Patent Application
20120264072
This invention relates to photovoltaic materials and manufacturing methods. More particularly, the invention provides a method and apparatus for performing reactive thermal treatment of thin film photovoltaic materials, and provides a method and apparatus for improving temperature uniformity and reducing process time during reactive thermal processes.
Energy comes in forms such as petrochemical, hydroelectric, nuclear, wind, biomass, solar, and more primitive forms such as wood and coal. Over the past century, modern civilization has relied upon petrochemical energy as an important energy source. Petrochemical energy includes gas and oil. Gas includes lighter forms such as butane and propane, commonly used to heat homes and serve as fuel for cooking Gas also includes gasoline, diesel, and jet fuel, commonly used for transportation purposes. Heavier forms of petrochemicals can also be used to heat homes in some places.
More recently, environmentally clean and renewable energy sources are desired. One type of clean energy is solar energy. Solar energy technology generally converts electromagnetic radiation from the sun to other forms of energy. Although solar energy is environmentally clean and has been successful to a point, many limitations remain to be overcome before it becomes widely used throughout the world. As an example, one type of solar cell uses crystalline materials derived from semiconductor material. These crystalline materials can be used to fabricate optoelectronic devices that include photovoltaic and photodiode devices that convert electromagnetic radiation to electrical power. Crystalline materials, however, are costly and difficult to make on a large scale; and devices made from such crystalline materials have low energy conversion efficiencies. Other types of solar cells use “thin film” technology to form a thin film of photosensitive material to be used to convert electromagnetic radiation into electrical power. Similar limitations exist with the use of thin film technology. Additionally, film reliability is often poor and cannot be used for extensive periods of time in conventional environmental applications. Thin films are often difficult to mechanically integrate with each other.
As an effort to improve thin film solar cell technology, processes of manufacturing an advanced CIS and/or CIGS based photovoltaic film stack on substrates with planar, tubular, cylindrical, circular or other shapes have been developed. There are various manufacturing challenges in forming the photovoltaic film stack, such as maintaining structure integrity of substrate materials, controlling chemical compositions of the ingredients in one or more precursor layers, carrying out proper thermal treatment of the one or more precursor layers within a reactive gaseous environment, ensuring uniformity and granularity of the thin film material on substrates during reactive thermal treatment, etc. Especially, when manufacturing the thin film based photovoltaic device on large sized substrates, temperature uniformity across the whole substrate surface is desired. It is desirable to have an improved system and method for processing thin film photovoltaic devices on planar or non-planar shaped, fixed or flexible substrates.
This invention provides a method and apparatus for thermal treatment of thin film solar cells with improved temperature uniformity and reduced process time. The method and apparatus provide a dual door cover for enhancing both conduction and convection cooling and substrate temperature uniformity during reactive thermal treatment processes. The invention provides an apparatus for performing reactive thermal treatment of thin film photovoltaic devices. The apparatus includes a furnace having a tubular body surrounded by heaters and cooling devices. The tubular body encloses an interior volume from a first end to a second end. A first door structure covers the first end with a first plate facing the interior volume. The first plate is coupled to a first coil pipe within the door structure. A similar structure is provided at the opposite end of the furnace. In addition, the apparatus includes a removable rack fixture within the furnace. The rack fixture allows an array of substrates to be loaded into the interior volume from either end of the furnace. Baffle members disposed in the interior volume control interior convection.
Preferably, the furnace is made of quartz which is substantially chemically inert and has good thermal conductivity characteristics. The substrates, each having a dimension ranging from about 20 cm to 156 cm, are loaded using the rack fixture. The large industrial thin-film substrates are maintained at one process temperatures for annealing in an reactive gaseous environment. Combined effects of thermal conduction through the quartz body and controlled convection induced by the door structures result in a temperature variation typically no more than 10° C. during the process period, across the substrates of as large as 156 cm and greater.
In an alternative embodiment of the present invention, a method for performing a reactive thermal treatment of photovoltaic material with enhanced temperature uniformity is provided. The method includes providing a furnace enclosing a tubular volume between a first end cover and a second end cover for holding one or more substrates therein. The furnace is then heated to a process temperature range, and held with a variation less than 10 degrees Centigrade for performing a reactive thermal treatment of the substrates. The furnace is then cooled to reduce the temperature of the substrates from the process temperature range to near room temperature, at a rate of about 1 degree per minute or faster.
The method provides reactive thermal treatment of a thin-film precursor to form an absorber of photovoltaic devices on large glass substrates. The apparatus for performing the thermal treatment in a reactive gaseous environment preferably requires the furnace itself to be chemically inert and thermally conductive. In a specific embodiment, the apparatus is a quartz tube with end covers for facilitating convection of working gases therein. Baffle members are used to retain the working gases around the substrates as necessary. The end covers are symmetrical disposed with built-in heat-exchanger structures to keep a cool plate to serve as both cold traps for residue particles and heat sinks Thus the large substrates can be placed in the furnace tube and maintained at a process temperature range with high temperature uniformity. This enables the reaction between the working gases and the precursor material on the substrates to be performed with improved temperature uniformity, leading to formation of a photovoltaic absorber with higher conversion efficiency.
This invention provides a method and apparatus for processing thin film solar cells on substrates with improved temperature uniformity. The method and structure are applied for the manufacture of copper indium gallium selenide thin film photovoltaic devices on glass substrates, but the invention can be used in other processes.
The tubular body 110 preferably is a material having good thermal conductivity, with heating elements 160 in contact with the tubular body 110 so that thermal energy can be directly conducted into the furnace. For example, the chamber 100 can be quartz, which is resistive to reactive gases and a good thermal conductor. Other techniques can be used to heat the quartz tubular body. In addition, the cooling elements may be provided by a refrigerant gas flowing around the outside of the quartz tubular body. This allows the temperature of the furnace chamber 100 to be controlled as necessary for the thermal processing.
In
The two end covers 120A and 120B are designed to seal the chamber 100 to form a vacuum system. Either of the end cover 120A or 120B can also include vacuum pipes—one pipe 1221 for supplying desired gas species into the chamber interior volume 111 from an external source and one pipe 1222 connected to a vacuum pump to empty the chamber before a process or purge the chamber after a process. In a process, the work gases include selenide gas or sulfide gas, often mixed with inert gases as carrier gas. The selenium and sulfur species are commonly used for forming thin-film photovoltaic materials in a reactive thermal treatment process.
The furnace chamber 100 is designated for performing thermal treatment of thin-film materials on substrates. Substrates can be loaded from either the first end region 115 or the second end region 116 by opening the corresponding door structure or end cover 120A or 120B. The substrates 101 can be loaded in a boat structure 138 which is inserted into the interior volume 111 of the furnace chamber. The substrates 101 are usually large panels of glass or other material designated for forming thin-film photovoltaic devices thereon. Typically the substrates are rectangular shaped glass substrates having a dimension as large as 156 cm. Each substrate has usually been preprocessed to form films stacks overlying the glass surface. A thin-film precursor material such as a copper species, an indium species, a gallium species, and/or sodium dopants mixed by various depositing or doping techniques, can be formed on top of the film stacks. In one embodiment, the copper-indium-gallium mixed precursor is intended for reacting with selenium or sulfur gaseous species to form a thin-film photovoltaic absorber. The boat structure 138, loaded with the substrates 101, is supported by a rack fixture 135 inside the chamber. In an embodiment, the rack fixture 135 is a removable via the door structures using a shaft 132. In another embodiment, the rack fixture 135 is loaded and unloaded by a robot loader (not shown) associated the apparatus 100. The rack fixture 135 and boat structure 138 inside the interior volume are exposed to the reactive work gases, so they each are preferably chemically inert. In an implementation, all are quartz material.
The furnace chamber 100 can also includes baffles near the ends 115 and 116. These baffle members assist in keeping heated gases within the interior volume where the substrates are treated while keeping cooler gases in regions 111A and 111B near the end covers. The baffles include a first group of baffles 140 substantially covering a major portion of the cross section area of the tubular body and a baffle 141 covering the lower edge portion. The baffles 140 are disk shaped and positioned near the middle part of tubular interior volume. The baffle 141 is crescent shaped to partially cover the lower edge portion of the disk baffles. Near the other end 116, a group of disk shape baffles 150 cover of the tube cross-section and a crescent baffle 151 is attached to the tubular furnace body. All these baffles can be quartz.
As used herein, “crescent” means a “shape produced when a circular disk has a segment of another circle removed from its edge, so that what remains is a shape enclosed by two circular arcs of different diameters which intersect at two points.” For example, some descriptions or definitions can be found in public information website such as http://en.wikipedia.org/wiki/Crescent.
The substrates are usually planar shaped, e.g. like a panel of glass. Typical sizes are a 20×20 cm glass square, a 20×50 cm glass rectangle, a 65×156 cm glass rectangle. The glass is usually soda lime glass, widely used for substrates of solar cells. Of course the substrate can be made of other materials including fused silica, quartz, or others. The substrates can be other planar shapes, including rectangular, square, disk, as well as non-planar shapes such as a rod, tube, semi-cylindrical tile, or even flexible foil, depending on applications.
The substrates usually have overlayers formed by earlier processes. For example, a precursor layer including a copper species, an indium species, and/or an indium-gallium species may be formed on a surface of the substrate using sputtering techniques. In a subsequent reactive thermal treatment process, the precursor layer is reactively treated in a gaseous environment within the furnace tube containing a selenide species, or a sulfide species, and a nitrogen species. When the furnace tube is heated, the gaseous selenium reacts with the copper-indium-gallium species in the precursor layer. As a result of the reactive thermal treatment, the precursor layer is transformed to a photovoltaic film stack containing copper indium (gallium) diselenide (CIGS) compound, which is a p-type semiconductor and serves as an absorber layer for forming photovoltaic cells. Further description of the thermal treatment process for forming the CIGS photovoltaic film stack of thin film solar cells is found in U.S. Patent Application Ser. No. 61/178,459 entitled “Method and System for Selenization in Fabricating CIGS/CIS Solar Cells” filed on May 14, 2009, by Robert Wieting, assigned to Stion Corporation of San Jose and hereby incorporated by reference.
In another embodiment, each disk shaped baffle 240 has a ring-shaped gap 211 between its peripheral rim and an inner wall of the furnace tube 210. This gap 211 allows a convection flow of the work gases from the central region 111 to the end cover region 111A (see
Substrates are loaded in a boat structure supported on a rack fixture within the interior volume of the furnace tube. Usually each substrate is arranged vertically and in parallel to other substrates to facilitate work gas circulation. As shown in
During temperature ramping stage and a treatment stage at the processing temperature, the cool convection current is restricted so that the temperature around the substrate is more uniform. By optionally providing crescent shaped baffle 241, the lower portion of the gap, which is a major path for the cooled gases, is substantially blocked. The cooled gases are largely maintained in the end cover region, but may pass through the gap at the higher portion above the crescent shaped baffle gap, where the gases become warmer. In one embodiment, the arc length of the baffle 241 is one half of the perimeter of the furnace tube or smaller, e.g. 40% of the perimeter or smaller, however, it can be 50% to 66% of the perimeter, or larger, depending on the application. By reducing convection, the heated gases remain in the central region, accelerating heating operations.
During a temperature cooling stage (usually after the processing stage), however, an enhanced convection current flow is desired. Cooling of the furnace tube is achieved by first cooling the tubular body via thermal conduction, and secondly cooling the work gases inside furnace via interior convection with enhanced heat exchange between the work gases and the end cover plates. Cooling can be achieved by use of cooling elements 170 (see
Another way of cooling is achieved by enhancing the convection current flow to move the warmer work gases within the central region faster towards the cooler face plates, and then back to the central region. Therefore, optionally, the crescent shaped baffle may be moved to re-open the lower portion of the gap. Of course, other approaches can be used to alter the convection to enhance the cooling. Using two door structures makes a symmetric configuration relative to the loaded substrates, and helps enhance temperature uniformity across the substrates in addition to obtaining a faster cooling rate.
In another embodiment, the cold face plate serves as a cold trap for absorbing un-reacted residue particles formed during the reactive thermal treatment processes. In such an example, the work gases include hydrogen selenide gas or hydrogen sulfide gas. When the temperature is increased to about the processing temperature range of 420° C., the hydrogen selenide gas can be subjected to thermal cracking and break into hydrogen gas and selenium particles. A portion of the Se particles may not complete a reaction with the precursor material on the substrate and are thus carried by the flow of work gases. Other gases or particles may be released from the substrate surfaces or precursor material mixtures as well, including un-reactive particles. An undesirable fate for these particles is to deposit onto the substrate surface, causing degradation of the photovoltaic absorber. By being kept cool during the process dwelling stage, the face plates of the end covers become major absorbing places for such un-reactive particles.
As shown, the above method provides an improved technique of treating a thin-film photovoltaic material in a reactive gas environment. In a preferred embodiment, the method uses a quartz furnace tube with end covers to provide stable heating and cooling, yet allow ramping of temperatures with faster rates by controlling both thermal conduction and internal convection. The two end covers can be kept cool, providing a cold trap for residue particles, and a heat exchange plate to induce healthy internal convection.
As shown, the method 400 for treating photovoltaic materials in a reactive thermal process starts with a step 402, which include preparing substrates with a thin-film precursor material. The thin-film precursor material includes a mixture of copper species, indium species, or gallium species, and sometimes sodium species. The method 400 follows with a step 404 providing a furnace tube as a processing apparatus. Substrates are then loaded (step 406), and the end covers seal the furnace. Usually the substrates are loaded into a substrate holder (or boat structure), and then the substrate holder is inserted into the furnace tube supported by a rack fixture. The substrate loading process can also be used to install baffles for altering internal convection as needed.
After the furnace is sealed, the method 400 includes a step 408 of supplying work gases via pipes through the end covers. In step 408 the method 400 provides a gaseous environment in the interior volume of the furnace tube ready for conducting reactive thermal treatment processes having a predetermined temperature profile. The work gases are supplied to the furnace tube from a gas supply device, such as a valve or injector coupled to the end covers of the furnace tube. The working gases usually include a chemical precursor species designed to react with the thin-film precursor material overlying the substrate. The working gases can include a carrier gas such as nitrogen, helium, argon, and other gases. Of course, the gas step usually is preceded with a purge process, either for preparing a vacuum before introducing the work gases, or purging the furnace after the process ends.
The method 400 includes a step 410 to increase temperature of the substrates. Following the ramping stage, the method 400 includes step 412 for maintaining the process temperature by controlling thermal transfer via both conduction and convection, as described above.
Method 400 next includes step 414 for cooling the furnace by conduction and convection to reduce temperature from the process temperature range to near room temperature. In an example, the substrate can be cooled in a rate of 1 degree per minute, or 3 degrees per minute or faster, while still keep a reasonably uniform temperature across the large substrate.
Subsequently, other steps 416 may be followed to purge the chamber with nitrogen gas and remove all the reactive gases, to handle the treated substrates for continuing other processes for manufacturing a photovoltaic device on the large sized substrate according to an embodiment of the present invention.
After the stage P2, the process requires a first cooling stage C1. The first cooling process is preferred to be carried out with a relatively slow cooling rate. With a cooling rate of about a half degree drop per minute, or slower, the glass maintains sufficient viscosity to relax internal stress up to time t6, when the temperature reaches about 430° C. Beyond this point, the glass will not have much retained strain, so a further drop in temperature would not cause damage. Then, an accelerate cooling stage C2 is started, with cooling of 1 to 3 degrees per minute. This enhanced cooling rate can substantially reduce process time and increase productivity.
While the present invention has been described using specific embodiments, it should be understood that various changes, modifications, and variations may be effected without departing from the spirit and scope of the invention as defined in the appended claims. For example, while a tubular shaped furnace is illustrated, other shapes of furnace and baffles can be used. Additionally, although the above embodiments are applied to reactive thermal treatment for forming CIS and/or CIGS photovoltaic devices, other thermal processes can also be used.
This application claims the benefit of U.S. Provisional Application No. 61/439,079, filed Feb. 3, 2011, entitled “Method and Apparatus for Performing Reactive Thermal Treatment of Thin Film PV Material.” The entire disclosure of which is incorporated herein.
| Number | Date | Country | |
|---|---|---|---|
| 61439079 | Feb 2011 | US |
