction region  520 , and the third junction region  530 . Each junction region may include an emitter region, a base region, and a junction between the emitter region and the base region. The solar cell device  500  may include any suitable number of via holes, such as via holes  591 ,  592  and  593 . According to the illustrated exemplary embodiments, the first via hole  591  terminates at or within the emitter region of the first or top junction region  510 , the second via hole  592  terminates at or within the emitter region of the second or middle junction region  520 , and the third via hole  593  terminates at or within the emitter region of the third or bottom junction region  530 . The illustrated via holes  591 ,  592  and  593  provide electrical connections to different emitter regions within the device  500 . 
     Those of ordinary skill in the art will appreciate that the emitter contact via holes  591 ,  592  and  593  are illustrative and a plurality of emitter contact via holes may terminate in different regions or layers of the various junction region. 
     An insulating material  570  is formed on the sidewalls of the via holes  591 ,  592  and  593  and on at least a portion of a bottom surface of the base region of the third junction region  530 . Conductive layers  581 ,  582  and  583  are formed on the insulating layer  570  and provide electrical contacts to the emitter layers of the first, second and third junction regions, respectively. An antireflection coating  560  may be deposited over the emitter region of the first junction region  510  to minimize losses due to reflections. 
     In the illustrative embodiment, the device  500  includes multiple via holes that contact different emitter layers in different junction regions. In this structure, multiple junctions may be combined with series-parallel connections rather than the conventional series connection. The above structure may improve spectral robustness and efficiency of the solar cell device  500 . 
       FIG. 6  is a schematic flow chart diagram of an exemplary method for fabricating the solar cells in accordance with the teachings of the present application. In accordance with an embodiment of the present application, multiple p-n junctions regions are provided in series to form a solar cell device (step  610 ). Each p-n junction region may include an emitter region and a base region to form a p-n junction between the regions. A via hole is formed to extend from the bottom of the device into a selected emitter region of the multiple p-n junctions regions (step  620 ). An insulating layer is formed on the inner surface of the via hole (step  630 ). A conductive layer is formed on the insulating layer to electrically couple the emitter region and the base region that contacts the bottom of the device (step  640 ). 
     Those skilled in the art will appreciate that any number of the p-n junctions regions can be employed in the solar cell device of the present application. Furthermore, those skilled in the art will appreciate that any suitable number of via holes may be formed in the solar cell device of the present application, as described below with reference to  FIG. 7 . 
       FIG. 7  is a schematic flow chart diagram of another exemplary method for fabricating the solar cells in accordance with the teachings of the present application. In accordance with this embodiment of the present application, a first junction region having an emitter region and a base region, and a second junction region having an emitter region and a base region are provided in series to form a solar cell device (step  710 ). A first via hole is formed to extend from the base region of the second junction region into the emitter region of the first junction region and to terminate in the emitter region of the first junction region (step  720 ). A second via hole is provided to extend from the base region of the second junction region into the emitter region of the second junction region and to terminate in the emitter region of the second junction region (step  730 ). An insulating layer is formed on an inner surface of the first via hole and the second via hole (step  740 ). A first conductive layer is formed on the insulating layer on the inner surface of the first via hole to electrically couple the emitter region of the first junction region and the base region of the second junction region (step  750 ). A second conductive layer is formed on the insulating layer of the second via hole to electrically couple the emitter region of the second junction region and the base region of the second junction region (step  760 ). 
       FIG. 8A  shows an exemplary single junction solar cell suitable for use with the teachings of the present application. The single junction solar cell includes, for example, a junction between the emitter region  810  and the base region  820 . The emitter region  810  may be deposited over the base region  820 . 
     The emitter region  810  may include an emitter contact layer  811 , a window layer  812 , and/or an emitter layer  813 . The doped emitter contact layer  811  may be deposited over the window layer  812  to enhance the electrical contact with a metal conductive material, such as the conductive layer  280  depicted in  FIG. 2 . The window layer  812  may be deposited over the emitter layer  813  to allow the light to reach the emitter layer  820  and help effectively separate the electrons and holes before they can recombine. 
     The base region  820  may include a base layer  821 , a backside field (BSF) layer  822 , and/or a base contact layer  823 . The base layer  821  may be deposited over the BSF layer  822 . The BSF layer  822  may be deposited over the base contact layer  823  to reduce recombination loss at the backside of the solar cell. The doped base contact layer  823  may enhance contact with a metal material, such as the backside contact  250  in  FIG. 2 . 
     In the illustrative embodiment, one or more via holes terminate at or within any of the regions in the illustrated junction region. Preferably, one or more via holes terminate at or within any of the layers in the emitter region  810 . Most preferably, one or more via holes terminate at or within the emitter contact layer of the emitter region  810 . 
       FIG. 8B  shows an exemplary multi-junction solar cell device suitable for use with the teachings of the present invention. The multi-junction solar cell device may include a first junction region  830  and a second junction region  850 . The first junction region  830  may include an emitter contact layer  831 , a window layer  832 , an emitter layer  833 , a base layer  841 , and/or a BSF layer  842 , which can be separated into emitter regions and base regions as shown in  FIG. 8A . The second junction region  860  may include an emitter contact layer  861 , a window layer  862 , an emitter layer  863 , a base layer  871 , a BSF layer  872 , and/or a base contact layer  873 , which can be separated into emitter regions and base regions as shown in  FIG. 8A . A tunneling junction layer  850  is deposited between the first junction region  830  and the second junction region  850 . The tunneling junction layer  850  may form a tunneling diode to allow the flow of electrons between the first junction region  830  and the second junction region  850 . 
     In the illustrative embodiment, one or more via holes may terminate at or within any of the junction regions. Preferably, one or more via holes terminate at or within any of the layers in the emitter regions. Most preferably, one or more via holes may terminate at or within the emitter contact layers in the emitter regions. Those of ordinary skill will readily recognize that the via holes can terminate in any other region or layer of the junction regions. 
     One of ordinary skill in the art will appreciate that the structure of the junction region is not limited to the configuration illustrated in  FIGS. 8A and 8B , and the junction region may include one or more different layers. For example, the junction region may not include the window layer or the BSF layer. 
     One of the advantages of the present application is that the reduction of shadow losses is possible. Furthermore, the device series resistance may also be reduced while the shadow losses are reduced. 
     Numerous modifications and alternative embodiments of the present application will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode for carrying out the present application. Details of the structure may vary substantially without departing from the spirit of the present application, and exclusive use of all modifications that come within the scope of the appended claims is reserved. It is intended that the present application be limited only to the extent required by the appended claims and the applicable rules of law. 
     It is also to be understood that the following claims are to cover all generic and specific features of the invention described herein, and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.