Patent Application
20120027921
The subject matter disclosed herein relates generally to the field of thin film deposition processes wherein a thin film layer, such as a semiconductor material layer, is deposited on a substrate. More particularly, the subject matter is related to a vapor deposition apparatus and associated process for depositing a thin film layer of a photo-reactive material on a glass substrate in the formation of photovoltaic (PV) modules.
Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy (sunlight) to electricity. For example, CdTe has an energy bandgap of 1.45 eV, which enables it to convert more energy from the solar spectrum (sunlight) as compared to lower bandgap (1.1 eV) semiconductor materials historically used in solar cell applications. Also, CdTe converts light more efficiently in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in low-light (i.e., cloudy) conditions as compared to other conventional materials.
Solar energy systems using CdTe PV modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner.
Certain factors greatly affect the efficiency of CdTe PV modules in terms of cost and power generation capacity. For example, CdTe is relatively expensive and, thus, efficient utilization (i.e., minimal waste) of the material is a primary cost factor. In addition, the energy conversion efficiency of the module is a factor of certain characteristics of the deposited CdTe film layer. Non-uniformity or defects in the film layer can significantly decrease the output of the module, thereby adding to the cost per unit of power. Also, the ability to process relatively large substrates on an economically sensible commercial scale is a crucial consideration.
CSS (Close Space Sublimation) is a known commercial vapor deposition process for production of CdTe modules. Reference is made, for example, to U.S. Pat. No. 6,444,043 and U.S. Pat. No. 6,423,565. Within the vapor deposition chamber in a CSS system, the substrate is brought to an opposed position at a relatively small distance (i.e., about 2-3 mm) opposite to a CdTe source. The CdTe material sublimes and deposits onto the surface of the substrate. In the CSS system of U.S. Pat. No. 6,444,043 cited above, the CdTe material is in granular form and is held in a heated receptacle within the vapor deposition chamber. The sublimated material moves through holes in a cover placed over the receptacle and deposits onto the stationary glass surface, which is held at the smallest possible distance (1-2 mm) above the cover frame. The cover is heated to a temperature greater than the receptacle.
While there are advantages to the CSS process, the related system is inherently a batch process wherein the glass substrate is indexed into a vapor deposition chamber, held in the chamber for a finite period of time in which the film layer is formed, and subsequently indexed out of the chamber. The system is more suited for batch processing of relatively small surface area substrates. The process must be periodically interrupted in order to replenish the CdTe source, which is detrimental to a large scale production process. In addition, the deposition process cannot readily be stopped and restarted in a controlled manner, resulting in significant non-utilization (i.e., waste) of the CdTe material during the indexing of the substrates into and out of the chamber, and during any steps needed to position the substrate within the chamber.
Accordingly, there exists an ongoing need in the industry for an improved vapor deposition apparatus and process for economically feasible large scale production of efficient PV modules, particularly CdTe modules.
Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.
An apparatus is generally provided for vapor deposition of a sublimated source material as a thin film on a photovoltaic (PV) module substrate. The apparatus includes at least one receptacle disposed in a deposition head. Each receptacle is configured for receipt of a granular source material (e.g., cadmium telluride). A heating system is configured to heat the receptacle(s) to sublimate the source material. A substantially vertical distribution plate is disposed between the receptacle(s) and a substrate conveyed through the apparatus. The distribution plate is positioned at a defined distance from a vertical conveyance plane of a deposition surface of the substrate. The distribution plate comprises a pattern of passages therethrough that distribute the sublimated source material for deposition onto the deposition surface of the substrate.
Variations and modifications to the embodiments of the vapor deposition apparatus discussed above are within the scope and spirit of the invention and may be further described herein.
A process is also generally provided for vapor deposition of a sublimated source material to form thin film on a photovoltaic (PV) module substrate. According to one embodiment, source material can be supplied to at least one receptacle within a deposition head. Each receptacle can be heated with a heating system to sublimate the source material, and the sublimated source material can be directed through a distribution plate having a substantially vertical orientation. Individual substrates can be conveyed in a substantially vertical arrangement past the distribution plate such that the sublimated source material passing through the distribution plate is deposited onto a deposition surface of the substrates.
Variations and modifications to the embodiment of the vapor deposition process discussed above are within the scope and spirit of the invention and may be further described herein.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims, or may be obvious from the description or claims, or may be learned through practice of the invention.
A full and enabling disclosure of the present invention, including the best mode thereof, is set forth in the specification, which makes reference to the appended drawings, in which:
Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention encompass such modifications and variations as come within the scope of the appended claims and their equivalents.
The thin film may be, for example, a film layer of cadmium telluride (CdTe). As mentioned, it is generally recognized in the art that a “thin” film layer on a PV module substrate is generally less than about 10 microns (μm). It should be appreciated that the present vapor deposition apparatus 100 is not limited to use in the system 10 illustrated in
For reference and an understanding of an environment in which the vapor deposition apparatus 100 may be used, the system 10 of
Referring to
The process chamber 12 also includes a plurality of interconnected cool-down modules 20 downstream of the vapor deposition apparatus 100. The cool-down modules 20 define a cool-down section within the process chamber 12 through which the substrates 14 having the thin film of sublimated source material deposited thereon are conveyed and cooled at a controlled cool-down rate prior to the substrates 14 being removed from the system 10. Each of the modules 20 may include a forced cooling system wherein a cooling medium, such as chilled water, refrigerant, gas, or other medium, is pumped through cooling coils (not illustrated) configured with the modules 20.
In the illustrated embodiment of system 10, at least one post-heat module 22 is located immediately downstream of the vapor deposition apparatus 100 and upstream of the cool-down modules 20 in a conveyance direction of the substrates. The post-heat module 22 maintains a controlled heating profile of the substrate 14 until the entire substrate is moved out of the vapor deposition apparatus 100 to prevent damage to the substrate, such as warping or breaking caused by uncontrolled or drastic thermal stresses. If the leading section of the substrate 14 were allowed to cool at an excessive rate as it exited the apparatus 100, a potentially damaging temperature gradient would be generated longitudinally along the substrate 14. This condition could result in breaking, cracking, or warping of the substrate from thermal stress.
As diagrammatically illustrated in
Still referring to
In operation of the system 10, an operational vacuum is maintained in the process chamber 12 by way of any combination of rough and/or fine vacuum pumps 40. Additionally, one or more process gasses can be added to these chambers to control the atmosphere within. In order to introduce a substrate 14 into the process chamber 12, the load module 28 and buffer module 30 are initially vented (with the valve 34 between the two modules in the open position). The valve 34 between the buffer module 30 and the first heater module 16 is closed. The valve 34 between the load module 28 and load conveyor 26 is opened and a substrate 14 is moved into the load module 28. At this point, the first valve 34 is shut and the rough vacuum pump 32 then draws an initial vacuum in the load module 28 and buffer module 30. The substrate 14 is then conveyed into the buffer module 30, and the valve 34 between the load module 28 and buffer module 30 is closed. The fine vacuum pump 38 then increases the vacuum in the buffer module 30 to approximately the same vacuum in the process chamber 12. In another embodiment, after pumping down the remaining atmosphere to a sufficiently low level so as to not contaminate the process chamber 12, the buffer module 30 is then backfilled with a process gas or mixture of process gases to a pressure matched with that of the vacuum chamber. At this point, the valve 34 between the buffer module 30 and process chamber 12 is opened and the substrate 14 is conveyed into the first heater module 16.
An exit vacuum lock station is configured downstream of the last cool-down module 20, and operates essentially in reverse of the entry vacuum lock station described above. For example, the exit vacuum lock station may include an exit buffer module 42 and a downstream exit lock module 44. Sequentially operated valves 34 are disposed between the buffer module 42 and the last one of the cool-down modules 20, between the buffer module 42 and the exit lock module 44, and between the exit lock module 44 and an exit conveyor 46. A fine vacuum pump 38 is configured with the exit buffer module 42, and a rough vacuum pump 32 is configured with the exit lock module 44. The pumps 32, 38 and valves 34 are sequentially operated to move the substrates 14 out of the process chamber 12 in a step-wise fashion without loss of vacuum condition within the process chamber 12.
System 10 also includes a conveyor system configured to move the substrates 14 into, through, and out of the process chamber 12. In the illustrated embodiment, this conveyor system includes a plurality of individually controlled conveyors 48, with each of the various modules including a respective one of the conveyors 48. It should be appreciated that the type or configuration of the conveyors 48 may vary. In the illustrated embodiment, the conveyors 48 are roller conveyors having rotatably driven rollers that are controlled so as to achieve a desired conveyance rate of the substrates 14 through the respective module and the system 10 overall.
As described, each of the various modules and respective conveyors in the system 10 are independently controlled to perform a particular function. For such control, each of the individual modules may have an associated independent controller 50 configured therewith to control the individual functions of the respective module. The plurality of controllers 50 may, in turn, be in communication with a central system controller 52, as diagrammatically illustrated in
Referring to
Each receptacle 116 is configured for receipt of a granular source material 117. As shown, the three receptacles 116 are aligned substantially vertically within the deposition head 110. This arrangement of the receptacles 116 can allow for a more uniform distribution of the source vapors 119 upon sublimation of the source material 117.
A heating system can be positioned within the deposition head 110 to sublimate the source material 117 within each receptacle 116. As shown, a plurality of heating elements 115 can be utilized in one particular embodiment. In one particular embodiment, a heating element 115 can be positioned in close proximity to each receptacle (e.g., underneath) such that each receptacle 116 is primarily heated via its respective heating element 115. As such, the temperature of each receptacle 116 can be independently controlled by its respective heating element 115. In the illustrated embodiment, at least one thermocouple 122 is operationally positioned to monitor temperature within or near each receptacle 116. This independent control of the heating of each receptacle 116 can help control the vapor pressure of the sublimated source material within the deposition head 110 by allowing for independent adjustment of the temperature of each receptacle 116, and thus the sublimation rate of the source material 117 within each receptacle 116. This independent control of the temperature of each receptacle 116 can help control the vapor pressure of the source vapors within the deposition head 110 and lessen the vapor pressure gradient within the deposition head 110 before passing through the distribution manifold 124 and distribution plate 152.
As mentioned, the granular source material may be supplied by a feed device or system 24 (
Referring to
A distribution manifold 124 is disposed between the receptacles 116. This distribution manifold 124 may take on various configurations within the scope and spirit of the invention, and serves to distribute the sublimated source material that flows from the receptacles 116.
In the illustrated embodiment, the distribution manifold 124 can be heated to inhibit that source vapors from depositing thereon, and may also indirectly heat the receptacles 116. As shown, the distribution manifold 124 has a clam-shell configuration that includes a first shell member 130 closer to the receptacles 116 and a second shell member 132 closer to the substrates 14. Each of the shell members 130, 132 includes recesses therein that define cavities 134 when the shell members are mated together as depicted in
Still referring to
In the illustrated embodiment, a distribution plate 152 is disposed between the distribution manifold 124 at a defined distance from the deposition surface of an underlying substrate 14 (i.e., the surface of the substrate 14 facing the distribution plate 152). This distance may be, for example, between about 0.3 cm to about 4.0 cm. In a particular embodiment, the distance is about 1.0 cm. The conveyance rate of the substrates past the distribution plate 152 may be in the range of, for example, about 10 mm/sec to about 40 mm/sec. In a particular embodiment, this rate may be, for example, about 20 mm/sec. The thickness of the CdTe film layer that deposits onto the deposition surface of the substrate 14 can vary within the scope and spirit of the invention, and may be, for example, between about 1 μm to about 5 μm. In a particular embodiment, the film thickness may be about 1.5 μm to about 4 μm.
The distribution plate 152 includes a pattern of passages, such as holes, slits, and the like, therethrough that further distribute the sublimated source material passing through the distribution manifold 124 such that the source material vapors are uninterrupted in the transverse direction. In other words, the pattern of passages are shaped and staggered or otherwise positioned to ensure that the sublimated source material is deposited completely over the substrate 14 in the transverse direction so that longitudinal streaks or stripes of “un-coated” regions on the substrate are avoided. In one embodiment, the distribution plate 152 can be heated, such as via the distribution manifold 124, to inhibit the source material from depositing on the distribution plate.
As previously mentioned, a significant portion of the sublimated source material will flow out of the receptacles 116 source vapors (depicted by arrows 119). Although these curtains of vapor will diffuse to some extent in the longitudinal direction prior to passing through the distribution plate 152, it should be appreciated that it is unlikely that a uniform distribution of the sublimated source material will be achieved as the vapors pass through the distribution manifold. However, the distribution plate 152 can aid in the further distribution of the source vapors contacting the substrate 14 to ensure substantially uniform deposition of the thin film layer.
As illustrated in the figures, it may be desired to include a debris shield 150 between the receptacle 116 and the distribution manifold 124. This shield 150 includes holes defined therethrough (which may be larger or smaller than the size of the holes of the distribution plate 152) and primarily serves to retain any granular or particulate source material from passing through and potentially interfering with operation of the movable components of the distribution manifold 124. In other words, the debris shield 150 can be configured to act as a breathable screen that inhibits the passage of particles without substantially interfering with vapors flowing through the shield 150. Thus, this shield 150 can protect the distribution manifold 124, the distribution plate 152, and/or the substrate 14 from unvaporized source material that can be in the deposition head 110 (e.g., cracking and/or popping of the source material may occur during sublimation, resulting in unvaporized source material being ejected from the receptacle 116).
A cold trap 153 is positioned under the substrate 14 and within the deposition head 110 to collect errant source vapors 119. As shown, the cold trap 153 is positioned along the lower surface of the deposition head 110. For example, the cold trap 153 can have a trap temperature that is below the sublimation temperature of the source material (e.g., about 0° C. to about 300° C. for CdTe vapors). As such, any errant source vapors that contact the cold trap 153 will plate onto the surface of the cold trap 153. Additionally, the cold trap can collect any particles that fall to the bottom of the chamber. This collected errant source vapors can be recycled as source material for later use. Although shown under only the substrate 14, the cold trap can be extended to cover the entire lower surface of the deposition head 110 in certain embodiments.
Referring to
Referring to
The shutter plate 136 configuration illustrated in
The present invention also encompasses various process embodiments for vapor deposition of a sublimated source material to form a thin film on a PV module substrate. The various processes may be practiced with the system embodiments described above or by any other configuration of suitable system components. It should thus be appreciated that the process embodiments according to the invention are not limited to the system configuration described herein.
In a particular embodiment, the vapor deposition process includes supplying source material to a plurality of receptacles within a deposition head (e.g., vertically arranged receptacles), and heating each receptacle to sublimate the source material. The sublimated source material is directed out of the receptacle and through the distribution plate. Individual substrates are conveyed substantially vertically past the distribution plate. The sublimated source material that passes through the distribution plate and is distributed onto a deposition surface of the substrates.
In yet another unique process embodiment, the passages for the sublimated source material through the heat source may be blocked with an externally actuated blocking mechanism, as discussed above.
Desirably, the process embodiments include continuously conveying the substrates at a constant linear speed during the vapor deposition process.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
