By Prof. Dr. Susan Schorr, Helmholtz-Zentrum Berlin, Germany.
Kesterite-type compound semiconductors (Cu2ZnSnS4 – CZTS, Cu2ZnSnSe4 – CZTSe) are probably the most interesting material under development considering the aspects of earth abundance, low cost, durability and sustainability of a solar cell technology. Nevertheless the performance of kesterite-based devices has been limited by a low open circuit voltage (VOC), the origin of which has been the subject of intense debate.
The kesterite type structure (space group I-4) can be described by a stacking sequence of cation layers Cu/Sn – Cu/Zn – Sn/Cu – Cu/Zn – Cu/Sn perpendicular to the crystallographic c-direction. As the most likely origin of the band tailing the disorder in the Cu-Zn plane in the kesterite structure is discussed, but also recombination of charge carriers at deep defects attributes to band tailing. Such defects are resulting from deviations from the stoichiometric composition. The best performing kesterite-based thin film solar cells were obtained with a material quite different from the stoichiometric compound, especially with a Cu-poor/Zn-rich composition. Thus besides Cu-Zn disorder, formed by CuZn and ZnCu antisites, various kind of intrinsic cationic point defects (vacancies, antisites, interstitials) occur in off-stoichiometric kesterite-type semiconductors. The formation of such defects is driven thermodynamically by minimizing the Gibbs free energy of the crystal.
Neutron scattering – in this case elastic coherent scattering (diffraction) was applied – allows a differentiation between the electronic similar elements copper and zinc in the crystal structure. In a systematic study based on neutron powder diffraction, applying the average neutron scattering length analysis method, we were able to evaluate experimentally the off-stoichiometry type and intrinsic point defect concentrations as well as the Cu-Zn disorder in kesterite-type semiconductors.
The presentation will focus on the correlation between off-stoichiometry, structural disorder and physical properties of kesterites.
Susan Schorr, Professor for Geo-Materials Research, Freie Universitaet Berlin and head of the Department Structure and Dynamics of Energy Materials at the Helmholtz-Zentrum Berlin for Materials and Energy (HZB), received 1991 her Diploma degree in crystallography from the Humboldt University Berlin. After she obtained her Ph.D. degree in physics from the Technical University Berlin in 1995, she joined the inelastic neutron scattering group at the Hahn-Meitner-Institute Berlin as a Postdoctoral Associate. In 1997-98 she was visiting scientist at the Los Alamos National Laboratory, US. In 2001 she started as a Research Associate at the University Leipzig, Institute of Mineralogy, Crystallography and Materials Science, where she finished her Habilitation in 2006. In this time she started to work on ternary and multinary compound semiconductors for photovoltaic applications and developed the average neutron scattering length analysis method to evaluate the materials intrinsic point defects. S. Schorr went to the Hahn-Meitner-Institute Berlin (now HZB) to join the Institute of Technology in the Solar Energy Division as a group leader. In 2008 she was appointed as Professor for Geo-Materials Research at the Institute of Geological Sciences, Freie Universitaet Berlin and became 2011 Department Head at the HZB. Since 2013 she is the speaker of the HZB’ Energy Materials Research Division and in 2015 she was elected chair of the Scientific-Technical Council of the HZB.