Patent Application
20100319759
This invention pertains generally to nanotechnology and particularly to nano-scale structures and processes for making these structures.
Solar panels that harness solar energy and convert it to electrical energy are well known. A typical solar electricity system includes the following components: solar panels, charge controller, inverter, and often batteries. A typical solar panel, often referred to as a photovoltaic (PV) module, consists of a one or more interconnected PV cells environmentally sealed in protective packaging consisting of a glass cover and extruded aluminum casing.
The PV cell may be a p-n junction diode capable of generating electricity in the presence of sunlight. It is often made of crystalline silicon (e.g., polycrystalline silicon) doped with elements from either group 13 (group III) or group 15 (group V) on the periodic table. When these dopant atoms are added to the silicon, they take the place of silicon atoms in the crystalline lattice and bond with the neighboring silicon atoms in almost the same way as the silicon atom that was originally there. However, because these dopants do not have the same number of valence electrons as silicon atoms, extra electrons or “holes” become present in the crystal lattice. Upon absorbing a photon that carries an energy that is at least the same as the band gap energy of the silicon, the electrons become free. The electrons and holes freely move around within the solid silicon material, making silicon conductive. The closer the absorption event is to the p-n junction, the greater the mobility of the electron-hole pair.
When a photon that has less energy than silicon's band gap energy strikes the crystalline structure, the electrons and holes are not mobilized. Instead of the photon's energy becoming absorbed by the electrons and holes, the difference between the amount of energy carried by the photon and the band gap energy is converted to heat.
While the idea of converting solar energy to electrical power has much appeal, conventional solar panels have limited usage because their efficiencies are generally only around 15% and are manufactured using costly silicon wafer manufacturing processes and materials. This low efficiency is due in part to the planar configuration of current PV cells, as well as the relatively large distances between the electrodes and the P-N junction. Low efficiency means that larger and heavier arrays are needed to obtain a certain amount of electricity, raising the cost of a solar panel and limiting its use in large-scale structures.
The most common material for solar cells is silicon. Crystalline silicon generally comes in three categories: single-crystal silicon, polycrystalline silicon, and ribbon silicon. Solar cells made with single or monocrystalline wafers have the highest efficiency of the three, at about 20%. Unfortunately, single crystal cells are expensive and round so they do not completely tile a module. Polycrystalline silicon is made from cast ingots. They are typically made by filling a large crucible with molten silicon and carefully cooling and solidifying them. The polycrystalline silicon is less expensive than single crystal, but is only about 10-14% efficient depending on the process conditions and resulting imperfections in the material. Ribbon silicon is the last major category of PV grade silicon. It is typically formed by drawing flat, thin films from molten silicon, and has a polycrystalline structure. Silicon ribbon's efficiency range of 11-13% is also lower than monocrystalline silicon due to more imperfections. Most of these technologies are based on wafers about 300 μm thick. The PV cells are fabricated then soldered together to form a module.
Another technology under development is multijunction solar cells, which is expected to deliver less than 18.5% efficiency in actual use. The process and materials to produce multijunction cells are enormously expensive. Those cells require multiple gallium/indium/arsenide layers. The best is believed to be a sextuple-junction cell. Current multijunction cells cannot be made economical for large-scale applications.
A promising enabler of PV cells and other technology is nanotechnology. However, one problem with implementing nanotechnology is that the minute conductors may not be able to withstand their own formation, much less subsequent processing conditions or conditions of use in the end product. For example, the metal forming the nanoconductors may be soft, making it prone to bending or breaking during application of additional layers.
Further, it has heretofore proven difficult and even impossible to create nanoarrays having structures of uniform size and/or spacing.
Thus, as alluded to, the technology available to create PV cells and other electronic structures is limited to some extent by processing limitations as well as the sheer fragileness of the structures themselves.
Therefore, it would be desirable to enable creation of nanostructures having high aspect ratios and yet are durable enough for practical use in industry.
It would also be desirable to enable fabrication of a solar cell that has a higher than average efficiency, and in some embodiments, higher than about 30%.
A method according to one embodiment includes adding a template to a substrate; depositing conductive material in the template thereby forming an array of conductive nanocables on the substrate; removing at least part of the template; and depositing at least one layer of photovoltaic material on exposed portions of the conductive nanocables.
A nanostructure according to one embodiment includes an array of nanocables extending from a substrate, the array of nanocables having physical characteristics of having been formed using an at least partially removed template; an insulating layer extending along the substrate; and at least one layer of photovoltaic material overlaying portions of the nanocables.
In one approach, the template is created by forming a membrane on a patterned surface, and removing the membrane from the patterned surface, wherein the membrane is subsequently coupled to the substrate for being the template. The membrane may include a dielectric, polymer or a combination thereof.
In one approach, the template is formed by embossing. In another approach, the template is added to the substrate in a continuous process. In yet another approach, the template is formed at least in part from a photoresist that is patterned without a hard mask.
The at least one layer of photovoltaic material may be electroplated, formed by chemical vapor deposition and etching, and/or any suitable process. In one embodiment, depositing one of the layers of photovoltaic material includes performing multiple chemical bath depositions with at least one of a thermal anneal and a densification performed between the chemical bath depositions.
The nanocables, or groups thereof, may be elongated, and may have substantially uniform peripheries. Axes of the nanocables are tilted from a direction normal to a plane of the substrate in some embodiments.
In one embodiment, a first group of the nanocables has a different composition, thickness, and/or height than a second group of the nanocables. In another embodiment, the at least one layer of photovoltaic material overlaying a first group of the nanocables has a different composition and/or thickness than the at least one layer of photovoltaic material overlaying a second group of the nanocables.
The template may partially remain during the deposition of the at least one layer of photovoltaic material. At least a portion of the template may remain in the final structure as an insulating layer.
In one embodiment, the substrate is electrically conductive, and at least a portion of the conductive substrate is segmented for forming electrically isolated segments thereof. In one approach, in a first deposition, electricity is conducted only to a first group of the conductive nanocables for depositing the at least one layer of photovoltaic material thereon; wherein in a second deposition, electricity is conducted only to a second group of the conductive nanocables for depositing the at least one layer of photovoltaic material thereon; wherein the first and second groups include at least some different conductive nanocables. As a result, a composition, thickness, and/or height of structures formed by the first deposition may be different than a composition, thickness, and/or height of structures formed in the second deposition.
In one embodiment, the substrate is conductive, and is treated for enhancing electrical contact between the substrate and the conductive nanocables.
A front contact in communication with an uppermost of the at least one layer of photovoltaic material may be created.
In another embodiment, a conductive layer is formed over the at least one layer of photovoltaic material, and is segmented into electrically isolated segments.
In one embodiment, the array of conductive nanocables with photovoltaic material thereon may be coupled to another photovoltaic device, the photovoltaic device being at least semi-transparent, wherein the array is positioned relative to the photovoltaic device such that light passing through the photovoltaic device strikes the array.
In another embodiment, at least some of the conductive nanocables with photovoltaic material thereon have a portion with a wider diameter than in another portion thereof. The portion having the wider diameter may be positioned towards the substrate or away from the substrate.
In one embodiment, a dielectric overcoat, e.g., of ethyl vinyl acetate may be deposited over the structure.
One embodiment includes photovoltaically activating the conductive nanocables with photovoltaic material thereon using a pulsating laser. In another embodiment, a layer is deposited over the nanocables, wherein a material of the layer is of sufficient temperature at deposition thereof to photovoltaically activate the nanocables with the at least one layer of photovoltaic material thereon. In one approach, the layer is a transparent conductive oxide that also acts as a front contact for the nanocables with photovoltaic material thereon.
Voids are present in the array in one embodiment. A conductive layer is formed over the array and a conductor is coupled to the conductive layer in the void.
The following description is the best mode presently contemplated for carrying out the present invention. This description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each and any of the various possible combinations and permutations.
Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.
It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified.
Embodiments of the invention are described herein in the context of solar cells. However, it is to be understood that the particular application provided herein is just an exemplary application, and the nanocable arrangement of the invention is not limited to the application or the embodiments disclosed herein.
This disclosure also relates to nano arrays of thin film solar cells. Solar modules constructed using thin film systems tend to use a single larger single plane thin films solar cell, rather than an array of smaller interconnected nano-scale solar cells. The entire module can use a laser scribe to mark individual cells. It is important to note that nano systems will be processed differently than current technology thin films. Four main thin film material system types are amorphous silicon (A-Si), copper indium selenide (CuInSe2 commonly referred to as CIS), copper indium gallium selenide (CuInxGa1-xSex) commonly referred to as CIGS), and CdTe/CdS. A-Si films are typically fabricated using plasma enhanced chemical vapor deposition (PE-CVD).
The term “nanocable” denotes any elongated body whose one dimension (e.g., diameter or width) is of nano or micro scale or size and the other dimension is larger, potentially much larger. A “nanostructure” may include one or more nanocables. A nanocable may be fabricated with dissimilar materials, either as a core rod or wire that is laterally enveloped by one or more layers of material(s), as a nanotube that is filled with one or more layers of material(s), as a single structure of one material, etc. Nanocables are also interchangeably referred to as nanotubes, nanorods, nanowires, filled nanotubes and bristles. The functional element of the nanocable in each case is the interface(s) between the two (or more) materials. In various alternative configurations and modes of growth, a succession of layers of different materials, alternating materials or different thicknesses of materials can be deposited to form nested cylinder nanocables.
The term “photovoltaically active p-n junction” denotes any p-n junction with an adequate p-layer and n-layer thickness to generate electricity.
A method according to one general embodiment includes adding a template to a substrate; depositing conductive material in the template thereby forming an array of conductive nanocables on the substrate; removing at least part of the template; and depositing at least one layer of photovoltaic material on exposed portions of the conductive nanocables.
A nanostructure according to one general embodiment includes an array of nanocables extending from a substrate, the array of nanocables having physical characteristics of having been formed using an at least partially removed template; an insulating layer extending along the substrate; and at least one layer of photovoltaic material overlaying portions of the nanocables.
Bristle angles can be created by heating a polymer membrane and creating an asymmetric drag to get a template with tilted apertures into which a material may be formed, e.g., by electroplating. Deformation of the template may be performed, in one embodiment, by having a heat source, a source of drag, and an optional cooling source. One example would be a doctor blade scraping the heated top of a polymer membrane while the substrate is cooled with a cooling block, cool air, cool water, etc. A heated air knife may be used to replace the doctor blade. This also may be done with two contact rolls where one roll is cooled and moving at slow speed and another roll is heated and moving at a slightly faster speed. Additionally, seeding processes/vapor processes may be used on tilted surfaces to grow nanowire arrays at angles. Various shapes may be obtained using asymmetric pore membranes, according to various approaches. One or more electrically conductive strips 33 may extend across the array or portion thereof to assist in carrying electricity away from the array, thereby improving the overall efficiency of the brush. The efficiency gains are more pronounced in larger arrays. Such strips 32 are preferably very thin to block minimal light, in one approach.
In some approaches, the first and/or second conductive layers 14, 18 (or other layer or set of layers) are each contiguous, thereby simplifying manufacture, maximizing conductivity, reducing the chance of construction defects, etc. In other approaches, the second conductive layer 18 (front contact layer), first conductive layer 14 (back contact layer), and/or other layer or set of layers is segmented for forming electrically isolated segments thereof. Thus, a defect such as a short in a small section of the cell can be isolated and prevented from affecting neighboring portions of the PV cell. Segmentation of any of the layers may be effected using any known technique, such as etching, ablation, mechanical scoring, etc. Sections may be individually tested with only good portions of the device bussed together.
Separate groups of nanocables, such as rows, can be bussed or combined with metal and dielectric deposition so that any number of rows can he isolated for plating. For example, the segmented areas of conductive layer 18 may be bussed prior to plating such that energy may be delivered to one part of the array and not others. By switching segments on and off, two or more different compositions (different materials, material sets having different concentrations or elemental ratios, etc.), layers having differing thicknesses, etc., may be plated on different portions of the array, resulting in groups of bristles or portions thereof having different characteristics and/or properties such as different compositions, diameters, heights, affinity to certain wavelengths of light, etc. The segmentation may bus not only individual rows, but with patterning techniques to those skilled in the art, one may bus every other bristle in a basket weave type design, or any other possible division of bristles down to individual bristles. There can be any number of materials electroplated in a basket weave design, and moreover bristles can be optimized to PV thicknesses or material types which are especially suited for parts of the spectrum, according to one embodiment.
In addition to modifying or treating the back contact from the conductive base, the bristles themselves can also be modified with materials that can serve as improved back contacts. Such materials include Sn, An, Cu, C, Sb, Au, Te polymers, metal oxides, Si, SiO2, S, NiO, Ni2O5, NiS2, Zn, Sb2Te3, Ni, NiTe2, Si, SiO2, Cu, Ag, Au, Mo, Al and Te/C or combinations thereof. One may perform electrochemical deposition (ECD) of the nano or micro cables with an alloy of Ni and Cu, according to one embodiment. The alloying may minimize the diffusion of Cu during the heat activation step. One may also ECD coat a thin layer of Te on top of the alloyed nano or micro cable as a diffusion barrier, according to one embodiment. Nickel can be etched with a 10% HCl solution so that oxide is removed but the nano or micro cable stays intact. With that clean layer, CuSO4 solution may be applied and heat treated to cause Cu diffusion into the nano or micro rod prior to plating. Varying degrees of oxides such as SiO2 can also be added to the microrod surface effect to modify the plated composition, according to one embodiment.
In a further approach, a thin metal oxide layer may be formed or allowed to remain over the metal contact, e.g., Ni layer, to create a diffusion barrier to the lower metal contact and the overlying PV materials. Generally, one would expect an oxide layer to detrimentally affect performance by creating too much electrical resistance for proper operation of the array. Surprisingly, and counter to conventional wisdom, such an oxide layer was formed in an experiment, and was found not to cause an overly-detrimental effect on electric performance of the array. In the experiment, a thin layer of NixOy formed on the Ni lower contact due to exposure to oxygen. A CdTe layer was formed thereover. Surprisingly, the array functioned well. Moreover, it was found that the layer of NixOy beneficially prevented diffusion of the Ni into the CdTe layer. Accordingly, in some embodiments, a layer of metal oxide may be formed between the lower contact and the PV materials. Such layer of metal oxide may be formed, e.g., by exposing the lower contact to an oxygen-containing environment (e.g., air, ozone rich atmosphere, etc.) preferably while being heated (e.g., to greater than about 100° C.); barrel ashing; to etc.
The closer the photon absorption event is to the p-n junction, the more likely the event will result in usable electricity. In the case of a nanobrush, a reflective back contacting layer is not required because the photon can continue along the linear path so that it can contact the material on the opposite side of the cell thereby achieving a double pass in each nanobristle. In
The substrate 12 may be a conductive material or a nonconductive material (coated with a conductive material), rigid or flexible. For example, the substrate 12 may be glass, doped silicon, diamond, metal, polymer, ceramics, or a variety of composite materials, etc. Thin metal foil or certain polymers may be used where flexibility is desired. Flexible materials may be processed, e.g., by vacuum, attaching them to a structure that has the correct dimensions or flatness during the process, according to one embodiment. Moreover, a template may be formed by depositing a template material such as a binary template material continuously on a substrate such as a metal foil being unwrapped from a roll, forming the template, and creating PV nanostructures as described herein using the template. Structural integrity of the nanocable will vary with material choices. In the case of brittle or easily deformable bristles, a flexible substrate material may be used if attached to a rigid or semi-rigid surface. The molded surface/flexible membrane may be of particular help when PV cells are desired for an aerodynamic surface such as an airplane part, the roof of a car, the surface of other vessels or portable devices. Adequately, thin metals may also be used and plated using a semi continuous reel to reel plating. In such systems, degrease, positive electropolish, negative electropolish, electrochemical depositions made semicontinuous via dancer rollers, rinse and drying operations are all feasible in this case. One advantage to such a system is a formation of a semiflexible matt analogous to a window blind.
Moreover, the continuous nature of such an array allows one to tailor a power output of the array to meet specific needs, rather than the current approach of providing a number of modular arrays to achieve at least the desired power output, and in most instances, more than needed. The approach may be achieved by any number of techniques such as cutting the array to a size that provides the desired power output, forming the array with characteristics tailored to the specific parameters desired, etc. For example, assume a customer needs a 20 kW output. If each 1-inch band of array provides 5 kW, a four inch section would be provided.
Each of the bristles 20 is a discrete nanoscale PV cell. Compared to conventional flat PV cells design where only a single “xy” planar surface is exposed to light, the solar brush 10 has a “xyz” or a three-dimensional surface. Thus, for a given volume, the solar brush 10 has a useful surface area that can be several times to thousands of times greater than the “xy” surface area of conventional PV cells. The area between solar bristles 20 may be sufficiently wide as to make the brush absorptive to the majority of photons. Additionally, the bristles may be thin enough to be partially transparent. This effective transparency and bristle spacing increases effective energy generation to happen from sunrise to sunset while flat PV cells work optimally when the sun is straight above the PV surface, according to one embodiment. Because the effective energy generation from the solar brush is expected to be many times higher than conventional PV cell technology, the weight per kilowatt generated would be many times lower. The thickness of the PV material is as important as the height and the spacing of the bristles. If the materials are sufficiently thin, electron- hole recombination ceases to damage cell efficiency, and up to a 15% gain above that of 29% theoretical efficiency of a single junction cell becomes possible. This would allow use in small applications such as charging electronic devices (cell phone, computer, PDA, etc.), use in medium scale applications such as light weight roof-top energy for industrial and agricultural power generation, and use in large applications such as a light weight energy source for transportation (automobile, aircraft, barges, etc.). The efficiency of the cell would also enable improved power generation in low light conditions. The wide range of spectrum adsorption may also generate power from infra-red light at night time.
In addition, the film thickness to grain size ratio may be tuned to capture a particular wavelength of light, as disclosed in U.S. Provisional Patent Application No. 61/310,227, filed Mar. 3, 2010, and which is herein incorporated by reference. Moreover, some approaches use a mixture of grain diameters to capture multiple spectrums of light. For example, larger grains may he used to capture longer wavelengths of light, while smaller grains are used to capture the higher energy/lower wavelength photons and produce higher-energy events.
Another advantage of using nano or micro cable structures is that the p-n junction associated with each nanocable has a smooth interface that results in a sharper junction. The smoothness is improved at nanoscale as the roughness (measured as rms-root mean square for instance) increases as the scale increases.
It should also be noted that though the axes of the bristles 20 are oriented normal (perpendicular) to the plane of the array in the drawings, the axes of the bristles may be tilted slightly (a few degrees from normal, e.g., 1, 2, 5, 10 or more degrees) or pronouncedly (e.g., 40-89 degrees). One reason why a tilted configuration may be desirable is to reduce unimpeded penetration of light into the array when the light is traveling in a direction normal to the array, according to one embodiment.
The layered brush structure may also be used to increase photovoltaic cell efficiency by using a high- and low-band gap material and semiconductor thicknesses tuned for spectral selection. A high band gap material may be used to coat the upper photovoltaic brush, and the low band gap material may be used to coat the lower photovoltaic brush. The upper material may convert higher-energy light to electricity and dissipate much heat. The lower material may convert lower-energy light. This increases both the efficiency and the life span of the lower brush A, according to one embodiment.
The layered structure does not need to be made of the same material or by the same process. For example, the upper brush may be produced using a conductive/transparent core of silicon and a silicon substrate made from photolithography and chemical vapor deposition and the lower brush made with organic dye technology. This way, the low-band-gap light can easily pass through the upper layer and reach, an organic nanocable base. The base may be made from an anodized aluminum template, carbon nanojacket, and wet polymer process. The layers may or may not have the same dimensions and/or composition. The design need not be limited to two types of photovoltaic cells. A multitude of cells can be included with a multitude of photovoltaic materials so long as each cell has adequate transparency for light to reach the cell below. Adequate light is determined by whether or not the brush structures at the lower level are capable of producing energy based on the amount of light that is received in operation.
Thin, minimal reflectance metals such as gold may be layered along with the n- and p-layers to conduct the current so further gains in efficiency might be achieved. Other metals may also be used, as would be known to one of skill in the art.
One benefit over current technology is, as previously mentioned, that the maximum efficiency for a given material may be achieved, according to various embodiments. Another potential benefit may be achieved by layering material with different band gaps (energies required to excite electrons). The idea is to have a high band gap material such as GaAs (max efficiency ˜20%, band gap ˜1.4 eV) or CdTe (max efficiency ˜30%; band gap ˜1.6 eV) at the tip of the bristle and a reduced band gap material further down the bristle such as CIS or CIGS type PV material further down (max efficiency of ˜24%; band gap ˜0.8 eV). Photons with low energy will not react with high band gap material but will be available to react with low band gap material further down the bristle at further penetration depths. This may he achieved by CVD of CIS material on a nanocable, followed by etching to the top metal core of the nanocable, followed by catalytic growth on top of the nanocable, and the cable would be finished up by electroplating of CdTe/CdS, according to one embodiment. The solar brush PV cell design may also be a multijunction cell and is a superior architecture for such.
A flexible nanopore substrate can be used as the substrate 12 for deposition of metal. The substrate 12 may be a membrane applied to or constructed on a thin conductive sheet, and may he made into any desired shape. After metal deposition in the membrane pores occurs, the bristles 20 are formed. While other PV tapes and films have XY flexibility and strength, they are limited and no other technology allows for XYZ design of a rigid or flexible long lasting solar cell. The varied geometry of the solar brush allows the PV cells to he optimized for solar exposure from a fixed location, optimal aesthetic appeal, and minimal aerodynamic drag for transportation applications, according to one embodiment. Specific geometries combined with reflective substrates can effectively produce a combined PV film and solar concentrator.
There are many combinations of materials that may be used for the solar brush 10. One configuration is to use a Si thin film. Other configurations include, but are not limited to, CdTe/CdS (CdTe/CdS/SnO2/Indium Tin Oxide(ITO)/glass), GaAs/GaInP, CuInGaSe2, Cu(InxGa1-x)(S,Se)2, CuIn1-xGaxSe1-ySy, CGSe/CdS, CuInxGa1-xTe2/n-InSe, CdS/CIGS interface, ZnS/CIGS, Cu2S—CdS, CuInS2 or a mix of CuxS, CuInS2 and CuIn5S8, Cu(In,Ga)Se2/CdS, CIS/In2Se3, InN, CIS/In2Se3, ZnSxSe1-x. GaInP/GaAs, GaInP/GaAs/Ge, GaAs/CIS, a-Si/CIGS (a-Si is amorphous Si/hydrogen alloy), FeS2, Cu2O, ITO/a-CNx (Al Schottky thin-film carbon nitride solar cells), and MoS2 based solar cells or more general: MX2 (M=Mo, W; X═S, Se) thin films with Ni and Cu additives layers may be used as well. An Al2O3 layer may be used as a diffusion barrier with the CuInGaSe2 type PV cells, according to one embodiment. The manufacturing step may include heat annealing at high temperatures to allow for the consolidation of polycrystalline deposits to form a single crystal material or improve the structural integrity and regularity or geometry of the materials. Alternatively, single crystalline growth of layers may be favored by slow growth of the layers at moderate temperatures. Single crystalline deposits are important for optimum electron transport and photon absorption.
Deposition of the various materials may include chemical vapor deposition, solution phase deposition, electrochemical deposition, electrochemically induced sol-gel deposition, electrochemical atomic layer epitaxy, electroless deposition; e-beam evaporation, sol-gel with electrophoresis or centrifugation, electron beam lithography, scanned probe lithography, pressure injections, polymerization and electro-polymerization, pyrolytic decomposition, etc. Nanocables may also be grown from catalyst sites from chemical vapor deposition, wet or dry etched from a substrate, etc.
When designing a PV cell, one of the considerations is the photon flux. The number of photons that make it through the atmosphere at a given point remains relatively constant regardless of modifications in the PV cell that receives them. When determining the appropriate geometry for a PV cell, it is convenient to start by calculating the area of the gaps and the area of the bristle-tops.
A
total
=A
top
+A
gap
During the same calculation, it is useful to determine if the spacing for a given cable density is viable for given geometries. Presented by way of example only, and not meant to be limiting in any way, when the diameter of the nanocable 22 (Dnanoable) is 50 nm, the minimum PV bristle diameter D is about 220 nm. When Dnanocable=150 nm, the minimum PV bristle optical thickness is about 320 nm. The physical diameter of the bristles 20 will be 100-500 nm larger than the diameter of the nanocable 22, but these numbers should be used for the optical diameter calculations because the outer shell is transparent. The optical diameter is used for calculating the solar efficiency, and the physical diameter is used for determining process limits.
One preferred density (ρ) range for nanocables is:
ρ=106-109 pores/cm2=1010-1013 pores/m2
when using track etched membranes. When using metal oxide templates the density range shifts to:
ρ=1012-1015 pores/m2
For the low density case, there is 1 cable per 10−10 m2, or 1 cable in the center of a 10−5×10−5 m square, so the separation between the center of cables is 10−5 m or 10000 nm. From that number, the diameter of the bristle from its center axis (which extends through the length of the nanocable 22) to the n-layer is subtracted. The spacing may not be smaller than the cable and is preferably larger, so cases involving unrealistic physical spacing were eliminated from calculations in Table 1. Optical spacing, S, is given by the following formula:
S=cable separation (center pt. to center pt.)−diameter a bristle (semitransparent material)
After optical spacing is determined, the areas of the top of the PV bristles (Atop) as well as areas between the bristles (Agap) may be determined, according to one embodiment. Table 1 shows that majority of the planar surface area lies within the gaps of the PV cell, not the bristle tops. However, there are design points that have significant levels of top surface area.
Planar area and mass per area are useful to determine back reflection. For planar cells, reflection bounces much of the light out of the PV cell before it has a chance to be absorbed and generate electricity. However, back reflection can benefit the planar cell by bouncing the light off of the back of the cell to give the cell two opportunities to absorb photons from the same stream of light. However, while the back reflection increases the number of absorptive events in the planar cell, it also increases the amount of heat generated per unit volume. In the case of the solar brush 10, only a fraction of the photons that hit the bristle tops can reflect away from the PV cell.
In many cases with the solar brush 10, over 96% of the light falls into Agap. Several things happen to the light that falls into the gap: (a) the light is absorbed, (b) the light continues straight through the bristle into the next nearest bristle (as shown in
The depth and areas of penetrated light may also be calculated. This is a measure of how uniformly the light can be dispersed throughout the PV brush. The penetration of light is governed by the following formula:
T
pen=penetration thickness=S tan Θ
The thickness or bristle height is related to the maximum penetration. The average penetration for a light stream in many cases would be about Θ/2. However, as Θ approaches 90°, the bottom of the cell may be theoretically flooded with light. However, in reality, this flooding effect is minimal or nonexistent because the light is affected by irregularities in the bristle geometry and can be eliminated by tilting the bristles slightly, according to one embodiment.
The penetration % is a useful design criterion. For example, for transparent cables, if there is 10% penetration, the light will have as few as 10 passes through PV cables, and the average photon would have up to 20 passes through the p-n junction since the photon may pass through the p-n junction twice per bristle. It is probably best to set design criteria to target less than 20% for most of the day to ensure adequate absorption opportunities for the light stream. When Θ goes to 90°, tan Θ goes to ∞, temporarily making the penetration level 100%. Optimization, however, will generally be a function of field testing results.
The total PV absorption area is much greater for the sides of the bristles 20 than for the tops. Acell is the surface area available by PV brush which is given by:
A
cell
=T(π)(Dρ/2)
where T is the height of the cable, D is the optical diameter of the PV bristle, and ρ is the number of bristles per unit area. The quantity is divided by 2 because it is assumed that most light absorption will come from the sun which is shining on half of the cell at one time. There will be significant absorption events from scattered light as well, but the majority of photons typically come directly from the sun.
The penetration area is proportional to the penetration depth, as shown by the following formula:
A
pen=area initially penetrated by light=Tpen(π)(Dρ)
Where Agap>>Atop the dilution of light is represented by the following formula:
A
pen
=T
pen
/T*A
total
From Apen and Agap (Table 1), a calculation that shows the amount of light dilution that occurs in the cell can be made. The light dilution is useful for opportunities for solar absorption events and uniform heating. More dilute light leads to lower maximum temperatures or fewer hot spots in the cell, resulting in improved overall efficiency. Wherever there are hot spots, there is rapidly degrading conversion efficiency. Wherever there is concentrated light that tends to create hot spots, the ratio of opportunities for an absorption event to the number of photons decreases.
The substrate 12 may be a conductive material (e.g., metal) or a nonconductive material (e.g., glass, polymer, etc.) that is coated with a conductive layer.
Alternatively, a template or membrane can be continuously produced using a cylinder with metallic posts that has a polymer sprayed on it such as those membranes created by 10X Technologies. Another alternative would be to emboss a polymeric (e.g., thick polypropylene) film in a continuous or batch embossing process using conventional embossing techniques like those employed by paper and diaper manufacturers.
Selection of the membrane 30 depends on the particulars of the PV cell that is being fabricated. Different p/n combinations have different thickness requirements and therefore different cable size requirements, according to one embodiment. Before electrochemical deposition, the membrane 30 is cleaned and air bubbles expelled from the pores by submerging the membrane 30 in methanol and sonicating for 5 minutes.
Final etching through the insulator film may still be required as would oxide removal, according to some embodiments.
Following the template formation, nano or micro cable may be formed using any number of solutions. Nickel is widely used in MEMS devices and is compatible as a back contact for many PV devices, and is therefore a good choice. Other metals are also available to use, as would be known to one of skill in the art. There is a wide range of commercial nickel baths that are appropriate for the template. Constant potential deposition is generally preferred for precision bristles, but processes can easily be built for constant current deposition as well.
In one approach, after the micro or nanocable is formed, the polymer is ashed away using a barrel asher or similar plasma and wet etched to remove residue, according to one embodiment. Prior to plating, the oxide removal process is repeated on the nano or micro bristles to create a high quality back contact, according to one embodiment. Acidic CdTe plating may be performed at a pH of about 1.75, saturated Te from TeO2 and CdSO4 at about 100 times the concentration of the Te. Care should be taken to keep the bath clean and keep any contaminants from the bath. Constant potential deposition of CdTe may be performed anywhere between about 60° C. and 90° C. Potentiostats can be easily adjusted between 0.3V and 0.8V to deliver slightly P type CdTe, according to one embodiment.
Following CdTe deposition, electrochemical or immersion CdS plating may be performed using well known materials, such as thiourea, thiosulfate baths, etc., at about 65C to 80C. Finally, CdCl2 at about 1% in water or methanol is heated in contact or proximity of the surface at about 100° C. to 500° C. from about 30 sec to 45 minutes, depending on the thickness of the CdTe, according to various embodiments. Lastly an indium tin oxide or fluorinated tin oxide coating is applied to the cell and the fabrication of the cell is complete, according to one embodiment.
The pore walls and the membrane 30 become coated with discrete nanoscopic Ag particles. The membrane is rinsed with ethanol and immersed in water. Then the membrane is immersed in a 7.9 mM Na3Au(SO3)2/0.127M Na2SO3/0.625 M formaldehyde solution that has a temperature of about 0° C. Gold plating is continued for 10 to 24 hours (time is dependant on pore size), at which time the nanocables are fully formed in the membrane, according to one embodiment.
An alternative way to deposit materials inside membranes entails using electrophoresis or centrifugation sol-gel methods, electrochemical atomic layer epitaxy, chemical vapor deposition, sputtering, E-beam evaporation, thermal evaporation, electron beam lithography, and scanned probe lithography. Alternatively, well known additives can be dissolved in the solution to impart nanocable strength or better electrical connections to the n-layer conductor. Preferably, metal covers all exposed areas of the membrane, substrate, and fills the pores, according to one embodiment. After the gold deposition, the membrane is soak with water and rinsed 4 times over a 3-hour period and immersed in 25% nitric acid for 12 hours to remove residual Sn or Ag. Finally, the membrane is rinsed with water and air dried. Evaporative metal deposition can also take place in the same manner as in
Alternatively, the membrane may be placed into the electroless plating solution by itself. The top, bottom, sides and pores become metalized. The membrane 30 may be glued or otherwise coupled, as mentioned above, to the metalized substrate 12.
If desired, atomic layer epitaxy may be used to build a protective cover over the membrane 30. Atomic layer epitaxy may be used as an alternative to electrochemical epitaxy.
As shown in
The insulating layer 16 may keep the current from the n-layer and p-layer from short circuiting. The insulator can also limit deposition of PV material to the nanocables. Because insulation eliminates the effects of defects of one cable from affecting its neighbors, processes like electroplating become feasible.
Electroplating is a desirable process because of low equipment costs and relatively good material conservation relative to other processes such as sputtering and CVD which deposit material throughout the chamber in addition to in the desired area. The thickness may easily be determined by using various exposure times to dichloromethane and verifying the membrane thickness with scanning electron Microscopy.
If all of the membrane 30 is removed, excessive material is consumed. This process may be used if a thinner insulating material or a material other than the material the membrane 30 is made of is desired to form the insulating layer 16. In this case, the desired material may be spin-coated on the substrate 12 with polymethylmethacrolate (PMMA) to a thickness of about 1 μm, according to one embodiment. The PMMA may function as a membrane glue and/or an insulator. Any insulating material that can be applied to the PV cell be it polymers, silicon dioxide, or any insulator that can have adequate dimensional control during application. The PC membrane may be placed on top of the PMMA and baked at around 100° C. for about an hour.
In some embodiments, the insulating layer is eliminated altogether. As long as the p and n layers are adequately produced, direct contact with the conducting layers is possible.
In other embodiments, holes are made in the insulating layer after attachment of the membrane. For example, reactive ion etching (RIE) with oxygen and/or wet etching may be used to drill through the insulating layer 16 to allow the nanocables 32 to connect with the first conductive layer 14.
In other embodiments, membrane can actually be used as a masking layer to etch pores in the underlayer, which is the insulating layer 16 in this case.
In another illustrative approach, CdS deposition is performed in 1.5 mM SC(NH2)2, 1.5 mM Cd SO4, and 2 mM NH4OH heated to a temperature of about 40° C. to about 70° C. Under these conditions, a 4.5 minute exposure would lead to a CdS layer of about 30 nm.
The CdS layer also can be deposited by ECALE (electrochemical atomic layer epitaxy), ALD (atomic layer deposition in chemical vapor deposition system), sol-gel processing, low pressure CVD, etc. Again, when non-electroplating processes such as CVD-related methods are used, etching can be used to remove the p-type layer 24 at the base of the structure to expose the insulation layer and create isolation between the nanostructures.
Under some circumstances it may be economical and desirable to build a conductive grid (e.g., parallel conductors, hub and spoke configuration, crossed conductors, etc.) above the insulating layer and between the cables to augment the current carrying capacity of the top conductive layer (e.g., TCO). The grid may be formed of any conductive material, a metal or metallic alloy being preferred. The grid preferably does not contact the PV layers of the bristles, and may be formed under or above the upper conductive layer. The grid may be coupled to a collection circuit, coupled to a bus, etc. Such grid may be used in some embodiments to overcome challenges with optimizing TCO transparency and conductivity and/or to enable use of a conductive polymer that might otherwise be adequate. Preferably, such grid is located where it does not block light. Moreover, if the grid is reflective, it may provide a nominal amount of reflection that may be beneficial. In one approach, the grid may be deposited during building the insulating layer(s) before patterning the bristles. In another approach, the grid may be formed after the n-layer is applied and the bristles are segmented. Other formation techniques may also be used.
Additionally, gel electrolytes may be used to make the electrical contact for the n-layer as shown in U.S. Patent Application No. 2004/0025933, which is herein incorporated by reference. The electrolyte solution may be a combination of poly(4-vinylpyrinidine), poly(2-vinylpyrinidine), polyethylned oxide, polyurethanes, polyamides and a lithium salt. The salt may be lithium iodide, lithium bromide, lithium perchlorate, lithium thiocyanate, lithium trifluormethyl sulfonate, and lithium hexafluorophosphate to name a few.
Although
Also, any membrane with micropores may be applied to the substrate 12 to produce the PV brush, according to some embodiments. Also, any metal deposition may work with nanopores be it chemical vapor deposition, plasma vapor deposition, metal organic vapor deposition, electrochemical deposition (electrochemical epitaxy, under-potential deposition), liquid phase epitaxy, molecular beam epitaxy, hot wall epitaxy, sputtering, E-beam and thermal evaporation, electroless deposition, chemical bath deposition, sol gel and solution methods, vapor-liquid solid methods, sonochemistry methods, microwave methods, etc.
In any of the embodiments described herein a transparent or substantially transparent dielectric overcoat may be applied to the solar brush. The deposition thickness may be less than, equal to, or greater than the height of the nanocables. Any known material may be used. Preferably, the dielectric overcoat is applied in liquid form, according to one embodiment, thereby reducing the risk of breaking any of the nanocables during application thereof. Particularly preferred materials include resins and polymeric materials, such as liquid ethyl vinyl acetate (EVA), which is typically deposited and cured. Moreover, any known deposition technique may be used, including dipping, spraying, spin coating, etc. Preferably, the dielectric overcoat is applied after the front electrical contacts are made, thereby sealing the cell from the elements.
As noted elsewhere herein, the overcoat material may he heated prior to the deposition to a sufficient temperature to photovoltaically activate the nanostructures such that application of the overcoat activates the photovoltaic nanostructures, according to one embodiment.
In one approach, the PV cell can be fabricated on a nonconductive, heat tolerant sheet with sufficient room to apply EVA, rubber, or combinations thereof to the surface. The sheet can then be heat sealed to any relatively transparent, impact resistant, and able to with stand environmental damage such as moisture penetration, physical impact, extremes in temperature, etc.
Nanoporous structures of certain metal oxides can be obtained with the metal anodization process instead of, or as a variation of, the method illustrated in
In one experiment, tin oxide was anodized. Before electro deposition, a thin Au film was sputtered on one side of the aluminum anodically oxidized (AAO) membrane to serve as the conductive layer. Electro deposition of Sn into the pores of the AAO membrane was carried out at a constant current density of 0.75 mA/cm2 for 1 hour in electrolyte containing sodium tricitrate of 25 g/L and tin dichloride of 7 g/L. The Sn embedded in the AAO membrane was anodized at 10 V in 0.2 M boric acid, whose pH value was adjusted to 8.6 by 0.5 M NaOH(aq). The anodization proceeded until the current density dropped to almost zero. The AAO membrane was then removed through wet etching with 0.5 M NaOH(aq), leaving behind an array of nanoporous tin oxide nanorods. Finally, the samples were calcinated at 500° C. for 3 hours in air.
The bristles 20 may be shaped to increase the surface area. For example, the bristles 20 may have “branches” or holes in the nanocable. Holes may be created by depositing the Cd/Au alloy as just described and anodizing.
As another alternative to the method of
To create an insulating layer in between nanocables, the top metal contact 74 can be removed to expose the composite surface made of conductive nanowire and p layer. Then, the p layer and n layer can be selectively etched leaving the conductive nanowires sticking out the top surface. Then, an insulating layer can be applied and the surface is then polished to result in a composite surface made of conductive nanowires and insulating layer. Then, a conductive layer is applied which will create the contact with the nanowires.
As shown in
With the titanium dioxide cap 139 removed, the carbon nanotube 136 is burned off by exposure to air at about 600° C. (
As mentioned above, there are numerous advantages of the solar brush 10 over conventional PV cells. The solar brush 10 demonstrates a high thermal stability. Unlike nanoparticles, where the linear thermal expansion coefficient increases with the reduction of the average grain size (Cu, for instance), Cu nano-wires show a smaller thermal expansion coefficient than that of the bulk Cu. The high thermal stability is related to the grain boundary structure and high aspect ratio of the nanostructure.
Daisy chain connections may also combat potential thermal expansion/contraction issues by minimizing chip size and then connecting them opposed to having a large sheet that would have a higher potential for stress cracking due to thermal expansion contraction. Daisy chains between cells may also add flexibility to a PV brush array. To accomplish this, the cells may have special interlocking mechanism to serve the dual purpose of a being a robust carrier of the film during processing and to speed assembly.
Because the method describes growth of conductors on a conductive sheet, the failure rate that plagues current PV cell manufacturing will be greatly improved giving further cost/efficiency advantages.
A further advantage of the PV brush is that the distance electrons diffuse through the semiconductive layer to the conductive layer is shorter than that of conventional PV cells thereby reducing internal resistance of the PV cells to deliver further power generation efficiencies, according to one embodiment. Because the PV bristles are thin, they use a small fraction of the material required for planar cells. A variety of organic and inorganic semiconductors can he applied to the conductive core and thicknesses can easily be optimized for power generation and stability.
Besides solar panels, nanoelectronic assemblies can also be used for light generation in optical chips, according to one embodiment. Optical chips are widely thought to be the replacement for semiconductor chips. Optical chips have narrow pathways that light can travel unhindered while semiconductor chips are limited by electric field effects between on circuit and the next. A micro light source with unique color attributes may be used in optical chip technologies. The nanoelectric assemblies may also be used as a nanolight source for such chips. Additionally, the nanodiodes can be used in a flat screen display for an ultra sharp video monitor. Additionally, the nanodiodes can be used for very energy efficient lighting.
The PV brush has flexible manufacturing options including membrane manufacturing technologies or photolithography e-beam, low density layered mechanical scoring, nanoporous templated, electroplating, and electrical arcing. These manufacturing methods may be used on a variety of membrane/nanoporous media which allows cell to be shaped and hardened to geometry that has maximum solar efficiency, maximum aerodynamic efficiency, maximum aesthetic appeal or a combination of the aforementioned attributes. Flexible units can also be achieved by daisy chain connection between small rigid units or from the use of a flexible substrate, according to one embodiment. At high temperatures, uneven thermal expansion can cause cracking and wear as well. High temperature degradation is mitigated because each component of this PV cell can be sized to minimize thermal expansion and can be further optimized with flexible expansion joint conductive connections between PV arrays. Additionally, the greater surface area of the solar brush will reduce thermal heat generated under the PV solar cell compared to the conventional flat unit which may greatly reduce unwanted heat buildup. One further advantage is that micro conductors often have reduced resistance at higher temperatures; therefore, the PV brush may be able to transfer energy more effectively than conventional PV cells at higher operating temperatures.
Finally, the geometry can be used to trap or release heat, according to one embodiment. If heat were found to be detrimental to energy above a certain point, the unit may be designed with vents. However, it should be noted that performance of nanocables may be different that than large scale wires. While large scale wires/cables have higher resistance to electrical flow at high temperatures, energy flow may improve due to improved flow through grain boundaries in nano-scale structures.
Power generation is a function of average power per day. The median sun hours for various cities in California is about 6.18 kW/(day*m2) according to a Go Solar® Company web page at www.solarexpert.com/Pvinsolation.html. On average, solar energy is drawn from about 6 hours per day based on the data made publicly available by National Renewable Energy laboratory findings. The distribution is commonly given as a Gaussian curve, which has the following distribution:
Assuming an average of μ=6 hours, a standard deviation of σ=1 hour, and integrated power of 6.18 kwh/m2 for an average day gives a maximum energy. When x=μ, the theoretical maximum power generated is about 4.933 kWhr/m2. Based on EU studies of layering, the importance of having each solar event near the p-n junction, and reduced hot spots, the CdTe system may approach its theoretical efficiency limit. Efficiency could get as high as 30% or more with the single layer systems and potentially higher, especially if a high and low band gap system (discussed earlier) are combined. The distributions are shown in
The power calculation works out as follows:
P=6.18 kWh/(m2×d) from the mean values for a California city
P
Brush
=P×E×O
Thus, where E=29 (29% efficiency) for a CdTe/CdS PV cell and O=the orientation gain 1.44 (44% gain), PBrush=2.60 kWhr/(m2×d) (average day in the mean city in CA).
However, it should be noted that the brush can pick up about a 44.8% gain in efficiency by because it would require little if any sun orientation adjustments. The orientation of the solar brush 10 may have a large effect on performance. Planar PV modules lose up to 44% power from poor orientation and often need to be reoriented using a “solar compass”. Due to its unique design, the solar brush 10 does not require reorientation.
If electrical current through the PV device is sufficiently high, a cooling systems that may either be used to generated thermoelectric power (i.e. steam turbine type of power generation) or water heating systems for home use may also be possible.
A majority of the light from the sun is scattered from the atmosphere. Collecting scattered light using the solar brush 10 should lead to even higher energy production. Further energy gains from multi-junction solar cells may bump the efficiency to double what is believed to be currently possible.
The solar brush 10 will probably approach the theoretical maximum efficiency for a given material. Because the brush can be made nearly transparent, most of the light continues to travel through the cell. For practical purposes, the brush would appear to be of ∞ thickness. Because the bristles can be designed just thick enough for stable solar absorption, each absorptive event would happen near the p-n junction. The occurrence of the absorptive event near the p-n junction improves cell efficiency. Another key to improving cell efficiency is to reduce localized heating. Each time there is solar absorption, part of the energy ejects the electron and part of the energy heats the cell. The heating reduces the efficiency of the cell. When cells rely on back reflection, they are also doubling the heat load for a given areas. As the sun moves across the sky, the penetration angle is changing and the trajectory of the solar stream is changing so there is a greater quantity of “fresh” material for the photons to impact. With the solar brush 10, more of the absorption events can be made to occur near the p-n junction through control of the layer thicknesses, and the light stream will pass through greater amounts of PV material. Multiple junction material is believed to be the key to maximum efficiency in the future. Table 2 shows efficiency potential, band gap, and field efficiencies for several materials, any of which may be used in an embodiment of the present invention.
Efficiency compares favorably with current technologies to give the maximum power increases. Table 3 shows the potential energy efficiency and power generation capability in the state of California.
Power generation and effective areas for the brush can be significantly boosted through the use of a solar concentrator. A solar concentrator may redirect large areas of light perpendicular to the surface, thereby utilizing the surface area at the depths of the brush. Only light angles close to 90° can penetrate a high area shell. The penetration depth in shown by
Konarka uses a technology where printed polymers generate energy from all visible spectra. As described in http://www.konarkatech.com/about/, PV polymers are printed on polymer sheets. Materials are produced by injecting a dye into titanium dioxide and printing the material on to polymers. The Konarka technology is expected to yield 10% efficiency and last about 8 years. In comparison, the materials disclosed herein that are used for the solar brush 10 have a lifespan in the 25 to 30 year time frame. Konarka's process may be 100 times less expensive than the solar brush 10 but produces PV cells of only around 2% efficiency. Furthermore, these PV cells would not have a form that is compatible with concentrators. Therefore, the maximum power Konarka's PV film would expect to generate on a given day would be about 0.11 kW/m2, and the brush may generate between 450 and 2500 times the power that the Konarka system generates.
Table 4 illustrates the power generation for 8″ disk PV cells.
Solar brushes 10 may be made from disks of any commercially-available diameter, or can be grown from any dimension films using oxide templates, according to various embodiments. They can use existing photolithography and sputtering machines. If an 8″ diameter disk is used, it would generate the power equivalent of 0.97 to 5.58 m2 planar photovoltaic cells. If a perfect reflector were used in the solar collector, the minimum dish size would range from a diameter of 1.1 m to 14.8 m for full utilization of the PV cell area. Because perfect reflectors do not exist, some of the energy would be lost to absorption and misdirected reflections. A 2 m to 25 m diameter may be used to generate the maximum energy. Smaller units can be produced if desired, the size being a function of the power requirements and the installation location. The 8″ disk may generate 1.6 to 24.42 kW/day depending on the final area and thickness of material on a disk. The system is also preferably sized to allow proper current conduction without undue system heating of the substrate metal.
The small disk size will allow easy cleaning and reduce efficiency losses over time. Since the area of the central disk is so small, it may be designed to snap in and out to be cleaned in a way that is impractical for larger cells.
The wide range of methods to form nanocables on either flexible or rigid substrate that is shaped to a given specification then hardened impacts the efficiency of the film.
Hard coatings such as TiN, ZrN, or HfN that have melting points around 3,000° C. may be used for certain layers to minimize reflectance or as a reinforcement “jacket” to increase the hardness of the nanocables.
One useful technique that allows improved geometric control of the brush is over etching the top or bottom of the template so that the top or bottom of the bristle curves or steps outward (away from its axis), thereby having a larger diameter towards one end thereof than the other. The increased diameter of some or all of the bristles towards the bottom of the array (substrate end) or top of the array may be effected in many other ways, such as selectively adding more material to the top or bottom portion of the bristle, plating in a bath only covering a portion of the bristle length, etc. The larger lower diameter may be only along a portion of the bristle, thereby creating a step in diameter change somewhere along the bristle's length, may be a gradual tapering of the outer surface, a tapered portion and one or more portions having a constant diameter along the axis of the bristle, etc. The larger diameter at the bottom of the bristle is useful for many reasons. For example, this geometry provides greater stability of the bristle (less likely to break off). This geometry allows bristles to be both taller and placed closer to each other while maintaining a uniform, continuous TCO coating. This geometry also enables many TCO deposition techniques to be effective. Illustrative TCO deposition techniques include spraying, dipping, spin-on application, etc.
In other approaches, where the nanocable is overplated to form a conductive layer above the template, as disclosed in U.S. Provisional Patent Application No. 61/310,227, filed Mar. 3, 2010, and which has been incorporated by reference, it may be beneficial to have the tapering located near the overplated area, the overplated area being the conductive substrate for subsequent processing.
TCO may also be applied in a heated liquid form. Accordingly, in one embodiment, the liquid is so hot that the TCO deposition and the heat activation of the cell are combined in one step. In other words, the heat from the TCO activates the PV cells.
Heat activation can also be performed with a laser. One preferred approach uses a pulsating laser, which affords good control over both the amount of energy applied to the array as well as allowing tailored activation of particular sections of the array. Another advantage of laser activation is that very little energy is wasted and the carbon footprint is minimized. Often modules are activated in ovens where most of the energy is lost to the environment. Another advantage is that the correct amount of energy is applied to the PV cell. When cells get to much or too little energy, the cell performance is reduced. Finally, the lasers can be pulsed such that some nanocables receive more energy than others. This can be particularly helpful when multiple materials with differing activation requirements are found in the PV array.
Arrays can be formed or patterned is such a way as to leave voids in the array where conductors can be placed in communication with the front contact layer to reduce the current load on the bulk of the front contact layer. Voids can be patterned also to allow control the voltage of the module. In one approach, assume the nanocables are about uniformly arranged in a two-dimensional array as in
Additional techniques which may be used in portions of various embodiments are disclosed in copending U.S. patent application Ser. Nos. 11/466,411 and 11/466,416, both filed Aug. 22, 2006, and which are both incorporated by reference.
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
This application claims priority to U.S. Provisional Patent Application No. 61/219,277 filed Jun. 22, 2009, and which is herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61219277 | Jun 2009 | US |
