Patent Application
20110290654
The invention relates generally to electrodeposition apparatus and methods. Methods and apparatus described herein find particular use in solar cell fabrication.
Electrodeposition is generally a plating process that uses electrical current to reduce or oxidize chemical species of a desired material from a solution and coat a conductive substrate with a thin layer of that material. An electroplating system typically includes two electrodes and an electrolyte. Additionally, a reference electrode may also sometimes be employed. In an electrodeposition process, typically the part to be coated is one of the electrodes and the coating material is supplied from the electrolyte in which the electrodes are immersed. In electroplating, the electrolyte is replenished periodically with the chemical species being deposited on the substrate. Sometimes, the electrode that is not being coated can be a source of the chemical species in order to replenish the electrolytic solution.
Solar or photovoltaic cells are devices that convert photons into electricity by the photovoltaic effect. Solar cells are assembled together to make solar panels, solar modules, or photovoltaic arrays. Thin film solar cells are stacked structures, having layers of materials, including photovoltaic materials, stacked on a substrate for support of the stack. There are many fabrication techniques used for fabricating the individual layers of the stack. One particularly useful method is electrodeposition, however there are drawbacks to conventional apparatus and methods in this respect. For example, when electrodepositing a material onto an electrically insulating substrate, such as glass, a conductive coating must be applied to the substrate in order to allow electric currents to pass. These conductive coatings are typically thin and can have high sheet resistance which produces a voltage drop and current non-uniformities when electroplating over a large area. In these cases uniform deposition of the electroplated film is problematic.
What is needed, therefore, are improved apparatus and methods for electrodeposition on large area, resistive substrates. Given the demand for renewable energy, improved apparatus and methods are particularly important for solar cell fabrication where the typical substrate is glass coated by a thin layer of transparent conductive oxide.
The invention relates generally to electrodeposition apparatus and methods. The inventors have found that when depositing films via electrodeposition, where the substrate has an inherent resistivity, for example, sheet resistance in a thin film, methods and apparatus described herein can be used to electrodeposit materials onto the substrate by forming a plurality of ohmic contacts through the substrate surface to an underlying conducting layer, for example a transparent conductive oxide, and thereby overcome the inherent resistance to electrodeposit uniform films thereon. Methods and apparatus described herein find particular use in solar cell fabrication.
One embodiment is an apparatus for electrodeposition, including: (i) a counter electrode including a plurality of apertures normal to a surface of the counter electrode that faces a substrate surface during electrodeposition; and (ii) a plurality of contact pins, each contact pin of said plurality of contact pins registered with, and configured to pass through, each aperture of said plurality of apertures and establish electrical contact with the substrate surface while being electrically isolated from the counter electrode during electrodeposition. Some embodiments described herein employ spring-type contact pins, compliant pins or rigid pins, depending upon the application. Particular materials and configurations of apparatus in accord with embodiments of the invention are described in more detail below.
Another embodiment is a method of electrodeposition, including: (a) establishing a plurality of ohmic contacts through a substrate film to an underlying electrically conductive film using a plurality of contact pins, said plurality of contact pins electrically isolated from a counter electrode; and (b) electrodepositing a material from an electrolyte onto the substrate film. Methods described herein are meant to address films that, although having some intrinsic conductivity, have a resistivity that must be overcome in order for uniform plating to occur, and therefore ohmic contacts are established through the film to an underlying electrically conductive film. One aspect of methods described herein is establishing such ohmic contacts via the substrate film from the front side, that is, the side facing a counter electrode during deposition onto the substrate film. In one embodiment, ohmic contact is established by exploiting materials that allow such contact at or around the plating voltage. In other embodiments, a breakdown voltage is applied to establish ohmic contact without the need for more expensive materials that match well to the substrate to allow ohmic contact at or around the plating voltage.
Using methods described herein, higher plating currents can be used without sacrificing film uniformity, either in thickness of the film or the chemical stoichiometry of the film. Particular aspects of methods are described in more detail below.
Back encapsulation can generally serve to provide encapsulation for the cell and provide mechanical support. Back encapsulation can be made of many different materials that provide sufficient sealing, moisture protection, adequate mechanical support, ease of fabrication, handling and the like. In many thin film solar cell implementations, back encapsulation is formed from glass although other suitable materials may be used.
A substrate layer can also be used to provide mechanical support for the fabrication of the solar cell. The substrate can also provide electrical connectivity. In many thin film solar cells, the substrate and back encapsulation are the same. Glass plate is commonly used in such instances.
A back contact layer can be formed from a thin film of material that provides one of the contacts to the solar cell. Typically, the material for the back contact layer is chosen such that the contact resistance for the electrons/holes flowing from/to the absorber layer is minimized. This result can be achieved by fabricating an ohmic or a tunneling back contact layer. This back contact layer can be formed from many different materials depending on the type of thin film solar cell. For example, in copper indium gallium diselenide (CIGS) solar cells, this layer can be molybdenum. In cadmium telluride (CdTe) thin film solar cells, this back contact layer can be made, for example, of nickel or copper. These materials are merely illustrative examples. That is, the material composition of the back contact layer is dependent on the type of absorber material used in the cell. The thickness of a back contact layer film is typically in the range of a few microns.
The absorber layer is a thin film material that generally absorbs the incident photons (indicated in
A window layer is also typically a thin film of semiconducting material that creates a p-n junction with the absorber layers and, in addition, allows the maximum number of photons in the energy regime of interest to pass through to the absorber layer. The window layer can be an n or p-type semiconductor, depending on the material used for the absorber layer. For example, the window layer can be formed from a cadmium sulphide (CdS) n-type semiconductor for CdTe and CIGS thin film solar cells. The typical thickness of this layer is of the order of hundreds to thousands of angstroms.
A top contact is typically a thin film of material that provides one of the contacts to the solar cell. The top contact is made of a material that is transparent to the photons in the energy regime of interest for the solar cell. This top contact layer is typically a transparent conducting oxide (TCO). For CdTe, CIGS, and amorphous silicon thin film solar cells, the top contact can be formed from, for example, indium tin oxide (ITO), aluminum doped zinc oxide (ZnO) or flourine doped tin oxide (SnO2). The top contact layer thickness can be of the order of thousands of angstroms.
A top encapsulation layer can be used to provide environmental protection and mechanical support to the cell. The top encapsulation is formed from a material that is highly transparent in the photon energy regime of interest. This top encapsulation layer can be formed from, for example, glass.
Thin film solar cells are typically connected in series, in parallel, or both, depending on the needs of the end user, to fabricate a solar module or panel. The solar cells are connected to achieve the desired voltage and current characteristics for the panel. The number of cells connected together to fabricate the panel depends on the open circuit voltage, short circuit current of the cells, and on the desired voltage and current output of the panel. The interconnect scheme can be implemented, for example, by laser scribing for isolation and interconnection during the process of the cell fabrication. Once these panels are made, additional components such as bi-pass diodes, rectifiers, connectors, cables, support structures etc. are attached to the panels to install them in the field to generate electricity. The installations can be, for example, in households, large commercial building installations, large utility scale solar electricity generation farms and in space, for example, to power satellites and space craft.
As mentioned above, electrodeposition is an attractive methodology for depositing various layers of thin film solar cells. Processes have been developed for the deposition of the back contact, absorber, window and top contact layers using electrodeposition.
For illustration purposes, electrodeposition is described herein as being used in the fabrication of CdTe-based solar cells although electrodeposition can be used to fabricate any number of other types of solar cells or other types of thin films products and/or devices. That is, the invention is not limited to this exemplary electrodeposition chemistry.
Solar cell photovoltaic stacks are conventionally constructed in an order starting from, for example, a top encapsulation layer, a top contact layer, a window layer, an absorber layer, a back contact layer and so on, that is, in an order opposite of the description of the layers with reference to
Referring again to
Methods described herein are exemplified as being carried out on substantially flat substrates, such as conventional glass substrates. However, methods described herein can also be employed substrates with non-planar geometries, such as cylinders, curved and/or irregular contoured surfaces, depending on the desired configuration of the final product photovoltaic device. One embodiment is any method described herein wherein the substrate comprises a curved architecture, for example a cylinder, a parabola, a cone, a hemisphere, and the like. The curved architecture can be convex, concave or have both components, depending upon the need.
Once the TCO coated glass is cleaned, a CdS layer, 215, may then be deposited, for example, by using an aqueous solution of, for example, a cadmium salt and elemental sulfur composition. The solution does not have to be aqueous. That is, other solvents, such as dimethylsulfoxide (DMSO), can be used. This deposition can be done using electrodeposition. For electrodeposition, the ITO coated glass can form one of the electrodes. The other electrode can be, for example, made of graphite, and the electrolyte can be, for example, a DMSO solution of a cadmium salt and elemental sulfur. Potential is applied between the electrodes so that CdS is deposited from the solution onto the ITO coated glass substrate. Another method of depositing the CdS layer is chemical deposition, for example via wet chemistry or dry application such as CVD. The CdS deposited is an n-type semiconductor and its thickness is typically between 500 Å and 1 μm. Subsequent to the deposition, the layer can be annealed, for example under an inert atmosphere such as argon, to achieve film densification and grain growth to improve the electrical and mechanical properties of the CdS film.
A cadmium telluride layer, 220, can then be electrochemically deposited on the CdS/TCO/Glass stack (now a substrate for electrodeposition), for example, from an acidic or basic media containing a cadmium salt and tellurium oxide. In this process, the CdS/TCO/Glass substrate forms one of the electrodes and platinum or other materials can be used as the other electrode. The electrolyte can contain an acidic or basic media, in solvents such as water, DMSO or other solvents, with a cadmium salt and tellurium oxide, for example. Films of thickness ranging from 1 to 10 μm are typically deposited. Cadmium telluride films may then be annealed at approximately 400° C. in an air or oxygen or CdCl2 environment so as to improve the electrical properties of the film and also to convert the CdTe film to a p-type semiconductor. It is believed that these methods optimize grain size and thus improve the electrical properties of the films.
After this CdTe deposition and annealing, a laser scribing process is typically performed to remove CdS and CdTe from specific regions (not shown). In this scribing operation, the laser scribing is utilized such that CdS and CdTe are removed from specific regions of the solar panel. However, the conductive oxide (for example, Al doped ZnO or ITO) is not removed by the laser scribe. Then a second laser scribing step is performed in which CdS, CdTe and TCO are removed from specified regions.
A back contact layer, 225, can then be deposited on the CdTe layer, using for example sputtering or electrodeposition. For example, copper, nickel and/or other metals, alloys and composites can be used for the back contact layer. This back contact fabrication step can be followed by an anneal, for example, at temperatures of between about 150° and about 200° C. to form an ohmic contact. The back contact layer can cover the CdTe layer and also fill the vias (not shown) created in the CdTe/CdS layer by the laser scribing process.
After back contact layer deposition and annealing, laser scribing can typically be used to remove the back contact layer material from specific areas, but the CdTe layer is not etched away in this process. This removal step can complete the process for isolation and interconnecting the solar cells in series in the solar panel/module.
After the deposition of the back contact layer, an encapsulation layer, 230, can be applied, for example, using ethylvinyl acetate (EVA). Encapsulation protects the photovoltaic stack. Glass, 235, can be added for further structural support (and protection) of the stack.
The above described fabrication process represents a brief outline and many variants of this process can be employed for the fabrication of CdTe thin film solar cells. For other types of thin film solar cells, different chemicals, etc. can be employed. In this description, example process steps have been described for illustrative purposes. Other steps would typically include additional details of the laser scribing and ablation steps employed for the fabrication of the interconnect schemes and cell isolations, multiple clean and drying steps between the different layer depositions and the like. Values for the layer thicknesses, anneal temperatures, chemical composition etc. described herein are merely illustrative and are not meant to limit the scope of the invention. These values can vary across a wide range as processes are optimized for many different output variables.
In a typical configuration electrodeposition contacts, 335, to the working electrode are made at the edges of the working electrode as depicted. This configuration works well when the working electrode is highly conductive, for example metallic, and therefore has little sheet resistance. However when the electrodeposition is performed on, for example, CdS/TCO/glass, where CdS/TCO is the working electrode, this configuration is problematic. For example, when using electrodeposition to achieve high quality, stochiometrically-correct films, the potential at the surface of the working electrode has to be kept fairly uniform. For example for electrodeposition of CdTe on a CdS/TCO, the potential across the full surface of the working electrodes can not vary by more than of the order of milli-volts. The thickness of the film deposited in electrodeposition is proportional to the total charge that flows through the system, and the total charge flowing through the system is a function of the current and the time for which the current flows. Since electrodeposition on large area substrate working electrodes is desirable, and potential drop across such substrate's large surfaces occurs due to only peripheral supply of potential, deposited film uniformity suffers unless steps are taken to mitigate potential drop across the substrate and/or underlying electrically conductive layer, for example, a transparent conductive oxide.
In order to minimize the time that it takes to achieve a given thickness of film, the current flowing through the system has to be increased. For example, for electrodeposition of CdTe on a CdS/TCO/Glass substrate the sheet resistance of the TCO is on the order of 2-20 ohms/square. The area of a typical substrate is on the order of square meters. For this resistance and area, if the substrate, for example via the TCO, is contacted only from the periphery, and the potential drop across the substrate surface has to be maintained to within milli-volt tolerances, then the total current is limited to a range on the order of tens to hundreds of micro-amps per square centimeter. At these currents, for example if a few microns of CdTe film is to be deposited, it can take on the order of several hours to deposit the CdTe film. This severely limits the throughput of conventional electrodeposition equipment and significantly increases the cost of production of solar cells. If the current is increased during the deposition in an attempt to improve the throughput of the equipment, then the result is significantly higher potential drops and corresponding non-uniformities in the CdTe film thickness and composition across the surface of the substrate, which results in poor quality solar cells.
The inventors have found that many of the above-described limitations of conventional electrodeposition can be overcome. In certain embodiments, the substrate is contacted in a manner that alleviates the potential drop constraints and permits the use of significantly higher deposition currents to improve throughput while maintaining high-quality uniform films.
As mentioned above, one embodiment is an apparatus for electrodeposition, including: (i) a counter electrode including a plurality of apertures normal to a surface of the counter electrode that faces a substrate surface during electrodeposition; and (ii) a plurality of contact pins, each contact pin of the plurality of contact pins registered with and configured to pass through each aperture of the plurality of apertures and establish electrical contact with the substrate surface but be electrically isolated from the counter electrode during electrodeposition.
The contact pins can include at least one of a rigid pin, a compliant pin and a spring-type pin. That is, some embodiments of the invention include apparatus with combinations of pin type, depending on the desired outcome of the deposition. A rigid pin is a pin that is relatively rigid, that is, the pin does not deform or bend substantially upon contact with the substrate. A compliant pin is a pin that does have some give, that is, it can deform or bend upon contact with the substrate. Compression contact between a compliant pin and the substrate can be varied in force by, for example, using compliant pins made with varying amounts of compliancy, for example, by varying thickness of pins made of a single material and/or by making pins from different materials and or making flexures on the pins that provide compliance. A spring-type pin is a pin, with a rigid or compliant component, that specifically can deform or otherwise move or be displaced vertically with respect to the substrate. That is, a spring-type pin makes a compression contact with the substrate via a mechanism such as a spring, a pneumatic device, an elastomeric member and the like. Thus a spring-type pin can have a rigid pin component with, for example, a spring device that allows the rigid pin to move normally to the surface of the substrate upon engagement with the substrate such that a compression contact is made with the substrate. One embodiment of the invention is an apparatus as described above with spring-type contact pins.
The contact pins can be made of materials that are chemically resistant to the electrolyte and/or are coated with a material that protects them from the electrolyte and also may serve as an insulating material to electrically isolate the pins from the counter electrode. Contact pins can be made of a variety of metallic materials or coatings. Suitable materials for contact pins of the invention include one of gold, titanium, tungsten, steel, titanium nitride, and indium or alloys of these and other metallic materials. In one embodiment, the contact pins include a material, for example made of or coated with, that does not dissolve in the electrolyte nor plate under the plating conditions employed. That is, materials need not necessarily be coated with an additional material to protect the pins from the corrosive electrolyte and/or protect the substrate from contamination from material dissolved from the pins by the electrolyte. Suitable materials for this embodiment include gold, tungsten, titanium, titanium nitride, steel, and indium or alloys of these and other metallic materials.
Preferably, the pins are made of material that makes good electrical contact with the substrate. Thus the material used in the tip or contact area of the pin can be tailored to the particular needs of the deposition system and chemistry. For example, if the substrate consists of CdS/TCO/Glass, where deposition is to occur on a film of CdS, then the tips of the contact pins can be coated with or made of indium and/or an alloy of indium. Indium makes a good ohmic contact with CdS under plating conditions without the need to apply higher potentials to break down resistance to ohmic contact. “Ohmic contact” means a region on the substrate where the current-voltage (I-V) curve of the substrate in the localized contact region is linear and symmetric. Put another way, an ohmic contact is a contact with voltage independent resistance, that is, a contact having a negligible resistance regardless of the polarity of the applied voltage. Thus, since the resistance is negligible at the ohmic contacts, plating potential can be supplied to an underlying electrically conductive layer without substantial resistance from the substrate layer. Also, the contact pins should be mechanically robust to minimize wear and tear and reduce operating costs and down time of the tool, and, as mentioned, should be chemically compatible (either coated with insulator or not) with the electrolyte being used, and preferably are cost effective.
In the example in
Coating the pins is but one approach to electrically isolating the contact pins from the counter electrode. There are many other ways in which the contact pins can be electrically isolated from the counter electrode. In one embodiment, the apertures in the counter electrode through which the contact pins pass are coated with an insulating material as described above. Embodiments of the invention include combinations of electrical isolation configurations as described above. For example, electrically insulating the pins, apertures in the counter electrode, using appropriate spacing between the contact pins and the counter electrode, etc. can be used in combination to provide suitable electrical isolation of the contact pins from the counter electrode. Any of the above electronic isolation methods can be employed in any combination.
The counter electrode can be made of many different materials as would be understood by one of ordinary skill in the art. In general, the counter electrode is electrically conductive, chemically compatible with the electrolytic solution, and meets any cost considerations. In one embodiment, the counter electrode includes at least one of platinum, graphite, titanium, tungsten, titanium suboxide (for example as sold under the trade name, Ebonex™, by Atraverda of South Wales, UK) and titanium nitride.
The spacing of contact pins can be optimized to achieve the best throughput possible which will be dictated by the particulars of the process. For example, if the conductive layer (for example, the TCO layer of a CdS/TCO/glass substrate) of the substrate has a sheet resistance of 10 ohms per square and the current desired during the deposition is 2 mA per square centimeter, then a contact pin spacing of, for example, 2 cm per square will result in a maximum potential drop across the whole substrate of less than 20 mV. Thus, precise control of the potential drops across the surface of the substrate can be tailored by appropriate pin spacing (which also depends on pin materials and configurations, for example, contact area) even at high deposition currents so as to allow the fabrication of uniform layers at high throughputs. In one embodiment, the plurality of contact pins includes a pin density of between about 100 pins/m2 and about 10,000 pins/m2, in another embodiment between about 500 pins/m2 and about 1000 pins/m2, in another embodiment between about 550 pins/m2 and about 750 pins/m2, and in yet another embodiment between about 650 pins/m2 and about 675 pins/m2, in another embodiment about 667 pins/m2.
As mentioned above, optimal pin spacing can depend on, for example, the contact area of the contact pins, that is, where a pin interfaces with the surface of the plating substrate. Since each pin may contact the substrate slightly differently, the contact area of the individual pins may be expressed in terms of an average contact area. Contact pins can have surfaces that make contact with the substrate where the surfaces can have various shapes to optimize contact, for example, flat surfaces or pointed or wedge-shaped surfaces that dig into the substrate to establish better electrical contact. The contact pins can have various cross-sections, for example, to facilitate manufacturing and/or electrolyte flow around the pins. In many cases, contact pins will be relatively thin, so that the average contact area is reflected in the average diameter of the pin. In one embodiment, each pin of the plurality of pins has an average diameter of between about 10 microns and about 1000 microns, in another embodiment between about 100 microns and about 800 microns, in another embodiment between about 150 microns and about 750 microns, in another embodiment between about 200 microns and about 600 microns, and in yet another embodiment between about 250 microns and about 500 microns.
Smaller diameter pins are useful for a number of reasons, one of which is creating smaller “dead” areas in the deposited film. That is, where the pins contact a substrate, deposition of the new film is blocked and thus “voids” or holes are created after the pins and the substrate are disengaged. These holes in the newly deposited film must be appropriately addressed in order to create, for example, a functional photovoltaic stack. This aspect is described in more detail below.
Contact pin configurations as described herein can be used for static bath deposition equipment or for equipment in which the electrolyte is flowing through the equipment. For example,
Since the contact pins have volume, in such a laminar flow scenario, it is possible that the contact pins can create a shadowing effect. That is, due to the contact pin's leading side or edge interaction with a substantially unidirectional laminar flow of electrolyte, an area adjacent to the contact pin and opposite the side of the pin that encounters the electrolyte first, there can be a differential fluid pressure at that area adjacent to the pin and this can create a localized different deposition rate than that on the rest of the substrate. If the contact pins have a small enough average diameter, then these effects can be minimized or made insignificant. Also, the cross-section of the pins can be made more aerodynamic so that there is substantially laminar flow around the entire pin (rather than laminar flow at the leading edge or side and turbulent flow at the opposite edge or side). Also, this shadowing effect can be overcome by flowing the electrolyte in a turbulent fashion, where the parameters of the electrolyte and the process permit operation in the turbulent flow regime. Thus, one embodiment is an apparatus as described herein configured to flow an electrolyte between the substrate surface and the counter electrode in a substantially laminar flow. Another embodiment is an apparatus as described herein configured to flow an electrolyte between the substrate surface and the counter electrode in a turbulent fashion. In one embodiment, the counter electrode includes apertures through which electrolyte flows normally to the surface of the counter electrode and encounters the substrate surface normally, for example, a shower head type counter electrode. That is, for uniform deposition on the substrate, the counter electrode need not have a continuous surface, for example, the apertures for the contact pins do not prevent uniform deposition on the substrate and therefore additional apertures can be included for electrolyte flow as described.
Other apparatus for performing electrodeposition will typically include a mechanism for placing the substrate in the appropriate location and for engaging the substrate with the contact pins. Electrodeposition can be commenced once the pins make suitable electrical contact with the substrate and the electrolyte is present. The composition of the electrolyte depends on the material to be deposited. Examples of electroplating solutions that can be used for fabricating different layers of CdTe solar cells are described above.
As mentioned, the substrate need not be planar or substantially flat, it can be curved. In the event plating is to be performed on a curved substrate, for example a cylinder, the counter electrode and contact pins are configured appropriately to carry out methods described herein.
The electrodeposition apparatus may also include a controller system for managing the different components of the system. By way of example, the controller may be configured or programmed to select the potential difference that is applied between the substrate and the electrode, control electrolytic flow rate and fluid management, control movement mechanisms, register contact pins with a counter electrode, verify connectivity of contact pins with the substrate, apply voltages to individual pins, and the like. Any suitable hardware and/or software may be utilized to implement the controller system. For example, the controller system may include one or more microcontrollers and microprocessors such as programmable devices (for example, complex programmable logic devices (CPLD's) and field programmable gate arrays (FPGA's) and unprogrammable devices such as gate array application specific integrated circuits (ASIC's) or general-purpose microprocessors and/or memory configured to store data, program instructions for the general-purpose processing operations and/or the inventive techniques described herein.
Another embodiment is a method of electrodeposition, including: (a) establishing a plurality of ohmic contacts through a substrate film to an underlying electrically conductive film using a plurality of contact pins, the plurality of contact pins electrically isolated from a counter electrode; and (b) electrodepositing a material from an electrolyte onto the substrate film. As described above, methods of the invention find particular use where the substrate film has limited conductivity and thus an instrinsic sheet resistance, especially where deposition is to be performed on substrates having large areas. By establishing a plurality of ohmic contacts to an underlying conductive layer, higher plating currents can be used while addressing potential drops across large plating areas. In the scenario where the underlying electrically conductive layer is, for example, a relatively thin transparent conducting oxide, its sheet resistance is addressed via the ohmic contacts through the substrate layer and thus higher plating currents can be used without large potential drops across the transparent conductive oxide.
“Substrate film” means a film or layer that is part, or will be a part, of an electronic device, such as a photovoltaic device. In one embodiment, a substrate film has a thickness of between about 0.01 μm and about 10 μm, in another embodiment between about 0.03 μm and about 5 μm, in another embodiment between about 0.03 μm and about 0.3 μm, and in another embodiment between about 0.1 μm and about 0.3 μm. For example, CdS can be the substrate film. Under, and adjoining the substrate film is an electrically conductive layer to which ohmic contacts are made through the substrate film. The electrically conductive layer has an inherent sheet resistance that is compensated for during electrodeposition methods of the invention so that higher plating currents can be used without sacrificing uniformity (which would result if potential is applied only via the periphery of the electrically conductive layer as in conventional methods). In one embodiment, the electrically conductive layer has a sheet resistance of between about 1 ohm per square and about 30 ohms per square, in another embodiment between about 2 ohms per square and about 20 ohms per square, in another embodiment between about 5 ohms per square and about 15 ohms per square.
In one embodiment, establishing the plurality of ohmic contacts includes at least one of using contact pins, of the plurality of contact pins, that include a contact area which comes in contact with the substrate film, the contact area including a conductor capable of establishing ohmic contact with the substrate film at or about the plating voltage. For example, if the substrate film includes CdS, then a conductor that would allow ohmic contact within the plating voltage regime is indium. Thus in one embodiment, the contact pins are coated with and/or include indium at least in their contact area, that is, where they adjoin the substrate film upon engagement with the substrate film. Other conductors that allow such ohmic contact include, but are not limited to, aluminum, gallium, and zinc. One potential drawback of this method is the cost of the conductor as described above. For example, indium is relatively expensive. However, in the example of a CdS substrate film, the amount of indium needed is relatively small, as only the contact area of the pins need contain indium, and the contact pins typically have a small cross-section and/or tip configuration.
One embodiment is a method of electrodeposition, including: (a) establishing a plurality of ohmic contacts with a TCO via a CdS film using a plurality of contact pins, the plurality of contact pins electrically isolated from a counter electrode; and (b) electrodepositing a material from an electrolyte onto the CdS film; where (a) includes at least one of using contact pins coated with indium at least at the contact point and applying a breakdown voltage to each of the plurality of contact pins. The breakdown voltage is that as appropriate to form the ohmic contacts with the TCO. In one embodiment the electrodeposited material includes cadmium telluride.
It can be beneficial if the material for the contact pins is not constrained by requiring establishing an ohmic contact with an underlying electrically conductive layer via the substrate at or around the plating voltage. For example, for contacting CdS, the metals described above, for example indium, for forming the ohmic contacts at or around the plating voltage are typically expensive and/or not commercially available. However a large number of conductors, for example common metals, make ohmic contact with the materials commonly used, for example, in transparent conductive oxides which are under the substrate layer. In one embodiment, when electrodepositing on substrate films that have, for example, an underlying TCO, after engaging the contact pins with the substrate film, a breakdown voltage is applied to the contact pins to establish an ohmic contact to the underlying TCO. This can be done prior to introduction of electrolyte to the apparatus and/or after. That is, a breakdown voltage is applied to establish the ohmic contacts with the underlying layer rather than, for example, coating the contact pins with a material, for example indium, that allows establishment of the ohmic contacts at or near the plating potential.
“Breakdown voltage” is a term of art generally meaning the minimum voltage that causes a portion of an insulator to become electrically conductive. Substrate films, for example CdS and the like, have some conductivity, but also some inherent resistance. The breakdown voltage is the minimum voltage required to overcome the resistive component of the substrate film and allow electrical flow to the underlying conductive layer, for example, a TCO. The breakdown voltage, for example when CdS is the substrate film, is on the order of a few volts, when the CdS layer is on the order of a 1000 Å thick. This potential locally perturbs the CdS creating a conductive path to the TCO, creating an ohmic contact between the contact pins and the TCO. This embodiment makes a much wider choice of conductive materials available for the contact pins, at least for the portion configured to make contact with the substrate during deposition. In one embodiment, the breakdown voltage is high enough to breakdown the substrate film's resistance, but not so high as to reach the breakdown voltage of the underlying TCO. In one embodiment, the breakdown voltage is between about 0.5 volts and about 10 volts, in another embodiment between about 1 volt and about 5 volts, and in another embodiment the breakdown voltage is between about 2 volts and about 3 volts.
When the contact pins are engaged with the substrate film, and electrolyte is flowing, there is the possibility, depending on the materials and configuration of the contact pins and if they penetrate the substrate film, that the contact pins' position on the substrate film surface may change. That is, the electrolyte flow can physically displace the pin from its original position along the surface of the substrate film to a new position. Embodiments of the invention contemplate pin displacement from a first contact area to another contact area. Also, the breakdown voltage can change the physical characteristics of the substrate film where a portion of the substrate film in contact with the pin can be changed sufficiently so as to facilitate physical displacement of the pin's contact, for example, by the electrolyte flow. In one embodiment, a breakdown voltage is applied prior to electrolyte flow. In another embodiment, a breakdown voltage is applied after electrolyte flow. In yet another embodiment, a breakdown voltage is applied before and after electrolyte flow.
In a specific embodiment, where a breakdown voltage is applied to a CdS substrate film and deposition potential is not reached in the CdS film at the breakdown voltage, then the breakdown voltage is applied after electrolyte flow so that pin movement, for example due to the pins first encountering electrolyte flow, is irrelevant. That is, if there is little possibility of deposition at the breakdown voltage, then pin movement due to the breakdown voltage along with electrolyte flow is irrelevant, since pin movement due to these forces will have occurred prior to any deposition on the substrate film.
In another embodiment, illumination of the substrate can be used to lower its resistivity and thus aid in forming ohmic contacts. That is, since photovoltaic substrate films, for example CdS, are photoactive, then shining intense light on the substrate (layer on which deposition is to occur) lowers the resistance of the film and thus can lower the resistance to making ohmic contact, without need to apply a breakdown voltage. In one implementation the light source can be integrated with the plating apparatus. The light source can be a bright white light source or specific wavelengths of between about 400 nm and about 900 nm can be used. In one embodiment, blanket illumination of the substrate film is performed through the CdS/TCO/glass substrate with the light incident from the glass side (side opposite of where electrodeposition is to take place) of the substrate. The illumination would be applied at the beginning of the deposition to lower the contact resistance to the CdS substrate and would be turned off at or near the end of the deposition or after the deposition is complete.
In another embodiment, the physical characteristics of the substrate film are modified so as to form better ohmic contacts. For example, it has been observed that nanocrystalline cadmium sulfide films can be altered by anneal and/or swift heavy ion (SHI) irradiation to lower resistivity in the films (for example, see: Engineering of nanocrystalline cadmium sulfide thin films by using swift heavy ions, by R. R. Ahire et al., 2007 J. Phys. D: Appl. Phys. 40 4850, which is incorporated herein by reference for all purposes). One embodiment of the invention includes exposing the substrate film to at least one of an anneal and irradiation with ions to aid in creation of the ohmic contacts. In one embodiment, the substrate film is irradiated in at least the areas where the contact pins make contact with the substrate film. This may include specific contact point irradiation, that is, coinciding with the contact points only and/or on slight larger areas than the contact points centered on the contact areas. In another implementation of this embodiment, a grid pattern of light, where the illuminated grid on the substrate includes the contact pin areas on the substrate, is used. In another embodiment, the substrate film is irradiated substantially across its surface so that selective irradiation at the contact pin's point of contact is not necessary.
Embodiments of the invention are meant to include combinations of the above methods of forming ohmic contacts, that is, particular materials as part of the contact pins to make ohmic contact at or near the plating potential, applying a break down voltage, exposing the substrate film to high intensity light, and preconditioning the film's physical characteristics toward better ohmic contact.
Embodiments of the invention also include contacting the underlying conductive layer, the layer under the substrate to which ohmic contacts are made, at the periphery, that is, voltage is applied to the periphery of the underlying conductive layer as well as via ohmic contacts through the substrate film.
After the electrodeposition on the substrate film, the contact pins are removed. By virtue of the pins presence during electrodeposition, the pins block electrodeposition on the substrate film at the locations of the contact pins. Therefore when the pins are removed, voids remain in the newly deposited layer.
After the holes are filled with the insulating material, subsequent layers can be deposited, as depicted in
Methods of the invention can be used for depositing more than one material layer prior to filling with insulating material. Another embodiment is a method of electrodeposition as described above, further including: (c) electrodepositing a second material onto the (first) material, without first disengaging contact between the plurality of contact pins and the substrate film; (d) withdrawing the plurality of contact pins from the material and the second material; and (e) filling the holes in the (first) material and the second material thus formed with an insulating material. Insulating materials as described above for hole filling are suitable for hole filling in this method as well.
Some methods of the invention obviate the need to fill holes created in a newly electrodeposited layer resulting from electrodeposition followed by disengaging the contact pins from the substrate. One embodiment is an electrodeposition method as described above, where holes are exposed upon disengagement of the contact pins and the substrate, further including arranging the plurality of contact pins so that the areas where each of the plurality of contact pins make contact with the substrate film substantially coincide with one or more laser scribes that will be carried out during formation of one or more photovoltaic cells which include the substrate film. In one example, material (for example CdTe) is removed using laser ablation from certain regions in order to make interconnects and isolation trenches for creation of solar cells in a grid. If the contact probes are placed in a manner such that they are coincident with the areas that would be removed eventually for interconnecting and/or isolating individual cells, then the lack of deposition in the areas where the contact pins leave voids does not result in any additional loss of photoelectrically active area. That is, using this method, there is no need to fill the holes, but rather make them part of, for example, a planned isolation trench or interconnect scheme.
Embodiments described above include scenarios where the contact pins touch a substrate film in order to make ohmic contacts to an underlying conductive layer. It is important to note that transparent conductive oxides, for example, have an inherent sheet resistance, therefore methods of the invention are well suited for laying down, for example, cadmium sulfide layers on a TCO. Even though currently there are more cost effective methods of depositing CdS on a TCO, for example by chemical deposition, these homogeneous nucleation chemical depositions create large waste streams. Electrodeposition methods described herein make less waste, and therefore it is contemplated that due to the true cost of current homogeneous nucleation chemical depositions, methods of the invention may replace them. One embodiment is a method of electrodeposition, including: (a) establishing a plurality of ohmic contacts to a transparent conductive oxide film using a plurality of contact pins, said plurality of contact pins electrically isolated from a counter electrode; and (b) electrodepositing a material from an electrolyte onto the transparent conductive oxide film.
Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Therefore, the present embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.
