Inkjet systems for use in photovoltaic production

Cost savings, efficiency improvements and the environmentally friendly nature of inkjet make it an exciting breakthrough technology for solar cell manufacturing.

Cost savings, efficiency improvements and the environmentally friendly nature of inkjet make it an exciting breakthrough technology for solar cell manufacturing.

Ross N. Mills, Philip Branning, iTi Solar, a division of imaging Technology international (iTi) Corp., Boulder, CO USA

The need to reduce the cost of extracting electricity from solar energy in order to achieve cost parity with nonrenewable energy sources is driving a search for increased module efficiency and for savings at all points of the photovoltaic (PV) supply chain. Inkjet materials deposition has multiple unique benefits that can be leveraged for efficiency gains, as well as environmentally responsible cost reduction at the basic material and cell processing nodes of the supply chain applied across multiple types of PV technology. As a digital manufacturing process with properties of both printing and coating processes [1], inkjet has been applied to the patterned deposition of conductive fluids for contact formation on silicon cells, but inkjet also has the capability to manufacture entire thin film cells.

Figure 1. Force and deflection limits for breaking silicon wafers.

Thinner wafers

Materials cost. Silicon makes up 50% of a module’s cost [2]. There is a clear motivation to reduce silicon consumption per cell, and the most obvious way to accomplish this is to use a thinner wafer. As wafers are thinned, the brittle nature of the material and stress related flexure and distortion lead to decreased yield [3] when processed on equipment designed for a 300µm rigid wafer. This is a challenge for all forms of silicon, whether it is sawed into wafers or grown into a ribbon. Yet, it appears that handling is the only barrier as cells built from wafers as thin as 47µm are viable [4].

Handling challenges. Figure 1 shows the breaking point due to contact force and deflection for handling of silicon wafers as a function of wafer thickness [5]. Each step in the cell manufacturing process needs to be optimized to handle these thin, flexible silicon wafers, and while this is possible for the transport steps, it is quite challenging for the screen-printed contacts. While the squeegee does not need excessive force, this step is nevertheless responsible for many broken cells, even at 270µm thickness [6]. In addition to actual breakage, micro and macro cracking can occur at significantly lower forces and deflections. Wafers with microcracks may be visually normal with a slight decrease in performance. However, a microcracked wafer may degrade and the microcracks may propagate due to temperature cycling in the field. Replacing a contact process with a non-contact step presents an opportunity to increase process yield.

Why inkjet?

Non-contact. Inkjet deposition is a non-contact process, and so can be applied to silicon wafers of any thickness without stressing the wafer. In addition to screen printing pressure issues, the low speed of the inkjet drops impinging on the surface eliminates potential damage to previous deposited thin films that could be caused by high speed pressurized jets of other non-contact processes, such as aerosol jet or continuous inkjet.

Figure 2. a) XY Solar MDS installed at the National Renewable Energy Laboratory in Golden, CO; and b) Lab Fab XY MDS 300.

Current crystalline silicon (c-Si) cell processing involves screen printing top contacts onto sliced wafers. As the wafer itself comprises up to half of the processing cost [7], it is advantageous to decrease wafer thickness and increase wafer yield per ingot. Yet these thinner wafers are prone to edge chips and fractures caused by cell handling. While these small fractures can reduce cell efficiency, they can also contribute to yield losses during top contact formation, when the pressure applied during screen printing can cause complete breakage of the cell. When this occurs, production must be stopped while the shards are removed and the spilled paste is cleaned. The non-contact nature of inkjet alleviates these challenges.

Digital image control. Inkjet is digitally controlled. Each pattern to be printed is loaded into the control computer’s memory, and as a result can be digitally manipulated. In contrast, screen printing requires creation of a relatively expensive template, which cannot be changed. Printing a second pattern requires a second template. As a consequence, the process is inflexible and expensive to modify. Digital image control is initially useful in pilot production, where the space of control parameters can be searched to find optimal jetting conditions.

The digital benefit of inkjet also extends past optimization of static factors like substrate and fluid properties, and can be used to dynamically adjust the printed pattern for better alignment to pre-existing features and distortions introduced by an upstream process. Digital image adjustment together with precision motion control can result in fewer wafer handling steps by rotating and aligning the image rather than rotating and aligning the wafer.

Precise volume deposition. With inkjet, very little ink is wasted in the printing process as the inkjet drop ejection process repeatably produces tiny deposition volumes and is digitally controlled. Compared to screen printing, which can waste up to 20% of the metallic paste, inkjet wastes only 1% of fluid in the deposition process. The metallic and conductive fluids used for contacts are expensive, measured in thousands of dollars per liter, and so this reduced waste makes a significant contribution to lower costs. In addition to contact formation, the precise volume deposition is useful for uniform film formation; films can be formed with thickness on the order of a hundred nanometers or less depending on the type of inkjet technology.

PV cell production advantages

Substrate compatibility. Inkjet deposition is a versatile process that can be applied to both rigid and flexible substrates. In a scanning configuration, a small number of printheads can be used to deposit high resolution patterns on nearly any type of rigid or flexible substrate, ranging from individual silicon wafers to square meter sized glass or plastic superstrates. By increasing the number of printheads and arranging them perpendicular to the process direction, flexible substrates can be used in a roll-to-roll, single pass process. Metallic foils and plastic films can be passed under the printhead array, and precise, variable patterns can be applied in a continuous fashion.

Figure 3. VJET large substrate printer.

Atomospheric operation. Inkjet deposition can operate under atmospheric pressure in an inert or reactive environment, in contrast to all types of vapor deposition which require a vacuum. This eliminates the need for the specialized equipment necessary to achieve and maintain a vacuum, including the pumps, thick steel enclosures and control systems.

Continuous operation. In a single pass configuration, inkjet deposition can improve throughput by continuously processing a stream of cells, which can be either discrete wafers or a rolled substrate that is later diced into individual cells. This is an advantage over batch processes for film deposition, which can include spin coating and vacuum deposition. Single pass systems also have a throughput advantage over batch processes for top contact printing, including single nozzle spraying techniques such as aerosol printing.

Inkjet process development

Development tools and services. Anyone contemplating ink jet as a production technology must rely on personal experience and multiple knowledge resources such as consultants, integration companies and manufacturers [8]. In addition, it is important to have access to an inkjet development lab where feasibility can be proven prior to investing in production tools and process development.

Figure 4. a) Digital web press and b) single pass system.

The ink jet components consist of the print head, print head drive electronics, print head maintenance, fluid supply, and substrate. Motion control hardware, electronics and software are necessary to position the print head and substrate. A curing or drying system may be required to fix the jetted fluid to the substrate or to obtain the desired material properties such as conductivity. Data interface hardware and software, and application unique software is required to glue the system together and provide a user interface.

Development tools for ink jet technology can generally be classified as visualization systems or printing and deposition systems. Visualization systems are used to study the behavior of print heads, fluids and substrates under simulated operating conditions. The printing and deposition systems are used to develop production processes and produce sample output for evaluating fluid and substrate interaction.

Characterizing the formation of the droplet during the ejection process is of principal importance to understanding the performance limitations of the print head and the fluid in any given application. Instruments like the iTi Drop Watcher can be used to estimate important parameters such as the maximum drop ejection rate, drop volume, fluid usage, drop placement error and consistency. In addition, the user will be able to determine the operating window in terms of the drive waveform, environment, and fluid properties.

Two systems for deposition on rigid and flexible non-porous substrates are shown in Fig. 2. This XY MDS 300 and XY Solar MDS can accommodate multiple print heads and rigid or non-porous substrate up to 50mm thick with process speeds up to 1.5m/s.

Production Systems. The use of development tools often leads directly to a production system specification. iTi Solar has developed two production system platforms The addition of custom options for automated maintenance and material handling that can be specific to a particular process.

Non-porous substrate production systems. The VJET shown in Fig. 3 is a derivative of the XY MDS that prints on rigid substrates such as silicon, glass, ceramic, metal, and non porous plastics at up to 1200 dots per inch. Conductive fluids can also be used to print 30µm lines and pads with 20µm spacing. The VJET uses a scanning print head cluster on a moving gantry with a stationary substrate.

Flexible substrate and single pass production systems. An Ink Jet Digital Web Press and Single Pass systems for working with flexible substrates such as paper, thin metal, and plastic are shown in Fig. 4. FujiFilm Dimatix, Xaar and other manufactures’ print heads can deposit multiple fluids on a moving web. In addition, curing or drying devices can be adapted to either system. The machines can operate as a production systems or stand alone prototypes.

Conclusion

Future solar applications of inkjet are already coming out of research labs to provide an achievable path to grid parity. Cost savings, efficiency improvements and the environmentally friendly nature of inkjet make it an exciting breakthrough technology for solar cell manufacturing. One of the most exciting is the prospect for using inkjet for an entire thin-film solar cell production process. Progress has been made in printing the active layer of organic polymer based cells [9], as well as the electrode layer [10], and entire cells have been made using screen printing [11]. The groundwork has been laid for replacing screen printing, spin coating and vacuum deposition with inkjet.

References

  1. 1. Frederik C. Krebs, “Fabrication and Processing of Polymer Solar Cells: A Review of Printing and Coating Techniques,” Solar Energy Materials & Solar Cells, vol. 93, pp. 394-412, 2009.
  2. 2. Meng Tao, “Inorganic Photovoltaic Solar Cells: Silicon and Beyond,” Electrochemical Society Interface, no. Winter, pp. 30-35, 2008.
  3. 3. J. Wohlgemuth, M. Narayanan, “Large-Scale PV Module Manufacturing Using Ultra-Thin Polycrystalline Silicon Solar Cells,” Final Subcontractor Report NREL/SR-520-40191, 2006.
  4. 4. Michael Reuter, Willi Brendle, Osama Tobail, and Jurgen H. Werner, “50µm thin solar cells with 17.0% efficiency,” Solar Energy Materials & Solar Cells, vol. 93, pp. 704-706, 2009.
  5. 5. Per Arne Wang, “Industrial Challenges for Thin Wafer Manufacturing,” in IEEE World Conference on Photovoltaic Energy Conversion, vol. 1, Waikoloa, Hawaii, 2006, pp. 1179-1182.
  6. 6. P.S. Dominguez, J.M. Fernandez, “Introduction of Thinner Monocrystalline Silicon Wafers in an Industrial Cell-manufacturing Facility,” in 20th European Photovoltaic Solar Energy Conference, Barcelona, 2005.
  7. 7. M.A. Green, “Photovoltaics: Technology Overview,” Energy Policy, vol. 28, pp. 989-998, 2000.
  8. 8. Ross N. Mills, William F. Demyanovich, “Materials and Process Development for Digital Fabrication Using Ink Jet Technology,” IS&T Digital Fabrication, 2005.
  9. 9. T. Aernouts, T. Aleksandrov, C. Girotto, J. Genoe, and J. Poortmans, “Polymer-based Organic Solar Cells Using Ink Jet Printed Active Layers,” Applied Physics Letters, no. 92, 2008.
  10. 10. K. Xerxes Steirer, et al., “Ultrasonically Sprayed and Inkjet Printed Thin Film Electrodes for Organic Solar Cells,” Thin Solid Films, vol. 517, pp. 2781-2786, 2009.
  11. 11. Frederik C. Krebs et al., “A Complete Process for Production of Flexible Large Area Polymer Solar Cells Entirely Using Screen Printing—First Public Demonstration,” Solar Energy Materials & Solar Cells, vol. 93, pp. 422-441, 2009.

Ross N. Mills received his PhD from the U. of California Berkeley and is chief technology officer for iTi Corp.; rmills@iticorp.com; www.itisolar.com, www.iticorp.com. Also from iTi is Philip Branning, who received his BS from Vanderbilt U. and is lead software engineer for development tools.

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