Timely introduction of capable manufacturing equipment and support infrastructure is critical for rapid growth of the PV industry
Dan Crowley and Peter Cronin
Who can argue about the enormous benefits of solar energy? Our planet’s Sun produces a renewable source of clean energy. While the price of many other energy sources will continue to increase, the cost of the Sun’s energy is fixed at free. All that is needed to harness this energy is economical photovoltaic (PV) systems that convert solar energy into electrical energy.
FIGURE 1. Historically PV module prices drop as production increases.
While the benefits of solar energy are undeniable, PV-produced energy remains a relatively small, although growing, percentage of worldwide energy usage. This is partly due to the relatively high cost of producing solar cells. Industry and governmental incentives are helpful and lower the effective cost, thus creating demand for PV systems. Ultimately, however, it comes down to the fact that solar cells need to be produced at a lower cost. Today, researchers continue to study materials, including silicon and thin-film-based substrates, to find new ways to increase efficiencies and to reduce cost (see Figure 1).
Total cost to manufacture can be segmented into two components: materials and production. While much research has gone into studying materials, very little has been written about how to achieve the economies of scale required in PV manufacturing.
Reduced time to production
As the global demand for PV-generated power increases, so does the need for PV manufacturing equipment. It is not a race to bring the technology to market. It is a race for cost-effective volume production. The timely introduction of capable manufacturing equipment and the support infrastructure is critical for rapid growth.
Companies are adopting technologies from industries such as photonics, flat-panel display, and semiconductor to bring to market PV manufacturing-equipment solutions. These technologies include motion platforms, lasers, optics, and optomechanics. These components are integrated into proven high-volume capital equipment that builds turnkey systems for various processes such as edge isolation and isolation scribing.
Several components are common to the manufacturing of all solar cells. These include motion control and motion platforms for moving products within a process step, lasers and beam-delivery optics for materials processing, and light sources for testing the output and efficiency of solar cells.
The motion-control and motion-platform requirements for processing solar cells and panels are strikingly similar to those of other industries. Existing motion control and platforms in a variety of form factors and sizes provide a cost-effective solution for material movement in PV manufacturing.
FIGURE 2. Laser wavelength vs. absorption depth
For example, most silicon-based solar cells are 100 to 150 mm, with some solar-cell manufacturers looking to produce cells up to 200 mm. This form factor requires motion platforms similar to those used in the semiconductor and microelectronics industry. Requirements for material handing of large thin-film panels fall within the size of motion platforms used in the flat-panel-display industry.
In addition to size, other critical parameters of existing motion platforms, such as accuracy and speed, match the requirements of manufacturing equipment for solar cells and panels. These platforms provide a readily available source for material movement and are well suited to use in PV-manufacturing processes such as isolation scribing of thin-film solar panels.
Sophisticated motion-control systems are used in laser-scribing equipment to achieve high accuracy for handling cells and panels. High accuracy is important for silicon solar cells because the location of the scribe line contributes to cell surface area and efficiency. Scribing as closely as possible to the edge frees up valuable surface “real estate” for exposure to the Sun. Thin-film panels are scribed to segment and link the panel into many individual cells.
FIGURE 3. Machined silicon at 1064 nm. Bottom of scribe (left) and top of scribe (right).
Typically a three-scribe pattern is used to isolate and interconnect cells. The area used for the three scribe lines does not contribute to power generation so it is necessary to scribe the lines as closely together as possible. Precision motion control and a stable platform maintain line spacing across the large panel.
High accuracy is achieved through careful platform design. The selection of materials and components contribute to overall accuracy. Granite and engineered composites are used for stability and vibration damping. Thermally matched components allow for accuracy to be maintained despite environment or process temperature changes. Encoders, linear motors, and programmable motion-control systems provide for fast and precise motions.
The principle of laser processing is straightforward. A laser beam is focused and used for material removal-a process known as ablation. This is commonly referred to as micromachining. The laser must be selected to match specific properties (absorption, melting temperature, thermal diffusivity, and so forth) of the material being processed.
Pulsed lasers remove material by ablating very small regions with each pulse. Given a certain spot size, the pulse-repetition rate can be controlled so that the pulses are overlapped and create a continuous line of scribed material. The repetition rate is determined by considering the interrelationship between the laser and material properties.
The parameters of the laser include power, wavelength, pulse length, absorption coefficient, and stability. Together these characteristics and parameters determine the quality and speed of material removal. It is important to deliver optimum power density for fast laser scribing without causing damage to the adjacent material. Considering process requirements, reliability, stability, serviceability, and cost of operation, diode-pumped lasers are the usual choice for PV laser scribing (see Figures 2 and 3).
A complete line of lasers is required to process the various materials used for PV manufacturing. Full capability application laboratories at the equipment supplier are useful for developing process recipes. Both the lasers and the processes can be adapted from the photonics industry for photovoltaics.
FIGURE 4. Optics or galvo scanners transport the laser beam to the solar cell.
Laser beam quality is a critical parameter for process control and speed. Beam uniformity and optical transport efficiency must be maximized for the best use of the laser power. Optics enable the efficient delivery of the beam from the laser source to the work surface. The beam travels through free space optics or a fiber. Polarizers, filters, and mirrors manage the beam to ensure the optimal beam shape at the work surface. At the end of the optical path, fixed optics or a galvanometer scanner focus and direct the energy to scribe the solar cell or panel. For scanners, it is important to have a sufficient field of view and resolution to match the laser and the process parameters at the material surface. Multiple lasers or split beams can be used to increase production speeds (see Figure 4).
Integrated solutions pull it all together by providing turnkey capital equipment. This equipment consists of the integration of motion control, lasers, optics, automation, and software. The obvious benefit to the PV manufacturer is that the technical challenges of each subsystem and the integration of these subsystems are handled by one supplier. Another major benefit of one company designing and manufacturing the equipment is that established application support and field service organizations can be leveraged to provide customer support.
Designing and engineering PV equipment requires engineering capabilities with core competency in all areas of analysis, design, software, and control. In the race for cost-effective volume production, an equipment supplier must use the latest manufacturing technologies to quickly and cost-effectively configure and produce systems.
Laser-based machining tools provide an ideal solution for many of the complex processes required in the manufacturing of solar cells. These systems achieve very fast cycle rates. Common examples include laser edge isolation and laser isolation scribing machines.
High-rate laser scribing is a key process in the production of large monolithic thin-film panels. Monolithic thin-film substrate solar panels require scribing for segmentation into smaller cells and to create a series interconnection. A laser isolation scribing system is used for fast scribing to produce this series connection. This is done to produce an efficient panel by maximizing the power output potential.
For silicon-based solar cells, a laser edge-isolation system ablates material to create a groove that electrically isolates the top surface from the sides of the substrate. This eliminates leakage from high resistance shorting caused by edge contamination from the antireflective coating. This isolation increases the solar-cell efficiency.
These turnkey systems consist of a number of subsystems, including material handling, advanced machine vision, motion control/platform, and laser and laser-beam delivery. Material handling automatically transfers substrates to the work area. The particular application dictates the material-handling configuration that can be batch mode, in-line (single lane/dual lane), cassette-to-cassette, or roll-to-roll. The vision system is used to find fiducial and substrate corners so that the laser can process the material at a programmed offset from the desired feature. The system’s motion platform moves and precisely locates the beam delivery unit over the work surface.
A laser beam is focused and used as a tool to remove material. The laser-beam delivery unit directs the laser for micromachining. Multiple lasers or split laser beams can be used to increase production speeds.
Achieving economies of scale
The cost of producing photovoltaic systems must be reduced for the use of this technology to expand. A large part of the solution will be to achieve economies of scale through the use of other industries’ existing manufacturing infrastructure. A successful model has been demonstrated for utilizing and modifying existing industry components including lasers and optics to quickly and successfully bring to market high-volume capital equipment and processes for solar cell manufacturing. One major benefit of this model is being able to leverage established application support and field service organizations to provide on-going customer support.
Dan Crowley is director of sales (dan.crowley @newport.com) and Peter Cronin (firstname.lastname@example.org) is product specialist at Newport Corp., North Billerica, MA, USA; www.newport.com.