Transparent photovoltaic module

Abstract: A thin film photovoltaic module has transparent elec­ trodes on front and back surfaces of a semiconductive layer. In one embodiment, both transparent electrodes pass light to the semiconductive layer for photovoltaic purposes. In another embodiment, the back electrode passes light which enters through the front electrode and is not absorbed by the semiconductive layer, so that the module is selectively trans­ parent to the unabsorbed light. The spectral makeup of the transmitted light is controlled by controlling the optical char­ acteristics of the layers of the module. The index of refraction of the back electrode is a primary source of transmission control along with the refractive index of the front electrode and the thickness of the semiconductive layer itself. Light transmitted by the module can be used for nonphotovoltaic purposes, such as illuminating the interior of a vehicle or dwelling, or may be directed back into the semiconductive layer by a reflective surface behind the back electrode.

Description: The present invention relates generally to the photovoltaic art and, more particularly, to a thin film photovoltaic module having transparent front and back electrode arrangements. Photovoltaic cells of the prior art have typically been opaque, extending from a first, light-incident surface having a transparent conductive electrode to a second, back surface covered by a metallic electrode film. Metallic back electrodes have been favored because they are both conductive and reflective, enhancing collection efficiency of the device and reflecting unused light back through the semiconductive layer for use in generating electricity. A significant problem associated with photovoltaic panels is the space required to locate them in full view of the sun. If an opaque module casts a shadow on another module for part or all of a day, the shaded area of the second module is effectively removed from the current generating system. One approach to solving the problem of module size and bulk is to form stacked modules in which one array of cells is positioned over another array of cells so that light passing unabsorbed through the upper array is absorbed by the lower array. United States Patent No. 4,461,922 discloses a stacked arrangement in which an upper thin film panel corresponding basically to the embodiment shown in Figures 1-3 of the present application has a pair of transparent electrodes for passing unused light to a lower panel for photovoltaic purposes. However, such modules are more expensive to manufacture than single panels and are opaque to incident light. Moreover, specific back electrode structures attenuating transmission of infrared light are not disclosed. A "bifacial" array able to receive solar energy at opposite surfaces thereof is disclosed in Cuevas, et al., "50 Percent More Output Power From An Albedo-Collecting Flat Panel Using Bifacial Solar Cells", Solar Energy, 29 -5, pages 419-420 (1982). The array disclosed therein comprises a plurality of p -type crystalline substrates with diffused regions on opposite major surfaces thereof. Each substrate forms a pair of independent solar cells able to receive light from opposite directions. The cells are made according to costly crystalline silicon technology, including the use of grid-type contacts, and absorb all radiation that reaches the semiconductive material from either direction. Thin film cells having two transparent contacts are described in Konagai, et al., "The Effect of Residual Impurity B or P on Photovoltaic Properties of Amorphous Silicon Solar Cells", 16th IEEE Photovoltaic Specialists Conference, San Diego, September 27-30, 1982, but only as an experimental expedient for examining the generation of carriers by independent illumination of the cell either through its p -layer or its n -layer. The article does not propose a practical use for a transparent cell and the two sides of the cells are not illuminated simultaneously. Therefore, it is desirable in many applications to provide an inexpensive solar module which alleviates problems associated with cell positioning and makes better use of light over the solar spectrum. This problem is solved by a photovoltaic module according to one of claims 1 to 3. Further embodiments of the invention are disclosed in the subclaims. A photovoltaic module according to the present invention includes: a thin film semiconductive layer containing at least one photovoltaic region and having first and second oppositely directed major surfaces; first transparent electrode means for establishing electrical connection to the first surface of the semiconductive layer and for transmitting incident light to the semiconductive layer over a portion of the solar spectrum; and second transparent electrode means for establishing electrical connection to the second surface of the semiconductive layer and for optically coupling the semiconductive layer to the surrounding atmosphere to permit transmission by the module of a portion of the light transmitted by the first electrode means and not absorbed by the semiconductive layer; wherein the second electrode means is formed to attenuate transmission of infrared light and comprises an even number of layers alternating between a high reflection metal layer having a thickness between 5 and 20 nm and a metal oxide layer having a thickness between 40 and 100 nm, an index of refraction less than 1.7 and an extinction coefficient greater than 0.05 over most of the infrared spectrum, with the or one of the high reflection metal layers being directly adjacent the semiconductive layer. An other preferred embodiment of the invention comprises a solar module including: a thin film semiconductive layer containing at least one photovoltaic region and having first and second oppositely directed major surfaces; first transparent electrode means for establishing electrical connection to the first surface of the semiconductive layer and for transmitting incident light to the semiconductive layer over a portion of the solar spectrum; and second transparent electrode means for establishing electrical connection to the second surface of the semiconductive layer and for optically coupling the semiconductive layer to the surrounding atmosphere to permit transmission by the module of a portion of the light transmitted by the first electrode means and not absorbed by the semiconductive layer; wherein the second electrode means is formed to attenuate transmission of infrared light and comprises a thin film composite layer having, in order, a first transparent conductive oxide layer in electrical contact with the semiconductive layer; a first barrier layer for chemical isolation; a central metallic layer having a thickness between 5 and 20 nm and consisting preferably of Ag, a second barrier layer for chemical isolation and a second transparent conductive oxide layer, the transparent conductive oxide layers being preferably of doped zinc oxide and having a thickness between 50 and 150 nm and the barrier layers having a thickness of no more than 10 nm and preferably of approximately 5 nm and consisting preferably of TiO 2 . An other preferred embodiment of the invention comprises a solar module including: a thin film semiconductive layer containing at least one photovoltaic region and having first and second oppositely directed major surfaces; first transparent electrode means for establishing electrical connection to the first surface of the semiconductive layer and for transmitting incident light to the semiconductive layer over a portion of the solar spectrum; and second transparent electrode means for establishing electrical connection to the second surface of the semiconductive layer and for optically coupling the semiconductive layer to the surrounding atmosphere to permit transmission by the module of a portion of the light transmitted by the first electrode means and not absorbed by the semiconductive layer; wherein the second electrode means is formed to attenuate transmission of infrared light and comprises a symmetrical stack formed of a pair of doped zinc oxide (ZnO) layers and a silver (Ag) layer, with ZnO layers on both outer surfaces of the stack, the silver layer having a thickness of 5 - 20 nm, the ZnO layers having a thickness of 40 - 100 nm and being doped with a group III element or hydrogen. In a preferred emobidment, the electrodes can be fabricated to maximize transmission of light by the module and to selectively attenuate transmission of the infrared component of the light. The latter objective may be accomplished by careful control of the optical properties of all layers of the module, such as their indicies of refraction, transparencies, extinction coefficients, physical thicknesses, texture and band gaps. At the same time, the first electrode, the semiconductive layer and the second electrode are fabricated to maximize transmission of visible light. Because the modules of the present invention are transparent to light over part of the solar spectrum, they can be used in place of conventional glass so that the transmitted light serves a separate, nonphotovoltaic purpose. Examples of such uses include windows, skylights and vehicle sunroofs, in which it is desired to transmit visible light without the heat associated with infrared light. The spectral response of the semiconductive layer and the refractive characteristics of its transparent electrodes can be adjusted to "tune" the module to the needs of the application. Accordingly, the module performs the photovoltaic function of current generation as well as the filtering function of a window having a refractive coating. Because the module is used in place of conventional glass, placement and storage is not a problem and the cost of the module is at least partially offset by the alternative cost of glass with a comparable coating. The above and other features of the present invention may be more fully understood from the following description, together with the accompanying drawings, wherein similar reference characters refer to similar elements throughout and in which: FIGURE 1 is a generalized perspective view of a transparent photovoltaic panel as basically described in US-A-4 461 922 and constructed in accordance with a preferred embodiment of the present invention; FIGURE 2 is a fragmentary exploded vertical sectional view taken along the line 2-2 of FIGURE 1; FIGURE 3 is a fragmentary vertical sectional view of the structure of FIGURE 2 in assembled form; FIGURE 4 is a fragmentary perspective view of the module of FIGURE 1 in use as a sunroof of an automobile; FIGURE 5 is a fragmentary perspective view of the module of FIGURE 1 in use as a transparent hatch cover of a boat; FIGURE 6 is a fragmentary side elevational view of the module of FIGURE 1 in use as a window of a building; FIGURE 7 is a fragmentary perspective view of the module of FIGURE 1 in use as a skylight; FIGURE 8 is a graphical representation of the refractive indices (n) and extinction coefficients (k) of zinc oxide and indium tin oxide as functions of the wavelength of light; FIGURE 9 is a graphical representation of relative light transmittance data generated by computer modeling of a thin film solar cell having zinc oxide electrodes and a comparable cell having indium tin oxide electrodes, over the solar spectrum; FIGURE 10 is a fragmentary vertical sectional view of a portion of a composite transparent electrode film prepared according to a preferred embodiment of the present invention and in contact with a portion of the semiconductive layer of the module; FIGURE 11 is a fragmentary vertical sectional view of a portion of another composite transparent electrode film prepared according to a preferred embodiment of the invention and in contact with a portion of the semiconductor layer of the module. The transparent photovoltaic module of the present invention, depicted generally in FIGURES 1-3 by the numeral 10, has a semiconductive layer 12 with a first (front) transparent electrode arrangement 14 at a front surface 15 and a second (back) transparent electrode arrangement 16 at a back surface 17. The module is deposited in thin film form on a substrate 18, which may be a suitable transparent glass or plastic material, so that the module is able to receive incident light (hv) from both above and below the module. The photovoltaic layer 12 absorbs a portion of the spectrum of light transmitted to it by either of the electrode arrangements and transmits the remainder through the module, subject to any filtering properties of the opposite electrode arrangement. The module 10 has a variety of applications, including use in place of panes of window glass having a refractive coating. Examples are illustrated in FIGURES 4-7, wherein the module takes the form of an automotive sunroof, a boat hatch, a window of an office building and a skylight, respectively. Such uses typically require that the infrared component of the incident light be filtered out by the pane while allowing most of the visible component to pass. A key attribute of the module 10 is its ability to be "tuned" to selectively transmit or filter light in different portions of the solar spectrum, depending upon application. The two electrode arrangements and the semiconductive layer can be tuned to filter out virtually all of the undesired infrared component in the applications of FIGURES 4-7. A parameter having a significant effect upon transmission of infrared light is the index of refraction of the layers of the module, and particularly the index of the back electrode arrangement from which light must pass to the surrounding air if it is to be transmitted by the module. However, the refractive index of a transparent conductor often varies widely over the solar spectrum. This phenomenon is illustrated graphically in FIGURE 8, wherein the upper line 20 represents the refractive index of substantially pure zinc oxide (no additives added) over the solar spectrum and the lower line 22 represents the refractive index of substantially pure indium tin oxide. Whereas the index of zinc oxide is approximately 1.7 or higher over the entire solar spectrum, the index of indium tin oxide drops off drastically in the infrared range (wavelengths of 0.7 µm and above). It is difficult for infrared radiation to escape outwardly from the back electrode arrangement at an index of refraction below approximately 1.7 µm, reducing the infrared transmission of the module. Thus, a module having a thin film of substantially pure indium tin oxide or a similar transparent material as one of its electrodes effectively filters out much of the infrared component of incident light. This is particularly true when the low index material is used in the back electrode, from which light passes outwardly to the surrounding atmosphere. The thin film photovoltaic module of the present invention may be constructed of monolithic material containing solar cells interconnected in various ways and deposited by various processing techniques, of which the module 10 is a preferred embodiment. It will, of course, be understood that the following description is given by way of example only and not limitation. Referring now specifically to FIGURE 1, the module 10 constructed according to a preferred embodiment of the present invention defines a plurality of elongated solar cells 30 connected electrically in series between a pair of external leads 32-32'. The cells are shaped as narrow strips connected along opposite longitudinal edges to minimize series resistance losses. Connection between cells is accomplished through an unpatterned active film of the solar panel, without interrupting the film. Current generated by the cells in response to incident light (hv) travels a very short distance within each cell electrode before passing to the opposite electrode of an adjacent cell. With reference now to FIGURE 2, the electrode arrangement 16 comprises a plurality of back electrode portions 34 formed of a transparent conductive layer 35 separated by a nonconducting spacing 36 and arranged to substantially overlie elongated photovoltaic regions of the thin film. The front electrode arrangement 14 includes a transparent conductive layer 38 and a series of thickened contact portions or "stitch bars" 40. The layer 38 is patterned to form a plurality of transparent electrode portions 42 separated by spacings 44 and substantially underlying the respective photovoltaic regions. The photovoltaic regions are thus effectively sandwiched between the back electrode portions 34 and the front electrode portions 42 for collection of current generated within the regions. In addition, each front electrode portion partially overlaps the back electrode portion of an adjacent photovoltaic region over a preselected area 46. A major feature of this construction is the provision of conductive paths substantially transversely through the active thin film 12 between each front electrode portion and the back electrode portion of an adjacent photovoltaic region. Interconnection is accomplished at the area of electrode overlap to connect the photovoltaic regions electrically in series without patterning or otherwise interrupting the film. In the embodiment as illustrated, the stitch bars 40 are tall enough and narrow enough, in comparison to the thin film 12, to electrically short through the film. The final configuration of the solar module 10 is illustrated best in FIGURE 3, wherein the stitch bars cause the subsequently applied film 12 to be distorted in a manner producing relatively thin regions 48 which are unable to withstand the cell voltage. It is advantageous that the stitch bars be as rough as possible at their upper surfaces to focus the applied electric field and further reduce the resistance of the regions 48. The stitch bars 40 are preferably approximately 25 µm thick, and the semiconductive thin film 12 and the patterned transparent conductive layers 35 and 38 are each approximately 500 nm (5,000 angstroms) thick. At the locations of the stitch bars 40, the film 12 is sandwiched between a pair of conductive elements and the stitch bars 40 are at least two times thicker than the thickest portion of the film 12. This produces an effective short circuit through the film in the area of each stitch bar, but does not short out the cells 30 because the film 12 has a very high sheet resistance. The sheet resistance virtually eliminates current within the plane of the film, leaving only substantially transverse currents developed within the photovoltaic regions and passed between the electrodes at the areas of overlap. Referring to FIGURES 2 and 3 in more detail, the various layers and electrode portions of the panel 10 are sequentially deposited on a major surface 50 of the substrate 18. The substrate is preferably glass or other suitable transparent material which is compatible with the materials of the stitch bars 40 and the transparent conductive layer 38. The stitch bars may be applied to the substrate either before or after the transparent conductive layer, by screen printing, electroplating, evaporation through a mask, or other techniques well known to the art. The material of the stitch bars may be silver, aluminum, nickel or other material providing a high quality contact. Although the stitch bars 40 are illustrated as grid lines or sections of grid lines passing through the preselected areas 46 of electrode overlap, it is not necessary that they be continuous or that they be lines at all. If the stitch bars are screen printed, they should be at least approximately 25 µm in height to operate satisfactorily. If evaporated, they should be at least 2 microns, and preferably 10 µm, in height. In each case, the aspect ratio (height divided by width) and the roughness of the stitch bars are the parameters responsible for the localized short circuit that permits the panel 60 to operate. The transparent conductive (TC) layer 38 is preferably deposited, in the first instance, as a continuous layer. ITO for example, may be deposited at approximately 300 degrees Celsius by vacuum evaporation of indium and tin in an oxygen atmosphere with glow discharge assistance. The glow discharge activates the oxygen to produce a high quality film. After deposition, the transparent conductive layer 38 is patterned using conventional techniques, such as laser scribing. In the case of the panel 10, the patterning operation entails removal of the transparent conductive layer along a series of parallel lines near the stitch bars 40, yielding the front electrode portions 42 separated by spacing 44. The front electrode portions are thus shaped as parallel strips corresponding generally in area to the cells 30 of FIGURE 1. However, the front electrode portions 42 and the cells 30 need not be shaped as strips, as long as each front electrode portion is provided with an interconnecting portion 52 which overlaps part of the back electrode portion of an adjacent photovoltaic region. Although the layer 38 can be applied either before or after the stitch bars 40, it is preferably not patterned until after the stitch bars have been applied. The stitch bars 40 then act as a guide for patterning the layer 38. The thickness of the transparent conductive layer 38 is selected to minimize reflection from the back surface of the layer and absorption of light by it. According to established optical principles, internal reflection losses within a transparent body are minimized when the thickness of the body is an odd multiple of one-quarter of the wavelength of incident light for the case in which light travels from a material of low refractive index to a material of high refractive index. For present purposes, the relevant wavelength is that at the peak of the spectral response of the photovoltaic material making up the film 12. The TC layer will also be chosen to be thermally compatible with the layer 12. As above noted, the thin film 12 may contain any suitable photovoltaic material defining a photojunction for conversion of light to electrical energy. In the preferred embodiment, the film 12 is thin film silicon-hydrogen alloy (TFS) having n +, i and p + layers 54, 56 and 58, respectively, as illustrated in FIGURE 2. In the case of TFS, the film 12 is deposited by conventional glow discharge techniques without patterning or masking. The film 12 extends continuously and completely across the stitch bars 40, the transparent conductive layer 38 and the spacings 44. The thickness of the TFS film 12, complete with all three layers of different conductivity types, is typically between approximately 100 and 1000 nm (1,000 and 10,000 angstroms). The thickness of the TFS in a particular application is dependent on the particular material used and on the desired level of light transmittance in the visible range. The back electrode arrangement 16 serves as the back contact of the cells 30 and is preferably a TC layer formed by following the same parameters as above-described. However, the material of the back electrode arrangement can be different, as required to meet the filtering requirements of a particular application. Although shorting through the film 12 can be achieved in many circumstances with the structure described above, it is sometimes desirable to enhance localized conduction through the thin film 12 by applying heat to the solar panel 10. It is usually desirable to apply heat only at localized areas within the preselected areas of overlap 46 by directing a laser beam 60 or other suitable source of localized heat onto the module. In the case in which the stitch bars 40 are relatively high, narrow and rough, such heating can cause dispersal of the back electrode material, and possibly the material of the stitch bars 40, through the thickness of the semiconductive film 12. The resulting composite region 62 is much more conductive than the bulk of the film and enhances interconnection of the cells. In constructing the monolithic module 10 by deposition on the substrate 18, a layer of transparent conductive material (TC) is deposited on the substrate 18 and subsequently on the semiconductive layer 12 after it is deposited on the first layer 38. The TC layer, which is found on both sides of the monolithic layer 12, may be formed of any transparent conductive material known to the art and having optical, electrical and mechanical properties compatible with the semiconductor material being utilized. For example, indium-tin-oxide (ITO), tin oxide (TO), indium oxide (IO), zinc oxide (ZnO) or other similar materials well known in the art may be utilized. By choosing and tailoring the properties of the TC layers, the voltages and optics of the system may be tailored in accordance with the particular semiconductor materials and the particular application for which the module 10 is intended. Compensational adjustments and inclusion of additives are useful in this regard. Another factor in choosing a particular TC layer is that its coefficient of thermal expansion be compatible with that of the monolithic thin film. This keeps the film from being stressed during heating through absorption of the solar light. Perhaps most importantly, by choosing the appropriate index of refraction, extinction coefficient, thickness, conductivity and band gap of the TC layer, one may control the effective filter characteristics of the TC layer; that is, to reflect all or part of the solar spectrum or to permit it to pass through the TC layer. The tailoring or "tuning" of the various layers of the module 10 to a particular application can be best understood by way of example. Referencing FIGURES 4-7, the embodiments shown are similar in that each transmits visible light to the interior of an associated structure while filtering out infrared light. The module 70 of FIGURE 4 is configured as a partially transparent sunroof of an automobile 72 which admits visible light to the interior of the automobile but filters out the infrared. This gives the occupant an open feeling without subjecting him or her to the heat of ordinary sunlight. At the same time, electrical power is generated to operate accessories or trickle charge a battery required to maintain the memory of an on-board computer. If it is desired to introduce light from the entire solar spectrum, the sunroof can be removed or slid rearwardly, as known in the art, depending upon its design. FIGURE 5 illustrates a similar module 74 configured as a partially transparent hatch of a sailboat 76, and FIGURES 6 and 7 illustrate window modules 78 of a building 80 and a skylight module 82 of a dwelling 84, respectively. In each case, the modules 70, 72, 78 and 82 generate electricity according to photovoltaic principles while transmitting useful visible light and filtering infrared light to help insulate a structure. In the embodiments of FIGURES 4-7, the goal of passing substantial visible light requires that the semiconductive layer 12 be thin and that the electrode arrangements 14 and 16 be transparent to visible light. At the same time, the infrared portion of the spectrum must be selectively decoupled from the incident light. That is most easily accomplished by forming the back electrode of a material having a refractive index below approximately 1.7 and an extinction coefficient of at least 0.05 over much of the infrared ranqe. Materials suitable for this purpose will typically have extinction coefficients varying from approximately zero to a maximum as high as 0.4 in the infrared range. When the back electrode has the specified index of refraction and extinction coefficient, a significant proportion of the infrared light passed by the semiconductive layer is blocked. Much of it is internally reflected by the outer surface of the electrode and transmitted back through the semiconductive layer. Substantially pure ITO has the characteristics described above, as indicated in the graph of FIGURE 8. Thus, the refractive index (n) of ITO at wavelengths over 0.7 µm decreases steadily from approximately 1.7 to less than 1.0, while the extinction coefficient (k) increases from approximately 0.02 to more than 0.2 within the same range. In fact, the extinction coefficient of ITO exceeds 0.05 at all wavelengths over approximately 0.9 µm. Thus, ITO is a prime candidate for selectively attenuating transmission of infrared light by the module of the present invention. The behavior of a thin film silicon-based solar module similar to the module 10 described above has been modeled on a computer. The model had a layer of ZnO as a front electrode (light-incident side) and a layer of ITO as a rear electrode (transmitting side), and was clad with glass on both sides. Both the front and rear electrodes of the model were 356 nm (3560 angstroms) thick, which thickness was chosen based on the known optical constants of the materials and the electrical requirements of using them as current collection electrodes. The semiconductive layer 12 was assumed to be a homogeneous layer 300 nm (3000 angstroms) thick of thin film silicon-hydrogen alloy having a band gap of 1.72 (E o ). For purposes of the model, the n and p -layers of the semiconductive material were assumed to have the same optical properties as the i -layer. This is justified because the n and p -layers are thin and their optical contributions are small, accounting for no more than small perturbations in the transmission of light. As in the case of any transparent module designed according to the teachings of the present invention, the thicknesses of all layers of the model were determined by weighing the tradeoffs between desired electrical and optical performance of the module. Thus, modules designed with different criteria in mind would typically have layer dimensions different from those of the model, but the dimensions would be arrived at by a similar performance analysis. The transmittance characteristics predicted by the model are represented by the lower curve 84 of FIGURE 9, in which transmittance by the module in the infrared region (wavelengths of 0.7 µm and above) is attenuated. Transmittance in the wavelength range 0.8 to 1.1 µm is approximately 50 percent, while that above 1.1 µm falls rapidly to approximately 10 percent. By contrast, the results of a similar model of a module having a pair of electrodes of substantially pure ZnO is illustrated by the curve 88 which shows a significantly higher transmittance of infrared light (80 to 100 percent between 0.8 and 1.2 µm). If desired, the infrared transmittance of the module having ITO electrodes can be reduced even further by modification of the electrode material. Techniques which produce an increase in the conductivity of ITO tend to create a corresponding decrease in its index of refraction. However, this attenuates the visible transmittance as well as the infrared transmittance of the material and must be carefully controlled. TO is also useful as a front or back electrode material in the embodiments of FIGURES 4-7 because it has an index of refraction of approximately 1.9. Much like ITO, the index of refraction of TO can be decreased by adding fluorine or antimony to tune the cell to filter out an undesired portion of the solar spectrum. In accordance with an aspect of the invention, suitable thin film composites of high reflectance metals and metal oxides, laid up in symmetrical stacks, serve as one or both of the transparent electrodes of the module 10. Such materials are conductive, somewhat transparent, and selectively reflect infrared light. A preferred form of such a composite is a zinc oxide (ZnO)/silver (Ag) stack having a total thickness of approximetly 50-200 nm (500-2000 angstroms) with ZnO films on both outer surfaces of the stack. For example, the stack may comprise a pair of ZnO layers approximately 40-100 nm (400-1000 angstroms) thick on either side of a 5-20 nm (50-200 angstrom) layer of Ag. The zinc oxide in contact with the semiconductor must be electrically conductive to permit low resistance contacts. Electrical conductivity of ZnO for this purpose can be enhanced by addition of group III elements or hydrogen. The preparation and use of such composites are described in greater detail in Fan, et al., "Transparent Heat Mirrors For Solar Energy Applications", Applied Optics, 15 -4, pages 1012-1017 (1976). The Fan, et al., article describes the use of such composites as selective light absorbers but does not propose use as a transparent conductive electrode of a solar cell. The light transmittance of a composite stack of the type described above can be controlled over the solar spectrum by choice of appropriate materials and thicknesses for the metal and metal oxide layers. In accordance with another aspect of the invention, a suitable electrode can be prepared by depositing the first metallic layer of the above-described stack in contact with the semiconductive layer, omitting the first metal oxide layer and the steps required to deposit it. The remaining layers are then unchanged. A two-layer stack embodying this concept is illustrated at 132 in FIGURE 10, wherein the layer directly adjacent to the semiconductive layer 12 is a thin metallic layer 134. The metal may be aluminum, although silver is preferred for its superior optical properties in the short wavelength region. The thickness of the metal layer typically ranges between 5 and 20 nm (50 and 200 angstroms). The next layer is a metal oxide layer 136 which is similar in make-up to the oxide layers of the composite stack discussed above and has optical properties chosen to tune the light transmittance of the cell. Thus, the layer 136 may be ITO having an index of refraction less than 1.7 and an extinction coefficient greater than 0.05 over most of the infrared spectrum when it is desired to filter out a large proportion of the infrared light. In that case, the thickness of the layer would typically be between 40 and 100 nm (400 and 1000 angstroms). In such two-layer stacks, the semiconductive layer acts as an integral part of the optical electrode element and as the material that generates photocurrent. The two layer assembly 132 of FIGURE 10 may be repeated a number of times to increase conductivity. With appropriate choices of layer materials and thicknesses, light transmission need not be significantly reduced. For maximum transmittance it is preferred that each oxide layer be an integral number of half wavelengths thick. A thin film composite in accordance with still a further aspect of the invention and suitable for use as a transparent electrode of the module 10 is shown at 110 in FIGURE 11. The stack 110 is in electrical contact with a surface of the semiconductive layer 12 for transmission of light therebetween, and is preferably symmetrical about a thin central layer 112 of Ag or other suitable metal approximately 5-20 nm (50 to 200 angstroms) thick. When the semiconductive layer 12 is a thin film silicon-containing layer, the silver layer is chemically isolated from it by a pair of TiO 2 layers 114 which may be approximately 5 nm (50 angstroms) thick and act as a barrier to diffusion and oxidation of the silver. The combination of the silver and TiO 2 layers is sandwiched between a pair of relatively thick transparent conductive (TC) layers 116. The TC layers 116 may be made of ZnO, ITO, TO or other material of suitable conductivity, transmissivity, index of refraction and extinction coefficient, and which is compatible with the silicon-containing material of the semiconductive layer 12. The preferred thickness of the TC layers 116 is approximately 90 nm (900 angstroms) in the case of ZnO or ITO, but may vary between approximately 50 and 200 nm (500 and 2000 angstroms). The conductivity, thickness and index of refraction of the material will, of course, be chosen to optimize cell output and to selectively transmit light within a preselected wavelength range. However, the composite structures described above will always reflect infrared light to some degree. In any of the above-described composite structures, the layer adjacent to the semiconductive layer 12 must make good electrical contact to the semiconductive layer if the cell is to operate. When the nearest layer is ZnO or ITO, its conductivity must be relatively high to make a good contact with thin film silicon-based materials. In the case of ZnO, this is done by including boron, aluminum or other suitable additive. Other parameters important to satisfactory operation of the embodiments of FIGURES 4-7 are the thickness and make-up of the semiconductive layer 12. In the case of thin film silicon (TFS) having an average band gap of at least approximately 1.7, the thickness can be anywhere between approximately 100 and 1000 nm (1000 angstroms and 10,000 angstroms), with transmittance of visible light being noticeably lower at thicknesses above 500 nm (5000 angstroms). The preferred range of thickness of a thin film silicon-based semiconductive layer for these purposes is 200-500 nm (2000 to 5000 angstroms) and the optimal thickness is believed to be approximately 350 nm (3500 angstroms). At that thickness, the material transmits a significant amount of visible light while maintaining a suitable electric field within the photoconductive region. The conductivity of ZnO can be increased, and its index of refraction decreased, by inclusion of suitable additives, such as boron, aluminum, gallium, indium or thallium. Thus, the relevant transmittance properties of the cell can be tuned by controlled addition of the listed elements to the ZnO electrode material. In addition, ZnO has a lower optical absorption edge than TO, causing it to filter out some of the blue light. A variety of other materials can be used for the front and/or back contacts of the modules, some having even higher indices of refraction than zinc oxide over the solar spectrum. One such material is cadmium sulphide (CdS), which has an index of refraction of approximately 2.1 to 2.5. However, when CdS is used in conjunction with a semiconductive layer of a thin film silicon-based material, it is necessary to dope the CdS with indium (In) or other suitable material to increase its conductivity to the point at which it makes good electrical contact to silicon. Because the utilization of incoming radiation to generate electricity is particularly important in the embodiment of FIGURE 11, it is desirable in most cases to maintain the thickness of the semiconductive layer at approximately 500 nm (5000 angstroms) or more. However, thickness is only one of many factors which determine the operating characteristics of the device, and must be weighed with the other factors to arrive at a preferred arrangement. Practical devices can be obtained at thicknesses of 100-1000 nm (1000 to 10,000 angstroms), or more. When the semiconductive layer 12 is of the p - i - n type, the p -type region is typically a "window layer" having a band gap high enough to be transparent to incoming light. Accordingly, virtually all light of the relevant wavelength range passes through the p -layer for absorption by the i -layer. In such cells, it is not necessary for the n -layer to be a window layer, as well. However, the transmissivity of the n -layer is significant because any absorption in the n -layer reduces transmitted light without contributing to the photovoltaic affect. To be suitably transparent for these purposes, a thin film silicon-type n -layer should have a band gap of at least approximately 1.85 electron volts. Such a band gap can be obtained by inclusion of carbon or other suitable additive within the n -layer. From the above, it can be seen that there has been provided a stand-alone transparent solar module which can be illuminated on both sides and is tunable in its light transmitting properties over the solar spectrum to perform a wide variety of functions. The controllable transparency of the module enables it to be substituted for glass in conventional applications or used in conjunction with other energy systems to reduce the obtrusiveness and space requirements usually associated with solar modules. While certain specific embodiments of the invention have been disclosed as typical, the invention is not limited to these particular forms, but rather is applicable broadly to all such variations as fall within the scope of the appended claims. As an example, the photovoltaic module of the present invention need not have a silicon-based semiconductive layer, but rather may incorporate any known photovoltaic material for which suitable contact materials exist. The compositions, thicknesses and preparation of the semiconductive layer and the transparent electrodes must then be adapted to that material.

Claims: A photovoltaic module (10) comprising a thin film semiconductive layer (12) containing at least one photovoltaic region and having first and second oppositely directed major surfaces; first transparent electrode means for establishing electrical connection to the first surface of the semiconductive layer (12) and for transmitting incident light to the semiconductive layer over a portion of the solar spectrum; and second transparent electrode means (132) for establishing electrical connection to the second surface of the semiconductive layer (12) and for optically coupling the semiconductive layer to the surrounding atmosphere to permit transmission by the module of a portion of the light transmitted by the first electrode means and not absorbed by the semiconductive layer (12); wherein the second electrode means (132) is formed to attenuate transmission of infrared light and comprises an even number of layers alternating between a high reflection metal layer (134) having a thickness between 5 and 20 nm and a metal oxide layer (136) having a thickness between 40 and 100 nm, an index of refraction less than 1.7 and an extinction coefficient greater than 0.05 over most of the infrared spectrum, with the or one of the high reflection metal layers (134) being directly adjacent the semiconductive layer (12). A photovoltaic module (10) comprising a thin film semiconductive layer (12) containing at least one photovoltaic region and having first and second oppositely directed major surfaces; first transparent electrode means for establishing electrical connection to the first surface of the semiconductive layer (12) and for transmitting incident light to the semiconductive layer over a portion of the solar spectrum; and second transparent electrode means (110) for establishing electrical connection to the second surface of the semiconductive layer (12) and for optically coupling the semiconductive layer to the surrounding atmosphere to permit transmission by the module of a portion of the light transmitted by the first electrode means and not absorbed by the semiconductive layer (12); wherein the second electrode means (110) is formed to attenuate transmission of infrared light and comprises a thin film composite layer having, in order, a first transparent conductive oxide layer (116) in electrical contact with the semiconductive layer (12); a first barrier layer (114) for chemical isolation; a central metallic layer (112) having a thickness between 5 and 20 nm and consisting preferably of Ag, a second barrier layer (114) for chemical isolation and a second transparent conductive oxide layer (116), the transparent conductive oxide layers being preferably of doped zinc oxide and having a thickness between 50 and 150 nm and the barrier layers having a thickness of no more than 10 nm and preferably of approximately 5 nm and consisting preferably of TiO 2 . A photovoltaic module (10) comprising a thin film semiconductive layer (12) containing at least one photovoltaic region and having first and second oppositely directed major surfaces; first transparent electrode means for establishing electrical connection to the first surface of the semiconductive layer (12) and for transmitting incident light to the semiconductive layer over a portion of the solar spectrum; and second transparent electrode means for establishing electrical connection to the second surface of the semiconductive layer (12) and for optically coupling the semiconductive layer to the surrounding atmosphere to permit transmission by the module of a portion of the light transmitted by the first electrode means and not absorbed by the semiconductive layer (12); wherein the second electrode means is formed to attenuate transmission of infrared light and comprises a symmetrical stack formed of a pair of doped zinc oxide (ZnO) layers and a silver (Ag) layer, with ZnO layers on both outer surfaces of the stack, the silver layer having a thickness of 5 - 20 nm, the ZnO layers having a thickness of 40 - 100 nm and being doped with a group III element or hydrogen. A photovoltaic module according to any one of the preceding claims, wherein one or both of the electrode means are formed to enhance or preferably to maximise transmission of visible light. A structure (72, 76, 80, 84) comprising a plurality of walls defining an interior volume; at least one opening in said walls and a photovoltaic module (70, 74, 78, 82) according to any one of the preceding claims forming part of said structure and covering the opening. A structure according to claim 5 comprising a vehicle. A structure according to claim 6, wherein the vehicle is an automobile (72) having a sun roof; the opening in said walls is closed by the sun roof, and the photovoltaic module (70) is disposed within the sun roof to convert incident light falling within a first portion of the solar spectrum to electricity and transmit light within a second portion of the spectrum to the interior of the automobile. A structure according to claim 6, wherein the vehicle is a vessel (76) for navigation on water and has a hatch cover; the opening in said walls is closed by the hatch cover; and the photovoltaic module (74) is disposed within the hatch cover to convert incident light falling within a first portion of the solar spectrum to electricity and transmit light falling within a second portion of the solar spectrum to the interior of the vessel. A structure according to claim 5, wherein the structure is a building (80, 84); the opening in the walls is a window opening; and the photovoltaic module (78, 82) is disposed within the window opening to convert incident light falling within a first portion of the solar spectrum to electricity and acts as a window to transmit light falling within a second portion of the solar spectrum to the interior of the building.

Inventors: BERMAN ELLIOT, GAY CHARLES F, MILLER STEPHEN C, TANNER DAVID P, BERMAN, ELLIOT, GAY, CHARLES F., MILLER, STEPHEN C., TANNER, DAVID P.
Applicants: SIEMENS SOLAR IND LP, SIEMENS SOLAR INDUSTRIES L.P.
Application year: 1986
Main classification: H01L27
IPC codes: H01L31, H01L27