Continuing on the theme of PV manufacturing, Dan Crowley and Peter Cronin explain how various existing advanced technologies and infrastructure could be utilized to reduce the cost of volume production in photovoltaics manufacturing.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 economic photovoltaic (PV) systems that convert solar energy into electrical energy.
Production floor where the Newport MRSI-175PV is assembled
The benefits of solar energy are undeniable, yet solar PV only accounts for a tiny, albeit growing, percentage of worldwide energy usage. In part this is due to the high cost of producing solar cells. Industry and government incentives help to lower the effective cost and to create demand for PV systems but, ultimately, it comes down to the fact that solar cells need to be produced more cheaply. Researchers continue to study materials – including silicon and thin film-based substrates – to find new ways to increase efficiencies and reduce cost. Historically, PV module prices have dropped as production increases (Figure 1).
Figure 1. PV module prices drop as production increases
Total cost-to-manufacture can be divided into two components – materials and production. Although there has been considerable research on PV materials, very little has been written about how to achieve the economies of scale required in PV manufacturing. This article examines the economics of implementing a cost-effective manufacturing infrastructure for PV production.Reduced time to cost-effective volume 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 but a race for cost-effective volume production. The timely introduction of capable manufacturing equipment and the support infrastructure is critical for rapid growth.
Example motion platform HybrYX
Companies are adopting technologies from industries such as photonics, flat panel displays and semiconductors to bring PV manufacturing equipment solutions to market. These technologies include motion platforms, lasers, optics and optomechanics. Such components are integrated into proven, high-volume capital equipment which is used to build turnkey systems for various processes such as edge isolation and isolation scribing (see below).Best-in-class components
A number of 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 material processing
- 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 can provide a cost-effective manufacturing solution for material movement in PV systems.
For example, most silicon-based solar cells are 100-150 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 (e.g. accuracy and speed) match the requirements of manufacturingequipment for solar cells and panels. These platforms provide a readily available source for material movement and are well suited for use in PV manufacturing processes such as isolation scribing of thin film solar panels
Newport Hippo Q-Switched diode pumped family of lasers
Sophisticated motion control systems are used in laser scribing equipment in order 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. 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 close 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 will 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.
Laser and optics
The principle of laser processing is straightforward: a laser beam is focused and used for material removal in a process known as ablation. This is commonly referred to as micromachining. The laser must be selected to match specific properties of the material being processed (absorption, melting temperature, thermal diffusivity, etc.).
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 overlap 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. Figure 2 shows the relationship between laser wavelength and absorption depth.
Figure 2. Laser wavelength versus absorption depth
It is important to deliver optimum power density for fast laser scribing without damaging the adjacent material. Diode-pumped lasers are the usual choice for PV laser scribing given the process requirements, reliability, stability, serviceability and cost of operation.
A complete line of lasers is required to process the various materials used for PV manufacturing. Full capability application laboratories at equipment suppliers are useful for developing process recipes. Both the lasers and the processes can be adapted for photovoltaics from the photonics industry.
Laser beam quality is a critical parameter for process control and speed. It is essential to maximize beam uniformity and optical transport efficiency 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 fibre. 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 (Figure 3) focus and direct the energy to scribe the solar cell or panel.
Figure 3. Optics or galvo scanners are used to transport the laser beam to the solar cell
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.Integrated solutions – turnkey capital equipment
Integrated solutions bring it all together by providing turnkey capital equipment containing motion control, lasers, optics, automation and software.
The obvious benefit to the PV manufacturer is that one supplier handles the technical challenges of each subsystem and the integration of these subsystems. Another major benefit of one company designing and manufacturing the equipment is that established application support and field service organizations can be brought together to provide enhanced customer support.
Designing and engineering PV equipment requires engineering capabilities with core competencies in analysis, design, software and controls. In the race for cost-effective volume production, an equipment supplier needs to use the latest manufacturing technologies to configure and produce systems both quickly and cost-effectively.
Example of integrated solutions – laser edge isolation and laser isolation scribing
Laser-based machining tools provide an ideal solution for many of the complex processes required in the manufacture 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. Maximizing the power output potential produces an efficient panel.
For silicon-based solar cells, a laser edge isolation system is used to ablate material to create a groove which isolates the top surface electrically from the sides of the substrate. This eliminates leakage from high resistance shorting caused by edge contamination from the anti-reflective (AR) coating. This isolation increases the efficiency of the solar cells.
These turnkey systems consist of a number of subsystems including material handling, advanced machine vision, motion control/platform, laser and laser beam delivery.
Example of an integrated solution product
Material handling transfers substrates automatically to the work area. The particular application dictates the material handling configuration, which 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 locates the beam delivery unit precisely 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.In summary
The cost of producing photovoltaic systems must be reduced to enable use of this technology to expand. A large part of the solution will be to achieve economies-of-scale through the use of existing manufacturing infrastructure from other industries.
This article describes a successful model for utilizing and modifying existing industry-leading components including lasers and optics to bring high-volume capital equipment and processes for solar cell manufacturing to market both quickly and successfully. A major benefit of this model is the ability to utilize established application support and field service organizations to provide ongoing customer support.
Dan Crowley and Peter Cronin work for Newport Corporation
e-mail: [email protected]