Sandwiching an oxygen-rich layer of silicon between a solar cell and its metal contact has allowed researchers in Europe to break performance records for the efficiency with which silicon solar cells convert sunlight into electricity. But the challenge now is how to make these so-called passivating contacts suitable for mass production.
‘There is currently a lot of excitement about passivating contacts among the solar cell community,’ said Dr Byungsul Min at the Institute for Solar Energy Research in Hamelin (ISFH), Germany. This year, the technology allowed his laboratory to set a new record efficiency of 26.1% for the kind of solar cells the kind that dominates the photovoltaics market. Commercial solar panels currently operate with an efficiency of around 20%.
Passivating contacts consist of two thin layers of oxidised and crystallised silicon sandwiched between a solar cell and its metal contact. Speaking to a packed hall this September at the European Photovoltaics Solar Energy Conference in Brussels, Belgium, Dr Min said that the layers work by healing broken atomic bonds on the silicon surface and reducing the risk of electric charges getting trapped as they flow out of the solar cell.
The design was developed in 2013 by ISFH and the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany. In recent years, it has driven the energy conversion efficiency of silicon photovoltaics above 25% – a ceiling that had limited the efficiency that researchers could achieve in the lab for over a decade.
Still, Dr Min says that few manufacturers have so far adopted passivating contacts in industry. As part of a project called DISC, he is now coordinating work with research institutes and equipment manufacturers across Europe to streamline their design for mass fabrication.
Making record-setting solar cells with passivating contacts has so far required costly materials and complex laboratory techniques that Dr Min says cannot be adopted in factory assembly lines. However, by getting rid of these sophisticated approaches and substituting them with tools that are already common in the solar cell industry, the DISC consortium expects to bring down manufacturing costs for the technology.
ISFH has notably replaced an expensive and highly conductive indium-containing layer that is deposited on the cell surface to better collect electrical charges out of the passivating contact. By fine-tuning pressure and temperature conditions during production, Dr Min can now form a zinc-containing layer that presents comparable physical properties while using abundant materials.
Dutch equipment provider Meco is swapping complex lithography steps with plating techniques that can metallise the electrical contacts of passivating contact solar cells in throughputs high enough for factory assembly lines.
Over the past year, DISC samples have shuttled across France, Germany, Switzerland and the Netherlands as partners play their part in an international supply line. Each laboratory adds a layer of silicon or other materials in which it specialises, gradually building up the stack of semiconductors into a functioning solar cell.
‘This August, we completed our first industry-sized solar cells,’ said Dr Min. ‘They have already reached energy conversion efficiencies above 21%.’ This falls within the range of solar cells on the market today.
Over the coming year, Dr Min expects that fine-tuning the layers in these factory-friendly devices will help edge their performance above that of the competition. In an industry where a difference of just half a percentage can make or break companies, a technology with a proven potential of over 25% efficiency in the laboratory offers enticing prospects for manufacturers.
‘We have to go to higher solar cell efficiencies,’ agreed Dr Martin Hermle, one of the pioneers of passivating contacts at Fraunhofer ISE. His research group is now developing industrial deposition methods for the solar cells produced in DISC, and developing ways of further boosting their energy conversion efficiency in another project called Nano-Tandem.
‘The cost of solar panels is largely dictated by their surface area. If you can make cells with 30% efficiency instead of 20% or 15%, that really helps reduce the overall cost of solar energy.’