Photovoltaic cells developed by IMT can change the world
With the energy issue now front-page news, Professor Christophe Ballif invites us to take a look at his high-technology laboratory, which manufactures high-performance photovoltaic cells.
Microengineering’s Neuchâtel building doesn’t look much from the outside. Yet it houses one of the world’s most important academic research groups involved in the area of solar energy: the Photovoltaics Laboratory (PV-LAB), headed by Professor Christophe Ballif. “We are working toward the creation of low-cost photovoltaic cells with a very high yield”, explains the head of the laboratory, with characteristic modesty.
An innocuous statement, behind which hides, however, very sophisticated manufacturing processes. The scientists are in fact working on the development of silicon-based solar cells, which necessitates advanced know-how. “We use silicon because this material is plentiful and non-toxic”, says the Professor. The speciality of the laboratory is the depositing of thin layers of silicon. These are either based on amorphous silicon (which absorbs visible light) or on microcrystalline silicon (with an absorption spectrum up to infra-red). The layers are integrated into three categories of solar cells.
Up to 21% yield
The first method consists of depositing these layers directly on to a glass plate, using plasma-based processes. It is possible to create either amorphous or microcrystalline cells, or to combine both in a micromorph cell which makes better use of the solar spectrum. With this process, a silicon thickness of less than two microns is enough to obtain solar cells with a 12% yield, with potentially very competitive manufacturing costs. A second group of scientists is working on applying these layers, not on glass, but on plastic (PET) – a flexible material. The resulting cells produce a close to 10% yield in the laboratory. As for the third process, “it is very dear to my heart” says Christophe Ballif. An understandable enthusiasm, since it was he, who, on arriving in Neuchâtel, initiated the development of this technique, enabling the PV-LAB to position itself as one of the pioneer laboratories worldwide. It consists of depositing very thin layers of amorphous silicon directly on to a small plate of crystalline silicon. This results in “heterojunction cells” with a spectacular yield, reaching 21%. “It is currently the only way of proceeding which remains simple and enables the obtention of such a yield”, explains Christophe Ballif. This type of solar panel will probably remain, at the moment, more costly than modules consisting of pure thin layers, since it requires silicon plates. “If a large surface is available in a sunny region, thin-layer cells (12% yield) will without doubt be better value than these with a high yield for producing cheap kWh. On the other hand, if the surface is more limited, high-yield technologies can present a strong advantage. Thus a person with only a 20 m2 roof, aiming to produce 4000 kWh per year, will be able to use this new technology.” The development potential is therefore huge, and industry has now understood this. The company Roth und Rau (DE), for example, set up a branch in Neuchâtel to cooperate with the PV-LAB in 2008. It is now marketing equipment for the manufacture of high-yield solar cells.
A complete chain
This type of cooperation requires that the photovoltaic laboratory remain very much aware of the needs of industry. This is why the scientists are studying all the problems linked to the new technologies, and at all the levels of the pre-industrial chain. This concerns the manufacture of solar cells, as well as the production of photovoltaic panels, whose every single property is studied. “The interdisciplinary aspect is both interesting and motivating for the various teams”, states Christophe Ballif. “Even module design is examined, and one of our groups is working on a way to integrate solar panels in the urban environment in an esthetic manner. For example, it’s possible to change the color of a module by working with colored polymers”. The goal is also to ensure that solar panels last for 25 years. “We are also performing tests to check how they resist to the various weather elements.” In order to simulate heavy snow layers, for example, a 500 kg/m2 load is applied to the modules in order to test their resistance. Nothing is left to chance.
"Technically, it’s possible to do without nuclear energy"
Will these efforts one day be rewarded by an extended practical application, at a time where nuclear energy is being seriously challenged? This is what Christophe Ballif earnestly hopes. For this scientist, the technical potential to do without nuclear energy exists, and photovoltaic panels are part of the solution. “If all the roofs in Switzerland that are well oriented were covered with our 21%-yield cells, this would enable the production of 45% of the country’s electricity. The photovoltaic potential is therefore enormous. For example, we will soon be able to manage to produce electricity at a cost of 4 to 5 centimes per kWh in the sunny regions of Europe”, he states. However, the scientist insists on the fact that to work with energies which are more respectful of the environment, the effort would have to be global. “It also means concentrating on more efficient ways of storing and transporting electricity, as well as creating intelligent networks and making sure that the different forms of electricity generation are complementary. In the near future, recourse to natural gas for power stations would probably be difficult to avoid in the event of the abandoning of nuclear energy. It’s also a question of improving energy efficiency, insulating houses, working on developing methods for storing and transporting electricity, and also finding a way of reducing the consumption of gas-fuelled vehicles.” What remains to be done is to convince the politicians and authorities to invest massively in research and in sustainable energies. And this won’t be easy. As regards solar energy, in particular, scientists have difficulty finding the necessary means to remain competitive. This is why CSEM (the Swiss Center for Electronics and Microtechnology) and EPFL have launched an initiative to build a national center focusing on the development of photovoltaic technologies. “This would enable us to go beyond the normal mission of an academic laboratory like ours”, says Professor Ballif – a man with a dream.
Photos: PVLab (Alain Herzog); Heterojunction cells from the PVLab (D. Houncheringer)