# Patent

## Title
Customized color patterning of photovoltaic cells

## Abstract

Photovoltaic cells and photovoltaic modules, as well as methods of making and using such photovoltaic cells and photovoltaic modules, are disclosed. More particularly, embodiments of the photovoltaic cells selectively reflect visible light to provide the photovoltaic cells with a colorized appearance. Photovoltaic modules combining colorized photovoltaic cells may be used to harvest solar energy while providing a customized appearance, e.g., an image or pattern.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of the earlier filing date of U.S. Provisional Patent Application No. 61/790,950, filed Mar. 15, 2013, entitled “CUSTOMIZED COLOR PATTERNING OF PHOTOVOLTAIC CELLS.” The aforementioned application is hereby incorporated by reference, in its entirety, for all purposes.


STATEMENT OF GOVERNMENT RIGHTS
This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention.


FIELD
The present invention relates in general to photovoltaic modules and in particular to flexible photovoltaic modules having miniaturized photovoltaic cells.
BACKGROUND
The adoption of photovoltaics for generating electricity from sunlight is largely driven by cost considerations. At present, photovoltaic systems are not competitive with fossil-fuel generated electricity. Thus, there is a need to reduce the overall photovoltaic system cost. This entails reducing the cost of photovoltaic modules as well as module assembly costs with enhanced functionality.
One way to reduce the cost of photovoltaic modules is to reduce the size of photovoltaic cells. In this aspect, small and thin photovoltaic cells have been developed that reduce photovoltaic material use dramatically. These thin photovoltaic cells are typically formed using microelectronic and micro-electromechanical systems (MEMS) techniques. Once formed, the cells may be assembled into photovoltaic modules. The miniaturized photovoltaic cells enable new photovoltaic module applications that traditional photovoltaic cells may not allow.
Traditional photovoltaic cells have been coated with antireflection coatings to minimize the reflectance of light incident on a cell surface. Accordingly, antireflection coatings are typically applied with an optical thickness to maximize absorption and conversion efficiency. As a result, traditional photovoltaic cells appear black or dark blue, due to the nearly complete absorption of incident light by the cells.
SUMMARY
Photovoltaic cells and photovoltaic modules, as well as methods of making and using such photovoltaic cells and photovoltaic modules, are disclosed. In an embodiment, a photovoltaic cell is provided having a device layer with a cell surface and a film with a film surface over the cell surface. The film may be configured to reflect a predetermined wavelength of visible light from the film surface and the cell surface in a direction away from the device layer. The film may include an optical thickness such that rays of the visible light respectively reflected in the direction from the film surface and the cell surface interfere constructively. Furthermore, the optical thickness of the film may vary such that the optical thickness at a first location of the film differs from a second optical thickness at a second location of the film. As a result, rays of a second predetermined wavelength of the visible light may respectively reflect in the direction from the film surface and the cell surface and interfere constructively at the second location.
In an embodiment, a photovoltaic module is provided having a first set of photovoltaic cells and a second set of photovoltaic cells. Each photovoltaic cell in the first set may include a first cell surface under a first film and each photovoltaic cell in the second set may include a second cell surface under a second film. Furthermore, the second film may differ from the first film such that the first set of photovoltaic cells and the second set of photovoltaic cells respectively reflect a first wavelength and a different second wavelength of visible light in a direction away from the photovoltaic module. For example, the first wavelength and the second wavelength may differ by at least 10 nm. In one embodiment, the first set of photovoltaic cells is horizontally spaced apart from the second set of photovoltaic cells on a plane of the photovoltaic module, forming an array of cells over the plane. In another embodiment, the first set of photovoltaic cells is stacked above the second set of photovoltaic cells out of a plane of the photovoltaic module, forming one or more multi-junction cells.
In an embodiment, the first film of a photovoltaic module may include a first optical thickness and the second film may include a second optical thickness different from the first optical thickness. For example, the first optical thickness may equal half of the first reflected wavelength and the second optical thickness may equal half of the second wavelength. The first film may have a first film thickness corresponding to the first optical thickness and the second film may have a second film thickness corresponding to the second optical thickness. Furthermore, at least one of the first film thickness and the second film thickness may be greater than about 75 nm. Alternatively or additionally, the first film may include a first material and the second film may include a second material different from the first material. The first material and the second material may be selected from the group consisting of nitrides and oxides. Furthermore, in an embodiment, at least one of the first film and the second film includes multiple layers.
In an embodiment, the first set of photovoltaic cells is electrically coupled with the second set of photovoltaic cells. The first set of photovoltaic cells may include a plurality of photovoltaic cells electrically coupled in series, and the second set of photovoltaic cells may include a plurality of photovoltaic cell subsets electrically coupled in parallel. Furthermore, each of the photovoltaic cell subsets may include a plurality of photovoltaic cells electrically coupled in series.
In an embodiment, a photovoltaic module includes a receiving substrate with a support surface that supports the first set of photovoltaic cells and the second set of photovoltaic cells. Respective reflections from the first set of photovoltaic cells and the second set of photovoltaic cells may be visually combined into an image on the photovoltaic module. For example, the image may be a camouflage pattern on the photovoltaic module. Furthermore, the receiving substrate may be configured to flex such that the support surface includes a bend radius of less than 20 mm.
In an embodiment, a method includes depositing a first film over a first cell surface of a photovoltaic cell in a first set of photovoltaic cells, depositing a second film over a second cell surface of a photovoltaic cell in a second set of photovoltaic cells, and electrically coupling the first set of photovoltaic cells with the second set of photovoltaic cells to form a photovoltaic module. The second film may differ from the first film such that the first set of photovoltaic cells and the second set of photovoltaic cells respectively reflect a first wavelength and a different second wavelength of visible light. For example, the first film and the second film may include different thicknesses. Alternatively or additionally, the first film and the second film may include different materials selected from the group consisting of nitrides and oxides. In an embodiment, the first set of photovoltaic cells includes a plurality of photovoltaic cells electrically coupled in series, and the second set of photovoltaic cells includes a plurality of photovoltaic cell subsets electrically coupled in parallel. Each photovoltaic cell subset may include a plurality of photovoltaic cells electrically coupled in series. Furthermore, the first set of photovoltaic cells and the second set of photovoltaic cells may be attached to a support surface of a receiving substrate in an arrangement such that respective reflections from the first set of photovoltaic cells and the second set of photovoltaic cells visually combine into an image on the photovoltaic module. Furthermore, the receiving substrate may be configured to flex such that the support surface includes a bend radius of less than 20 mm.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
FIG. 1 is a top view of a photovoltaic module having a plurality of photovoltaic cells in accordance with an embodiment of the invention.
FIG. 2 is a section view, taken about line A-A of FIG. 1, of a plurality of photovoltaic cells on a receiving substrate of a photovoltaic module in accordance with an embodiment of the invention.
FIG. 3 is a detail view, taken from Detail A of FIG. 2, of a photovoltaic cell having a film in accordance with an embodiment of the invention.
FIG. 4 is a section view, taken about line B-B of FIG. 3, of a film portion of a photovoltaic cell in accordance with an embodiment of the invention.
FIG. 5 is a graph illustrating power loss versus film thickness for a photovoltaic cell in accordance with an embodiment of the invention.
FIG. 6 is a graph illustrating film thickness to obtain a constructive fringe for reflecting various wavelengths of visible light from a photovoltaic cell in accordance with an embodiment of the invention.
FIG. 7 is a section view, taken about line C-C of FIG. 1, of a plurality of photovoltaic cells having different films in accordance with an embodiment of the invention.
FIG. 8 is a section view, taken about line D-D of FIG. 7, of a photovoltaic cell having a first film in accordance with an embodiment of the invention.
FIG. 9 is a section view, taken about line E-E of FIG. 7, of a photovoltaic cell having a second film in accordance with an embodiment of the invention.
FIG. 10 is a section view of a plurality of photovoltaic cells in a stacked configuration and having different films in accordance with an embodiment of the invention.
FIG. 11 is a schematic view of a photovoltaic module having a first set of photovoltaic cells electrically connected with a second set of photovoltaic cells in accordance with an embodiment of the invention.
FIG. 12 is a pictorial view of a photovoltaic module having an image formed from a plurality of photovoltaic cells in accordance with an embodiment of the invention.
FIG. 13 is a pictorial view of an integrated photovoltaic module having an image formed from a plurality of photovoltaic cells in accordance with an embodiment of the invention.



DETAILED DESCRIPTION
In an aspect, embodiments describe a photovoltaic cell having a film for selectively reflecting visible light of a predetermined wavelength away from the photovoltaic cell. For example, in an embodiment, a photovoltaic cell includes a film over a cell surface that results in constructive interference of light rays having similar wavelengths in a visible spectrum. Accordingly, the light rays reflect from the photovoltaic cell, causing the photovoltaic cells to appear colored to an onlooker.
In another aspect, embodiments describe a photovoltaic module including a plurality of photovoltaic cells for reflecting colored light. In an embodiment, two or more photovoltaic cells have different films for reflecting visible light of different colors. In an embodiment, the color properties of the individual photovoltaic cells vary based on the characteristics of the different films. For example, a first photovoltaic cell may have a film with a first optical thickness such that a first wavelength of visible light constructively interferes to reflect from the photovoltaic cell, imparting a first color, e.g., red brown, to the photovoltaic cell. Similarly, a second photovoltaic cell may have a film with a second optical thickness such that a second wavelength of visible light constructively interferes to reflect from the photovoltaic cell, imparting a second color, e.g., dark green, to the photovoltaic cell. Accordingly, when viewed from a viewing angle, a photovoltaic module may appear to have differently colored photovoltaic cells. In an embodiment, a plurality of photovoltaic cells and/or sets of photovoltaic cells on a photovoltaic module may be colorized to provide an image, e.g., a camouflage pattern, on the photovoltaic module.
In an aspect, embodiments describe a photovoltaic module having a plurality of photovoltaic cells that are electrically connected in a manner to facilitate energy harvesting from photovoltaic cells that purposefully reflect incident light of a preselected wavelength. In an embodiment, a photovoltaic module includes multiple sets of photovoltaic cells. At least one of the sets includes a plurality of photovoltaic cells interconnected in series and at least one set includes subsets of photovoltaic cells electrically connected in parallel. Accordingly, energy harvested from the plurality of photovoltaic cell sets is not limited to the worst performing photovoltaic cell in the subsets, but rather, energy from the subsets may be harvested independently from each other and accumulated without constraining the energy conversion efficiency of the entire photovoltaic module.
Referring to FIG. 1, a top view of a photovoltaic module having a plurality of photovoltaic cells is shown in accordance with an embodiment of the invention. In an embodiment, a photovoltaic module 100 includes one or more photovoltaic cells 102 supported on receiving substrate 104. More particularly, a plurality of photovoltaic cells 102 may be attached to receiving substrate 104 in an arrangement. For example, photovoltaic cells 102 may have a polygonal shape or any other shape. In an embodiment, photovoltaic cells 102 include a hexagonal shape and are meshed together in a honeycomb pattern to cover an area of photovoltaic module 100 that will subsequently be illuminated, e.g., by sunlight.
In an embodiment, receiving substrate 104 may include a conductive material and/or may include a substantially flexible polymer. For example, receiving substrate 104 may have the structure and characteristics of a flexible circuit. That is, receiving substrate 104 may include a conductive circuit pattern affixed, coated with, and/or encapsulated within a flexible insulating polymer film. Accordingly, conductive circuitry of receiving substrate 104 may be electrically connected with one or more of photovoltaic cells 102. In alternative embodiments, receiving substrate 104 may include a rigid material to stiffen photovoltaic module 100. Thus, receiving substrate 104 material may be selected according to the intended application of photovoltaic module 100.
Photovoltaic cells 102 may be electrically connected with circuitry of receiving substrate 104 in numerous manners. For example, solder bumps, conductive epoxy, or electroplated bumps having metal materials, e.g., metal alloy, titanium, indium, etc., may be applied to an underside of photovoltaic cells 102 and bonded with electrical contacts of receiving substrate 104 circuitry using known techniques, e.g., soldering.
Optionally, photovoltaic module 100 may further include a base substrate 106 structurally connected with receiving substrate 104. Base substrate 106 may be a flexible or rigid sheet of material, depending on the application. For example, base substrate 106 may be a thin sheet of polymer, elastomer, fabric, or any other material that is flexible and/or resilient. Alternatively, base substrate may be a structure formed from glass, metal, or any other material that is rigid and/or stiff. Base substrate 106 may provide structure to photovoltaic module 100 that compensates or complements the structure of receiving substrate 104. Thus, base substrate 106 may be sufficiently stiff to allow receiving substrate 104 to be formed solely from electrical wiring and/or elastic tape, yet still achieve acceptable overall structural integrity for the desired application. As an example, base substrate 106 may be ripstop forming a wall of a tent or a jib of a sailboat, poster panel for a billboard, etc.
In an embodiment, the resulting photovoltaic module 100 is a flexible solar module having a plurality of solar cells. The term “flexible” as used herein refers to the ability of photovoltaic module 100 to bend, as opposed to a rigid structure which does not bend in its natural state. As such, the flexibility of photovoltaic module 100 may be defined by a bend radius of from about 0.75 mm to about 20 mm, for example, from about 3 mm to about 12 mm, or from about 5 mm to about 7 mm. Such a small bend radius may be attributed in part to the dimensions and positioning of photovoltaic cells 102 within photovoltaic module 100 as well as the overall thickness of photovoltaic module 100.
Representatively, photovoltaic cells 102 may have a thickness of, for example, less than about 50 μm and preferably from about 5 to about 10 μm. As such, an overall thickness of photovoltaic module 100 remains small, for example, less than about 0.5 mm. More particularly, photovoltaic module 100 may have an overall thickness in a range of about 0.2 mm to about 0.4 mm. In addition, each of photovoltaic cells 102 may have profile dimensions of, for example, up to about 5 mm. More particularly, a lateral dimension of photovoltaic cell 102 may be about 1 mm or less. The smaller the thickness and lateral dimensions of photovoltaic cells 102, the smaller may the bend radius be.
In addition to its size, photovoltaic cell 102 may also include a shape that contributes to the overall flexibility of photovoltaic module 100. For example, photovoltaic cell 102 may include a polygonal shape (e.g. hexagonal or square) that allows for photovoltaic cells 102 to be tightly packed while maintaining spacing between them. This spacing may accommodate bending of photovoltaic module 100 in a variety of directions. For example, photovoltaic cells 102 may be arranged within photovoltaic module 100 in a side by side honeycomb like pattern such that each of the side walls of photovoltaic cells 102 may be juxtaposed with a sidewall of an adjacent photovoltaic cell as illustrated in FIG. 1. Since each of photovoltaic cells 102 may be independently attached to receiving substrate 104 and may have length and width dimensions of less than 1 mm, photovoltaic module 100 may flex or bend at multiple points around and between photovoltaic cells 102 without causing contact between photovoltaic cells 102.
Although a flexible photovoltaic module 100 is described, it is further contemplated that in some embodiments the photovoltaic module 100 may be a substantially rigid structure. For example, where photovoltaic cells 102 are attached to a rigid receiving substrate 104 or base substrate 106, photovoltaic module 100 may form a rigid module structure. Furthermore, additional stiffening structures may be used, such as ribbing, epoxy films, etc., to further stiffen an originally flexible photovoltaic module.
Referring to FIG. 2, a section view, taken about line A-A of FIG. 1, of a plurality of photovoltaic cells on a receiving substrate of a photovoltaic module is shown in accordance with an embodiment of the invention. Each photovoltaic cell 102 may be supported on a support surface 203 of receiving substrate 104. For example, a device layer 202 of photovoltaic cell 102 may be fixed to support surface 203. In an embodiment, device layer 202 is at least partially covered by a film 204. As used here, device layer 202 generally refers to a portion of photovoltaic cell 102 that receives incident light through film 204. That is, device layer 202 may include a portion of photovoltaic cell 102 that converts energy from absorbed light into electricity using the photovoltaic effect. Thus, device layer 202 may have numerous physical configurations and be formed from various materials known in the art.
Referring to FIG. 3, a detail view, taken from Detail A of FIG. 2, of a photovoltaic cell having a film is shown in accordance with an embodiment of the invention. In an embodiment, device layer 202 includes a bulk material such as monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, gallium arsenide, indium phosphide, or copper indium selenide/sulfide, to name a few. Device layer 202 may also include semiconductor junction regions formed in the bulk material, such as p-type region 302 and n-type region 304. As an example, p-type region 302 may include boron or gallium, and n-type region 304 may include phosphorous or arsenic. Accordingly, semiconductor junction regions may be formed using known implant-based techniques, such as consecutively applying p+ implants and n+ implants through openings in a photoresist layer. Alternatively, non-implant based approaches may be used to create p-type region 302 and n-type regions 304, such as dopant diffusion processes.
In an embodiment, p-type region 302 and n-type region 304 are electrically connected to electrical contacts 306. Electrical contacts 306 may be associated with receiving substrate 104, or may be separately formed from receiving substrate 104. For example, electrical contacts 306 may be solder bumps or aluminum paste annealed over semiconductor junction regions. Electrical contacts 306 may further be placed in electrical connection with electrical leads 308. For example, electrical leads 308 may be circuitry of receiving substrate 104 that interconnect with power equipment, such as a power inverter. Thus, electricity generated in device layer 202 may be harvested through electrical leads 308 of photovoltaic module 100.
Film 204 may be applied over a cell surface 310 of device layer 202. More specifically, film 204 may be applied over an entire cell surface 310 and side wall 311 of device layer 202, or alternatively, film 204 may be applied over only a portion of cell surface 310. In an embodiment, film 204 may be formed over cell surface 310 using deposition techniques. For example, film 204 may be deposited by sputtering, atomic layer deposition, electron beam evaporation, chemical vapor deposition, etc. In an embodiment, film 204 is deposited over cell surface 310 to a film thickness 312. In an embodiment, film thickness 312 may be controlled during the selected deposition process, for example, by controlling timing of deposition. Thickness of film 204 may also be measured during deposition to ensure that a desired thickness is achieved. Thus, films may be controllably applied using known processes and process controls.
Film 204 may be formed from numerous optical materials. For example, film 204 may be formed from various nitrides or oxides. In an embodiment, film 204 includes either silicon nitride (Si3N4) or silicon dioxide (SiO2). However, other materials may be suitable to form film 204, including a variety of transparent dielectric materials such as alumina, magnesium fluoride, diamond-like carbon, etc. Furthermore, exotic methods such as nanostructured films, self-assembled microspheres, and organic materials, may be used to form film 204 over cell surface 310. In an embodiment, film 204 may also include a semiconductor material.
In an embodiment, film 204 material may be chosen based on a refractive index value of the material. In particular, a film material may be selected that has an index value between the refractive index values of the surrounding medium, e.g., air, and device layer 202, e.g., silicon. An index value in this range may provide for the reflection of light from both film surface 314 and cell surface 310. Accordingly, in an embodiment, a refractive index of film 204 is in a range of about 1.0 to 6.5 for the transmission of light across the visible spectrum. For example, refractive index for film 204 of silicon nitride transmitting light spanning the visible spectrum may be between about 2.01 to 2.09. Alternatively, the refractive index for film 204 of silicon dioxide transmitting light across the visible spectrum may be in a range of about 1.45 to 1.47.
As described below, film 204 may have one or more layers formed from similar or different materials. Each layer may include a similar or different optical thickness. As used here, optical thickness refers to an optical path length or optical distance of a film or a film layer. More specifically, optical thickness is a product between a refractive index of a film and a dimensional thickness of the film. For example, in the case of two films, one formed from silicon nitride and one formed from silicon dioxide, a dimensional thickness of the films to ensure that light of the same wavelength that is coherent before entering the films will also be in phase when exiting the films, is the ratio between the refractive indices of the materials. That is, the thickness of the silicon dioxide film must be about 1.4 times thicker than the thickness of the silicon nitride film to ensure that transmitted light rays of the same wavelength, entering the films simultaneously, will remain in phase when exiting the films.
Reflection of a ray of light from a film depends on optical thickness. More particularly, for two light rays of a like wavelength to constructively interfere and reflect from film 204, the rays may be in phase as they exit film surface 314. Thus, an optical thickness of the film through which one of the rays travels may be equal to a half wavelength of the incident light ray such that the light ray will travel a full wavelength as it traverses through film twice (once in each direction). Accordingly, the dimensional thickness of film will equal a half wavelength divided by the refractive index of the film. As an example, for film of silicon nitride having a refractive index of 2.09 when transmitting light with a wavelength of 380 nm, a dimensional thickness of the film will be a half wavelength (190 nm) divided by the refractive index (2.09), or about 91 nm, to ensure that the incident light constructively interferes at the film surface. From the discussion above, it is apparent that constructive interference and reflectance of light from a film depends in part on optical thickness, and optical thickness may be controlled not only through the choice of material with a particular refractive index, but also through the dimensional thickness of the film and/or film layers.
Still referring to FIG. 3, dimensional film thickness 312 may be defined by a distance between cell surface 310 and a film surface 314. For example, dimensional film thickness 312 may be a length orthogonally between cell surface 310 and film surface 314. Accordingly, incident light 316 passing through film surface 314 travels at least film thickness 312 before contacting cell surface 310. Incident light 316 may be sunlight, for example, and thus may be modeled as including a broad spectrum of light rays having different wavelengths. In an embodiment, a ray from incident light 316 having a first wavelength, i.e., film ray 318, reflects from film surface 314 as incident light 316 passes through the interface of film 204 and a surrounding medium. Similarly, a ray from incident light 316 having the same first wavelength, i.e., cell ray 320, reflects from cell surface 310 as incident light 316 passes through the interface of film 204 and device layer 202. Accordingly, film ray 318 and cell ray 320 of like type may propagate in the same direction away from device layer 202. As explained below, whether film ray 318 and cell ray 320 propagate away from device layer 202 depends in part on whether the rays constructively interfere.
Referring to FIG. 4, a section view, taken about line B-B of FIG. 3, of a film portion of a photovoltaic cell is shown in accordance with an embodiment of the invention. In an embodiment, film 204 includes multiple layers. For example, film 204 may include a bottom film layer 402 deposited over cell surface 310 of device layer 202. Furthermore, film 204 may include a top film layer 404 deposited over bottom film surface 406 of bottom film layer 402. Accordingly, incident light 316 directed toward device layer 202 passes through top film surface 408, bottom film surface 406 and cell surface 310 before entering a bulk material of device layer 202 to be converted into electricity. Furthermore, in an embodiment, as broad spectrum incident light 316 passes through film 204, a top film ray 410 reflects from top film surface 408, a bottom film ray 412 reflects from bottom film surface 406, and cell ray 320 reflects from cell surface 310. Accordingly, top film ray 410, bottom film ray 412, and cell ray 320 may propagate in the same direction away from device layer 202.
The embodiments illustrated in FIG. 3 and FIG. 4 provide a simplified model of film ray 318 and cell ray 320 for a single wavelength of impinging light, but it will be appreciated that a corresponding film ray 318 and cell ray 320 may be present for all wavelengths of light incorporated in incident light 316. Thus, whether any film ray 318 and cell ray 320 for a given wavelength are reflected away from photovoltaic cell 102 as light energy depends on optical interference between the rays. Furthermore, in certain film configurations, different wavelengths of light may be reflected from the same photovoltaic cell 102, and the rays from these reflected light wavelengths may be visually recognized as a combination color. For example, in a case of multi-layered film 204 having top film layer 404 and bottom film layer 402, top film ray 410 and bottom film ray 412 may constructively interfere to reflect a ray of blue light from a first layer of film 204 while bottom film ray 412 and cell ray 320 may constructively interfere to reflect a ray of red light from a second layer of film 204. These reflected rays of different wavelengths may visually combine to be recognized by an onlooker as a magenta color reflecting from the photovoltaic cell 102. Combinations of reflected light from a single photovoltaic cell may be exploited through the addition of more and different layers of film 204, which provides for control over the types and intensities of reflected visible light wavelengths.
In a first configuration, for any wavelength of incident light 316, film ray 318 and cell ray 320 may respectively reflect from film surface 314 and cell surface 310 in phase with each other, and thus, the rays may constructively interfere such that a resultant light ray of the given wavelength is emitted from photovoltaic cell 102 as a reflection. Alternatively, in a second configuration, film ray 318 and cell ray 320 of the given wavelength may respectively reflect from film surface 314 and cell surface 310 out of phase with each other, and thus, the rays may destructively interfere such that no resultant light ray of the given wavelength is reflected from photovoltaic cell 102. As explained above, whether film ray 318 and cell ray 320 will be in phase or out of phase may be determined by an optical thickness of film 204. Thus, reflection of a resultant light ray of a given wavelength may be controlled by controlling optical thickness of film 204 and/or film 204 layers. More specifically, reflection of a color from photovoltaic cell 102 may be controlled by adjusting a material and/or film thickness 312 for each layer in film 204.
Referring to FIG. 5, a graph illustrating power loss versus film thickness for a photovoltaic cell is shown in accordance with an embodiment of the invention. The example is provided for a single layer film, but one skilled in the art may extrapolate the discussion below to multi-layered films also. As film thickness 312 increases from zero, reflected power decreases from a maximum to a minimum at a first destructive fringe 502. This first destructive fringe 502 represents a film thickness 312 at which reflection losses from device layer 202 are minimal, i.e., a point of maximum solar energy conversion for a cell. Thus, for traditional solar cells, film thickness 312 at destructive fringe 502 may represent a preferred option. Film thickness 312 at the first destructive fringe 502 may vary based on the material used for a film and the number of layers in film 204. As an example, for a device layer 202 formed from silicon and a single film layer formed from silicon nitride (refractive index ̃2.04 at 535 nm), the first destructive fringe 502 for an incident wavelength of light of 535 nm may occur with a film 204 having an optical thickness of a quarter wavelength, i.e., corresponding to a film thickness 312 of approximately 65 nm. In fact, for the same solar cell over the entire spectrum of visible light, the first destructive fringe 502 may occur with film thicknesses 312 ranging from about 44 nm to 93 nm. However, in an embodiment, film thickness 312 may be chosen to be between about 65 nm to 75 nm in order to take advantage of a destructive fringe 502 near the middle range of the visible spectrum.
As mentioned above, the first destructive fringe 502 may correspond to different film thicknesses 312 where multiple film layers are employed. For example, in a case in which device layer 202 is formed from silicon and a two-layer film 204 is employed having a silicon dioxide top film layer 404 (refractive index ̃1.46 at 535 nm) and a silicon nitride bottom film layer 402 (refractive index ̃2.04 at 535 nm), a destructive fringe 502 for an incident wavelength of light of 535 nm may occur with a top film layer thickness and a bottom film layer thickness corresponding to an optical thickness of a quarter wavelength each. Accordingly, a destructive fringe 502 may occur with a top film layer thickness of approximately 91 nm and a bottom film layer thickness of approximately 65 nm. In an alternative embodiment, top film layer 404 may be designed to create a destructive fringe for a first wavelength and bottom film layer 402 may be designed to create a destructive fringe of a second wavelength. More layers may be employed to create even more destructive fringes centered at different wavelengths.
As film thickness 312 increases beyond the optical thickness associated with destructive fringe 502 for a given wavelength, percent power reflected for that wavelength increases. More specifically, as film thickness 312 increases so does the path length over which cell ray 320 travels, causing film ray 318 and cell ray 320 to shift back into phase until a maxima at constructive fringe 504 is achieved. At constructive fringe 504, film ray 318 and cell ray 320 are in phase and combine to reflect away from device layer 202. Accordingly, energy of film ray 318 and cell ray 320 are lost as reflected visible light and the power absorption of device layer 202 decreases as compared to absorption at destructive fringe 502. As an example, for a device layer 202 formed from silicon and a single layer film 204 formed from silicon nitride, constructive fringe 504 may occur over the entire spectrum of visible light with film thickness 312 ranging from about 88 nm to 186 nm.
In an embodiment, for a photovoltaic cell 102 having a single layer film 204, destructive fringe 502 and constructive fringe 504 for a given wavelength repeat in a sinusoidal manner with intermittent maxima occurring at fringe spacing 506. More specifically, fringe spacing 506 corresponds with half of the wavelength of the impinging light ray, given that for every increase in film thickness 312 by one half wavelength, the optical path length travelled by cell ray 320 may be a full wavelength, and thus, the film ray 318 and cell ray 320 interference may repeat. Accordingly, for longer wavelength light, e.g., red light, fringe spacing 506 is greater than for shorter wavelength light, e.g., violet light.
Referring to FIG. 6, a graph illustrating film thickness to obtain a constructive fringe for reflecting various wavelengths of visible light from a photovoltaic cell is shown in accordance with an embodiment of the invention. Based on the above description, in order to ensure that a given wavelength of light is reflected from photovoltaic cell 102, film 204 may be deposited in a configuration corresponding to constructive fringe 504 for that wavelength. As an example, photovoltaic cell 102 with silicon device layer 202 and single layer film 204 of silicon nitride may include film thickness 312 of between about 88 nm to 186 nm to ensure that at least one wavelength of light across the visible spectrum constructively interferes and is selectively reflected away from photovoltaic cell 102. Thus, film configuration may be manipulated to selectively reflect a given wavelength of light such that the cell appears to have a predetermined color. More particularly, optical thickness of film 204 may be controlled such that photovoltaic cell 102 reflects a wavelength of light greater than 380 nm, greater than 450 nm, greater than 570 nm, greater than 590 nm, and/or greater than 620 nm, corresponding to the visible spectrum of light.
Film configuration may be manipulated in numerous ways to achieve the desired selective reflectance and photovoltaic cell color. For example, in a simplified embodiment employing a silicon device layer 202 covered by a silicon nitride film 204, film thickness 312 may be deposited to a thickness greater than a film thickness corresponding to destructive fringe 502 for a given wavelength. More specifically, film thickness 312 may be deposited to achieve an optical thickness of at least a quarter wavelength of the selected colored light wavelength and more preferably at least three-eighths of the wavelength of the selected colored light. Further still, film thickness 312 may be varied to achieve an optical thickness of film 204 in a range between even multiples of three-eighths to five-eighths of a wavelength corresponding to the selected color. Such a range corresponds to constructive region 508 in which rays tend to mostly interfere constructively. For example, a second constructive fringe 504 for a wavelength may be centered within constructive region 508 between optical thicknesses of film 204 ranging from six-eighths to ten-eighths, and more particularly between optical thicknesses ranging from seven-eighths to nine-eighths of the wavelength corresponding to the selected color. Thus, film thickness 312 may be configured to include an optical thickness within constructive region 508 in order to take advantage of second constructive fringe 504, or other higher order constructive fringes, and to reflect the wavelength of light in that range.
In an embodiment, film thickness 312 is deposited to a thickness corresponding to constructive fringe 504 for the desired wavelength of light. For example, for a photovoltaic cell 102 having silicon device layer 202 and single layer film 204 of silicon nitride, in order to reflect a green color, film 204 may be deposited to achieve an optical thickness of about half of the wavelength of the selected green colored light. For example, film 204 may be deposited to a thickness of about 130 nm to reflect green light with a wavelength of about 530 nm and thus color photovoltaic cell 102 green.
From the discussion above, it will be apparent that optimal film thickness 312 may vary based on the color sought, film material, number of layers, etc. However, as an example, in an embodiment in which a single layer film 204 of silicon nitride is used, film thickness 312 may be greater than about 75 nm to achieve a thickness above a first destructive fringe 502 corresponding to a violet color. Alternatively, film thickness 312 may be greater than about 100 nm to achieve a thickness above a first destructive fringe 502 corresponding to a blue color. In another embodiment, film thickness 312 may be greater than about 125 nm to achieve a thickness above a first destructive fringe 502 corresponding to a green color. Additionally, film thickness 312 may be greater than about 200 nm to achieve a thickness above a first destructive fringe 502 corresponding to a yellow color, and so on.
In addition to a minimum film thickness 312 to achieve a photovoltaic cell 102 color, film thickness 312 may also be selected based on different power generation efficiencies centered around different constructive fringes 504. More particularly, a photovoltaic cell 102 may appear of a certain color when film 204 is applied to a thickness corresponding to one of the constructive fringes 504 associated with the color. For example, photovoltaic cell 102 may appear to be colored a similar shade of green when single layer film 204 includes an optical thickness of 130 nm, 260 nm, 390 nm, and so on, corresponding to each constructive fringe 504 shown in FIG. 5. However, even though cell color may be similar, the overall power efficiency of photovoltaic cell 102 with film thickness of 390 nm may differ as compared to the overall power efficiency of photovoltaic cell 102 with film thickness of 130 nm. This may be the case, for example, because reflectance of other wavelengths may be minimized at a higher order constructive fringe as compared to other constructive fringes for a selected wavelength or associated color. Thus, a designer wanting a green photovoltaic cell 102 may choose a film thickness corresponding to a constructive fringe 504 at which overall power efficiency for the colorized photovoltaic cell 102 is highest, even if the chosen thickness is more costly to apply.
The discussion above primarily assumes a perpendicular angle of incidence of a light ray entering film 204. However, in practice, this idealization may not be the case. For example, onlookers may view photovoltaic module 100 at an angle, rather than straight on. Nonetheless, one skilled in the art may adjust for non-perpendicular viewing angles through variations in film thickness 312. For example, if an onlooker is expected to view photovoltaic module 100 at an angle of 45 degrees, film thickness 312 may be expected to decrease approximately by the cosine of 45 degrees times a half wavelength of the incident light 316. That is, for a same color appearance of photovoltaic module 100, film thickness 312 may be decreased by about 30% as compared to film thickness 312 when onlooker is expected to view photovoltaic module 100 perpendicularly. These adjustments are derivable by one skilled in the art and may be exploited to ensure that the desired color is reflected in the desired direction from photovoltaic cells 102 that are placed on curved surfaces. For example, photovoltaic cells 102 covering a curved parabolic reflector may include different film thickness 312. More particularly, film thickness 312 of photovoltaic cells 102 near the center of the reflector may be thicker than film thickness 312 of photovoltaic cells 102 near an outer edge of the reflector, when an onlooker is expected to view the center of the reflector straight on. As a result, the entire curved surface may appear to have approximately the same color to the onlooker, even though the optical thicknesses of the photovoltaic cells 102 across the photovoltaic module 100 differ for the different angles of incidence.
Referring to FIG. 7, a section view, taken about line C-C of FIG. 1, of a plurality of photovoltaic cells having different films is shown in accordance with an embodiment of the invention. Selective coloring of photovoltaic cells based on film selection may be applied to a plurality of photovoltaic cells 102 in a photovoltaic module 100 in order to achieve a desired overall appearance and aesthetic of the photovoltaic module 100. In an embodiment, a first photovoltaic cell 702 and a second photovoltaic cell 704 of photovoltaic module 100 are illuminated by incident light 316. More specifically, a broad spectrum incident light 316 having a first incident light 706 of a first wavelength and a second incident light 708 of a second wavelength is directed to the photovoltaic module 100. First film 710 of first photovoltaic cell 702 may be configured in a manner described above to selectively reflect first reflection 712 of a first wavelength of visible light away from first photovoltaic cell 702, while second film 714 of second photovoltaic cell 704 may be configured to reflect second reflection 716 of a second wavelength of visible light away from second photovoltaic cell 704. In an embodiment, the first wavelength and the second wavelength may be the same, and thus, first photovoltaic cell 702 and second photovoltaic cell 704 may have a similar color appearance. However, in another embodiment, the first wavelength and the second wavelength may differ, resulting in differently colored photovoltaic cells. For example, the first wavelength may differ from the second wavelength by at least 10 nm, creating a perceptible change in color hue, even where the photovoltaic cells are in the same color range. In an embodiment, the first wavelength and the second wavelength differ by more than about 30 nm. Thus, when viewed from the same direction, photovoltaic module 100 may appear to have two differently colored pixels, i.e., first photovoltaic cell 702 and second photovoltaic cell 704 arranged as a display. Wavelengths of light in incident light 316 not reflected by either of the photovoltaic cells may be absorbed by the photovoltaic cells for the creation of electricity.
Referring to FIG. 8, a section view, taken about line D-D of FIG. 7, of a first photovoltaic cell having a first film is shown in accordance with an embodiment of the invention. In an embodiment, first film 710 is deposited over device layer 202 in a configuration to selectively reflect first incident light 706 away from first photovoltaic cell 702. For example, first incident light 706 may be red light with a wavelength of about 700 nm. As described above, in a case of a single layer film 204 of silicon nitride over a silicon device layer 202, first film thickness 802 may be varied such that the optical thickness of film 204 is a half wavelength of the red light to cause the red light to constructively interfere and reflect away from first photovoltaic cell 702. Thus, since silicon nitride has a refractive index of about 2.02 at the desired wavelength, first film thickness 802 may be deposited to about 174 nm to cause constructive film ray 804 and constructive cell ray 806 to combine and reflect away from device layer 202, giving first photovoltaic cell 702 a reddish color.
First photovoltaic cell 702 may be selectively colored by altering first film 710 to reflect away certain wavelengths of light, and similarly, it may be selectively receptive to other wavelengths of light. For example, second incident light 708 may correspond to yellow light with a wavelength of about 580 nm. Thus, in the example described above in which first photovoltaic cell 702 is manipulated to appear red, first film 710 of silicon nitride with a thickness of about 174 nm will have an optical thickness of about three-fifths of the yellow light wavelength. Accordingly, destructive film ray 808 and destructive cell ray 810 of second incident light 708 will substantially destructively interfere, and thus, most of yellow light will become absorbed light 718 within device layer 202, rather than being reflected away to alter the color of first photovoltaic cell 702.
Referring to FIG. 9, a section view, taken about line E-E of FIG. 7, of a second photovoltaic cell having a second film is shown in accordance with an embodiment of the invention. In an embodiment, second photovoltaic cell 704 is configured to preferentially reflect second incident light 708 while substantially absorbing first incident light 706. Second film 714 may include a single layer of silicon dioxide over a silicon device layer 202. Second film thickness 902 may be deposited to achieve an optical thickness of a half wavelength of second incident light 708. For example, where second incident light 708 corresponds to yellow light with a wavelength of about 580 nm, second film thickness 902 with a refractive index of about 1.46 at the desired wavelength may be approximately 142 nm to cause constructive film ray 804 and constructive cell ray 806 to constructively interfere and reflect away from device layer 202, giving second photovoltaic cell 704 a yellowish color.
Second photovoltaic cell 704 may also be selectively receptive to other wavelengths of incident light 316, such as first incident light 706. In an example in which first incident light 706 is red light with a wavelength of 700 nm, second film 714 with a thickness of 198 nm will have an optical thickness of about two-fifths of the red light wavelength. Accordingly, destructive film ray 808 and destructive cell ray 810 of first incident light 706 will substantially destructively interfere, and thus, most of red light will become absorbed light 718 within device layer 202, rather than being reflected away to alter the appearance of second photovoltaic cell 704.
In an embodiment, an optical thickness across a film may be varied to cause multiple visible reflections to emanate from the same film surface. For example, the optical thickness at a first location on a film may be greater than an optical thickness at a second location on the film. Thus, as incident light passes through the film, a first wavelength of light will constructively interfere at the first location and a second wavelength of light will constructively interfere at the second location. Accordingly, the first wavelength of light will reflect from the first location and the second wavelength of light will reflect from the second location. Therefore, the visible color of a photovoltaic cell may be altered across the cell surface by closely controlling optical thickness of a film over the cell surface. This phenomenon may be used, for example, to give a single photovoltaic cell surface an iridescent visible appearance.
Referring to FIG. 10, a section view of a plurality of photovoltaic cells in a stacked configuration and having different films is shown in accordance with an embodiment of the invention. In addition to having multiple photovoltaic cells 704 with different reflective properties arranged in-plane relative to each other, e.g., separated horizontally from each other on a plane of photovoltaic module 100 as shown in FIG. 7, a plurality of photovoltaic cells 704 may also be separated vertically from each other. For example, mechanically stacked photovoltaic cell designs may be used in microsystems enable photovoltaics (MEPV). In an embodiment, a multi junction cell includes first photovoltaic cell 702 stacked above second photovoltaic cell 704. Thus, the stack of photovoltaic cells may project out of a plane of photovoltaic module 100. First photovoltaic cell 702 may include first film 710 and second photovoltaic cell 704 may include second film 714. Thus, first film 710 may be above both first photovoltaic cell 702 and second photovoltaic cell 704, while second film 714 may be sandwiched between first photovoltaic cell 702 and second photovoltaic cell 704. Optionally, second film 714 may function as a glue layer to adhere the photovoltaic cells together, or in another embodiment, a separate junction film may be applied to bond the photovoltaic cells into a stacked configuration. The cells may be nominally isolated electrically and interconnected using various interconnection architectures, as described further below.
In an embodiment, incident light 706 may pass through first film 710 to be partially reflected as first visible reflection 712 and partially absorbed as absorbed light 718. Absorbed light 718 may also travel through first photovoltaic cell 702 toward second photovoltaic cell 704. More specifically, absorbed light 718 is not only partially absorbed by first photovoltaic cell 702, but portions may also reflect from sandwiched second film 714 as second reflection 716, and still other portions may enter second photovoltaic cell 704 to become absorbed light 718 therein.
Unlike FIG. 7, in which reflected light 712 and 716 may be independently optimized and spatially separated, reflected light from first film 710 and second film 714 in a stacked cell configuration is not independent from each other. This phenomenon is due to the multiple partial reflections that can occur between layers. For example, in an embodiment, portions of reflected light 716 emanate away from the stacked multi junction cell as visible light 1002, but still other portions are reflected from a lower surface of first film 710 back toward second film 714 as partial internal reflection 1004. Partial reflection 1004 may further enter second photovoltaic cell 704 as absorbed light 718.
Although the presence of internal reflections and the combinations of internal reflections with absorbed light may complicate the photovoltaic cell model, the design of a multi junction cell may nonetheless be performed using well known optical design techniques. For example, a structure such as this may be referred to as a multilayer dielectric stack, for which design tools exist to optimize reflections. Thus, first film 710 and second film 714 may be optimized to achieve the desired visible reflections 712 and 716 over the optical spectrum of interest. A spatial variation of first film 710 may allow a spatial dependence of reflection across the stacked photovoltaic cell 704. Furthermore, multiple stacked photovoltaic cells 704 may be arranged in-plane across photovoltaic module 100 as described above with respect to FIG. 7.
Referring to FIG. 11, a schematic view of a photovoltaic module having a first set of photovoltaic cells electrically connected with a second set of photovoltaic cells is shown in accordance with an embodiment of the invention. Other embodiments of interconnect architecture may be used, such as one or more of the embodiments described in U.S. patent application Ser. No. 13/164,483. As described above, selective coloring of photovoltaic cells 102 may cause reflective loss in the range of light wavelengths that are reflected away from device layer 202. As an example, overall efficiency of photovoltaic cells colored in a manner as described above may result in a loss of energy conversion efficiency of up to about 20% as compared to a photovoltaic cell that incorporates a purely anti-reflective coating. To accommodate such power loss in individual photovoltaic cells 102, photovoltaic module 100 may be electrically connected in a manner that allows for efficient overall harvesting of power from a plurality of differently colorized photovoltaic cells 102.
In an embodiment, photovoltaic module 100 includes a first set 1102 of one or more first photovoltaic cells 702 that are electrically connected in series with each other. For example, first set 1102 may include a plurality of first photovoltaic cells 702 that are each configured to have the same energy conversion efficiency. For example, each of first photovoltaic cells 702 may be configured to reflect the same color light in a direction, e.g., red light, and therefore appear to have the same color as described above. Alternatively, each of first photovoltaic cells 702 may be differently colored or have different film configurations, but may nonetheless have approximately the same power efficiency. For example, a first photovoltaic cell 702 in the first set 1102 may appear as a first color and reflect light at a first constructive fringe for the first color, while a second photovoltaic cell 704 in the first set 1102 may appear as a second color and reflect light at a second constructive fringe for the second color. Although the reflected colors may differ, the power loss for the different colors may be similar at the different selected constructive fringes, and thus, differently colored photovoltaic cells 102 in the first set 1102 may have about the same conversion efficiency.
Although first set 1102 is shown with six photovoltaic cells in series, first set 1102 may include more or fewer photovoltaic cells. For example, in an embodiment, first set 1102 includes between ten and twelve photovoltaic cells electrically connected in series. Typically, a series of photovoltaic cells are limited in efficiency by the least efficient cell in the series. However, since in an embodiment every cell in first set 1102 includes the same configuration, e.g., reflects the same wavelength of light, the power production efficiency of first set 1102 may not be significantly constrained by any one of the first photovoltaic cells 702 as compared to another.
In an embodiment, photovoltaic module 100 includes a second set 1104 of photovoltaic cells divided into subsets. For example, second set 1104 may include a first subset 1106 and a second subset 1108. As shown, second set 1104 may include six subsets. However, this is illustrative and more or fewer subsets may be included in second set 1104. Rather than being electrically connected in series, subsets of second set 1104 may be connected in parallel. For example, each subset may include one or more photovoltaic cells electrically connected in series, and the subsets may then be electrically connected in parallel. Consequentially, the efficiency of each subset will be limited by the least efficient photovoltaic cell in the subset. Nonetheless, the overall efficiency of second set 1104 may not be so limited. More particularly, voltage harvesting from each subset may be accumulated independently from parallel subsets, and thus, accumulation of voltage may not be limited by any one of the subsets. Optionally, each photovoltaic cell in a subset may be similarly configured, i.e., may be similarly colored. However, in an embodiment, similar coloring of sets may be unnecessary except to the degree it is necessary to achieve similar energy conversion efficiencies in the photovoltaic cells of first set 1102.
Photovoltaic module 100 may include numerous combinations of sets. For example, second set 1104 may be electrically connected in series with a third set 1110, which like first set 1102, may include a plurality of photovoltaic cells electrically connected in series. Furthermore, like first set 1102, the photovoltaic cells in third set 1110 may be similarly colored so as to not limit the efficiency of third set 1110 to a single low efficiency photovoltaic cell. Additional sets like second set 1104 in which subsets of serially connected photovoltaic cells are connected in parallel may be added to build out photovoltaic module 100 further. Combinations of sets may be made in any manner, with sets connected in series, as shown, or with two or more sets of photovoltaic module 100 connected in parallel.
Referring to FIG. 12, a pictorial view of a photovoltaic module having an image formed from a plurality of photovoltaic cells is shown in accordance with an embodiment of the invention. In an embodiment, photovoltaic module 100 having a plurality of colorized photovoltaic cells 102 may be created such that reflections from the photovoltaic cells 102 visually combine to provide an image 1202. For example, photovoltaic cells 102 may essentially provide pixels of a displayed image 1202. In an embodiment, photovoltaic module 100 includes about 786,000 pixels arranged in a matrix fashion, such that an image resolution of 1024×768 may be achieved. Given that photovoltaic cells 102 may be individually formed and colored prior to being applied to receiving substrate 104, any size of image 1202 and/or resolution may be provided on photovoltaic module 100.
As an example, photovoltaic cells 102 may be arranged to display an image 1202, such as a company logo, on a photovoltaic module 100. In an embodiment, first set 1102 of photovoltaic cells 102 may include a film that preferentially reflects a background color and second set 1104 of photovoltaic cells 102 may include a film that preferentially reflects a logo color. As described above, second set 1104 may also include some photovoltaic cell pixels of background color electrically connected in parallel with photovoltaic cell pixels of logo color. Thus, the combination of first set 1102 and second set 1104 in an arrangement will display a logo image 1202 to an onlooker. It will be readily apparent that although a photovoltaic module 100 that reflects a logo image 1202 may be less efficient than one with a purely anti-reflective coating, the aesthetic value provided by such a photovoltaic module 100 may offset the lower energy conversion efficiency. Furthermore, color customization of photovoltaic cells 102 may invite architects, industrial designers, and marketers to integrate microscale photovoltaic cells 102 into products that presently do not use commercial photovoltaic cells 102 because of their traditional size and appearance.
In addition to providing a logo image 1202 as shown in FIG. 12, images 1202 of varying complexities may be achieved through the individual manipulation of photovoltaic cell reflective properties and the combination of colorized photovoltaic cells into an arrangement on a photovoltaic module 100. More particularly, by treating each photovoltaic cell as a pixel, additive color models may be employed to combine different color lights into a wide range of colors. Those additive color models may be implemented in a single photovoltaic cell design or achieved by combining photovoltaic cells reflections.
In an embodiment, additive color models may be realized through the use of multi-layered films. More specifically, multiple film layers may be tuned to cause constructive interference and reflection of multiple light wavelengths. For example, in the case of a photovoltaic cell 102 having a two-layered film with a top film layer 404 and a bottom film layer 402, the top film layer 404 may be tuned to include an optical thickness of about 240 nm, i.e., a half wavelength of blue light. Furthermore, the bottom film layer 402 may be tuned to include an optical thickness of about 110 nm, and thus, the combination of the upper layer and lower layer may create a total optical thickness for the film 204 of about 350 nm, i.e., a half wavelength of red light. Accordingly, constructive interference and reflection may occur for both blue light and red light, and the reflected light rays may visually combine to give the photovoltaic cell 102 a magenta color.
Alternatively, photovoltaic cells 102 adjacently positioned that respectively reflect red, green, and blue may be manipulated to cause the combination of reflected light to be recognized as a combined color, such as red brown or forest green. Manipulation to achieve this effect may be made according to the principles described above. For example, the intensity of reflected red, green, and blue light from individual photovoltaic cells may be controlled by controlling film properties such that film ray 318 and cell ray 320 constructively interfere to a greater or lesser degree. Intensity of a color will be greatest where film ray 318 and cell ray 320 have no phase shift, but lower intensities may also be achieved by shifting film ray 318 and cell ray 320 out of phase slightly. Then, by reflecting different light and light intensities from adjacent cells, the individual colors from multiple photovoltaic cells 102 may visually combine into a resultant color when viewed by a distant onlooker.
Referring to FIG. 13, a pictorial view of an integrated photovoltaic module having an image formed from a plurality of photovoltaic cells is shown in accordance with an embodiment of the invention. In contrast with macroscale photovoltaic cells, miniaturized photovoltaic cells 102 may fit within intricate shapes and contours of objects. Patterning and coloring of photovoltaic cells 102 placed on these objects may contribute to the overall design and function of the object. In an embodiment, photovoltaic module 100 may be applied over a flexible or curved surface to provide an aesthetic look and feel to an object. Given that photovoltaic module 100 may be formed with high flexibility, as described above, photovoltaic module 100 may conform to the movement and shape of the object that is to be visually modified. For example, photovoltaic module 100 may cover a curved surface, such as a helmet or parabolic reflector, or it may be adhered to a fabric substrate that flexes during use, such as clothing. As a further example, a tent may be covered by photovoltaic module 100 having first set 1102 of photovoltaic cells 102 of a first color and second set 1104 of photovoltaic cells 102 of a second color. The first color may be a red brown and the second color may be a forest green. Accordingly, when arranged in a particular pattern, the photovoltaic cells 102 may provide a camouflage pattern 1302 on the tent surface. It will be apparent that these implementations are non-limiting examples and that the integration of photovoltaic modules 100 as described above into product or structural form factors is virtually boundless.
In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments. It will be apparent however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. The particular embodiments described are not provided to limit the invention but to illustrate it. The scope of the invention is not to be determined by the specific examples provided above but only by the claims below. In other instances, well-known structures, devices, and operations have been shown in block diagram form or without detail in order to avoid obscuring the understanding of the description. Where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated in the figure to indicate corresponding or analogous elements, which may optionally have similar characteristics.
It should also be appreciated that reference throughout this specification to “one embodiment”, “an embodiment”, “one or more embodiments”, or “different embodiments”, for example, means that a particular feature may be included in the practice of the invention. Similarly, it should be appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects may lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of the invention.

## Claims

1. A photovoltaic cell comprising:
a device layer having a cell surface; and
a film having a film surface over the cell surface, wherein the film is configured to reflect a predetermined wavelength of visible light from the film surface and the cell surface in a direction away from the device layer,
wherein the film includes a first optical thickness at a first location such that rays of the visible light respectively reflected in the direction from the film surface and the cell surface interfere constructively at the first location,
wherein the film includes a second optical thickness at a second location of the film such that rays of another predetermined wavelength of the visible light respectively reflected in the direction from the film surface and the cell surface interfere constructively at the second location, and
wherein the first optical thickness is different from the second optical thickness.

2. A photovoltaic module comprising:
a first set of photovoltaic cells, each photovoltaic cell in the first set including a first cell surface under a first film;
a second set of photovoltaic cells, each photovoltaic cell in the second set including a second cell surface under a second film, wherein the second film differs from the first film such that the first set of photovoltaic cells and the second set of photovoltaic cells respectively reflect a first wavelength and a different second wavelength of visible light in a direction away from the photovoltaic module; and
a receiving substrate supporting the first set of photovoltaic cells and the second set of photovoltaic cells on a support surface, wherein respective reflections from the first set of photovoltaic cells and the second set of photovoltaic cells visually combine into an image on the photovoltaic module.

3. The photovoltaic module of claim 2, wherein the first set of photovoltaic cells is horizontally spaced apart from the second set of photovoltaic cells on a plane of the photovoltaic module.

4. The photovoltaic module of claim 2, wherein the first set of photovoltaic cells is stacked above the second set of photovoltaic cells out of a plane of the photovoltaic module.

5. The photovoltaic module of claim 2, wherein the first wavelength and the second wavelength differ by at least 10 nm.

6. The photovoltaic module of claim 5, wherein the first film includes a first optical thickness and the second film includes a second optical thickness different from the first optical thickness.

7. The photovoltaic module of claim 6, wherein the first optical thickness equals half of the first wavelength and the second optical thickness equals half of the second wavelength.

8. The photovoltaic module of claim 7, wherein the first film has a first film thickness corresponding to the first optical thickness and the second film has a second film thickness corresponding to the second optical thickness, and wherein at least one of the first film thickness and the second film thickness is greater than 75 nm.

9. The photovoltaic module of claim 6, wherein the first film includes a first material and the second film includes a second material different from the first material.

10. The photovoltaic module of claim 9, wherein the first material and the second material are selected from the group consisting of nitrides and oxides.

11. The photovoltaic module of claim 10, wherein at least one of the first film and the second film includes multiple layers.

12. The photovoltaic module of claim 5, wherein the first set of photovoltaic cells is electrically coupled with the second set of photovoltaic cells, wherein the first set of photovoltaic cells includes a plurality of photovoltaic cells electrically coupled in series, wherein the second set of photovoltaic cells includes a plurality of photovoltaic cell subsets, wherein the photovoltaic cell subsets are electrically coupled in parallel, and wherein each of the photovoltaic cell subsets includes a plurality of photovoltaic cells electrically coupled in series.

13. The photovoltaic module of claim 2, wherein the image is a camouflage pattern.

14. The photovoltaic module of claim 2, wherein the receiving substrate is configured to flex such that the support surface includes a bend radius of less than 20 mm.