Producing high performance solar cells at lower cost

Dr. Sebastian Brückner graduates “summa cum laude” on solar cells made from III-V semiconductors

Solar cells made from III-V semiconductors achieve the highest efficiency of all solar cells. Researchers recently celebrated a new world record in which HZB was involved: They successfully developed a solar cell that reaches 44.7 percent efficiency. And this technology holds even greater potential if the production subprocesses for these high performance cells could be more precisely controlled. Sebastian Brückner of Helmholtz Zentrum Berlin found the solution to an important challenge while completing his doctorate. He was studying the atomic surface structure of silicon and germanium, which are candidate substrate materials for such solar cells. Brückner gave compelling arguments as to how silicon and germanium substrates must be optimally prepared in the process gas environment in order to avoid defects in the subsequent III-V layers. For his thesis, which he submitted to Prof. Dr. Recardo Manzke of the Physics Institute at Humboldt-Universität zu Berlin, he received the highest academic distinction – a summa cum laude.

The high efficiency of III-V semiconductor solar cells makes them the first choice of energy source for satellites in space and for concentrator photovoltaic systems (which concentrate light using lenses, for example). Industrially, germanium is the established substrate for manufacturing these cells. It would, however, be less expensive and easier to work with silicon instead of germanium, especially because there is a much broader knowledge base on handling the latter, more widely used wafer material. Sebastian Brückner’s work now shows how we could in principle change over to silicon.

Sebastian Brückner prepared the substrate of silicon and germanium by metal-organic chemical vapour deposition (MOCVD) and closely examined the behaviour at the surfaces and interfaces to the III-V semiconductor materials. To do so, he used various ultra-high-vacuum-based methods and a special in-situ spectroscopic method (in-situ reflection anisotropy spectroscopy) to compare the atomic surface structures of the two materials and prepare them in controlled fashion. The critical step here was to prepare double-layer stepped substrates to avoid anti-phase disorder. The junior researcher also managed to identify important process parameters (including temperature and pressure) for producing silicon and germanium substrates with exactly the desired atomic structure. This method for silicon substrates has now been patented at HZB.

These results could also be significant for solar fuel production, Sebastian Brückner concludes: “Solar water splitting systems require tandem configurations, where absorbers made of a silicon/III-V semiconductor combination would be especially effective. If silicon was the substrate used, such tandem cells could even be produced at low cost.”

Sebastian Brückner has been part of the workgroup of Prof. Thomas Hannappel at TU Ilmenau since June 2011, and is delegated to HZB in the scope of a cooperative project. Thomas Hannappel moved to Ilmenau in the summer of 2011, where he has put together a team of 16.

The title of Sebastian Brückner’s thesis is: “Atomic scale in situ control of Si(100) and Ge(100) surfaces in CVD ambient”. You can download the summary via the link on the right.