Improving the production of solar cells is one of the pillars of ECN’s research into energy technologies. Recently, a brand-new machine was delivered in Petten. “This is it,” says Ingrid Romijn of ECN Solar Energy, “Our MAiA XS. It may be ‘extra small’, but it can do almost anything. Just think up a solar cell concept and it will deposit the right layers for you!”
With its vacuum chambers, plasma sources and pumps, the MAiA (multiple action in-line plant) takes up a considerable space. But much less than a factory hall, even though it can turn out 200 solar cells per hour: enough for a pilot production run of industrial processes.
“It is the successor to our SiNA,” says Romijn. “For that one, we spent ten years developing processes that are now used by 60 per cent of the solar cell manufacturers in Europe and Asia. We built the prototype in collaboration with the German firm Roth & Rau, who have now supplied us with the MAiA; an improved version that can deposit dielectric layers such as silicon nitride or silicon dioxide on both sides of a cell. It can also be used for silicon etching.”
€ per watt-peak
A solar cell with a higher yield converts a greater part of the light into electrical capacity. This is important despite the fact that sunlight is free, because the price of solar cells increases in proportion to their surface dimensions. A higher yield means that a smaller and therefore cheaper installation can deliver the same performance – as long as the improved solar cells are not (that much) more expensive than the standard cells. The trick is to make a quality cell using as little silicon and other raw materials as possible.
The other important element is reducing production costs. The ideal production line is inexpensive to purchase and run and makes a lot of cells per hour. With the MAiA, ECN aims to tick all these boxes in an integrated way.
The solar cell
Each solar cell consists of two layers of a semi-conductive material, usually silicon. Electron donors are applied to one of the layers, usually through diffusion. Conversely, the other layer is equipped with electron acceptors. In this way an n-layer is created on top of a p-layer. Electron acceptors accept negatively charged electrons, thereby donating an ‘electron hole’, which acts like the carrier of a positive charge. Part of the donated electrons and their counterparts move over and cross from the p-layer to the n-layer. This creates an electric field, which slows down and eventually halts the crossing over of electrons. This field comes in handy when the cell absorbs sunlight and extra pairs of negative and positive charge carriers are created throughout the material. These pairs are split at the interface: from a pair in the p-layer, the electron passes through to the n-layer while its counterpart does not. Conversely, of the pairs in the n-layer the positive charge is the one that is passed on to the other side. This gives rise to a difference in electric potential. If the n-layer and p-layer are connected via an external load, a current is created and this supplies the solar cell with its electric potential.
Room for improvement
Part of the incoming light is reflected by the solar cell. To keep this loss to a minimum, the surface is roughened somewhat and covered with an anti-reflective layer. Another part of the light passes through the cell without being absorbed. For this reason it is a good idea to make the back of the cell reflective on the inside, so that the light is given a second chance. In addition, some of the pairs are lost through ‘recombination’: negative and positive charge carriers cancel each other out before the electric field can split them up.
Recombination occurs most frequently in parts where there are irregularities in the crystalline structure of the silicon. This is on the surface, on the border between two crystalline fields and in places where the silicon has been contaminated by other substances. That is why the extremely pure and blemish-free monocrystalline silicon, which is used in the electrical industry, produces the most efficient solar cells. That material’s relatively high price means that the slightly less pure and less blemish-free multicrystalline silicon is an interesting option too – if one can manage to ‘passivate’ it sufficiently, that is: treat it so that the potential losses resulting from recombination are reduced.
Above: In the PUM concept (on the left, registered by Solland Solar) the broad contact strips of the classic solar cell have been moved to the back, but the remaining contact strips still cover five per cent of the surface.
Above: The MAiA is crucial to research into cell concepts such as these two, which could both result in an increased yield. Both concepts include a passivating, dielectric layer at the front and rear. The one at the top is called the ASPIRe (All Sides Passivated and Interconnected at the Rear) and has already produced good results, including a sixteen per cent yield. MWT stands for Metallisation Wrap-Through.
The second concept is a follow-up which will receive more attention in coming years. In the EWT (Emitter Wrap-Through) the front of the cell is entirely free of contact strips, which increases the effective surface of the cell by five per cent compared to the ASPIRe cell.
A new back
“Hydrogenated silicon nitrate can be used very effectively as a passivating layer on the front of the cell,” says Romijn. “It also works as an effective anti-reflective layer.” And it also works well as a base for the contact strips that form one of the cell’s electrical poles. A happy side-effect of the heat used in baking those metal strips is that it drives the hydrogen further into the cell, thereby passivating any faults that may lie deeper in the silicon material.
Another method for increasing a solar cell’s yield is through improved metallisation. Thanks to the MAiA, the contact strips can be moved from the front to the rear of the cell, resulting in a seven per cent increase in its effective surface.
This also makes it possible to do away with the full-metal backing, which forms the cell’s second electrical pole.
Romijn: “The MAiA opens up a whole new playing field for us, because we can now deposit good ‘in-line’ passivation layers on both the front and back. And it opens up many more possibilities: apart from silicon nitrate, we can also deposit silicon dioxide and double layers, for instance. The idea is to replace the metal at the back with isolating layers such as silicon dioxide and an open network of contact strips.”
On a good track
Moreover, this technology is also in line with processes that are already being used in the industry, so that the step from ‘lab to fab’ can be made quickly. Can we expect more major developments, or will the silicon cell remain dominant, and is it only a matter of time before technology such as MAiA develops improved concepts for production?
“You can never be sure,” says Romijn. “Other types of solar cell appear to be gaining ground, but up until now there haven’t been any alternatives that could match the silicon cell for yields. For the time being, we are on a really good track.”
Contact
Ingrid Romijn
ECN Solar Energy
Tel. +31 22 456 4309
E-mail: Ingrid Romijn
Text: Steven Bolt
Info
Inline processing of crystalline silicon solar cells: the holy grail for large-scale manufacturing?
Reaching 16.4% module efficiency with back-contacted mc-Si solar cells
An overview of MWT cells and evolution to the ASPIRE concept: a new integrated mc-Si cell and module design for high-efficiencies
PASHA: a new industrial process technology enabling high efficiencies on thin and large mc-Si wafers
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