Patent Application
20090260675
1. Field of the Invention
The present invention generally relates to solar module design and fabrication and, more particularly, to packaging techniques for solar modules such as solar modules employing Group IBIIIAVIA absorbers.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications. It should be noted that although the chemical formula for CIGS(S) is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in
Manufactured solar cells are interconnected by stringing them or shingling them to form solar modules. Such modules are constructed using various packaging materials to mechanically support and to protect the solar cells against environmental degradation. The most common packaging technology involves lamination of solar cell strings or circuits in transparent encapsulants. In a lamination process, in general, electrically interconnected solar cells are sandwiched between layers of encapsulants and front and back protective sheets; and all the components are subjected to heat and pressure to bond module components together forming a module package. The front protective sheet is typically glass, but may also be a transparent flexible polymer film such as TEFZEL® The back protective sheet may be a sheet of glass or a polymeric sheet such as TEDLAR®. Light enters the module through the front protective sheet.
A variety of materials are used as encapsulants for packaging solar cell modules, such as ethylene vinyl acetate copolymer (EVA). Although EVA is typically the best known encapsulant material in the solar cell industry, it has certain limitations. One well known limitation of EVA is its decomposition under sunlight and moisture from prolonged use, which results in releasing of acetic acid. Acetic acid production and glass transition concerns with ethylene-vinyl acetate used in photovoltaic devices, Michael D. Kempe; Gary J. Jorgensen; Kent M. Terwilliger; Tom J. McMahon; Cheryl E. Kennedy; Theodore T. Borek, Solar Energy Materials and Solar Cells, Volume 91, Issue 4, 15 Feb. 2007, Pages 315-329) (Applications of Ethylene Vinyl Acetate as an Encapsulation Material for Terrestrial Photovoltaic Modules, E. F. Cuddihy. C.D. Coulbert, R. H. Liang, A. Gupta, P. Willis, B. Baum, DOE/JPL/1012-87,) Acetic acid is especially harmful for thin film solar cell structures employing CIGS absorbers. Lamination of solar cells using EVA as the encapsulant leaves unreacted peroxides even after obtaining very high gel contents. Unreacted peroxides may catalyze the discoloration of the EVA layers and corrosion of the solar cells throughout the lifetime of the solar module. Under the influence of acetic acid and unreacted chemicals, the CIGS absorber and other layers of the solar cell such as the transparent conductive oxide layer, the buffer layer such as Cd(Zn)S, etc. may degrade which in turn may lead to degradation, discoloration and eventually malfunction of the module.
A recently marketed thermoplastic material known as thermoplastic polyurethane (TPU) is one of the promising materials without the aforementioned shortcomings of EVA. Etimex, an encapsulant supplier from Germany, is presently offering a photovoltaic-grade TPU film. Lamination of TPU film does not involve incomplete curing reactions that would leave unreacted peroxides inside the laminated solar panel. For TPU, there is also no acetic acid generation during the lamination process or later upon exposure to sunlight and moisture, therefore it may be used as a substitute for EVA in the module structure. However, very high cost of TPU (almost double or triple the price of EVA) limits its use in the conventional manner of laminating solar cells, and as such there is a strong reason not to use it.
From the foregoing, there is a need in the solar cell manufacturing industry, especially in thin film photovoltaics, for better packaging techniques that can provide reliable performance at reduced cost. It should be noted that thin film technologies such as CIGS photovoltaics are being developed for cost reduction and they are very sensitive to the cost of module packaging.
Present invention provides a method for encapsulating interconnected solar cells including Group IBIIIAVIA absorbers and an apparatus according to the same, whereby a light receiving side encapsulant layer including a thermoplastic polyurethane is used to cover the light receiving side of the interconnected solar cells. The back side of the interconnected solar cells are covered with a back side encapsulant layer that is different from the light receiving side encapsulant layer.
Present invention provides a packaging method for manufacturing IBIIIAVIA solar cell modules using a combination of more than one type of encapsulants to cover the front and back of the solar cells within the module to obtain a synergistic advantage in combination with reduced packaging cost and increased module lifetime. In one embodiment, a solar cell module having one or more solar cells may be encapsulated using at least two different layers of encapsulant materials. Accordingly, back of the interconnected solar cells within the module structure, which would not be exposed to light and its degrading affects, can be covered with a first material layer that is low cost and soft therefore providing a low-stress cushion for the devices. The first material may or may not be a transparent material. A front side of the interconnected solar cells, where the light is received, may be covered with a second material layer which is a transparent material layer that does not release harmful chemicals during lamination and is not prone to chemical degradation when exposed to light although it may be a higher cost material. In one embodiment, light receiving side of the solar cells may be coated with a transparent polyurethane material layer such as thermoplastic polyurethane (TPU), and the back side of the solar cells may be coated with a copolymer material layer such as ethylene vinyl acetate (EVA).
As shown in
In the solar cell device 102, the base portions 112 of the solar cells 108 form the back side 124 of the solar cell device and likewise front portions 114 of the solar cells 108 form the front side 126 or the light receiving side of the solar cell device. Direction of the incoming light is depicted by arrows A. Furthermore, surface area of the front side 126, i.e., a combination of the areas of the front surfaces 115, forms the front surface or light receiving surface of the solar cell device, and similarly the surface area of the back side, i.e., a combination of areas of the back surfaces 113, forms the back surface of the solar cell device 102. Back surface and the front surface of the solar cell device 102 may include insulation spaces among the solar cells 102. As will be described more fully below, using a packaging process such as a lamination process, the back encapsulant layer 104 is coated over the back side 124, and the front encapsulant layer 106 is coated over the front side 126, entirely sealing the solar cell device 102, as shown in
As mentioned above, the packaging method of the invention advantageously uses different materials as encapsulants to coat the backside 124 and the front side 126 to minimize chemical incompatibility issues between the packaging materials and the solar cells as well as reducing the cost and increasing the solar cell and module stability, output and life time. Referring to
Referring back to
Such stacked components of the solar cell module are placed in a laminator and heat treated for about 10-20 minutes in a temperature range of 120°-160° C. The laminator may be a clamp-type laminator or it may be a roll-to-roll laminator. The clamp-type laminator may contain upper and lower chambers, and a rubbery diaphragm. The stacked components of the solar cell module will be placed on a heating plate in the lower chamber at a temperature range of 120°-160° C. During the lamination process, the upper and lower chambers are de-aired (vacuum) to 0.1-10 mbar pressure level in 3-20 seconds and kept at this pressure level for 3-7 minutes. The process de-airs and melts the front encapsulate and the back encapsulant around the solar cell device 102. After the melting step, the upper chamber pressure will be brought up to atmospheric pressure in several seconds while the lower chamber pressure will be kept at its previous pressure level. This step will apply pressure to the stacked components by the diaphragm and last 7-13 minutes to complete a uniform, bubble-free solar cell lamination. The lamination process yields a packaged solar cell module as shown in
Adding to this, another advantage of using EVA at the bottom of the module is that the residual chemicals generated during lamination do not hurt the active part of the solar cells which are located at the front side. Besides, since the EVA is behind the solar cells (where the substrate is) its exposure to sunlight is blocked off by the solar cells on top of the EVA layer. Thus, the EVA layer is not exposed to sunlight and residual chemicals generated as a result of this exposure during the operation are eliminated, except at the edges of the cells which occupy a relatively small area.
Further more, the modulus of elasticity for TPU is higher than that is for EVA (about 4000 PSI for TPU and 1000 PSI for EVA), making TPU a more rigid material than EVA. The solar cells in a module will be exposed to mechanical and thermal stresses in the outdoors as they are heated up by sun during the day and cooled down during the night or when they are exposed to windy situations. Cuddihy et al. (Applications of Ethylene Vinyl Acetate as an Encapsulation Material for Terrestrial Photovoltaic Modules, E. F. Cuddihy. C. D. Coulbert, R. H. Liang, A. Gupta, P. Willis, B. Baum, DOE/JPL/1012-87,) have shown that the encapsulant thickness needed to protect solar cells in a solar module depends on the encapsulant material's modulus of elasticity. The lower the modulus of elasticity, the lesser the thickness of encapsulant required to dampen the stresses on the solar cells inside a solar module. It has been found that the thickness (in units of mils) to modulus of elasticity ratio (in units of klb/inch2) should be equal to or greater than 4 for encapsulant material to dampen the stress due to wind deflection. (E. F. Cuddihy, Encapsulant Selection and Durability Testing Experience, Jet Propulsion Laboratory, Reliability and Eng. of Thin-Film Photovoltaic Modules; p 249-274, 1985) Typically, 12 to 18 mils thickness range is the most commonly used material specification for EVA layers in the solar cell industry. From a t/E ratio point of view, the 18 mil thickness is very well sufficient even for the reduced EVA thickness (˜20%) after lamination. A typical TPU thickness range in the industry may be 20-25 mils. Minimum 20 mil thicknesses is required for TPU encapsulant to be able to dampen the stresses on solar cells taking into considerations of t/E, and reduced encapsulant thickness after lamination. TPU(front encapsulant)/TPU (back encapsulant) packages need to use thicker TPU layers (total ≧40 mils) than the EVA layers in an EVA/EVA packages (˜20-36 mils) to dampen the deflective and thermal stresses in the sealed module structure. Since EVA is less rigid than TPU material, by using a thin and flexible EVA on the bottom of the cell, the overall TPU/EVA package thickness is reduced. 20 mil thick TPU for front and 12 mil thick EVA for the back layer encapsulant, a total of 32 mil, will be sufficient to protect the solar cells from the deflective stresses. This is especially important for flexible module packages. Thinner package has more mechanical flexibility.
Another advantage of using a combination of a TPU layer along with a EVA layer is that this combination can be manufactured using a roll-to-roll process in which the combination TPU/EVA layers are sufficiently flexible to be rolled upon completion, whereas an EVA only construction is too rigid for use in a roll-to-roll process manufacturing.
Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.
