Institute of Technology
Research at the Institute of Technology focuses on the next generation of solar cells, including new kinds of materials and innovative cell structures. Long-term goals are to develop efficient and competitive thin film solar cells. Thin-film technologies are developed to a stage where industrial applications can follow as the next step. Currently research is focused on the deposition and analysis of chalcopyrite (Cu(In,Ga)Se2, Cu(In,Ga)S2) and kesterite (Cu2ZnSnS4) thin film solar cells. Because the formation of these compound semiconductors during coevaporation or from precursor layers is complex, innovative structural in-situ analysis by energy-dispersive X-ray diffraction and fluorescence is applied to reveal the key reaction mechanisms for the film formation. The polycrystalline compound semiconductor layers as well as complete solar cells are analyzed by a wide array of structural, electrical and optical characterization techniques such as grazing incidence X-ray diffraction, EXAFS, XANES, admittance, photoluminescence, electroluminescence, Raman spectroscopy, and different electron microscopy-based analytical techniques such as EDX, EBSD, and EBIC.
Coevaporation of Cu(In,Ga)Se2 (CIGSe) thin film absorber layers on glass or flexible substrates yields solar cells with conversion efficiencies > 20%. To achieve such high efficiencies, complex multi-stage PVD processes are used, which are still not completely understood. In addition, these solar cells comprise a large number of functional layers, e.g. the stack ZnO:Al/ZnO/CdS/CIGSe/MoSe2/Mo/glass, including elemental gradients and the presence of defect chalcopyrite phases at the heterojunction interface. Our research aims at furthering the understanding of the formation and function of the material properties of these layers and the ensuing device characteristics, with the goal to control and tailor specific material and device properties by optimizing the deposition processes.
Detailed insights into the reaction processes during the formation of thin film compound semiconductors such as Cu(In,Ga)(S,Se)2 and Cu2ZnSn(S,Se)4 can be gained by synchrotron-based in situ energy-dispersive diffraction and fluorescence analysis (EDXRD/XRF) in combination with numerical modeling. For these measurements rapid thermal annealing or coevaporation of thin film absorber materials is performed in dedicated custom-built process chambers at the synchrotron beamline at BESSY II. The understanding of the complex reactions taking place during the thin film deposition is essential for improvements of existing processes as well as for the development of novel processes and solar cell concepts.
Fast absorber growth for thin film solar cells may be achieved by rapid thermal processing (RTP). Here, metal precursor layers are rapidly heated up by infrared radiation and react chemically in a sulfur or selenium atmosphere to form polycrystalline semiconductors such as Cu(In,Ga)S2, Cu(In,Ga)Se2, or related materials.
The class of adamantine - or diamond-like – semiconductors comprises a multitude of binary, ternary and quaternary materials with widely varying optical and electronic properties. To utilize the full potential of the sun’s spectrum in thin film solar cells, band gaps between 1 and 1.7eV are necessary. Sustainable development of photovoltaic electricity on the terawatt scale requires the use of abundant and non-toxic elements. Recently the quaternary kesterite-type materials Cu2ZnSnS4 and Cu2ZnSnSe4 have attracted growing interest because of their optimal band gaps between 1-1.5eV, abundance of the constituent elements and its close relationship with chalcopyrite-type materials. We focus on investigating the fundamental properties of these materials as well as on developing suitable deposition methods for efficient solar cells based on these semiconductors.
The Electron Microscopy (EM) unit at the Solar Energy Division of the HZB, runs several scanning and transmission electron microscopes equipped with state-of-the-art analytical methods such as electron back scatter diffraction (EBSD), microprobe (EDX), and EBIC. Specialized sample preparation including focused ion beam (FIB) methods is essential to characterize absorber and functional layers as well as interfaces of thin film solar cell materials. Focus of the scientific work is revealing structure-property relationships for polycrystalline thin film solar cells.
(030) 8062 - 42931
