solar cells, silicon casting and the patent bargain
At its most fundamental a patent is a bargain between the state and an inventor, where the state receives an enabling disclosure of the invention in return for a limited monopoly. The state enters this bargain to provide the disclosed information to the public for the benefit of further invention and building of the next generation of technology.
Over the span of three articles; Cheaper High- … (Suntech), How BP Blew … (BP), and Advance Could … (GT) Technology Review has unwittingly presented an interesting case study of this bargain in operation.
First, let’s explore the technology in a bit more detail. In both the Suntech and BP articles there are bits and pieces referring to the work of Schmid at Crystal Systems. Digging back through US patents to Schmid we find 3,898,051 (‘051), detailing a system and method for growing a crystal. The charge or feedstock material is contained within a crucible, which is itself contained within a cylindrical resistance heater. The system further includes a heat exchanger located under the bottom center of the crucible. It is indicated that a seed crystal may or may not be used, where the seed, if used, would be placed above the heat exchange unit. As presented roughly in columns four and five of the description the heat exchanger is initially used to ensure at least a portion of the seed crystal remains solid during melting of the charge. As disclosed, initial crystal growth begins with increased gas flow rate to the heat exchanger i.e. increasing the amount of material with a temperature below the melting temperature. At a point later in the growth cycle the temperature of the crucible side walls is also lowered to further crystal growth. The first example of ‘051 details the growth of a sapphire crystal. At the moment I do not want to argue whether the detailed system and method outlined in ‘051 worked for the growth of a silicon single crystal. For the sake of the current article let us assume this is the “first generation” technology for silicon crystal growth that in fact provided multi-crystalline structures when used according to the disclosed embodiments.
Back to the bargain. The ‘051 patent expired in the early ‘90s and come the turn of the century there is renewed interest in solar power. Particular interest in systems and methods of reducing the production costs of the single or mono-crystalline silicon active material. With the assumption that the ‘051 system did not work for silicon, what might be improved, or how might the base process be modified to yield better results for Si? In general one wants to promote the growth of a single, oriented, crystal while mitigating or at least controlling nucleation and growth of “secondary” grains. The nature of the seed crystal and the location, amount and nature of heat flow from the charge during crystal growth are two parameters that come to mind for tinkering.
In the above articles we are introduced to various inventors and/ or assignees including Nathan Stoddard, GT Advanced Technologies and Suntech. With reasonably straight forward searches USP 8,048,221 (‘221) to Stoddard and US patent application 12/999,439 (‘439) assigned to GT Solar came to light. No published documents assigned to Suntech that appeared related to their work in the area were found, suggesting any related patent applications are still in the 18 month pre-publication window. We do however have published images of silicon that purportedly results from their casting process.
From the above we do in fact see work around the number and nature of seed crystals and the nature of heat flow from the furnace. With regard to seeds both a single and multiple seeds are disclosed. In a case of more than one seed the seeds may be tiled into a specific pattern on at least one surface of the crucible. With regard to heat flow, there are examples where heat input or extraction is a function of the particular surface in the furnace and another case where the furnace insulation is movable to control heat flow from the crucible. With regard to the Suntech process we will be looking for correlation between any disclosed information in patent documents and the observed microstructure.
There is much more that could be said about the current crop of casting technologies. That though is for another forum. It would also be interesting to consider if the technologies can co-exist, i.e. what are the freedom to operate considerations for the second generation and where further patent data points might fall if and when patent coverage around the casting technologies is expanded. With regard to the US patents that are now beginning to emerge it may ultimately be qualified US attorneys that opine on the various issued claims, and if there is room for all of these second generation casting technologies to operate freely.