At the international SiliconPV conference, which took place this year from April 8 to 10 in Leuven, Belgium, the 10 best submitted abstracts are traditionally honored. This year, too, we congratulate two of our colleagues whose work has been selected from over 200 abstracts from over 20 countries. Dr. Sören Schäfer and Dr. Dominic Walter received the award for their contributions “26%-efficient and 2 cm narrow interdigitated back contact silicon solar cells with passivated slits on two edges” and “Easy-to-Apply Methodology to Measure the Hydrogen Concentration in Boron-Doped Crystalline Silicon”.
Dr. Sören Schäfer studied physics at the Philipps University of Marburg and joined ISFH in 2012 as a PhD student. Since completing his dissertation in 2018 on the development of novel light trapping structures for Si solar cells, he has been working as a postdoc on the implementation of passivating “poly-Si on oxide” (POLO) contacts in industrial Si solar cells.
In February 2018, ISFH and the MBE Institute of Leibniz Universität Hannover set a new world record for solar cells on boron-doped silicon material with an efficiency of 26.1 % (see press release of February 6, 2018). Solar cells with such high energy conversion efficiencies are particularly suitable for applications in which a maximum energy yield must be achieved with a very limited surface area. An example of this is the roof of an electric car, some of whose energy requirements are to be covered by photovoltaics (“vehicle integrated PV”).
For maximum energy yield on car roofs, however, more than very good solar cells are needed: firstly, as little space as possible should be “given away” between the individual solar cells, and secondly, the solar cells must follow the curvature of the car roof. With silicon-based solar cells, which are actually inflexible, shingle technology offers an attractive solution. Similar to a mosaic, the curvature of the roof is reproduced by many narrow cell strips that overlap on one side.
For practical reasons, large solar cells are first produced for this purpose, which are then subsequently divided into many small strips. This splitting process, which is usually carried out by a laser, leads to damaged or unpassivated cut edges and thus to recombination losses and a significant reduction in cell efficiency. These losses are all the higher the narrower the cell strips become and the higher the initial cell efficiency is.
These edge losses were systematically investigated as the focus of the award-winning SiliconPV conference contribution by Dr. Sören Schäfer. He was able to show that the edge losses can be almost completely suppressed with suitable process control. Even on a Si solar cell that was only 2 cm narrow, an efficiency of 26% could be achieved.
Dr. Dominic Walter studied physics at the University of Freiburg. From 2011 to 2016, he completed his PhD thesis at ISFH on the permanent deactivation of boron-oxygen-correlated defects in silicon. After completing his doctorate, he took over the management of a project at ISFH aimed at the industrial implementation of this deactivation.
An important question with defects in silicon is to what extent there are interactions with hydrogen. Such interactions have been reported by several groups, but mostly without direct detection of hydrogen in silicon. This was mainly due to the fact that the current methods for detecting and measuring the concentration of hydrogen are very elaborate. In addition, concentrations in silicon are often below the corresponding detection limits.
In his winning paper, Dr. Dominic Walter presented that the hydrogen concentration in silicon can be determined by means of simple measurements of the resistivity of silicon wafers. Depending on the accuracy of the resistance measurement, a detection limit of 2 × 1013 cm-3 can be expected. The method is based on the theoretical understanding of the formation of boron-hydrogen pairs and the associated change in the conductivity of silicon.
Further details will soon be available in the renowned journal “Solar Energy Materials and Solar Cells”, in which the SiliconPV Award winning papers will be published.
The international SiliconPV conference is organized and alternately hosted by the renowned European research institutes EPFL, Fraunhofer ISE, imec, CEA-INES, University of Constance, ECN and ISFH. Next year, the SiliconPV conference will take place for the first time outside Europe – in Hangzhou, China.