Solar cells are not used to their full potential from a technical point of view

2014 - international year of crystallography

For the research on new suitable crystalline compounds scientists use the Inorganic Crystal Structure Database (ICSD) produced by FIZ Karlsruhe.

It is a long process from basic research to the release of a marketable product. Therefore, it is important for basic researchers to have access to reliable and professionally substantiated data. The precise knowledge of physical and chemical characteristics of the selected source material is indispensable for developing new compound classes. However, only the clever and innovative combination of all properties of substances gives the scientist the opportunity to come up with new ideas and areas of application. With a little luck a new substance will be used for industrial application.

The photovoltaics technology is an example for the progress of the basic research in material science and how it follows new directions. Yet, current solar cells mainly consist of silicon. Silicon is well known and its crystalline characteristics are well documented in ICSD the Inorganic Crystal Structure Database produced by FIZ Karlsruhe – Leibniz Institute for Information Infrastructure. All materials for the photovoltaics technology have one thing in common: They consist of layers made of photoelectric materials the production of which requires a good knowledge of the crystalline properties. In this context, ICSD is an indispensable source for scientists. With currently over 170,000 records it is the biggest database for inorganic crystal structures worldwide and dates back to 1913.

The image shows a three-dimensional crystal structure obtained from ICSD. This is the crystalline compound mercury rubidium tin telluride (HgRb2SnTe4)

The photovoltaics technology is still in an ongoing process of innovation. In addition to silicon, new crystal systems are explored. Basic research has identified several materials which could replace silicon in the future. Especially promising are materials made out of copper, indium, gallium and selenium which are also known as CIGS. These are crystal systems consisting of the above elements. These elements could be the future elements of the photovoltaics technology. Compared to silicon, the second generation of semiconductor materials has numerous advantages. The material can be applied very thinly on the substrate allowing for new areas of application for the photovoltaics technology to be created. This includes, for example, the vaporization of foils with a thin layer. These foils could then be used to coat house walls.. Therefore, heavy and thick silicon solar cells could soon become obsolete. Compared to silicon, CIGS materials are more expensive. However, they may be more economic in the long run because of the reduction in material usage and a higher degree of efficiency. The characterization of a specific research project is shown in the BINE portal¹ operated by FIZ Karlsruhe on: http://www.bine.info/en/topics/renewable-energy-sources/photovoltaics/news/duennschicht-module-guenstiger-produzieren/

Also, Zintl phases such as magnesium silicide or magnesium silicon hydride (MgSi or MgSiH systems) have big potential for future materials used in photovoltaics. Prof. Dr. Stefanie Dehnen from Phillips-Universität Marburg researches in this field. She explains her basic research in the interview below and reveals which new materials could play an important role in the future of photovoltaics.

FIZ-KA: Prof. Dehnen, you hold a chair in inorganic chemistry at Phillips-Universität Marburg. There your research is about crystalline compounds such as multicore Zintl phases, zeolite analogues or functionalized chalcogenide cluster. 2014 is the year of crystallography and you have published a highly regarded article with the title “Von salzartigen Metallen zu maßgeschneiderten Nanoteilchen” (“From saline metals to tailored nano particles”) in the magazine Spektrum der Wissenschaft. Also, the publication is about new fields of application for so-called Zintl phases in which you describe a possible use/application of this crystalline compounds in photovoltaics. Do you believe that in the PV new materials will appear, which will replace silicon as main material and, if so, why?

Prof. Stefanie Dehnen (SD): There are always new and promising potential materials for the photovoltaics technology. However, I am cautious with the opinion that silicon will be replaced as the main material in the near future. Other crystalline compounds such as copper indium selenide (CulnSe2) or cadmium telluride (CdTe) are more efficient than silicon but more critical from a toxicological perspective and more expensive. Eventually, economic factors will determine if other compounds have a chance to enter the market. Generally, I think that there will be a larger offer in the future. However, in addition to silicon there will be other materials, which enable special applications, for instance solar panels that can be applied very thinly on the substrate. In my opinion, a complete displacement of silicon will not take place because of its unparalleled availability.

FIZ-KA: Which role do the professional databases play in your daily research?

SD: During our basic research, crystallography is our central analysis method. Professional databases save us time: We do not have to visit libraries. With the help of professional databases it is possible for us to retrieve necessary information for crystalline compounds very fast.

FIZ-KA: ICSD is a professional database for inorganic crystal structures. You use ICSD. What do you think is the biggest benefit for your research using the mentioned database?

SD: Yes, we regularly use ICSD and CSD (Cambrigde Structure Database), so to speak, the organometallic pendant. My research group produces many new crystalline compounds and the first step is always to check if these crystal structures already exist. In ICSD, we can specifically research for cell parameters and make selective use of the data at a very early stage. If it turns out that the crystal structure already exists, we will stop the further analysis. Sometimes this also affects the synthesis route. This saves us time and work.

FIZ-KA: What is the relevance of the database for your discipline in general?

SD: ICSD plays an important role for us. On the one hand we can specifically sort out the crystals which have been described by other scientists very fast. On the other hand we get important information from ICSD before we publish a paper. When we discover a new crystal structure, we search in ICSD for related crystalline structures. Also, we investigate papers on the subject and cite them in our own publications.

FIZ-KA: Would you be able to do your research without ICSD?

SD: No, we could not do our work without ICSD. For our research we need the quickly retrievable and scientifically based information. Another decisive factor is the linking of data with the particular references. It is not only important for us to know if the crystal structures already exist, we also need easy access to the specific references, where the data will be described. This link exists in ICSD. The possibility to display crystal structures three-dimensionally and to turn them interactively is ideal. Hence, one glance is enough to decide if the displayed structure is relevant or not. This would be much more difficult if only the cell parameters and intervals were specified in numerical form.

FIZ-KA: Prof. Dehnen, we thank you very much for the interview.

The interview was conducted by Dr. Babett Bolle, Department of Press and Public Relations at FIZ Karlsruhe.

Editorial: HAU/BAB

Translation: RKA

¹ The BINE Information Service is provided by FIZ Karlsruhe and sponsored by the Federal Ministry for Economic Affairs and Energy.