Cu2ZnSnS4 Nanocrystal Ink Absorber Layers for Thin Film Solar Cells

Abstract

A promising approach to low-cost photovoltaic devices is to fabricate thin film solar cells using solution-processable nanocrystal inks. This is applicable to the material class of kesterites having favourable opto-electronic properties and the potential for high efficiencies. This study looks at the chalcogenide compound Cu2ZnSnS4 (CZTS) in particular which consists of earth-abundant elements and therefore provides means for truly sustainable energy generation. Re-dispersed in an organic solvent, the nanoparticles can easily be deposited in thin films on top of glass substrates. Subsequent annealing is used to promote grain growth for a dense, polycrystalline absorber layer. The resulting films (or as-synthesised particles) are analysed with respect to their crystal structure, phase purity, morphology, chemical composition, and opto-electronic properties. Combining a variety of methods helps to disentangle the complex material characteristics of the quaternary compound.

The reproducibility of CZTS nanocrystal synthesis by the hot-injection method is briefly discussed. The focus of this thesis is on the influence of the annealing temperature held at a constant value in a range between 400°C and 600°C for 1h on the film development. This is supplemented by an investigation of the influence of different annealing times between 0.5h and 3.0h at a moderate temperature of 500°C and by an exemplary analysis of cross-sections through samples of a CZTS absorber on a molybdenum rear contact layer. The experiments suggest that temperatures around 600°C are preferable in order to enhance the quality of the crystal structure and to obtain large-grained films which is seen as a major area of improvement in this field of research. Increasing the annealing time, in contrast, does induce hardly any changes in the material and is therefore less effective in achieving high-quality absorber layers. The final state of the films is reached after only about 1h of annealing.