METHOD AND APPARATUS FOR ELECTRODEPOSITING LARGE AREA CADMIUM TELLURIDE THIN FILMS FOR SOLAR MODULE MANUFACTURING
Embodiments of the inventions provide methods and apparatus to electroplate films of tellurides such as CdTe, or its alloys on multiple large area workpieces. In one embodiment a method of forming a solar cell absorber film on multiple work pieces uses a self adjusting mechanism taking advantage of the high resistivity of the solar cell absorber film. Larger deposits of the plating material onto one workpiece, due for example, to non-uniformity of solution flow, results in larger resistance thus decreasing the current flowing through that workpiece. The decreased current then deposits less material over that workpiece. In another embodiment multiple workpieces can be electroplated using a single power supply in a single plating bath.
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This application claims benefit of U.S. Provisional Application No. 61/401,632 filed on Aug. 17, 2010, the contents of which are hereby incorporated by reference in their entirety for all purposes.
FIELD OF THE INVENTIONThe present inventions relate to methods and apparatus for preparing thin films of Group IIB-VIA compound semiconductor films, specifically CdTe films, for radiation detector and photovoltaic applications. Inventions are applicable to large scale deposition of thin films on large substrates for manufacturing thin film solar module.
BACKGROUND OF THE INVENTIONSolar cells and modules are photovoltaic (PV) devices that convert sunlight into electrical power. The most common solar cell material is silicon (Si). However, lower cost PV cells may be fabricated using thin film growth techniques that can deposit solar-cell-quality polycrystalline compound absorber materials on large area substrates using low-cost methods.
Group IIB-VIA compound semiconductors comprising some of the Group IIB (Cd, Zn, Hg) and Group VIA (O, S, Se, Te, Po) materials of the periodic table are excellent absorber materials for thin film solar cell structures. Especially CdTe has proved to be a material that can be used in manufacturing high efficiency solar panels at a cost below $1/W.
In fabricating the “super-strate” structure 10 of
In the “sub-strate” structure 17 of
The CdTe absorber film 14 of
While electrodepositing a CdTe film over a large workpiece, such as a large transparent sheet (such as a glass substrate with dimensions of 60 cm×60 cm or larger) comprising a transparent conductive layer and a junction partner layer, there are voltage drops that need to be taken into consideration. During deposition, the electroplating current flows; i) through an electrical contact made to the transparent conductive layer, ii) through the transparent conductive layer (such as a transparent conductive oxide layer) flowing parallel to its surface, iii) through the junction partner layer (such as a CdS layer) flowing perpendicular to its surface, and iv) through an already deposited portion of the CdTe film, flowing perpendicular to its surface. This is schematically shown in
The electrodeposition system 100 comprises a container 101 holding a plating solution 102 comprising Cd and Te species. The stack comprising the transparent sheet 11, the transparent conductive layer 12, and the junction partner layer 13 is used as the cathode 107, which at the same time is the workpiece. There is a conductive anode 103 across from the cathode 107. Both the anode 103 and the cathode 107 are immersed in the plating solution 102. A voltage “V” is applied between the anode 103 and the cathode 107 through an electrical contact 105 made to the transparent conductive layer 12, using a power supply 104, such that the cathode 107 is made more negative with respect to the anode 103. The electrical contact 105 is protected from the plating solution 102 by a protective seal 106 which may be made of a non-conductive polymeric material. Upon application of the voltage “V” a plating current “I” starts to flow through the conductive wires connecting the power supply 104 to the anode 103 and the cathode 107. The plating current “I” flows from the anode 103 to the cathode 107 through the plating solution 102 while depositing a CdTe film over the junction partner layer 13. The plating current “I” flows through the conductive portions of the cathode 107 in a distributed manner. For example, at or around a location “A” on the cathode 107, the plating current flows through an already deposited portion 14A of the CdTe film, and then it flows through the junction partner layer 13 and into the transparent conductive layer 12. Then it flows horizontally to the electrical contact 105 as shown by arrow 108. On the other hand, at or around a location “B”, which may be farther away from the electrical contact 105, the current flow path within the transparent conductive layer 12 between the location “B” and the electrical contact 105 is much longer corresponding to a larger voltage drop. As a result, the surface potential of the CdTe film 14A at the location “B” is expected to be different (more positive) than it is at the location “A” if the plating current distribution over the surface of the cathode 107 is uniform. It should be noted that the sheet resistance of a typical transparent conductive oxide used in solar cell structure may be in the range of 5-20 ohms per square.
In experimental deposition systems employing small area substrates (such as 10 cm×10 cm substrates) the voltage drops across the substrate do not constitute a big problem and the plating voltage and current values are closely controlled by a power supply, typically employing a reference electrode, assuring that the deposition conditions fall within the region where stoichiometric CdTe is deposited over the whole substrate. In a manufacturing environment, however, where hundreds of large area substrates need to be processed, it may become very expensive to build hundreds of plating systems such as the one shown in
As the above review demonstrates, there is a need to develop methods and apparatus to deposit high quality stoichiometric CdTe layers on multiple large substrates in a cost effective way.
The chemical composition of an electrodeposited CdTe film is a function of its surface potential during the electrodeposition period. Potentials close to or more negative than the deposition potential of Cd yield Cd-rich deposits, whereas potentials close to the deposition potential of Te yield Te-rich compositions. What is required for high efficiency solar cell fabrication is a stoichiometric CdTe layer with a Cd/Te molar ratio of near 1.0, which can be obtained only within a specific voltage range between the deposition potentials of Te and Cd.
In the example of
In general, the present invention forms high quality CdTe thin films on multiple large area substrates using one plating system and one power supply. This is achieved by defining the geometry of the electroplating tool and by carefully selecting the process conditions as will be described below.
Unlike electrodeposited metals, electrodeposited Group IIB-VIA compound layers, such as CdTe layers, have high electrical resistivities. In their as deposited forms CdTe layers may have resistivity values in a range of 104-107 ohm-cm, compared to metal resistivities, which may be in the range of 10-4-10-6 ohm-cm. This means that as a CdTe layer is electrodeposited on a junction partner layer, such as a CdS layer, the voltage drop across the deposited CdTe film increases as the thickness of the CdTe layer increases. Present inventions utilize this fact in a beneficial way to deposit CdTe layers over many workpieces in a single electrodeposition tool.
The workpieces over which CdTe films are electrodeposited have a first dimension which is larger than 50 cm. A second dimension of the workpieces may be larger than or equal to the first dimension, preferably larger than the first dimension. A sketch of an exemplary workpiece 39 comprising a transparent sheet 40, a transparent conductive layer 45 and a junction partner layer 48 is shown in
During process, power is applied between the anode busbar 309 and the cathode busbar 307, initiating cathodic CdTe deposition over the front surfaces of all workpieces at the same time. Power may be applied through application of a controlled voltage or controlled current by a single power supply because the deposition process is self correcting. If for example, the current density increases for a specific workpiece in the group for any reason (such as non-uniformity of solution flow), the thickness of the CdTe film deposited over that workpiece would also increase. As indicated above, the resistivity for deposited CdTe layers is at least about 104 ohm-cm, and typically falls in the range of about 104-107 ohm-cm. Increased CdTe thickness would increase the resistance of the electrical circuit for that specific workpiece. Increased resistance would, in turn, lower the deposition current, therefore self adjusting the process back to normal where all the workpieces receive substantially the same current density, and therefore substantially the same stoichiometric CdTe layer (e.g., same thickness and same compositional constituents). The above mentioned self adjustment mechanism allows use of a single power supply and eliminates the need for employing one power supply for each workpiece and continually monitoring the voltage-current values. It should be noted that if the plated material was a low resistivity metallic film, a thickness change of the deposited film over one specific substrate would not introduce any significant change in resistance and therefore, the self adjustment mechanism would not work.
The preferred conditions of running the process of the present inventions are as follows: If the number of large area workpieces is given by “N”, the length of the short edges of each workpiece (41A and 41B in
In general, electrodepositing CdTe films over multiple large size workpieces, each with a short edge dimension of “W”, a plating current density of less than about 1000/W2 is preferred, wherein W is given in units of centimeters and the current density is given in the units of milliamps per square centimeter (mA/cm2). However, the plating current density should be more than about 300/W2 to avoid excessive time loss during deposition. Accordingly, the preferred current density for processing 20 workpieces, each with a short edge dimension of 80 cm would be less than 1000/6400=0.16 mA/cm2, and more than about 0.05 mA/cm2. The total current applied would be less than (0.16×20×80×L), where L is the dimension of the longer edge of each workpiece.
The techniques described above are also applicable to the formation of Group IIB-VIA absorber layers that include other elements, such as films comprising alloys of CdTe with materials such as Zn, Hg, Mn and Mg. The technique are also applicable for absorber layers containing Te wherein the electroplating solution comprises Te.
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- Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.
Claims
1. A method of forming a solar cell absorber film on multiple work pieces in a self-adjusting manner, each workpiece having a conductive surface, two short edges substantially parallel to each other and two contacting edges substantially parallel to each other, the length of the contacting edges being longer than or equal to the length of the short edges, the method comprising;
- making electrical contacts to the conductive surface of each workpiece along the two contacting edges,
- connecting the electrical contacts of each workpiece to a cathode busbar,
- immersing each workpiece into a solution comprising Te,
- immersing at least one anode into the solution,
- applying a potential difference between the at least one anode and the cathode busbar making the cathode busbar more negatively charged compared to the at least one anode thereby causing a current density to flow through each of the multiple workpieces, wherein
- the solar cell absorber film has a resistivity of at least 104 ohm-cm such that the current density received by each workpiece is self-adjusted by the resistivity, to be substantially the same.
2. The method of claim 1 wherein the conductive surface of each workpiece comprises a transparent conductive layer and a junction partner layer and the solar cell absorber film is formed on the junction partner layer.
3. The method of claim 2 wherein the junction partner layer comprises cadmium sulfide and the solution further comprises Cd.
4. The method of claim 1 wherein the current density is less than about 1000/W2, wherein the current density is given in units of mA/cm2 and W is the length of the short edge of the multiple workpieces given in units of centimeters.
5. The method of claim 4 wherein the number of workpieces is at least 20 and the current density is more than about 300/W2.
6. The method of claim 4 wherein the value of W is at least 50 cm.
7. The method of claim 1 wherein the solution further comprises at least one of Zn, Hg, Mn and Mg.
8. The method of claim 3 wherein the solution further comprises at least one of Zn, Hg, Mn and Mg.
9. The method of claim 1 wherein the size of a plurality of the workpieces is such that their length and width are each greater than or equal to 60 cm.
10. An electrodeposition system to form a CdTe containing film over multiple workpieces, each workpiece having a conductive surface, two short edges substantially parallel to each other and two contacting edges substantially parallel to each other, the system comprising;
- a tank for holding a solution comprising Cd and Te,
- electrical contact strips configured to make contact to the conductive surface of each workpiece along the two contacting edges,
- a cathode busbar for connecting to the electrical contact strips,
- at least one anode configured for immersion into the tank, and
- a single power supply configured to apply a potential difference between the at least one anode and the cathode busbar.
Type: Application
Filed: Aug 17, 2011
Publication Date: Feb 23, 2012
Applicant: Encoresolar, Inc. (Fremont, CA)
Inventor: Bulent M. Basol (Manhattan Beach, CA)
Application Number: 13/211,619
International Classification: C25D 3/56 (20060101); C25D 17/00 (20060101);