Abstract:
A photovoltaic cell may include a semiconductor base, a semiconductor mesa extending from the semiconductor base, a dielectric and a conductive material. The semiconductor mesa includes a top surface and a side wall, and a first portion of the dielectric is disposed on the top surface, a second portion of the dielectric is disposed on the side wall, and a third portion of the dielectric is disposed on the base. The conductive material is disposed on the top surface of the mesa and on the dielectric, and the conductive material covers the first portion of the dielectric, the second portion of the dielectric, and a portion of the third portion.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    Some embodiments generally relate to the conversion of solar radiation to electrical energy. More specifically, embodiments may relate to improved photovoltaic cells for use in conjunction with solar collectors. 
         [0003]    2. Brief Description 
         [0004]    A photovoltaic (or, “solar”) cell generates charge carriers (i.e., holes and electrons) in response to received photons. Many types of solar cells are known, which may differ from one another in terms of constituent materials, structure and/or fabrication methods. A solar cell may be selected for a particular application based on its efficiency, electrical characteristics, physical characteristics and/or cost. 
         [0005]    The semiconductor material (e.g., silicon) of a solar cell contributes significantly to total solar cell cost. Accordingly, many approaches have been proposed to increase the output of a solar cell for a given amount of semiconductor material. A concentrating solar radiation collector, for example, may receive solar radiation (i.e., sunlight) over a first surface area and direct the received sunlight to an active area of a solar cell. The active area of the solar cell is several times smaller than the first surface area, yet receives substantially all of the photons received by first surface area. The solar cell may thereby provide an electrical output equivalent to a solar cell having the size of the first surface area. 
         [0006]    Other approaches include increasing the size of the active photon-receiving surface area for a given amount of semiconductor material.  FIG. 1A  is a perspective view and  FIG. 1B  is a top view of one conventional solar cell. Solar cell  100  includes semiconductor base  110  and semiconductor mesa  120 . Semiconductor mesa  120  may include one or more optically-responsive p-n junctions. Each junction may cause generation of charge carriers in response to different photon wavelengths. 
         [0007]    Mesa  120  is covered with conductor  130  for collecting current generated by solar cell  100  in response to received photons. Conductor  130  is disposed in a pattern which allows suitable collection of the generated current. Conductor  130  is also disposed over the optically-active area of solar cell  100  and defines field  140  to receive photons into the optically-active area. Field  140  includes the areas within the pattern which are not covered by conductor  130 , and is symmetrical about center point  150 . Field  140  therefore represents optically-active areas of solar cell  100  which receive photons during operation. 
         [0008]    It is desirable to increase a size of a field such as field  140  as a percentage of the total solar cell area. A larger field may allow a solar cell to accept more photons per unit time than a smaller field, leading to increased power generation. A larger field may also increase a tolerance for errors in guiding solar radiation to a desired position on the solar cell. Consequently, increasing a size of an active area as a percentage of the total solar cell area may increase power generation and/or error tolerance for a given amount of semiconductor material, or may allow the maintenance of existing generation and tolerance levels using less semiconductor material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The construction and usage of embodiments will become readily apparent from consideration of the following specification as illustrated in the accompanying drawings, in which like reference numerals designate like parts. 
           [0010]      FIG. 1A  is a perspective view and  FIG. 1B  is a top view of a solar cell. 
           [0011]      FIG. 2  is a top view of a solar cell according to some embodiments. 
           [0012]      FIG. 3  is a three-dimensional cutaway view of a portion of the  FIG. 2  solar cell according to some embodiments. 
           [0013]      FIG. 4  is a cross-sectional view of a contact of the  FIG. 2  solar cell according to some embodiments. 
           [0014]      FIG. 5  is a top view of a solar cell according to some embodiments. 
           [0015]      FIG. 6  is a three-dimensional cutaway view of a portion of the  FIG. 5  solar cell according to some embodiments. 
           [0016]      FIG. 7  is a cross-sectional view of a first polarity contact of the  FIG. 5  solar cell according to some embodiments. 
           [0017]      FIG. 8  is a cross-sectional view of a second polarity contact of the  FIG. 5  solar cell according to some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The following description is provided to enable any person in the art to make and use the described embodiments and sets forth the best mode contemplated by for carrying out some embodiments. Various modifications, however, will remain readily apparent to those in the art. 
         [0019]      FIG. 2  is a top view of solar cell  200  according to some embodiments. Solar cell  200  may comprise a III-V solar cell, a II-VI solar cell, a silicon solar cell, or any other type of solar cell that is or becomes known. Solar cell  200  may comprise any number of active, dielectric and metallization layers, and may be fabricated using any suitable methods that are or become known. 
         [0020]    Solar cell  200  comprises semiconductor base  210  and semiconductor mesa  220 , an outer edge of which is represented by a dashed line in  FIG. 3 . Semiconductor mesa  220  and all other semiconductor mesas discussed herein may include one or more p-n junctions deposited using any suitable method. According to some embodiments, the junctions are formed using molecular beam epitaxy and/or metal organic chemical vapor deposition. The junctions may include a Ge junction, a GaAs junction, and a GaInP junction. Each junction exhibits a different band gap energy, which causes each junction to absorb photons of a particular range of energies and generate charge carriers in response thereto. 
         [0021]    Conductive material  230  is disposed in a pattern over an optically-active area of top surface  222  of mesa  220 . Conductive material  230  may comprise a metal or any suitable conductor. Material  230  is disposed in a pattern over surface  222  to allow suitable collection of the current generated by solar cell  200 . Conductive material  230  also defines field  240  to receive photons into the optically-active area of mesa  220 . Field  240  is circumscribed by a substantially rectangular (e.g., square) area and includes areas which are not covered by material  230 . Field  240  represents optically-active areas of solar cell  200  which receive photons during operation. 
         [0022]    Contact material  226  is disposed upon conductive material  230 . Contact material  226  may facilitate electrical connections between material  230  and external circuitry. Each of contact material  226  on conductive material  230  may exhibit a same polarity, therefore a lower side of solar cell  200  may comprise contact material (not shown) having an opposite polarity. By virtue of the foregoing arrangement, current may flow between the “top side” and “bottom side” contact material while solar cell  200  generates charge carriers. 
         [0023]    Contact material  226  may provide a wettable spot for solder subsequently placed thereon. Contact material  226  may comprise a barrier between such solder and conductive material  230  to prevent intrusion of the solder into material  230  before and after soldering. In some embodiments, a solder mask (not shown) may be deposited on conductive material  230  to further prevent solder from contacting material  230 . Contact material  226  may comprise a wirebonding pad in some embodiments. 
         [0024]    Conductive material  230  also overlaps the outer edge of mesa  220  and a portion of dielectric  260 . As shown, dielectric  260  extends from an inner perimeter represented by a dotted line to an outer edge of base  210 . Additional detail and explanation of the arrangement of conductive material  230 , dielectric  260  and an outer edge of mesa  220  according to some embodiments will be provided with respect to  FIGS. 3 and 4 . 
         [0025]    In comparison with solar cell  100 , the outer perimeter of the photon-receiving field has been moved closer to the mesa edge. Accordingly, the total area of the field as a percentage of semiconductor material has increased. A perimeter of corresponding field  140  according to conventional designs is illustrated as a dashed line for comparative purposes. 
         [0026]    In some embodiments, many mesas such as semiconductor mesa  220  are formed on a single semiconductor wafer. For example, p-n junctions may be fabricated on specific areas of the wafer, conductive material may be deposited as shown in  FIG. 3  on each area, and semiconductor material between each area may be removed to result in an array of raised mesas on the wafer. The wafer may then be singulated into individual cells as shown in  FIG. 2 . 
         [0027]      FIGS. 3 and 4  are three-dimensional cutaway views to show an arrangement of solar cell  300  according to some embodiments. The cutaway views also depict the respective portions of solar cell  200  indicated in  FIG. 2 . Accordingly, solar cell  300  may be identical to solar cell  200  of  FIG. 2 , but embodiments are not limited thereto. 
         [0028]    Dielectric  360 , which may comprise any suitable dielectric material, is disposed on semiconductor base  310 , on side wall  324  of semiconductor mesa  320 , and on top surface  322  of mesa  320 . Moving from the left to the right of  FIG. 3 , conductive material  330  is disposed directly on top surface  322  in the field-defining pattern, overlaps dielectric  360  on top surface  322 , overlaps dielectric  360  on side wall  324 , and overlaps dielectric  360  on a portion of base  310 . 
         [0029]    Dielectric  360  may prevent shorting of the p-n junctions of mesa  320  by insulating side wall  324  from conductive material  330 . Embodiments may therefore allow conductive material  330  to extend past the edge of mesa  320  and to thereby increase the active area of cell  300  expressed as a percentage of the total chip area. By moving conductive material  330  closer to the edge of solar cell  300  and across the edge of mesa  320 , otherwise wasted regions of solar cell  300  are utilized more efficiently than in conventional arrangements. 
         [0030]    In some embodiments, dielectric  360  and/or conductive material  330  are continuous around a perimeter of semiconductor mesa  320 . Embodiments are not limited thereto. In this regard, dielectric  360  may be disposed only at locations where conductive material  330  traverses over the mesa edge to insulate mesa side wall  324  from any such material  330 . 
         [0031]    The  FIG. 4  cross-section is taken across a contact material  326  of mesa top surface  322 .  FIG. 4  shows dielectric  360  overlapping side wall  324  and conductive material  330  overlapping dielectric  360  as shown in  FIG. 3 . 
         [0032]    The embodiments pictured in  FIGS. 2 through 8  each include a frame of conductive material which defines an outer limit of an active area and which is at least partially disposed on top of a semiconductor mesa. In some embodiments, no such frame is disposed on top of the semiconductor mesa. Instead, a dielectric is disposed from above the mesa over a mesa edge and to the chip edge (as shown in  FIG. 3 ) and the conductive grid lines are extended across the mesa edge to a contact ring placed on the dielectric above the semiconductor base. Such an arrangement may further increase the size of the active area as a percentage of semiconductor material. 
         [0033]      FIG. 5  is a top view of solar cell  500  according to some embodiments. Solar cell  500  provides conductive contacts of opposite polarities on a same side of solar cell  500 . Accordingly, a complete electrical circuit may be established via connections to one side of solar cell  500 . 
         [0034]    Conductive material  530  is disposed in a pattern over an optically-active area of mesa  520 . The pattern defines a field to receive photons into the optically-active area. Similar to solar cell  200  of  FIG. 2 , conductive material  530  overlaps an outer edge (represented by a dashed line) of mesa  520 . Dielectric  560  extends from an inner perimeter (represented by a dotted line) to an outer edge of base  510 . In some embodiments, dielectric  560  and/or conductive material  530  are continuous around a perimeter of semiconductor mesa  520 . 
         [0035]    Conductive material  570  is disposed on a top surface of base  510 . Conductive material  570  may be used establish a conductive contact having a polarity opposite from a polarity of a contact electrically coupled to material  530  on mesa  520 . In some embodiments, base  510  defines lip  580  (represented by a dashed and dotted line) adjacent to conductive material  570 . Features of lip  580  will be described below with respect to  FIG. 8 . 
         [0036]      FIGS. 6 through 8  are three-dimensional cutaway views to show an arrangement of solar cell  600  according to some embodiments. The cutaway views also depict the respective portions of solar cell  500  indicated in  FIG. 5 . Solar cell  600  may be identical to solar cell  500  of  FIG. 5 , but embodiments are not limited thereto. 
         [0037]    The  FIGS. 6 and 7  views are similar to those depicted in  FIGS. 3 and 4  with respect to solar cell  300 . With reference to  FIG. 6 , dielectric  660  is disposed on semiconductor base  610 , on side wall  624  of semiconductor mesa  620 , and on top surface  622  of mesa  620 . Conductive material  630  is disposed directly on top surface  622  in the field-defining pattern, overlaps dielectric  660  on top surface  622 , overlaps dielectric  660  on side wall  624 , and overlaps dielectric  660  on a portion of base  610 . As described above, dielectric  660  may prevent shorting of the p-n junctions of mesa  620  by insulating side wall  624  from conductive material  630 , and, in some embodiments, may allow conductive material  630  to extend past the edge of mesa  620  and to thereby increase the active area of cell  600  expressed as a percentage of the total chip area. 
         [0038]    The  FIG. 7  cross-section shows dielectric  660  overlapping side wall  624  and conductive material  630  overlapping dielectric  660 . A conductive contact having a first polarity may be coupled to contact material  626 . 
         [0039]      FIG. 8  is a cross-sectional view of a portion of solar cell  600  including conductive contact  670 . Conductive contact  670  may exhibit a polarity opposite from a polarity of a contact electrically coupled to material  630 .  FIG. 8  illustrates dielectric material  660  and conductive material  630  overlapping an edge of mesa  620  as described above. However, an opening exists in dielectric  660  at the top surface of base  610 . Conductive contact  670  is disposed in this opening, thereby establishing electrical contact with base  610 . 
         [0040]      FIG. 8  also illustrates lip  680  defined by base  610  in some embodiments. Dielectric  680  overlaps side wall  685  of lip  680  to insulate and protect exposed semiconductor material. In the absence of lip  680  and dielectric  660  disposed thereon, conductive contact  670  would be adjacent to an exposed side wall of semiconductor base  610 . Accordingly, lip  680  and dielectric  660  disposed thereon allow solar cell  600  to be singulated directly adjacent to conductive contact  670 . 
         [0041]    Lip  680  may protect mesa  620  against micro-cracks propagating to within the active region during singulation. The likelihood of micro-cracks may be insignificant depending on the materials system and the dimensions chosen for the particular design of cell  600 . Since fabrication of lip  680  may add an additional masking layer and a set of related fabrication steps, some embodiments do not include lip  680 . 
         [0042]    The several embodiments described herein are solely for the purpose of illustration. Embodiments may include any currently or hereafter-known versions of the elements described herein. Therefore, persons skilled in the art will recognize from this description that other embodiments may be practiced with various modifications and alterations.