Photovoltaics research with synchrotron radiation


© Amran Al-Ashouri /HZB

Using solar energy

Photovoltaics is the direct conversion of light energy, mostly from sunlight, into electrical energy by means of solar cells. The potential of photovoltaics is enormous - especially near the equator, of course, where the irradiation density is high and the fluctuations are small. But according to studies, 20 to 30 % of the electricity demand in Central Europe could also be covered by photovoltaics. Currently, mainly static systems are used to feed electricity into the grid, but mobile use at the point of generation would also be conceivable, e.g. on cars, bags or clothing.

Research goals

For the end consumer, the costs and the performance of a photovoltaic system are the most decisive factors. Research is therefore aimed at developing cost-effective solar cells with a high degree of efficiency. To achieve this, existing technologies such as silicon cells can be improved - but more promising are newer systems that use other materials completely or in addition. In this way, sustainable raw materials can be used at the same time, the weight can be reduced and/or other properties such as thickness and flexibility can be optimised. For industrial use, it is also a matter of durability (stability), because the solar cells have to withstand wind and weather, and scalability for industrial-scale production. The material properties can be studied with synchrotron radiation.

Synchrotron radiation for the understanding of solar materials

Synchrotron radiation for understanding solar materialsSynchrotron radiation is extremely intense radiation up to the X-ray range. It shows us how materials are constructed and function at their core. With this knowledge, we can better understand the materials for photovoltaics and adapt them accordingly. Synchrotron radiation is generated at large-scale research facilities such as storage rings or free-electron lasers, in which charged particles accelerated to almost the speed of light emit photons, intense light (synchrotron radiation). In Germany, there are the synchrotron radiation sources PETRA III and FLASH at the German Electron Synchrotron (DESY) in Hamburg, BESSY II at the Helmholtz-Zentrum Berlin (HZB), the European XFEL near Hamburg, with German participation the ESRF in Grenoble, France, the KIT light source in Karlsruhe and DELTA in Dortmund.

Types of solar cells

The properties of a solar cell depend primarily on the material of the semiconductor, in which positive and negative electrical charge carriers are created through the absorption of light particles, and through whose transport to the electrodes of the solar cell an electrical voltage is generated. With a market share of over 90 %, monocrystalline, polycrystalline and amorphous silicon-based solar cells are the most widespread. Since only the wavelength range that lies within the size of the band gap of the semiconductor material is absorbed, these cells can only be optimised to a certain extent. In the search for other materials, various other cell types have been developed that have different advantages (e.g. chalcogenide solar cells, perovskite solar cells, thin-film solar cells and organic solar cells). Thin-film solar cells often consist of cadmium telluride or copper indium gallium diselenide. Here, research is being conducted on materials without critical raw materials, e.g. copper zinc sulphide selenide (kesterite). Organic solar cells do not yet achieve such high efficiencies, but they are light, felxible and can also be produced semi-transparently. The inner structure of the active layer is of central importance for organic solar cells, and this can be studied in operation with synchrotron radiation on the nanometre scale.

Perovskite solar cells

Perovskite solar cells, whose semiconductor material perovskite is cheap to produce and absorbs light with longer wavelengths than silicon, are particularly promising and very effective. In combination with silicon cells (so-called tandem solar cells), high efficiencies are achieved and the potential is far from exhausted. However, the stability still needs to be improved. To better understand the material, defects, the movement of charge carriers, the crystal structure, the stability and phase diagrams are being studied. Synchrotron radiation is predestined for this because it can be used to measure at different wavelengths. In addition, the intense light of the synchrotron allows fast temperature-dependent measurements. Interfaces can also be studied well. This is important because charge carriers are easily lost there. An overview of research on perovskite materials is provided by the Perovskite Database Project.

Photovoltaics research in Germany

Basic research on semiconductors for photovoltaics is carried out in Germany at a large number of universities and other research institutions, using different methods. At the German Conference for Research with Synchrotron Radiation, Neutrons and Ion Beams at Large Facilities 2018 (SNI2018) in Garching, this was reflected in diverse contributions from TU Munich, FAU Erlangen-Nuremberg, University of Tübingen and University of Jena - often dealing with perovskites. The electricity property of a semiconductor depends on the crystal structure, which can be determined very precisely with synchrotron radiation. However, light elements, especially hydrogen, hardly scatter at all. Therefore, it makes sense to use neutron scattering in materials containing hydrogen.

One centre of photovoltaics research in Germany is the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), which operates the synchrotron radiation source BESSY II.From understanding the material to optimised manufacturing processes, research is carried out there in various departments and there is also cooperation with companies, e.g. in the Competence Centre Photovoltaics Berlin in cooperation with the TU Berlin and in the Helmholtz Innovation Laboratory HySPRINT, which focuses on (opto)electronic materials and components at an early stage of technological development. Tandem solar cells are developed and optimised here. The efficiency of the laboratory-scale perovskite tandem solar cell was almost 30% at the end of 2021, and further progress has been made since then: the use of standard silicon solar cells and silicon bottom cells from the company Q CELLS that are ready for series production.

With its materials research at the Institute for Photon Science and Synchrotron Radiation, the Organic Photovoltaics Working Group, the Perovskite Photovoltaics Taskforce, and the KeraSolar project, there is also a focus on this research area at the Karlsruhe Institute of Technology (KIT).

Examples from research

The improvement of the proven silicon solar cells, the preparation of new cell types for the market - above all perovskite tandem solar cells - and the research on completely new materials for photovoltaics build on the results from research with synchrotron radiation. Below are some press releases and other information on this:

European pilot line for innovative photovoltaic technology based on tandem solar cells, HZB/Qcells, 23 Nov. 2022

A perfect match: perovskite meets perovskite. HZB, 7 November 2022

Tandem solar cells with perovskite: nanostructures help in many ways. HZB, 24 October 2022

Photovoltaics: Fully Scalable All-Perovskite Tandem Solar Modules. KIT, 13 July 2022

Environmental impact of perovskite-on-silicon solar PV modules lower than silicon alone, Oxford PV, TU Berlin, HZB, 11 July 2022

Thin-film Photovoltaic Technology Combines Efficiency and Versatility, KIT, 14 June 2022

From Lab to Fab: World Record Solar Cell Goes from Lab to Industry. HZB / Hanwha Q CELLS GmbH, 07 March 2022

How can the energy transition succeed? Helmholtz, 05 January 2022

Royal Society of Chemistry praises HZB team’s paper on hybrid perovskite structures. HZB / Univ. Potsdam, FU Berlin, 17 September 2021

Lead-free perovskite solar cells - How fluoride additives improve quality. HZB, 26 July 2021

Perovskite Solar Cells: Insights into early stages of structure formation. HZB, 18 June 2021

Tandem Solar Modules: One-Two Combination Packs a More Powerful Punch. KIT, Capitano project with ZSW and NICE Solar Energy, 02 September 2019

Perowskit-Material-„Kombo“ bereitet Weg zu hocheffizienten Solarzellen. (KIT Kompakt 05/2019)