US20070240760A1

Movatterモバイル変換

Info

Publication number: US20070240760A1
Application number: US11/809,274
Authority: US
Inventors: Chris Gronet
Original assignee: Solyndra Inc
Current assignee: Solyndra Inc
Priority date: 2005-06-20
Filing date: 2007-05-31
Publication date: 2007-10-18
Also published as: US7196262B2; US20070181176A1; US20060283498A1

Abstract

A solar cell assembly comprising a plurality of elongated solar cells is provided. Each solar cell in the plurality of cells comprises a conductive core configured as a first electrode, a semiconductor junction circumferentially disposed on the conductive core, and a TCO layer disposed on the semiconductor junction. The plurality of solar cells is arranged in a parallel manner in pairs on a transparent insulating substrate such that (i) the solar cells in each respective pair are joined together lengthwise by a corresponding counter-electrode, and (ii) solar cells adjacent pairs of solar cells do not touch each other. Each solar cell pair is affixed to the transparent insulating substrate. A first and second solar cell pair is electrically connected in series by an electrical contact that electrically connects the first electrode of each cell in the first pair to the corresponding counter-electrode of the second pair.

Description

1. FIELD OF THE INVENTION

This invention relates to solar cell assemblies for converting solar energy into electrical energy and more particularly to improved solar cell assemblies.

2. BACKGROUND OF THE INVENTION

Interest in photovoltaic cells has grown rapidly in the past few decades. Photovoltaic cells comprise semiconductor junctions such as p-n junctions. It is well known that light with photon energy greater than the band gap of an absorbing semiconductor layer in a semiconductor junction is absorbed by the layer. Such absorption causes optical excitation and the release of free electrons and free holes in the semiconductor. Because of the potential difference that exists at a semiconductor junction (e.g., a p-n junction), these released holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. The flow of carriers into the external circuit constitutes a electrical current density, J amp cm⁻², which, under short-circuit conditions, is known as the short-circuit current density, J_sc. At the same time, the separation of the charges (holes and electrons) sets up a potential difference between the two ends of the material, φ, which under open circuit conditions is known as the open-circuit voltage, φ_OC. It is desirable to maximize both J_scand φ_OC. For interaction with the solar spectrum, J_scand φ_OCare optimized when the junction semiconductor absorber has a band gap of about 1.4 electron volts (eV).

It is presently common practice to provide an array of solar cells to generate electrical energy from solar radiation. Many solar cells are made of silicon. However, cells made of other materials, e.g., cadmium sulfide and gallium arsenide, have also been developed and tested. Crystalline silicon has traditionally been a favored material since it has a band gap of approximately 1.1 eV and thus favorably responds to the electromagnetic energy of the solar spectrum. However, because of the expense in making crystalline silicon-based cells, thin film solar cells made of materials other than silicon have been explored and used.

Presently solar cells are fabricated as separate physical entities with light gathering surface areas on the order of 4-6 cm²or larger. For this reason, it is standard practice for power generating applications to mount the cells in a flat array on a supporting substrate or panel so that their light gathering surfaces provide an approximation of a single large light gathering surface. Also, since each cell itself generates only a small amount of power, the required voltage and/or current is realized by interconnecting the cells of the array in a series and/or parallel matrix.

A conventional prior art solar cell structure is shown inFIG. 1. Because of the large range in the thickness of the different layers, they are depicted schematically. Moreover,FIG. 1 is highly schematized so that it will represent the features of both “thick-film” solar cells and “thin-film” solar cells. In general, solar cells that use an indirect band gap material to absorb light are typically configured as “thick-film” solar cells because a thick film of the absorber layer is required to absorb a sufficient amount of light. Solar cells that use a direct band gap material to absorb light are typically configured as “thin-film” solar cells because only a thin layer of the direct band-gap material is need to absorb a sufficient amount of light.

The arrows at the top ofFIG. 1 show the direction of the solar illumination on the cell. Layer (element)102 is the substrate. Glass or metal is a common substrate. In thin-film solar cells,substrate102 can be-a polymer-based backing, metal, or glass. In some instances, there is an encapsulation layer (not shown)coating substrate102.Layer104 is the back electrical contact for the solar cell. It makes ohmic contact with the absorber layer ofsemiconductor junction106.

Layer

106 is the semiconductor absorber layer. In many but not all cases it is a p-type semiconductor.Absorber layer106 is thick enough to absorb light.Layer108 is the semiconductor junction partner—that completes the formation of a p-n junction, which is a common type of junction found in solar cells. In a solar cell based on a p-n junction, when absorber106 is a p-type doped material,junction partner108 is an n-type doped material. Conversely, whenlayer106 is an n-type doped material,layer108 is a p-type doped material. Generally,junction partner108 is much thinner than absorber106. For example, in someinstances junction partner108 has a thickness of about 0.05 microns.Junction partner108 is highly transparent to solar radiation.Junction partner108 is also known as the window layer, since it lets the light pass down to absorberlayer106.

In a typical thick-film solar cell,

layers

106 and108 can be made from the same semiconductor material but have different carrier types (dopants) and/or carrier concentrations in order to give the two layers their distinct p-type and n-type properties. In thin-film solar cells in which copper-indium-gallium-diselenide (CIGS) is absorberlayer106, the use of CdS to formlayer108 has resulted in high efficiency cells. Other materials that can be used forlayer108 includes, but are not limited to, SnO₂, ZnO, ZrO₂and doped ZnO.

Layer

110 is the top transparent electrode, which completes the functioning cell.Layer110 is used to draw current away from the junction sincejunction partner108 is generally too resistive to serve this function. As such,layer110 should be highly conductive and transparent to light.Layer110 can in fact be a comb-like structure of metal printed ontolayer108 rather than forming a discrete layer.Layer110 is typically a transparent conductive oxide (TCO) such as zinc oxide (ZnO), indium-tin-oxide (ITO), or tin oxide (SnO₂). However, even when a TCO layer is present, abus bar network114 is typically needed to draw off current since the TCO has too much resistance to efficiently perform this function in larger solar cells.Network114 shortens the distance charger carriers must move in the TCO layer in order to reach the metal contact, thereby reducing resistive losses. The metal bus bars, also termed grid lines, can be made of any reasonably conductive metal such as, for example, silver, steel or aluminum. In the design ofnetwork114, there is design a tradeoff between thicker grid lines that are more electrically conductive but block more light, and thin grid lines that are less electrically conductive but block less light. The metal bars are preferably configured in a comb-like arrangement to permit light rays throughTCO layer110. Busbar network layer114 andTCO layer110, combined, act as a single metallurgical unit, functionally interfacing with a first ohmic contact to form a current collection circuit. In U.S. Pat. No. 6,548,751 to Sverdrup et al., hereby incorporated by reference in its entirety, a combined silver (Ag) bus bar network and indium-tin-oxide layer function as a single, transparent ITO/Ag layer.

Layer

112 is an antireflection (AR) coating, which can allow a significant amount of extra light into the cell. Depending on the intended use of the cell, it might be deposited directly on the top conductor (as illustrated), or on a separate cover glass, or both. Ideally, the AR coating reduces the reflection of the cell to very near zero over the spectral region that photoelectric absorption occurs, and at the same time increases the reflection in the other spectral regions to reduce heating. U.S. Pat. No. 6,107,564 to Aguilera et al., hereby incorporated by reference in its entirety, describes representative antireflective coatings that are known in the art.

Solar cells typically produce only a small voltage. For example, silicon based solar cells produce a voltage of about 0.6 volts (V). Thus, solar cells are interconnected in series or parallel in order to get a reasonable voltage. When connected in series, voltages of individual cells add together while current remains the same. Thus, solar cells arranged in series reduce the amount of current flow through such cells, compared to analogous solar cells arrange in parallel, thereby improving efficiency. As illustrated inFIG. 1, the arrangement of solar cells in series is accomplished usinginterconnects116. In general, aninterconnect116 places the first electrode of one solar cell in electrical communication with the counterelectrode of an adjoining solar cell.

As noted above and as illustrated inFIG. 1, conventional solar cells are typically in the form of a plate structure. Although such cells are highly efficient when they are smaller, larger planar solar cells have reduced efficiency because it is harder to make the semiconductor films that form the junction in such solar cells uniform. Furthermore, the occurrence of pinholes and similar flaws increase in larger planar solar cells. These features can cause shunts across the junction.

A number of problems are associated with solar cell designs present in the known art. A number of prior art solar cell designs and some of the disadvantages of each design will now be discussed.

As illustrated inFIG. 2, U.S. Pat. No. 6,762,359 B2 to Asia et al. discloses asolar cell210 including a p-type layer12 and an n-type layer14. Afirst electrode32 is provided on one side of the solar cell.Electrode32 is in electrical contact with n-type layer14 ofsolar cell210.Second electrode60 is on the opposing side of the solar cell.Electrode60 is in electrical contact with the p-type layer of the solar cell. Light-transmitting

layers

200 and202 form one side ofdevice210 whilelayer62 forms the other side.

Electrodes

32 and60 are separated by

insulators

40 and50. In some instances, the solar cell has a tubular shape rather than the spherical shape illustrated inFIG. 2. Whiledevice210 is functional, it is unsatisfactory.Electrode60 has to pierceabsorber12 in order to make an electrical contact. This results in a net loss in absorber area, making the solar cell less efficient. Furthermore, such a junction is difficult to make relative to other solar cell designs.

As illustrated inFIG. 3A, U.S. Pat. No. 3,976,508 to Mlavsky discloses a tubular solar cell comprising a cylindrical silicon tube2 of n-type conductivity that has been subjected to diffusion of boron into its outer surface to form an outer p-conductivity type region4 and thus ap-n junction6. The inner surface of the cylindrical tube is provided with a first electrode in the form of an adherent metal conductive film8 that forms an ohmic contact with the tube. Film8 covers the entire inner surface of the tube and consists of a selected metal or metal alloy having relatively high conductivity, e.g., gold, nickel, aluminum, copper or the like, as disclosed in U.S. Pat. Nos. 2,984,775, 3,046,324 and 3005862. The outer surface is provided with a second electrode in the form of a grid consisting of a plurality of circumferentially extendingconductors10 that are connected together by one or more longitudinally-extendingconductors12. The opposite ends of the outer surface of the hollow tube are provided with two circumferentially-extending

terminal conductors

14 and16 that intercept the longitudinally-extendingconductors12. The spacing of the circumferentially-extendingconductors10 and the longitudinally-extendingconductors12 is such as to leaveareas18 of the outer surface of the tube exposed to solar radiation.

Conductors

12,14 and16 are made wider than the circumferentially-extendingconductors10 since they carry a greater current than any of the latter. These conductors are made of an adherent metal film like the inner electrode8 and form ohmic contacts with the outer surface of the tube. While the solar cell disclosed inFIG. 3 is functional, it is also unsatisfactory.

Conductors

12,14, and16 are not transparent to light and therefore the amount of light that the solar cell receives is proportionally reduced by the amount of surface area occupied by the cells.

U.S. Pat. No. 3,990,914 to Weinstein and Lee discloses another form of tubular solar cell. Like Mlavsky, the Weinstein and Lee solar cell has a hollow core. However, unlike Mlavsky, Weinstein and Lee dispose the solar cell on a glass tubular support member. The Weinstein and Lee solar cell has the drawback of being bulky and expensive to build.

Referring toFIGS. 3B and 3C, Japanese Patent Application Kokai Publication Number S59-125670, Toppan Printing Company, published Jul. 20, 1984 (hereinafter “S59-125670”) discloses a rod-shaped solar cell. The rod shaped solar cell is depicted in cross-section in Figure. A conducting metal is used as thecore1 of the cell. A light-activated amorphous silicon semiconductor layer3 is provided oncore1. An electrically conductive transparentconductive layer4 is built up on top of semiconductor layer3. The transparentconductive layer4 can be made of materials such as indium oxide, tin oxide or indium tin oxide (ITO) and the like. As illustrated inFIG. 3B, alayer5, made of a good electrical conductor, is provided on the lower portion of the solar cell. The publication states that this goodconductive layer5 is not particularly necessary but helps to lower the contact resistance between the rod and aconductive substrate7 that serves as a counter electrode. As such,conductive layer5 serves as a current collector that supplements the conductivity ofcounter electrode7 illustrated inFIG. 3C.

As illustrated inFIG. 3C, rod-shapedsolar cells6 are multiply arranged in a row parallel with each other, andcounter electrode layer7 is provided on the surface of the rods that is not irradiated by light so as to electrically make contact with each transparentconductive layer4. The rod-shapedsolar cells6 are arranged in parallel and both ends of the solar cells are hardened with resin or a similar material in order to fix the rods in place.

S59-125670 addresses many of the drawbacks associated with planar solar cells. However, S59-125670 has a number of significant drawbacks that limit the efficiency of the disclosed devices. First, the manner in which current is drawn off the exterior surface is inefficient becauselayer5 does not wrap all the way around the rod (e.g., seeFIG. 3B). Second,substrate7 is a metal plate that does not permit the passage of light. Thus, a full side of each rod is not exposed to light and can thus serve as a leakage path. Such a leakage path reduces the efficiency of the solar cell. For example, any such dark junction areas will result in a leakage that will detract from the photocurrent of the cell. Another disadvantage with the design disclosed inFIGS. 3B and 3C is that the rods are arranged in parallel rather than in series. Thus, the current levels in such devices will be large, relative to a corresponding serially arranged model, and therefore subject to resistive losses.

Referring toFIG. 3D, German Unexamined Patent Application DE 43 39 547 A1 to Twin Solar-Technik Entwicklungs-GmbH, published May 24, 1995, (hereinafter “Twin Solar”) also discloses a plurality of rod-shaped solar cells2 arranged in a parallel manner inside atransparent sheet28, which forms the body of the solar cell. Thus, Twin Solar does not have some of the drawbacks found in S59-125670.Transparent sheet28 allows light in from both

faces

47A and47B.Transparent sheet28 is installed at a distance from awall27 in such a manner as to provide anair gap26 through which liquid coolant can flow. Thus, Twin Solar devices have the drawback that they are not truly bifacial. In other words, only face47A of the Twin Solar device is capable of receiving direct light. As defined here, “direct light” is light that has not passed through any media other than air. For example, light that has passed through a transparent substrate, into a solar cell assembly, and exited the assembly is no longer direct light once it exits the solar cell assembly. Light that has merely reflected off of a surface, however, is direct light provided that it has not passed through a solar cell assembly. Under this definition of direct light, face47B is not configured to receive direct light. This is because all light received byface47B must first traverse the body of the solar cell apparatus after entering the solar cell apparatus throughface47A. Such light must then traverse coolingchamber26, reflect offback wall42, and finally re-enter the solar cell throughface47B. The solar cell assembly is therefore inefficient because direct light cannot enter both sides of the assembly.

Discussion or citation of a reference herein will not be construed as an admission that such reference is prior art to the present invention.

3. SUMMARY OF THE INVENTION

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates interconnected solar cells in accordance with the prior art.

FIG. 2 illustrates a spherical solar cell including a p-type inner layer and an n-type outer layer in accordance with the prior art.

FIG. 3A illustrates a tubular photovoltaic element comprising a cylindrical silicon tube of n-type conductivity that has been subjected to diffusion of boron into its outer surface to form an outer p-conductivity type region and thus a tubular solar cell in accordance with the prior art.

FIG. 3B is a cross-sectional view of an elongated solar cell in accordance with the prior art.

FIG. 3C is a cross-sectional view of a solar cell assembly in which a plurality of elongated solar cells are affixed to an electrically conductive substrate in accordance with the prior art.

FIG. 3D is a cross-sectional view of a solar cell assembly disposed a distance away from a reflecting wall in accordance with the prior art.

FIG. 4A is a cross-sectional view of elongated solar cells electrically arranged in series and geometrically arranged in a parallel or near parallel manner on counter-electrodes that contact a substrate in order to form a bifacial assembly, in accordance with an embodiment of the present invention.

FIG. 4B is a cross-sectional view taken aboutline4B-4B ofFIG. 4A depicting the serial electrical arrangement of tubular solar cells in a bifacial assembly in accordance with an embodiment of the present invention.

FIG. 4C is a blow-up perspective view ofregion4C ofFIG. 4B, illustrating various layers in elongated solar cells in accordance with one embodiment of the present invention.

FIG. 4D is a cross-sectional view of an elongated solar cell taken aboutline4D-4D ofFIG. 4B, in accordance with an embodiment of the present invention.

FIG. 4E is a cross-sectional view taken aboutline4B-4B ofFIG. 4A that depicts the serial arrangement of tubular solar cells in a bifacial assembly in accordance with an alternative embodiment of the present invention.

FIG. 4F is a cross-sectional view of a elongated solar cell taken aboutline4F-4F ofFIG. 4E, in accordance with an embodiment of the present invention.

FIGS. 5A-5D depict semiconductor junctions that are used in various elongated solar cells in various embodiments of the present invention.

FIG. 6A is a cross-sectional view of elongated solar cells electrically arranged in series in a bifacial assembly where counter-electrodes form interfaces between solar cell pairs, in accordance with another embodiment of the present invention.

FIG. 6B is a cross-sectional view taken aboutline6B-6B ofFIG. 6A that depicts the serial arrangement of tubular solar cells in a bifacial assembly in accordance with an embodiment of the present invention.

FIG. 6C is a cross-sectional view of an elongated solar cell taken aboutline6C-6C ofFIG. 6B, in accordance with an embodiment of the present invention.

FIG. 7A is a cross-sectional view of elongated solar cells electrically arranged in series in a bifacial assembly where counter-electrodes abut individual solar cells, in accordance with another embodiment of the present invention.

FIG. 7B is a cross-sectional view taken about line7B-7B ofFIG. 7A that depicts the serial arrangement of tubular solar cells in a bifacial assembly in accordance with an embodiment of the present invention.

FIG. 8 is a cross-sectional view of elongated solar cells electrically arranged in series in a bifacial assembly where counter-electrodes abut individual solar cells and the outer TCO is cut, in accordance with another embodiment of the present invention.

FIG. 9 is a cross-sectional view of elongated solar cells electrically arranged in series in a bifacial assembly in which the inner metal electrode is hollowed, in accordance with an embodiment of the present invention.

FIG. 10 is a cross-sectional view of elongated solar cells electrically arranged in series in a bifacial assembly in which a groove pierces the counter-electrodes, transparent conducting oxide layer, and junction layers of the solar cells, in accordance with an embodiment of the present invention.

FIG. 11 illustrates how the solar cell assemblies of the present invention can be used in conjunction with one type of static concentrator.

FIG. 12 illustrates how the solar cell assemblies of the present invention can be used in conjunction with another type of static concentrator.

FIG. 13 illustrates a solar cell made by a roll method in accordance with an embodiment of the present invention.

Like reference numerals refer to corresponding parts throughout the several views of the drawings. Dimensions are not drawn to scale.

5. DETAILED DESCRIPTION

Disclosed herein are solar cell assemblies for converting solar energy into electrical energy and more particularly to improved solar cells and solar cell arrays. The solar cells of the present invention have a wire shape and are arranged in parallel but are electrically connected in series.

5.1 Basic Structure

The present invention provides asolar cell assembly400 in which elongatedsolar cells402, shown in cross-section inFIG. 4A, serve to absorb light. A conductive core (elongated conductive core)404 serves as the first electrode in the assembly and a transparent conductive oxide (TCO)412 on the exterior surface of each solar cell serves as the counter electrode.

In general,conductive core404 is made out of any material such that it can support the photovoltaic current generated by solar cell with negligible resistive losses. In some embodiments,conductive core404 is composed of any conductive material, such as aluminum, molybdenum, steel, nickel, silver, gold, or an alloy thereof. In some embodiments,conductive core404 is made out of a metal-, graphite-, carbon black-, or superconductive carbon black-filled oxide, epoxy, glass, or plastic. In some embodiments,conductive core404 is made of a conductive plastic. As defined herein, a conductive plastic is one that, through compounding techniques, contains conductive fillers which, in turn, impart their conductive properties to the plastics system. The conductive plastics used in the present invention to formconductive core404 contain fillers that form sufficient conductive current-carrying paths through the plastic matrix to support the photovoltaic current generated by solar cell with negligible resistive losses. The plastic matrix of the conductive plastic is typically insulative, but the composite produced exhibits the conductive properties of the filler.

Asemiconductor junction410 is formed aroundconductive core404.Semiconductor junction410 is any photovoltaic homojunction, heterojunction, heteroface junction, buried homojunction, or p-i-n junction having an absorber layer that is a direct band-gap absorber (e.g., crystalline silicon) or an indirect band-gap absorber (e.g., amorphous silicon). Such junctions are described inChapter 1 of Bube,Photovoltaic Materials,1998, Imperial College Press, London, which is hereby incorporated by reference in its entirety. Details of exemplary types ofsemiconductors junctions410 in accordance with the present invention are disclosed in Section 5.2, below. In addition to the exemplary junctions disclosed in Section 5.2, below,junctions410 can be multijunctions in which light traverses into the core ofjunction410 through multiple junctions that, preferably, have successfully smaller bandgaps.

Optionally, there is a thin intrinsic layer (i-layer)415 betweensemiconductor junction410 and an outer transparent conductive oxide (TCO)layer412. The i-layer415 can be formed using any undoped transparent oxide including, but not limited to, zinc oxide or indium-tin-oxide.

The transparent conductive oxide (TCO)layer412 is built up on top of the semiconductor junction layers410 thereby completing the circuit. As noted above, in some embodiments, there is a thin i-layer coating thesemiconductor junction410. In such embodiments,TCO layer412 is built on top of the i-layer. In some embodiments,TCO layer412 is made of tin oxide SnO_x(with or without fluorine doping), indium-tin oxide (ITO), doped zinc oxide (ZnO) or any combination thereof. In some embodiments,TCO layer412 is either p-doped or n-doped. For example, in embodiments where the outer semiconductor layer ofjunction410 is p-doped,TCO layer412 can be p-doped. Likewise, in embodiments where the outer semiconductor layer ofjunction410 is n-doped,TCO layer412 can be n-doped. In general,TCO layer412 is preferably made of a material that has very low resistance, suitable optical transmission properties (e.g., greater than 90%), and a deposition temperature that will not damage underlying layers ofsemiconductor junction410 and/or optional i-layer415. In some embodiments,TCO412 is an electrically conductive polymer material such as a conductive polytiophene, a conductive polyaniline, a conductive polypyrrole, a PSS-doped PEDOT (e.g., Bayrton), or a derivative of any of the foregoing. In some embodiments, TCO comprises more than one layer, including a first layer comprising tin oxide SnO_x(with or without fluorine doping), indium-tin oxide (ITO), zinc oxide (ZnO) or a combination thereof and a second layer comprising a conductive polytiophene, a conductive polyaniline, a conductive polypyrrole, a PSS-doped PEDOT (e.g., Bayrton), or a derivative of any of the foregoing. Additional suitable materials that can be used to form TCO layer are disclosed in United States Patent publication 2004/0187917A1 to Pichler, which is hereby incorporated by reference in its entirety.

Rod-shaped (elongated)solar cells402 are lined up multiply parallel. The entire assembly is sealed between electrically resistanttransparent substrate406 and a covering422 using a sealant such as ethyl vinyl acetate. Covering422 is generally made from the same materials assubstrate406. Suitable materials for covering422 andsubstrate406 include, but are not limited to glass or polyvinyl fluoride products such as Tedlar (DuPont, Wilmington, Del.).

FIG. 4B provides a cross-sectional view with respect toline4B-4B ofFIG. 4A. As can be seen withFIGS. 4A and 4B, eachelongated cell402 has a length that is great compared to the diameter d of its cross-section. An advantage of the architecture shown inFIG. 4A is that there is no front side contact that shadessolar cells402. Such a front side contact is found in known devices (e.g.,elements10 ofFIG. 3). Another advantage of the architecture shown inFIG. 4A is thatelongated cells402 are electrically connected in series rather than in parallel. In such a series configuration, the voltage of eachelongated cell402 is summed. This serves to increase the voltage across the system, thereby keeping the current down, relative to comparable parallel architectures, and minimizing resistive losses. A serial electrical arrangement is maintained by arranging all or a portion of the elongatedsolar cells402 such that they do not touch each other, as illustrated inFIGS. 4A and 4B. The separation distance betweensolar cells402 is any distance that prevents electrical contact betweensolar cells402. For instance, in some embodiments, the distance between adjacentsolar cells402 is 0.1 micron or greater, 0.5 microns or greater, or between 1 and 5 microns.

Another advantage of the architecture shown inFIG. 4A is that the resistance loss across the system is low. This is because each electrode component of the circuit is made of highly conductive material. For example, as noted above,conductive core404 of eachsolar cell402 is made of a conductive metal. Furthermore, eachconductive core404 has a diameter that is large enough to carry current without an appreciable current loss due to resistance. While largerconductive cores404 ensure low resistance, TCO layers encompassing such largerconductive cores404 must carry current further to contacts (counter-electrode strip)420. Thus, there is an upper bound on the size ofconductive cores404. In view of these and other considerations, diameter d is between 0.5 millimeters (mm) and 20 mm in some embodiments of the present invention. Thus,conductive cores404 are sized so that they are large enough to carry a current without appreciable resistive loss, yet small enough to allowTCO412 to efficiently deliver current to leads420. With this balanced design, resistive loss is minimized and an efficientsolar cell assembly400 is realized.

The advantageous low resistance nature of the architecture illustrated inFIG. 4A is also facilitated by the highly conductive properties ofleads420. In some embodiments, for example, leads420 are composed of a conductive epoxy (e.g., silver epoxy) or conductive ink and the like.

There are a number of different ways in which elongatedcells402 can be packaged in order to formsolar cell assembly400. For example, in one embodiment, leads420 are formed by depositing a thin metallic layer onsubstrate406 and then patterning the layer into a series of parallel strips, where each strip runs the length of asolar cell402. Then, elongatedsolar cells402 are affixed tosubstrate406 byleads420 using a conductive epoxy. In some embodiments, leads420 are formed directly onsolar cells402 andsolar cells402 are not affixed tosubstrate406. In such embodiments, there are at least two different ways in which elongatedsolar cells402 can be packaged to formsolar cell assembly400. In a first approach, elongatedsolar cells402, having leads420 as illustrated inFIG. 4A, rest onsubstrate406 but are not affixed to the substrate. In a second approach, elongatedsolar cells402, having leads420 as illustrated inFIG. 4A, do not contactsubstrate406. This second approach is not illustrated. In this second approach, a layer of ethyl vinyl acetate or some other suitable transparent material separatescontacts420 fromsubstrate406.

Still another advantage of the architecture illustrated inFIG. 4A is that the path length through the absorber layer (e.g.,

layer

502,510,520, or540 ofFIG. 5) ofsemiconductor junction410 is, on average, longer than the path length through of the same type of absorber layer having the same width but in a planar configuration. Thus, the elongated architecture illustrated inFIG. 4A allows for the design of thinner absorption layers relative to analogous planar solar cell counterparts. In the elongated architecture, the thinner absorption layer absorbs the light because of the increased path length through the layer. Because the absorption layer is thinner relative to comparable planar solar cells, there is less resistance and, hence, an overall increase in efficiency in the cell relative to analogous planar solar cells. Additional advantages of having a thinner absorption layer that still absorbs sufficient amounts of light is that such absorption layers require less material and are thus cheaper. Furthermore, thinner absorption layers are faster to make, thereby further lowering production costs.

Another advantage of elongatedsolar cells402 illustrated inFIG. 4A is that they have a relatively small surface area, relative to comparable planar solar cells, and they possess radial symmetry. Each of these properties allow for the controlled deposition of doped semiconductor layers necessary to formsemiconductor junction410. The smaller surface area, relative to conventional flat panel solar cells, means that it is easier to present a uniform vapor across the surface during deposition of the layers that formsemiconductor junction410. The radial symmetry can be exploited during the manufacture of the cells in order to ensure uniform composition (e.g., uniform material composition, uniform dopant concentration, etc.) and/or uniform thickness of individual layers ofsemiconductor junction410. For example, theconductive core404 upon which layers are deposited to makesolar cells402 can be rotated along its longitudinal axis during such deposition in order to ensure uniform material composition and/or uniform thickness.

The cross-sectional shape ofsolar cells402 is generally circular inFIG. 4B. In other embodiments,solar cell402 bodies with a quadrilateral cross-section or an elliptical shaped cross-section and the like are used. In fact, there is no limit on the cross-sectional shape ofsolar cells402 in the present invention, so long as thesolar cells402 maintain a general overall rod-like or wire-like shape in which their length is much larger than their diameter and they possess some form of cross-sectional radial symmetry.

As illustrated inFIG. 4B,assembly400 comprises many elongatedsolar cells402 geometrically arranged in parallel fashion and electrically connected in series. For example, a first and second elongated solar cell (rod-shaped solar cell)402 are electrically connected in series by anelectrical contact433 that connects the conductive core404 (first electrode) of the first elongatedsolar cell402 to the correspondingcounter-electrode strip420 electrode strip of the second elongated solar cell. Thus, as illustrated inFIG. 4A, elongatedsolar cells402 are the basic unit that respectively forms thesemiconductor layer410, theTCO412, and the metalconductive core404 of the elongatedsolar cell402. The elongatedsolar cells402 are multiply arranged in a row parallel or nearly parallel with respect to each other and rest upon independent leads (counter electrodes)420 that are electrically isolated from each other. Advantageously, in the configuration illustrated inFIG. 4A, elongatedsolar cells402 can receive direct light either throughsubstrate406, covering422, or bothsubstrate406 and covering422.

In some embodiments, not all elongatedsolar cells402 inassembly400 are electrically arranged in series. For example, in some embodiments, there are pairs of elongatedsolar cells402 that are electrically arranged in parallel. A first and second elongated solar cell can be electrically connected in parallel, and are thereby paired, by using a first electrical contact (e.g., an electrically conducting wire, etc., not shown) that joins theconductive core404 of a first elongated solar cell to the second elongated solar cell. To complete the parallel circuit, theTCO412 of the first elongatedsolar cell402 is electrically connected to theTCO412 of the second elongatedsolar cell402 either by contacting the TCOs of the two elongated solar cells either directly or through a second electrical contact (not shown). The pairs of elongated solar cells are then electrically arranged in series. In some embodiments, three, four, five, six, seven, eight, nine, ten, eleven or more elongatedsolar cells402 are electrically arranged in parallel. These parallel groups of elongatedsolar cells402 are then electrically arranged in series.

In some embodiments, rather than packagingsolar cells402 between asubstrate406 and cover422 using a sealant such as ethyl vinyl acetate,solar cells402 arranged in the same planar parallel configuration illustrated inFIGS. 4A and 4B are encased in a rigid transparent film. Suitable materials for such a rigid transparent film include, but are not limited to, polyvinyl fluoride products such as Tedlar (DuPont, Wilmington, Del.).

FIG. 4C is an enlargement ofregion4C ofFIG. 4B in which a portion ofconductive core404 and transparent conductive oxide (TCO)412 have been cut away to illustrate the positional relationship betweencounter-electrode strip420,elongated cell402, and electrically resistanttransparent substrate406. FurthermoreFIG. 4C illustrates howelectrical contact433 joins theconductive core404 of one elongatedsolar cell402 to the counter-electrode420 of anothersolar cell402.

One advantage of the configuration illustrated inFIG. 4 is thatelectrical contacts433 that serially connectsolar cells402 together only need to be placed on one end ofassembly400, as illustrated inFIG. 4B. Thus, referring toFIG. 4D, which is a cross-sectional view of a elongated solar402 cell taken aboutline4D-4D ofFIG. 4B, it is possible to completely seal far-end455 ofsolar cell402 in the manner illustrated. In some embodiments, the layers in this seal are identical to the layers circumferentially disposed lengthwise onconductive core404, namely, in order of deposition onconductive core404,semiconductor junction410, optional thin intrinsic layer (i-layer)415, and transparent conductive oxide (TCO)layer412. In such embodiments, end455 can receive sun light and therefore contribute to the electrical generating properties of thesolar cell402.

FIG. 4D also illustrates how the various layers deposited onconductive core404 are tapered atend466 whereelectrical contacts433 are found. For instance, a terminal portion ofconductive core404 is exposed, as illustrated inFIG. 4D. In other words,semiconductor junction410, optional i-layer415, andTCO412 are stripped away from a terminal portion ofconductive core404. Furthermore, a terminal portion ofsemiconductor junction410 is exposed as illustrated inFIG. 4D. That is, optional i-layer415 andTCO412 are stripped away from a terminal portion ofsemiconductor junction410. Such a configuration is advantageous because it prevents a short from developing betweenTCO412 andconductive core404. InFIG. 4D, elongatedsolar cell402 is positioned oncounter-electrode strip420 which, in turn, is positioned onto electrically resistanttransparent substrate406. However, there is no requirement thatcounter-electrode strip420 make contact with electrically resistanttransparent substrate406. In fact, in some embodiments, elongatedsolar cells402 and their corresponding electrode strips420 are sealed between electrically resistanttransparent substrate406 and covering422 in such a manner that they do not contactsubstrate406 and covering422. In such embodiments, elongatedsolar cells402 and corresponding electrode strips420 are fixedly held in place by a sealant such as ethyl vinyl acetate.

FIG. 4D further provides a perspective view ofelectrical contacts433 that serially connect elongatedsolar cells402. For instance, a first electrical contact433-1 electrically interfaces withcounter-electrode420 whereas a second electrical contact433-2 electrically interfaces with conductive core404 (the first electrode of elongated solar cell402). First electrical contact433-1 serially connects the counter-electrode of elongatedsolar cell402 to theconductive core404 of another elongatedsolar cell402 inassembly400. Second electrical contact433-2 serially connects theconductive core404 of elongatedsolar cell402 to the counter-electrode420 of another elongatedsolar cell402 inassembly400.

FIG. 4E provides a cross-sectional view with respect toline4B-4B ofFIG. 4A in accordance with another embodiment of the present invention.FIG. 4E is similar toFIG. 4B. However, inFIG. 4E, elongatedsolar cells402 facingend455 are not sealed as they are inFIG. 4B andFIG. 4D. Thus, the ends of elongatedsolar cells402 facingend455 cannot contribute to the photovoltaic potential ofsolar cell402. However, the embodiment illustrated inFIG. 4E has the advantage of being easier to make than the embodiment illustrated inFIGS. 4B and 4D. Furthermore, in many instances, the loss of contribution to the photovoltaic potential fromend455 is negligible because the surface area of such ends is so small.FIG. 4F is a cross-sectional view of a elongated solar402 cell taken aboutline4F-4F ofFIG. 4E which further illustrates the configuration ofend455 of elongatedsolar cell402 in accordance with the embodiment of the invention illustrated inFIG. 4E.

FIG. 6 illustrates asolar cell assembly600 in accordance with the present invention. Specifically,FIG. 6A is a cross-sectional view of rod-shaped (elongated)solar cells402 electrically arranged in series in abifacial assembly600 wherecounter-electrodes420 form interfaces between solar cell pairs402. As illustrated inFIG. 6A,solar cell assembly600 comprises a plurality of elongatedsolar cells402. There is no limit to the number ofsolar cells402 in this plurality (e.g., 1000 or more, 10,000 or more, between 5,000 and one millionsolar cells402, etc.). As in the embodiment of the invention illustrated inFIG. 4 and described above, each elongatedsolar cell402 comprises aconductive core404 with asemiconductor junction410 circumferentially disposed on the conductive core. A transparentconductive oxide layer412 circumferentially disposed on thesemiconductor junction412 completes the circuit.

As illustrated inFIGS. 6A and 6B, the plurality of elongatedsolar cells402 are geometrically arranged in a parallel or near parallel manner as a plurality of solar cell pairs so as to form a planar array having a first face (onside633 ofassembly600 as illustrated inFIG. 6A) and a second face (onside655 ofassembly600 as illustrated inFIG. 6A).Solar cells402 in a pair of solar cells do not touch thesolar cells402 in an adjacent pair of solar cells. However, in the embodiment illustrated inFIG. 6,solar cells402 within a given pair of solar cells are in electrical contact with each other through theircommon counter-electrode420. Accordingly,assembly600 comprises a plurality ofmetal counter-electrodes420. Each respective metal counter-electrode in the plurality of metal counter-electrodes joins together, lengthwise, elongatedsolar cells402 in a corresponding solar cell pair in the plurality of solar cell pairs. As such, elongatedsolar cells402 in a solar cell pair are electrically arranged in parallel, not series.

In some embodiments there is a first groove677-1 and a second groove677-2 that each runs lengthwise on opposing sides ofsolar cell402. InFIG. 6A, some but not allgrooves677 are labeled. In some embodiments, the counter-electrode420 of each pair ofsolar cells402 is fitted between opposinggrooves677 in the solar cell pair in the manner illustrated inFIG. 6A. The present invention encompassesgrooves677 that have a broad range of depths and shape characteristics and is by no means limited to the shape of thegrooves677 illustrated inFIG. 6A. In general, any type ofgroove677 that runs along the long axis of a firstsolar cell402 in a solar cell pair and that can accommodate all or part ofcounter-electrode420 in a pairwise fashion together with an opposing groove on the secondsolar cell402 in the solar cell pair is within the scope of the present invention.

As illustrated inFIG. 6A, a transparent electrically insulatingsubstrate406 covers all or a portion offace655 of the planar array of solar cells. In some embodiments,solar cells402

touch substrate

406. In some embodiments,solar cells402 do not touchsubstrate406. In embodiments in whichsolar cells402 do not touchsubstrate406, a sealant such as ethyl vinyl acetate is used to sealsubstrate406 ontosolar cells402.

FIG. 6B provides a cross-sectional view with respect toline6B-6B ofFIG. 6A. As can be seen inFIGS. 6A and 6B, each elongatedsolar cell402 has a length that is great compared to the diameter of its cross-section. Typically eachsolar cell402 has a rod-like shape (e.g., has a wire shape). Each solar cell pair is electrically connected to other solar cell pairs in series by arranging the solar cell pairs such that they do not touch each other, as illustrated inFIGS. 4A and 4B. The separation distance between solar cells pairs is any distance that prevents electrical contact between the cells. For instance, in some embodiments, the distance between adjacent solar cell pairs is 0.1 micron or greater, 0.5 microns or greater, or between 1 and 5 microns. Serial electrical contact between solar cell pairs is made byelectrical contacts677 that electrically connect theconductive cores404 of each elongated solar cell in a one solar cell pair to the corresponding counter-electrode120 of a different solar cell pair as illustrated inFIG. 6B.FIG. 6B further illustrates a cutaway ofconductive core404 andsemiconductor junction410 in onesolar cell402 to further illustrate the architecture of the solar cells.

Referring back toFIG. 6A, in some embodiments,solar cell assembly600 further comprises a transparent insulatingcovering422 disposed onface633 of the planar array ofsolar cells402, thereby encasing the plurality of elongatedsolar cells402 between the transparent insulatingcovering422 and the transparent electrically insulatingsubstrate406. In such embodiments, transparent insulatingcovering422 and the transparent insulatingsubstrate406 are bonded together by a sealant such as ethyl vinyl acetate. Although not illustrated inFIGS. 6A and 6B, in preferred embodiments, there is an intrinsic layer circumferentially disposed between thesemiconductor junction410 andTCO412. In some embodiments, this intrinsic layer is formed by an undoped transparent oxide such as zinc oxide, indium-tin-oxide, or a combination thereof.

In some embodiments, thesemiconductor junction410 ofsolar cells402 inassembly600 comprise an inner coaxial layer and an outer coaxial layer, where the outer coaxial layer comprises a first conductivity type and the inner coaxial layer comprises a second, opposite, conductivity type. In some embodiments, the inner coaxial layer comprises copper-indium-gallium-diselenide (CIGS) and the outer coaxial layer comprises CdS, SnO₂, ZnO, ZrO₂, or doped ZnO. In some embodiments,conductive core404 and/orelectrical contacts677 and/or counter-electrodes420 are made of aluminum, molybdenum, steel, nickel, silver, gold, or an alloy thereof. In some embodiments, transparentconductive oxide layer412 is made of tin oxide SnO_x, with or without fluorine doping, indium-tin oxide (ITO), zinc oxide (ZnO) or a combination thereof. In some embodiments, transparent insulatingsubstrate406 and transparent insulatingcovering422 comprise glass or Tedlar. Although not shown inFIG. 6, in some embodiments,conductive core404 is hollowed as depicted inFIG. 9.

FIG. 6C illustrates a cross-sectional view of an elongated solar402 cell taken aboutline6C-6C ofFIG. 46.FIG. 6C illustrates how the various layers deposited onconductive core404 are tapered at either end687 or688 (end687 as illustrated inFIG. 6C). For instance, a terminal portion ofconductive core404 is exposed, as illustrated inFIG. 6C. In other words,semiconductor junction410, an optional i-layer (not shown), andTCO412 are stripped away from a terminal portion ofconductive core404 at an end of the solar cell. Furthermore, a terminal portion ofsemiconductor junction410 is exposed as illustrated inFIG. 6C. That is, optional i-layer (not shown) andTCO412 are stripped away from the terminal portion ofsemiconductor junction410 at an end of the solar cell (end687 inFIG. 6C). Such a configuration is advantageous because it prevents an electrical short from developing betweenTCO412 andconductive core404. InFIG. 6C, elongatedsolar cell402 is positioned on electrically resistanttransparent substrate406. However, there is no requirement that elongatedsolar cell402 make direct contact with electrically resistanttransparent substrate406. In fact, in some embodiments, elongatedsolar cells402 are sealed between electrically resistanttransparent substrate406 and covering422 in such a manner that they do not contactsubstrate406 and covering422. In such embodiments, elongatedsolar cells402 are fixedly held in place by a sealant such as ethyl vinyl acetate.

In some embodiments, not all elongated solar cell pairs inassembly600 are electrically arranged in series. For example, in some embodiments, two or more pairs of elongated solar cells are themselves paired such that all the elongated solar cells in the paired pairs are electrically arranged in parallel. This can be accomplished by joining theconductive core404 of each of the solar cells by a common electrical contact (e.g., an electrically conducting wire, etc., not shown). To complete the parallel circuit, theTCO412 of each of the elongatedsolar cell402 are electrically joined together either by direct contact or by the use of a second electrical contact (not shown). The paired pairs of elongated solar cells are then electrically arranged in series. In some embodiments, three, four, five, six, seven, eight, nine, ten, eleven or more pairs of elongated solar cells are electrically arranged in parallel. These parallel groups of elongatedsolar cells402 are then electrically arranged in series.

FIG. 7 illustratessolar cell assembly700 in accordance with another embodiment of the present invention.Solar cell assembly700 comprises a plurality of elongatedsolar cells402. Each elongatedsolar cell402 in the plurality of elongated solar cells has aconductive core404 configured as a first electrode, asemiconductor junction410 circumferentially disposed on theconductive core402 and a transparentconductive oxide layer412 disposed on thesemiconductor junction410. The plurality of elongatedsolar cells402 are geometrically arranged in a parallel or a near parallel manner thereby forming a planar array having a first face (facingside733 of assembly700) and a second face (facingside766 of assembly700). The plurality of elongated solar cells is arranged such that one or more elongated solar cells in the plurality of elongated solar cells do not contact adjacent elongated solar cells. In preferred embodiments, the plurality of elongated solar cells is arranged such that each of the elongated solar cells in the plurality of elongated solar cells does not directly contact (through outer the TCO layer412) adjacent elongatedsolar cells402.

In some embodiments there is a first groove777-1 and a second groove777-2 that each runs lengthwise on opposing sides ofsolar cell402. InFIG. 7A, some but not all grooves777 are labeled. In some embodiments, there is a counter-electrode420 in one or both grooves of the solar cells. In the embodiment illustrated inFIG. 6A, there is a counter-electrode fitted lengthwise in both the first and second grooves of each solar cell in the plurality of solar cells. Such a configuration is advantageous because it reduces the pathlength of current drawn off ofTCO412. In other words, the maximum length that current must travel inTCO412 before it reaches a counter-electrode420 is a quarter of the circumference of the TCO. By contrast, in configurations where there is only asingle counter-electrode420 associated with a givensolar cell402, the maximum length that current must travel inTCO412 before it reaches a counter-electrode420 is a full half of the circumference of the TCO. The present invention encompasses grooves777 that have a broad range of depths and shape characteristics and is by no means limited to the shape of the grooves777 illustrated inFIG. 7A. In general, any groove shape777 that runs along the long axis of asolar cell402 and that can accommodate all or part ofcounter-electrode420 is within the scope of the present invention. For example, in some embodiments not illustrated byFIG. 7A, each groove777 is patterned so that there is a tight fit between the contours of the groove777 and the counter-electrode420.

As illustrated inFIG. 7A, there are a plurality ofmetal counter-electrodes420, and each respective elongatedsolar cell402 in the plurality of elongated solar cells is bound to at least a firstcorresponding metal counter-electrode420 in the plurality of metal counter-electrodes such that the first metal counter-electrode lies in a groove777 that runs lengthwise along the respective elongated solar cell. Furthermore, in the solar cell assembly illustrated inFIG. 7A, each respective elongatedsolar cell402 is bound to a secondcorresponding metal counter-electrode420 such that the second metal counter-electrode lies in a second groove777 that runs lengthwise along the respective elongatedsolar cell402. As further illustrated inFIG. 7A, the first groove777 and the second groove777 are on opposite or substantially opposite sides of the respective elongatedsolar cell402 and run along the long axis of the cell.

Further illustrated inFIG. 7A, is a transparent electrically insulatingsubstrate406 that covers all or a portion offace766 of the planar array. The plurality of elongatedsolar cells402 are configured to receive direct light from both face733 and face766 of the planar array.Solar cell assembly700 further comprises a transparent insulatingcovering422 disposed onface733 of the planar array, thereby encasing the plurality of elongatedsolar cells402 between the transparent insulatingcovering422 and the transparent electrically insulatingsubstrate406.

FIG. 7B provides a cross-sectional view with respect to line7B-7B ofFIG. 7A.Solar cell402 are electrically connected to other in series by arranging the solar cells such that they do not touch each other, as illustrated inFIGS. 7A and 7B and by the use of electrical contacts as described below in conjunction withFIG. 7B. The separation distance betweensolar cells402 is any distance that prevents electrical contact between the TCO layers412 ofindividual cells402. For instance, in some embodiments, the distance between adjacent solar cells is 0.1 micron or greater, 0.5 microns or greater, or between 1 and 5 microns.

Referring toFIG. 7B, serial electrical contact betweensolar cells402 is made byelectrical contacts788 that electrically connect the metalconductive core404 of one elongatedsolar cell402 to the corresponding counter-electrodes120 of a differentsolar cell402 as illustrated inFIG. 7B.FIG. 7B further illustrates a cutaway of metalconductive core404 andsemiconductor junction410 in onesolar cell402 to further illustrate the architecture of thesolar cells402.

The solar cell assembly illustrated inFIG. 7 has several advantages. First, because of the positioning ofcounter-electrodes420 and the transparency of bothsubstrate406 and covering422, there is almost zero percent shading in the assembly. For instance, the assembly can receive direct sunlight from both face733 andface766. Second, in embodiments where a sealant such as EVA is used tolaminate substrate406 and covering422 onto the plurality of solar cells, the structure is completely self-supporting. Still another advantage of the assembly is that is easy to manufacture. Unlike solar cells such as that depicted inFIG. 3A, no complicated grid or transparent conductive oxide on glass is needed. For example, to assemble asolar cell402 and itscorresponding counter-electrodes420 together to complete the circuit illustrated inFIG. 7A, counter-electrode420, when it is in the form of a wire, can be covered with conductive epoxy and dropped in the groove777 ofsolar cell402 and allowed to cure. As illustrated inFIG. 7B,conductive core404,junction410, andTCO412 are flush with each other atend789 of elongatedsolar cells402. In contrast, atend799 conductive core protrudes a bit with respect tojunction410 andTCO412 as illustrated.Junction410 also protrudes a bit atend799 with respect toTCO412. The protrusion ofconductive core404 atend799 means that the sides of a terminal portion of theconductive core404 are exposed (e.g., not covered byjunction410 and TCO412). The purpose of this configuration is to reduce the chances of shorting counter-electrode420 (or the epoxy used to mount the counter-electrode in groove777) withTCO412. In some embodiments, all or a portion of the exposed surface area ofcounter-electrodes420 are shielded with an electrically insulating material in order to reduce the chances of electrical shortening. For example, in some embodiments, the exposed surface area ofcounter-electrodes420 in the boxed regions ofFIG. 7B is shielded with an electrically insulating material.

Still another advantage of the assembly illustrated inFIG. 7 is that the counter-electrode420 can have much higher conductivity without shadowing. In other words, counter-electrode420 can have a substantial cross-sectional size (e.g., 1 mm in diameter whensolar cell402 has a 6 mm diameter). Thus, counter-electrode420 can carry a significant amount of current so that the wires can be as long as possible, thus enabling the fabrication of larger panels.

The series connections betweensolar cells402 can be between pairs ofsolar cells402 in the manner depicted inFIG. 7B. However, the invention is not so limited. In some embodiments, two or moresolar cells402 are grouped together (e.g., electrically connected in a parallel fashion) to form a group of solar cells and then such groups of solar cells are serially connected to each other. Therefore, the serial connections between solar cells can be between groups of solar cells where such groups have any number of solar cells402 (e.g., 2, 3, 4, 5, 6, etc.). However,FIG. 7B illustrates a preferred embodiment in which eachcontact788 serially connects only a pair ofsolar cells402.

In some embodiments, there is a series insulator that runs lengthwise between eachsolar cell402. In one example, this series insulator is a 0.001″ thick sheet of transparent insulating plastic. In other examples this series insulator is a sheet of transparent insulating plastic having a thickness between 0.001″ and 0.005″. Alternatively, a round insulating clear plastic separator that runs lengthwise betweensolar cells402 can be used to electrically isolate thesolar cells402. Advantageously, any light that does enter the small gap betweensolar cells402 will be trapped and collected in the “double-divet” area formed by facing grooves777 of adjacentsolar cells402.

Yet another embodiment ofsolar cell assembly700 is that there is no extra absorption loss from a TCO or a metal grid on one side of the assembly. Further,assembly700 has the same performance or absorber area exposed on both

sides

733 and766. This makesassembly700 symmetrical.

Still another advantage ofassembly700 is that allelectrical contacts788 end at the same level (e.g., in the plane of line7B-7B ofFIG. 7A). As such, they are easier to connect and weld with very little substrate area wasted at the end. This simplifies construction of thesolar cells402 while at the same time serves to increase the overall efficiency ofsolar cell assembly700. This increase in efficiency arises because the welds can be smaller. Smaller welds take up less of the electrically resistanttransparent substrate406 surface area that is otherwise occupied bysolar cells402.

Although not illustrated inFIG. 7, in some embodiments in accordance withFIG. 7, there is an intrinsic layer circumferentially disposed between thesemiconductor junction410 and the transparentconductive oxide412 in an elongatedsolar cell402 in the plurality of elongatedsolar cells402. This intrinsic layer can be made of an undoped transparent oxide such as zinc oxide, indium-tin-oxide, or a combination thereof. In some embodiments, thesemiconductor junction410 ofsolar cells402 inassembly700 comprise an inner coaxial layer and an outer coaxial layer where the outer coaxial layer comprises a first conductivity type and the inner coaxial layer comprises a second, opposite, conductivity type. In an exemplary embodiment the inner coaxial layer comprises copper-indium-gallium-diselenide (CIGS) whereas the outer coaxial layer comprises CdS, SnO₂, ZnO, ZrO₂, or doped ZnO. In some embodiments not illustrated byFIG. 7, theconductive cores404 insolar cells402 are hollowed.

FIG. 8 illustrates asolar cell assembly800 of the present invention that is identical tosolar cell assembly700 of the present invention with the exception thatTCO412 is interrupted bybreaks810 that run along the long axis ofsolar cells402 and cut completely throughTCO412. In the embodiment illustrated inFIG. 8, there are twobreaks810 that run the length ofsolar cell402. The effect ofsuch breaks810 is that they electrically isolate the twocounter-electrodes420 associated with eachsolar cell402 insolar cell assembly800. There are many ways in which breaks800 can be made. For example, a laser or an HCl etch can be used.

In some embodiments, not all elongatedsolar cells402 inassembly800 are electrically arranged in series. For example, in some embodiments, there are pairs of elongatedsolar cells402 that are electrically arranged in parallel. A first and second elongated solar cell can be electrically connected in parallel, and are thereby paired, by using a first electrical contact (e.g., an electrically conducting wire, etc., not shown) that joins theconductive core404 of a first elongated solar cell to the second elongated solar cell. To complete the parallel circuit, theTCO412 of the first elongatedsolar cell402 is electrically connected to theTCO412 of the second elongatedsolar cell402 either by contacting the TCOs of the two elongated solar cells either directly or through a second electrical contact (not shown). The pairs of elongated solar cells are then electrically arranged in series. In some embodiments, three, four, five, six, seven, eight, nine, ten, eleven or more elongatedsolar cells402 are electrically arranged in parallel. These parallel groups of elongatedsolar cells402 are then electrically arranged in series.

FIG. 9 illustrates asolar cell assembly900 of the present invention in whichconductive cores402 are hollowed. In fact,conductive cores402 can be hollowed in any of the embodiments of the present invention. One advantage of such ahollowed core402 design is that it reduces the overall weight of the solar cell assembly.Core402 is hollowed when there is a channel that extends lengthwise through all or a portion ofcore402. In some embodiments,conductive core402 is metal tubing.

In some embodiments, not all elongatedsolar cells402 inassembly900 are electrically arranged in series. For example, in some embodiments, there are pairs of elongatedsolar cells402 that are electrically arranged in parallel. A first and second elongated solar cell can be electrically connected in parallel, and are thereby paired, by using a first electrical contact (e.g., an electrically conducting wire, etc., not shown) that joins theconductive core404 of a first elongated solar cell to the second elongated solar cell. To complete the parallel circuit, theTCO412 of the first elongatedsolar cell402 is electrically connected to theTCO412 of the second elongatedsolar cell402 either by contacting the TCOs of the two elongated solar cells either directly or through a second electrical contact (not shown). The pairs of elongated solar cells are then electrically arranged in series. In some embodiments, three, four, five, six, seven, eight, nine, ten, eleven or more elongatedsolar cells402 are electrically arranged in parallel. These parallel groups of elongatedsolar cells402 are then electrically arranged in series.

FIG. 10 illustrates asolar cell assembly1000 of the present invention in which counterelectrodes420,TCOs412, andjunctions410 are pierced, in the manner illustrated, in order to form two discrete junctions in parallel.

5.2 Exemplary Semiconductor Junctions

Referring toFIG. 5A, in one embodiment,semiconductor junction410 is a heterojunction between anabsorber layer502, disposed onconductive core404, and ajunction partner layer504, disposed onabsorber layer502.

Layers

502 and504 are composed of different semiconductors with different band gaps and electron affinities such thatjunction partner layer504 has a larger band gap thanabsorber layer502. In some embodiments,absorber layer502 is p-doped andjunction partner layer504 is n-doped. In such embodiments,TCO layer412 is n⁺-doped. In alternative embodiments,absorber layer502 is n-doped andjunction partner layer504 is p-doped. In such embodiments,TCO layer412 is p⁺-doped. In some embodiments, the semiconductors listed in Pandey,Handbook of Semiconductor Electrodeposition, Marcel Dekker Inc., 1996,Appendix 5, hereby incorporated by reference in its entirety, are used to formsemiconductor junction410.

5.2.1 Thin-Film Semiconductor Junctions Based on Copper Indium Diselenide and Other Type I-III-VI Materials

Continuing to refer toFIG. 5A, in some embodiments,absorber layer502 is a group I-III-VI₂compound such as copper indium di-selenide (CuInSe₂; also known as CIS). In some embodiments,absorber layer502 is a group I-III-VI₂ternary compound selected from the group consisting of CdGeAs₂, ZnSnAs₂, CuInTe₂, AgInTe₂, CuInSe₂, CuGaTe₂, ZnGeAs₂, CdSnP₂, AgInSe₂, AgGaTe₂, CuInS₂, CdSiAs₂, ZnSnP₂, CdGeP₂, ZnSnAs₂, CuGaSe₂, AgGaSe₂, AgInS₂, ZnGeP₂, ZnSiAs₂, ZnSiP₂, CdSiP₂, or CuGaS₂of either the p-type or the n-type when such compound is known to exist.

In some embodiments,junction partner layer504 is CdS, ZnS, ZnSe, or CdZnS. In one embodiment,absorber layer502 is p-type CIS andjunction partner layer504 is n-type CdS, ZnS, ZnSe, or CdZnS.Such semiconductor junctions410 are described inChapter 6 of Bube,Photovoltaic Materials,1998, Imperial College Press, London, which is hereby incorporated by reference in its entirety.

In some embodiments,absorber layer502 is copper-indium-gallium-diselenide (CIGS). In some embodiments,absorber layer502 is copper-indium-gallium-diselenide (CIGS) andjunction partner layer504 is CdS, ZnS, ZnSe, or CdZnS. In some embodiments,absorber layer502 is p-type CIGS andjunction partner layer504 is n-type CdS, ZnS, ZnSe, or CdZnS.

5.2.2 Semiconductor Junctions Based on Amorphous Silicon or Polycrystalline Silicon

In some embodiments, referring toFIG. 5B,semiconductor junction410 comprises amorphous silicon. In some embodiments this is an n/n type heterojunction. For example, in some embodiments,layer514 comprises SnO₂(Sb),layer512 comprises undoped amorphous silicon, andlayer510 comprises n+ doped amorphous silicon.

In some embodiments,semiconductor junction410 is a p-i-n type junction. For example, in some embodiments,layer514 is p⁺ doped amorphous silicon,layer512 is undoped amorphous silicon, andlayer510 is n⁺ amorphous silicon.Such semiconductor junctions410 are described in Chapter 3 of Bube,Photovoltaic Materials,1998, Imperial College Press, London, which is hereby incorporated by reference in its entirety.

In some embodiments of the present invention,semiconductor junction410 is based upon thin-film polycrystalline. Referring toFIG. 5B, in one example in accordance with such embodiments,layer510 is a p-doped polycrystalline silicon,layer512 is depleted polycrystalline silicon andlayer514 is n-doped polycrystalline silicon. Such semiconductor junctions are described in Green,Silicon Solar Cells: Advanced Principles&Practice, Centre for Photovoltaic Devices and Systems, University of New South Wales, Sydney, 1995; and Bube,Photovoltaic Materials,1998, Imperial College Press, London, pp. 57-66, which is hereby incorporated by reference in its entirety.

In some embodiments of the present invention,semiconductor junctions410 based upon p-type microcrystalline Si:H and microcrystalline Si:C:H in an amorphous Si:H solar cell are used. Such semiconductor junctions are described in Bube,Photovoltaic Materials,1998, Imperial College Press, London, pp. 66-67, and the references cited therein, which is hereby incorporated by reference in its entirety.

5.2.3 Semiconductor Junctions Based on Gallium Arsenide and Other Type III-V Materials

In some embodiments,semiconductor junctions410 are based upon gallium arsenide (GaAs) or other III-V materials such as InP, AlSb, and CdTe. GaAs is a direct-band gap material having a band gap of 1.43 eV and can absorb 97% of AM1 radiation in a thickness of about two microns. Suitable type III-V junctions that can serve assemiconductor junctions410 of the present invention are described inChapter 4 of Bube,Photovoltaic Materials,1998, Imperial College Press, London, which is hereby incorporated by reference in its entirety.

Furthermore, in someembodiments semiconductor junction410 is a hybrid multijunction solars cells such as a GaAs/Si mechanically stacked multijunction as described by Gee and Virshup, 1988, 20^thIEEE Photovoltaic Specialist Conference, IEEE Publishing, New York, p. 754, which is hereby incorporated by reference in its entirety, a GaAs/CuInSe₂MSMJ four-terminal device, consisting of a GaAs thin film top cell and a ZnCdS/CuInSe₂thin bottom cell described by Stanbery et al., 19^thIEEE Photovoltaic Specialist Conference,IEEE Publishing, New York, p. 280, and Kim et al., 20^thIEEE Photovoltaic Specialist Conference,IEEE Publishing, New York, p. 1487, each of which is hereby incorporated by reference in its entirety. Other hybrid multijunction solar cells are described in Bube,Photovoltaic Materials,1998, Imperial College Press, London, pp. 131-132, which is hereby incorporated by reference in its entirety.

5.2.4 Semiconductor Junctions Based on Cadmium Telluride and Other Type II-VI Materials

In some embodiments,semiconductor junctions410 are based upon II-VI compounds that can be prepared in either the n-type or the p-type form. Accordingly, in some embodiments, referring toFIG. 5C,semiconductor junction410 is a p-n heterojunction in which layers520 and540 are any combination set forth in the following table or alloys thereof.



	Layer 520	Layer 540

	n-CdSe	p-CdTe
	n-ZnCdS	p-CdTe
	n-ZnSSe	p-CdTe
	p-ZnTe	n-CdSe
	n-CdS	p-CdTe
	n-CdS	p-ZnTe
	p-ZnTe	n-CdTe
	n-ZnSe	p-CdTe
	n-ZnSe	p-ZnTe
	n-ZnS	p-CdTe
	n-ZnS	p-ZnTe

Methods for manufacturingsemiconductor junctions410 are based upon II-VI compounds are described inChapter 4 of Bube,Photovoltaic Materials,1998, Imperial College Press, London, which is hereby incorporated by reference in its entirety.

5.2.5 Semiconductor Junctions Based on Crystalline Silicon

Whilesemiconductor junctions410 that are made from thin semiconductor films are preferred, the invention is not so limited. In someembodiments semiconductor junctions410 is based upon crystalline silicon. For example, referring toFIG. 5D, in some embodiments,semiconductor junction410 comprises a layer of p-type crystalline silicon540 and a layer of n-type crystalline silicon550. Methods for manufacturing crystallinesilicon semiconductor junctions410 are described in Chapter 2 of Bube,Photovoltaic Materials,1998, Imperial College Press, London, which is hereby incorporated by reference in its entirety.

5.3 Albedo Embodiments

The solar cell assemblies of the present invention are advantageous because they can collect light through either of their two faces. Accordingly, in some embodiments of the present invention, theses bifacial solar cell assemblies (e.g.,

solar cell assembly

400,600,700,800,900, etc.) are arranged in a reflective environment in which surfaces around the solar cell assembly have some amount of albedo. Albedo is a measure of reflectivity of a surface or body. It is the ratio of electromagnetic radiation (EM radiation) reflected to the amount incident upon it. This fraction is usually expressed as a percentage from 0% to 100%. In some embodiments, surfaces in the vicinity of the solar cell assemblies of the present invention are prepared so that they have a high albedo by painting such surfaces a reflective white color. In some embodiments, other materials that have a high albedo can be used. For example, the albedo of some materials around such solar cells approach or exceed ninety percent. See, for example, Boer, 1977, Solar Energy 19, 525, which is hereby incorporated by reference in its entirety. However, surfaces having any amount of albedo (e.g., five percent or more, ten percent or more, twenty percent or more) are within the scope of the present invention. In one embodiment, the solar cells assemblies of the present invention are arranged in rows above a gravel surface, where the gravel has been painted white in order to improve the reflective properties of the gravel.

In some embodiments, the bifacial solar cell assemblies of the present invention are placed in a manner such that one surface (e.g., face633 of solar cell assembly600) is illuminated in a way similar to a conventional flat-panel solar cell panel. For example, it is installed facing South (in the northern hemisphere) with an angle of inclination that is latitude dependent (e.g., in general is not very different from the latitude). The opposing surface of the bifacial solar cell assembly (e.g., face655 of solar cell assembly600) of the present invention receives a substantial amount of diffuse light reflected from the ground and neighboring walls in the vicinity of the solar cell assembly.

By way of example, in some embodiments of the present invention, the bifacial solar cell assemblies (panels) of the present invention have a first and second face and are placed in rows facing South in the Northern hemisphere (or facing North in the Southern hemisphere). Each of the panels is placed some distance above the ground (e.g., 100 cm above the ground). The East-West separation between the panels is somewhat dependent upon the overall dimensions of the panels. By way of illustration only, panels having overall dimensions of about 106 cm×44 cm are placed in the rows such that the East-West separation between the panels is between 10 cm and 50 cm. In one specific example the East-West separation between the panels is 25 cm.

In some embodiments, the central point of the panels in the rows of panels is between 0.5 meters and 2.5 meters from the ground. In one specific example, the central point of the panels is 1.55 meters from the ground. The North-South separation between the rows of panels is dependent on the dimensions of the panels. By way of illustration, in one specific example, in which the panels have overall dimensions of about 106 cm×44 cm, the North-South separation is 2.8 meters. In some embodiments, the North-South separation is between 0.5 meters and 5 meters. In some embodiments, the North-South separation is between 1 meter and 3 meters.

In some embodiments of the present invention, the panels in the rows are each tilted with respect to the ground in order to maximize the total amount of light received by the panels. There is some tradeoff between increasing the amount of light received by one face versus the amount of light received on the opposing face as a function of tilt angle. However, at certain tilt angles, the total amount of light received by the panels, where total amount of light is defined as the sum of direct light received on the first and second face of the bifacial panel, is maximized. In some embodiments, the panels in the rows of panels are each tilted between five degrees and forty-five degrees from the horizontal. In some embodiments, the panels of the present invention are tilted between fifteen degrees and forty degrees from the horizontal. In some embodiments, the panels of the present invention are tilted between twenty-five degrees and thirty-five degrees from the horizontal. In one specific embodiment, the panels of the present invention are tilted thirty degrees from the horizontal.

In some embodiments, models for computing the amount of sunlight received by solar panels as put forth in Lorenzo et al., 1985, Solar Cells 13, pp. 277-292, which is hereby incorporated by reference in its entirety, are used to compute the optimum horizontal tilt and East-West separation of the solar panels in the rows of solar panels that are placed in a reflective environment.

5.4 Dual Layer Core Embodiments

Embodiments of the present invention in whichconductive core404 of thesolar cells402 of the present invention is made of a uniform conductive material have been disclosed. The invention is not limited to these embodiments. In some embodiments,conductive core404 in fact has an inner core and an outer conductive core. The outer conductive core is circumferentially disposed on the inner core. In such embodiments, the inner core is typically nonconductive whereas the outer core is conductive. The inner core has an elongated shape consistent with other embodiments of the present invention. For instance, in one embodiment, the inner core is made of glass fibers in the form of a wire. In some embodiments, the inner core is an electrically conductive nonmetallic material. However, the present invention is not limited to embodiments in which the inner core is electrically conductive because the outer core can function as the electrode. In some embodiments, the inner core is tubing (e.g., plastic tubing).

In some embodiments, the inner core is made of a material such as polybenzamidazole (e.g., Celazole®, available from Boedeker Plastics, Inc., Shiner, Tex.). In some embodiments, the inner core is made of polymide (e.g., DuPont™ Vespel®, or DuPont™ Kapton®, Wilmington, Del.). In some embodiments, the inner core is made of polytetrafluoroethylene (PTFE) or polyetheretherketone (PEEK), each of which is available from Boedeker Plastics, Inc. In some embodiments, the inner core is made of polyamide-imide (e.g., Torlon® PAI, Solvay Advanced Polymers, Alpharetta, Ga.).

In some embodiments, the inner core is made of a glass-based phenolic. Phenolic laminates are made by applying heat and pressure to layers of paper, canvas, linen or glass cloth impregnated with synthetic thermosetting resins. When heat and pressure are applied to the layers, a chemical reaction (polymerization) transforms the separate layers into a single laminated material with a “set” shape that cannot be softened again. Therefore, these materials are called “thermosets.” A variety of resin types and cloth materials can be used to manufacture thermoset laminates with a range of mechanical, thermal, and electrical properties. In some embodiments, the inner core is a phenoloic laminate having a NEMA grade of G-3, G-5, G-7, G-9, G-10 or G-11. Exemplary phenolic laminates are available from Boedeker Plastics, Inc.

In some embodiments, the inner core is made of polystyrene. Examples of polystyrene include general purpose polystyrene and high impact polystyrene as detailed in Marks'Standard Handbook for Mechanical Engineers, ninth edition, 1987, McGraw-Hill, Inc., p. 6-174, which is hereby incorporated by reference in its entirety. In still other embodiments, inner core is made of cross-linked polystyrene. One example of cross-linked polystyrene is Rexolite® (available from San Diego Plastics Inc., National City, Calif.). Rexolite is a thermoset, in particular a rigid and translucent plastic produced by cross linking polystyrene with divinylbenzene.

In some embodiments, the inner core is a polyester wire (e.g., a Mylar® wire). Mylar® is available from DuPont Teijin Films (Wilmington, Del.). In still other embodiments, the inner core is made of Durastone®, which is made by using polyester, vinylester, epoxid and modified epoxy resins combined with glass fibers (Roechling Engineering Plastic Pte Ltd. (Singapore).

In still other embodiments, the inner core is made of polycarbonate. Such polycarbonates can have varying amounts of glass fibers (e.g., 10%, 20%, 30%, or 40%) in order to adjust tensile strength, stiffness, compressive strength, as well as the thermal expansion coefficient of the material. Exemplary polycarbonates are Zelux® M and Zelux® W, which are available from Boedeker Plastics, Inc.

In some embodiments, the inner core is made of polyethylene. In some embodiments, inner core is made of low density polyethylene (LDPE), high density polyethylene (HDPE), or ultra high molecular weight polyethylene (UHMW PE). Chemical properties of HDPE are described in Marks'Standard Handbook for Mechanical Engineers, ninth edition, 1987, McGraw-Hill, Inc., p. 6-173, which is hereby incorporated by reference in its entirety. In some embodiments, the inner core is made of acrylonitrile-butadiene-styrene, polytetrfluoro-ethylene (Teflon), polymethacrylate (lucite or plexiglass),

nylon

6,6, cellulose acetate butyrate, cellulose acetate, rigid vinyl, plasticized vinyl, or polypropylene. Chemical properties of these materials are described in Marks'Standard Handbook for Mechanical Engineers, ninth edition, 1987, McGraw-Hill, Inc., pp. 6-172 through 1-175, which is hereby incorporated by reference in its entirety.

Additional exemplary materials that can be used to form the inner core are found inModern Plastics Encyclopedia, McGraw-Hill; Reinhold Plastics Applications Series, Reinhold Roff,Fibres, Plastics and Rubbers, Butterworth; Lee and Neville,Epoxy Resins, McGraw-Hill; Bilmetyer,Textbook of Polymer Science, Interscience; Schmidt and Marlies,Principles of high polymer theory and practice, McGraw-Hill; Beadle (ed.), Plastics, Morgan-Grampiand, Ltd., 2 vols. 1970; Tobolsky and Mark (eds.),Polymer Science and Materials, Wiley, 1971; Glanville,The Plastics's Engineer's Data Book, Industrial Press, 1971; Mohr (editor and senior author), Oleesky, Shook, and Meyers,SPI Handbook of Technology and Engineering of Reinforced Plastics Composites, Van Nostrand Reinhold, 1973, each of which is hereby incorporated by reference in its entirety.

In general, outer core is made out of any material that can support the photovoltaic current generated by solar cell with negligible resistive losses. In some embodiments, outer core is made of any conductive metal, such as aluminum, molybdenum, steel, nickel, silver, gold, or an alloy thereof. In some embodiments, outer core is made out of a metal-, graphite-, carbon black-, or superconductive carbon black-filled oxide, epoxy, glass, or plastic. In some embodiments, outer core is made of a conductive plastic. In some embodiments, this conductive plastic is inherently conductive without any requirement for a filler.

In embodiments where an inner core and an outer core is present,semiconductor junction410 andTCO412 are stripped from the inner core at a terminal end of the solar cell where an electrical contact serially joins the solar cell to another solar cell. For example, in some embodiments, thesemiconductor junction410 and TCO are stripped in the manner illustrated inFIGS. 4D, 4F,6B,6C, and7B.

5.5 Exemplary Dimensions

The present invention encompasses solar cell assemblies having any dimensions that fall within a broad range of dimensions. For example, referring toFIG. 4B, the present invention encompasses solar cell assemblies having a length l between 1 cm and 50,000 cm and a width w between 1 cm and 50,000 cm. In some embodiments, the solar cell assemblies have a length l between 10 cm and 1,000 cm and a width w between 10 cm and 1,000 cm. In some embodiments, the solar cell assemblies have a length l between 40 cm and 500 cm and a width w between 40 cm and 500 cm.

5.6 Solar Cells Manufactured Using a Roll Method or Having an Inner TCO

In some embodiments, copper-indium-gallium-diselenide (Cu(InGa)Se₂), referred to herein as CIGS, is used to make the absorber layer ofjunction110. In such embodiments,conductive core404 can be made of molybdenum. In some embodiments,core404 comprises an inner core of polyimide and an outer core that is a thin film of molybdenum sputtered onto the polyimide core prior to CIGS deposition. On top of the molybdenum, the CIGS film, which absorbs the light, is evaporated. Cadmium sulfide (CdS) is then deposited on the CIGS in order to completesemiconductor junction410. Optionally, a thin intrinsic layer (i-layer) is then deposited on thesemiconductor junction410. The i-layer can be formed using any undoped transparent oxide including, but not limited to, zinc oxide or indium-tin-oxide. Next,TCO412 is disposed on either the i-layer (when present) or the semiconductor junction410 (when the i-layer is not present). TCO can be made of a material such as aluminum doped zinc oxide (ZnO:Al).

ITN Energy Systems, Inc., Global Solar Energy, Inc., and the Institute of Energy Conversion (IEC), have collaboratively developed technology for manufacturing CIGS photovoltaics on polyimide substrates using a roll-to-roll co-evaporation process for deposition of the CIGS layer. In this process, a roll of molybdenum-coated polyimide film (referred to as the web) is unrolled and moved continuously into and through one or more deposition zones. In the deposition zones, the web is heated to temperatures of up to ˜450° C. and copper, indium, and gallium are evaporated onto it in the presence of selenium vapor. After passing out of the deposition zone(s), the web cools and is wound onto a take-up spool. See, for example, 2003, Jensen et al., “Back Contact Cracking During Fabrication of CIGS Solar Cells on Polyimide Substrates,” NCPV and Solar Program Review Meeting 2003, NREL/CD-520-33586, pages 877-881, which is hereby incorporated by reference in its entirety. Likewise, Birkmire et al., 2005, Progress in Photovoltaics: Research and Applications 13, 141-148, hereby incorporated by reference, disclose a polyimide/Mo web structure, specifically, PI/Mo/Cu(InGa)Se₂/CdS/ZnO/ITO/Ni—Al. Deposition of similar structures on stainless foil has also been explored. See, for example, Simpson et al., 2004, “Manufacturing Process Advancements for Flexible CIGS PV on Stainless Foil,” DOE Solar Energy Technologies Program Review Meeting, PV Manufacturing Research and Development, P032, which is hereby incorporated by reference in its entirety.

In some embodiments of the present invention, an absorber material is deposited onto a polyimide/molybdenum web, such as those developed by Global Solar Energy (Tucson, Ariz.), or a metal foil (e.g., the foil disclosed in Simpson et al.). In some embodiments, the absorber material is any of the absorbers disclosed herein. In a particular embodiment, the absorber is Cu(InGa)Se₂. In some embodiments, the elongated core is made of a nonconductive material such as undoped plastic. In some embodiments, the elongated core is made of a conductive material such as a conductive metal, a metal-filled epoxy, glass, or resin, or a conductive plastic (e.g., a plastic containing a conducting filler). Next, thesemiconductor junction410 is completed by depositing a window layer onto the absorber layer. In the case where the absorber layer is Cu(InGa)Se₂, CdS can be used. Finally, an optional i-layer415 andTCO412 are added to complete the solar cell. Next, the foil is wrapped around and/or glued to a wire-shaped or tube-shaped elongated core. The advantage of such a fabrication method is that material that cannot withstand the deposition temperature of the absorber layer, window layer, i-layer or TCO layer can be used as an inner core for the solar cell. This manufacturing process can be used to manufacture any of thesolar cells402 disclosed in the present invention, where theconductive core402 comprises an inner core and an outer conductive core. The inner core is any conductive or nonconductive material disclosed herein whereas the outer conductive core is the web or foil onto which the absorber layer, window layer, and TCO were deposited prior to rolling the foil onto the inner core. In some embodiments, the web or foil is glued onto the inner core using appropriate glue.

An aspect of the present invention provides a method of manufacturing a solar cell comprising depositing an absorber layer on a first face of a metallic web or a conducting foil. Next a window layer is deposited on to the absorber layer. Next a transparent conductive oxide layer is deposited on to the window layer. The metallic web or conducting foil is then rolled around an elongated core, thereby forming an elongatedsolar cell402. In some embodiments, the absorber layer is copper-indium-gallium-diselenide (Cu(InGa)Se₂) and the window layer is cadmium sulfide. In some embodiments, the metallic web is a polyimide/molybdenum web. In some embodiments, the conducting foil is steel foil or aluminum foil. In some embodiments, the elongated core is made of a conductive metal, a metal-filled epoxy, a metal-filled glass, a metal-filled resin, or a conductive plastic.

In some embodiments, a transparent conducting oxide is deposited on a wire-shaped or tube-shaped elongated core rather than wrapping a metal web or foil around the elongated core. In such embodiments, the wire-shaped or tube-shaped elongated core can be, for example, a plastic rod, a glass rod, a glass tube, or a plastic tube. Such embodiments require some form of conductor in electrical communication with the interior face of the semiconductor junction. In some embodiments, divits in the wire-shaped or tube-shaped elongated core are filled with a conductive metal in order to provide such a conductor. The conductor can be inserted in the divits prior to depositing the transparent conductive oxide onto the wire-shaped or tube-shaped elongated core.

More specific embodiments will now be disclosed. In some embodiments the elongated core is a glass tubing having a divet that runs lengthwise on the outer surface of the glass tubing, and the manufacturing method comprises depositing a conductor in the divit prior to the rolling step. In some embodiments the glass tubing has a second divit that runs lengthwise on the surface of the glass tubing. In such embodiments, the first divit and the second divit are on approximate or exact opposite circumferential sides of the glass tubing. In such embodiments, accordingly, the method further comprises depositing a conductor in the second divit prior to the rolling or, in embodiments in which rolling is not used, prior to the deposition of an inner TCO, junction, and outer TCO onto the elongated core.

In some embodiments the elongated core is a glass rod having a first divet that runs lengthwise on the surface of the glass rod and the method comprises depositing a conductor in the first divit prior to the rolling. In some embodiments the glass rod has a second divit that runs lengthwise on the surface of the glass rod and the first divit and the second divit are on approximate or exact opposite circumferential sides of the glass rod. In such embodiments, accordingly, the method further comprises depositing a conductor in the second divit prior to the rolling or, in embodiments in which rolling is not used, prior to the deposition of an inner TCO, junction, and outer TCO onto the elongated core. Suitable materials for the conductor are any of the materials described as a conductor herein including, but not limited to, aluminum, molybdenum, steel, nickel, silver, gold, or an alloy thereof.

FIG. 13 details a cross-section of asolar cell402 in accordance with the present invention. Thesolar cell402 can be manufactured using either the rolling method or deposition techniques. Components that have reference numerals corresponding to other embodiments of the present invention (e.g.,410,412, and420) are made of the same materials disclosed in such embodiments. InFIG. 13, there is anelongated tubing1306 having a first and second divit running lengthwise along the tubing (perpendicular to the plane of the page) that are on circumferentially opposing sides oftubing1306 as illustrated. In typical embodiments,tubing1306 is not conductive. For example,tubing1306 is made of plastic or glass in some embodiments.Conductive wiring1302 is placed in the first and second divit as illustrated inFIG. 13. In some embodiments the conductive wiring is made of any of the conductive materials of the present invention. In some embodiments,conductive wiring1302 is made out of aluminum, molybdenum, steel, nickel, silver, gold, or an alloy thereof. In embodiments where1304 is a conducting foil or metallic web, theconductive wiring1302 is inserted into the divits prior to wrapping the metallic web or conductingfoil1304 around theelongated core1306. In embodiments where1304 is a transparent conductive oxide, theconductive wiring1302 is inserted into the divits prior to depositing the transparentconductive oxide1304 onto elongatedcore1306. As noted, in some embodiments the metallic web or conductingfoil1304 is wrapped aroundtubing1306. In some embodiments, metallic web or conductingfoil1304 is glued totubing1306. In someembodiments layer1304 is not a metallic web or conducting foil. For instance, in some embodiments,layer1304 is a transparent conductive oxide (TCO). Such a layer is advantageous because it allow for thinner absorption layers in the semiconductor junction. In embodiments wherelayer1304 is a TCO, the TCO,semiconductor junction410 andouter TCO412 are deposited using deposition techniques.

One aspect of the invention provides a solar cell assembly comprising a plurality of elongatedsolar cells402 each having the structure disclosed inFIG. 13. That is, each elongatedsolar cell402 in the plurality of elongated solar cells comprises anelongated tubing1306, a metallic web or a conducting foil (or, alternatively, a layer of TCO)1304 circumferentially disposed on theelongated tubing1306, asemiconductor junction410 circumferentially disposed on the metallic web or the conducting foil (or, alternatively, a layer of TCO)1304 and a transparentconductive oxide layer412 disposed on thesemiconductor junction410. The elongatedsolar cells402 in the plurality of elongated solar cells are geometrically arranged in a parallel or a near parallel manner thereby forming a planar array having a first face and a second face. The plurality of elongated solar cells is arranged such that one or more elongated solar cells in the plurality of elongated solar cells do not contact adjacent elongated solar cells. The solar cell assembly further comprises a plurality of metal counter-electrodes. Each respective elongatedsolar cell402 in the plurality of elongated solar cells is bound to a firstcorresponding metal counter-electrode420 in the plurality of metal counter-electrodes such that the first metal counter-electrode lies in a first groove that runs lengthwise on the respective elongatedsolar cell402. The apparatus further comprises a transparent electrically insulating substrate that covers all or a portion of said the face of the planar array. A first and second elongated solar cell in the plurality of elongated solar cells are electrically connected in series by an electrical contact that connects the first electrode of the first elongated solar cell to the first corresponding counter-electrode of the second elongated solar cell. In some embodiments, theelongated tubing1306 is glass tubing or plastic tubing having a one or more grooves filled with aconductor1302. In some embodiments, each respective elongatedsolar cell402 in the plurality of elongated solar cells is bound to a secondcorresponding metal counter-electrode420 in the plurality of metal counter-electrodes such that the second metal counter-electrode lies in a second groove that runs lengthwise on the respective elongatedsolar cell402 and such that the first groove and the second groove are on opposite or substantially opposite circumferential sides of the respective elongatedsolar cell402. In some embodiments, the plurality of elongatedsolar cells402 is configured to receive direct light from the first face and said second face of the planar array.

5.7 Static Concentrators

In some embodiments, static concentrators are used to improve the performance of the solar cell assemblies of the present invention. The use of a static concentrator in one exemplary embodiments is illustrated inFIG. 11, wherestatic concentrator1102, with aperture AB, is used to increase the efficiency of bifacial solar cell assembly CD, where solar cell assembly CD is any of400 (FIG. 4),600 (FIG. 6),700 (FIG. 7),800 (FIG. 8),900 (FIG. 9), or1000 (FIG. 10).Static concentrator1102 can be formed from any static concentrator materials known in the art such as, for example, a simple, properly bent or molded aluminum sheet, or reflector film on polyurethane.Concentrator1102 is an example of a low concentration ratio, nonimaging, compound parabolic concentrator (CPC)-type collector. Any (CPC)-type collector can be used with the solar cell assemblies of the present invention. For more information on (CPC)-type collectors, see Pereira and Gordon, 1989, Journal of Solar Energy Engineering, 111, pp. 111-116, which is hereby incorporated by reference in its entirety.

Additional static concentrators that can be used with the present invention are disclosed in Uematsu et al., 1999, Proceedings of the 11^thInternational Photovoltaic Science and Engineering Conference, Sapporo, Japan, pp. 957-958; Uematsu et al., 1998, Proceedings of the Second World Conference on Photovoltaic Solar Energy Conversion, Vienna, Austria, pp. 1570-1573; Warabisako et al., 1998, Proceedings of the Second World Conference on Photovoltaic Solar Energy Conversion, Vienna, Austria, pp. 1226-1231; Eames et al., 1998, Proceedings of the Second World Conference on Photovoltaic Solar Energy Conversion, Vienna Austria, pp. 2206-2209; Bowden et al., 1993, Proceedings of the 23^rdIEEE Photovoltaic Specialists Conference, pp. 1068-1072; and Parada et al., 1991, Proceedings of the 10^thEC Photovoltaic Solar Energy Conference, pp. 975-978, each of which is hereby incorporated by reference in its entirety.

In some embodiments, a static concentrator as illustrated inFIG. 12 is used. The bifacial solar cells illustrated inFIG. 12 can be any of the bifacial solar cell assemblies of the present invention, including but not limited to assembly400 (FIG. 4),600 (FIG. 6),700 (FIG. 7),800 (FIG. 8),900 (FIG. 9), or1000 (FIG. 10). The static concentrator uses two sheets of cover glass on the front and rear of the module with submillimeter V-grooves that are designed to capture and reflect incident light as illustrated in the Figure. More details of such concentrators is found in Uematsu et al., 2001, Solar Energy Materials & Solar Cell 67, 425-434 and Uematsu et al., 2001, Solar Energy Materials & Solar Cell 67, 441-448, each of which is hereby incorporated by reference in its entirety.

7. REFERENCES CITED

All references cited herein are incorporated herein by reference in their entirety and for all purposes to the same extent as if each individual publication or patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety for all purposes.

Many modifications and variations of this invention can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. For example, in some embodiments theTCO412 is circumferentially coated with an antireflective coating. In some embodiments, this antireflective coating is made of MgF₂. The specific embodiments described herein are offered by way of example only, and the invention is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

1-114. (canceled)

115. A method of manufacturing a solar cell comprising:

depositing an absorber layer on a face of a metallic web or a conducting foil;

depositing a window layer on said absorber layer;

depositing a transparent conductive oxide layer on said window layer; and

rolling said metallic web or conducting foil around an elongated core, thereby forming an elongated solar cell.

116. The method ofclaim 115, wherein said absorber layer is copper-indium-gallium-diselenide (Cu(InGa)Se₂) and said window layer is cadmium sulfide.

117. The method ofclaim 115, wherein the metallic web is a polyimide/molybdenum web.

118. The method ofclaim 115, wherein the conducting foil is steel foil or aluminum foil.

119. The method ofclaim 115, wherein the conducting foil is a transparent conducting material.

120. The method ofclaim 115, wherein the elongated core is wire-shaped or tube-shaped.

121. The method ofclaim 115, wherein the elongated core is made of a nonconductive material.

122. The method ofclaim 121, wherein the nonconductive material is plastic or glass.

123. The method ofclaim 115, wherein the elongated core is made of a conductive metal, a metal-filled epoxy, a metal-filled glass, a metal-filled resin, or a conductive plastic.

124. The method ofclaim 115, wherein an i-layer is deposited on the window layer prior to depositing the transparent conductive oxide layer.

125. The method ofclaim 115, wherein said rolling step further comprises gluing said metallic web or conducting foil around said elongated core.

126-130. (canceled)

131. The method ofclaim 115, wherein the elongated core is a plastic rod, a glass rod, a glass tube, a plastic tube, or a metal tube.

132. The method ofclaim 115, wherein the elongated core is hollowed.

133. The method ofclaim 115, wherein the solar cell is rod-shaped.

134. The method ofclaim 115, the method further comprising:

circumferentially depositing an antireflective coating on said transparent conductive oxide layer.

135. The method ofclaim 134, wherein said antireflective coating is made of MgF₂.

136. The method ofclaim 115, wherein said elongated core is made of polybenzamidazole, polyimide, polytetrafluoroethylene, polyetheretherketone, polyamide-imide, glass-based phenolic, polystryrene, cross-linked polystyrene, polyester, polycarbonate, polyethylene, acrylonitrile-butadiene-styrene, polytetrafluoro-ethylen, polymethacrylate, nylon 6,6, cellulose acetate butyrate, cellulose acetate, rigid vinyl, plasticized vinyl, or polypropylene.

137. The method ofclaim 115, wherein the solar cell is between 1 cm and 50,000 cm in length.

138. The method ofclaim 115, wherein the solar cell is between 1 cm and 50,000 cm in width.