Neldner, Kai: Structural trends and phase relations in off-stoichiometric kesterite type compound semiconductors. , 2016
Freie Universität Berlin
Open Access Version

The finite resources of fossil fuels, the security risk of atomic power plants and the absence of a permanent repository for nuclear waste have greatly increased the importance of renewable energies like wind power and solar energy. Photovoltaic materials have seen a rapid development in the recent years. Especially in the thin film solar cell research multiple new technologies and absorber materials have emerged. In a short period of time the efficiency of the thin films based on CZTS have reached a record efficiency of 12.6% [97]. The main advantages of thin film solar cells are fast (vacuum free) fabrication processes, fewer material usage and therefore lower fabrication costs. To further increase the efficiency of CZTS based thin film solar cells it is important to understand the limiting factors of the material system to reach similar efficiency as the CIGS system of 22.6% [8] by avoiding the use of toxic and scarce elements like Cd and In. This work presents a systematic and fundamental study on the structural variations and chemical trends in off-stoichiometric CZTS type compound semiconductors. Therefore CZTS powder samples are synthesized by solid state reaction and the chemical composition is determined by WDX analyses at an electron micro probe analyzer. Several advanced diffraction techniques like X-ray diffraction, anomalous X-ray diffraction and neutron diffraction are used to obtain lattice parameters and site occupations of the Wyckoff positions. The goal is to study intrinsic point defects and the corresponding defect concentration in relation to the distinct off-stoichiometric CZTS types. In total six different off-stoichiometric types are identified by introducing the E- [29], F-type [96] to the already postulated A-, B-, C- and D-type [45]. The chemical characterization of the CZTS powders shows that single-phase samples are formed preferably in the off-stoichiometric B-type region, with mixtures of A- and F-type, which correspond mainly to Cu-poor / Zn-rich conditions. X-ray diffraction results show that CZTS is the main phase in all samples even if secondary phases like copper, zinc and tin sulfides are present. Furthermore the evaluation of the tetragonal deformation and unit cell volume show a strong relation to the Cu and Zn content of the samples. The tetragonal deformation and unit cell volume decrease from Cu-poor / Zn-rich to Cu-rich / Zn-poor conditions (no tetragonal deformation = 1). Also the intrinsic point defects, defect complexes and defect concentrations determined by neutron diffraction in combination with the average neutron scattering length method, change by chemical composition and correspond to the proposed off-stoichiometric CZTS types. Furthermore the analyzed Cu-Zn disorder is independent of the off-stoichiometric CZTS types and is the dominating defect complex in the majority of samples.