The present invention relates to methods and apparatus for preparing thin films of Group IBIIIAVIA compound semiconductor films for radiation detector and photovoltaic applications.
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in
In a thin film solar cell employing a Group IBIIIAVIA compound absorber, the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se)2 absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good device performance Cu/(In+Ga) molar ratio is kept at around or below 10.0. As the Ga/(Ga+In) molar ratio increases, on the other hand, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current typically may decrease. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition. It should be noted that although the chemical formula is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
The first technique used to grow Cu(In,Ga)Se2 layers was the co-evaporation approach which involves evaporation of Cu, In, Ga and Se from separate evaporation boats onto a heated substrate, as the deposition rate of each component is carefully monitored and controlled.
Another technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where at least two of the components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CuInSe2 growth, thin sub-layers of Cu and In are first deposited on a substrate to form a precursor layer and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere contains sulfur, then a CuIn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer, i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of a Cu(In,Ga)(S,Se)2 absorber. Other prior-art techniques include deposition of Cu—Se/In—Se, Cu—Se/Ga—Se, or Cu—Se/In—Se/Ga—Se stacks and their reaction to form the compound. Mixed precursor stacks comprising compound and elemental sub-layers, such as a Cu/In—Se stack or a Cu/In—Se/Ga—Se stack, have also been used, where In—Se and Ga—Se represent selenides of In and Ga, respectively.
Sputtering and evaporation techniques have been used in prior art approaches to deposit the sub-layers containing the Group IB and Group IIIA components of metallic precursor stacks. In the case of CuInSe2 growth, for example, Cu and In sub-layers were sequentially sputter-deposited from Cu and In targets on a substrate and then the stacked precursor film thus obtained was heated in the presence of gas containing Se at elevated temperatures as described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositing a stacked precursor film comprising a Cu—Ga alloy sub-layer and an In sub-layer to form a Cu—Ga/In stack on a metallic back electrode and then reacting this precursor stack film with one of Se and S to form the compound absorber layer. U.S. Pat. No. 6,092,669 described sputtering-based equipment and method for producing such absorber layers.
One prior art method described in U.S. Pat. No. 4,581,108 utilizes an electrodeposition approach for metallic precursor preparation. In this method a Cu sub-layer is first electrodeposited on a substrate. This is then followed by electrodeposition of an In sub-layer and heating of the deposited Cu/In precursor stack in a reactive atmosphere containing Se. This technique was found to require very high plating current densities resulting in non-uniformities and problems of adhesion to the substrate as discussed in reference publications (Kapur et al., “Low Cost Thin Film Chalcopyrite Solar Cells, Proceedings of 18th IEEE Photovoltaic Specialists Conf., 1985, p. 1429; “Low Cost Methods for the Production of Semiconductor Films for CIS/CdS Solar Cells”, Solar Cells, vol. 21, p. 65, 1987).
As the brief review above demonstrates there is still a need to develop high-throughput, low cost techniques to manufacture thin film solar cells and modules.
The present invention provides a roll to roll system to form solar cell absorbers by continuously processing a surface of a flexible foil as the flexible foil is advanced through processing units of the roll to roll system.
An aspect of the present invention provides a system for forming an absorber structure for solar cells on a front surface of a continuous flexible workpiece as the continuous flexible workpiece is advanced through units of the system. The system includes a conditioning unit to condition the front surface of the continuous flexible workpiece to form activated surface portions.
The system further includes a first electroplating unit to form a first layer of a precursor stack by electroplating a metal belonging to one of Group IB and Group IIIA of the periodic table on an activated surface portion of the continuous flexible workpiece as the continuous flexible workpiece is advanced through the first electroplating station. A first cleaning unit of the system is to clean the first layer deposited in the first electroplating unit.
The system further includes a second electroplating unit to form a second layer of the precursor stack by electroplating a metal belonging to one of Group IB and Group IIIA of the periodic table onto the first layer as the continuous flexible foil is advanced through the first and the second electroplating units and while the first layer is continued to be electroplated onto a following activated surface portion of the surface of the continuous flexible foil in the first electroplating unit. The first layer is different from the second layer. A second cleaning unit of the system is to clean the second layer deposited in the second electroplating unit.
The system further includes a third electroplating unit to form a third layer by electroplating a metal belonging to one of Group IB and Group IIIA of the periodic table onto the second layer to complete the precursor stack as the flexible foil is advanced through the first, second and third electroplating stations and while the second layer is continued to be electroplated in the second electroplating station on the first layer that is electroplated on the following activated portion of the surface of the flexible foil, and while the first layer is continued to be electroplated onto another following activated portion of the surface of the flexible foil in the first electroplating station. The third layer is different from the first and second layers. The system further includes a moving assembly to hold and linearly move the continuous flexible workpiece through the units of the system, wherein the moving assembly comprises a feed spool to unwrap and feed unprocessed portions of the continuous flexible workpiece into the system and a take-up spool to receive the processed portions and wrap them around.
Present invention provides a low-cost, high throughput two-stage process for fabrication of CIGS(S) type absorber layers for manufacturing of solar cells.
Flow chart 100 shown in
It should be noted that the surface activation step is very important because electrodeposition efficiency on a surface depends on the nature of that surface on which a material is deposited. An activated surface is a material surface that is electrochemically active and can be electroplated with efficiency. If the surface is electrochemically passive, electrodeposition efficiency is generally low and adhesion is poor. However, on an active, or activated, surface electrodeposition efficiency is higher and more consistent. Consistent electrodeposition efficiency yields consistent thickness for the electrodeposited material. In the present invention CIGS type absorber layers are formed employing precursor stacks such as Cu/Ga/In or Cu/Ga/Cu/In stacks. The thicknesses of the layers within the stack need to be tightly controlled to be able to control the Cu/(In+Ga) and Ga/(In+Ga) molar ratios which are typically below 1 and which are important for the quality of the resulting absorbers and the performance of solar cells fabricated on such absorbers. A typical target ratio for Cu/(In+Ga) may be in the range of 0.8-0.95. In a roll-to-roll system the contact layer on which a first layer such as a Cu layer would be deposited may be exposed to the atmosphere for different periods of time depending on the location on the roll. For example, in a roll that may be 5000 ft long, the contact layer at the beginning of the roll may be coated with Cu within a few minutes whereas a portion of the contact layer at the end of the roll may be coated after 41 hours if the continuous flexible workpiece moves at a rate of 2 ft/minute. Such variation in exposure of the contact layer to atmosphere may induce differences in the condition of the contact layer surface due to oxidation, exposure to chemical fumes etc. Plating efficiency of the Cu layer on the contact layer may then be different on portions of the contact layer at the beginning of the roll and at the end of the roll. Such differences in efficiency, in turn, cause differences in the thickness of the Cu layer throughout the flexible workpiece and thus cause a change in the Cu/(In+Ga) molar ratio. As a result, process yields are reduced, and manufacturability of high efficiency solar cells at high yields cannot be achieved. By employing an activation chamber and activation process step before the electrodeposition of the first layer on the contact layer, consistence of electrodeposition efficiency of the first layer on the contact layer is assured throughout the roll and yield for consistent Cu/(In+Ga) ratio is assured.
Conditioning process of the present invention results in an electroplating efficiency of more than 90% when a subsequent electroplating process is performed and the first metal layer such as a copper layer is electroplated onto the activated surface. For example, an activated surface formed on the contact layer by a cathodic conditioning process provides more than 90% electroplating efficiency for the subsequent electroplating process, such as copper electroplating. However, if the surface is electrochemically passive, the electroplating efficiency is low, less than 90%, maybe even as low as 20-50%.
Boxes 104 through 108 show a process sequence to form a precursor stack of the present invention. As shown in box 104, in a first electrodeposition step, a Group IB material such as copper, may be electrodeposited on the conditioned and cleaned surface of the contact layer. This step is followed by a cleaning step to clean the surface of the electrodeposited Group IB material, box 105. As shown in box 106, in a second electrodeposition step, a first Group IIIA material, such as gallium, may be electrodeposited on the surface of the cleaned Group IB material layer. This step is followed by a cleaning step to clean the surface of the electrodeposited first Group IIIA material layer, box 107. As shown in box 108, in a third electrodeposition step, a second Group IIIA material, such as indium, may be electrodeposited on the surface of the cleaned first Group IIIA material layer, which completes the precursor stack. The precursor stack may be cleaned and dried following step, box 109. The precursor stack may be reacted in presence of Group VIA materials, such as selenium and sulfur with gas phase delivery, to form an absorber, box 10.
Alternately, the precursor layer in box 108 may be just cleaned without drying, as shown in box 111, to electrodeposit a Group VIA material onto the precursor stack as shown in box 112. Following the electrodeposition process, the precursor stack with the Group VIA layer is cleaned, box 113, and reacted to form an absorber, box 114. During the reaction, optionally, additional Group VIA materials may be introduced to the forming absorber.
The roll-to-roll processing approach of the present invention offers several advantages. Electrodeposition is a surface sensitive process. Defects in electrodeposited layers mostly originate from the surface they are plated on. Therefore, it is preferable to minimize handling of substrates in an electroplating approach. Surfaces to be plated need to be protected from physical contact, particles etc. that may later cause defectivity in the films deposited on such surfaces. Plating efficiency and the thickness uniformity of electroplated layers are also affected by the condition of the surface they are plated on. For example, electrodeposition of Cu, Ga or In on a chemically active, fresh surface is a much more repeatable process compared to electrodeposition on a surface that may be exposed to air, chemical vapors or, in general, to outside environment for varying amounts of time. In a roll-to-roll process all depositions are done in a controlled environment (enclosure for the roll not shown in figures) and the time between depositions are minimized unlike a batch process that requires several loading and unloading steps to deposit a stack of materials on a base. In the present roll-to-roll process a material, such as Cu is plated on a section of the base. The surface of this plated material is fresh and active after plating and after the water rinse step. Therefore, when section moves into the next plating bath, for example a Ga or In plating bath, within a few seconds or minutes, deposition initiates on this active surface. If the velocity of the foil base is constant, then the Ga or In plating always operates on the same Cu surface in terms of activity. This provides highly repeatable results in terms of thickness and uniformity of the In and Ga layers. Same is true for the Cu layer also.
If the Cu layer is first to be deposited on the flexible foil base, the surface of the flexible foil base may first be activated by passing it through a pre-deposition electrolyte and applying a pre-deposition process step or conditioning to the surface. The predeposition process step may be an etching step or an electrotreating step such as a cathodic conditioning step comprising applying a cathodic voltage to the base with respect to an electrode in the pre-deposition electrolyte or an anodic conditioning step comprising applying an anodic voltage to the base with respect to an electrode in the pre-deposition electrolyte. Conditioning step may also include a pickling step; or a deposition step comprising depositing a fresh layer on the base before the deposition of Cu. In all such cases, an active surface may be provided to the Cu electrodeposition step so that this step yields repeatable results in terms of Cu layer thickness and uniformity. As described before, thickness and uniformity control for deposited Cu, In and/or Ga layers are of utmost importance since Cu/(In+Ga) and Ga/(In+Ga) molar ratios need to be controlled throughout the base.
In the electroplating system 30 of
In this example, electrodeposition is carried out on the free surface 46A of the conductive layer 46. The back surface 45B of the flexible foil substrate 45 may optionally be covered with a secondary layer 47 (shown with dotted line) to protect the flexible foil substrate 45 during annealing/reaction steps that will follow to form the CIGS(S) compound, or to avoid buckling of the flexible foil substrate 45. It is important that the material of the secondary layer 47 be stable in chemistries of the Cu, In and Ga plating baths, i.e. not dissolve into and contaminate such baths, and also be resistant to reaction with Group VIA elements. Materials that can be used in the secondary layer 47 include but are not limited to Ru, Os, Ir, Ta, W etc. Use of a secondary layer 47 comprising at least one of Ru, Ir and Os has an added benefit. Such materials are very resistant to reaction with Se, S and Te. Therefore, after any reaction step that forms CIGS(S) compound layer on the free surface 46A of the conductive layer 46, the secondary layer protects the flexible foil substrate 45 from reaction with Se, S or Te and leaves a surface that can be soldered easily. In prior art devices Mo was used as the secondary layer 47. During selenization and/or sulfidation processes or during the growth of the CIGS(S) absorber, this Mo layer reacted with Se and/or S forming a Mo(S,Se) surface layer. After solar cells are completed, they need to be interconnected to form modules. Interconnection involves soldering or otherwise attaching back surface of each cell to the front surface of the adjacent cell. A Mo(S,Se) layer on the back of the cell cannot be soldered effectively, therefore physical removal of the selenized and/or sulfidized Mo surface is needed. However, a surface comprising at least one of Ru, Ir and Os can be soldered easily without the added step of removing a selenized or sulfidized surface layer because these materials do not appreciably selenize or sulfidize.
Referring back to
Once a section of the free surface 46A of the conductive layer 46 is conditioned and cleaned it moves into the Cu electroplating unit 31. Within the Cu electroplating unit 31, the free surface 46A (or the surface of the seed layer if a seed layer has been deposited in the conditioning unit 34) is exposed to a Cu plating bath 36A which may be circulated between a first reservoir 36AA and a first chemical cabinet 36A′. The Cu plating bath 36A may be filtered and replenished during circulation or while in the first chemical cabinet 36A′. Measurement and control of various bath parameters, such as additive content, Cu content, temperature, pH etc. may be continuously or periodically carried out within the first chemical cabinet 36A′ to assure stability of the Cu deposition process. Electrical connection to the conductive layer 46 (or to the flexible foil substrate 45 if the foil substrate itself is conductive) may be achieved by various means including through rollers 39 which may be touching the flexible foil base 22 at, at least part of its back or front surfaces. Preferably, front surface contacts are made at the two edges avoiding physical contact with most of the front surface which may be damaged or contaminated by contacts. A first anode 40A is placed in the Cu plating bath 36A and a potential difference is applied between the first anode 40A and the portion of the conductive layer 46 within the Cu electroplating unit 31, to deposit Cu on the portion of the free surface 46A that is exposed to the Cu plating bath 36A as the flexible foil base 22 is moved.
The portion of the flexible foil base 22 processed in the Cu electroplating unit 31, passes through the Cu cleaning unit 37A and enters into the Ga electroplating unit 32. Within the Ga electroplating unit, the surface of the already deposited Cu layer is exposed to a Ga plating bath 36B which may be circulated between a second reservoir 36BB and a second chemical cabinet 36B′. The Ga plating bath 36B may be filtered and replenished during circulation or while in the second chemical cabinet 36B′. Measurement and control of various bath parameters, such as additive content, Ga content, temperature, pH etc. may be continuously or periodically carried out within the second chemical cabinet 36B′ to assure stability of the Ga deposition process. Electrical connection to the conductive layer 46 (or to the flexible foil substrate 45 if the flexible foil substrate itself is conductive) may be achieved by various means including through rollers 39 which may be touching the base at, at least part of its back or front surfaces. Preferably, front surface contacts are made at the two edges avoiding physical contact with most of the front surface which may be damaged or contaminated by contacts. A second anode 40B is placed in the Ga plating bath 36B and a potential difference is applied between the second anode 40B and the portion of the conductive layer 46 within the Ga electroplating unit 32, to deposit Ga on the portion of the Cu surface that is exposed to the Ga plating bath 36B as the flexible foil base 22 is moved.
The portion of the flexible foil base processed in the Ga electroplating unit 32, passes through the Ga cleaning unit 37B and enters into the In electroplating unit 33. Within the In electroplating unit, the surface of the already deposited Ga layer is exposed to an In plating bath 36C which may be circulated between a third reservoir 36CC and a third chemical cabinet 36C′. The In plating bath 36C may be filtered and replenished during circulation or while in the third chemical cabinet 36C′. Measurement and control of various bath parameters, such as additive content, In content, temperature, pH etc. may be continuously or periodically carried out within the third chemical cabinet 36C′ to assure stability of the In deposition process. Electrical connection to the conductive layer 46 (or to the flexible foil substrate 45 if the flexible foil substrate itself is conductive) may be achieved by various means including through rollers 39 which may be touching the flexible foil base at, at least part of its back or front surfaces. Preferably, front surface contacts are made at the two edges avoiding physical contact with most of the front surface which may be damaged or contaminated by contacts. A third anode 40C is placed in the In plating bath 36C and a potential difference is applied between the third anode 40C and the portion of the conductive layer 46 within the In electroplating unit 33, to deposit In on the portion of the Ga surface that is exposed to the In plating bath 36C as the base 22 is moved. After In electrodeposition the portion of the flexible foil base comprising the all-electroplated Cu/Ga/In stack is passed through a cleaning/drying unit 38 and moved to the return spool 21.
It should be noted that additional process units may be added to the electroplating system 30 of
We so far described an example of a system and process for roll-to-roll electrodeposition of stacks comprising Group IB and Group IIIA materials. Other processing units may be added to the electroplating system of
The roll-to-roll processing system of
The examples above employed a flexible foil base 22 such as the one depicted in
In two-stage techniques, which involve deposition of a metallic precursor film comprising Cu, In and Ga and then reaction of the metallic precursor film with at least one of Se and S, individual thicknesses of the Cu, In and Ga layers need to be well controlled because they determine the final stoichiometry or composition of the compound layer after the reaction step. The roll-to-roll deposition approach of the present invention lends itself well for smart process control so that these thicknesses may be monitored and controlled using in-situ measurement devices such as X-ray fluorescence (XRF). XRF probes may be placed at various positions in the systems of
Once the metallic precursor films, or the “metallic precursor/Group VIA material” stacks, or the reacted precursor layers of the present invention are formmed, reaction or further reaction of these layers with Group VIA materials may be achieved by various means. For example, these layers may be exposed to Group VIA vapors at elevated temperatures. These techniques are well known in the field and they involve heating the layers to a temperature range of 350-600° C. in the presence of at least one of Se vapors, S vapors, and Te vapors provided by sources such as solid Se, solid S, solid Te, H2Se gas, H2S gas etc. for periods ranging from 5 minutes to 1 hour. In another embodiment a layer or multi layers of Group VIA materials may be deposited on the metallic precursor layers and then heated up in a furnace or in a rapid thermal annealing furnace and like. Group VIA materials may be evaporated on, sputtered on or plated on the metallic precursor layers in a separate process unit. Alternately inks comprising Group VIA nano particles may be prepared and these inks may be deposited on the metallic precursor layers to form a Group VIA material layer comprising Group VIA nano particles. Dipping, spraying, doctor-blading or ink writing techniques may be employed to deposit such layers. Reaction may be carried out at elevated temperatures for times ranging from 1 minute to 30 minutes depending upon the temperature. As a result of reaction, the Group IBIIIAVIA compound is formed. It should be noted that reaction chambers may also be added to the apparatus of
In the examples above, systems with horizontal web geometry have been discussed. It should be noted that the concepts of the present invention may be applied to systems where the flexible foil base travels in a vertical position or at any angle with respect to the horizontal plane. Depositions may be carried out on the horizontal web in either “deposit up” or “deposit down” manner. The flexible foil substrate may move from left to right or vice-versa. It may move continuously or in a stepwise manner. It may also move with an oscillating “back-and-forth” motion. It is possible to deposit some layers onto the flexible foil base as it is moved in one direction and then deposit more layer(s) as the foil is moved back in the reverse direction. DC, AC, pulsed or pulse-reverse type power supplies, among others, may be used for the electrodeposition steps.
Solar cells may be fabricated on the Group IBIIIAVIA compound layers of the present invention using materials and methods well known in the field. For example a thin (<0.1 microns) CdS layer may be deposited on the surface of the compound layer using the chemical dip method. A transparent window of ZnO may be deposited over the CdS layer using MOCVD or sputtering techniques. A metallic finger pattern is optionally deposited over the ZnO to complete the solar cell.
Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.
This application claims priority to Provisional Application Ser. No. 60/862,164 filed on Oct. 19, 2006.
Number | Date | Country | |
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60862164 | Oct 2006 | US |