KR20160037984A - Laminated backplane for solar cells - Google Patents

Abstract

A back-contacting solar cell structure comprising a front side and a metallized back side for receiving sunlight, the rear side contact solar cell structure comprising an on-cell patterned base electrically connected to the base and emitter regions on the back side of the back- . The backplane laminate layer, which is composed of resin and fibers, and has a thermal expansion coefficient that is relatively matched to the thermal expansion coefficient of the back contact solar cell semiconductor substrate, has an on-cell base, an emitter metallization portion and an on- Contact solar cell semiconductor substrate that is not covered by the back-contacting solar cell semiconductor substrate.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a laminated backplane,

Relevant Application Cross Reference

This application claims the benefit of U.S. Provisional Application No. 61 / 860,216, filed June 30, 2013, which is incorporated herein by reference in its entirety.

This application claims the benefit of U.S. Provisional Application No. 61 / 521,743, filed on August 9, 2011, and U.S. Provisional Application No. 61 / 521,754, filed on August 9, 2011, This application is a continuation-in-part of U.S. Patent Application No. 13 / 807,631, which is incorporated herein by reference.

This application is a continuation-in-part of U.S. Patent Application No. 13 / 433,280, filed March 28, 2012, which claims the benefit of U.S. Provisional Application No. 61 / 468,548, filed March 28, 2011, This application is a continuation-in-part of U.S. Patent Application No. 13 / 204,626, filed on August 5, 2011, which claims the benefit of U.S. Provisional Application No. 61 / 370,956, filed on even date herewith.

Technical field

The present disclosure relates generally to the field of solar cells, and more specifically to solar cells with backplanes.

Traditionally, crystalline silicon (polycrystalline silicon and monocrystalline silicon) has the largest market share in the PV industry and currently accounts for about 85% of the global PV market. While it has long been understood that changing to thin crystalline silicon solar cells is one of the most powerful and effective ways to reduce the cost of PV (the cost of crystalline silicon wafer materials used in solar cells as part of the overall PV module is relatively low Thin wafers are very susceptible to mechanical breakage during wafer handling and cell processing and are limited by problems such as reduced yields due to thin and fragile silicon wafers.

Thin crystalline solar cells may be advantageous as permanent backplane supports, but solar cell fabrication often involves high temperature processing, which increases mechanical stresses on thin silicon solar cell substrates and underscores problems with thermal expansion mismatch, adhesion failure do. Conventional solar cells often include a solar cell package, a capsule material, a module, and a sealing sheet for protecting and supporting the solar cell within the field, The inherent structural properties are lacking for use as permanently adhered and bonded layers.

Therefore, there is a demand for a back-contacting solar cell having a backplane attached to a solar cell that provides an improved structural support to the back-contacting solar cell. According to the disclosed subject matter, a backplane-attached rear-facing solar cell provides that the disadvantages and problems associated with previously developed solar cells are eliminated or reduced.

According to one aspect of the disclosed subject matter, there is provided a back contact solar cell structure having a photoreceptor front side and a metallized back side. The on-cell patterned base and emitter metallization are electrically connected to the base and emitter regions on the back-contacting solar cell semiconductor substrate. A backplane laminate layer composed of resin and fibers and having a thermal expansion coefficient that is relatively matched to the thermal expansion coefficient of the back contact solar cell semiconductor substrate is attached to the on-cell base and emitter metallization portions, Contact solar cell semiconductor substrate that is not covered by the portion of the back-contacting solar cell semiconductor substrate.

Further disclosed novel features of these and other forms of the disclosed subject matter will become apparent from the description provided herein. This summary is not intended to be a comprehensive description of the claimed subject matter, but rather to provide a brief review of some of the subject matter's functions. Other systems, methods, features, and advantages provided herein will become apparent to those skilled in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within the scope of the present invention, as embodied within the claims.

BRIEF DESCRIPTION OF THE DRAWINGS The features, nature, and advantages of the disclosed subject matter, along with the drawings, will be apparent from the description set forth below, wherein like reference numerals designate like features,
1 is a cross-sectional view of a backplane-attached rear-facing solar cell comprising a backplane layer having fiber and resin content;
2 is a cross-sectional view of a back-contacting solar cell with a dual level backplane;
3 is a cross-sectional view of a back-contacting solar cell with a two-level metallization backplane attached comprising a backplane layer having fiber and resin content;
4 is a cross-sectional view of a back-contacting solar cell with a two-level metallized dual-level backplane;
5 is a photograph of a woven aramid fiber substrate 20-fold magnified;
6 is a photograph of a nonwoven aramid fiber substrate 20 times magnified;
FIG. 7 is a graph showing data on the coefficient of thermal expansion (or CTE) vs. resin content for an aramid fiber prepreg material;
Figure 8 is a rheological graph schematically illustrating the rheology profile of a mixed resin system;
Figure 9 is a diagram illustrating the diffusion of an additive in a resin and the coupling of silane to silicon oxide;
Figures 10a, 10b, and 10c are graphs illustrating the lamination process profile of a mixed resin system;
11 is a graph showing a series of experimental outlines for a sample with mixed resin;
Figs. 12A and 12B are SEM images emphasizing voids in a solar cell structure occurring during non-vacuum lamination; Fig.
Figure 13 is a high level solar cell and module fabrication process flow embodiment using starting crystalline (monocrystalline or polycrystalline);
14 is a cross-sectional view of a backplane-mounted back-contact solar cell according to the disclosed subject matter.

The following description is intended to be illustrative of the general principles of the disclosure, rather than of limitation. The scope of the present disclosure should be determined with reference to the claims. Exemplary embodiments of the present disclosure are shown in the figures, wherein like numerals are used to refer to the same and corresponding parts of the various figures. Importantly, exemplary dimensions and computations disclosed in the embodiments are provided as a detailed description of specific embodiments and are used as general guidelines when forming and designing solar cells according to the disclosed subject matter.

The present application is based on the finding that a brittle material can be used that is biocompatible (e.g., having an area ranging from about 100 cm ² to less than a square meter), thin (e.g., ranging from a few microns to a few hundred microns) Providing a unique structure solution that supports using a prepreg backplane structure comprising fibers. The disclosed organic or polymer backplane structures and fabrication methods have been developed for matching with the thermal expansion coefficient (CTE) of semiconductor materials (e.g., crystalline silicon) of solar cells used in high efficiency solar cells.

Innovative backplane and backplane lamination process characteristics (for applications including, but not limited to, high-efficiency solar cells) are provided, which include material and method descriptions of manufacture for those skilled in the art. Although the present disclosure has been described with respect to particular embodiments, such as interdigitated back-contact (IBC) solar cells using, for example, monocrystalline silicon substrates and the fabrication materials described therein, Other solar cells, including but not limited to IBC back-contact solar cells (e.g. metallized wrap-throw or MWT back-contact solar cells, conventional front contact batteries) (Such as silicon, gallium arsenide, germanium, gallium nitride, other binary and ternary semiconductors, etc.), technological areas, and / or the principles discussed herein, have.

The innovative material properties and processing requirements of the backplane are used in the manufacture of flexible semiconductor electronics and photovoltaic devices, particularly for high efficiency crystalline silicon solar cells. The example design rule is applicable to a back contact / rear bonded crystalline silicon solar cell support comprising a cell made of epitaxially grown single crystal silicon, or a cell made of a crystalline silicon wafer or a polycrystalline wafer made using CZ . However, it should be understood that the embodiments provided herein may be applied to other crystalline semiconductor solar cells and modules.

In one of the simplest embodiments, the backplane is a silicon wafer or silicon substrate (or other semiconductor material such as GaAs, GaN, etc.) that is thin (e.g., in the range of a few microns to 250 microns thick) (E. G., Coupled) to a < / RTI > 1 is a cross-sectional view of a backplane mounted back-contacting solar cell comprising a backplane layer having fiber and resin content. 2 is a cross-sectional view of a back-contacting solar cell with a dual level backplane. Although the focus is on application to back contact / rear junction solar cells using thin crystalline silicon absorbers, embodiments of the present invention may be applied to various other solar cell designs (e.g., front contact solar cells) and other semiconductor absorber materials As will be understood by those skilled in the art.

The backplane structure can perform two primary functions. First, a wafer or thin wafer that can be handled and processed without damage or breakage and that can be laminated into the final solar cell module without damage or breakage, or a structural support over the entire area of the thin semiconductor absorber, to be. Second, the backplane can be fabricated by extracting the cell level power of the cell level contact metallization portion (referred to as metal-1 or M1 or on-cell metallization portion) on a silicon wafer or substrate (on-cell) And serves as a dielectric or electrically insulating layer connecting between the second or more highly conductive metallization portions (referred to as Metal-2 or M2), which are used to interconnect the first and second conductive metallization portions. Thus, the backplane can perform the construction of a two-level metallized portion of the back contact / back junction solar cell for inexpensive manufacture of high efficiency solar cells and modules. 3 is a cross-sectional view of a back-contacting solar cell with a two-level metallization backplane attached including a backplane layer having fiber and resin content. 4 is a cross-sectional view of a back-contacting solar cell with a two-level metallized double-level backplane. The disclosed subject matter relates to a back-contacting solar cell with a prepreg backplane supported and to fabrication thereof, and in U. S. Patent Application Serial No. 13 / 433,280, filed March 28, 2012 No. 2009/0000715), 13 / 807,631 filed on December 28, 2012 (PCT International Publication No. 2013022479, published February 14, 2013), filed on April 2, 2013 No. 13 / 822,657 (U.S. Patent No. 2013/0213469, published Aug. 22, 2013), and No. 13 / 869,928, filed Apr. 24, 2013 2013/0228221), which is incorporated herein by reference.

In addition, the backplane structure as a selectively and thermally and mechanically separated structure from the solar cell substrate can place power semiconductor electronics components per solar cell for various battery level applications, for example, Is a rectifier switch for distributed shade management and maximum output point tracking (MPPT) power optimization devices for extracting maximum power from solar cells.

According to the disclosed subject matter, the backplane is a resin-impregnated fiber substrate directly laminated to the back side of a thin silicon wafer or solar cell absorber (generally from about 25 microns to about 250 microns thick, among other considerations, About 75 microns to 150 microns). Thus, it provides a simple, durable, reliable and cost effective solar cell design. 1 and 2 are sectional views of a back-contacting solar cell to which a backplane is attached.

The following design considerations have been established for the manufacture and evaluation of backplanes and are provided as detailed design requirements and guidelines for backplane material selection and testing. The major backplane embodiments provided herein meet key design objectives for end product certification and product commercialization. Suitable material ranges for backplanes to support thin solar cell absorber materials (e.g., ranging from a few microns to 250 microns or less in thickness) can be used to ensure compatibility with final solar cell applications and solar cell / module manufacturing processes Can be defined by a set of requirements (essentially optional requirements). The boundary conditions help define the material selection process, the material developmet process, and the technology evaluation process. The high level criteria for backplane material and performance may include, but are not limited to, combinations of critical and selective criteria:

A bonding system for attaching to a wide variety of different materials used in solar cell substrates and a passivation, metallization, and module encapsulation material (e.g., silicon, silicon oxide, aluminum, aluminum oxide, aluminum / silicon fired paste, Aluminum foil, tin), and any other encapsulating material used in module capsule materials (e.g., ethylene vinyl acetate (EVA) or polyolefin) or any module lamination. Adhesion reduction is improved, and the risk of tearing of the backplane after the laminating process is prevented.

- All component backplane materials must be suitable for a variety of environments (depending on the solar cell process flow) in the solar cell manufacturing flow. Solar cell process environments include, for example, high vacuum, rapid heat rise, plasma etching, thermal annealing (e.g., below about 300 ° C), wet processing Wet silicon etching, texturing, cleaning, and / or metal plating when used after lamination). For example, in some instances, the solar cell process flow may not require wet processing after lamination, and certain environmental compatibility requirements may not apply to a particular process flow.

- All constituent backplane materials must support relatively high processing temperatures of about 250 ° C to 300 ° C. In some cases, the backplane should be designed so that the resin is not pyrolyzed at a processing temperature of at least 350 캜.

- All constituent backplane materials must have reasonable or sufficient thermal conductivity to facilitate heat sinking of energy degraded from the solar cell absorbing layer.

- All constituent backplane materials can be subjected to laser drilling (e.g., laser drilling of the via holes through stacked backplane sheets for M2-M1 conductive via plugs formed by patterned M2 formation, for example, To be used with laser cutting and drilling as required.

The entire backplane system must have a CTE that matches or is comparably close to the CTE of the semiconductor substrate and the semiconductor substrate is crystalline silicon, for example, a crystalline silicon solar cell having a CTE of about 2.6 to 2.8 ppm / 占 폚. That is, the average CTE through the backplane thickness or the expansion control layer of the backplane should be relatively matched to the CTE of the semiconductor substrate. Also, the CTE matching in the case of two levels of metallized portions is particularly important for via protection.

- All constituent backplane materials should be non-reactive with wet chemical materials as needed for solar cell processing. Wet chemicals include, for example, acids, bases, solvents and oxidizing agents. In addition, depending on the required or selected solar cell processing method, this requirement can be achieved without using wet processing after lamination in a cell manufacturing process flow embodiment (e.g., all wet processing must be performed prior to solar cell backplane lamination) , Can not be applied or can be mitigated.

- The entire backplane system must contain a resin flow, since the contained resin flow can prevent a long range of resin flow.

The entire backplane system should provide a relatively flat battery with less than ± 3 mm center-to-edge substrate bend / deflection during and after backplane lamination. Small cell bending facilitates solar cell fabrication after lamination to fabricate solar cells and subsequently deposit solar cells attached to the backplane in a solar module.

The entire backplane system is designed to have a relative size (e.g., from about 20 캜 to about 250 캜) and a temperature reduction (from about 250 캜 to about 20 캜), such that relative planarity or planarity changes little or no change Should be stable.

- The entire backplane system should have a dielectric or relative electrical insulation and serve as an efficient electrical insulation layer between the solar cell metallization layers (eg, to enable a two-level solar cell metallization structure with M1-M2 stacks for).

The entire backplane system must be at least partially transparent in the IR spectrum during laser processing (e.g., laser cutting)

- The entire backplane system should have improved battery and module aesthetic design. Particularly important considerations for application may be advantageous to the aesthetic appearance of solar cells and modules such as residence or automatic roof tops and portable / movable solar power applications.

- The entire backplane system must meet the field life requirements of solar modules and generally meet the field life of solar panels within the field for at least 25 years.

- The entire backplane system is cost effective, for example, including backplane sheet material should be less than about $ 0.04 per watt.

The provided backplane solution uses a resin system as an adhesive / binder in a backplane material in which a battery structure is connected together after stacking backplane sheets on a solar cell substrate. Available resin systems include, for example, epoxies, polyimides and all commercially available mixed resins, each having properties that meet the requisite and / or optional criteria for use in the application of the disclosed subject matter. Although the resin is used together with the supporting fibrous substrate described herein, only the resin can be used for the gap filling uniquely, that is, the resin is used as the gap filler.

A high efficiency back contact / back joining (also known as inter-fitted back contact or IBC) solar cell with a thin crystalline silicon absorber using a backplane can be fabricated with an epoxy and mixed resin system and mixed with an appropriate fiber substrate , It can be used for a feasible, cost-effective backplane solution, and each material system takes into account certain advantages and disadvantages.

Epoxy-based resin systems are common in the printed circuit board (PCB) industry. Epoxy-based resin systems have proven to be a cost-effective and flexible solution for PCB applications and can be used efficiently in the manufacture of flexible, high-efficiency solar cells. Epoxies are typically used to provide excellent adhesion to other solar cell layers, wide stacking process tolerances to lamination temperatures and pressures, relatively good chemical resistance, good hole drilling uniformity, and the ability to withstand localized heating in conventional soldering processes There is an advantage to provide. However, the epoxy drawback that can limit the ability of the epoxy to achieve all design goals for a particular solar cell application is the considerable resin flow during deposition (with a resin side outlet flow scale much larger than the thickness of the backplane) High temperature resistance, a relatively high coefficient of thermal expansion (much higher than the CTE of silicon) of 35 to 45 ppm / 占 폚, continuous curing (crosslinking) of the resin after lamination, high moisture absorption, and adhesion to some solar cell layer surfaces The epoxy resin may need to mechanically roughen the surface. Nonetheless, solar cells fabricated using epoxy resin backplane systems have proven to meet all reliability requirements in the final solar cell application.

Mixed resin systems using high molecular weight resins and thermosets, such as thermoplastics (e.g., organic or synthetic thermoplastic rubbers), are used in the PCB industry, but are less common. Mixed resins are often custom made for use in certain applications and can provide benefits for epoxy resins for application in certain flexible high efficiency solar cells. The mixed resin system provided herein is a mixed resin system that can be a "non-flowing" material and has a significantly extended temperature operating window, for example, high molecular weight resins and thermosetting properties. Such a mixed resin system can be used for re-curing during reheating, for limited resin flow during lamination (hence limited side-plane outflow of resin), high thermal budget capability, low temperature absorption, Metal-one layer, such as a metallized portion), CTE lower than epoxy resin, additional resin bridging limited at elevated temperatures, and good chemical resistance. For epoxy resins, a non-flowing (or less flowable) resin may require an adhesion promoter to achieve adhesion to the silicon oxide (which may cause chemical compatibility limitations by the addition of an adhesion promoter) Low laser via hole drilling uniformity, and a narrow laminating process window for temperature and pressure. However, a further advantage of mixed resin systems using thermoplastic materials in some solar cell backplane applications is that the thermoplastics of the material heat and re-mold the laminate sufficiently cured to obtain a particular curved target in the final part. High efficiency solar cells made of mixed resin systems (for example, resins mixed with thermoplastic materials such as thermoplastic rubber) are fabricated and assembled in a module structure and have positive results. In addition, the mixed resin system is the main pathway of high efficiency solar cells used and mass produced in the main embodiments provided herein.

The use of the resin alone is not a viable solution for a solar cell structure support (for example, a support for a thin silicon solar cell) in manufacturing, but in reality a resin material is impregnated into the fiber substrate and a combination of resin and fiber is used To achieve the desired final backplane material properties, which is referred to as a prepreg (the standard PCB industry term for a partially cured resin impregnated fiber substrate is a prepreg). The primary silicon solar cell embodiments provided herein employ a prepreg backplane having a CTE value that is equal to or comparable to the CTE of the silicon substrate.

In general, prepregs are pre-impregnated reinforcing materials with resin and are ready for use to create a composite part (a wet lay up system where a prepreg can be used to produce composites faster and easier have). Prepreg can be fabricated using equipment designed to ensure consistency and combining reinforced fibers or fabric with specially formulated pre-promoted resins. The prepregs covered with flexible backing paper can be handled easily and can be bent during a certain period of time (out-life) at room temperature. Further, with the development of the prepreg, a material that does not require refrigeration during storage, a long-life prepreg, and a product that is cured at low temperature were obtained. The prepreg laminate can be cured by heating under pressure. Conventional prepregs are formulated for autoclave curing, but the low temperature prepreg can be sufficiently cured using vacuum bag pressures at much lower temperatures. The prepreg sheet can be used as a building block of a printed circuit board and can be made from a combination of resin and CTE reducing fibers or particles. The prepreg backplane material is inexpensive and can be used as a solar cell substrate in a thin (e.g., 50 to 250 microns, specifically CTE < 10 ppm / About 50 to 150 microns) prepreg sheet that is chemically resistant to the texturing chemicals and is thermally stable at least 180 degrees Celsius (or at least 280 degrees Celsius).

The fibrous substrate is used to achieve the general mechanical properties of the prepreg material. The fibers determine many properties of the laminate, which includes cured sheet thickness, in-plane (X and Y axis) CTE, mechanical strength, and dielectric properties. The fibrous substrate is made of roll-stock, which can be manufactured in high volume using continuous rollers and dip coating processes. Choosing the type of fiber used in the substrate is important to achieve the final properties of the final prepreg laminate.

The aramid fiber substrate has a niche in the PCB industry in applications requiring low CTE and high total assembly reliability / lifetime under stressed environmental conditions. The backplane layer provided herein is a unique and novel (not surprising) example of aramid fibers and resins with a relatively low obtained prepreg CTE (to match or closely match that of the solar cell absorber layer) as the backplane sheet material during permanent thermal lamination to the solar cell Combinations are applied. The aramid fiber has a unique property that the fiber has a positive CTE in the radial direction and a negative CTE in the axial direction (-4.5 ppm / DEG C). Substantially, the fibers decrease in length with increasing temperature. Thus, prepreg systems, including fibers and resins, can be finely tuned to achieve specific CTE targets that are arranged according to requirements for final application (e.g., as a backplane in a crystalline semiconductor solar cell). The aramid is further advantageous in that it is compatible with CO ₂ laser drilling and cutting, which is an important requirement in backplane-attached backplane solar cells with multistage metallized portions such as the M1 / M2 metallized sub- This can be. In general, aramid fiber substrates can be used as woven and nonwoven (paper) substrates.

The woven aramid fiber substrate is made of fiber bundle woven with a fabric having exactly 90 degrees between bundles. This orientation provides the advantage of balancing the in-plane CTE in the X and Y directions and also provides excellent mechanical strength. However, the woven aramid may have a number of significant limitations in solar cell applications attached to the backplane, which may result in relatively high material costs, potential future supply limitations (materials may be classified and allocated as strategic materials by the US government Inconsistent laser hole drilling by various fiber densities through weaves, and high resin volume requirements to fill gaps and voids between fiber bundles. 5 is a photograph of a woven aramid fiber substrate 20 times magnified.

The nonwoven aramid fiber substrate is composed of short fibers pressed into a paper-like material in the direction of random fibers in the X-Y plane. 6 is a photograph of a 20-fold enlarged nonwoven fabric aramid fiber substrate. The nonwoven aramid fibers also exhibit negative CTE in the X and Y directions, but due to the random orientation of the fibers the shrinkage is somewhat less than that of the woven material. The mechanical strength may be less than the woven material but is sufficiently high compared to solar cells attached to a backplane such as the embodiment provided herein. All the above-mentioned limitations described in woven aramid materials can be resolved when using a nonwoven construction. However, a unique limitation to nonwoven materials includes the maximum sheet thickness limit to 6 mils, and tighter process uniformity requirements during prepreg manufacture. Significantly, such limitations may have no or minimal impact on solar cell backplane applications where thin backplanes (e.g., ranging from 2 to 6 mils or thicknesses from about 50 microns to 150 microns) are often provided as desirable.

In addition, expensive experiments using prepregs made of nonwoven aramid fibers indicate that they can be matched directly with the CTE of silicon when mated with compatible resin materials. A completed back contact silicon solar cell using a backplane comprising nonwoven aramid fibers and mixed resin prepregs indicates that it meets solar cell product reliability requirements and the use of a nonwoven aramid fiber substrate is a high efficiency solar cell attached to a backplane Is considered to be a major fiber prepreg option for mass production and commercialization.

In yet another embodiment, carbon fibers may be used. Carbon fiber substrates are widely used in aerospace and top end consumer products requiring high strength, high rigidity, high temperature durability and low weight, but carbon fiber substrates are generally not used in the PCB industry. Carbon fiber substrates are only available in woven configurations and can be very costly for very expensive and cost-effective solar cell applications. Carbon fibers, like aramid, can have a very low CTE (typically 1-2 ppm / 占 폚) and provide a low CTE laminate with a CTE comparable to a semiconductor absorbent such as silicon when paired with a commercially available resin system. However, it may not be possible to obtain an exact match with the CTE of silicon due to the amount of resin volume required to fill the voids and gaps in the woven carbon fiber substrate. A further consideration of carbon fibers may be the limiting factor in the maximum efficiency achieved with backplane laminated solar cell designs due to the lack of electrical conductivity or complete electrical insulation, i.e. the electrical conductivity of the carbon fibers is not high.

In some instances, the carbon fiber backplane substrate is not considered a good fit for a particular low cost, high efficiency solar cell due to the limitations described, especially from a cost standpoint. However, carbon fiber backplane materials are useful and applicable in backplane-attached solar cells where a very thin (e.g., about 20 to 100 micron thick) backplane with very high stiffness is desirable.

In another embodiment, a glass fiber substrate may be used. Glass fiber substrates are the most commonly used material for prepregs in the PCB industry. Fiberglass offers attractive cost-competitive solutions in most applications and can be combined with many resin systems. However, limitations of glass fiber prepregs in solar cell applications attached to high efficiency backplanes include relatively high CTE (especially for silicon) in the range of about 5-7 ppm / 占 폚, and (if necessary) Includes difficulties of sexuality. With these limitations, glass fiber substrates may be less desirable for applications as backplanes in high efficiency solar cell structures according to additional considerations. However, in one embodiment, a prepreg produced by glass fibers, when an additional layer of different material is interposed between the glass fiber prepreg and the silicon, to eliminate the relatively mismatched effects with the CTE, As shown in FIG.

As described above, prepregs are fiber substrates that are saturated with partially "cured" resin systems during coating operations. In practice, the fiber substrate is coated with a resin on a coater. Specifically, the material is passed through a fan filled with a resin solution in a solvent and then passed through a series of "metering bars" to precisely control the amount of resin deposited on the fibers. The resin-saturated fibers are passed through a series of temperature-controlled oven zones, then the solvent is removed and the resin is partially reacted (or "B-step").

As described herein, a prepreg that matches (or nearly matches the CTE of) the silicon's CTE supports a thin crystalline semiconductor solar cell, such as a back contact / back junction solar cell, using a thin crystalline silicon absorber layer. Lt; RTI ID = 0.0 &gt; backplane &lt; / RTI &gt; In addition, pre-matched with the CTE of silicon allows a thin silicon semiconductor layer and a mechanically enhanced flexible solar cell and forms a two-level metallized portion in a back-contacting solar cell interconnect design. Since the prepreg is only partially cured, it can be permanently laminated directly to the backside of the back-contacting solar cell and can be sufficiently cured in situ (e.g., in a vacuum heat lamination apparatus). Thus, the prepreg resin forms a permanent bond with the solar cell backside surface (which also covers the uncovered portion of the solar cell substrate also includes the patterned metal-1 or M1 contact metallization portion), additional adhesive is needed The cost is reduced and the reliability is improved. Since the maximum prepreg curing temperature is less than 250 占 폚, the solar cell wafer is not affected by the temperature reached during the prepreg curing. Lamination pressure can be used to "flow" the resin during lamination cure, but a suitable laminate fixing design reduces the risk of wafer breakage due to pressure.

A number of important control parameters must be considered during prepreg fabrication that affects the backplane lamination quality. The most important guideline is to obtain a very specific ratio of the amount of resin to the amount of resin, referred to as resin content (RC), and is generally expressed as a percentage. The range in which the RC value can be achieved in the prepreg is determined by a plurality of factors; This includes the shape of the resin, the shape of the fiber substrate, reduced defects for "dry fibers " on the low RC side, and reduced defects for bubbles / ripples on the high RC side. A crystalline silicon solar cell with a backplane using a thin (50-200 [mu] m) prepreg sheet having an RC value of about 20% to 65% was fabricated. RC must be relatively uniform over the width of the prepreg sheet (typically 50 inches or less) and the length of the prepreg roll (hundreds of yards) of the roll to obtain the consistency required for backplane-attached back-contact solar cell applications . Significantly, the prepreg backplane for the back-contact silicon solar cell described herein is quite different from the unique and conventional PCB industry prepregs. For example, the requirements for prepreg defects for solar cell backplane applications are less stringent than in the PCB industry, resulting in reduced manufacturing costs and reduced prepreg material costs. However, in order to obtain the required CTE target, the RC uniformity should be maintained at a tolerance of less than ± 5%, preferably greater than ± 1%, in the preparation of the prepreg.

The second important factor in the prepreg for making backplane supported solar cells is the uniformity of the tension on the fiber substrate web as it is drawn through the coating apparatus during the prepreg roll manufacturing process. Uneven tension causes harmful distortions and distortion of the prepreg material consistency. For example, materials that contain excessive warpage will cause unacceptable axial deflections in the stacked solar cell portions. The material with warpage will be twisted at the laminated part. Warping and twisting are detrimental to achieving the planar portion. Thus, for backplane applications in solar cells attached to backplanes (including, but not limited to, back-contact solar cells), prepreg roll-type materials are manufactured to have an RC in the range of 20% to 65%, and non- And there is no excessive bending and distortion.

A third important factor with regard to prepreg rolls for making backplane-supported solar cells is the production of fiber substrates prior to processing through coating lines. The aramid material may be sensitive to moisture absorption that is detrimental to the coating and B-step curing process. In practice, the substrate must be maintained under certain low humidity conditions prior to resin coating. This same problem is applied to the prepreg sheet before lamination on the rear side of the solar cell. Improperly stored prepregs can cause separation defects after lamination if the aramid fibers absorb environmental moisture prior to lamination. Thus, for solar cells (including, but not limited to, rear-facing solar cells) attached to the backplane, the prepreg rolls must be fabricated under certain low humidity conditions prior to resin coating of the fiber substrate, It should be kept relatively dry before the laminating process.

The ratio of resin to fiber (or resin content RC) is an important factor in certain applications, especially in prepreg customization for solar cells attached to backplanes. In conventional PCB applications, the RC value can be selected to balance a) the total resin volume for gap filling, and b) the CTE value of the final stack. RC values of 50% to 75% are common for prepregs used in PCB manufacture. At this level, it is enough resin to fill all the gaps and voids in the board structure. Excess resin is pressed out of the board assembly during lamination and removed during board trimming. However, the typical CTE value at this high RC value for the 7 to 9 ppm / C epoxy / woven aramid prepreg range is too high for the solar cell structure attached to the backplane. For example, a backplane with a value of RC 51% may cause solar cell warping outside the tolerance range.

For the epoxy / woven aramid prepreg, a 5% reduction in RC is consistent with a reduction in CTE of about 1 ppm / 占 폚. Thus, an RC value of about 25% for an epoxy / woven aramid prepreg is needed to achieve a near CTE value for silicon, and a backside topography (e.g., by spray coating or screen printing coating before the laminating process) This value is too small to be considered a resin insufficient to fill gaps and voids on the metallized rear side of the solar cell &lt; RTI ID = 0.0 &gt;

As described above, the backplane structures provided herein focus on prepregs that use a "non-flow" (or less flow or less flow) resin system, preferably bonded to a nonwoven aramid fiber substrate. With this design, the CTEs of the cured prepreg and silicon are matched relatively closely and a flexible solar cell attached to a relatively flat stacked backplane is obtained. A narrow range of about 30% and 40%, specifically about 33% of target RC values, has been established as a requirement for the CTE of the prepreg and silicon to be relatively matched. According to experimental testing and fabrication, RC values of less than or equal to about 32% were found to be able to obtain negative partial warpage, and RC values of greater than or equal to about 34% could achieve positive partial warpage. In some examples, the optimum range of RC for the aramid fiber prepreg material used as the backplane of the silicon solar cell with backplane is in the range of about 32% to 34%, and may be in a wide range (e.g., 30 &Lt; / RTI &gt; to 45%). 7 is a graph showing the thermal expansion coefficient or CTE versus resin content for the aramid fiber prepreg material. It is noted that the cured 33% RC prepreg has a CTE which essentially or closely matches the CTE of silicon.

The specific RC requirements for a close match between the CTE of the cured prepreg and silicon are determined, and additional resin content considerations can be applied according to gap fill requirements. A solar cell attached to a backplane having an aramid fiber prepreg backplane can be used when a minimum gap filling is required on the rear side of the solar cell and when a significant gap filling is required on the rear side of the solar cell.

For example, if a minimum gap fill is desired, for example, a relatively thin layer of the patterned metallization layer (Metal-1 or M1) is formed on the back side of the solar cell prior to lamination, and / The total volume of resin in the prepreg may be used to fill the entire structure and fill any voids in the prepreg fiber substrate if it is substantially planarized by a layer of spray coating dielectric for surface planarization or by screen printing application . The target 33% RC for prepregs in mixed resin and nonwoven aramid fiber substrates proved to fill design requirements with 1ply and 2ply prepreg structures. Solar cells attached to the backplanes provided herein may use single ply or double ply or multiple ply prepreg structures (where each ply may have a different RC value for multiple ply such as a 2 ply backplane). Analysis of the cross-sectional area from a complete backplane laminated solar cell shows a uniform, void-free and relatively flat backplane laminated solar cell structure. In addition, the prepreg laminating process window for temperature, pressure and curing time is very wide and is flexible in the selection of production stacking equipment for solar cell backplane stacking.

If a significant gap fill is required, for example in the case of considerable backside surface topography by thick patterned M1, the total volume of resin is between the resin "leached" in the gap on the solar cell backside topography and the resin in the fiber substrate Balance must be achieved. According to experiments of solar cell backplane lamination having a prepreg consisting of a mixed resin and a nonwoven aramid fiber substrate, the prepreg structure consisting of 2 plits of the prepreg each had RC values of ply different from each other, Which can be used to manufacture solar cells. Based on an embodiment of the present invention, a prepreg ply in contact with a region requiring gap filling requires a sufficient resin volume to fill any gaps in the solar cell rear structure and fill any voids in the fiber substrate . For example, an RC value of 45% to about 50% for the gap fill ply is needed to laminate the high aspect ratio M1 contact metallized portion and the screen printed back contact cell structure. In order for the CTEs of the cured prepreg and silicon to be relatively matched, the high RC ply must be laminated together with the low RC ply such that the total stacked structure is about 33% (or the entire range of 31% to 35%) of the final RC. This forms a 2nd ply with an RC value of about 16% to about 21%. The prepreg second layer should contain sufficient resin to "wet" the first substrate to remove voids and encapsulate the individual aramid fibers and to bond the reliable continuous resin to the first ply. Analysis of the cross section from a solar cell attached to a complete backplane indicates that a uniform, void-free solar cell structure is achieved using this 2 ply backplane lamination configuration. However, the lamination process window for temperature, pressure and cure time is significantly narrower than this embodiment, and flexibility in selection of the product stacking device for application is reduced.

In addition, a single ply of the fibrous substrate can be made to have an asymmetric or graded resin coating over the thickness from the prepreg side to one side for considerable gap fill applications. The side of the prepreg in contact with the cell structure should have sufficient resin (high resin content or resin RC) to fill any gaps, but the opposite side of the prepreg should be filled with a small amount of resin sufficient to ensure that the fibers are fully encapsulated Content or low RC). Coating with this asymmetric resin with RC graded over the prepreg thickness produces a material having about 33% of the average (volume average) RC and requires the same relatively narrow processing window as the 2ply pass described above.

Importantly, the gap filling can be accomplished by pushing the resin from the prepreg substrate into the gap using a high pressure lamination during lamination and allowing the backplane to be thermally cured with the on-cell metallization, the aramid fibers and the mixed resin Having a 100% resin content portion flowing between the second prepreg portion containing the resin The second prepreg portion has a resin content in the range of 28%, so that the total resin content for the backplane is in balance with, for example, 39% of the resin content. Thus, the second prepreg portion acts as an expansion control layer that cooperates with a semiconductor substrate, for example, a silicon semiconductor substrate. This backplane structure is shown schematically in the backplane supported solar cell cross section of FIGS. 2 and 4.

At the widest level, the backplane used in backplane stacked solar cells is the structural support of a thin silicon absorber layer of a solar cell attached to the backplane. If the backplane meets certain dielectric or electrical isolation requirements, the electrical function of the cell does not depend on the specific thickness of the backplane. Thus, the choice of the target backplane thickness can be performed primarily in accordance with the goals of solar cell mechanical durability and backplane attachment product cost. The final prepreg backplane thickness of the solar cell attached to the backplane may range from about 2 mils (or about 50 microns) to about 10 mil (or about 250 microns), and in some instances, the preferred prepreg thickness is about In the range of 3 mils to 8 mils (or in the range of about 75 microns to 200 microns).

A solar cell attached to a flexible (or bendable) backplane may be a prepreg having a total thickness of about 8 mils (or about 200 microns) (e.g., one prepreg ply having a thickness of 8 mils, Two prepregs ply), and prepregs having a total thickness of 4 mils (or about 100 microns) (e.g., 1 ply at 4 mils thickness).

In addition, the solar cell manufacturing process steps can be optimized for prepreg thickness. For the 8 mil and 4 mil prepreg ply, solar cells having an 8 mil thickness backplane from a mechanical durability standpoint are semi-rigid and may be manually passaged stepwise to minimize the risk of cell damage according to appropriate handling protocols . Solar cells with a 4 mil thick backplane are relatively flexible and flexible, and strict handling protocols may be required to prevent damage to the cell during processing, and thus the 4 mil design may be more suitable for automated handling and delivery. Another consideration is the cost of materials, thinner backplanes may be desirable to reduce material costs. For the 8 mil and 4 mil prepreg ply, the backplane design can use a 4 mil solution to reduce raw material costs.

For backplane attachment, the curing properties of the resin system are important for determining the optimal processing conditions for temperature rise and lamination pressure. Figure 8 is a rheological graph that schematically illustrates the rheology profile of a mixed resin system. The rheology curve describes the specific properties of the resin system since the material proceeds from the B step through the resin melt to the resin through the resin gelatinization and finally to complete curing. The epoxy resin system is well understood and certain conditions can be optimized for specific processes to optimize uniform lamination thickness by gap fill, void removal, and solar cell laminating processes. Mixed resin systems require a rheological curve to perform complex curing cycles and to optimize the lamination process.

8, the curved line (n * circle) shows the viscosity change of the resin to the temperature. The minimum viscosity in this system reaches 175 캜. The crossover of the G 'and G &quot; lines is generally referred to as the resin system (G') and the shear modulus (G ' The crossover point is not well defined but shows that the gelation of the resin starts at a temperature lower than the minimum viscosity. The lamination process is developed using a fast heating rate of temperature to quickly reach the minimum viscosity and the resin can flow into the gap and voids before the gelling process is sufficient to prevent resin flow.

The glass transition temperature is that the cured resin transitions from a hard, fragile state to a soft, rubber-like state. These parameters are important considerations in the PCB industry, but are less important for solar cells attached to the backplane. When the Tg temperature is reached, most abrupt changes are three to four times greater than the CTE below the Tg point. The increase in CTE is not an important consideration in the X-Y plane due to the suppression effect of the fiber substrate, but has a significant effect on the Z-axis. In conventional PCB applications, the increased Z-axis CTE did not electrically connect to the plated through vias. Due to the Z axis expansion of the prepreg, the stress on the plating is sufficient to break the connection.

However, this failure mode can be prevented in a solar cell attached to a backplane. For example, the maximum processing temperature after deposition (and thus after M2 metallization) is less than about 300 占 폚, although it is a relatively high temperature step (e.g., PECVD passivation deposition at temperatures below about 300 占 폚) Is limited. In addition, the solar cell attached to the completed backplane may be exposed to temperatures close to the Tg point during lamination within the module assembly. The module lamination can expose the cell to about 130 &lt; 0 &gt; C or less, for example, to heat the thermosetting resin in the module structure. However, this relatively low heating event is less prone to failure of metallized structures (e.g., M2 structures).

The use of additives in resin systems in the PCB industry is common. For example, common additives include flame retardants, adhesion promoters, crosslinking inhibitors / accelerators, pigments, fillers to achieve specific CTE, electrical or thermal conductivity requirements, UV stabilizers, and others. The resin system is controlled by additives for a particular application. The solar cell backplane requirements are unparalleled in the application of common prepregs with adjustable additives.

For example, the use of a nonwoven aramid fiber substrate prevents the use of fillers in the resin system. The substrate fiber density is high enough to act as a filler and prevent the filler from being evenly dispersed into the prepreg structure. The properties of such a fiber substrate limit the options of using additives to match the CTE of the backplane and the solar cell substrate (e.g., silicon), and also limit the ability to regulate the thermal conductivity of the backplane. Fillers are not required in solar cells attached to the backplane provided herein.

Second, as a final product, solar cells are sufficiently encapsulated in a module encapsulant material (e.g., EVA, polyolefin, or other suitable encapsulating material), hermetically sealed in the solar cell area, and also blocking most unwanted UV light . Such a capsule material eliminates the need to include UV stabilizers and flame retardants in the prepreg. No flame retardant or stabilizer is used in the solar cell attached to the backplane provided herein.

Third, the adhesion of the resin is highly dependent on the polarity of the resin and its ability to form molecular level bonds with other materials. Epoxy-type resins are highly polar and form strong bonds. Backplanes using epoxy resin systems have been very successful in many generations of solar cell designs attached to backplanes. When a mixed resin system is used, a problem arises in connection with adhesion of silicon and silicon oxide. Thus, a silane-based adhesion promoter can optionally be added to the resin system to address adhesion problems that are dependent on additional considerations and structural design. The silane coupling agent contains inorganic reactive groups on the silicon and binds most inorganic substrates, especially when it contains silicon or aluminum, as in the case of the backside surface of a back contact solar cell embodiment in which the substrate is provided. 9 is a diagram showing diffusion of an additive in a resin and coupling of silane to a silicon oxide. The bonding problem was solved when silane was used. Thus, in a solar cell embodiment attached to one backplane, a silane adhesion promoter is added to the prepreg in contact with silicon oxide on the back surface of the metallized solar cell.

Fourth, final product solar cells may require specific aesthetics. A pigment may be added to the resin system to soften the natural color of the aramid fiber substrate. For example, the requirements for pigments include transparency in the infrared (IR) spectrum, compatibility with CO ₂ laser cutting and via drilling, and compatibility with all chemicals used in the solar cell manufacturing process.

Thus, although mixed resin systems can be controlled by the addition of silane adhesion promoters and pigments, other additives are not required to meet the solar cell constructs attached to the backplanes disclosed herein and the functional requirements of the application embodiments.

The resin can be used as an independent component to fill the gaps in the back metal structure of the solar cell attached to the backplane. This strategy can be used to form a rear surface contact solar cell surface topography with a high aspect ratio due to the very thick (e.g., between about 20 microns and 50 microns) height of the M1 or contact metallized metal structure on the back side, It is most useful when it is not preferable to use the resin in the prepreg. Indeed, a wide range of materials can be used with very low modulus thermosetting resins that can be applied by using an effective gap filler, e.g., various forms of UV curable resin / adhesive using screen printing or stencil techniques directly on the solar cell. CTE matching of the gap fill material is not a major consideration since the sheet of low CTE prepreg is used as the primary structural support of the solar cell attached to the backplane. The CTE of the prepreg is such that the two layers (prepreg and silicon solar cell substrate) act as an inhibiting layer for the gap fill material, that is, the prepreg sheet and the solar cell substrate are the gap filling material and the on- To act as an inflation control layer interposed between the layers. Considering cost, it is also a viable option for the market to use gap fillers and prepregs of single-layer sheets.

Importantly, the same resin system used in the prepreg in the main embodiment can be used as a gap filler. This requires optimization of the lamination process to push the resin out of the resin from the prepreg in order to flow within the on-cell metallization portion. This backplane structure is schematically illustrated in the backplane supported solar cell cross-section of Figs. 2 and 4.

The laminating process can be used to bond the prepreg to the solar cell. The laminate is heated and pressed to complete the curing of the resin in the prepreg sheet bonded with the cell. 10A, 10B, and 10C are graphs showing the lamination process profile of the mixed resin system. FIG. 10A is a graph showing the temperature profile, FIG. 10B is a graph showing the pressure profile (note the temperature decrease during the lamination process), and FIG. 10C is a graph showing the vacuum condition. The following describes key considerations for the selection of process conditions to ensure acceptable lamination results.

The maximum temperature used in the cured prepreg depends on three major factors that reach the target cure level of the resin, establish the thermal history of the prepreg structure, and achieve an efficient process cycle time.

For epoxy or mixed resin systems, the minimum cure temperature (e.g., the minimum cure temperature recommended by most manufacturers) is often in the range of 180 ° C to 190 ° C for 90 minutes. For epoxy resins, a curing temperature of up to about 220 캜 can be used without damaging the prepreg. Mixed resin systems can have high durability at high temperatures and stacking can be performed to temperatures below 275 ° C. At higher temperatures, the lamination should be conducted in a vacuum or inert gas environment to prevent the surface of the prepreg resin from becoming dark by oxidation.

The step of establishing a "thermal history" in the cured prepreg is an important consideration in solar cells attached to the backplane. During resin curing, the stress conditions in the prepreg structure are set based on the reached thermal profile and the maximum temperature. The internal stress may be different for the laminated portion at 200 ° C versus the laminated portion at 250 ° C. Since the solar cell is exposed to an additional high temperature environment after lamination, it may be necessary to expose this part to a high target temperature during lamination. The general rule of thumb is to laminate this part at a temperature 25 ° C above the maximum temperature to be exposed after lamination.

A particular advantage of laminating at high temperatures is that the resin is completely cured. The amount of thermal crosslinking of the resin is a function of time and temperature. By processing at higher lamination temperatures, this portion will be less cured after lamination when exposed to elevated temperatures during fabrication.

The current lamination process cycle time can be set from 110 to 120 minutes until the load is removed from the lamination pressure load. This cycle period is designed to cure the resin sufficiently at a target temperature of 250 占 폚 (110 min cycle) to 275 占 폚 (120 min cycle). Cure time optimization is very beneficial in the process engineering that is currently being done.

The heating up and cooling down rates vary widely in the PCB industry. In general, the heating rate is adjusted for a particular resin system in order to balance the molten resin flow to the initiation of the resin gel. The cooling rate is used to manage residual stress buildup in the multi-layer PCB due to CTE mismatching of the layers. In general, a heating temperature rise of 3 ° C to 7 ° C / min and a cooling down temperature of 3 ° C to 5 ° C / min are recommended. In a solar cell attached to a backplane using a mixed resin, an aggressive heat up ramp may be desirable to reach the maximum resin flow before the gel develops. Excellent results were obtained at a high heating rate of about 10 캜 / min. With regard to cooling, CTE matching of the prepreg and silicon reduces the problems associated with the accumulation of residual stresses during cooling. Thus, in some instances, a rapid cooling down of 8 [deg.] C / min (limited by process equipment) may be used.

The pressure applied during lamination serves to distribute the resin within the structure for gap fill and void reduction. Operating at very low pressures results in a void structure in the prepreg structure and a final structure with low adhesion between the layers. Operating at very high pressures can result in a uniform structure, but potentially destroys / warps the fiber substrate, increases residual stresses in the final structure, and can destroy the wafer. In determining the pressure used in the stack, in solar cells attached to the backplane, the three critical parameters, minimum pressure, maximum pressure, and profiled pressure during stacking must be balanced.

The first important minimum pressure parameter is the elimination of gaps and voids. Gaps and voids deteriorate the structural integrity of the assembly and yield losses in the manufacturing process. Determining the minimum pressure required requires finding the pressure required to prevent voids in the prepreg material and it is necessary to find the pressure required to flow the molten resin within the gap within the laminate.

The helium leak test method was developed to find the minimum pressure to ensure voids were blocked in the prepreg. In practice, a single ply, large area sample (1.5 times the surface area of the solar cell) of the prepreg material is cured under certain pressure and temperature conditions. The cured sample is then analyzed for voids using a pressurized helium leak test. When voids are present, helium leaks through the cured sheet and is detected. The runoff determines the severity of voids. The acceptance criterion is zero voids. 11 is a graph showing the mixed resin system and a series of test results performed on the sample. This particular investigation indicated that a minimum lamination pressure of 80 psi is required to ensure voids are not present in the cured laminates.

The second important minimum pressure parameter is the resin flow in the cell structure. The back contact solar cell design uses a metal structure (e.g., M1) on the backside surface of the passivated silicon cell to electrically contact and collect current. These metal structures (e. G., Interfitting M1 metallization fingers) are at a height of about 1 micron to about 50 microns above the passivated silicon back surface. Since the prepreg contacts only on the metal structure, the prepreg at the start of lamination hangs away from the cell surface. The pressure applied during the process should ensure that the resin flows into the gap and the void-free final structure. For example, a pressure of about 100 psi may be required in a gap filled with a metal structure in a height range of less than 10 microns, and a pressure of about 200 psi filled in a thick M1 metal structure (e.g., within a 50 micron height range) You may need it.

The maximum pressure limit can be determined by reaching the damage threshold of various parts of the solar cell structure. For example, a) a thin semiconductor (e.g., silicon) layer coupled to a backplane, b) a wafer carrier used to support a thin layer prior to backplane deposition (e.g., for a thin epitaxially grown silicon semiconductor layer Silicon template or silicon wafer), or c) damage to the solar cell metal structure. In some cases, the damage to the metal structure may be a gating or crystal factor. For example, a back-contacting solar cell metal structure (e.g., an on-cell base and emitter metallization portion M1) can withstand a maximum stacking pressure of about 300 psi before a desorption failure mode occurs. On the other hand, the damage threshold for an exemplary wafer carrier and a thin semiconductor (e.g., a silicon layer epitaxially grown on a silicon template) may exceed 800 psi to have no or zero or minimal problems.

A strong link between the amount of warpage in the final section and the pressure applied during the duration of the lamination cycle was determined. If the ideal part is to be machined, this part will be bent by a process in which the pressure is applied and remains constant throughout the stacking cycle. However, this part will be formed without warping by the process in which the pressure is applied and removed during the lamination cycle. Thus, using a process with varying pressures during lamination can be a requirement to achieve the design goals of the final solar cell, as illustrated by the stacking pressure drop schematically illustrated in FIG. 10B.

The vacuum in the stack is a selective stacking parameter in the PCB industry. Hundreds of PCB assemblies are manufactured each day by a lamination process using only lamination pressure to ensure a void free structure. In general, a non-vacuum process is used when a sufficient amount of resin readily flows into the prepreg and discards any trapped air outside the assembly through the resin flow.

In a solar cell attached to a backplane using a "non-flowing" mixed resin, it may be desired to use a vacuum during the deposition. That is, the resin may be insufficient to "push " the bubble out of the assembly such that excess air is exhausted using a vacuum. 12A and 12B are SEM images emphasizing voids in a solar cell structure showing a portion generated during non-vacuum lamination and from which silicon is removed from a resin cured by trapped bubbles. The post-lamination preparation step in which this part is placed in a high vacuum at an elevated temperature can be used to bring out any air bubbles filled in the laminate. The portions shown in Figures 12A and 12B were intentionally made without the use of vacuum and air was trapped between the silicon and prepreg layers during deposition. For backplane lamination of a prepreg in a solar cell, the laminating apparatus has a vacuum capability and this portion is held under vacuum for the entire duration of the laminating cycle.

The total curing time is an important parameter in obtaining the mechanical properties of the cured laminate. If the curing time is short, there is a risk that the resin will be incompletely cured, which in turn will result in a low mechanical property of the final part and a further resin curing step in the later solar cell manufacturing step. The long curing time is inefficient and is only necessary if the curing temperature is at a minimum (e. G., The minimum recommended curing temperature). For a backside solar cell, selecting a resin and a maximum curing temperature, it was shown that a 60 minute curing period was efficient, which provides a cured assembly that is substantially quicker than conventional lamination but meets all the mechanical requirements.

In conventional PCB manufacturing, the platen used to hold parts during the lamination cycle is generally made of stainless steel. The advantages of stainless steel include, for example, low CTE, good heat transfer properties, and stainless steel can be machined to have high flat durability. In addition to the platen, various types of spacers, release sheets, and conformal pressure transfer layers are added within the layup to form the "drum" of the part. These drums are placed in a lamination press and all parts in the drum are machined simultaneously.

The prepreg substrate may be laminated to a back-contacting solar cell using a process very similar to that from the PCB industry. One notable exception is that the platen can be made of aluminum or stainless steel. Large scale experiments with aluminum platens showed excellent heat transfer properties and were lighter in weight than stainless steel. If the layer is desorbed in the north, the high CTE value of aluminum is not critical in meeting the design requirements for the final solar cell product. The stack material for forming the drum is important to ensure that various parts are removed to account for CTE mismatching.

The choice of release sheet and conformal pressure delivery material can be adjusted to account for a significantly higher temperature during lamination. Standard materials can be sourced from the PCB industry for these high temperature layers to help reduce manufacturing costs.

Solar cells made of crystalline silicon are fragile and can easily be damaged due to incorrect handling. However, the provided backplane design is durable and bendable, and a complete solar cell using a very thin silicon layer is easily handled, and there is no risk of damage or movement to a minimum. With this feature, a simple device automation tool and a simple cassette to cassette transfer tool can be used during manufacture. After the backplane is deposited on the silicon layer of the solar cell, the manufacturing yield loss is the smallest due to the component breakage. As the thickness of the backplane decreases, the solar cell becomes very flexible. Handling practice is a set-up that can be adjusted to reduce the risk of folding or wrinkling the part and ensure that the handling tool has a soft touch.

The disclosed prepreg backplane supported solar cell structures provide excellent process manufacturing reliability as well as reliability and durability in solar cell end-use operational limitations with respect to temperature and humidity. This reliability performance supports the feasibility of solar cell designs (backplane materials and lamination processes) used in high efficiency solar cells. For example, thermal cycling is used to determine the module's ability to withstand thermal mismatch, fatigue, and other stresses due to repeated changes in temperature. According to important and unusual requirements for esting (example thermal cycle, humidity, and humidity cooling test processed per International Electrotechnical Commission requirements in IEC standard 61215, rev 2), a solar cell with a prepreg backplane supported Indicating that they have substantially exceeded certification requirements. For example, a solar cell with a prepreg backplane supported may have 400 column cycles of a standard test requiring a module temperature of -40 ° C for a minimum residence time of 10 minutes and a module temperature of 85 ° C for a minimum residence time of 10 minutes (Maximum 100 ° C / h temperature changes with temperature). The humidity test is used to determine the ability of the module to withstand long-term penetration effects of humidity. The solar cell with the prepreg backplane supported is capable of withstanding the absorption test for a time period of 1300 ° C where the module is maintained at a relative humidity of 85% ± 5 at a temperature of 85 ° ± 2 ° C. for several hours, A backsheet material is used which is known to allow a simple configuration with modular lamination in the worst case and allow moisture penetration under long-term high humidity conditions. The humidity cooling test is used to determine the module's high temperature and humidity and therefore its ability to withstand the effects of temperatures below zero. The photovoltaic cell with the prepreg backplane supported lasted 38 cycles of this IEC certification test, and the sample did not cause problems within about 4X certification requirements.

The hysteresis failure mode understood from the PCB industry is used to evaluate a potential failure mode, which can occur for a solar cell design in which the prepreg backplane is supported. Several types of failures caused by solar cells can be determined through measurable loss in performance. In a single lost connection of the PCB, the entire assembly may fail, and the power output by the connection loss in the solar cell will decrease. The reliability test is based on the amount of power loss seen over the duration of the experiment. The loss of electrical connections in PCB and solar cell applications is due to joint failures that can result from CTE mismatching, fatigue failure due to thermal or mechanical cycling, or segregation by moisture ingress. Each of these failure modes was investigated and neglected as a problem in performing the reliability test.

To solve the CTE mismatch, the innovative method provided matches the CTE of the silicon solar cell with the prepreg backplane. This CTE matching prevents stresses on the electrical connection and removes this conventional PCB failure mode as temperature changes.

Failure by thermal and mechanical cycling is solved through a combination of resin selection and a very low CTE by a Tg point that exceeds the operating range of the solar cell. This design feature ensures that there is no tension or shear in the electrical connection. For example, the high CTE (about 17 ppm / 占 폚) of copper plating and other metals is limited by the low CTE of the prepreg and the copper is kept under compression as the temperature rises. In addition, the inherent flexibility of the prepreg backplane can bend and fail during mechanical loading to prevent accumulation of interlaminar shear stresses. For additional margins, thousands of electrical contact points (e.g., M2 electrical connections to M1) for the solar cell can be fabricated through vias in the backplane. The aramid fiber substrate has been demonstrated to be serviced during floods in severe environments to provide significant margin when preventing micro-crack formation around the via location. The high via count provides a redundancy level and reduces the effect if the 25-year field life expectancy of the solar cell is reduced by a small number of connections.

To solve water intrusion, the mixed resin system can reduce the moisture absorption of the prepreg by a factor of the epoxy resin system. As a result, it has less sensitivity to high humidity environments and has added margins to products in field installations. The two high humidity tests performed during the reliability demonstration strongly indicate that the humidity is not a problem.

Thus, conventional failure modes from the PCB industry can be solved through prepreg material selection and electrical contact redundancy in the solar cell.

In operation, the unique solutions provided herein to support thin silicon-based solar cells using excellent and cheap prepreg backplanes vary from industry to industry. The prepreg backplane opened up a pathway that handles very thin silicon, eliminates barriers to processing and leads to very competitive product costs. In addition, the potential application of this technology is broad. The key backplane features of low cost, flexibility, simple manufacturing process, and durability reliability can be easily adjusted for use with a number of different solar cell materials. Flexible battery capability initiates a number of potential applications outside conventional module designs, including home electronics, Building Integrated PV (BIPV) products, and automotive applications.

The M1 and M2 metallization layers described herein are separated by a dielectric prepreg layer and are connected by a conductive via through a prepreg and the M1 and M2 metallization layers are electrically connected by a conductive post. Figures 3 and 4 are cross-sectional views of a back contact solar cell having M1 and M2 metallization portions. The metal layer M1 close to the solar cell can be laminated using a physical method (PVD, paste printing) or chemical technique (CVD). The prepreg layer is laminated together with the mixed resin. The vias can then be drilled by backplane lamination / attachment using mechanical, chemical, or laser drilling techniques. The vias can be cleaned using plasma sputtering, reactive species plasma or wet chemistry. In one M2 fabrication embodiment, a seed layer is deposited for the upper level metal layer M2 using PVD, inkjet, or screen printing, then a thick metal is plated over the seed layer to reduce resistance and line resistance in the via. The front side of the cell (e.g. with anti-reflective coating ARC layer) must be protected from the plating and etch bath. In one embodiment, one side fixture is metal etched to define the M2 line after being attached to the front side of the cell during the wet chemical step during plating.

The dual level metallized back-contacting solar cells described herein can be formed on a thin film silicon substrate formed using an epitaxial growth process or a monocrystalline, pseudocrystalline or polycrystalline silicon CZ wafer. Tables 1 and 2 demonstrate that the epitaxially grown substrate (starting from porous layer formation in a porous silicon / epitaxial substrate deposition / release process, Table 1) and the dual level metallization portion from CZ wafer (Table 2) Two-step flow embodiments and corresponding processing steps to form a thin crystalline rear-facing back-junction solar cell (having a thickness in the range of &lt; RTI ID = 0.0 &gt; 100.

Epitaxial-based thin monocrystalline (5 탆 - 100 탆) rear-facing rear junction solar cell with dual level metallization process flow

Harmonize Process Flow for CZ Wafer Based Thin Monocrystalline, Pseudocrystalline, or Polycrystalline Back-Contact Rear Junction Solar Cells with Double-Level Metallization Portions

A more detailed epitaxial based back-contacting solar cell process flow is provided below. Many of these process flows and specifically the process flow for metallization portions are applicable to non-epitaxial based back-contacting solar cells. For example, starting with a reusable silicon template made of a p-type single crystal silicon wafer, a porous silicon substrate can be fabricated that is porous (e.g., by an electrochemical etching process through a surface modification process in an HF / IPA wet chemical in the presence of an electrical current) A thin sacrificial layer of silicon is formed. The starting material or reusable template may be a monocrystalline silicon wafer formed, for example, using a crystal growth method such as FZ, CZ, MCZ (magnetostabilized CZ), and may further include an epitaxial layer growing on such a silicon wafer . The semiconductor doping form may be p or n and a wafer shape, and the most generally rectangular shape may be any geometric shape or non-geometric shape, such as a pseudo-square or round shape.

At the formation of the sacrificial porous silicon layer, it acts as a high-quality epitaxial seed layer and the next separation / lift-off layer and forms a thin layer of in-situ doped monocrystalline silicon (e.g., from a few microns to less than about 70 microns Or a layer thickness in the range of less than about 50 microns) is formed and is also referred to as epitaxial growth. The in-situ doped monocrystalline silicon layer can be formed by atmospheric pressure epitaxy, for example, using a chemical vapor deposition or CVD process in an atmosphere comprising silicon gas and hydrogen, such as trichlorosilane (or TCS).

Prior to backplane lamination, the solar cell base and emitter contact metallization pattern may be formed by a thin layer of, for example, screen printing or sputtering (PVD) or evaporated aluminum (or aluminum silicon alloy or Al / NiV / Sn stack) And is directly formed on the rear side of the battery. This metallized first layer (referred to herein as Ml) defines the solar cell contact metallization pattern and, for example, a fine pitch intertwined rear contact (IBC) conductor finger has a base and emitter area of the IBC cell define. The M1 layer extracts the solar cell current and voltage and transfers the electrical power of the solar cell to a second level / layer (referred to herein as M2) of the highly conductive solar cell metallization portion formed after M1.

After completion of the majority of solar cell processing steps, a very inexpensive backplane layer can be bonded to a thin epi layer for supporting and strengthening the permanent cell as well as for supporting the high conductive metal metallization portion of the solar cell. The prepreg backplane material may be thin (e.g., in the range of about 50 to 250 microns, in some instances in the range of 50 to 150 microns), flexible, and electrically insulating. (Eg solar cell area of at least 125 mm x 125 mm, 156 mm x 156 mm or more) Solar cell is mechanically weak from the template along the sacrificial porous silicon layer (eg, For example, through a mechanical emission MR process), and the template can be reused several times to reduce the manufacturing cost of the solar cell. The final cell processing is performed on the sun side of the solar cell and the solar side is exposed after being released from the template. The sun side processing may include, for example, antireflective coating deposition process steps after front side texturing and passivation.

After forming the backplane (above or within the M1 layer) backplane, the backplane supported solar cell is then removed from the template along the mechanically weak sacrificial porous silicon layer, and after completion of the front side texturing and passivation process, a high conductivity M2 Layer is formed. Via holes (up to hundreds or thousands of via holes in some instances) are drilled into the backplane (e.g., by laser drilling) and may have a diameter in the range of about 50 to 500 microns. This via hole is disposed over a predetermined area of M1 for electrical connection through a conductive plug formed in such a via hole between the patterned M2 and M1 layers. Al2O3, Al / NiV / Sn, or copper is then used (e. G., By plasma sputtering, plating, evaporation or a combination thereof) followed by or together with via hole filling and conductive plug formation Thereby forming a patterned highly conductive metallization layer M2. For an interfitted rear contact (IBC) solar cell with fine pitch IBC fingers on M1 (e.g., for hundreds of fingers), the patterned M2 layer may be designed to be orthogonal to the M1 layer, Or tapered M2 fingers are essentially perpendicular to the M1 fingers. Due to this vertical deformation, the M2 layer can have much more IBC fingers than the M1 layer (e.g., by a factor of about 10 to 50 M2 fingers). The M2 layer may be formed in a thicker pattern with IBC fingers wider than the M1 layer. The solar cell bus bar is located on the M2 layer and not on the M1 layer (i.e., there is no bus bar in M1), eliminating the electrical shading losses associated with the on-cell bus bar. Because the base and emitter interconnects and bus bars are located on the M2 layer on the solar cell backplane, electrical access is provided from the back of the solar cell to the base and emitter terminals of the solar cell on the backplane.

The backplane material formed between M1 and M2 may be a thin sheet of prepreg material with a sufficiently low coefficient of thermal expansion (CTE) to prevent excessive thermal inductive stress on the thin silicon layer. In addition, the prepreg backplane must meet the process integration requirements for the post-stage battery fabrication process, and the integration requirements are thermal stability during PECVD deposition of the front passivation and ARC layers, especially chemical resistance during wet texturing of the front side of the cell. Electrical insulating prepreg backplane materials must meet the module level stacking process and long term reliability requirements.

Figure 13 is a high level solar cell and module fabrication process flow embodiment using starting crystalline (monocrystalline or polycrystalline) silicon wafers, according to the disclosed subject matter. 13 illustrates a high level Icel process flow for fabricating a backplane-attached back contact / backside junction (IBC) using two layers of metallized portions M1 and M2. The first layer or level of the patterned cell metallization portion M1 is formed as the last process step in a plurality of shear cell fab processes prior to backplane lamination in an essentially partially fabricated solar cell. 14 is a cross-sectional view of a solar cell according to the disclosed subject matter, which can be manufactured according to the process steps.

The prepreg sheet can be attached to the rear side of the solar cell while being held on the template (before the battery lift-off process) using a vacuum laminator. While applying heat and pressure, the thin prepreg sheet is laminated or adhered to the back side of the permanently processed solar cell. The lift-off emission boundary is then defined around the solar cell (for example, near the edge of the template) using a pulsed laser scribing tool, followed by a backplane laminated solar cell with a mechanical emission or lift-off process And is separated from the reusable template. The following process steps are performed: (i) completion of the texture and passivation process on the sun side of the solar cell, (ii) after completion of the solar cell, a high conductive metal on the back side of the cell (which may include parts of the solar cell backplane) May include a text portion. A layer of high-conductivity metallization M2 (including, for example, aluminum, copper, or silver) containing emitter and base polarity is formed on the laminated solar cell backplane.

The foregoing description of the exemplary embodiments is provided to enable any person skilled in the art to make or use the claimed subject matter. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of innovative facilities. Accordingly, the claimed subject matter is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (35)

A rear-facing solar cell structure comprising a front side and a metallized rear side for receiving sunlight,
Cell patterned base and emitter metallization portions electrically connected to the base and emitter regions on the rear side of the back-contacting solar cell semiconductor substrate, and an on-cell patterned base and emitter metallization portion electrically connected to the on- A back side contact solar cell semiconductor substrate rear portion; And
And a backplane laminate layer attached to the back-contacting solar cell substrate rear region that is not covered by the on-cell patterned base and emitter metallization portions Wherein the backplane laminate is comprised of a resin and fibers and has a thermal expansion coefficient that is matched relatively close to the back contact solar cell semiconductor substrate.
The method according to claim 1,
Wherein the backplane laminate layer is a fiber-reinforced prepreg layer.
The method according to claim 1,
Wherein the fibers in the backplane laminate layer are made of aramid fibers.
The method according to claim 1,
Wherein the fibers in the backplane laminate layer comprise nonwoven aramid fibers.
The method according to claim 1,
Wherein the fibers in the backplane laminate layer comprise woven aramid fibers.
The method according to claim 1,
Wherein the backplane laminate layer is a semi-rigid backplane having a thickness in the range of about 75 [mu] m to 1000 [mu] m.
The method according to claim 1,
Wherein the backplane laminate layer is a flexible backplane having a thickness of about 50 [mu] m to 150 [mu] m.
The method according to claim 1,
Wherein the backplane laminate layer is a flexible backplane having a thickness ranging from about 25 [mu] m to about 100 [mu] m.
The method according to claim 1,
Wherein the resin in the backplane laminate layer is a mixed resin comprising at least two different resin components.
The method according to claim 1,
Wherein the resin in the backplane laminate layer is a mixed resin comprising at least one thermoplastic resin component.
The method according to claim 1,
Wherein the resin in the backplane laminate layer comprises an epoxy or thermosetting or B-staged resin.
The method according to claim 1,
Wherein the backplane laminate further comprises a silane-based adhesion promoter.
The method according to claim 1,
Wherein the rear contact solar cell substrate is a crystalline silicon back contact solar cell substrate.
The method according to claim 1,
Wherein the back-contacting solar cell substrate is a back-contacting solar cell substrate fitted to each other, and the on-cell base and the emitter metallization portion are formed in a pattern of a base and emitter metallization fingers fitted to each other, .
The method according to claim 1,
A plurality of via holes drilled in the backplane laminate; And
A base and emitter metallization second layer formed on the backplane laminate and providing electrical connection to the on-cell base and the emitter metallization through the via holes in the backplane laminate;
Wherein the back-contacting solar cell structure further comprises:
A rear-facing solar cell structure comprising a front side and a metallized rear side for receiving sunlight,
An on-cell patterned base and emitter metallization portion electrically connected to the base and emitter regions of the back-contacting solar cell semiconductor substrate, and an on-cell patterned base and emitter metallization portion electrically connected to the backside A contact solar cell substrate rear portion; And
And a backplane laminate layer attached to the back-contacting solar cell substrate rear region that is not covered by the on-cell patterned base and emitter metallization portions, Wherein the backplane laminate has a first portion comprised of a resin and the resin flows between the patterned on-cell base and the emitter metallization portions and is attached to the rear side of the back-contacting solar cell substrate,
The backplane laminate having a second portion comprised of a resin and a fiber on the first portion and providing a thermal expansion control layer having a thermal expansion coefficient (CTE) sufficiently low as compared to the back contact solar cell semiconductor substrate, Battery structure.
17. The method of claim 16,
Wherein the backplane laminate layer is an electrically insulating layer.
17. The method of claim 16,
Wherein the fibers in the second portion of the backplane laminate layer are aramid fibers.
17. The method of claim 16,
Wherein the fibers in the second portion of the backplane laminate layer are nonwoven aramid fibers.
17. The method of claim 16,
Wherein the fibers in the second portion of the backplane layer are woven aramid fibers.
17. The method of claim 16,
Wherein the backplane laminate layer is a rigid backplane having a thickness ranging from about 75 [mu] m to about 1000 [mu] m.
17. The method of claim 16,
Wherein the backplane laminate layer is a flexible backplane having a thickness in the range of about 50 [mu] m to 150 [mu] m.
17. The method of claim 16,
Wherein the backplane laminate layer is a flexible backplane having a thickness ranging from about 25 [mu] m to about 100 [mu] m.
17. The method of claim 16,
Wherein the resin in the backplane laminate layer is a mixed resin containing at least two resin components.
17. The method of claim 16,
Wherein the resin in the backplane laminate layer is a mixed resin comprising at least one thermoplastic resin component.
17. The method of claim 16,
Wherein the resin in the backplane laminate layer comprises an epoxy or thermosetting or B-staged resin.
17. The method of claim 16,
Wherein the backplane laminate further comprises a silane-based adhesion promoter.
17. The method of claim 16,
Wherein the rear contact solar cell substrate is a crystalline silicon back contact solar cell substrate.
17. The method of claim 16,
Wherein the back-contacting solar cells are back-contact solar cells fitted to each other and the on-cell patterned base and the emitter metallization portions are formed in a pattern of a base and emitter metallization fingers interdigitated with each other, .
17. The method of claim 16,
A plurality of via holes drilled in the backplane laminate; And
A base and an emitter metallization second layer formed on the backplane laminate and providing electrical connection to the on-cell base and the emitter metallization through the via hole in the backplane;
Wherein the back-contacting solar cell structure further comprises:
17. The method of claim 16,
Wherein the backplane laminate layer has a resin content in the range of 30% to 45%.
A fine electronic semiconductor structure formed on a semiconductor substrate,
A patterned metallized first layer electrically connected to select an area of the semiconductor substrate, and a portion of the semiconductor substrate not covered by the patterned metallized first layer; And
And a backplane laminate layer attached to the semiconductor substrate region that is not covered by the patterned metallized first layer, the backplane laminate having a first portion comprised of a resin Wherein the resin flows between the patterned metallized first layers and is attached to the semiconductor substrate, the backplane laminate having a second portion comprised of resin and fibers on the first portion, Together with a thermal expansion control layer having a sufficiently low coefficient of thermal expansion (CTE).
1. A semiconductor device structure formed on a semiconductor substrate,
A patterned metallized first layer electrically connected to select the semiconductor substrate region, and a semiconductor substrate portion not covered by the patterned metallized first layer; And
And a backplane laminate layer attached to the semiconductor substrate region that is not covered by the patterned metallized first layer, the backplane laminate having a first portion comprised of a resin Wherein the resin flows between the patterned metallized first layers and is attached to the semiconductor substrate, the backplane laminate having a second portion comprised of resin and fibers on the first portion, Together with a thermal expansion control layer having a sufficiently low coefficient of thermal expansion (CTE).
1. A semiconductor device structure formed on a semiconductor substrate,
A patterned metallized first layer electrically connected to select the semiconductor substrate region, and a semiconductor substrate portion not covered by the patterned metallized first layer; And
An electrically insulated backplane laminate layer attached to an area of the semiconductor substrate that is not covered by the patterned metallized first layer and the patterned metallized first layer,
Wherein the backplane laminate is combined with a resin having a thermal expansion coefficient (CTE) matched relatively similar to the semiconductor substrate.
35. The method of claim 34,
A plurality of via holes drilled in the backplane laminate; And
A patterned metallized second layer formed on the backplane laminate and providing electrical connection to the metallized first layer patterned through the via hole in the backplane laminate;
Wherein the back-contacting solar cell structure further comprises:

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