Mercoledì 31 ottobre settembre 2018
Sala Seminari, Piano I, Edificio U5 – via Roberto Cozzi 55, Milano
Relatore: Alberto Lomuscio – Laboratory for Photovoltaics, Physics and Materials Science Research Unit; University of Luxembourg, 44, rue du Brill, 4422-Belvaux, Luxembourg
Titolo: Quasi Fermi level splitting of Cu-poor and Cu-rich CuInS2 absorber layers
Abstract. Cu(In,Ga)S2-based solar cells are interesting tandem partners for Si or chalcopyrite solar cells, but suffer from a low open circuit voltage. Recently, record efficiencies have been achieved by using higher growth temperatures for the absorber. To gain further insight, the study of the structural and electronic quality of only the CuInS2 absorber is essential as recombination mechanisms at the interface of absorber and buffer layer in the device are excluded. We have studied the growth temperature dependence of the absorber quality itself by investigating electronic and structural properties. We show that the quasi Fermi level splitting, which indicates the maximum open circuit
voltage an absorber is capable of, increases with higher growth temperature. The quasi Fermi level splitting is limited by a deep defect and its concentration decreases with higher growth temperature and is less prominent in Cu-rich films. Thus, we demonstrate that the open circuit voltage of CuInS2-based solar cells can be limited to below 850 mV by the absorber, independent of the interface. In contrast to the changes in the electronic properties, the structural properties are rather independent of temperature within the range investigated but are significantly influenced by the composition.