US20090242024A1 - Photovoltaics with interferometric back side masks - Google Patents

Abstract

An interferometric mask covers reflective conductors on the back side of a photovoltaic device. Such an interferometric mask may reduce reflections of incident light from the conductors. In various embodiments, the mask reduces reflections, so that a front and back electrode pattern appears black or similar in color to surrounding features of the device. In other embodiments, the mask may modulate reflections of light such that the electrode pattern matches a color in the visible spectrum.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application claims the benefit of U.S. Provisional Patent Application No. 61/014,405, filed on Dec. 17, 2007, the entire contents of which are incorporated by reference and should be considered a part of this specification.
• The present application is related to U.S. patent application Ser. No. 11/950,392, filed Dec. 4, 2007 (Atty. Docket No. QMRC.007A/080171), to U.S. Provisional Application No. 61/002,198, filed on Nov. 7, 2007 (Atty. Docket No. QMRC.007PR/080171P1), and to U.S. patent application Ser. No. Unknown, filed on even date herewith (Atty. Docket No. QMRC.008A2/080419U2).
BACKGROUND
• 1. Field of the Invention
• The present invention relates generally to the field of optoelectronic transducers that convert optical energy into electrical energy, such as for example photovoltaic cells.
• 2. Description of the Related Technology
• For over a century fossil fuel such as coal, oil, and natural gas has provided the main source of energy in the United States. The need for alternative sources of energy is increasing. Fossil fuels are a non-renewable source of energy that are depleting rapidly. The large scale industrialization of developing nations such as India and China has placed a considerable burden on the available fossil fuels. In addition, geopolitical issues can quickly affect the supply of such fuels. Global warming is also of greater concern in recent years. A number of factors are thought to contribute to global warming; however, widespread use of fossil fuels is presumed to be a main cause of global warming. Thus there is an urgent need to find a renewable and economically viable source of energy that is also environmentally safe. Solar energy is an environmentally safe renewable source of energy that can be converted into other forms of energy such as heat and electricity.
• Photovoltaic (PV) cells convert optical energy to electrical energy and thus can be used to convert solar energy into electrical power. Photovoltaic solar cells can be made very thin and modular. PV cells can range in size from a few millimeters to 10's of centimeters. The individual electrical output from one PV cell may range from a few milliwatts to a few watts. Several PV cells may be connected electrically and packaged in arrays to produce sufficient amount of electricity. PV cells can be used in a wide range of applications such as providing power to satellites and other spacecraft, providing electricity to residential and commercial properties, charging automobile batteries, etc.
• While PV devices have the potential to reduce reliance upon hydrocarbon fuels, the widespread use of PV devices has been hindered by inefficiency and aesthetic concerns. Accordingly, improvements in either of these aspects could increase usage of PV devices.
SUMMARY OF CERTAIN INVENTIVE ASPECTS
• In one embodiment, a photovoltaic device, defining a front side on which light is incident and a back side opposite the front side, includes a photovoltaic active material. A conductor is positioned on the back side of the photovoltaic material. An optical interferometric mask is positioned over the back side of the conductor.
• In another embodiment, a photovoltaic device includes a means for generating an electrical current from incident light on a side of said means. The device also includes a front side means and a back side means for conducting the generated electrical current, as well as a means for interferometrically masking the back side conducting means from the incident side of the photovoltaic device.
• In another embodiment, a method of manufacturing a photovoltaic device includes providing a photovoltaic generator with a photovoltaic active layer, a front side conductor and a back side conductor. The front side conductor is configured to allow light through the front side to the photovoltaic active layer. A plurality of layers is formed over the back side conductor of the photovoltaic generator to define an interferometric modulator configured to mask the back side conductor.
• In another embodiment, a method of manufacturing a photovoltaic device, having a front side on which light is incident and a back side opposite the front side, includes forming an interferometric modulator configured to reduce visible reflections from the back side of the photovoltaic device.
BRIEF DESCRIPTION OF THE DRAWINGS
• Example embodiments disclosed herein are illustrated in the accompanying schematic drawings, which are for illustrative purposes only.
• FIG. 1 schematically illustrates a theoretical optical interferometric cavity.
• FIG. 2 schematically illustrates a plurality of layers forming one implementation of an optical interferometric modulator.
• FIG. 3A is a block diagram of an interferometric modulator (“IMOD”) stack, similar to that ofFIG. 2, comprising an absorber layer, an optical resonant cavity and a reflector.
• FIG. 3B schematically illustrates an IMOD where the optical cavity includes an air gap formed by posts or pillars between the absorber and reflector layers.
• FIG. 3C illustrates an embodiment of an IMOD, wherein the optical resonant cavity can be adjusted electromechanically, in an “open” state.
• FIG. 3D illustrates an IMOD, wherein the optical resonant cavity can be adjusted electromechanically, in a “closed” state.
• FIG. 4 shows the total reflection versus wavelength of an interferometric light modulator with an optical cavity configured to reflect yellow for normally incident broadband white light.
• FIG. 5 shows the total reflection versus wavelength with an optical cavity configured to minimize visible reflections for normally incident broadband white light.
• FIG. 6 shows the total reflection versus wavelength of an interferometric light modulator like that ofFIG. 5 when the angle of incidence or view angle is approximately 30 degrees to normal.
• FIG. 7 schematically illustrates a photovoltaic cell comprising a p-n junction.
• FIG. 8 is a block diagram that schematically illustrates a photocell comprising a deposited thin film or deposited photovoltaic active material.
• FIGS. 9A and 9B are schematic plan and isometric sectional views depicting an example of a solar photovoltaic device with visible reflective electrodes on the front side.
• FIGS. 10A-10G are schematic cross-sectional views illustrating steps in a process of manufacturing an embodiment of an interferometric modulator (IMOD) mask integrated with a photovoltaic device, where the IMOD mask is patterned together with photovoltaic device front electrodes.
• FIG. 10H is a schematic cross-sectional view of the photovoltaic device ofFIG. 10G after formation of a protective film over the IMOD mask.
• FIGS. 11A-D are schematic cross-sectional views illustrating steps of adding an IMOD mask over a photovoltaic device in accordance with another embodiment, wherein at least one layer defining a optical resonant cavity for the IMOD mask remains unpatterned.
• FIG. 12 is a schematic cross-sectional view of a photovoltaic device with an IMOD mask covering electrodes in accordance with another embodiment, wherein the IMOD mask comprises layers that are patterned to be slightly wider than the photovoltaic device front electrodes.
• FIGS. 13A-13E are schematic cross-sectional views illustrating steps in a process of manufacturing a thin film photovoltaic device on a transparent substrate, with an integrated IMOD mask.
• FIG. 13F is a schematic cross-sectional view of another embodiment of an IMOD mask integrated with a thin film photovoltaic device on a transparent substrate, wherein at least one layer defining a optical resonant cavity for the IMOD mask remains unpatterned.
• FIG. 13G is a schematic cross-sectional view of another embodiment of an IMOD mask integrated on a front side of a transparent substrate, opposite the side of the substrate with active photovoltaic material.
• FIGS. 14A and 14B are schematic cross-sectional views of a photovoltaic device formed with a single crystal semiconductor photovoltaic device, with and without an IMOD mask formed over the front electrodes.
• FIG. 15 is a schematic cross-sectional view of an embodiment of an interferometrically-enhanced photovoltaic device with an integrated IMOD mask.
• FIG. 16 is a schematic cross-sectional view of an embodiment of an interferometrically-enhanced photovoltaic device in which an integrated IMOD mask is operatively coupled to the back electrode.
• FIG. 17A is a schematic cross-sectional view of an embodiment of a conductive ribbon with an integrated IMOD mask on one side.
• FIG. 17B is a schematic cross-sectional view of a photovoltaic device with masked ribbons, like that ofFIG. 17A, attached to electrodes of the photovoltaic device, for interconnection of multiple photovoltaic devices.
• FIG. 18A is a schematic cross-sectional view of another embodiment of a conductive ribbon with an integrated IMOD mask covering both sides of the ribbon conductor.
• FIG. 18B is a schematic cross-sectional view of a photovoltaic device with masked ribbons, like that ofFIG. 18A, attached to electrodes of the photovoltaic device, for interconnection of multiple photovoltaic devices.
• FIG. 19 is a schematic cross-sectional view of an embodiment of an interferometrically-enhanced photovoltaic ribbon in which the ribbon conductor and the photovoltaic layers are manufactured separately for integration on a photovoltaic device.
• FIG. 20 is a schematic top-down view of a plurality of photovoltaic devices, wafers or cells interconnected by a conductive ribbon.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS
• One issue hindering widespread adoption of photovoltaic (PV) devices on available surfaces for conversion of light energy into electric energy or current is the undesirable aesthetic appearance of conductors or electrodes on the PV devices. The high reflectivity of common front electrode materials contrasts with the darker appearance of the active PV material itself, and furthermore hinders the blending of PV devices with surrounding materials. Embodiments described herein below employ interferometric modulator (IMOD) constructions designed to darken, hide or blend electrodes, thus providing an IMOD mask over conductors for PV devices. Light incident on the IMOD mask results in little or no visible reflection in the region of the electrodes due to the principles of optical interference. The interferometric masking effect is governed by the dimensions and fundamental optical properties of the materials making up the IMOD mask. Accordingly, the masking effect is not as susceptible to fading over time, compared to common dyes or paints.
• Although certain preferred embodiments and examples are discussed herein, it will be understood that the inventive subject matter extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. It is intended that the scope of the invention disclosed herein should not be limited by the particular disclosed embodiments. Thus, for example, in any method or process disclosed herein, the acts or operations making up the method/process may be performed in any suitable sequence and are not necessarily limited to any particular disclosed sequence unless specifically noted. Various aspects and advantages of the embodiments have been described where appropriate. It is to be understood that not necessarily all such aspects or advantages may be achieved in accordance with any particular embodiment. Thus, for example, it should be recognized that the various embodiments may be carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other aspects or advantages as may be taught or suggested herein. The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. The embodiments described herein may be implemented in a wide range of devices that include photovoltaic devices for collection and conversion of optical energy to electrical energy.
• In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. As will be apparent from the following description, the embodiments may be implemented in a variety of devices that comprise photovoltaic active material.
• FIG. 1 illustrates an optical resonant cavity. An example of such an optical resonant cavity is a soap film which may produce a spectrum of reflected colors. The optical resonant cavity shown inFIG. 1 comprises two interfaces or surfaces101 and102. The twosurfaces101 and102 may be opposing surfaces on the same layer. For example, the twosurfaces101 and102 may comprise surfaces on a glass or plastic plate or sheet or a film of glass, plastic, or any other transparent material. Air or other media may surround the plate, sheet, or film. In the illustrated example, light partially reflects and partially transmits at each ofinterfaces101,102.
• A ray oflight103 that is incident on thefront surface101 of the optical resonant cavity is partially reflected as indicated by thelight path104 and partially transmitted through thefront surface101 alonglight path105. The transmitted light may be partially reflected alonglight path107 and partially transmitted out of the resonant cavity alonglight path106. The amount of light transmitted and reflected may depend on the refractive indices of the material that forms the optical resonant cavity and of the surrounding medium. The example is simplified by omission of multiple internal reflections, as will be appreciated by the skilled artisan.
• For purposes of the discussions provided herein, the total intensity of light reflected from the optical resonant cavity is a coherent superposition of the two reflectedlight rays104 and107. With such coherent superposition, both the amplitude and the phase of the two reflected beams contribute to the aggregate intensity. This coherent superposition is referred to as interference. The two reflectedrays104 and107 may have a phase difference with respect to each other. In some embodiments, the phase difference between the two waves may be 180 degrees and cancel each other out. If the phase and the amplitude of the twolight rays104 and107 are configured so as to reduce the intensity then the two light beams are referred to as interfering destructively. If on the other hand the phase and the amplitude of the twolight beams104 and107 are configured so as to increase the intensity then the two light rays are referred to as interfering constructively. The phase difference depends on the optical path difference of the two paths, which depends both on the thickness of the optical resonant cavity, the index of refraction of the material(s) between the twosurfaces101 and102, and whether the indices of surrounding materials are higher or lower than the material forming the optical resonant cavity. The phase difference is also different for different wavelengths in theincident beam103. Accordingly, in some embodiments the optical resonant cavity may reflect a specific set of wavelengths of theincident light103 while transmitting other wavelengths of theincident light103. Thus some wavelengths may interfere constructively and some wavelengths may interfere destructively. In general, the colors and the total intensity reflected and transmitted by the optical resonant cavity thus depend on the thickness and the material(s) forming the optical resonant cavity and surrounding media. The reflected and transmitted wavelengths also depend on viewing angle, different wavelengths being reflected and transmitted at different angles.
• InFIG. 2, an optical resonant cavity is defined between two layers. In particular, anabsorber layer201 defines the top orfront surface101 of the optical resonant cavity while abottom reflector layer202 defines the bottom or backsurface102 of the optical resonant cavity. The thicknesses of the absorber and reflector layers may be substantially different from each other. For example, theabsorber layer201 will typically be thinner than thebottom reflector layer202 and is designed to be partially transmissive. The absorber and reflector layers may comprise metal. As shown inFIG. 2, the ray oflight203 that is incident on theabsorber layer201 of the optical interference cavity is partially reflected out of the optical interference cavity along each of thepaths204 and207. The illumination field as viewed by an observer on the front or incident side is a superposition of the two reflectedrays204 and207. The amount of light substantially absorbed by the device or transmitted out of the device through thebottom reflector202 can be significantly increased or reduced by varying the thickness and the composition of the reflector layers201,202, whereas the apparent color of reflections is largely determined by the interference effect governed by the size or thickness of the optical resonant cavity between them and the material properties of theabsorber layer201.
• In some embodiments, the optical cavity between the front andback surfaces101,102 is defined by a layer, such as an optically transparent dielectric layer, or plurality of layers. In other embodiments, the optical resonant cavity between the front andback surfaces101,102 is defined by an air gap or combination of optically transparent layer(s) and an air gap. The size of the optical interference cavity may be tuned to maximize or minimize the reflection of one or more specific colors of the incident light. The color or colors reflected by the optical interference cavity may be changed by changing the thickness of the cavity. Accordingly, the color or colors reflected by the optical interference cavity may depend on the thickness of the cavity. When the cavity height is such that particular wavelength(s) are maximized or minimized by optical interference, the structure is referred to herein as an interferometric modulator (IMOD). An IMOD may be static (fixed) or dynamic (active).
• In certain embodiments, the optical resonant cavity height between the top absorber and the bottom reflector may be actively varied for example by microelectromechanical systems (MEMS). MEMS include micromechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and/or other micromachining processes that etch away or remove parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. Such MEMS devices include IMODs having an optical resonant cavity that can be adjusted electromechanically. An IMOD selectively absorbs and/or reflects light using the principles of optical interference. In certain embodiments, an interferometric modulator may comprise a pair of conductive plates, one of which is partially reflective and partially transmissive and the other of which is partly or totally reflective. The conductive plates are capable of relative motion upon application of an appropriate electrical signal. In a particular embodiment, one plate may comprise a stationary layer deposited on a substrate and the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. As described herein in more detail, the position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. In this manner, the color of light output by a MEMS interferometric modulator (IMOD) can be actively varied.
• Using such a MEMS-adjustable optical interference cavity, or MEMS IMOD, it is possible to provide at least two states. A first state comprises an optical interference cavity of a certain dimension whereby light of a selected color (based upon the size of the cavity) interferes constructively and is reflected out of the cavity. A second state comprises a visible black state produced due to constructive and/or destructive interference of light, such that visible wavelength are substantially absorbed. Alternatively, the two states can be colored and broad spectrum (white) reflective.
• FIG. 3A is a simplified schematic of anIMOD stack300. As illustrated, theIMOD stack300 comprises anabsorber layer301, areflector303, and an opticalresonant cavity302 formed between theabsorber layer301 and thereflector303. Thereflector303 may, for example, comprise a metal layer, such as aluminum, and is typically thick enough to be opaque (e.g., 300 nm). The opticalresonant cavity302 may comprise an air gap and/or one or more optically transparent materials. If the opticalresonant cavity302 is defined by a single layer between thereflector303 and theabsorber layer301, a transparent conductor or transparent dielectric may be used. In some embodiments, the opticalresonant cavity302 can comprise a composite structure comprising multiple materials that may include two or more of an air gap, a transparent conducting material, and a transparent dielectric layer. A possible advantage of multiple layers and/or air gaps is that selected layers of the stack may serve multiple functions, such as device passivation or scratch resistance in addition to its optical role in theIMOD stack300. In some embodiments, the optical resonant cavity may comprise one or more partially transparent materials, whether conductive or dielectric. Exemplary transparent materials for theoptical interference cavity302 may comprise a transparent conductive oxide (TCO), such as indium tin oxide (ITO), and/or a dielectric, such as silicon dioxide (SiO₂).
• In this embodiment light passes through theIMOD stack300 first by passing into theabsorber layer301. Some light passes through the partiallytransmissive absorber layer301, through theoptical interference cavity302, and is reflected off thereflector303 back through the opticalresonant cavity302 and through theabsorber layer301.
• With reference toFIG. 3B, in other embodiments, the thickness of the opticalresonant cavity302 may comprise anair gap302 supported byspacers311, such as rails, posts or pillars. Within theIMOD300, the optical resonant orinterference cavity302 may be an air gap that is static, or one that is dynamic, i.e., variable using, for example, MEMS technology.
• An interferometric modulator (IMOD) structure such as shown inFIG. 3A or3B selectively produces a desired reflection output using optical interference. This reflected output may be “modulated” by selection of the thickness and optical properties of the opticalresonant cavity302, as well as the thickness and optical properties of theabsorber301 and thereflector303. The reflected output may also be varied dynamically using a MEMS device to change the size of the opticalresonant cavity302. The color observed by a viewer viewing the surface of theabsorber301 will correspond to those frequencies that are substantially reflected out of the IMOD and are not substantially absorbed or destructively interfered by the various layers of the IMOD. The frequencies that interfere and are not substantially absorbed can be varied by selecting the thickness of the opticalresonant cavity302.
• FIGS. 3C and 3D show an embodiment of an IMOD wherein the optical resonant cavity (302 inFIG. 3B) includes an air gap and can be electromechanically changed using MEMS technology.FIG. 3C illustrates an IMOD configured to be in the “open” or “relaxed” state andFIG. 3D illustrates an IMOD configured to be in the “closed” or “collapsed” state. The IMOD illustrated inFIGS. 3C and 3D comprises asubstrate320, athin film stack330 and areflective membrane303. Thethin film stack330 may comprise an absorber (corresponding to303 inFIGS. 3A and 3B) as well as other layers and materials, such as a separate transparent electrode layer and a dielectric layer to keep the electrodes insulated from one another. In some embodiments, thethin film stack330 may be attached to thesubstrate320. In the “open” state, thethin film stack330 is separated from thereflective membrane303 by agap340. In some embodiments, for example, as illustrated inFIG. 3C, thegap340 may be an air gap, supported byspacers311, such as rails, pillars or posts. In the “open” state, the thickness of thegap340 can vary, for example, between 120 nm and 400 nm (e.g., approximately 260 nm) in some embodiments. Hence, in the “open” state, the optical resonant cavity ofFIGS. 3A and 3B comprises the air gap together with any transparent layers over the absorber within thethin film stack330.
• In certain embodiments, the IMOD can be switched from the “open” state to the “closed” state by applying a voltage difference between thethin film stack330 and thereflective membrane303 as illustrated inFIG. 3D. In the “closed” state, the optical cavity over the absorber between thethin film stack330 and thereflective membrane303 is defined by, e.g., a dielectric layer overlying the absorber in thethin film stack330, and is typically configured to reflect “black” or minimal visible reflections. The thickness of the air gap in general can vary between approximately 0 nm and approximately 2000 nm, for example, between “open” and “closed” states in some embodiments.
• In the “open” state, one or more frequencies of the incident light interfere constructively above the surface of thesubstrate320. Accordingly, some frequencies of the incident light are not substantially absorbed within the IMOD but instead are reflected from the IMOD. The frequencies that are reflected out of the IMOD interfere constructively outside the IMOD. The display color observed by a viewer viewing the surface of thesubstrate320 will correspond to those frequencies that are substantially reflected out of the IMOD and are not substantially absorbed by the various layers of the IMOD. The frequencies that interfere constructively and are not substantially absorbed can be varied by changing the thickness of the optical cavity (which includes the gap340), thereby changing the thickness of the optical resonant cavity.
• FIG. 4 illustrates a graph of total reflection of an IMOD (for example, theIMOD300 ofFIG. 3A or3B) versus wavelength as seen from a direction normal or perpendicular to the front surface of the IMOD. The graph of total reflection shows a reflection peak at approximately 550 nm (yellow). A viewer viewing the IMOD will observe the IMOD to be yellow. As mentioned previously, the location of the peak of the total reflection curve can be shifted by changing either the thickness or material of the opticalresonant cavity302 or by changing the material and thickness of one or more layers in the stack.
• FIG. 5 illustrates a graph of total reflection of the IMOD versus wavelength over a wavelength range of approximately 400 nm to 800 nm for an IMOD with an optical cavity thickness selected to minimize reflections in the visible range. It is observed that the total reflection is uniformly low in the entire visible wavelength range. Thus very little light is reflected out of the interferometric modulator. The color observed by a viewer looking perpendicularly at the front surface of the IMOD may generally be black, reddish black or purple in some embodiments.
• Generally, an IMOD stack can have a view angle dependency. However, when an optical resonant cavity is selected to minimize IMOD reflection in the visible range, the angle dependency tends to be fairly low.FIG. 6 illustrates total reflection versus wavelength for an IMOD with an optical resonant cavity, optimized to minimize visible reflections, when the angle of incidence or view angle is 30 degrees. It is observed that the total reflection is uniformly low in the entire visible wavelength range. Thus very little visible light is reflected out of the interferometric modulator. A comparison ofFIGS. 5 and 6 shows that the spectral response of the IMOD with acavity302 chosen or modulated to minimize visible reflection is approximately the same for normal incidence and when the angle of incidence or view angle is 30 degrees. In other words, the spectral response of a “black” IMOD, with a cavity selected to minimize visible reflections, does not exhibit a strong dependency on the angle of incidence or the view angle.
• FIG. 7 shows a typical photovoltaic (PV)cell700. A typical photovoltaic cell can convert light energy into electrical energy or current. A PV cell is an example of a renewable source of energy that has less impact on the environment, e.g., a small carbon footprint. Using PV cells can provide possible cost benefits, such as reducing the cost of energy generation. PV cells can have many different sizes and shapes, e.g., from smaller than a postage stamp to several inches across. Several PV cells can often be connected together to form PV cell modules that may be up to several feet long and a few feet wide. Modules, in turn, can be combined and connected to form PV arrays of different sizes and power output.
• The size of an array can depend on several factors, such as the amount of sunlight available in a particular location and the needs of the consumer. The modules of the array can include electrical connections, mounting hardware, power-conditioning equipment, and batteries that store solar energy for use when the sun is not shining. A PV device can be a single cell with its attendant electrical connections and peripherals, or a PV module or a PV array. A PV device can also include functionally unrelated electrical components, e.g., components that are powered by the PV cell(s).
• A typical PV cell comprises a PV active region disposed between two electrodes. In some embodiments, the PV cell comprises a substrate on which a stack of layers is formed. The PV active layer of a PV cell may comprise a semiconductor material such as silicon. In some embodiments, the active region may comprise a p-n junction formed by contacting an n-type semiconductor material703 and a p-type semiconductor material704 as shown inFIG. 7. Such a p-n junction may have diode-like properties and may therefore be referred to as a photodiode structure as well.
• The PV active layer(s)703,704 are sandwiched between two electrodes that provide an electrical current path. Theback electrode705 can be formed of aluminum, silver, molybdenum or some other conducting material. The back electrode can be rough and unpolished. Thefront electrode701 is designed to cover a significant portion of the front surface of the p-n junction so as to lower contact resistance and increase collection efficiency. In embodiments wherein thefront electrode701 is formed of an opaque material, thefront electrode701 is configured to leave openings or windows over the front of the PV active layer to allow illumination to impinge on the PV active layer. In some embodiments, the front electrodes can include a transparent conductor, for example, transparent conducting oxide (TCO) such as tin oxide (SnO₂) or indium tin oxide (ITO). The TCO can provide good electrical contact and conductivity and simultaneously be transparent to the incoming light. In some embodiments, the PV cell can also comprise a layer of anti-reflective (AR) coating702 disposed over thefront electrode701. The layer of AR coating702 can reduce the amount of light reflected from the front surface of the PV active layer(s)703,704.
• When the front surface of the active PV material is illuminated, photons transfer energy to electrons in the active region. If the energy transferred by the photons is greater than the band-gap of the semiconducting material, the electrons may have sufficient energy to enter the conduction band. An internal electric field is created with the formation of the p-n junction. The internal electric field operates on the energized electrons to cause these electrons to move, thereby producing a current flow in anexternal circuit707. The resulting current flow can be used to store charge or directly power various electrical devices, such as alight bulb706 as shown inFIG. 7.
• In some embodiments, the p-n junction shown inFIG. 7 can be replaced by a p-i-n junction wherein an intrinsic or undoped semiconducting layer is sandwiched between a p-type and an n-type semiconductor. A p-i-n junction may have higher efficiency than a p-n junction. In some other embodiments, the PV cell can comprise multiple junctions.
• The PV active layer(s) can be formed by any of a variety of light absorbing, photovoltaic materials such as crystalline silicon (c-silicon), amorphous silicon (α-silicon), cadmium telluride (CdTe), copper indium diselenide (CIS), copper indium gallium diselenide (CIGS), light absorbing dyes and polymers, polymers dispersed with light absorbing nanoparticles, III-V semiconductors such as GaAs, etc. Other materials may also be used. The light absorbing material(s) where photons are absorbed and energy transferred to electrical carriers (holes and electrons) is referred to herein as the PV active layer of the PV cell, and this term is meant to encompass multiple active sub-layers. The material for the PV active layer can be chosen depending on the desired performance and the application of the PV cell.
• In some embodiments, the PV cell can be formed by using thin film technology. For example, in one embodiment, where optical energy passes through a transparent substrate, the PV cell may be formed by depositing a first or front electrode layer of TCO on a substrate. PV active material is deposited on the first electrode layer. A second electrode layer can be deposited on the layer of PV active material. The layers may be deposited using deposition techniques such as physical vapor deposition techniques, chemical vapor deposition techniques, electrochemical deposition techniques, etc. Thin film PV cells may comprise amorphous or polycrystalline materials such as thin-film silicon, CIS, CdTe or CIGS. Some advantages of thin film PV cells are small device footprint and scalability of the manufacturing process, among others.
• FIG. 8 is a block diagram schematically illustrating a typical thinfilm PV cell800. Thetypical PV cell800 includes aglass substrate801 through which light can pass. Disposed on theglass substrate801 are afirst electrode layer802, a PV active layer803 (shown as comprising amorphous silicon), and asecond electrode layer805. Thefirst electrode layer802 can comprise a transparent conducting material such as ITO. As illustrated, thefirst electrode layer802 and thesecond electrode layer805 sandwich the thin film PVactive layer803 therebetween. The illustrated PVactive layer803 comprises an amorphous silicon layer. As is known in the art, amorphous silicon serving as a PV material may comprise one or more diode junctions. Furthermore, an amorphous silicon PV layer or layers may comprise a p-i-n junction wherein a layer of intrinsic silicon is sandwiched between a p-doped layer and an n-doped layer.
• As illustrated inFIGS. 9A and 9B, many PV devices employ specular orreflective conductors910,911 on a front, or light-incident, side of the device, as well as on a back side of thePV device900. Conductors on the front or light-incident side can comprisebus electrodes910 orgridline electrodes911. When optical energy is absorbed by the PVactive material903, electron-hole pairs are generated. These electrons and holes can generate current by moving to one or the other of thefront electrodes910,911 or backelectrodes905, as shown inFIG. 9B. The front conductors orelectrodes910,911 are patterned to both reduce the distance an electron or hole must travel to reach an electrode while also allowing enough light to pass through to the PVactive layer903. However, the lines of bright reflections generated by these electrodes are often considered to be unattractive, such that PV devices are often not employed in visible locations.
• Accordingly, some embodiments below describe methods of covering unsightly electrodes so that the electrode pattern appears dark or black to better match the appearance of exposed PV active regions. Furthermore, some embodiments described below provide photovoltaic devices that are uniform in appearance so that they can better blend in with surrounding structures (e.g., rooftop tiles). This may be achieved either by darkening the portion of the front of the PV device that has patterned electrodes, or by rendering the entire front surface (electrodes and active regions) of the photovoltaic device dark.
• One way of darkening or otherwise masking the electrode so as to suppress reflections from a conducting layer or electrode is to use an interferometric modulator (IMOD) as a mask, with reflectance tuned to darken the electrodes and/or blend with the color appearance of exposed PV active regions. In the IMOD stack, the function of the IMOD reflector (e.g.,reflector303 ofFIG. 3A or3B) can be served by the conductor being masked (e.g.,front bus electrodes910 orgrid line electrodes911 ofFIGS. 9A and 9B). Light incident on the IMOD mask results in little or no visible reflection in the region of the electrodes due to the principles of optical interference discussed above. Advantageously, the interferometric effect is governed by the thickness and material(s) of the absorber and optical resonant cavity. Accordingly, the masking effect is not as susceptible to fading over time compared to common dyes or paints.
• FIGS. 10A-10G illustrate one example of a process for fabricating a PV device incorporating an IMOD mask on front electrodes. The example employs a deposited thin film of PV active material. In one embodiment, such a photovoltaic device may be formed on asubstrate1010 such as plastic, glass or another suitable workpiece. As illustrated inFIG. 10A, a method of manufacturing such a device can comprise forming aback electrode1020 on asubstrate1010 using known methods. A metal layer may be deposited to serve as theback electrode1020 for a photovoltaic device, but non-metal conducting materials can also be used.
• With reference toFIG. 10B, the method further includes formation of a photovoltaicactive material1030. In the illustrated embodiment, the photovoltaic (PV)active material1030 comprises a deposited thin film, although in other arrangements portions of single crystal, semiconductor substrates and/or epitaxial layers thereover are employed. A deposited PV active material can comprise, for example, an amorphous silicon thin film, which has recently been gaining in popularity. Amorphous silicon as thin films can be deposited over large areas by physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical vapor deposition, or plasma-enhanced chemical vapor deposition (PECVD), as well as by other methods known to those of skill in the art. As is known by those with skill in the art, PV active materials comprising amorphous silicon layers may include one or more junctions with n-doped and/or p-doped silicon and may further comprise p-i-n junctions. Other appropriate materials for the PVactive material1030 include germanium (Ge), Ge alloys, and alloys like copper indium gallium selenide (CIGS), cadmium telluride (CdTe), as well as III-V semiconductor materials, or tandem multi-junction photovoltaic materials and films. III-V semiconductor materials include such materials as gallium arsenide (GaAs), indium nitride (InN), gallium nitride (GaN), boron arsenide (BAs). Semiconductor alloys like indium gallium nitride may also be used. Other photovoltaic materials and devices are also possible. Methods of forming these materials are known to those having skill in the art. As an illustrative example, alloys like CIGS can be formed by a vacuum-based process where copper, gallium, and indium are co-evaporated or co-sputtered then annealed with a selenide vapor to form the final CIGS structure. Non-vacuum-based alternative processes are also known to those of skill in the art.
• InFIG. 10C, a transparent conducting oxide (TCO)1040 is optionally deposited over the PVactive material1030. TCO layers are often used with photovoltaic materials, particularly thin film photovoltaic materials, in order to improve electrode contact to the PVactive layer1030. Functionally theTCO1040 forms a part of the front electrodes completing a circuit for carrying current generated by the PVactive material1030, but conventionally the more conductive metal conductors that overlie theTCO1040 and connect the PV cell to a wider circuit are referred to as the front electrodes. As is known to those with skill in the art, a common TCO is indium tin oxide (ITO). Methods of forming or depositing ITO are well known in the art and include electron beam evaporation, physical vapor deposition, or sputter deposition techniques. Other TCO materials and processes of manufacture may also be used. The TCO layer can be omitted in other embodiments.
• InFIG. 10D, deposition of theTCO material1040 is followed by the forming of afront conductor layer1050. Thefront conductor layer1050 may comprise a metal or highly conductive material to serve as a front electrode and connect the PV cell into a circuit that carries current generated by the PV cell. As noted above, such conductors tend to be fairly reflective and can spoil the appearance of the PV device and hinder widespread use of PV devices. Typical reflective materials for thefront conductor layer1050 include aluminum (Al), molybdenum (Mo), zirconium (Zr), tungsten (W), iron (Fe), silver (Ag), and chromium (Cr).
• As shown inFIG. 10E, an opticalresonant cavity1060 is formed over thefront conductor1050. In the illustrated embodiment, the opticalresonant cavity1060 is a deposited transparent layer, although, as discussed above with respect toFIGS. 3A and 3B, in other arrangements the cavity can comprise an air gap (seeFIG. 3B) defined by spacers, such as posts, pillars or rails; a single transparent conductive or dielectric layer; a composite formed by multiple conductive or dielectric transparent layers; or a composite formed by combination of an air gap with one or more transparent layers. Optical resonant cavities of a single layer of transparent material can simplify manufacturing and reduce costs. Composite structures with multiple layers and/or air gaps can employ multiple layers to serve multiple functions, such as device passivation or scratch resistance, in addition to its optical role in the IMOD mask being formed.
• Air gaps or composite optical resonant cavities can also serve multiple functions, such as device ventilation or providing the ability to employ MEMS for either reflecting multiple colors (e.g., a color mode and a black mask mode) or for forming an actively (electrically) tunable IMOD mask. In the illustrated embodiments where thereflector303 of the IMOD mask also serves as a front electrode for a PV device, thereflector303 can be used as a stationary electrode for electrostatic actuation, for example, when the PV device is not active. Theabsorber301 can act as a movable electrode. The skilled artisan will appreciate that interconnection and external circuits for handling dual functions of electrostatic MEMS operation and current collection from a PV device can be integrated with the active IMOD mask of the PV device.
• The opticalresonant cavity1060 of one embodiment is formed by a layer of SiO₂or other transparent dielectric material. A suitable thickness for an SiO₂(or similar index) opticalresonant cavity1060 is between 300 Å (angstroms) and 1000 Å to produce an interferometric dark or black effect. Methods of depositing or forming SiO₂are known in the art, including CVD as well as other methods. Other suitable transparent materials for forming the opticalresonant cavity1060 include ITO, Si₃N₄, and Cr₂O₃. The opticalresonant cavity1060 of another embodiment is formed by an air gap layer and SiO₂or another transparent dielectric material. A suitable thickness for an air gap opticalresonant cavity1060 is between 450 Å and 1600 Å to produce an interferometric dark or black effect.
• Referring toFIG. 10F, anabsorber layer1070 is formed over the opticalresonant cavity1060. In the illustrated embodiment, where the IMOD mask being constructed is designed to interferometrically darken the appearance of the naturallyreflective front conductor1050, theabsorber layer1070 may comprise, for example, semitransparent thicknesses of metallic or semiconductor layers. The absorber layer may also comprise materials that have a non-zero n*k, i.e., a non-zero product of the index of refraction (n) and extinction coefficient (k). In particular, chromium (Cr), molybdenum (Mo), molybdenum-chromium alloy (MoCr), titanium (Ti), silicon (Si), tantalum (Ta) and tungsten (W) all form suitable layers. In one embodiment, the thickness of theabsorber layer1070 is between 20 Å and 300 Å. The thickness is selected to be less than that which produces an opaque layer.
• With reference toFIG. 10G, the stack illustrated inFIG. 10F is then patterned using, e.g., photolithographic patterning and etching or another suitable technique to form aPV device1000G as shown inFIG. 10G. The resultant interferometric modulator (IMOD)mask300 comprises a reflector303 (also serving as a front conductor or electrode for the PV device), an optical resonant cavity302 (referred to byreference number1060 prior to patterning), and apatterned absorber301. In the embodiment ofFIG. 10G, thereflector303, opticalresonant cavity302, and theabsorber301 are patterned together and hence aligned with one another. In other arrangements, components of theIMOD mask300 may have a pattern that differs in some fashion from the pattern of the conductor that serves as theIMOD mask reflector303, as will be better understood from the discussion ofFIG. 12 below. TheIMOD mask300 thus covers the front electrode orreflector303. Alignment of theIMOD mask300 with thereflectors303 that serve as front electrodes for the PV device risks some minimal reflections from the sides of thereflectors303 at acute viewing angles. On the other hand, theabsorber301 is patterned in a fashion that does not prevent any more light from reaching the PV active layer than thereflector303, which is present anyway as a front electrode, already does. Thus, theabsorber301 is patterned in a manner that avoids any further reduction in PV efficiency.
• The materials and dimensions of theabsorber301 and the opticalresonant cavity302 are selected to reduce reflectivity from theunderlying reflector303. Reflectivity is defined as a ratio of [the intensity of light reflected from the IMOD mask300] to [the intensity of incident light upon the top of the IMOD mask300] in the direction normal to the upper surface of themask300. Common PV device front electrode materials for thereflector303 exhibit reflectivity in the range of 30%-90%. TheIMOD mask300, however, is configured to interferometrically reduce the overall reflectivity to less than 10%. Thus, the reflectivity observable above theIMOD mask300 is for most common front electrode (reflector303) materials less than 10% (at which point the reflections tend to appear “gray”), and more typically less than 5%. The skilled artisan will appreciate, in view of the disclosure herein, that reflectivity can be reduced to as little as 1%-3%, thus truly appearing “black,” by proper selection of the materials and dimensions for the layer(s) in theabsorber301 and the opticalresonant cavity302.
• Thus, little or no light is seen reflecting from the front conductor of the PV device by an observer. Hence the pattern formed by theIMOD mask300 covering the electrode may appear dark or black. Alternatively, the structure of theIMOD mask300 is selected to reflect a color substantially matching the color of visible regions of the photovoltaic active material adjacent the front conductor. For most PV devices, the PV active area appears quite dark, such that reducing visible reflection by way of theIMOD mask300 effectively blends the conductors in with the appearance of the PV active area, making it difficult to distinguish the two regions of the PV device by sight. However, to the extent the visible regions of PV active material demonstrate color(s) other than dark or black, either due to unconventional PV materials or other coatings over the windows over the PV active material, theIMOD mask300 may be constructed to reflect other colors in order to match with the visible regions of the PV active area and produce a uniform color or appearance for the PV device.
• In one example, where the opticalresonant cavity302 comprises an air gap defined by spacers, such as posts, pillars or rails (seeFIG. 3B), a suitable height of the air gap for producing a dark orblack IMOD mask300 is between 450 Å and 1600 Å, depending in part on the other materials selected for theIMOD mask300. In another example, where the opticalresonant cavity302 comprises a dielectric with an index of refraction between 1 and 3 (e.g., SiO₂), a dark orblack IMOD mask300 can be produced with a dielectric thickness between 300 Å and 1000 Å.
• With reference toFIG. 10H, thePV device1000H can comprise additional layers, such as overlying hard coats, anti-reflection coatings or passivation layers, without detracting from the masking function of the IMOD mask. For example, adielectric layer1080 overlying theIMOD mask300 can comprise SiO₂or silicon nitride and can serve as a top passivation layer for the PV device. Furthermore, thedielectric layer1080 can be provided in a thickness suitable to serve as an antireflective (AR) layer which can further enhance the black state of the front electrode regions. Typical thicknesses for AR layers of silicon oxide or nitride are between about 300 Å and 1500 Å. To the extent other layers are positioned between the viewer and thefront electrode reflector303, adjustments may be called for in the choice of materials, optical properties, and thicknesses of the various layers to ensure that theinterferometric mask300 produces the desired reflectivity.
• FIGS. 11A-11D illustrate another embodiment in which an IMOD black mask is formed after patterning the front electrodes.FIG. 11A illustrates the PV device structure ofFIG. 10D after theconductor layer1050 ofFIG. 10D has been patterned, such as by photolithography and etching, to leave patterned front electrodes orreflectors303. Suitable materials for thefront conductor layer1050 are discussed above with respect toFIG. 10D. Patterning defines patterned conductors or front electrodes, which will also serve as thereflector303 for the IMOD mask to be formed. The structure ofFIG. 11A may represent, for example, a prefabricated photovoltaic (PV) device prior to packaging. Alternatively, in another embodiment, the PV device may be packaged and include, for example, a passivation layer (not shown) over the structure ofFIG. 11A prior to conducting the steps ofFIGS. 11B-11D. In such an arrangement, the selection of materials and dimensions for the subsequently formed optical resonant cavity and absorber should account for the optical effect of the passivation layer. Put another way, in such an unpictured embodiment, the passivation layer (not shown) can be considered a part of a composite optical resonant cavity being formed.
• FIG. 11B shows the structure ofFIG. 11A after forming a blanket layer or composite structure selected to define the opticalresonant cavity layer1060 for the IMOD mask. As noted in the discussion ofFIG. 10E, the opticalresonant cavity layer1060 can be an air gap (seeFIG. 3B) defined by spacers, such as posts, pillars or rails; a single transparent conductive or dielectric layer; a composite formed by multiple conductive or dielectric transparent layers; or a composite formed by combination of an air gap with one or more transparent layers.
• FIG. 11C illustrates the structure ofFIG. 11B after deposition of anabsorber layer1070. Suitable materials and thicknesses for thesemitransparent absorber layer1070 are discussed above with respect toFIG. 10F.
• FIG. 11D illustrates the structure ofFIG. 11C after patterning theabsorber layer1070 to leave apatterned absorber301. In the illustrated embodiment, the opticalresonant cavity layer1060 is left as a blanket or unpatterned layer. Hence the opticalresonant cavity layer1060 is blanketed over the PV cell. Theabsorber301 is patterned, such as by photolithographic masking and etching, to substantially cover the conductor/electrode303.
• The resultant structure ofFIG. 11D is aPV device1100 that comprises the interferometric orIMOD mask300, including the patternedreflector303 that also serves as a front conductor or front electrode for the PV device, a blanket opticalresonant cavity layer1060, and apatterned absorber301. The blanket opticalresonant cavity layer1060, which can represent a single layer or a composite structure as discussed above, can also serve other functions across the regions where PVactive layer1030 is visible or exposed, such as passivation or antireflection for the PVactive layer1030 or for an optionalintervening TCO layer1040. The regions of the opticalresonant cavity layer1060 that lie betweenpatterned reflector303 andabsorber301 form the opticalresonant cavity302 for theIMOD mask300. In the illustrated embodiment, theabsorber301 is patterned to be substantially aligned with thereflector303.
• FIG. 12 shows another embodiment of the invention, in which the opticalresonant cavity layer1060 and the absorber layer1070 (seeFIG. 11C), overlying the layers of a PV device as discussed with respect toFIG. 11C, are patterned together to cover the patternedreflectors303 yielding aPV device1200 as shown inFIG. 12. In this embodiment, theabsorber301 and the opticalresonant cavity302 are both patterned to cover the electrode, but extend slightly beyond theelectrode303. In such an embodiment, the patternedabsorber301 could extend laterally beyond the edge of the electrode by less than 10% of the width of the electrode on each side, and in one embodiment by less than 5% of the electrode width and the length of the absorber301 (not pictured) could independently be aligned with or longer (by less than 10% or less than 5%) than the electrode length. Thewider absorber301 better ensures covering to mask reflections from the front conductor/reflector303, and accommodates reasonable levels of mask misalignment between thereflector303 pattern and theabsorber301 pattern. On the other hand, by minimizing the extent that theabsorber301 is wider than thereflector303 that is being interferometrically masked, the amount of light reaching the PVactive layer1030, and thus overall PV device efficiency, can remain high.
• In other embodiments not illustrated, the absorber layer and optical resonant cavity structure can extend over all of the PV device, but in that case the absorber layer should be very thin (mostly transmissive) in order to minimize the reduction of light reaching the PV active layer. Thus, the extent of the dark or “black” effect is somewhat sacrificed when thinning a blanket absorber layer to maximize transmission. In that case it may also be desirable to employ an additional semitransparent reflector, with relatively high transmission, over the PV active layer in order to better match the reflected color with that of the IMOD in the front electrode regions.
• As discussed with respect toFIG. 10H, theinterferometric masks300 ofFIGS. 11D and 12 can also be protected or passivated by further layer(s) formed or deposited over the surface of the embodiments.
• FIGS. 13A-13E depict a process for manufacturing another embodiment, wherein layers of the PV device are formed over a transparent substrate through which light is transmitted into the PV active region.FIG. 13A begins with an appropriate opticallytransparent substrate1310, such as glass, plastic, or other appropriate substrate with useful optical properties. Anabsorber layer1320 is formed or deposited on the back side of the substrate, opposite the light-incident or front side. Hence, inFIGS. 13A-13E, light is incident from below. Suitable materials and thicknesses for thesemitransparent absorber layer1320 are discussed above with respect to theabsorber layer1070 ofFIG. 10F.
• FIG. 13B illustrates the structure ofFIG. 13A after forming or depositing an opticalresonant cavity layer1330 over theabsorber layer1320. As noted in the discussion ofFIG. 10E, the opticalresonant cavity layer1330 can be an air gap (seeFIG. 3B) defined by spacers, such as posts, pillars or rails; a single transparent conductive or dielectric layer; a composite formed by multiple conductive or dielectric transparent layers; or a composite formed by combination of an air gap with one or more transparent layers.
• FIG. 13C illustrates further formation or deposition of aconductor layer1340 over the opticalresonant cavity layer1330. Suitable materials for theconductor layer1340 are discussed above with respect to theconductor layer1050 ofFIG. 10D.
• With reference toFIG. 13D, patterning or etching thelayers1320,1330,1340 forms anIMOD mask300 pattern that is substantially similar to or covers thereflector303 pattern. Patterning the layer stack defines patterned conductors or front electrodes, which will also serve as thereflector303 for theIMOD mask300. Although formed on the back side of the substrate, thereflector303 is still frontward (closer to the light incident side) relative to the PV active layer, which has yet to be formed, and so thereflector303 is said to define “front conductors” for the PV device.
• FIG. 13E illustrates the result of depositing a thin film photovoltaic (PV)active layer1350 behind or opposite a light-incident side of theinterferometric mask300, followed by deposition of aback conductor layer1360. Suitable materials for thin film PV active layers are discussed above with respect toFIG. 10B, and in general PV active materials include numerous types of photosensitive semiconducting material, such as amorphous silicon. While not shown, a transparent conductor layer (TCO) such as ITO can be deposited prior to depositing the PVactive layer1350 in order to improve electrical contact between the PVactive layer1350 and thefront conductors303 and thus improve collection efficiency of thePV device1300E. Theback conductor layer1360 may comprise a metal conducting layer, and is typically formed to an opaque thickness.
• In the embodiment ofFIGS. 13A-13E, theinterferometric mask300 for a PV device is formed on the optical substrate prior to forming or depositing the PVactive material1350. In this embodiment, the photovoltaic device and theinterferometric mask300 are formed on a side of the optical substrate that is opposite the light-incident or front side of the substrate. Accordingly the sequence of layer formation can be opposite that ofFIGS. 10A-10G. Additional layers (not shown) can include TCO between the PVactive layer1350 and thesubstrate1310, and AR coatings or hard coats on the front side of thesubstrate1310.
• FIG. 13F illustrates another embodiment.FIG. 13F shows theabsorber layer1320 ofFIG. 13A being patterned prior to the formation of an opticalresonant cavity layer1370, leaving apatterned absorber301. Then the opticalresonant cavity layer1370 is deposited or formed over the patternedabsorber301. As noted in the discussion ofFIG. 10E, the opticalresonant cavity layer1370 can be an air gap (seeFIG. 3B) defined by spacers, such as posts, pillars or rails; a single transparent conductive or dielectric layer; a composite formed by multiple conductive or dielectric transparent layers; or a composite formed by combination of an air gap with one or more transparent layers. A layer of conductor material is deposited over the opticalresonant cavity layer1370. The conductor layer may then be patterned to form the front electrodes for thePV device1300F, also serving as thepatterned reflector303 for theIMOD mask300, while leaving the opticalresonant cavity layer1370 unpatterned over the PV cell. Subsequently the PVactive layer1350 is formed over the IMOD mask300 (including the front electrodes) and theback electrode1360 is formed over the PVactive layer1350.
• Use of a blanket opticalresonant cavity layer1370 in an embodiment where light is transmitted through the substrate, as shown inFIG. 13F, can have several advantages. As mentioned above, a transparent conductive oxide (TCO) is often used to improve contact between an electrode and a photovoltaic material. In the embodiment ofFIG. 13F, the optical resonant cavity structure can include or be formed by a TCO layer in contact with the front electrodes formed by thereflector303. As apparent from a comparison betweenFIG. 13E andFIG. 13F, the use of a blanketoptical cavity layer1370 also reduces the profile or topography of the device, and thus makes subsequent depositions (e.g., of the thin film PV active layer1350) easier.
• FIG. 13G illustrates another embodiment in which theinterferometric mask300 is formed on the light-incident or front side of thetransparent substrate1310, while thefront electrodes1390 and the photovoltaic (PV)active layer1350 are on the back side of thesubstrate1310, opposite the light-incident or front side. In such an embodiment, due to the thickness of thesubstrate1310 between thereflective front electrode1390 and theabsorber301, it is desirable for the frontside IMOD mask300 to include aseparate reflector303 on the front side of thesubstrate1310, patterned to cover thereflective front electrode1390 that is on the other side of thesubstrate1310. In this case, thePV device1300G can have a conventional construction on the back side of thesubstrate1310, including patternedfront electrodes1390,TCO layer1380, PVactive layer1350 andback electrode1360 formed in sequence over the back surface of thetransparent substrate1310. The front side of thesubstrate1310 includes anIMOD mask300 stack of aseparate reflector303, opticalresonant cavity302 andabsorber301, in sequence, formed on the front side of the light-transmissive substrate1310. As with the illustrated embodiments, this IMOD stack would preferably be patterned to cover thepatterned front conductor1390. Because it has itsown reflector303 andabsorber301, such an IMOD mask is electrically separated from the PVactive layer1350 and can accordingly be separately interconnected to form an electrostatic MEMS IMOD. In such an embodiment, theIMOD mask300 would be capable of opening and closing, as illustrated inFIGS. 3C and 3D. In this case the opticalresonant cavity302 may include an air gap (340 inFIG. 3C) through which the movable electrode (303 inFIGS. 3C and 3D) can move. As will be appreciated by the skilled artisan, in such an embodiment, dielectric layers and other layers, as well as support posts for spacing the movable electrode/reflector from the stationary electrode/absorber, may be formed in front of thesubstrate1310 to implement amovable IMOD mask300 on the light-incident side ofsubstrate1310.
• FIGS. 14A-14B illustrate an embodiment of integrating an IMOD mask with aPV device1400A in which photovoltaic material is a portion of a single crystal semiconductor substrate and/or epitaxial layer(s) formed over such a single crystal substrate.FIG. 14A depicts a photovoltaic (PV)device1400B comprising aback electrode1410, a p-type silicon layer1420, an n-type silicon layer1430, front conductors orelectrodes303, and ananti-reflective coating1450. As mentioned previously, it is desirable that the front electrodes303 (which can be, e.g., bus lines or grid lines for a PV array) be masked, or that reflections from them be reduced or minimized. Hence, aninterferometric mask300 may be formed on a light-incident or front side of the electrodes as shown inFIG. 14B. This can be accomplished in ways similar to those described above, using similar materials. In one embodiment, the process may begin with a silicon substrate or a single crystal silicon material comprising an active region withconductors303 already patterned, as inFIG. 14A, and theIMOD mask300 is formed thereover. In another embodiment, the process may begin with a silicon substrate or single crystal silicon material comprising an active region without a front conductor or electrode pattern, and the front conductors are formed asreflectors303 along with the opticalresonant cavity302 andabsorber301 using techniques similar to those discussed above with respect toFIGS. 10A-10G and11A-11D. As noted previously, theabsorber301 and the opticalresonant cavity302, or the absorber alone, may be patterned to be substantially aligned with the front electrode/reflector303 so as to cover thereflector303 as shown inFIG. 14B. In another embodiment, theabsorber301 and the opticalresonant cavity302, or the absorber alone, may be patterned so as to follow the pattern of the front electrode/reflector303 but be wider to cover a greater surface area than thereflector303. As inFIGS. 11D and 13F, the optical resonant cavity layer may be left unpatterned or blanketed over the PV cell, while the front electrode/reflector303 andabsorber301 are patterned. In yet another embodiment, theabsorber301, the opticalresonant cavity302, and/or the front electrode/reflector303 can be screen printed, in which case formation and patterning are conducted simultaneously. The layers that form the front electrode/reflector, optical resonant cavity and absorber can be screen printed together in any grouping or separately. Furthermore some layer(s) can be patterned by lithography and etch, while other layer(s) can be screen printed.
• The foregoing embodiments teach IMOD mask constructions that can be employed to interferometrically mask front electrodes of PV devices that have a wide variety of constructions. For example, in addition to the thin film and crystalline silicon PV cells and the transmissive substrate embodiments discussed above, an interferometric or IMOD mask may be used to mask reflections from the front electrodes of a thin film interferometrically enhanced photovoltaic cell or device.
• FIG. 15 illustrates an embodiment of aPV device1500 where aninterferometric mask300 masks reflections from areflector303 which may serve as a front conductor or electrode of an interferometrically enhanced cell formed on asuitable substrate1510. In the illustrated embodiment, theconductor303 is in electrical contact with a PVactive layer1540 through aTCO layer1550. In other embodiments, theconductor303 is directly in electrical contact with theactive layer1540, or is in electrical contact through other layers and materials not shown. The illustrated interferometrically tuned photovoltaic cells comprises areflector1520 and an opticalresonant cavity1530 disposed behind, or opposite a light-incident side, of the PVactive layer1540. The PVactive layer1540 may comprise a thin film photovoltaic material, such as amorphous silicon, CIGS or other thin semiconductor film photovoltaic material. The optical properties (dimensions and material properties) of thereflector1520 and opticalresonant cavity1530 are selected so that reflection from interfaces of thelayered PV device1500 coherently sum to produce an increased field in the PVactive layer1540 of the photovoltaic cell, where optical energy is converted into electrical energy. Such interferometrically enhanced photovoltaic devices increase the absorption of optical energy in the active region of the interferometric photovoltaic cell and thereby increase the efficiency of thedevice1500. In variations on this embodiment, multiple optical resonant cavities can be employed to separately tune different wavelengths of light and maximize absorption in the PV active layer(s). The buried opticalresonant cavity1530 may comprise transparent conductive or dielectric materials, air gaps, or combinations thereof.
• With reference toFIG. 16, in applications where the back side of the photovoltaic device may be visible, such as for window applications, theinterferometric mask300 may be integrated with or formed on the back side of the photovoltaicactive layer1350, and more particularly over the back side of the back electrode1660 (seeFIG. 16). As with thefront electrode1690, theback electrode1660 tends to be reflective and unseemly when visible. In some embodiments, theIMOD mask300 is configured such that the color of light reflected from the mask substantially matches a color of the surrounding features on the back side of the photovoltaic device. In some embodiments, theIMOD mask300 is configured such that the color of light reflected from the mask substantially matches a color in the visible spectrum. In some embodiments, the light reflected from theIMOD mask300 falls within the wavelengths associated with the visible spectrum. In some embodiments, theIMOD mask300 is configured such that little or no incident visible light is reflected from the back side of theIMOD mask300 such that the IMOD mask appears black from a normal viewing angle. TheIMOD mask300 therefore can greatly reduce the reflectivity from the back electrodes (normally 30-90%) to less than 10% or less than 5%. If a black appearance is desired, reflectivity can be about 1-3%.
• In some embodiments, the back sideinterferometric mask300 may be patterned across the wafer or across the cell to cover any pattern in theback electrode1660. From the standpoint of PV functionality, such patterning may not be needed, since the light to be converted comes through the front side and the back side can be opaque, such that the back side interferometric mask may blanket or cover the entire back side of the wafer. However, for applications where the backside is visible, often parts of the window or solar panel are translucent, transparent, or semi-transparent. In that case, patterning theIMOD mask300 to follow a pattern of theback electrode1660 can help maintain transparency in regions adjacent to theback electrode1660, thereby allowing some light through. As discussed above, with respect to the front electrodes, such IMOD mask patterns may be aligned with the back electrode patterns or maybe cover and extend beyond the surface of the back electrode by less than 10% or less than 5%. Also, similar to front side IMOD masks, the opticalresonant cavity302 of theback side mask300 can be formed with optically transparent films, including conductive or dielectric layers, an air gap, multiple layers, or combinations of layer(s) and an air gap.
• With reference toFIGS. 17A-20, an interferometric modulator (IMOD) mask can mask high reflectivity of a conductive ribbon or tab. The ribbon connects electrodes or buses across multiple photovoltaic (PV) devices, cells or wafers to form large solar panels, which greatly increases the amount of electrical charge that can be converted into direct current. The introduction of an IMOD mask over the conductive ribbon masks or blends the appearance of the reflective ribbon with the surrounding photovoltaic device, making the device more attractive to the consumer. For the typical PV device, the IMOD mask reduces reflections from ribbons on the front or back side of the PV device to darken the bright reflections from the ribbon. In some embodiments, the IMOD mask is configured such that the color of light reflected from the mask substantially matches a color of the surrounding features on the photovoltaic device. In some embodiments, the IMOD mask is configured such that the color of light reflected from the mask substantially matches a color in the visible spectrum. In some embodiments, the light reflected from the IMOD mask falls within the wavelengths associated with the visible spectrum. In some embodiments, the IMOD mask is configured such that little or no incident visible light is reflected from the IMOD mask such that the IMOD mask appears black from a normal viewing angle. As discussed above with respect to front electrodes, the IMOD mask preferably exhibits reflectivity in the visible range of less than 10% or even less than 5%. If a black appearance is desired, the IMOD mask can arrange reflectivity in the about 1-3% range. In other arrangements, “masking” may involve interferometrically producing a color in the visible spectrum that blends in with adjacent features on the device. “Color” IMOD masks can reflect a coherent band or bands of wavelengths to appear, e.g., red, green, blue, orange, etc.
• With reference toFIG. 17A, in one embodiment, the conductive ribbon is integrated with the interferometric mask stack on one side of the ribbon. The illustrated conductive ribbon comprises anelongated conductor layer303, an opticalresonant cavity302, and anabsorber layer301. In some embodiments, a separate metallic reflector or mirror layer (not shown) is deposited on theconductor layer303 before the deposition of the opticalresonant cavity302 and theabsorber layer301. The mirror layer may be formed from Al or Mo, or other similar materials. In the embodiment shown inFIG. 17A, theconductor layer303 itself functions as both the reflector for theIMOD mask300 and the conductive ribbon material carrying the bulk of current between PV cells. The conductor layer is generally dimensioned with a length sufficient to connect a plurality of PV devices, cells or wafers, and awidth1702 andthickness1704 sufficient to produce high conductivity.Typical thicknesses1704 are between 0.08 mm and 0.3 mm, whiletypical widths1702 are between 1.5 mm and 15 mm. The edges of theconductor layer303 may be angled or rounded. Theconductor layer303 generally comprises copper, but may be formed with other conductive materials. The opticalresonant cavity302 may be formed of an optically transparent dielectric layer, such as SiO₂, or an optically transparent conductive material, such as ITO or ZnO. As previously described for front and back side IMOD masks, the opticalresonant cavity302 may be formed with a single optically transparent layer, using multiple layers, air gap(s) and combinations of the same. The opticalresonant cavity302 is configured in dimension such that it optimizes constructive and/or destructive interference of select wavelengths of light, for example selecting certain wavelengths associated with the visible spectrum for constructive or destructive interference.
• One of the two major sides of theconductor layer303 may be coated or laminated with the opticalresonant cavity302 andabsorber layer301 as shown inFIG. 17A. One way of forming the embodiment ofFIG. 17A is to place the ribbon conductor on asupport layer1710, such as tape or another substrate, and coat it with additional materials to form the opticalresonant cavity302 andabsorber layer301 on the exposed surfaces. Another way of coating the opticalresonant cavity302 andabsorber layer301 onto theribbon conductor303 is to wind the ribbon conductor from roll to roll while depositing thelayers301,302 in a sputter tool.
• With reference toFIG. 20, the prefabricated ribbon with theintegrated IMOD mask300 can then be interconnected with the contacts, electrodes, or buses of multiplesolar cells2010 to form a largersolar panel2000. The wafers are unloaded from a stack and aligned. The photovoltaic ribbon is cut to length and provided with stress-relief bends. WhileFIG. 20 illustrates photovoltaic ribbon being applied to the back side of the wafers, this process can also be performed on the front side.
• FIG. 17B illustrates use ofsolder1720, or an adhesive layer of another material, applied to at least a portion of the conductors to be interconnected. Theconductor303 of theintegrated IMOD mask300 is then placed onto thesolder1720, which is disposed on top of thecell contacts1730 or on thefront electrodes1790, as illustrated inFIG. 17B. While illustrated with thesolder1720 contacting theback electrodes1760 by way of thecell contacts1730, it will be understood that solder can also directly contact back electrodes. This method is useful for soldering theribbon conductor303 to thefront electrodes1790,back electrodes1760, and/or oncontacts1730 through insulating material, in a single step or in a plurality of steps. In some embodiments, theIMOD mask300 may also be conductive, to facilitate electrical contact among the conductive features.
• The skilled artisan will appreciate that the structure ofFIG. 17B can also be obtained by standard formation of ribbon interconnection of multiple PV devices followed by shadow-masked deposition or deposition and patterning of IMOD mask layers. While solder or other conductive adhesive layers can be omitted by such a process, thin film deposition is in practice difficult on large interconnected solar panels.
• In another embodiment as illustrated inFIG. 18A, theribbon conductor303 is coated with the interferometric mask stack on both sides, for greater manufacturing ease, to form an integrated IMOD-masked ribbon1800 prior to attaching the ribbon on PV devices. In this embodiment, the opticalresonant cavity302 is preferably formed from a transparent conductive material, such as the transparent conductive oxides (TCOs), e.g., ITO or ZnO. Theabsorber layer301 is also preferably formed from a conductive material, such as semitransparent thicknesses of Mo, Cr, MoCr, or Ti, in order to facilitate an electrical contact between the conductors for the PV device and the ribbon conductor303 (which in the illustrated embodiment also serves as the reflector for the IMOD mask300). Accordingly, thecoated ribbon1800 can be readily soldered or otherwise electrically and mechanically connected to conductors of the PV device. As noted with respect toFIG. 17A, theIMOD mask300 of the surrounded ribbon embodiment can also include a separate reflective mirror layer between theribbon conductor303 and the opticalresonant cavity302.
• FIG. 18B illustrates a close-up view of one method of attaching the integrated IMOD-masked ribbon1800 with the photovoltaic device. In this embodiment, an integrated IMOD-masked ribbon1800, with an opticalresonant cavity302 and anabsorber layer301 coated on both sides of theribbon conductor303, is applied to afront electrode1890 of a PV device and attached by means of anadhesive layer1810. In the illustrated embodiment, thefront electrode1890 electrically communicates with the PVactive layer1850 by way of aTCO layer1880. This method is similar to the soldering method shown inFIG. 17B, except that theribbon conductor303 comes prefabricated with theintegrated IMOD mask300.
• In another embodiment as illustrated inFIG. 19, the IMOD mask layers (the opticalresonant cavity302 and the absorber layer301) and theribbon conductor303 are prefabricated but provided separately. For example, the layers forming the opticalresonant cavity302 andabsorber layer301 are formed on a tape or other substrate (not shown), optionally with a release layer (not shown). In this embodiment, theconductor ribbon303 and photovoltaic layers can be integrated on the PV device(s), either at the time of ribbon tabbing or soldering, or subsequently. With reference toFIGS. 17B and 19, according to one possible sequence,first solder1720 is placed on the front and back electrodes of the PV device that are to be interconnected. Next, theribbon conductor303 is aligned with the aforementioned cell contacts,1730,front electrodes1790 and/orback electrodes1760 and soldered or tabbed in place. Next, the IMOD mask layers (the opticalresonant cavity302 and the absorber layer301) are either coated or laminated onto theribbon conductor303.
• While the foregoing detailed description discloses several embodiments of the present invention, it should be understood that this disclosure is illustrative only and is not limiting of the present invention. It should be appreciated that the specific configurations and operations disclosed can differ from those described above, and that the methods described herein can be used in contexts other than fabrication of semiconductor devices.

Claims (26)

1. A photovoltaic device defining a front side on which light is incident and a back side opposite the front side, the photovoltaic device comprising:

a photovoltaic active material;

a conductor on the back side of the photovoltaic material; and

an optical interferometric mask over the back side of the conductor.

2. The photovoltaic device ofclaim 1, wherein the optical interferometric mask is configured such that a color of light reflected from the mask substantially matches a color of surrounding features on the back side of the photovoltaic device.

3. The photovoltaic device ofclaim 2, wherein the optical interferometric mask is configured such that the color of light reflected from the mask substantially matches a color in the visible spectrum.

4. The photovoltaic device ofclaim 1, wherein the light reflected from the optical interferometric mask falls within the wavelengths associated with the visible spectrum.

5. The photovoltaic device ofclaim 1, wherein the optical interferometric mask is configured such that little or no incident visible light is reflected from the back side of the interferometric mask such that the interferometric mask appears black from a normal viewing angle.

6. The photovoltaic device ofclaim 1, wherein the optical interferometric mask is configured such that the reflectivity in the visible range from the mask is less than 10%.

7. The photovoltaic device ofclaim 1, wherein the optical interferometric mask comprises:

a reflective surface;

an optical resonant cavity over the back of the reflective surface; and

an absorber over the back of the optical resonant cavity.

8. The photovoltaic device ofclaim 7, wherein the reflective surface is defined by a separate metallic reflector layer between the conductor and the optical resonant cavity.

9. The photovoltaic device ofclaim 7, wherein the reflective surface is defined by the back surface of the conductor.

10. The device ofclaim 1, wherein the photovoltaic device is incorporated into a window.

11. The photovoltaic device ofclaim 1, further comprising:

a display;

a processor that is configured to communicate with said display, said processor being configured to process image data; and

a memory device that is configured to communicate with said processor.

12. The photovoltaic device as recited inclaim 11, further comprising:

a driver circuit configured to send at least one signal to said display.

13. The photovoltaic device as recited inclaim 12, further comprising:

a controller configured to send at least a portion of said image data to said driver circuit.

14. A photovoltaic device comprising:

means for generating an electrical current from incident light on a side of said means;

front side means and back side means for conducting the generated electrical current; and

means for interferometrically masking said back side conducting means from the incident side of the photovoltaic device.

15. A method of manufacturing a photovoltaic device, comprising

providing a photovoltaic generator with a photovoltaic active layer, a front side conductor and a back side conductor, the front side conductor configured to allow light through the front side to the photovoltaic active layer; and

forming a plurality of layers over the back side conductor of the photovoltaic generator to define an interferometric modulator configured to mask the back side conductor.

16. The method ofclaim 15, wherein the interferometric modulator is integrated with the back side of the photovoltaic device.

17. The method ofclaim 16, wherein the interferometric modulator is deposited across the back side conductor.

18. The method ofclaim 17, wherein the interferometric modulator extends beyond the surface of the back side conductor by less than about 10%.

19. The method ofclaim 18, wherein the interferometric modulator extends beyond the surface of the back side conductor by less than about 5%.

20. The method ofclaim 15, wherein the interferometric modulator is patterned across the back side conductor.

21. The method ofclaim 20, wherein the interferometric modulator comprises an absorber, an optical resonant cavity, and a conductor, wherein the absorber is patterned to follow a pattern of the back side conductor.

22. The method ofclaim 21, wherein the optical resonant cavity is patterned to follow a pattern of the back side conductor.

23. The method ofclaim 21, wherein the absorber and optical resonant cavity are configured to reflect visible light substantially matching a visible spectrum reflected by exposed portions of the photovoltaic device.

24. A method of manufacturing a photovoltaic device having a front side on which light is incident and a back side opposite the front side, the method comprising forming an interferometric modulator configured to reduce visible reflections from the back side of the photovoltaic device.

25. The method ofclaim 24, wherein the interferometric modulator interferometrically produces a black appearance.

26. The method ofclaim 24, wherein the interferometric modulator interferometrically produces a coherent color in the visible spectrum.

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