by Katherine Derbyshire, Contributing Editor, Solid State Technology
At first glance, flat-panel displays and solar cells seem to have a lot in common. Both depend on diodes and transistors, which in both cases are usually implemented in silicon. Both are highly competitive markets where throughput and process control are essential to keep costs down and yields up.
Yet companies with products in both markets find that the similarities are relatively superficial, while understanding the differences is critical for success. For instance, while costs are important in both markets, they have very different ideas about the tradeoff between cost, process control, and device performance. The flat-panel display market is fundamentally performance-driven, whereas the electricity market is almost entirely cost-driven. With few exceptions, solar cells succeed only to the extent that they are cost-competitive with other sources of electricity. (In most cases, this is currently achieved by means of government subsidies or preferential tariffs.)
This emphasis on cost colors all aspects of solar cell manufacturing, Wolfgang Jeutter, VP of US operations for Manz Automation, explained to SST. Flat-panel display facilities use class 100 cleanrooms with particle-control mechanisms such as conveyors with encapsulated gears, while thin-film solar manufacturing is likely to use a class 10,000 cleanroom with handling equipment similar to that found in architectural glass manufacturing. On the other hand, a thin-film solar vapor deposition system might accept a fresh substrate every 40sec, and cost half as much as its flat-panel display counterpart, he noted.
This emphasis on cost over process cleanliness is possible in part because solar cell features are quite large by integrated circuit and flat-panel display standards. The most critical features on most thin-film solar panels, the scribe lines, occupy a 300μ stripe with 50μ between lines. The scribe lines themselves have a tolerance of about 20μ. The process control challenge arises because these lines must run the full length of the panel (nearly 3m) while maintaining the same separation from each other. Laser scribing is more precise than mechanical scribing, but the heat can warp the glass panel and cause dimensional variations. Heat can also melt the semiconductor in CIGS solar cells. (To address this problem, Synova and Manz Automation are target=”_blank”>integrating Synova’s water-guided laser technology with Manz’s automated scribing platforms — the water jet reduces heat damage, while the platform accounts for dimensional variation by using the first scribe line as a guide for the others.)
Cost concerns are also enough to drive adoption of new technologies. Interest in thin-film solar cells has exploded, even though their performance remains inferior to that of bulk silicon, because they are immune to the shortage of bulk silicon that has been plaguing the market for the last several years. Manufacturers are also intrigued by the potential for non-vacuum deposition of thin-film cells by methods such as printing or spray coating. Integrated circuit manufacturers look at printing processes and worry about film quality and alignment between layers. Solar cell manufacturers see the low costs that printed products achieve, and figure that the process quality issues can be solved and the cost savings are too big to pass up.
Yet what the silicon shortage gives, it can also take away. Several large polysilicon plants are currently under construction, and most analysts expect the shortage to ease within a year or two. The resulting drop in silicon prices will make bulk silicon cells more attractive, and thin-film cells relatively less so. Thus, thin-film cell manufacturers worry that they must make very rapid decisions to meet the closing market window, and expect their vendors to be able to move just as quickly. Dan Estrada of MES supplier Eyelit Inc. told SST that solar customers are making decisions in days or weeks that often take months in other markets. In return, vendors must be prepared to integrate a wide range of custom components quickly.
Knowing that cost is critical, thin-film solar manufacturers are implementing automation from the very beginning, Estrada noted. Eyelit’s IC customers, drawn mostly from the MEMS and specialty segments, are much more reluctant to make the investment. At the same time, implementing automation for solar panels is in some ways more difficult than for ICs. In the IC industry, SEMI standards govern the software and mechanical interfaces between automation components, wafer carriers, and process tools, but no such standards exist in the solar cell industry, and each company’s facility design is a closely guarded competitive weapon. This climate creates opportunities for vendors like Eyelit, with highly modular automation architectures, who can assemble solutions quickly without the need for substantial custom code.
A superficial view might hold that solar cell manufacturing is just like flat-panel display manufacturing, but simpler. A more nuanced understanding recognizes that the two segments have different needs, and a one-size-fits-all solution is unlikely to fit either. — K.D.