Kesterite Equilibrium Reaction and the Discrimination of Secondary Phases from Cu2ZnSnS4

Kesterite (Cu2ZnSnS4 - CZTS) based thin films are of increasing interest in the field of thin film photovoltaics. In recent developments, mixed S-Se kesterite devices have shown power conversion efficiencies of more than 10 %, but yet a major challenge is the fabrication of single phase kesterite material for high quality solar cells.
In this thesis, two aspects of the growth of single phase kesterite films have been studied in more detail. The first part focuses on the identification of secondary phases (e.g. ZnS and Cu2SnS3) and their discrimination from CZTS. Hereby, grazing incidence X-ray diffraction and Raman spectroscopy measurements were used to quantitatively identify secondary phases on a mixed, CZTS containing sample. Experimental results show, that in a measurement series a minimum of 7 % of ZnS or 28 % of Cu2SnS3 must be present in order to be identifiable. The second part of this thesis focuses on finding a way to grow absorber layers for high quality CZTS thin film solar cells. Hereby, it could be shown that it is important for the quality of the absorber layer to keep the equilibrium position of the formation reaction of CZTS close to the CZTS side throughout the annealing. To achieve this, it is suggested to anneal the precursors in the presence of sulfur and tin sulfide. A study showed that this novel annealing strategy helped to improve the power conversion efficiency of a device based on a vapor-deposited precursor from 0 % to 6.1 %. Within this context, a novel route to form CZTS from a simplified, only Cu and Zn containing precursor, has been introduced and a model of the formation reaction of CZTS from this simplified precursor has been suggested.