Patent Publication Number: US-9837571-B2

Title: Tandem nanofilm photovoltaic cells joined by wafer bonding

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application contains subject matter which is related to the subject matter of the following commonly assigned copending applications, including U.S. patent application Ser. No. 12/242,962 filed on 1 Oct. 2008 entitled “Optical Tandem Photovoltaic Cell Panels”; and U.S. patent application Ser. No. 12/243,995 filed on 2 Oct. 2008 entitled “Quantum Well Gap/Si Tandem Photovoltaic Cells”. Each of the above listed patent applications is hereby incorporated herein by reference in its entirety. 
     BACKGROUND 
     Definitions 
     Electromagnetic Radiation to Electric Energy Conversion Device (EREECD): A device that reacts with electromagnetic (optical) radiation to produce electrical energy Optical Radiation to Electric Energy Conversion Device (OREECD): A device that reacts with optical electromagnetic radiation to produce electrical energy. Such a device could be a radiation absorbing device, e.g. a photodetector/counter, photovoltaic cell (solar cell) or a radiation-driven electrolysis cell. 
     Optoelectronic Energy Device (OED): A device that reacts with optical radiation to produce electrical energy with an electronic device. 
     Photovoltaic cell: An electrical device (e.g. a semiconductor) that converts light or other radiant energy, in the range from ultraviolet to infrared radiation, incident on its surface into electrical energy in the form of power/voltage/current which has two electrodes, usually a diode with a top electrode and a bottom electrode with opposite electrical polarities. The photovoltaic cell produces direct current which flows through the electrodes. As employed herein, the term photovoltaic cell is generic to cells which convert radiant energy into electrical energy including EREECDs, OREECDs, and OEDs as defined above. 
     Solar cell: An electrical photovoltaic device (e.g. a semiconductor) that converts light incident on its surface into electrical energy which has two electrodes, usually a diode with a top electrode and a bottom electrode with opposite electrical polarities. The solar cell produces direct current which flows through the electrodes. As employed herein, the term solar cell is generic to cells which convert radiant energy into electrical energy. 
     This invention relates to tandem photoelectric devices formed from separate photovoltaic cells and more particularly to interconnections therebetween and methods of forming interconnected tandem photovoltaic cells. 
     Multijunction photovoltaic cells are devices containing two or more photovoltaic cells of different bandgaps connected in series and exposed to sunlight such that higher energy photons are absorbed in the upper-lying photovoltaic cells with their higher bandgaps while lower energy photons are transmitted to the lower bandgap photovoltaic cells in the stack. This combination makes more efficient use of the solar spectrum by converting more of it to electricity rather than heat. For example, single junction photovoltaic cells have a maximum efficiency of approximately 30% while multijunctions can exceed 50%, and 40% efficient triple junction photovoltaic cells have already been demonstrated. Such photovoltaic cells can be made in part by epitaxial growth of all the materials and layers. 
     For example, a triple junction photovoltaic cell, consists of a Ge N+/P junction, a P+/P/N+ GaAs photovoltaic cell grown upon the Ge, a P+/P/N+ GaInP junction grown upon the GaAs, with the P+/N+ boundaries acting as tunnel junctions, a AlInP passivating window layer upon the GaInP, a GaAlAs passivating window layer grown upon the GaAs, totaling twelve to seventeen layers of semiconductor material each of which must be lattice matched to all the others to prevent lattice mismatch defect formation, and each has to have an accurately determined thickness to ensure equal photocurrents in each photovoltaic cell (currents must be equal in each photovoltaic cell of an electrically series connection to obtain maximum power output and prevent any one of the cells acting as a power-draining efficiency-lowering load upon the others). A major problem can arise due to the need to prevent diffusion of dopant from any of the layers into adjacent regions. Such cross diffusion raises the resistance of the tunnel junctions and lowers efficiency. This sets a limit on the temperature/time profile during fabrication. In semiconductor fabrication, the simultaneous requirements of tight control of many variables (temperature, time, thicknesses, lattice matching, dopant densities) generally lowers yield and raises costs. 
     Wafer bonding represents an improved process for creating multijunction photovoltaic cells. By creating photovoltaic cells of different materials and bandgaps separately and joining them by bonding, each photovoltaic cell can be optimized and yield can be raised. Such bonded tandem photovoltaic cells are described, e.g. Zahler et al U.S. patent publication 2006/0021565, which discusses creating a GaAs/GaInP dual junction tandem photovoltaic cell by monolithic epitaxy means and bonding this to a silicon photovoltaic cell/substrate. Exfoliation (separation of the desired device volume from the handle substrate) is generally induced by low ion mass ion implantation which causes a weak zone in the material which cracks off upon heating if the ion density is sufficient. Bonding by electrostatic means (applying a high electric field) is described in Stambery U.S. Pat. No. 5,261,969. 
     One means of increasing the efficiency of photovoltaic cells is to incorporate quantum well layers within the depletion region of the photovoltaic cell p/n junction. Such quantum wells may be alternating layers of materials having a thickness less than ten nanometers with lower bandgaps than the host junction in order to increase the sunlight wavelength range over which the photovoltaic cell operates. Such quantum well incorporation has been discussed in Moustakas WO2005/104236, Moustakas et al. U.S. patent Publication 2005/0242364, and Suzuki U.S. Patent Publication 2002/0050288A1 which also describe texturing the surface of the upper photovoltaic cell to reduce light reflection. 
     Quantum well incorporation and tunnel junction formation in monolithic tandem photovoltaic cells are discussed in Freundlich U.S. Pat. No. 6,372,980. The above mentioned WO2005/104236, Park et al U.S. Pat. No. 6,663,944, Ji et al. U.S. Pat. No. 6,420,647, and Shaharyar U.S. Pat. No. 5,258,077 describe the use of textured surfaces to result in light trapping in thin films of semiconductors, such that light which is not absorbed in the first pass through the material will make multiple passes which increases its absorption probability. Fabricating tandem photovoltaic cells with a metal interconnect between intermediate cells in place of a tunnel junction is described in Manlass U.S. Pat. No. 5,322,572. 
     SUMMARY OF THE INVENTION 
     The present invention relates to means for fabricating multijunction photovoltaic cells with higher yield and lower cost than conventional monolithic fabrication that uses epitaxial deposition of materials. The invention makes use of 1) semiconductor wafer bonding 2) both conventional and novel interconnection schemes, and 3) combinations of different material properties to optimize tandem photovoltaic cell structures. Photovoltaic cells are made in two or more materials with different energy bandgaps. By fabricating such photovoltaic cells separately, each photovoltaic cell can be optimized for highest performance. After fabrication, a first photovoltaic cell which is to be located on the bottom made from a first material with the lowest bandgap is mounted by suitable means on a low cost substrate such as glass, metal, or plastic. The first photovoltaic cell may be thin although it does not need to be thin. The second photovoltaic cell composed of a second higher energy bandgap material (hereinafter material # 2 ) is wafer bonded onto the first photovoltaic cell with suitable mechanical and electrical interconnect means. Exfoliation is used to remove the main part of the substrate from the back surface of material # 2  leaving a thin device structure such as an N+/P/P+ junction. A third junction composed of a third, highest energy bandgap material (i.e. with a higher energy bandgap than material # 2 ) can be wafer bonded onto the back surface of photovoltaic cell # 2  to form a triple junction photovoltaic cell. If desired, subsequent junctions with yet higher bandgaps can be further wafer bonded onto the structure to form multijunctions with higher numbers of junctions. Each additional junction requires suitable interconnect means with the criteria that each interconnect material is low in electrical resistance, but high in optical transparency for all photon energies less than the bandgaps of the higher lying photovoltaic cells. Interconnects can be made by process steps comprising 1) bonding heavily doped semiconductor regions to create tunnel junctions; 2) first growing a heterojunction material on the front surface of the lower photovoltaic cell and/or back surface of the higher photovoltaic cell and wafer bonding the heterojunctions (the heterojunctions may also act as “window” layers (surface passivation layers) to prevent surface recombination; 3) depositing Transparent Conducting Oxides (TCO&#39;s) on one or both surfaces to be bonded; 4) depositing a thin metal grid on one surface of a photovoltaic cell and bonding the thin metal-gridded surface of one semiconductor to another semiconductor. 
     In accordance with this invention, a solar energy conversion multijunction device comprises thin film photovoltaic cells; with an upper photovoltaic cell of higher energy bandgap bonded on top of a lower photovoltaic cell of lower energy bandgap. Preferably, the upper photovoltaic cell has a thickness in the range from about 0.5 microns to about 10 microns; and/or the lower photovoltaic cell is mounted on a substrate composed of a material selected from the group consisting of silicon, glass, quartz, silica, alumina, ceramic, metal, graphite, and plastic. Preferably, there is an interface between the upper photovoltaic cell and the lower photovoltaic cell; wherein the interfaces between the photovoltaic cells comprises a structure selected from the group consisting of a tunnel junction, a heterojunction, a transparent conducting oxide, an alloying metal, and an alloyed metal grid. Additionally, the upper photovoltaic cell may contain a quantum well region; and/or at least one of the top surface or bottom surface of the energy conversion device contains light-trapping means. 
     In accordance with another aspect of this invention, a solar energy conversion device comprises three photovoltaic cells comprising individual, thin film devices; and two of the photovoltaic cells are bonded to a third one of the photovoltaic cells. Preferably, the photovoltaic cells have thicknesses in the range of about 0.5 microns to about 10 microns; and/or an upper one of the photovoltaic cells has an energy bandgap of at least about 1.7 electron volts, a middle one of the photovoltaic cells has a bandgap between about 1.2 electron volts and about 1.7 electron volts, and a lower one of the photovoltaic cells has an energy bandgap below about 1.2 electron-volts. It is preferred that all three of the photovoltaic cells have thicknesses in the range from about 0.5 microns to 10 microns; and/or a lower one of the photovoltaic cells is mounted on a substrate composed of a material selected from the group consisting of silicon, glass, quartz, silica, alumina, ceramic, metal, graphite, and plastic. 
     Preferably, each interface between the photovoltaic cells comprises a structure selected from the group consisting of a tunnel junction, a heterojunction, a transparent conducting oxide, and an alloying metal; and/or one or more of the thin film photovoltaic cells may contain quantum well or quantum dot regions. Quantum dots are nanoparticles less than 10 nanometers in size made from a material with a smaller bandgap than the host material, for example, Ge quantum dots in a Si host. Preferably light-trapping means is incorporated atop the energy conversion device. 
     In accordance with yet another aspect of this invention, an energy conversion device comprises photovoltaic cells formed on wafers fabricated separately and bonded together with interfaces therebetween; with the two or more of the photovoltaic cells being arranged in a hierarchical stack of decreasing order of energy bandgaps from top to bottom thereof. At least one of the photovoltaic cells comprises a thin film photovoltaic cell having a thickness in the range from about 0.5 microns to about 10 microns. Each of the interfaces between the photovoltaic cells comprises a structure selected from the group consisting of a tunnel junction, a heterojunction, a transparent conducting oxide, an alloying metal, and an alloyed metal grid; and the photovoltaic cells are mounted onto a substrate composed of a material selected from the group consisting of silicon, quartz, glass, silica, alumina, ceramic, metal, graphite, and plastic. Preferably, the energy conversion device includes an upper surface and/or a lower surface including light trapping means; and/or at least one of the photovoltaic cells may contain a quantum well or quantum dot region. 
     In accordance with yet another aspect of this invention at least two film photovoltaic cells are fabricated separately and joined by wafer bonding at interfaces between the photovoltaic cells. The photovoltaic cells are arranged in a hierarchical stack of decreasing energy bandgap thereof from top to bottom. Each of the thin film photovoltaic cells has a thickness in the range from about 0.5 microns to about 10 microns. The hierarchical stack is mounted upon a supporting substrate composed of a material selected from the group consisting of silicon, glass, quartz, silica, alumina, ceramic, metal, graphite, and plastic. Each of the interfaces between the photovoltaic cells comprises a structure selected from the group consisting of a tunnel junction, a heterojunction, a transparent conducting oxide, an alloying metal, and an alloyed metal grid; and an upper surface and/or a lower surface of the energy conversion device containing light-trapping means. Additionally, the photovoltaic cells may contain a quantum well or quantum dot region; and the bottom surface of the energy conversion device contains light-trapping means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a multijunction photovoltaic energy conversion device comprising two junctions made by separate photovoltaic cell fabrication and subsequent wafer bonding. 
         FIG. 2  shows a multijunction photovoltaic energy conversion device consisting of a stack of three separate photovoltaic cells made by separate photovoltaic cell fabrication and joined by subsequent wafer bonding. 
         FIG. 3  shows a multijunction photovoltaic energy conversion device consisting of a stack of three thin film photovoltaic cells mounted on a low cost substrate. 
         FIGS. 4A and 4B  show alternative process sequences by which wafer bonding of individual photovoltaic cells and exfoliation is accomplished. 
         FIG. 4C  shows a semiconductor layer coated by a passivating layer and a dielectric layer, the combination acting as an antireflective coating. 
         FIG. 5  shows a tandem photovoltaic cell with heterojunction layer interconnects. 
         FIG. 6  shows a photovoltaic energy conversion device formed with tandem photovoltaic cells with transparent conducting oxide layer interconnects. 
         FIG. 7  shows a photovoltaic energy conversion device formed with tandem photovoltaic cells with a metallic grid interconnect alloyed to semiconductor materials. 
         FIGS. 8A and 8B  show photovoltaic energy conversion devices formed with tandem photovoltaic cells with surface texturing to trap radiant energy including light, quantum wells, and heterojunction or TCO interconnects made by wafer bonding. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , a tandem, multijunction, photovoltaic energy conversion device  10  consisting of two separate, stacked, photovoltaic cells  10 A and  10 B composed of semiconductor materials and illuminated by radiant energy, e.g. sunlight  9 . The lower photovoltaic cell  10 A comprises a semiconductor wafer composed of material # 1  which contains an N+ doped region  16 , a p-type doped region  17 , and a P+ doped region  18 , which is mounted on a substrate  19  using suitable means, such as alloying, adhesive, or bonding. The substrate  19  is composed of a material such as silicon, glass, quartz, silica, alumina, ceramic, metal, graphite, and plastic. The upper photovoltaic cell  10 B comprises a semiconductor wafer which is made using a material # 2  with a higher bandgap than the material # 1  used to make photovoltaic cell  10 A, consists of an N+ doped region  13 , a P-type doped region  14 , and a P+ doped region  15 ; and the upper photovoltaic cell  10 B is mounted to the lower photovoltaic cell  10 A using wafer bonding. The process of wafer bonding may be accomplished by several means, for example 1) ion implantation of hydrogen or helium ions is performed first and then followed by steps of exfoliation, 2) growth upon a porous region and selective etching of the porous region, and other methods known in the art. Bonding takes place when two very clean, atomically flat surfaces are brought into contact so that atoms on the surfaces of the two materials interact to cause the materials to adhere to each other. Exfoliation is the separation of a portion of one of the materials which remains affixed to the other. The separation is accomplished by heat treatment or mechanical means such as a water jet, or by selective etching as mentioned above. 
     The photovoltaic cells  10 A and  10 B are made separately and the processes and procedures for making such photovoltaic cells can be optimized separately. The P+ doped region  15  and N+ doped region  16  on the borders between the photovoltaic cell  10 B and photovoltaic cell  10 A are so heavily doped that the boundary between them forms a tunnel junction providing a low resistance interconnect. 
       FIG. 2  shows a tandem, triple junction photovoltaic energy conversion device  20  illuminated by radiant energy, e.g. sunlight  9 , and consisting of three separate photovoltaic cells  10 A,  10 B, and  10 C. The multijunction solar device  20  can be fabricated, for example, by adding a third separate photovoltaic cell  10 C to the double junction tandem, photovoltaic cell device  10  of  FIG. 1 . The third photovoltaic cell  10 C, which is composed of a semiconductor wafer using a material # 3  with a higher bandgap than the material # 2  used to make photovoltaic cell  10 B, consists of an N+ doped region  11 , a P-type doped region  12 , and a P+ doped region  13 . Alternately, different materials may be used for photovoltaic cells  10 A and  10 B. Photovoltaic cells  10 A,  10 B, and  10 C may be fabricated and optimized separately and attached to substrate  19  by alloying, adhesives, or wafer bonding. Photovoltaic cell  10 B is mounted on photovoltaic cell  10 A by wafer bonding, and photovoltaic cell  10 C is mounted on photovoltaic cell  10 B by wafer bonding. Metal contacting grids  10 E and antireflective coatings  10 D are added to complete the photovoltaic cell, are well known in the photovoltaic cell art. 
     The lower photovoltaic cell  10 A in  FIGS. 1 and 2  may comprise a thick “bulk” semiconductor wafer such as silicon and substrate  19  may be glass, a metal such as aluminum or stainless steel, a ceramic material such as alumina or silica or graphite, or a plastic material, or it may be a silicon wafer such as low-cost metallurgical grade silicon. If the lower photovoltaic cell  10 A is a bulk wafer, the substrate  19  is optional and may be used for structural strength or is not necessary if the photovoltaic cell  10 A will be mounted onto a photovoltaic cell module directly using solder as is standard in the photovoltaic cell art. 
       FIG. 3  shows a second triple junction photovoltaic energy conversion device  24  illuminated by radiant energy, e.g. sunlight  9 , in which a set of three photovoltaic cells  14 A,  14 B, and  14 C from bottom to top are thin film photovoltaic cells with the lower photovoltaic cell  14 A mounted by wafer bonding onto a substrate  19 . Each of the photovoltaic cells  14 A,  14 B, and  14 C is mounted onto a surface therebelow by wafer bonding. The lower photovoltaic cell  14 A has N+ doped region  31  formed above a p-type doped region  32 , which is formed above a P+ doped region  33  which in turn is mounted on the substrate  19  which is described in more detail below. The middle photovoltaic cell  14 B has an N+ doped region  28  formed above a p-type doped region  29 , which is formed above a P+ doped region  30  which in turn is formed above the N+ doped region  31  of the lower photovoltaic cell  14 A. The upper photovoltaic cell  14 C has an N+ doped region  25  formed above a p-type doped region  26 , which is formed above a P+ doped region  27  which in turn is formed above the N+ doped region  28  of the middle photovoltaic cell  14 B. Each of the interconnect interfaces between the P+ region  27  at the bottom of the photovoltaic cell  14 C and the top N+ region  28  of the middle photovoltaic cell  14 B, and the P+ region  30  at the bottom of the photovoltaic cell  14 B and the top N+ region  31  of the bottom photovoltaic cell  14 A forms a tunnel junction interconnect interface. For example the thicknesses of photovoltaic cells  14 A,  14 B, and  14 C may be from about 0.5 microns to about 10 microns in thickness. The substrate  19  may be a metal, graphite, conductor-coated ceramic, or conductor-coated plastic. If the substrate  19 , which is shown as a metal for illustration, is composed of a conductor-coated ceramic, or conductor-coated plastic, the conductor coating is present on substrate  19  to make electrical contact to the bottom surface of the P+ region  33  of the bottom photovoltaic cell  14 A, and may be a metal, transparent conducting oxide (TCO) material, or a heavily doped semiconductor material. 
       FIGS. 4A and 4B  show alternative process sequences by which wafer bonding of individual photovoltaic cells and exfoliation is accomplished.  FIG. 4A  shows a process flow in which wafer bonding of the individual photovoltaic cells may be performed by a first method, including implantation and exfoliation. The process flow is shown for bonding a higher bandgap material # 1  onto a photovoltaic cell made from material # 2 . The higher bandgap material # 1 , for example, can be GaAs, GaP, GaInP, InAlP, GaAsP, CdTe, CdZnTe, or other semiconductor with a high bandgap of 1.4 eV or higher. For example, the lower bandgap material # 2 , can be silicon, germanium, SiGe alloys, CIGS (Copper Indium Gallium Selenide), or other semiconductor with a lower bandgap less than 1.4 eV. 
     In step A of  FIG. 4A , a wafer composed of a high bandgap material # 1  is chosen to fabricate an upper photovoltaic cell. 
     In step B, a narrow thickness of an N+ doped region is first formed in material # 1  by diffusion, by deposition (such as vapor growth or evaporation), or by ion implantation. 
     In step C, a region of lightly-doped p-type material is deposited on the top surface of the N+ doped wafer composed of material # 1  by a suitable means such as vapor growth, liquid phase epitaxy, or molecular beam epitaxy. 
     In step D, a region of P+ doping is then created in the layer deposited in step C. The P+ doping is performed by diffusion or ion implantation. 
     In step E, a hydrogen and/or helium implantation is made through the P+, P−, and N+ regions with energy at a high enough level to create a heavy hydrogen or helium concentration deeper below the surface than the N+ doped region. 
     In step F, material # 1  is then bonded to the surface of a previously-fabricated photovoltaic cell # 2  by methods well known in the bonding art, e.g. bringing the cleaned and atomically flat surfaces of the P+ region of material # 1  into contact with the clean, flat N+ surface of material # 2 . The photovoltaic cell # 2  may have been fabricated by bonding or by other techniques such as vapor growth. In  FIG. 1 , the two junction tandem photovoltaic cell  10  described above is shown wherein cell  10 A is formed of material # 2  and cell  10 B is formed of material # 1 . Materials # 1  and # 2  can be taken from the groups of materials described hereinabove. 
     Finally, in step G, a furnace anneal or Rapid Thermal Anneal (RTA) is carried out for simultaneously exfoliating the portion of photovoltaic cell # 1  lying above the hydrogen implant (the layers exfoliate by cracking off) and activating the ion implanted species. RTA is preferred from both a cost point of view and to prevent dopant movement which could degrade the tunnel junctions formed at the heavily doped interface between the two photovoltaic cells. The result is a photovoltaic tandem cell such as the photovoltaic energy conversion device  10  in  FIG. 1  comprised of a material # 1  forming cell  10 B and a material # 2  forming cell  10 A. 
     The bonding and exfoliation are best carried out with direct bandgap materials with very high absorption coefficients for light of photon energies greater than the bandgap. This allows the total thickness of the material to be exfoliated to be reduced. For example, the total thickness of the photovoltaic cell made from material # 1  can be as low as from about 0.5 microns to about 10 microns as hereinabove mentioned and reduces the required hydrogen or helium implant energy. For example, material # 1  could be GaInP, GaAlAs, CuGaSe, CdSeS, or GaAsP with a thickness of about 2 microns, while hydrogen or helium implantation can be carried out at an implanter voltage (potential) of about 350 kilovolts, commonly available in implant machines. If the photovoltaic cell of material  1  is from about 1 micron to about 1.5 microns thick, the implanter potential needed would be about 250 kilovolts. Thicker photovoltaic cell exfoliation would require greater potential. A photovoltaic cell with a thickness of about ten microns can be bonded and exfoliated with an implanter potential of about one million volts. 
     While  FIG. 4A  describes a first method in accordance with this invention for wafer bonding using hydrogen or helium implantation and exfoliation;  FIG. 4B  shows a process flow for wafer bonding of the individual photovoltaic cells by a second method which can be carried out by the use of porous silicon. 
     Step A is identical to step A in  FIG. 4A . In step B 2 , a heavily doped P+ region is created on the top surface of the wafer that allows the porous region in step C 2  to be formed, most commonly by anodization in hydrofluoric acid-water solution. A thicker, low doped P− region, that is deposited on the porous surface in step D 2  is followed by the N+ junction formation in step E 2 . After bonding the photovoltaic cell # 1  to previously fabricated photovoltaic cell # 2  in step F, exfoliation can be carried out in step G 2  by one of several means such as thermal cracking, chemical etching of the porous region, or mechanical means such as high power water jet or precision thin blades. Photovoltaic cell # 2  may have been made by means of liquid phase epitaxy, evaporation, or by other techniques such as vapor growth. In the case of  FIG. 1 , the finished tandem photovoltaic cell  10  can be formed with the photovoltaic cell  10 B being made from material # 1  and represents a photovoltaic cell  10 A made from material # 2 . 
     In one example of the process of  FIG. 4B , a wafer of material # 1 , which may be GaInP, GaAlAs, CuGaSe, CdSeS, or GaAsP, or Silicon contains a P+ surface obtained by deposition, diffusion, or ion implantation and is from about 0.5 microns to about 2 microns thick, is placed in a chemical bath of hydrofluoric acid in water. Electrical contact is made to the P+ surface by an electrode and to an inert metal electrode, and a voltage is placed between the electrodes until a desired current is reached. As a consequence, the P+ layer forms a porous region with a pore size dependent on the current. Following porous region formation, a low doped P− layer is grown by suitable means such as vapor growth or evaporation upon the surface, and an N+ region is created upon the surface of the P− layer by additional deposition, diffusion, or ion implantation. Bonding of this photovoltaic cell of material # 1  is carried out by the same means described hereinabove. Heat treatment can then be performed for simultaneously exfoliating the layer and activating the implant. Alternately, the porous region can be etched chemically or separated mechanically by, for example, high pressure water jet or precision thin blades which are inserted at the edge of the porous material with sufficiently-applied stress. 
     One advantage of the porous approach of  FIG. 4B  over the implantation/exfoliation method of  FIG. 4A  is that the allowed thicknesses of the layers are not limited by the available implanter potential. The exfoliation only depends on the depth of the porous region below the final surface of the photovoltaic cell, so that photovoltaic cells with thicknesses as much as about 10 microns in accordance with the invention can be easily implemented. 
     It is clear that variations in this bonding process flow can be made without changing the intent of the invention. It is also clear that the same process can be used to fabricate bonded multijunctions of three or more individual photovoltaic cells. 
     Photovoltaic cells have higher efficiency if the surface recombination is minimized by surface passivation means.  FIG. 4C  shows antireflective bilayer coating  10 D′ formed on the top surface  11 A of a semiconductor layer  11 B. The antireflective bilayer coating  10 D′ comprises a passivating layer  10 F and an additional dielectric layer  10 G. A plurality of metallic electrical contacts  10 E pass through the antireflective bilayer coating  10 D′ and the passivating layer  10 F into electrical and mechanical contact with the top surface  11 A of the semiconductor layer  11 B. This most often entails deposition of a semiconductor heterojunction layer with a higher bandgap onto a photovoltaic cell surface, where the heterojunction layer has a close lattice match to the photovoltaic cell to prevent interconnect interface defects. In  FIGS. 1 to 3 , the antireflective coating  10 D may be modified to contain a heterojunction passivation layer as well as one or more dielectric layers to reduce light reflection from the surface, as will be well understood by those skilled in the art. 
       FIG. 5  shows a third triple junction photovoltaic (PV) energy conversion device  48  illuminated by radiant energy, e.g. sunlight  9 , including a stack structure comprising a bottom photovoltaic cell  102  mounted on a substrate  19 , a middle photovoltaic cell  101 , and a top photovoltaic cell  100 . In this case, the interconnect interfaces comprise a set of heterojunction layers. A top heterojunction layer  35  is formed directly on a top surface of an N+ doped region  36  of the top photovoltaic cell  100 . A first intermediate heterojunction layer  39  is formed between the top photovoltaic cell  100  and the middle photovoltaic cell  101  in direct contact with the bottom P+ region  28  of the top photovoltaic cell  100  and in direct contact with the top surface of the N+ doped region  40  of the middle photovoltaic cell  101 . A second intermediate heterojunction layer  43  is formed between the middle photovoltaic cell  101  and the bottom photovoltaic cell  102 . The bottom photovoltaic cell  102  includes an upper N+ doped region  44  formed above a middle p-type doped region  45 , which is formed above a lower P+ doped region  46  which is mounted on the substrate  19  which is composed of a material such as silicon, quartz, glass, metal, graphite, or plastic. The middle photovoltaic cell  101  includes an upper N+ doped region  40  formed above a middle p-type doped region  41 , which is formed above a lower P+ doped region  42  that overlies the second (lower) intermediate heterojunction layer  43  which, in turn, is formed on top of the upper N+ doped region  44  of the bottom photovoltaic cell  102 . The top photovoltaic cell  100  includes an upper N+ doped region  36  formed above a middle p-type doped region  37 , which is formed above a lower P+ doped region  38  that overlies the first (middle) intermediate heterojunction layer  39  which in turn is formed on top of the upper N+ doped region  40  of the middle photovoltaic cell  101 . The upper heterojunction layer  35  is grown upon the top surface of the N+ doped region  36  of the top photovoltaic cell  100 . The middle heterojunction layer  39  is grown upon the top surface of the N+ doped region  40  of the middle photovoltaic cell  101 . Finally, the lower heterojunction layer  43  is present on the top surface of the upper N+ doped region  44  of the bottom photovoltaic cell  102 . 
     While  FIG. 5  shows a triple junction photovoltaic energy conversion device  48  formed with three photovoltaic cells, the same principle applies to two junction devices and multijunctions of more than three photovoltaic cells. Heterojunction layers for surface passivation may be deposited, for example, on as many photovoltaic cell surfaces as desired. Each photovoltaic cell may have such a heterojunction layer or a subset of the junctions in the tandem device may have them. If the heterojunction layers are heavily doped, they may also form part of the tunnel junction interconnect. For Ge and GalnAs photovoltaic cells suitable heterojunction materials include GaAs, GaAlAs, or ZnSe. For silicon photovoltaic cells suitable heterojunction materials include GaAsP, GaP or ZnS. For GaAs photovoltaic cells suitable heterojunction materials includes GaInP and GaAlAs. For GaInP suitable heterojunction materials photovoltaic cells includes AlInP. 
       FIG. 6  shows a triple junction photovoltaic cell device  49  illuminated by radiant energy, e.g. sunlight  9 , with an alternative to heterojunction layers at the bonding interconnect interfaces between individual photovoltaic cells including the top photovoltaic cell  103 , the middle photovoltaic cell  104  and the bottom photovoltaic cell  105 , which is formed on a substrate  19 . A set of Transparent Conducting Oxides (TCO&#39;s) layers  58 ,  62 , and  66  are formed above and between the individual photovoltaic cells  103 ,  104 , and  105 . Such TCO oxide layers  58 ,  62 , and  66  have much higher conductivities than semiconductors and form good electrical contacts to heavily doped semiconductors. The top TCO layer  58  is formed on the top surface of the N+ doped region  59  of the top photovoltaic cell  103 . The top photovoltaic cell  103  also contains p-type doped region  60  formed below the N+ doped region  59  and a P+ doped region  61  below the p-type doped region  60 . The P+ doped region  61  is formed on top of the middle TCO layer  62  which is formed on the top surface of the N+ doped region  63  of the middle photovoltaic cell  104 . The middle photovoltaic cell  104  also contains p-type doped region  64  formed below the N+ doped region  63  and a P+ doped region  65  formed below the p-type doped region  64 . The bottom TCO layer  66  is formed below the P+ doped region  65  and on the top surface of an N+ doped region  67  of the lower photovoltaic cell  105 , which also contains p-type doped region  68  formed below the N+ doped region  67  and an N+ doped region  69  formed below the N+ doped region  67  and on the top surface of the substrate  19 . The three photovoltaic cells  103 ,  104 , and  105  may be thin film photovoltaic cells as in  FIG. 3  or a combination of thick and thin photovoltaic cells. Bonding may be carried out with the TCO layers in place. Examples of TCO&#39;s include but are not limited to Indium Tin Oxide (ITO), tin oxide (SnOx), Zinc Oxide (ZnOx), Cadmium tin Oxide (CdSnOx), and oxides doped with Al, Sb, and F. 
     Tandem photovoltaic cell structures may be made with a combination of heterojunction and TCO layers. What remains the same in either case is the use of wafer bonding to accomplish the tandem photovoltaic cell fabrication. 
       FIG. 7  shows a photovoltaic energy conversion device formed  70  illuminated by radiant energy, e.g. sunlight  9 , formed of tandem photovoltaic cells  106 ,  107 , and  108  stacked from top to bottom with a set of metallic grid interconnect interfaces  81 A,  81 B and  81 C respectively alloyed to semiconductor materials forming a triple junction of photovoltaic cells. The device  70  illustrates yet another method of joining multiple photovoltaic cells into a tandem structure composed of the top photovoltaic cell  106 , the middle photovoltaic cell  107  and the bottom photovoltaic cell  108 . The individual, middle photovoltaic cell  107  is joined to the bottom P+ layer  73  of the top photovoltaic cell  106  by a metal grid  81 A. The metal grid  81 A may be applied to either the bottom surface of the bottom P+ layer  73  of the top photovoltaic cell  106  or to the top surface N+ layer  74  of the middle photovoltaic cell  107 . When heat treatment is performed as part of the bonding process, grid  81 A alloys to both layer  73  and layer  74 , forming an ohmic, low resistance contact to both layer  73  and layer  74 . Likewise, a grid  81 B may be placed either on the bottom surface of the bottom P+ layer  76  of the middle photovoltaic cell  107  or the top surface of the top N+ layer  77  of the photovoltaic cell  108 . Upon heat treatment, the metal grid  81 B alloys with both layer  76  and layer  77 , causing a low resistance contact connecting photovoltaic cell  107  and  108 . In similar fashion, a grid  81 C can be placed on the lower surface of layer  79  to bond photovoltaic cell  108  to substrate  19 . This allows the substrate  19  to be made of a metal which does not alloy with the semiconductor comprising photovoltaic cell  108 , by using a metal grid such as  81 C which alloys with photovoltaic cell  108 . The use of a single metal grid such as  81 A or  81 B or  81 C which alloys with two surfaces is a much simpler process than prior art methods of producing duplicate grids on both surfaces (for example, the surface of  73  and the surface of  74 ) and having to align the grids to mount the photovoltaic cells. 
     It will be apparent to one skilled in the art that a combination of tunnel junctions, heterojunctions, ICUs and/or alloying grids can be used in bonded tandem photovoltaic cells as described in  FIG. 3  and  FIGS. 5 to 7 . 
       FIG. 8A  shows a tandem photovoltaic cell device  80 A illuminated by radiant energy, e.g. sunlight  9 , consisting of individual three stacked photovoltaic cells  110 ,  111 , and  112 . The top photovoltaic cell  110  has an upper N+ doped region  84  above a middle p-type doped region  85  which is above a lower P+ doped region  86 . The lower P+ doped region  86  is bonded to the cell  111 . The middle photovoltaic cell  111  has N+ doped region  88 , p-type doped region  89 , and P+ doped region  90 . The bottom photovoltaic cell  112  has an upper N+ doped region  92  above a middle p-type doped region  94 , which is above a lower P+ doped region  95 . Interconnect interface regions  87  and  91  may be either tunnel junctions, heterojunctions, TCO&#39;s, or grid alloyed regions. A quantum well or quantum dot region  96  is included in the bottom photovoltaic cell  112 . The material used to form the quantum well or quantum dots in region  96  has a lower bandgap than material # 3  used to form the bottom photovoltaic cell  112 . 
     The top surface  83  of top N+ layer  84  of the top photovoltaic cell  110  is rough-textured to reduce light reflection from the top surface and cause light trapping. Similarly, a rough-textured surface  97  is incorporated onto the bottom surface of the substrate  19  to cause additional light trapping if the substrate  19  is transparent. On the other hand, if the substrate  19  is metallic, it will reflect light that reaches it back upward into the photovoltaic cells and also adds to light trapping. If the substrate  19  is a semiconductor, e.g. silicon, the textured surface  97  is easily created by plasma or wet etching. 
     Photovoltaic cells  110 ,  111 , and  112  may be thin films with thicknesses of from about 0.5 microns to about 10 microns; or the bottom photovoltaic cell  112  may be thick and mounted to substrate  19  by alloying, adhesive, or second metal layer alloying. A metal layer, not shown, can be applied on the surface of substrate  19  that will alloy with the bottom photovoltaic cell  112  during mounting even if the metal used for substrate  19  does not alloy with it. In addition, one or more layers of anti-reflective coating materials (not shown) may be added to the top of textured surface  83  to further reduce light reflection loss. 
       FIG. 8B  shows a tandem photovoltaic cell device  80 B in accordance with this invention (illuminated by radiant energy, e.g. sunlight  9 ) which is basically the same as  FIG. 8A  with the exception of a modification of the tandem photovoltaic cell device  80 A of  FIG. 8A  with two additional quantum well or quantum dot regions  96 A,  96 B in the photovoltaic cells  110 ,  111  formed below the N+ regions  84  and  88  and the respective upper surfaces of the p type regions  85  and  89  below the N+ regions  84  and  88  respectively of the two stacked upper and middle photovoltaic cells  110  and  111  in addition to a quantum well region  96  in the lower photovoltaic cell  112 . In each case the materials used in forming all of the several quantum wells or quantum dots  96 A,  96 B and  96  will have lower bandgaps than the host materials used to create photovoltaic cells  110 ,  111 , and  112 , respectively. 
     It is apparent to those skilled in the art that variations of the features outlined in the detailed description of the appended drawings can be made without deviating from the spirit of the invention. What is basic to the invention is the fabrication of thin film tandem, multijunction photovoltaic cells by separate photovoltaic cell formation and optimization with subsequent wafer bonding means to interconnect such thin film photovoltaic cells, together with the use of tunnel junctions, heterojunctions, transparent conductive oxides, or alloying grid metallurgy. The incorporation of light trapping and quantum wells enhances the performance of the thin film tandem photovoltaic cells, and the use of low cost substrate material reduces the overall cost of the devices.