Patent Application
20120052617
The subject matter disclosed herein relates generally to the field of thin film deposition processes wherein a doped 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 doped 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.
In accordance with an embodiment of the invention, an apparatus is provided for vapor deposition of a sublimated source material, such as CdTe, as a thin film on a photovoltaic (PV) module substrate. Although the invention is not limited to any particular film thickness, a “thin” film layer is generally recognized in the art as less than 10 microns (μm). The apparatus includes a deposition head, and a receptacle disposed in the deposition head. A first feed tube is configured to supply a source material (e.g., a granular CdTe material) into the deposition head, and a second feed tube is configured to supply a dopant material into the deposition head as a fluid. In one embodiment, a nozzle can be attached to the second feed tube for supplying the dopant material to the deposition head as a fluid. The receptacle is configured for receipt of the source material. A heated distribution manifold is disposed below the receptacle and includes a plurality of passages defined therethrough. The receptacle is indirectly heated by the distribution manifold to a temperature effective for sublimating the source material within the receptacle. The sublimated source material flows out of the receptacle and downward in the head chamber through the passages in the distribution manifold.
In a particular embodiment, a distribution plate is disposed below the distribution manifold and is at a defined distance above a horizontal plane of an upper surface of a substrate conveyed through the apparatus. The distribution plate includes a pattern of holes therethrough that further distribute the sublimated source material such that the material is deposited as a thin film having a substantially uniform desired thickness on the upper surface of the substrates. The substrates may be continuously conveyed at a constant (i.e., non-stop) linear conveyance rate through the apparatus.
In yet another embodiment, an apparatus for vapor deposition of a sublimated source material as a thin film on a photovoltaic (PV) module substrate includes a deposition head, and a receptacle disposed in an upper section of the deposition head for receipt of a source material. A first feed tube is configured to supply the source material (e.g., a granular CdTe material) into the deposition head, and a second feed tube is configured to supply a dopant material into the deposition head. A heated distribution manifold is disposed below the receptacle and includes a plurality of passages defined therethrough. The receptacle is indirectly heated by the distribution manifold to a degree sufficient to sublimate the source material within the receptacle. In a particular embodiment, the receptacle includes transversely extending end walls that are spaced from the walls of the deposition head a distance such that the sublimated source material flows primarily out and over the end walls of the receptacle and downwardly as transverse leading and trailing curtains towards and through the distribution manifold. The curtains of sublimated source material may be further distributed in the transverse and, to some extent, in the longitudinal directions before being deposited onto the upper surface of substrates conveyed through the apparatus. The substrates may be conveyed at a constant linear conveyance rate through the apparatus.
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.
In still another aspect, the invention encompasses a process for vapor deposition of a sublimated source material, such as CdTe, as a thin film on a photovoltaic (PV) module substrate. The process includes supplying source material to a receptacle within a deposition head, and supplying a dopant material into the deposition head as a fluid. The receptacle can be indirectly heated with a heat source member disposed below the receptacle to sublimate the source material. The sublimated source material is directed downwardly within the deposition head through the heat source. Individual substrates are conveyed below the heat source, and the sublimated source material that passes through the heat source is deposited onto an upper surface of the substrates such that leading and trailing sections of the substrates in a direction of conveyance are exposed to the same vapor deposition conditions in the head chamber to achieve a substantially uniform thickness of the thin film layer on the upper surface of the substrates. The substrates may be conveyed at a constant linear rate through the apparatus, with the sublimated source material being directed from the receptacle primarily as transversely extending leading and trailing curtains relative to the conveyance direction of the substrates. The curtains of sublimated source material may be further distributed transversely and, to some extent, longitudinally relative to the conveyance direction of the substrates after passing through the heat source member by, for example, being directed through a distribution plate disposed below the heat source member.
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.
Chemical elements are discussed in the present disclosure using their common chemical abbreviation, such as commonly found on a periodic table of elements. For example, hydrogen is represented by its common chemical abbreviation H; helium is represented by its common chemical abbreviation He; and so forth.
In the present disclosure, when a layer is being described as “on” or “over” another layer or substrate, it is to be understood that the layers can either be directly contacting each other or have another layer or feature between the layers. Thus, these terms are simply describing the relative position of the layers to each other and do not necessarily mean “on top of” since the relative position above or below depends upon the orientation of the device to the viewer.
Additionally, although the invention is not limited to any particular film thickness, the term “thin” describing any film layers of the photovoltaic device generally refers to the film layer having a thickness less than about 10 micrometers (“microns” or “μm”).
In addition to the source material for the thin film, a dopant or mixture of dopants (collectively referred to as “dopant(s)”) can be co-deposited on the substrate within the vapor deposition apparatus 100. As used herein, a “dopant” is an impurity element that is inserted into the thin film (in very low concentrations) in order to alter the electrical properties or the optical properties of the thin film. For instance, the atoms of the dopant can take the place of elements that were in the crystal lattice of the thin film. For example, using the proper types and amounts of dopant(s) in thin film semiconductors can produce p-type semiconductors and n-type semiconductors. In certain embodiments, the dopant(s) can be included in the thin film in trace concentrations, such as about 0.1 atomic parts per million (at ppm) to about 1,000 at ppm (e.g., about 1 at ppm to about 750 at ppm).
When the thin film is deposited from a source material of cadmium telluride (i.e., a cadmium telluride thin film layer) in the manufacture of a cadmium telluride thin film PV device, suitable dopants can include, but are not limited to, B, Al, Ga, In, Sc, Y, Cu, Au, N, As, P, Sb, Bi, Cl, F, Br, Li, Na, K, compound containing those elements, and mixtures thereof. In one particular embodiment, the cadmium telluride layer can include a p-type dopant(s), such as elemental Cu, Au, N, As, P, Sb, Bi, Cl, F, Br, Li, Na, K, compound containing those elements, or mixtures thereof.
In particular embodiments, the dopant material can include Cl, N, O, or mixtures thereof. However, other dopants or combination of dopants can be included as desired.
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 vacuum 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 vacuum 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
In addition, a second feed device 25 is configured with the vapor deposition apparatus 100 to supply dopant(s) material for including within the thin film on the substrate 14. The second feed device 25 may take on various configurations within the scope and spirit of the invention, and functions to supply the dopant(s) material without interrupting the continuous vapor deposition process within the apparatus 100 or conveyance of the substrates 14 through the apparatus 100.
Still referring to
In operation of the system 10, an operational vacuum is maintained in the vacuum chamber 12 by way of any combination of rough and/or fine vacuum pumps 40. In order to introduce a substrate 14 into the vacuum 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 vacuum chamber 12. At this point, the valve 34 between the buffer module 30 and vacuum 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 vacuum chamber 12 in a step-wise fashion without loss of vacuum condition within the vacuum chamber 12.
System 10 also includes a conveyor system configured to move the substrates 14 into, through, and out of the vacuum 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
While the first feed tube 148 is utilized to supply the source material to the receptacle 116, a second feed tube 151 is connected to a nozzle 153 for supplying the dopant material to the receptacle 116 as a fluid (i.e., a liquid and/or gas). In the embodiment shown in
In an alternative embodiment, the vapor deposition apparatus 100 shown in
In these embodiments, the dopant material may be a fluid (i.e., in a liquid or gaseous state) at room temperature and supplied to the deposition apparatus 100 without prior heating. For example, particularly suitable compounds that can be supplied as a fluid include but are not limited to HCl, Cl2, N2O, NH3, O2, NH4OH, NH4Cl, Br, Hg, a solution of CdCl2, or mixtures thereof. Alternatively, the dopant material can be heated or otherwise vaporized prior to supplying to the deposition apparatus 100.
The dopant material can be supplied in a pure form, or with a carrier fluid (e.g., a carrier liquid or carrier gas). In one particular embodiment, the carrier fluid is an inert material that will not affect the properties of the thin film layer formed on the substrate, and tends not to deposit into the layer. For example, when supplied as a liquid, the dopant material can be supplied in a carrier liquid, such as water, methanol etc., or mixtures thereof. Likewise, when supplied as a gas, the dopant material can be supplied in a carrier gas, such as an inert gas (e.g., argon), N2, O2, etc., or mixtures thereof.
In the illustrated embodiments, at least one thermocouple 122 is operationally disposed through the top wall 114 of the deposition head 110 to monitor temperature within the deposition head 110 adjacent to or in the receptacle 116.
The deposition head 110 also includes longitudinal end walls 112 and side walls 113 (
A heated distribution manifold 124 is disposed below the receptacle 116. This distribution manifold 124 may take on various configurations within the scope and spirit of the invention, and serves to indirectly heat the receptacle 116, as well as to distribute the sublimated source material that flows from the receptacle 116. In the illustrated embodiment, the heated distribution manifold 124 has a clam-shell configuration that includes an upper shell member 130 and a lower shell member 132. 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 below the distribution manifold 124 at a defined distance above a horizontal plane of the upper surface of an underlying substrate 14, as depicted in
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 in the transverse direction so that longitudinal streaks or stripes of “un-coated” regions on the substrate are avoided.
As previously mentioned, a significant portion of the sublimated source material will flow out of the receptacle 116 as leading and trailing curtains of vapor, as depicted in
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, as discussed in greater detail below. 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.
Referring to
Any manner of longitudinally extending seal structure 155 may also be configured with the apparatus 100 to provide a seal along the longitudinal sides thereof. Referring to
Referring to
The shutter plate 136 configuration illustrated in
Referring to
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 receptacle within a deposition head, and indirectly heating the receptacle with a heat source member to sublimate the source material. The sublimated source material is directed out of the receptacle and downwardly within the deposition head through the heat source member. Individual substrates are conveyed below the heat source member. The sublimated source material that passes through the heat source is distributed onto an upper surface of the substrates such that leading and trailing sections of the substrates in the direction of conveyance thereof are exposed to the same vapor deposition conditions so as to achieve a desired uniform thickness of the thin film layer on the upper surface of the substrates.
In a unique process embodiment, the sublimated source material is directed from the receptacle primarily as transversely extending leading and trailing curtains relative to the conveyance direction of the substrates. The curtains of sublimated source material are directed downwardly through the heat source member towards the upper surface of the substrates. These leading and trailing curtains of sublimated source material may be longitudinally distributed to some extent relative to the conveyance direction of the substrates after passing through the heat source member.
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.
