Alkali-induced surface defects and interface passivation in chalcogenide solar cells

• Applicants: Dr. Stefan Förster; Professor Dr. Roland Scheer
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429636549

The project addresses the ambivalent physical effects of alkali atoms acting on surfaces and interfaces of chalcogenide semiconductors Cu(In,Ga)(Se,S)2 and Cu2ZnSnSe4 and their solar cell devices. Starting point is the preliminary work of the applicants which shows that the alkali element Sodium can contribute to formation and passivation of surface and interface defects. These unexpected results are not yet understood although alkali doping of chalcogenide semiconductors is standard and many of its facets are well known. In the project, tailored sample preparation, surface spectroscopy, electronic defect spectroscopy, and computer simulation shall be combined to answer the following questions: (1) Which chemical species form the alkali-induced surface and interface defect?(2) What are the chemical and physical properties of the surface defect in terms of binding site, binding character, and area density?(3) What are electronic properties of the surface defects in terms of defect energy, capture cross section, and charge states?(4) What are the physical and chemical mechanisms of alkali-induced surface and interface passivation?(5) Are there long-term effects of defect formation and passivation?In the focus of the project are the semiconductors Cu(In,Ga)(Se,S)2 and Cu2ZnSnSe4, which are already commercially available as solar modules (Cu(In,Ga)Se2) or are emerging materials from earth-abundant elements (Cu2ZnSnSe4). The alkali-induced defect formation and passivation are of economic importance because they affect the energy conversion efficiency of the photovoltaic modules. Moreover, the alkali effects are of scientific importance because fundamental mechanisms of defect formation and passivation at the surface of chalcogenides are not understood.

• DFG Programme: Research Grants