Patent Publication Number: US-8536448-B2

Title: Zener diode within a diode structure providing shunt protection

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of Murali et al., U.S. patent application Ser. No. 12/980,424, “A Method to Form a Device by Constructing a Support Element on a Thin Semiconductor Lamina,” filed Dec. 29, 2010, now U.S. Pat. No. 8,173,452, issued May 8, 2012, owned by the assignee of the present invention and hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a method to prevent electronic shunting in a device. 
     A conventional prior art photovoltaic cell includes a p-n diode. An example is shown in  FIG. 1 . A depletion zone forms at the p-n junction, creating an electric field. Incident photons (incident light is indicated by arrows) will knock electrons from the valence band to the conduction band, creating free electron-hole pairs. Within the electric field at the p-n junction, electrons tend to migrate toward the n region of the diode, while holes migrate toward the p region, resulting in current, called photocurrent. Typically the dopant concentration of one region will be higher than that of the other, so the junction is either a p+/n− junction (as shown in  FIG. 1 ) or a n+/p− junction. The more lightly doped region is known as the base of the photovoltaic cell, while the more heavily doped region, of opposite conductivity type, is known as the emitter. Most carriers are generated within the base, and it is typically the thickest portion of the cell. The base and emitter together form the active region of the cell. The cell also frequently includes a heavily doped contact region in electrical contact with the base, and of the same conductivity type, to improve current flow. In the example shown in  FIG. 1 , the heavily doped contact region is n-type. 
     Many different schemes are used to make ohmic contact to the cell. In some schemes, due to defects in the cell, electrical shunts may inadvertently be formed. A method to avoid formation of shunts is desirable. 
     SUMMARY OF THE INVENTION 
     The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. In general, the invention is directed to a structure that minimizes or avoids shunt formation in a photovoltaic cell comprising a thin lamina. 
     A first aspect of the invention provides for a structure to prevent electrical shunting, the structure comprising: a monocrystalline semiconductor lamina having a first surface and a second surface opposite the first, and having a thickness between the first surface and the second surface less than about 50 microns; a first amorphous silicon layer on and in contact with the first surface, at least a portion of the first amorphous silicon layer heavily doped to a first conductivity type; a second amorphous silicon layer on and in contact with the second surface, at least a portion of the second amorphous silicon layer heavily doped to a second conductivity type, opposite the first conductivity type; a first contact in electrical contact with the first amorphous silicon layer; and a second contact in electrical contact with the second amorphous silicon layer, wherein the first amorphous silicon layer contacts the second amorphous silicon layer through an aperture in the lamina, forming a Zener diode, and in normal operation of the device, the second amorphous silicon layer is not in ohmic contact with the first contact. 
     Another aspect of the invention provides for a photovoltaic cell comprising: a monocrystalline silicon lamina having a thickness between about 2 and about 20 microns, the lamina having a first surface and a second surface opposite the first, the lamina comprising a base region of the photovoltaic cell; a first amorphous silicon layer on and in contact with the first surface, wherein at least half a thickness of the first amorphous silicon layer is heavily doped to a first conductivity type; a second amorphous silicon layer on and in contact with the first surface, wherein at least half a thickness of the second amorphous silicon layer is heavily doped to a second conductivity type opposite the first; a first contact in electrical contact with the first amorphous silicon layer; and a second contact in electrical contact with the second amorphous silicon layer, wherein the second amorphous silicon layer lines the sidewalls of an aperture in the lamina, and the first amorphous silicon layer does not line the sidewalls of the aperture in the lamina, and does not have a corresponding aperture aligned with the aperture in the lamina, and the first amorphous silicon layer and the second amorphous silicon layer are in contact through the aperture, forming a Zener diode, and wherein, during normal operation of the photovoltaic cell, the second amorphous silicon layer is not in ohmic contact with the first contact. 
     Each of the aspects and embodiments of the invention described herein can be used alone or in combination with one another. 
     The preferred aspects and embodiments will now be described with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a prior art photovoltaic cell. 
         FIGS. 2   a - 2   d  are cross-sectional views showing stages in formation of the photovoltaic device of Sivaram et al., U.S. Pat. No. 7,842,585. 
         FIGS. 3   a - 3   d  are cross-sectional views showing stages in formation of a photovoltaic device according to embodiments of the present invention. 
         FIGS. 4   a  and  4   b  are cross-sectional views showing stages in formation of a photovoltaic device according to embodiments of the present invention, illustrating formation of a Zener diode in an aperture. 
         FIG. 5  is a cross-sectional view provided for illustration showing an imagined alternative which is not an embodiment of the present invention. 
         FIG. 6  is a plan view of six photovoltaic cells connected in series. 
         FIGS. 7   a - 7   e  are cross-sectional views showing stage in formation of a photovoltaic device having a constructed metal support element according to embodiments of the present invention. 
         FIG. 8   a  is an IV curve of a prior art photovoltaic cell.  FIG. 8   b  is a plan view of a series string of such cells.  FIG. 8   c  is a series of IV curves for the string.  FIG. 8   d  is a circuit diagram for the string.  FIG. 8   e  shows IV curves when one cell is shaded, and  FIG. 8   f  shows a circuit diagram for this case. 
         FIG. 9   a  is a circuit diagram of a series string of cells according to an embodiment of the present invention, and  FIG. 9   b  shows the IV curves for the cells in this string when one cell is shaded. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Sivaram et al., U.S. Pat. No. 7,842,585, “Method to Form a Photovoltaic Cell Comprising a Thin Lamina,” describes fabrication of a photovoltaic cell comprising a thin semiconductor lamina formed of non-deposited semiconductor material. Referring to  FIG. 2   a , in embodiments of Sivaram et al., a semiconductor donor wafer  20  is implanted through first surface  10  with one or more species of gas ions, for example hydrogen and/or helium ions. The implanted ions define a cleave plane  30  within the semiconductor donor wafer. As shown in  FIG. 2   b , donor wafer  20  is affixed at first surface  10  to receiver  60 . Referring to  FIG. 2   c , an anneal causes lamina  40  to cleave from donor wafer  20  at cleave plane  30 , creating second surface  62 . In embodiments of Sivaram et al., additional processing before and after the cleaving step forms a photovoltaic cell comprising semiconductor lamina  40 , which is between about 0.2 and about 100 microns thick, for example between about 0.2 and about 50 microns, for example between about 1 and about 20 microns thick, in some embodiments between about 1 and about 10 microns thick or between about 3 and about 20 or between about 4 and about 12 microns thick, though any thickness within the named range is possible.  FIG. 2   d  shows the structure inverted, with receiver  60  at the bottom, as during operation in some embodiments. Receiver  60  may be a discrete receiver element having a maximum width no more than 50 percent greater than that of donor wafer  10 , and preferably about the same width, as described in Herner, U.S. patent application Ser. No. 12/057265, “Method to Form a Photovoltaic Cell Comprising a Thin Lamina Bonded to a Discrete Receiver Element,” filed on Mar. 27, 2008, owned by the assignee of the present application and hereby incorporated by reference. Alternatively, a plurality of donor wafers may be affixed to a single, larger receiver, and a lamina cleaved from each donor wafer. 
     Using the methods of Sivaram et al., photovoltaic cells, rather than being formed from sliced wafers, are formed of thin semiconductor laminae without wasting silicon through kerf loss or by fabrication of an unnecessarily thick cell, thus reducing cost. The same donor wafer can be reused to form multiple laminae, further reducing cost, and may be resold after exfoliation of multiple laminae for some other use. 
     Cleaving is most easily achieved by heating, for example to temperatures of 400 degrees C. or more. It has been found that the step of implanting to define the cleave plane may cause damage to the crystalline lattice of the monocrystalline donor wafer. This damage, if unrepaired, may impair cell efficiency. A relatively high-temperature anneal of the lamina following cleaving, for example at 900 degrees C., 950 degrees C., or more, will repair most implant damage in the body of the lamina. 
     In embodiments of Sivaram et al., a first surface of the donor body is permanently affixed to a support element before cleaving of the lamina. As described in Murali et al., earlier incorporated, it is also possible to cleave the lamina from the donor body when the first surface is not permanently affixed to a support element. 
     Turning to  FIG. 3   a , in the present invention, a donor wafer  20  is implanted through first surface  10  to form cleave plane  30 . First surface  10  may be placed adjacent to vacuum chuck  54 . As shown in  FIG. 3   b , a heating step is performed and lamina  40  separates from the donor wafer at the cleave plane, creating second surface  62 . Referring to  FIGS. 3   a  and  3   b , note that during the cleaving step, first surface  10  of the donor wafer is not permanently affixed to a support element. 
     An anneal, for example at 900 degrees C. or higher, is performed to repair damage caused to the crystal lattice of lamina  40  during the implant step that formed cleave plane  30  of  FIG. 3   a . Following this anneal, lamina  40  may be transferred from the vacuum chuck and affixed to a temporary support element  50 , as shown in  FIG. 3   c . As will be described in additional detail, one or more layers, including, for example, amorphous silicon layer  72 , transparent conductive oxide (TCO) layer  110 , and reflective metal layer  12 , are deposited on lamina  40 . A permanent support element  60  is constructed on lamina  40 ; in the embodiment shown, it is constructed directly on metal layer  12 , which is formed over second surface  62  of lamina  40 . A support element is considered to be “constructed” if it is formed in situ, rather than being provided as a pre-formed element. Examples include: a metal support element formed by plating, for example by electroplating or electroless plating; a ceramic support element formed by applying a ceramic mixture followed by curing in place; or a polymer support element formed by applying a the polymer in liquid or semi-liquid form and curing in place. Additional layers may be formed on or over amorphous silicon layer  72  before construction of support element  60 . Support element  60  will be made sufficiently thick to provide mechanical support to lamina  40 , which is too thin and fragile to survive much handling without such support. 
     Turning to  FIG. 3   d , following construction of support element  60 , lamina  40  is detached from temporary support element  50 .  FIG. 3   d  shows the structure inverted, with constructed support element  60  on the bottom, as in most embodiments. Additional layers, such as amorphous silicon layer  76  and ITO layer  112  are deposited, forming a photovoltaic cell. 
     Constructing support element  60  following the steps of cleaving and damage anneal, as in the present invention, provides several significant advantages. Note that the cell of  FIG. 3   d  includes heavily doped amorphous silicon layers,  72  and  76 , on opposing surfaces of lamina  40 . Lamina  40  is more lightly doped than the amorphous silicon layers  72  and  76 , and will serve as the base region of the photovoltaic cell. The base region of the cell absorbs incident light, and is where most charge carriers are generated. Lamina  40  is very thin, much thinner than the absorber region of a conventional photovoltaic cell. Thus, passivation of surfaces  10  and  62  of the lamina is particularly helpful to avoid loss of charge carriers by recombination at these surfaces. Amorphous silicon layers  72  and  76  are effective means of passivation, and their use is particularly advantageous with a very thin absorber. 
     Recall that in the embodiment of Sivaram et al., described above and referring to  FIG. 2   b , first surface  10  of donor wafer  20  is affixed to permanent support element  60  before cleaving and anneal. Thus, any layers deposited on first surface  10  before affixing to support element  60  will be exposed to the temperatures required by cleaving and anneal. This limits the materials that can be deposited on first surface  10 . As explained above, amorphous silicon is advantageous for passivation of first surface  10 . Amorphous silicon, however, cannot be exposed to the high temperature required to perform cleaving and damage anneal; such temperature will cause the amorphous silicon to crystallize. By cleaving and performing a damage anneal, both high temperature steps, before the lamina is permanently affixed to any support element, both surfaces of lamina  40  remain accessible for subsequent deposition of layers that cannot tolerate high temperature. This construction method also allows the use of other materials which cannot tolerate high temperature, such as aluminum or silver, which will tend to diffuse into silicon at high temperature, altering the electrical behavior of the cell. 
     The exfoliation process may be imperfect, and at times there may be holes in the lamina following cleaving. The use of amorphous silicon layers on opposing faces of the lamina provides protection against electrical shunting.  FIG. 4   a  shows lamina  40  affixed to temporary support element  50 , with second surface  62  exposed. An aperture  71  has been formed through lamina  40 . This aperture may have been formed unintentionally during cleaving, or may be formed intentionally. Heavily doped amorphous silicon layer  72  is deposited on second surface  62 , and lines the sidewalls of aperture  71 . Next TCO layer  110  is formed on heavily doped amorphous silicon layer  72 , followed by reflective layer or stack  12 , and finally by constructed permanent support element  60 , which may be, for example, a metal support element formed by plating. 
     Turning to  FIG. 4   b , the temporary support element may now be removed. This figure shows the structure inverted, with permanent support element  60  on the bottom, as during operation in this embodiment. Heavily doped amorphous silicon layer  76  is deposited on second surface  62 , followed by TCO layer  112  and wiring  57 , completing the photovoltaic cell. 
     Amorphous silicon layers  72  and  76  are heavily doped to opposite conductivity types, while lamina  40  is undoped, or lightly or moderately doped. Thus lamina  40  will serve as the base region of the cell. One of amorphous silicon layer  72  or  76  will comprise the emitter of the photovoltaic cell, while the other provides electrical contact to the base region of the cell. For example, lamina  40  may be lightly n-doped, while amorphous silicon layer  72  is heavily n-doped and amorphous silicon layer  76  is heavily p-doped; in this example, amorphous silicon layer  76  comprises the emitter of the cell, and is at the light-facing surface of the cell. Amorphous silicon layer  76  is in electrical contact with wiring  57 , which serve as first contacts. Amorphous silicon layer  72  is in electrical contact with metal layer  12 , which serves as a second contact. Note that in each case a TCO layer, which is conductive, intervenes. Amorphous silicon layer  72  and amorphous silicon layer  76  are in contact through aperture  71 . The contact between the p+and n+amorphous silicon layers forms a Zener diode  73 . During normal operation of the pictured cell, in this example photocurrent will flow from second contact  12  through TCO layer  110  and amorphous silicon layer  72 , entering lamina  40  at second surface  62  and exiting at first surface  10 , through amorphous silicon layer  76  and TCO layer  112 , to first contacts  57 . If the polarities were to be reversed, current flow would also be reversed. 
     Note that during normal operation, voltage across Zener diode  73  is below its turn-on voltage, so little or no current flows across it. Note further that the Zener diode is electrically between the first contact and the second contact. 
     By using heavily doped amorphous silicon layers at both first surface  10  and second surface  62 , both of which are deposited after formation of aperture  71 , the structure provides shunt protection. For example, referring to  FIG. 5 , if the contact to the base region of the cell, which resides in lamina  40 , were to be provided by heavily doped region  14 , for example formed by dopant diffusion, rather than by heavily doped amorphous layer  72  (of  FIG. 4   b ), it will be seen that amorphous layer  76  directly contacts TCO layer  110 , resulting in a shunt  77 . Current can flow directly between metal layer  12  and wiring  57  by way of TCO layer  110 , heavily doped amorphous layer  76 , and TCO layer  112 . This current path bypasses the diode formed by heavily doped p-type amorphous layer  76 , the emitter of the photovoltaic cell and lightly doped n-type lamina  40 , the cell&#39;s base region. 
     The structure shown in  FIG. 4   b  may also serve to provide protection in a set of cells connected in series when one cell of the string is suddenly shaded, as may happen when a leaf or some other object or substance blocks incident light. Referring to  FIG. 6 , cells A through F are connected in a series string. The same current must flow through all cells in the string. If one cell in the string, for example cell C, is shaded and suddenly produces much less photocurrent, it will match the current of the others by going into heavy reverse bias, which may damage the cell. To avoid this condition, it is known to include bypass diodes in a photovoltaic module. Such a bypass diode points in the opposite direction and has lower breakdown voltage. A Zener diode has low reverse breakdown voltage compared to a typical n+/p− or p+/n− diode. Referring to  FIG. 4   b , if the cell is placed under heavy reverse diode, the Zener diode  73  formed at the junction of amorphous layers  72  and  76  in aperture  71  will break down at a lower reverse voltage than the main cell, thus protecting the main cell. Thus, it may be desirable to deliberately form apertures  71  in a cell including a thin lamina in order to form the protective Zener diode shown in  FIG. 4   b . Such apertures may be formed using a laser. Alternatively, such apertures may be unintentionally formed during the cleaving process. 
     What has been described is a structure to prevent electrical shunting, the structure comprising: a monocrystalline semiconductor lamina having a first surface and a second surface opposite the first, and having a thickness between the first surface and the second surface less than about 50 microns; a first amorphous silicon layer on and in contact with the first surface, at least a portion of the first amorphous silicon layer heavily doped to a first conductivity type; a second amorphous silicon layer on and in contact with the second surface, at least a portion of the second amorphous silicon layer heavily doped to a second conductivity type, opposite the first conductivity type; a first contact in electrical contact with the first amorphous silicon layer; and a second contact in electrical contact with the second amorphous silicon layer, wherein the first amorphous silicon layer contacts the second amorphous silicon layer through an aperture in the lamina, forming a Zener diode, and in normal operation of the device, the second amorphous silicon layer is not in ohmic contact with the first contact. The lamina is disposed between the first contact and the second contact. There is no aperture in either the first or the second amorphous layer corresponding to the aperture in the lamina. 
     For clarity, detailed examples of a photovoltaic assembly including a lamina having thickness between 0.2 and 100 microns, in which a Zener diode is formed at the junction of amorphous silicon layers having opposite doping which meet in an aperture through a monocrystalline semiconductor lamina, will be provided. For completeness, many materials, conditions, and steps will be described. It will be understood, however, that many of these details can be modified, augmented, or omitted while the results fall within the scope of the invention. 
     EXAMPLE 
     The process begins with a donor body of an appropriate semiconductor material. An appropriate donor body may be a monocrystalline silicon wafer of any practical thickness, for example from about 200 to about 1000 microns thick or more. Typically the wafer has a &lt;100&gt; orientation, though wafers of other orientations may be used. Different crystalline orientations may be selected, for example, to avoid channeling during the implant step to come. In alternative embodiments, the donor wafer may be thicker; maximum thickness is limited only by practicalities of wafer handling, and may be, for example, 10,000 microns or more. Alternatively, polycrystalline or multicrystalline silicon may be used, as may microcrystalline silicon, or wafers or ingots of other semiconductor materials, including germanium, silicon germanium, or III-V or II-VI semiconductor compounds such as GaAs, InP, etc. In this context the term multicrystalline typically refers to semiconductor material having grains that are on the order of a millimeter or larger in size, while polycrystalline semiconductor material has smaller grains, on the order of a thousand angstroms. The grains of microcrystalline semiconductor material are very small, for example 100 angstroms or so. Microcrystalline silicon, for example, may be fully crystalline or may include these microcrystals in an amorphous matrix. Multicrystalline or polycrystalline semiconductors are understood to be completely or substantially crystalline. It will be appreciated by those skilled in the art that the term “monocrystalline silicon” as it is customarily used will not exclude silicon with occasional flaws or impurities such as conductivity-enhancing dopants. 
     The process of forming monocrystalline silicon generally results in circular wafers, but the donor body can have other shapes as well. For photovoltaic applications, cylindrical monocrystalline ingots are often machined to an octagonal, or pseudosquare, cross section prior to cutting wafers. Wafers may also be other shapes, such as square. Square wafers have the advantage that, unlike circular or hexagonal wafers, they can be aligned edge-to-edge on a photovoltaic module with minimal unused gaps between them. The diameter or width of the wafer may be any standard or custom size. For simplicity this discussion will describe the use of a monocrystalline silicon wafer as the semiconductor donor body, but it will be understood that donor bodies of other types and materials can be used. 
     Referring to  FIG. 7   a , donor wafer  20  is a monocrystalline silicon wafer which is lightly to moderately doped to a first conductivity type. The present example will describe a relatively lightly n-doped wafer  20  but it will be understood that in this and other embodiments the dopant types can be reversed. Wafer  20  may be doped to a concentration of between about 1×10 15  and about 1×10 18  dopant atoms/cm 3 , for example about 1×10 17  dopant atoms/cm 3 . Donor wafer  20  may be, for example, solar- or semiconductor-grade silicon. 
     In the next step, ions, preferably hydrogen or a combination of hydrogen and helium, are implanted into wafer  20  through first surface  10  to define cleave plane  30 , as described earlier. This implant may be performed using the teachings of Parrill et al., U.S. patent application Ser. No. 12/122,108, “Ion Implanter for Photovoltaic Cell Fabrication,” filed May 16, 2008; or those of Ryding et al., U.S. patent application Ser. No. 12/494,268, “Ion Implantation Apparatus and a Method for Fluid Cooling,” filed Jun. 30, 2009; or of Purser et al. U.S. patent application Ser. No. 12/621,689, “Method and Apparatus for Modifying a Ribbon-Shaped Ion Beam,” filed Nov. 19, 2009, all owned by the assignee of the present invention and hereby incorporated by reference. The overall depth of cleave plane  30  is determined by several factors, including implant energy. The depth of cleave plane  30  can be between about 0.2 and about 100 microns from first surface  10 , for example between about 0.5 and about 20 or about 50 microns, for example between about 1 and about 10 microns, between about 1 or 2 microns and about 5 or 6 microns, or between about 4 and about 8 microns. Alternatively, the depth of cleave plane  30  can be between about 5 and about 15 microns, for example about 11 or 12 microns. 
     Texture (not shown) may be formed at first surface  10  to minimize reflection. A method for forming advantageous low-relief texture is disclosed in Li et al., U.S. patent application Ser. No. 12/729,878, “Creation of Low-Relief Texture for a Photovoltaic Cell,” filed Mar. 23, 2010, owned by the assignee of the present invention and hereby incorporated by reference. The method of Li et al. includes buffing the surface, which may induce stress, tending to provide nucleation points for a crystallographically selective etch. Following texturing, at least 50 percent of first surface  10  has a peak-to-valley height between about 3000 angstroms and about 1 micron. In practice, nearly all of first surface  10  is textured. 
     Turning to  FIG. 7   b , following the texture etch, first surface  10  may be placed adjacent to vacuum chuck  54 , and a vacuum applied to hold donor wafer  20  in place. Vacuum chuck  54  and donor wafer  20  may be enclosed, for example in a quartz envelope. 
     Referring to  FIG. 7   c , a thermal step causes monocrystalline silicon lamina  40  to cleave from the donor wafer at the cleave plane. Cleaving is achieved in this example by exfoliation, which may be achieved at temperatures between, for example, about 350 and about 650 degrees C., for example, about 550 degrees C. In general, exfoliation proceeds more rapidly at higher temperature. During cleaving, pressure may be applied by using a plate attached to a piston that will press donor wafer  20  to vacuum chuck  54 . Pressures in the range of about 1 to about 100 psi, for example 40 psi, could be applied. The thickness of lamina  40  is determined by the depth of cleave plane  30 . In many embodiments, the thickness of lamina  40  is between about 1 and about 10 microns, for example between about 2 and about 5 microns, for example about 4.5 microns. In other embodiments, the thickness of lamina  40  is between about 4 and about 20 microns, for example between about 10 and about 15 microns, for example about 11 microns. Second surface  62  is created by cleaving. As shown, an aperture  71  may form in lamina  40  during cleaving. In some embodiments, aperture  71  may be formed intentionally, for example by laser. For simplicity, only one aperture  71  is shown, but in practice there may be many more. 
     An anneal step may be performed to repair damage caused to the crystal lattice throughout the body of lamina  40  during the implant step Annealing may be performed while lamina  40  remains in place on vacuum chuck  54 , for example, at 500 degrees C. or greater, for example at 550, 600, 650, 700, 800, 850 degrees C. or greater, at about 950 degrees C. or more. The structure may be annealed, for example, at about 650 degrees C. for about 45 minutes, or at about 800 degrees for about ten minutes, or at about 950 degrees for 120 seconds or less. In many embodiments the temperature exceeds 900 degrees C. for at least 60 seconds. 
     Next lamina  40  is removed from vacuum chuck  54 . This may be done, for example, using a vacuum paddle, not shown. To affect this transfer, the vacuum paddle is placed on second surface  62 , while the vacuum on first surface  10  is released. Following transfer to the vacuum paddle, second surface  62  is held by vacuum, while first surface  10  is exposed. Referring to  FIG. 7   d , lamina  40  is then transferred from the vacuum paddle to a temporary carrier  50 . It is affixed to temporary carrier  50 , for example using an adhesive. This adhesive must tolerate moderate temperature (up to about 200 degrees C.) and must be readily releasable. Suitable adhesives include, for example, polyester with maleic anhydride and rosin, which is hydrocarbon-soluble; or polyisobutylene and rosin, which is detergent soluble. Temporary carrier  50  may be any suitable material, for example glass, metal, polymer, silicon, etc. Following transfer, first surface  10  will be held to temporary carrier  50  by adhesive, while second surface  62  is exposed. 
     An etch step to remove damage caused by exfoliation may be performed, for example by a mix of hydrofluoric (HF) acid and nitric acid, or using KOH. It may be found that annealing is sufficient to remove all or nearly all damage and this etch is unnecessary. At minimum, the surface is cleaned of organic materials and residual oxide, using a dilute HF solution; for example, 10:1 HF for two minutes. Following this wet process, an amorphous silicon layer is deposited on second surface  62 . This layer  72  is heavily doped silicon and may have a thickness, for example, between about 50 and about 350 angstroms.  FIG. 7   d  shows an embodiment that includes intrinsic or nearly intrinsic amorphous silicon layer  74  between first surface  10  and doped layer  72 , and in immediate contact with both. In this case, layers  72  and  74  together make up an amorphous layer consisting of a first thickness of amorphous intrinsic silicon and a second thickness of heavily doped amorphous silicon. In other embodiments, layer  74  may be omitted. In this example, heavily doped silicon layer  72  is heavily doped n-type, the same conductivity type as lightly doped n-type lamina  40 . Lightly doped n-type lamina  40  comprises the base region of the photovoltaic cell to be formed, and heavily doped amorphous silicon layer  72  provides electrical contact to the base region. If included, layer  74  is sufficiently thin that it does not impede electrical connection between lamina  40  and heavily doped silicon layer  72 . 
     Note that heavily doped amorphous silicon layer  72 , and intrinsic layer  74 , line the sidewalls of aperture  71 . 
     A TCO layer  110  is formed on and in immediate contact with amorphous silicon layer  74 . Appropriate materials for TCO  110  include indium tin oxide and aluminum-doped zinc oxide. This layer may be, for example, about between about 500 to about 1500 angstroms thick, for example about 750 angstroms thick. This thickness will enhance internal reflection from a reflective layer to be deposited. In some embodiments this layer may be substantially thinner, for example about 100 to about 200 angstroms. 
     As will be seen, in the completed device, incident light will enter lamina  40  at first surface  10 . After passing through lamina  40 , light that has not been absorbed will exit lamina  40  at second surface  62 , then pass through TCO layer  110 . A reflective layer  12  formed on TCO layer  110  will reflect this light back into the cell for a second opportunity to be absorbed, improving efficiency. A conductive, reflective metal may be used for reflective layer  12 . Various layers or stacks may be used. In one embodiment, reflective layer  12  is formed by depositing a very thin layer of chromium, for example about 30 or 50 angstroms to about 100 angstroms, on TCO layer  110 , followed by about 1000 to about 3000 angstroms of silver. In an alternative embodiment, not pictured, reflective layer  12  may be aluminium, having a thickness of about 1000 to about 3000 angstroms. In the next step, a layer will be formed by plating. Conventional plating cannot be performed onto an aluminum layer, so if aluminum is used for reflective layer  12 , an additional layer or layers must be added to provide a seed layer for plating. In one embodiment, for example, a layer of titanium, for example between about 200 and about 300 angstroms thick, followed by a seed layer, for example of cobalt, which may have any suitable thickness, for example about 500 angstroms. 
     Metal support element  60  is formed by plating on reflective layer  12  (a chromium/silver stack in this embodiment). To form metal support element  60  by electroplating, temporary carrier  50  and lamina  40 , and associated layers, are immersed in an electrolyte bath. An electrode is attached to reflective layer  12 , and a current passed through the electrolyte. Ions from the electrolyte bath build up on reflective layer  12 , forming metal support element  60 . Metal support element  60  may be, for example, an alloy of nickel and iron. Iron is cheaper, while the coefficient of thermal expansion of nickel is better matched to that of silicon, reducing stress during later steps. The thickness of metal support element  60  may be as desired. Metal support element  60  should be thick enough to provide structural support for the photovoltaic cell to be formed. A thicker support element  60  is less prone to bowing. In contrast, minimizing thickness reduces cost. One skilled in the art will select a suitable thickness and iron:nickel ratio to balance these concerns. Thickness may be, for example, between about 25 and about 100 microns, for example about 50 microns. In some embodiments, the iron-nickel alloy is between about 55 and about 65 percent iron, for example  60  percent iron. 
     Turning to  FIG. 7   e , following construction of metal support element  60 , temporary carrier  50  can be removed, exposing first surface  10 .  FIG. 7   e  shows the structure inverted, with metal support element  60  on the bottom, as it will be during operation. Lamina  40  is detached from temporary support element  50 . In some embodiments, heating, for example to about 225 C, will cause the adhesive to soften, aiding removal. First surface  10  is cleaned, for example using a detergent, followed by a rinse. In some case a solvent may be used, such as photoresist stripper. Temporary support element  50  is also cleaned, using the same methods, and subsequently re-used. 
     Following cleaning, an amorphous silicon layer is deposited on first surface  10 . This layer  76  is heavily doped silicon and may have a thickness, for example, between about 50 and about 350 angstroms.  FIG. 7   e  shows an embodiment that includes intrinsic or nearly intrinsic amorphous silicon layer  78  between first surface  62  and doped layer  76 , and in immediate contact with both. In this case, layers  76  and  78  together make up an amorphous layer consisting of a first thickness of amorphous intrinsic silicon and a second thickness of heavily doped amorphous silicon. In other embodiments, layer  78  may be omitted. In this example, heavily doped silicon layer  76  is heavily doped p-type, opposite the conductivity type of lightly doped n-type lamina  40 , and serves as the emitter of the photovoltaic cell. If included, layer  78  is sufficiently thin that it does not impede electrical connection between lamina  40  and heavily doped silicon layer  76 . The term “heavily doped” when referring to amorphous silicon layers can be considered to mean doped to at least 10 18  dopant atoms/cm 3 . 
     Note that in aperture  71 , the amorphous silicon layer or layers formed on first surface  10 , layer  76  and layer  78  (if present), make immediate contact with the amorphous silicon layer or layers earlier formed on second surface  62 , layer  72  and layer  74  (if present). A Zener diode  73  is formed at this contact. If undoped amorphous silicon layers  78  and  72  are present, they are sufficiently thin so as not to impede the function of Zener diode  73 . 
     A transparent conductive oxide (TCO) layer  112  is formed on and in immediate contact with amorphous silicon layer  76 . Appropriate materials for TCO  112  include indium tin oxide and aluminum-doped zinc oxide. This layer may be, for example, about between about 700 to about 1800 angstroms thick, for example about 900 angstroms thick. In some embodiments, a layer having a refractive index between that of amorphous silicon layer  76  and TCO layer  112 , may be formed on amorphous silicon layer  76 , as described in Liang et al., U.S. patent application Ser. No. 12/894,254, “A Semiconductor with a Metal Oxide Layer Having Intermediate Refractive Index,” filed Sep. 30, 2010, owned by the assignee of the present application and hereby incorporated by reference. 
     Metal lines  57 , for example of silver paste, may be formed on TCO layer  112 , for example by screen printing, and cured at a relatively low temperature, for example about 180-250 degrees C. 
     A photovoltaic cell has been formed, including lightly doped n-type lamina  40 , which comprises the base of the cell, and heavily doped p-type amorphous silicon layer  76 , which serves as the emitter of the cell. Heavily doped n-type amorphous silicon layer  72  will provide good electrical contact to the base region of the cell. Electrical contact must be made to both faces of the cell. Contact to amorphous silicon layer  76  is made by gridlines  57 , by way of TCO layer  112 . Metal support element  60  is conductive and is in electrical contact with base contact  72  by way of conductive layer  12  and TCO layer  110 . A Zener diode is formed in aperture  71 , and in any other apertures, at the junction of heavily doped amorphous layers  72  and  76 , which are doped to opposite conductivity types. Intrinsic layers  74  and  78 , if present, are sufficiently thin that they do not prevent this junction from behaving as a Zener diode. 
       FIG. 7   e  shows completed photovoltaic assembly  80 , which includes a photovoltaic cell and metal support element  60 . In alternative embodiments, by changing the dopants used, heavily doped amorphous silicon layer  72  may serve as the emitter, while heavily doped silicon layer  76  serves as a contact to the base region. Incident light (indicated by arrows) falls on TCO  112 , enters the cell at heavily doped p-type amorphous silicon layer  76 , enters lamina  40  at first surface  10 , and travels through lamina  40 . Reflective layer  12  will serve to reflect some light back into the cell. In this embodiment, receiver element  60  serves as a substrate. Receiver element  60  and lamina  40 , and associated layers, form a photovoltaic assembly  80 . Multiple photovoltaic assemblies  80  can be formed and affixed to a supporting substrate  90  or, alternatively, a supporting superstrate (not shown). Each photovoltaic assembly  80  includes a photovoltaic cell. The photovoltaic cells of a module are generally electrically connected in series. 
     What has been described is a photovoltaic cell comprising: a monocrystalline silicon lamina having a thickness between about 2 and about 20 microns, the lamina having a first surface and a second surface opposite the first, the lamina comprising a base region of the photovoltaic cell; a first amorphous silicon layer on and in contact with the first surface, wherein at least half a thickness of the first amorphous silicon layer is heavily doped to a first conductivity type; a second amorphous silicon layer on and in contact with the first surface, wherein at least half a thickness of the second amorphous silicon layer is heavily doped to a second conductivity type opposite the first; a first contact in electrical contact with the first amorphous silicon layer; and a second contact in electrical contact with the second amorphous silicon layer, wherein the second amorphous silicon layer lines the sidewalls of an aperture in the lamina, and the first amorphous silicon layer does not line the sidewalls of the aperture in the lamina, and does not have a corresponding aperture aligned with the aperture in the lamina, and the first amorphous silicon layer and the second amorphous silicon layer are in contact through the aperture, forming a Zener diode, and wherein, during normal operation of the photovoltaic cell, the second amorphous silicon layer is not in ohmic contact with the first contact. 
       FIG. 8   a  depicts a typical IV curve for a prior art photovoltaic cell exposed to light. The cell operates at or near the maximum power point, which in this cell, for example, is at about 0.6 V. At negative voltage, current is nearly flat, until a very high reverse voltage, called the breakdown voltage, is reached. Note that the X-axis is not to scale. In the cell of  FIG. 8   a , the breakdown voltage is at about −12 V.  FIG. 8   b  shows three cells  100   a - 100   c  connected in series, though in normal operation, a string will generally contain more than three cells. In normal operation, the IV curves of the three cells are as shown in  FIG. 8   c . As these cells are connected in series, the voltages will add, producing 1.8 V for the string.  FIG. 8   d  is a circuit diagram of the string. 
     Referring to  FIG. 8   b , if cell  100   b  is unexpectedly shaded, photocurrent produced by this cell will drop dramatically. In order to match the current in the rest of the string, cell  100   b  will go into heavy reverse bias  FIG. 8   e  shows the IV curves of the two unshaded cells,  100   a  and  100   c , and of the shaded cell,  100   b . In this example, the current of the rest of the string, I S , is reached at about −12 V in the shaded cell,  100   b .  FIG. 8   f  shows a circuit diagram of the series string. The heavy reverse bias will likely damage cell  100   b.    
     A Zener diode will have a lower reverse breakdown voltage than the main diode. In a series string of cells formed according to the present invention, each cell is formed having a Zener diode formed in one or more apertures within each the cell. Each cell then has a Zener diode in parallel with the main cell, as shown in the circuit diagram of  FIG. 9   a . In a string including cells  80   a - 80   c  formed according to the present invention, when cell  80   b  is shaded, the Zener diode, under reverse bias, reaches current I S  at a much lower reverse voltage, in this example about −5 V. These voltages are shown in  FIG. 9   a . This will help prevent damage to the main cell. 
     In the embodiments just described, texturing was performed before plating. In other embodiments, texturing may be performed following plating. The texturing method of Li et al. calls for a buffing step thought to introduce surface stress, which creates nucleation points for etching. When texturing is performed following plating, it may be found that the stress induced by plating may render the buffing step unnecessary, and a timed etching step with a selective etchant such as TMAH or KOH or NaOH may be sufficient to form pyramids having a peak-to-valley height less than one micron. 
     In the embodiment just described, a metal support element was constructed on the lamina by plating. In alternative embodiments, a support element may be formed from other materials. For example, a ceramic mixture may be applied to a lamina and cured to form a ceramic support element, or a polymer support element may be formed by applying a the polymer in liquid or semi-liquid form and curing in place. Embodiments are described in detail in Murali et al., earlier incorporated. 
     A variety of embodiments has been provided for clarity and completeness. Clearly it is impractical to list all possible embodiments. Other embodiments of the invention will be apparent to one of ordinary skill in the art when informed by the present specification. Detailed methods of fabrication have been described herein, but any other methods that form the same structures can be used while the results fall within the scope of the invention. 
     The foregoing detailed description has described only a few of the many forms that this invention can take. For this reason, this detailed description is intended by way of illustration, and not by way of limitation. It is only the following claims, including all equivalents, which are intended to define the scope of this invention.