Patent Application
20140024172
The subject matter disclosed herein relates generally to methods and systems for depositing thin films during manufacture of cadmium telluride photovoltaic devices. More particularly, the subject matter disclosed herein relates generally to integrated systems for the deposition of a cadmium telluride layer and subsequent cadmium chloride treatment during manufacture of cadmium telluride photovoltaic devices, and their methods of use.
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 to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy 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 cloudy conditions as compared to other conventional materials.
The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer). Free carrier pairs are created by light energy and then separated by the p-n heterojunction to produce an electrical current.
During the production of CdTe PV modules, the surface of the CdTe PV module is typically cooled, transported to a subsequent treatment apparatus for cadmium chloride treatment (e.g., a cadmium chloride wash), and then subsequently annealed. This process of heating, cooling, and re-heating is inefficient in both energy consumption and cost. Additionally, the cadmium telluride layer is exposed to the environment during transport to the subsequent treatment apparatus. Such exposure can result in the introduction of additional atmospheric materials into the cadmium telluride layer, which can lead to the introduction of impurities in the CdTe PV module. Additionally, the room atmosphere naturally varies over time, adding a variable to a large-scale manufacturing process of the CdTe PV modules. Such impurities and additional variables can lead to inconsistent CdTe PV modules from the same manufacturing line and process.
Thus, a need exists for methods and systems for reducing the introduction of impurities and additional variables into a large-scale manufacturing process of making the CdTe PV modules, as well as increasing the energy efficiency of the process.
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.
Apparatus is generally provided for vapor deposition of a sublimated source material as a thin film on discrete photovoltaic module substrates conveyed in a continuous non-stop manner therethrough. In one embodiment, the apparatus can include a deposition head with a first sublimation compartment and a second sublimation compartment. The first sublimation compartment can be configured for receipt and sublimation of a first source material, and the second sublimation compartment can be configured for receipt and sublimation of a second source material. A first distribution plate can be positioned at a first defined distance above a horizontal conveyance plane of an upper surface of substrates conveyed through a first deposition area of the apparatus. The first distribution plate is generally positioned between the first sublimation compartment and the horizontal conveyance plane so as to define the first deposition area. A second distribution plate can be positioned at a second defined distance above a horizontal conveyance plane of an upper surface of substrates conveyed through a second deposition area of said apparatus. The first distribution plate is generally positioned between the first sublimation compartment and the horizontal conveyance plane so as to define the second deposition area.
In one embodiment, the first defined distance and the second defined distance can be substantially identical.
The first sublimation compartment and the second sublimation compartment can be isolated from each other such that the sublimated first source material is substantially prevented from mixing with the sublimated second source material, at least during sublimation (e.g., prior to passing through the respective distribution plate and/or within the first deposition area and the second deposition area). For example, in one embodiment, a seal member can be positioned between the first distribution plate and a second distribution plate such that source vapors are substantially prevented from mixing between the first deposition area and the second deposition area. Althernatively, the first deposition area and the second deposition area can define a single continuous deposition area.
Methods are also generally provided for depositing a thin film on a substrate. For example, a first source material can be heated in a first receptacle positioned within a first chamber of a deposition head to form first source vapors, which can then be directed through a first distribution plate. A second source material can be heated in a second receptacle positioned within a second chamber of the deposition head to form second source vapors, which can be directed through the distribution plate. substrate can be transported past the distribution plate such that a first majority of the first source vapors deposit on a deposition surface of the substrate prior to a second majority of the second source vapors.
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.
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”).
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 load vacuum station 11, the load vacuum chamber 28 and load buffer chamber 30 are initially vented (with the valve 34 between the two modules in the open position). The valve 34 between the load buffer chamber 30 and the first heater module 16 is closed. The valve 34 between the load vacuum chamber 28 and load conveyor 26 is opened, and a substrate 14 is moved into the load vacuum chamber 28. At this point, the first valve 34 is shut, and the rough vacuum pump 32 then draws an initial vacuum in the load vacuum chamber 28 and load buffer chamber 30. The substrate 14 is then conveyed into the load buffer chamber 30, and the valve 34 between the load vacuum chamber 28 and load buffer chamber 30 is closed. The fine vacuum pump 38 then increases the vacuum in the load buffer chamber 30 to approximately the same vacuum in the vacuum chamber 12 and the heating station 13. At this point, the valve 34 between the load buffer chamber 30 and heating station 13 is opened, and the substrate 14 is conveyed into the first heater module 16.
Thus, the substrates 14 are transported into the exemplary system 10 first through the load vacuum chamber 28 that draws a vacuum in the load vacuum chamber 28 to an initial load pressure. For example, the initial load pressure can be less than about 250 mTorr, such as about 1 mTorr to about 100 mTorr. Optionally, a load buffer chamber can reduce the pressure to about 1×10−7 Torr to about 1×10−4 Ton, and then backfilled with an inert gas (e.g., argon) in a subsequent chamber within the system 10 to a deposition pressure (e.g., about 10 mTorr to about 100 mTorr).
The substrates 14 can then be transported into and through a heating station 13 including heating chambers 16. The plurality of heating chambers 16 define a pre-heat section 13 of the system 10 through which the substrates 14 are conveyed and heated to a first deposition temperature before being conveyed into the vapor deposition chamber 19. Each of the heating chambers 16 may include a plurality of independently controlled heaters 18, with the heaters defining a plurality of different heat zones. A particular heat zone may include more than one heater 18. The heating chambers 16 can heat the substrates 14 to a deposition temperature, such as about 350° C. to about 600° C. Although shown with four heating chambers 16, any suitable number of heating chambers 16 can be utilized in the system 10.
The substrates 14 can then be transferred into and through the vapor deposition chamber 19 for deposition of a thin film onto the substrates 14 and subsequent treatment thereof. For example, the film can be a cadmium telluride thin film, and the treatment can be performed using cadmium chloride. The vapor deposition chamber 19 can include the deposition apparatus 80, such as shown in
After deposition and treatment in the vapor deposition chamber 19, the substrates 14 can be transported into and through a post-heat chamber 22, an optional annealing chamber 23, and a series of cooling chambers 20. In the illustrated embodiment of system 10, at least one post-heat chamber 22 is located immediately downstream of the vapor deposition apparatus 19. The post-heat chamber 22 maintains a controlled heating profile of the substrate 14 until the entire substrate is moved out of the vapor deposition chamber 19, in order to prevent damage to the substrate 14, such as warping or breaking caused by uncontrolled or drastic thermal stresses. If, for example, the leading section of the substrate 14 were allowed to cool at an excessive rate as it exited the apparatus 19, 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.
In certain embodiments, the anneal chamber 23 (or a series of annealing chambers) can be present to further heat the substrates 14 sufficient to anneal the deposited material thereon. For example, the substrates 14 can be annealed in the anneal chamber 23 by heating, in certain embodiments, to an anneal temperature of about 500° C. to about 800° C. after treatment of the deposited cadmium telluride layer formed using the first sublimation compartment 100 by deposition with the cadmium chloride vapors in the second sublimation compartment 200.
A cool-down chamber(s) 20 is positioned downstream of the vapor deposition chamber. The cool-down chamber 20 allows the substrates 14 having the treated thin film to be conveyed and cooled at a controlled cool-down rate prior to the substrates 14 being removed from the system 10. The cool down chamber 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 chamber 20. In other embodiments, a plurality of cool down chambers 20 can be utilized in the system 10.
An exit vacuum lock station 15 is configured downstream of the cool-down chamber 20, and operates essentially in reverse of the entry vacuum lock station 11 described above. For example, the exit vacuum lock station 15 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 47. 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 system 10 in a step-wise fashion without loss of vacuum condition within the system 10.
System 10 also includes a conveyor system 46 configured to move the substrates 14 into, through, and out of each of load vacuum station 12, the pre-heating station 13, the vapor deposition chamber 19, the post-heat chamber 22, and the cooling chambers 20. In the illustrated embodiment, this conveyor system 46 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
Referring to the first sublimation compartment 100, receptacle 102 is configured for receipt of a source material (not shown). As mentioned, the source material may be supplied by a first feed device 24 via a feed tube 104 (
The deposition head 82 also includes oppositely positioned lateral end walls 86, 87 and oppositely positioned longitudinal side walls 88, 89 (
Referring to
A first heated distribution manifold 120 is disposed below the first receptacle 102. This distribution manifold 120 may take on various configurations within the scope and spirit of the invention, and serves to indirectly heat the first receptacle 102, as well as to distribute the sublimated source material that flows from the first receptacle 102. In the illustrated embodiment, the heated distribution manifold 120 has a clam-shell configuration that includes an upper shell member 122 and a lower shell member 124. Each of the shell members 122, 124 includes recesses therein that define cavities 126 when the shell members are mated together, as depicted in
Still referring to
Similar to the discussion above, with respect to the first sublimation compartment 100, the second sublimation compartment 200 includes a second receptacle 202 configured for receipt of a second source material (not shown). As mentioned, the second source material may be supplied by a second feed device 25 via a second feed tube 204 (
The second receptacle 202 within the second sublimation compartment 200 has a shape and configuration such that the transversely extending end walls 212, 213 of the receptacle 202 are spaced from the internal lateral wall 90 and the lateral end wall 87, respectively. The side walls 214, 215 are lie adjacent to and in close proximity to the longitudinal side walls 88, 89, respectively, of the second sublimation compartment 200 so that very little clearance exists between the respective walls, as depicted in
A second heated distribution manifold 220 is disposed below the second receptacle 202. Similarly to the first distribution manifold 120, this second distribution manifold 220 may take on various configurations within the scope and spirit of the invention, and serves to indirectly heat the second receptacle 202, as well as to distribute the sublimated source material that flows from the second receptacle 202. In the illustrated embodiment, the heated distribution manifold 220 has a clam-shell configuration that includes an upper shell member 222 and a lower shell member 224. Each of the shell members 222, 224 includes recesses therein that define cavities 226 when the shell members are mated together as depicted in
Since the first heated distribution manifold 120 can be separate from the second heated distribution manifold 220, as shown in the embodiment of
In the illustrated embodiment, first and second distribution plates 130, 230 are disposed below the first and second sublimation compartments, respectively. These distribution plates 130, 230 are positioned at a defined distance above a horizontal plane of the upper surface of an underlying substrate 14, as depicted in
The first and second distribution plates 130, 230 include a pattern of passages 132, 232, respectively, such as holes, slits, and the like, therethrough that further distribute the sublimated source material passing through the distribution manifolds 120, 220 such that the source material vapors are substantially uninterrupted. In other words, the pattern of passages 132, 232 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 receptacles 102, 202 as leading and trailing curtains of vapor in both the first and second sublimation compartments 100, 200, as depicted in
As illustrated in the figures, it may be desired to include debris shields 150, 250 between the receptacles 102, 202 and the distribution manifolds 120, 220, respectively. These debris shields 150, 250 includes holes 152, 252 defined therethrough (which may be larger or smaller than the size of the holes 132, 232 of the distribution plates 130, 230) 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 manifolds 120, 220, as discussed in greater detail below. In other words, the debris shields 150, 250 can be configured to act as a breathable screen that inhibits the passage of particles without substantially interfering with vapors flowing therethrough.
Referring to
In addition, an optional middle seal 157 can be positioned between the first distribution plate 130 and the second distribution plate 230 to define a separation slot 159. Like end seals 154, the optional middle seal 157 can be disposed at a distance above the horizontal conveyance plane defined by the upper surface of the substrates 14 that is less than the distance between the horizontal conveyance plane defined by the substrates 14 and the distribution plate 130, 230, as is depicted in
In an alternative embodiment, the sublimated material may be allowed to intermix in the deposition area above the substrates 14. In other words, the sublimated vapor material is, in such an embodiment, allowed to intermix within a single, continuous deposition area defined under the first distribution plate 130 and the second distribution plate 230. However, such intermixing may be somewhat controlled by varying the distance between the distribution plates 130, 230 and the horizontal conveyance plane defined by the upper surface of the substrates 14. For instance, if the distance between the distribution plates 130, 230 and the horizontal conveyance plane defined by the upper surface of the substrates 14 is relatively small, then little intermixing will be realized in practice due to the tendency of the sublimated source vapors to deposit on the substrates 14 relatively quickly. As such, increasing the distance between the distribution plates 130, 230 and the horizontal conveyance plane defined by the upper surface of the substrates 14 can result in more intermixing.
As such, in one embodiment, the distribution plates 130, 230 can define a single distribution plate defining holes therethrough, and positioned such that the first source vapors from the first sublimation compartment 100 and the second source vapors from the second sublimation compartment 200 pass through the distribution plate 130, 230. As the substrates 14 are conveyed past the deposition head 80, a first majority of the first source vapors can deposit on the deposition surface of the substrate 14 prior to a second majority of the second source vapors.
Any manner of longitudinally extending seal structures 155 may also be configured with the apparatus 80 to provide a seal along the longitudinal sides thereof. Referring to
Referring to
In the embodiment shown, the shutter plates 136, 236 can be independently moved between the first and second operational positions. That is, the flow of sublimation material from either of the first sublimation compartment 100 and the second sublimation compartment 200 can be controlled, regardless of the operational position of the other compartment.
The shutter plate 136, 236 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, and subsequent vapor treatment. 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.
For example, the method for depositing a thin film on a substrate can include heating a first source material in a first receptacle positioned within a first chamber of a deposition head to form first source vapors, and directing the first source vapors through a distribution plate. A second source material can also be heated in a second receptacle positioned within a second chamber of the deposition head to form second source vapors, which can then be directed through the distribution plate. A substrate can be transported past the distribution plate such that a first majority of the first source vapors deposit on a deposition surface of the substrate prior to a second majority of the second source vapors.
In one particular embodiment of the method, the first source material can include cadmium telluride, and the second source material can include telluride. For example, the second source material can include telluride with a reduced stoichiometric amount (than CdTe), or substantially free from, cadmium. In one embodiment, the second source material can consist essentially of (e.g., consist of) telluride. As such, the second source material depositing onto the CdTe layer (formed from the deposited first source material) can be telluride-rich.
Alternatively, in one embodiment of the method, the first source material can include cadmium telluride, and the second source material can include cadmium chloride. As such, the second source material depositing onto the CdTe layer (formed from the deposited first source material) can treat the CdTe layer with cadmium chloride.
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.
