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Forschungsgebiete

Dünnschichtphotovoltaik

Untersuchung der Kristallisation von photovoltaisch aktiven Dünnschichten aus Chalkopyrit [Cu(InGa)SSe]2] und Kesterit [Cu2SnZn(SSe)4] mit Methoden der Röntgenbeugung und Röntgenfluoreszenz

Thin Films

Thin Film Photovoltaic Materials

The Materials

The compounds Cu(In,Ga)(S,Se)2 and Cu2ZnSn(S,Se)4 have attracted much attention as semiconductors especially due to their absorption coefficients for visible light exceeding that of crystalline silicon by two orders of magnitude. As a consequence an absorber layer of below two micrometers is already sufficient to absorb most of the incident solar power and so-called Thin Film Solar Cells become feasible. The decrease of the thickness of the absorber layer will save raw materials as well as energy input in the production process.

State of the Art

The compounds CuInSe2, CuGaSe2 and Cu(In,Ga)Se2 among other ternary compounds have shown good photovoltaic properties. First investigations of solar cell devices based on these materials have started 35 years ago. Meanwhile Cu(In,Ga)Se2 thin film solar cells are subject to active research all over the world. Although first manufactures are already producing and selling photovoltaic modules based on this technology there is still necessity for further research to clarify and to understand further material properties.

Cu2ZnSn(S,Se)4 thin films are another of the most promising materials for low-cost thin film solar cells and subject of increasing research. Compared with Cu(In,Ga)(S,Se)2 solar cells the knowledge about formation reactions, material properties and film growth is still insufficient and needs ongoing systematic investigations to enhance conversion efficiency and enable industrial fabrication.

Our Contribution (Publications)

The Thin Film Photovoltaics group focuses on investigations of Cu(In,Ga)(S,Se)2 , Cu2ZnSn(S,Se)4 and related materials by x-ray powder diffraction. We have developed a special experimental set-up which enables the detection of the semiconductor formation reactions in real time. Advanced data evaluation facilitates to determine chemical reaction equations of the synthesis of the absorber material. The experimental work is accompanied by theoretical input of thermodynamic as well as crystallographic data. These studies provide an overview on beneficial reaction paths for different chalcopyrite compounds in the phase field Cu(Al,Ga,In)(S,Se)2 and Cu2ZnSn(S,Se)4, which allows a further optimisation of the semiconductor production process. We collaborate with the Chair for Materials for Electronics and Energy Technology of the Materials Science Department (University of Erlangen-Nürnberg) and one industrial partner.

Materialphysik

Untersuchung von Kristallstruktur-Eigenschaftsbeziehungen an technologisch relevanten Materialien

Kooperationspartner

  • Department Werkstoffwissenschaften ‚Materials for Electronics and Energy Technology’
  • CENEM, Center for Nanoanalytics and Electron Microscopy, Arbeitsgruppe Elektronenmikroskopie
  • Energiecampus Nürnberg, Solarfabrik der Zukunft
  • Avancis GmbH München

Frequently used abbreviations

  • Chalcopyrite is the name for the natural mineral CuFeS2. The atoms are arranged in a tetragonal crystal structure, the so-called chalcopyrite structure. Apart from CuFeS2 many other compounds like CuInSe2 or CuGaSe2 crystallise in this structure.
  • CIS is a commonly used acronym, for example used in the term “CIS Solar Cells”. It stands for copper indium selenide, CuInSe2, but is also used for copper deficient compounds like Cu0.95InSe2.
  • CGS stands for the compound copper gallium selenide, CuGaSe2 (similar to CIS).
  • CIGS: The compounds CuInSe2 and CuGaSe2 form a solid solution. This means that any mixed crystal compound CuIn1-xGaxSe2 can be synthesised. The ideal composition concerning efficiency is represented by CIGS with an approximate gallium content of x = 0.3.
  • CZTS stands for the compound Cu2ZnSnS4, also named kesterite. The crystal structure of CZTS is similar to that of chalcopyrites such as CuInS2. A CZTS unit cell can be obtained from that of CuInS2 by replacing In with Zn and Sn. This replacement is desired in view of the scarce sources and therefore relatively high costs of In. Several research groups have attempted the use of CZTS thin film for photovoltaic solar energy conversion.
  • RTP – SEL: This shortcut means Rapid Thermal Processing (RTP) of Stacked Elemental Layers (SEL) and refers to one possible production method of the absorber material CIGS from the elements copper, indium, gallium and selenium. The sample temperature in this process is raised rapidly. Prior to annealing the Stacked Elemental Layers are deposited on a substrate (usually float glass) by sputtering or thermal evaporation.