Patent Publication Number: US-2016225928-A1

Title: Systems and processes for bifacial collection and tandem junctions using a thin-film photovoltaic device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 13/406,376 filed Feb. 27, 2012, which is a divisional of U.S. patent application Ser. No. 11/858,010 filed Sep. 19, 2007, now U.S. Pat. No. 8,124,870, which claims benefit of priority to U.S. Provisional Patent Application Ser. No. 60/845,705 filed Sep. 19, 2006. Each of the above-mentioned applications is incorporated herein by reference. 
    
    
     U.S. GOVERNMENT RIGHTS 
     This invention was made with Government support under National Aeronautics and Space Administration Grant No. NNC05CA41C. The Government has certain rights in this invention. 
    
    
     BACKGROUND 
     Photovoltaic (“PV”) devices generally consist of one or more active photovoltaic materials capable of generating an electric potential upon exposure to light, and electrical contacts constructed on a suitable substrate that are used to draw off electric current resulting from irradiation of the active PV material. PV devices are generally rigid, either because the active PV material itself is rigid, or because the substrate or other components of the device are inflexible. For example, glass, which is relatively inflexible, is frequently used as a substrate in thin film photovoltaic (“TFPV”) devices because of its strength, durability, tolerance to high processing temperatures and desirable optical properties. 
     TFPV devices are commonly distinguished from their thicker single-crystal PV counterparts in their ability to absorb light in relatively thin layers, and their ability to function well when fabricated using low-cost deposition techniques, and upon a variety of low-cost, lightweight and flexible substrates. Thus, TFPV devices are being considered for a variety of applications where weight and flexibility are important, such as for space satellites and high-altitude airships. 
     TFPV devices commonly include a solar absorber layer formed of a Group II-VI material, a Group I-III-VI.sub.2 material, or a Group III-V material. However, a solar absorber layer can be formed of other materials. The term Group II-VI material refers to a compound having a photovoltaic effect that is formed from at least one element from each of groups II and VI of the periodic table. In the context of this disclosure, Group II elements include Zinc, Cadmium, Mercury, and Magnesium and Group VI elements include Sulfur, Selenium, and Tellurium. The term Group I-III-VI.sub.2 material refers to a compound having a photovoltaic effect that is formed of at least one element from each of groups I, III, and VI of the periodic table, where there are two atoms of the group VI element for every one atom of the group I and III elements. In the context of this disclosure, Group I elements include Copper, Silver, and Gold, and Group III elements include Boron, Aluminum, Gallium, Indium, and Thallium. The term Group III-V material refers to a compound having a photovoltaic effect that is formed from at least one element from each of groups III and V of the periodic table. In the context of this disclosure, Group V elements include Nitrogen, Phosphorous, Arsenic, Antimony, and Bismuth. 
     Prior art TFPV devices with flexible substrates typically use metal foil or polyimide substrates. Metal foil substrates are capable of withstanding the high-temperatures and harsh thin-film deposition conditions needed for the highest efficiency TFPV devices, however, they are relatively heavy and are opaque. The latter characteristic does not allow for bifacial or backside visible light collection from reflected light sources, such as albedo light from either the moon or earth. Nor does this characteristic allow for transmission of undesirable infra-red (“IR”) light through the TFPV device; unused and untransmitted sub-bandgap light increases the operating temperature of the TFPV device and thereby decreases its efficiency. In one example, increased TFPV device operating temperature decreases efficiency by as much as 20%. Finally, opaque substrates do not allow for devices fabricated in the superstrate configuration, where the highest intensity visible light first passes through the substrate. Polyimide substrates are semi-transparent to IR light, however, they are only partially transparent to visible light or capable of withstanding the highest temperature thin-film deposition conditions required for certain CuInGaSe 2  (“CIGS”) based devices. 
     Attempts to provide PV devices on flexible and semi-transparent substrates are disclosed in U.S. Pat. No. 4,816,324, where tetrafluoroethylene-perfluoroalkoxy resin is used as a substrate for the PV device. However, tetrafluoroethylene-perfluoroalkoxy resin cannot withstand processing temperatures higher than 200-250° C., and therefore cannot be used for fabrication of high-efficiency TFPV devices that utilize Group II-VI and Group I-III-VI.sub.2 light-absorber materials, such as Cadmium Telluride (CdTe) and Copper Indium Gallium Di-Selenide (CIGS), respectively, since these materials require significantly higher processing temperatures. 
     Another example of a substrate that is lightweight, flexible, and that comprises materials such as silicon or silicone resin that are semi-transparent to visible light is described in U.S. Provisional Patent Application Ser. No. 60/792,852, and U.S. Non-Provisional Patent Application Ser. No. 11/737,119, each of which are incorporated herein by reference. This particular substrate is capable of withstanding processing temperatures up to 600° C., thereby enabling use of high-efficiency CIGS and CdTe materials in fabricating TFPV devices. However, to enable bifacial light collection, both the top and bottom contacts to the TFPV device must be at least semi-transparent to visible light. 
     To increase efficiency of TFPV devices through bifacial collection, semi-transparent substrates and/or semi-transparent back contacts are needed. For example, a semi-transparent back contact using a thin metal film (e.g., Cu) followed by a transparent conducting oxide (TCO) (e.g., Indium Tin Oxide) has been used with CdTe thin film Group II-VI semiconductor materials grown in a superstrate configuration on heavy, rigid glass substrates, as disclosed in a paper titled “Analysis of a Transparent Cu/ITO Contact and Heat Treatments on CdTe/CdS Solar Cells” by R. Birkmire, S. Hegedus, B. McCandless, J. Phillips and W. Shafarman [Proc. 19 th  IEEE PVSC (1987), p9671. However, a thin Cu layer would be difficult to implement with thin-film devices grown in a substrate configuration, because the back contact layer is deposited first and may be damaged and diffuse into other layers during the subsequent processing required for the solar absorber material. Application of the solar absorber material typically includes high heat, vacuum, and use of reactive elements such Se or S. Thus, transparent back contact materials utilized for superstrate configuration cannot necessarily be used for substrate configuration. 
     Semi-transparent back contact grids have been used along with a highly doped back semiconductor in Group II-VI solar cells materials (e.g., CdTe, ZnTe) in the superstrate configuration on heavy, rigid glass substrates for mechanically stacked four-terminal tandem TFPVs, as disclosed in a paper titled “Polycrystalline CdTe on CuInSe 2  Cascaded Solar Cells,” by P. Meyers, C. Liu, L. Russell, V. Ramanathan, R. Birkmire, B. McCandless and J. Phillips [Proc. 20 th  IEEE PVSC (1988), p 1448]. 
     Solar absorbing layers of typical CIGS TFPV devices are p-type and with back contacts formed by intimate connection to thick, opaque metals such as Mo, Ni, or Au that form a low resistance Schottky barrier contact. Thus, these typical high-performance back contacts do not enable visible light to pass through. 
     It has not been possible to fabricate TFPV devices with semi-transparent back contacts and acceptable performance where the TFPV device is based upon high-bandgap (greater than 1.4 eV) Group I-III-VI.sub.2 materials. For example, when standard semi-transparent TCOs are used without an interface layer as the back contact for wide-bandgap CuInGaSe 2  (CIGS) solar absorbing material, low efficiency (e.g., less than 4% efficient) devices result. However, the same back contact layer used with low-bandgap (less than 1.2eV) CIGS solar absorbing material produces high-efficiency devices (e.g., greater than 10% efficient). Thus, transparent back contact materials utilized for low-bandgap solar absorber materials cannot necessarily be used for wide-bandgap solar absorber materials, as needed for a transparent interconnect in monolithic two-terminal tandem devices. 
     Monolithic two-terminal tandem devices in the substrate configuration based on crystalline III-V materials and amorphous/microcrystalline Si have been fabricated and commercially sold. In the case of the crystalline III-V bottom cell, the tandem device is fabricated at temperatures greater than 700° C. using expensive deposition equipment for controlled crystal growth that is not amenable to very large area depositions. Thus this technology cannot be reasonably applied to relatively inexpensive large-area depositions using polycrystalline thin-films on low cost and/or flexible substrates. Furthermore, the transparent back contact design concepts of crystalline and amorphous/microcrystalline silicon devices such as tunnel junction interconnects cannot be readily transferred to devices based on polycrystalline CIS and related alloys. Such difficulty in transferring design concepts is due to difficulty in achieving tightly controlled spatial positioning required by tunnel junctions through doping and diffusion of impurities when in the presence of grain boundaries, which act as conduits for diffusion. In addition, very high levels of doping are difficult to achieve in CIS based alloy materials without also creating compensating defects. Thus, other device designs/structures may be preferred. 
     SUMMARY 
     A thin-film photovoltaic device includes a semi-transparent substrate for supporting the thin-film photovoltaic device. A semi-transparent back contact layer is disposed on the semi-transparent substrate. The semi-transparent back contact layer includes a semi-transparent contact layer disposed on the semi-transparent substrate, and a semi-transparent contact interface layer including a Cu(X)Te 2  material disposed on the semi-transparent contact layer. X is at least one of In, Ga, and Al. A solar absorber layer is disposed on the semi-transparent back contact layer, and the solar absorber layer includes one of a Group I-III-VI.sub.2 material and a Group II-VI material. A heterojunction partner layer disposed on the solar absorber layer, and a top contact layer is disposed on the heterojunction partner layer. 
     A thin-film photovoltaic device includes a semi-transparent substrate for supporting the thin-film photovoltaic device and a top contact layer disposed on the semi-transparent substrate. A heterojunction partner layer is disposed on the top contact layer, and a solar absorber layer is disposed on the heterojunction partner layer. The solar absorber layer includes one of a Group I-III-VI.sub.2 material and a Group II-VI material. A semi-transparent back contact layer is disposed on the solar absorber layer. The semi-transparent back contact layer includes a semi-transparent contact interface layer including a Cu(X)Te 2  material disposed on the solar absorber layer and a semi-transparent contact layer disposed on the semi-transparent contact interface layer. X is at least one of In, Ga, and Al. 
     A tandem thin-film photovoltaic device includes a substrate for supporting the device. A back contact layer is disposed on the substrate, and a bottom solar absorber layer is disposed on the back contact layer. A bottom heterojunction partner layer is disposed on the bottom solar absorber layer. A semi-transparent interconnect layer includes a semi-transparent contact layer disposed on the bottom heterojunction partner layer and a semi-transparent contact interface layer disposed on the semi-transparent contact layer. The semi-transparent contact interface layer includes a Cu(X)Te 2  material, where X is at least one of In, Ga, and Al. A top solar absorber layer is disposed on the semi-transparent interconnect layer, where the top solar absorber layer includes one of a Group I-III-VI.sub.2 material and a Group II-VI material. A top heterojunction partner layer is disposed on the top solar absorber layer, and a top contact layer is disposed on the top heterojunction partner layer. 
     A thin-film photovoltaic device includes a substrate for supporting the device. The substrate includes at least one of silicone, reinforced silicone, reinforced silicone resin, and silicone coated metal foil. A semi-transparent back contact layer is disposed on the substrate. The semi-transparent back contact layer includes a semi-transparent contact layer disposed on the substrate and a semi-transparent contact interface layer disposed on the semi-transparent contact layer. A solar absorber layer is disposed on the semi-transparent back contact layer, and a heterojunction partner layer is disposed on the solar absorber layer. A top contact layer is disposed on the heterojunction partner layer. 
     A thin-film photovoltaic device includes a semi-transparent silicone substrate for supporting the device. A top contact layer is disposed on the semi-transparent silicone substrate, and a heterojunction partner layer is disposed on the top contact layer. A solar absorber layer is disposed on the heterojunction partner layer, and a semi-transparent back contact layer is disposed on the solar absorber layer. The semi-transparent back contact layer includes a semi-transparent contact interface layer disposed on the solar absorber layer and a semi-transparent contact layer disposed on the semi-transparent contact interface layer. 
     A thin-film photovoltaic device includes a semi-transparent substrate for supporting the thin-film photovoltaic device, a semi-transparent back contact layer disposed on the semi-transparent substrate, a solar absorber layer disposed on a semi-transparent contact interface layer of the semi-transparent back contact layer, a heterojunction partner layer disposed on the solar absorber layer, and a top contact layer disposed on the heterojunction partner layer. The semi-transparent back contact layer includes (a) a semi-transparent contact layer disposed on the semi-transparent substrate, (b) a defect interface layer disposed on the semi-transparent contact layer, and (c) the semi-transparent contact interface layer, which includes a Cu(X)Te 2  material disposed on the defect interface layer, wherein X is at least one of In, Ga, and Al. The defect interface layer has a bandgap of less than 1.2 eV and is formed of a material selected from the group consisting of InTe, SnTe, InSnTe, and MoTe. The solar absorber layer includes one of a p-type Group I-III-VI.sub.2 material and a p-type Group II-VI material. The solar absorber layer has a different elemental composition from the semi-transparent contact interface layer. 
     A thin-film photovoltaic device includes a semi-transparent substrate for supporting the thin-film photovoltaic device, a top contact layer disposed on the semi-transparent substrate, a heterojunction partner layer disposed on the top contact layer, a solar absorber layer disposed on the heterojunction partner layer, and a semi-transparent back contact layer disposed on the solar absorber layer. The solar absorber layer includes one of a p-type Group I-III-VI.sub.2 material and a p-type Group II-VI material. The semi-transparent back contact layer includes (a) a semi-transparent contact interface layer including a Cu(X)Te 2  material disposed on the solar absorber layer, wherein X is at least one of In, Ga, and Al, (b) a defect interface layer disposed on the semi-transparent contact interface layer, and (c) a semi-transparent contact layer disposed on the defect interface layer. The semi-transparent contact interface layer has a different elemental composition from the solar absorber layer. The defect interface layer has a bandgap of less than 1.2 eV and is formed of a material selected from the group consisting of InTe, SnTe, InSnTe, and MoTe. 
     A process for forming a thin-film photovoltaic device includes the steps of (a) depositing a semi-transparent contact layer onto a semi-transparent substrate, (b) depositing a semi-transparent contact interface layer including a Cu(X)Te 2  material onto the semi-transparent contact layer, wherein X is at least one of In, Ga, and Al, such that Te of the semi-transparent contact interface layer interacts with one or more elements of the semi-transparent contact layer to form a defect interface layer, (c) depositing a solar absorber layer onto the semi-transparent contact interface layer, (d) depositing a heterojunction partner layer onto the solar absorber layer, and (e) depositing a top contact layer onto the heterojunction partner layer. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a cross-sectional schematic view of a structure forming one TFPV device in substrate configuration, in accordance with an embodiment. 
         FIG. 2  shows a cross-sectional schematic view of one monolithic tandem TFPV device, in accordance with an embodiment. 
         FIG. 3  shows a typical schematic spectral response of the TFPV device of  FIG. 2 . 
         FIG. 4  shows a cross-sectional schematic view of a structure forming one single junction flexible TFPV device capable of bifacial light collection, in accordance with an embodiment. 
         FIG. 5  shows a flowchart illustrating one process for fabricating a TFPV device in substrate configuration, in accordance with an embodiment. 
         FIG. 6  shows a flowchart illustrating one process for fabricating a TFPV device in superstrate configuration, in accordance with an embodiment. 
         FIG. 7  shows a cross-sectional schematic view of materials forming one TFPV device in superstrate configuration, in accordance with an embodiment. 
         FIG. 8  shows a cross-sectional schematic view of a structure forming one TFPV device in substrate configuration and including a defect interface layer, in accordance with an embodiment. 
         FIG. 9  shows a flowchart illustrating one process for fabricating a TFPV device in substrate configuration and including a defect interface layer, in accordance with an embodiment. 
         FIG. 10  shows a flowchart illustrating another process for fabricating a TFPV device in substrate configuration and including a defect interface layer, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure may be understood by reference to the following detailed description taken in conjunction with the drawings briefly described below. It is noted that, for purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale. 
     TFPV solar cells based upon Group II-VI and Group I-III-VI.sub.2 light-absorbing materials may provide low cost photovoltaic technology and may be deposited on lightweight and flexible substrates for high efficiency (W/m 2 ) and specific power (W/Kg) characteristics. A semi-transparent back contact with a high-temperature, lightweight and flexible semi-transparent substrate may enable higher efficiency TFPV devices, as compared to prior art devices, due to bifacial collection (e.g., above bandgap transmission into the device) and infrared (IR) light transmission out of the device (e.g., sub-bandgap). 
     A process is thus disclosed for forming a relatively low resistance semi-transparent back contact between p-type Group I-III-VI.sub.2 or Group II-VI solar absorbers and a Transparent Conducting Oxide (TCO), which can transmit sub-bandgap light and/or enable bifacial operation. This leads to a low cost, power efficient tandem TFPV device using a semi-transparent back contact formed by the process. Furthermore, a low cost, power efficient single junction TFPV device for bifacial operation may be produced using the semi-transparent back contact in conjunction with a semi-transparent, lightweight, and flexible substrate. Other features and advantages will become apparent in the following detailed description. 
       FIG. 1  shows a cross-sectional schematic view of a TFPV device  100  that is fabricated with Group I-III-VI.sub.2 (e.g., CuInGaSe 2 ), Group II-VI (e.g., CdSe), or Group III-V (e.g., GaAs) solar absorber materials 113 and a semi-transparent back contact  118 , which is semi-transparent to at least infrared light. TFPV device  100  also has a substrate  116 , a heterojunction partner layer  112  (sometimes referred to as a window layer), an optional buffer layer  119  and a top contact  111 . Back contact  118  is shown with two layers: a semi-transparent contact interface  114 ; and a semi-transparent contact  115 . Semi-transparent contact interface  114  and semi-transparent contact  115  are at least partially transparent to infrared light. In embodiments, semi-transparent contact interface  114  and semi-transparent contact  115  are also semi-transparent to visible light. However, semi-transparent contact interface  114  may be omitted for low bandgap Group I-III-VI.sub.2 solar absorber based bifacial devices where a high temperature and flexible substrate is used. Optional buffer layer  119  may, for example, represent an optional ‘buffer’ layer of insulating ZnO employed in many CIGS devices. 
     As shown, TFPV device  100  is formed in a substrate configuration with substrate  116  located below semi-transparent back contact  118  (relative to the direction of primary light incidence  120  upon a top surface  117  of TFPV device  100 ). However, a superstrate configuration may be provided for optically transparent substrates, by instead locating substrate  116  above top contact  111 , relative to the direction of primary light incidence  120 , in accord with another embodiment and without departing from the scope hereof. Substrate  116  may be rigid or flexible. Substrate  116  may, for example, be formed of at least one of glass, silicone, silicone resin, reinforced silicone, reinforced silicone resin, and high temperature polyimide. 
     Semi-transparent back contact layer  118  and substrate  116  may transmit sub-bandgap light  122  away from TFPV device  100  when the substrate is semi-transparent to sub-bandgap light, thereby reducing the operating temperature of TFPV device  100 . Semi-transparent back contact layer  118  and substrate  116  may also transmit above-bandgap light into the TFPV device  100 , thereby enabling bifacial light collection or absorption. Both of these mechanisms can increase the device operating efficiency (W/m 2 ), power output (W/kg), and output voltage (Voltage/cell). 
     Layers  111 ,  112 ,  113 ,  114  and  115  of TFPV device  100  may be used as a top cell of a tandem device (e.g., tandem device  200 ,  FIG. 2 ). 
     The electro-optical properties of CuInTe 2  and CuAlTe 2  have been considered for use in TFPV devices. More specifically, the electron affinity of CuInTe 2  is about 0.5 eV less than CuInSe 2  (CIS), but the bandgap of the telluride remains about the same as the selenide. Under certain circumstances, and considering theoretical band energies, this indicates that the band-edge discontinuity between the valence band of the telluride and the work function level of traditional opaque Mo back contacts may be 0.5 eV less than that of discontinuity of the selenide and traditional opaque Mo back contacts, which may result in lower contact resistance (lower Schottky barrier height) for the telluride compared to the selenide. Moreover, the telluride has the potential for being p-type and degenerately doped than selenide, which would also aid in the formation of a low resistance contact. This reasoning was then applied to the wider bandgap alloys with aluminum or CuAlSe 2  and CuAlTe 2 . As a semi-transparent back contact interface layer  114 , CuAlTe 2  has good optical transparency in the wavelength region of interest since as it has a bandgap of 2.06 eV. The higher valence band-edge energy of the telluride may be responsible for the good contact with Mo and may also be helpful in forming contacts to TCOs. 
     Semi-transparent back contact  118  may, for example, be fabricated by depositing semi-contact interface  114  onto semi-transparent contact  115 . Semi-transparent contact interface  114  may, for example, be a wide-bandgap alloy of Cu(X)(Te) 2  where X═In, Ga, or Al, or any combination of these three elements that results in a lower resistance between semi-transparent contact  115  and solar absorber  113 . Such semi-transparent contact interface formed of a wide-bandgap alloy of Cu(X)(Te) 2  is sometimes referred to as a CIGAT contact interface. Ideally, the bandgap of the Cu(X)(Te) 2  semi-transparent contact interface  114  would also be selected to be much higher than the bandgap of the solar absorber  113  to maximize the transparency of above-bandgap light to the solar absorber during bifacial operation. Alternatively, the semi-transparent contact interface layer  114  may be a thin (less than 100 Å, for example) metal layer (e.g., Mo) that is semi-transparent due to the low thickness or incomplete coverage, and relatively inert to the solar absorber deposition process. Semi-transparent contact interface  114  may provide low absorption of light due to its low thickness and/or incomplete coverage and/or its wide-bandgap characteristic. Semi-transparent contact  115  may, for example, be conductive electrodes consisting of TCOs such as ZnO:Al, Indium Tin Oxide (ITO), or SnO 2 , or a similarly transparent conducting material such as Stannates, or transparent layers with carbon nanotubes. This process of forming a low resistance and semi-transparent back contact  118  to Group I-III-VI.sub.2 materials (e.g., solar absorber  113 ) for use in TFPV devices (e.g., TFPV device  100 ) allows sub-bandgap light to be transmitted through semi-transparent back contact  118 , while also enabling bifacial operation of TFPV device  100 . Other semi-transparent back contact interface layers  114  such as thin Cu, or Cu doped ZnTe, which are detailed in published papers or patents, may be appropriate for CdTe based solar absorbers  113 . 
     In one example of fabrication of TFPV device  100 , a semi-transparent current carrying transparent contact  115  is deposited onto substrate  116  by means of sputtering, chemical vapor deposition, evaporation, or other thin-film deposition technique. Semi-transparent contact interface  114 , which may be formed of CuAlTe 2 , may be likewise be deposited onto semi-transparent contact  115  by means of sputtering, chemical vapor deposition, evaporation, or other thin-film deposition techniques. 
     A Group I-III-VI.sub.2 p-type material (e.g., solar absorber  113 ) may be deposited onto semi-transparent back contact  118  with an optional n-type semiconductor surface layer. Solar absorber  113  may optionally have a near surface region that is n-type. Deposition of solar absorber  113  may, for example, be achieved by means of co-evaporation, thermal evaporation, spraying, printing, or other thin-film deposition techniques and may contain selenides, sulfides, and tellurides of Cu, Ag, Al, Ga, In, Tl, and their alloys. In one example, solar absorber  113  may be a variation of Cu(In,Ga,Al)(Se,S) 2  such as CIGS. 
     A heterojunction partner layer  112  may be deposited by chemical bath deposition (CBD), chemical vapor deposition, sputtering, or other known techniques. Heterojunction partner layer  112  is, for example, CdS, ZnS, (Cd, Zn)S, ZnSe, ZnO, or SnO 2 . 
     Buffer Layer  119 , if included, may, for example, be deposited by chemical bath deposition (CBD), chemical vapor deposition, sputtering, or other technique. 
     Top contact layer  111  may be deposited onto heterojunction partner layer  112  or buffer layer  119  and may be mostly transparent to the solar spectrum. In one example, top contact layer  111  is a TCO (e.g., ITO or doped ZnO). A semi-transparent current carrying transparent contact  111  is deposited onto the buffer or heterojunction partner layer by means of sputtering, chemical vapor deposition or other thin-film deposition technique. 
       FIG. 2  shows a cross-sectional schematic view of a monolithic tandem device  200  having a top cell  218  containing a wide-bandgap solar absorber layer  212  and a bottom cell  220  containing a low-bandgap solar absorber  206  that are joined by a semi-transparent interconnect layer  210 . Top cell  218  is, for example, similar to TFPV device  100  in  FIG. 1 . Since top cell  218  may transmit sub-bandgap light, bottom cell  220  may be designed and fabricated to absorb and convert this light to electricity. Tandem device  200  may, for example, be based on thin-film high-efficiency and low cost CIS and related alloys that provide a higher efficiency (W/m 2 ), specific power (W/kg), and Voltage (Voltage/cell) than do existing single-junction CIGS devices. In addition, tandem device  200  may also have a lower cost per unit power output. In one embodiment, tandem device  200  is flexible (e.g., by using flexible material layers and substrate). In another embodiment, tandem device  200  is rigid (e.g., glass substrate or other rigid substrate). 
     Tandem device  200  is shown with a substrate  202 , a back contact  204 , a bottom solar absorber  206 , a bottom heterojunction partner layer  208  (sometimes referred to as a window layer), an optional buffer layer  222 , a semi-transparent contact  213 , a semi-transparent contact interface  211 , a top solar absorber  212 , a top heterojunction partner layer  214 , an optional buffer layer  219 , and a top contact  216 . Tandem device  200  may, for example, utilize high-efficiency, low cost, and easy to manufacture Group I-III-VI.sub.2 solar absorber materials, due to the ability for bandgap engineering. An efficient wide-bandgap top solar absorber  212  is desirable for top cell  218  as it is beneficial to the performance of good tandem devices. Top cell  218  is formed upon semi-transparent interconnect  210  (i.e., semi-transparent interconnect  210  functions as a back contact for top cell  218 ). Semi-transparent interconnect  210  may, for example, be similar to back contact  118  of  FIG. 1 , and is shown with a semi-transparent contact interface  211  and a semi-transparent contact  213 . Top contact  216 , top heterojunction partner layer  214 , top solar absorber  212 , and semi-transparent interconnect  210  form top cell  218 . TFPV device  100  of  FIG. 1  may be used (without substrate  116 ) as the top cell  218  of tandem device  200 . 
     Top cell  218  of tandem device  200  may be capable of transmitting unused above-bandgap light and sub-bandgap light to bottom cell  220 . Since absorption of light that is not converted to electricity may increase the operating temperature of top cell  218 , as it may with any single-junction device, allowing unused light to be transmitted to bottom cell  220  may increase the efficiency of top cell  218 . 
       FIG. 3  shows an illustration of an idealized spectral response graph  300  of TFPV device  200  of  FIG. 2 . A top cell spectral response curve  302  of top cell  218  has a spectral edge  306 . A bottom cell spectral response curve  304  of bottom cell  220  has a spectral edge  308 . Top cell spectral response  302  has a wide-bandgap, represented by spectral edge  306 , and bottom cell spectral response  304  has a smaller bandgap, represented by spectral edge  308  and at a higher light wavelength than the wide-bandgap spectral edge. 
       FIG. 4  shows a cross-sectional schematic view of a structure forming one exemplary embodiment of single junction flexible TFPV device  400  that is capable of bifacial light collection. TFPV device  400  is fabricated upon a semi-transparent substrate  402  and has a semi-transparent back contact  408  (which may, for example, be formed of a semi-transparent contact  404  and a semi-transparent contact interface  406  as shown), a solar absorber  410  (e.g., Group I-III-VI.sub.2, Group II-VI, Group III-V), a heterojunction partner layer  412  (sometimes referred to as a window layer), an optional buffer layer  413 , and a top contact  414 . Since semi-transparent back contact  408  components of TFPV device  400  are semi-transparent to visible and infrared light, bifacial light collection is possible, and heating that results from absorbing unused light is lowered. 
     The use of semi-transparent back contact  408  (also referred to herein as “transparent back contact  408 ” or “back contact  408 ”) and IR transparent substrate  402  may reduce the operating temperature of TFPV device  400 . The reduction in operating temperature may increase the operating efficiency (e.g., W/m 2 ), specific power output (W/kg), and output voltage (Voltage/cell) of TFPV device  400 . In addition, semi-transparent back contact  408  may enable bifacial operation of TFPV device  400  when substrate  402  is selected to be semi-transparent to above-bandgap light. In certain space or high altitude applications, bifacial operation may increase the output of TFPV device  400  by as much as 30%. 
     In  FIG. 4 , TFPV device  400  is shown in substrate configuration. However, layers  404 ,  406 ,  410 ,  412  and  414  may also be fabricated in a superstrate configuration without departing from the scope hereof. 
     Substrate  402  may be lightweight, flexible, and be formed of semi-transparent materials. For example, substrate  402  may comprise silicone or silicone resin. Substrate  402  may be reinforced, if necessary, as described in U.S. Provisional Patent Application Ser. No. 60/792,852 and in U.S. Non-Provisional patent application Ser. No. 11/737,119. Substrate  402  may be capable of withstanding processing temperatures up to 600° C., thereby enabling use of high-efficiency CIGS and CdTe materials in fabricating TFPV device  400 . 
     Semi-transparent contact  404  may be a semi-transparent current carrying TCO such as ZnO:Al, ITO, and SnO 2 , or a similarly transparent conducting materials such as Stannates or transparent layers with carbon nanotubes. Semi-transparent contact interface  406  may be a thin semi-transparent layer that is applied to semi-transparent contact  404  to lower resistance between solar absorber  410  and semi-transparent contact  404 . Semi-transparent contact  404  may be deposited by means of sputtering, chemical vapor deposition, or other thin-film deposition techniques. 
     In one embodiment, semi-transparent contact interface  406  is a discontinuous layer of metal, and/or is a very thin semi-transparent layer of metal, for use with CuInGaAlSe 2 (CIGAS) based solar absorbing material. In another embodiment, semi-transparent contact interface  406  is a wide-bandgap semiconductor fabricated from Cu(In,Ga,Al)(Te) 2  for use with CIGS based solar absorbers or from ZnTe for use with CdTe based solar absorbers. Semi-transparent contact interface  406  may, for example, be deposited by means of sputtering, chemical vapor deposition, co-evaporation, or other thin-film deposition techniques. 
     Solar absorber  410  may be a p-type semiconductor layer with an optional n-type semiconductor surface layer formed from the copper, or group I, deficient phase of the solar absorber. Deposition of solar absorber  410  may be by means of co-evaporation or other thin-film deposition techniques. 
     Heterojunction partner layer  412  may be deposited by chemical bath deposition (CBD) or other deposition techniques. The technique employed, however, should not damage the surface of solar absorber  410 . Heterojunction partner layer  412  may, for example, be an n-type semiconductor fabricated from either CdS, (Cd, Zn)S, ZnO, ZnOH, or ZnSe. 
     A top contact layer  414  makes contact with n-type heterojunction partner layer  412  and may be mostly transparent to the solar spectrum. In one embodiment, top contact layer  414  is a transparent ITO or ZnO:Al. Optionally, buffer layer  413  may be included between top contact layer  414  and heterojunction partner layer  412 . Buffer layer  413  may be a layer of insulating ZnO as found in many CIGS devices. 
     TFPV device  400  may be further fabricated using monolithic interconnection or by other means. 
     In one example of operation, incident light  418  (e.g., visible solar light or white light) primarily enters the top  416  of TFPV device  400  and passes through top contact layer  414 . Light  418  is then partially absorbed by heterojunction partner layer  412  and p-type solar absorber  410 . In particular, above-bandgap light is mostly absorbed by p-type solar absorber  410  and sub-bandgap light is mostly transmitted through solar absorber  410 , semi-transparent back contact  408 , and semi-transparent substrate  402 , and away from TFPV device  400  (shown as sub-bandgap light  420  ). Similarly, light  422  (e.g., reflected solar or white light) incident on the back of TFPV device  400  enters through semi-transparent substrate  402  and semi-transparent back contact  408 , and above-bandgap light is then absorbed by solar absorber  410  and heterojunction partner layer  412 . Below bandgap light  424  (i.e., unused light) is then mostly transmitted out through top contact  414 , and away from TFPV device  400 . 
       FIG. 5  is a flowchart illustrating one example of a process  500  of fabricating a bifacial TFPV device (e.g., TFPV device  400 ,  FIG. 4 ). A semi-transparent contact layer is deposited, in step  502 , onto a semi-transparent substrate. In one example of step  502 , semi-transparent contact layer  404  is ITO and is deposited onto semi-transparent substrate  402  made of silicone resin. A thin semi-transparent contact interface layer is deposited, in step  504 , onto the semi-transparent contact layer deposited in step  502 . In one example of step  504 , a semi-transparent contact interface layer  406  is a 1,000 Angstrom thick layer of CuAlTe 2 , with a Cu/Al ratio of 1, and is deposited onto semi-transparent contact layer  404 , which is ITO. A solar absorber layer is deposited, in step  506 , onto the semi-transparent contact interface layer deposited in step  504 . In one example of step  506 , solar absorber layer  410  is CuInGaAlSe 2 , and is deposited onto semi-transparent contact interface layer  406 , which is CuAlTe 2 . 
     In step  508 , a transparent heterojunction partner layer is deposited onto the solar absorber layer deposited in step  506 . In one example of step  508 , heterojunction partner layer  412 , is CdZnS and is deposited onto solar absorber layer  410 , which is CuInGaAlSe 2 . Step  509  is optional. In step  509 , a buffer layer is deposited onto the transparent heterojunction partner layer of step  508 . In one example of step  509 , buffer layer  413  is ZnO, and is deposited onto heterojunction partner layer  412 , which is CdZnS. A semi-transparent top contact layer is deposited, in step  510 , onto the transparent heterojunction partner layer of step  508  (or the buffer layer of step  509 , if included). In one example of step  510 , top contact layer  414  is ITO and is deposited onto heterojunction partner layer  412 , which is CdZnS. 
       FIG. 6  shows a flowchart illustrating one process  600  for fabricating a TFPV device in superstrate configuration.  FIG. 7  shows a cross-sectional schematic view of materials forming one example of a TFPV device  700  in superstrate configuration.  FIGS. 6 and 7  are best viewed together with the following description. 
     In step  602 , a semi-transparent top contact layer  714  is deposited onto a semi-transparent substrate layer  702 . In one example of step  602 , top contact layer  714  is a TCO, such as ITO, that is sputtered onto substrate  702 , which is made of silicone resin. Step  603  is optional. In step  603 , a buffer layer is deposited onto the semi-transparent contact layer of step  602 . In one example of step  603 , buffer layer  713  is ZnO, and is deposited onto top contact layer  714 , such as ITO. In step  604 , a transparent heterojunction partner layer  712  (sometimes referred to as a window layer) is deposited onto top contact layer  714  of step  602  or onto the buffer layer of step  603 . In one example of step  604 , heterojunction partner layer  712  is ZnO that is deposited onto top contact layer  714 , such as ITO, using a chemical bath deposition technique. 
     In step  606 , a solar absorber layer  710  is deposited onto heterojunction partner layer  712 . In one example of step  606 , solar absorber layer  710  is made of a Group I-III-VI.sub.2 p-type material, such as CIGS, which is deposited using a thermal evaporation technique onto heterojunction partner layer  712 . In another example of step  606 , a Group II-VI or a Group III-V material, such as CdSe, is deposited onto heterojunction partner layer  712 . In step  608 , a thin semi-transparent contact interface layer  706  is deposited onto solar absorber layer  710 . In one example of step  608 , a wide-bandgap telluride based Group I-III-VI.sub.2 p-type material, such as CuInGaAlTe 2 , is deposited onto solar absorber layer  710  using a sputtering technique. In step  610 , a semi-transparent contact layer  704  is deposited onto semi-transparent contact interface layer  706 . In one example of step  610 , a TCO, such as ITO, is deposited onto contact interface layer  706  using a sputter deposition technique. Layers  704  and  706  are illustratively shown together as back contact layer  708  in  FIG. 7 . 
     As appreciated, the CIGAT semi-transparent contact interface layer disclosed above may be used to improve contact to wide-bandgap solar absorbers in PV devices that have non-transparent back contacts without departing from the scope hereof. For example, such devices may have a CIGAT semi-transparent contact interface layer adjacent to a wide-bandgap solar absorber and have a thick opaque metal (e.g., Mo) adjacent to the CIGAT contact interface layer. Thus, TCO may not be needed as the back contact if bifacial operation is not required. The CIGAT semi-transparent contact interface layer may also be used with an opaque substrate (e.g., a silicone coated metal foil substrate) in embodiments of substrate configuration TFPV devices. 
     As appreciated, superstrate PV devices may use silicone or silicone resin (reinforced or not) as the substrate and may have non-transparent back contacts, without departing from the scope hereof. 
     Applicant has further discovered that a defect interface layer between the semi-transparent contact interface layer and the semi-transparent contact layer promotes high photovoltaic device efficiency. Any of the semi-transparent back contact layers or semi-transparent interconnect layers disclosed herein can be modified to include such defect interface layer. 
     For example,  FIG. 8  shows a cross-sectional schematic view of a TFPV device  800  including a defect interface layer to promote high photovoltaic device efficiency. TFPV device  800  is like TFPV device  100  of  FIG. 1  but includes a back contact layer  818  in place of back contact layer  118 . Back contact layer  818  includes a semi-transparent contact layer  815  disposed on substrate  116 , a defect interface layer  830  disposed on semi-transparent contact layer  815 , and a semi-transparent contact interface layer  814  disposed on defect interface layer  830 . Each of semi-transparent contact interface layer  814 , defect interface layer  830 , and semi-transparent contact layer  815  are at least partially transparent to infrared light, and in some embodiments, semi-transparent contact interface layer  814 , defect interface layer  830 , and semi-transparent contact layer  815  are also semi-transparent to visible light. Similar to semi-transparent contact interface layer  114  of  FIG. 1 , semi-transparent contact interface layer  814  is formed of a wide-bandgap alloy of Cu(X)(Te) 2  where X═In, Ga, or Al, or any combination of these three elements. Semi-transparent contact layer  815  is formed of a TCO, such as ITO, InO 2 , ZnInO, ZnSnO, or SnO 2 . 
     Applicant has further determined that defect interface layer  830  must be thin, to significantly promote efficiency of device  800 . In particular, a thickness  832  of defect interface layer  830  should be no more than 200 nm, though less than 10 nm is preferred. A thinner defect interface layer also promotes light transmission through this low-bandgap layer. Additionally, defect interface layer  830  must have a bandgap of less than 1.2 eV to positively impact efficiency of device  800 , though less than 1.0 eV is preferred to improve the electrical performance Applicant has determined that InTe, SnTe, InSnTe, or MoTe thin film of any stoichiometry that has a bandgap less than 1.2 eV (ex. In 2 Te 3 , SnTe 2 , SnTe, and MoTe 2 ) and is sufficiently defective is suitable for forming defect interface layer  830 . Defect interface layer  830  may form during the deposition of the Cu(X)Te layer on the transparent contact layer. Alternately, defect interface layer  830  may be explicitly deposited prior to the Cu(X)Te deposition, and in such case, defect interface layer  830  can react with the Cu(X)Te to form lower bandgap alloys of the Cu(X)Te (ex. CuInTe 2 ). Tellurium-based defect interface layers tend to have a lower bandgap than the Selenium-based materials, hence the advantage of the Cu(X)Te layer versus none (CIGS). In this document, a defect interface layer material is “sufficiently defective” if it has intra-bandgap defect states with energy levels at or near the middle of the bandgap that can act as effective recombination sites for electrons and holes for a given bandgap. Simulations show that for better performance, the intra-bandgap defect state concentrations in defect interface layer  830  should be higher if the bandgap of the defect layer is higher, or if the defect capture cross-section is smaller. 
     TABLE 1 below shows simulation results for many different defect interface layers in a device similar to photovoltaic device  800 , where (1) the solar absorber layer is formed of CIGS and has a bandgap of 1.4 eV and electron affinity of 4.15 eV, (2) the Cu(X)Te layer has a bandgap of 1.8 eV and electron affinity of 3.7 eV, and (3) the defect interface layer has properties as specified in the Table. The transparent back contact layer was modeled with a bandgap of 3.3 eV and electron affinity of 4.6 eV. As can be observed, the highest photovoltaic device efficiency values were generally obtained when the defect interface layer had low bandgap values. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Defect Interface layer (Na = 1e16, thickness = 10 nm) 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Donor 
                 Donor 
                 Donor 
                 Acceptor 
                 Acceptor 
                 Acceptor 
                   
               
               
                 electron 
                   
                 Defect 
                 Defect 
                 Defect 
                 Defect 
                 Defect 
                 Defect 
                 Device Parameters 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 affinity 
                 Bandgap 
                 Energy* 
                 Conc. 
                 Cap x-sec. 
                 Energy* 
                 Conc. 
                 Cap x-sec. 
                   
                   
                 Jsc 
                   
               
               
                 (eV) 
                 (eV) 
                 (eV) 
                 (cm−3) 
                 (cm−3) 
                 (eV) 
                 (cm−3) 
                 (cm−3) 
                 Eff. (%) 
                 Voc (mV) 
                 (mA/cm 2 ) 
                 FF 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 4.2 
                 0.8 
                 0.4 
                 4.E+16 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 9.2 
                 632 
                 26.8 
                 54.1 
               
               
                   
                 1 
                 0.5 
                 4.E+16 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 4.7 
                 476 
                 26.7 
                 37.2 
               
               
                   
                 1 
                 0.5 
                 4.E+16 
                 1.E−13 
                 0.5 
                 1.E+16 
                 1.E−13 
                 4.8 
                 494 
                 26.7 
                 36.3 
               
               
                   
                 1 
                 0.5 
                 4.E+17 
                 1.E−13 
                 0.5 
                 1.E+16 
                 1.E−13 
                 12.3 
                 739 
                 26.9 
                 61.7 
               
               
                   
                 1.2 
                 0.6 
                 4.E+16 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 1.3 
                 435 
                 24.4 
                 11.9 
               
               
                   
                 1.2 
                 0.6 
                 4.E+17 
                 1.E−13 
                 0.6 
                 1.E+16 
                 1.E−13 
                 8.5 
                 665 
                 26.8 
                 47.8 
               
               
                 4.4 
                 1 
                 0.5 
                 4.E+16 
                 1.E−13 
                 0.5 
                 1.E+16 
                 1.E−13 
                 11 
                 704 
                 26.9 
                 57.9 
               
               
                   
                 1 
                 0.5 
                 4.E+16 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 10.7 
                 693 
                 26.8 
                 57.5 
               
               
                   
                 1 
                 0.5 
                 4.E+17 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 12 
                 733 
                 26.9 
                 60.9 
               
               
                   
                 1 
                 0.5 
                 4.E+17 
                 1.E−15 
                 NA 
                 0.E+00 
                 NA 
                 8 
                 600 
                 26.8 
                 49.7 
               
               
                   
                 0.8 
                 0.4 
                 4.E+16 
                 1.E−13 
                 0.4 
                 1.E+15 
                 1.E−15 
                 14.4 
                 794 
                 26.9 
                 67.4 
               
               
                   
                 0.8 
                 0.4 
                 4.E+16 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 14.4 
                 794 
                 26.9 
                 67.4 
               
               
                   
                 0.6 
                 0.3 
                 4.E+16 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 15.8 
                 797 
                 26.9 
                 73.6 
               
               
                   
                 0.6 
                 0.3 
                 4.E+16 
                 1.E−15 
                 NA 
                 0.E+00 
                 NA 
                 11.7 
                 759 
                 26.9 
                 57.5 
               
               
                   
                 0.6 
                 0.3 
                 4.E+15 
                 1.E−15 
                 NA 
                 0.E+00 
                 NA 
                 9.8 
                 649 
                 26.8 
                 56.3 
               
               
                 4.6 
                 0.4 
                 0.3 
                 4.E+15 
                 1.E−15 
                 NA 
                 0.E+00 
                 NA 
                 15.4 
                 796 
                 26.9 
                 71.8 
               
               
                   
                 0.4 
                 0.3 
                 4.E+16 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 17.1 
                 798 
                 26.9 
                 79.5 
               
               
                   
                 0.4 
                 0.3 
                 1.E+14 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 15.6 
                 797 
                 26.9 
                 72.9 
               
               
                   
                 0.4 
                 0.3 
                 1.E+13 
                 1.E−13 
                 NA 
                 0.E+00 
                 NA 
                 13.7 
                 785 
                 26.9 
                 65 
               
               
                   
               
               
                 *From conduction band edge 
               
               
                 *From conduction band edge 
               
            
           
         
       
     
     Defect interface layer  830  is formed, for example, by reacting a thin layer of In, Sn, InSn, or Mo, in a Te ambient, or by co-depositing the material with Te at high substrate temperatures, or by co-depositing with Te by sputtering. In another example, InTe, SnTe, InSnTe, or MoTe could be deposited by sputtering or pulsed laser deposition. 
       FIG. 9  is a flowchart illustrating one example of a process  900  of fabricating a TFPV device including a defect interface layer (e.g., TFPV device  800 ,  FIG. 8 ). A semi-transparent contact layer is deposited, in step  902 , onto a semi-transparent substrate. In one example of step  902 , semi-transparent contact layer  815  is ITO and is deposited onto semi-transparent substrate  116  made of silicone resin. In step  903  a defect interface layer is deposited onto the semi-transparent contact layer deposited in step  902 . In one example of step  903 , one of In, Sn, InSn, or Mo is deposited onto semi-transparent contact layer  815  and is reacted in a Te ambient to form defect interface layer  830 . A thin semi-transparent contact interface layer is deposited, in step  904 , onto the defect interface layer formed in step  903 . In one example of step  904 , semi-transparent contact interface layer  814  is a 1,000 Angstrom thick layer of CuAlTe 2 , with a Cu/Al ratio of 1, and is deposited onto defect interface layer  830 . A solar absorber layer is deposited, in step  906 , onto the semi-transparent contact interface layer deposited in step  904 . In one example of step  906 , solar absorber layer  113  is CuInGaAlSe 2 , and is deposited onto semi-transparent contact interface layer  114 , which is CuAlTe 2 . 
     In step  908 , a transparent heterojunction partner layer is deposited onto the solar absorber layer deposited in step  906 . In one example of step  908 , heterojunction partner layer  112 , is CdZnS and is deposited onto solar absorber layer  113 , which is CuInGaAlSe 2 . Step  909  is optional. In step  909 , a buffer layer is deposited onto the transparent heterojunction partner layer of step  908 . In one example of step  909 , buffer layer  119  is ZnO, and is deposited onto heterojunction partner layer  112 , which is CdZnS. A semi-transparent top contact layer is deposited, in step  910 , onto the transparent heterojunction partner layer of step  908  (or the buffer layer of step  909 , if included). In one example of step  910 , top contact layer  111  is ITO and is deposited onto heterojunction partner layer  112 , which is CdZnS. 
     Applicant has further determined that in some cases it may be possible to form defect interface layer  830  in conjunction with deposition of semi-transparent contact interface layer  814 , thereby preventing the need to separately form defect interface layer  830 . In particular, Te from semi-transparent contact interface layer  814  may react with one or more elements of semi-transparent contact layer  815 , such as In, Sn, or InSn of semi-transparent contact layer  815 , or with a thin Mo layer (not shown) formed on semi-transparent contact layer  815 , to form defect interface layer  830  during deposition of semi-transparent contact interface layer  814 . For example, in a particular embodiment, semi-transparent contact layer  815  is formed of ITO, and defect interface layer  830  having an InTe composition is formed during deposition of semi-transparent contact interface layer  814  from a reaction Te of semi-transparent contact interface layer  814  with In of semi-transparent contact layer  815 . 
       FIG. 10  is a flowchart illustrating one example of a process  1000  of fabricating a TFPV device including a defect interface layer (e.g., TFPV device  800 ,  FIG. 8 ) without an explicit step for forming the defect interface layer. Process  1000  is like process  900  of  FIG. 9 , but with step  903  omitted and with step  904  replaced by step  1004 . Steps  902  and  906 - 910  are the same as described with respect to  FIG. 9 , and their description is not repeated. In step  1004 , a thin semi-transparent contact interface layer is deposited onto the semi-transparent contact layer deposited in step  902 , and Te of the semi-transparent contact interface layer reacts with one or more elements of the semi-transparent contact layer to form a defect interface layer between the semi-transparent contact interface layer and the semi-transparent contact layer. In one example of step  1004 , a semi-transparent contact interface layer  814  is a 1,000 Angstrom thick layer of CuAlTe 2 , with a Cu/Al ratio of 1, and is deposited onto semi-transparent contact layer  815 . The Te of semi-transparent contact interface layer  814  reacts with one or more elements of semi-transparent contact layer  815  to form defect interface layer  830 , in step  1004 . 
     Each of semi-transparent interconnect layer  210  ( FIG. 2 ), semi-transparent back contact  408  ( FIG. 4 ), and semi-transparent back contact  708  ( FIG. 7 ) may also be modified to include a defect interface layer similar to defect interface layer  830  of  FIG. 8 . For example, semi-transparent interconnect layer  210  of  FIG. 2  could be modified to include a defect interface layer disposed between semi-transparent contact interface layer  211  and semi-transparent contact layer  213 , where the defect interface layer is formed of one of InTe, SnTe, InSnTe, or MoTe. 
     Experimental Results 
     Bifacial light collection testing was performed on TFPV device  100  or  400  to determine if bifacial light collection occurs. AM 1.5 light (100 mW/cm 2 ) from a solar simulator was incident on top of each device, while a halogen lamp light source, also calibrated to 100 mW/cm 2 , was directed towards the substrate side of each device. Tests performed upon a CuInGaAlSe 2 (CIGAS) device with bandgap in the range 1.3 to 1.4 eV and fabricated on a glass substrate (substrate configuration) and using a semi-transparent back contact consisting of approximately 40 angstrom thick molybdenum contact interface layer on an ITO contact layer, showed that an additional 25% power may be achieved through bifacial operation (due to a 14% increase in current and an improved fill factor) of the device. Transparency measurements of semi-transparent back contact  408  on a glass substrate show a 40-60% transparency to visible light, indicating that there are additional reflectance losses and current collection losses within the device since only 25% increase in power output is observed. Nonetheless, during operation in space, if about 30% of the space solar intensity were available to the back of the CIGAS device (e.g., via albedo light), then an increase in output power of over 8% (i.e., 30% of 25%) may be expected from this prototype device. Bifacial light collection testing of a CIGAS device fabricated upon a lightweight and flexible silicone substrate, however, indicated only a slight increase in performance It was determined that only a small fraction of light was passing through back contact  408  and substrate  402 , possibly due to the thin metallic contact interface layer being too thick on the device tested. Thickness of semi-transparent contact interface  406  may have a large effect on light transmission through back contact  408 . Nonetheless, enhanced performance with bifacial collection was demonstrated. 
     A comparison of non-bifacial device performance (Efficiency and Open-circuit voltage) was performed on devices using lightweight and flexible silicone substrates and devices with similar composition on glass substrates for both opaque Mo back contacts and semi-transparent back contacts (bifacial capable) made from a very thin Mo contact interface layer followed by an ITO transparent contact. The device was in the substrate configuration and the solar absorber was low-bandgap CIGS. The non-bifacial testing results showed that the devices on the lightweight and flexible silicone substrates are capable of performing as well as devices on thick glass substrates, and likely even better than thick glass substrates if comparing just the results with semi-transparent back contacts. Device efficiencies over 11% were demonstrated with the traditional opaque Mo back contacts. Thus, lightweight and flexible substrates are compatible with the devices described herein and do not limit the CIGAS deposition temperature which leads to higher performance devices. 
     In another comparison of non-bifacial device performance in the substrate configuration, two semi-transparent contact interface layers were compared: a thin Mo layer, and a thin CuAlTe 2  layer. The solar absorbers were high-bandgap CIGAS with [Ga+Al]/III ratios in the 54 to 62 range, typically corresponding to bandgaps in the range of about 1.55-1.65 eV. The semi-transparent contact layer for these devices of this comparison was fluorine-doped SnO 2 , which together with the semi-transparent contact interface layer and glass substrate, make these devices capable of bifacial collection. The 1.16 cm2 devices were illuminated from the top (through the top contact) with AM 1.5 light (100 mW/cm 2 ) from a solar simulator and current-voltage measurements were performed. Three devices of each type were compared and the devices with the CuAlTe 2  interface layers were identified as 70816-1A-C3, 70822-1E-E3, and 70822-2A-05. The devices with the thin Mo interface layers were identified as 70816-1E-E3, 70822-1A-C3, and 70822-2E-E3. The photovoltaic device series resistance in the range of 2 to 2.5 volts was used as the measure of back contact electrical performance, as in this voltage range other device effects are minimized The series resistance for the device with the CuAlTe 2  interface layer was found to be 26.3, 5.8, and 19.2 ohm-cm 2 for an average of 17.1 ohm-cm 2 . The series resistance for the device with the thin Mo interface layer was found to be 6.1, 35.7, and 25.0 ohm-cm 2  for an average of 22.3 ohm-cm 2 . These values are typical for the high-bandgap solar absorbers. As discussed above, the thin Mo contact interface layers in combination with the TCO contact layer have already shown comparable performance to thick opaque Mo layers when using high-bandgap solar absorbers. However the usable thin Mo contact interface layers have also shown to have limited visible light transmission, in the range of 60-70%, prior to the CIGAS deposition. Given that the CuAlTe 2  is a thin high-bandgap semiconductor, then the absorption loss through this layer is negligible compared to the solar absorber itself. 
     Changes may be made in the above systems and processes without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present process and system, which, as a matter of language, might be said to fall there between.