Kesterites Cu2ZnSn(S,Se)4 are interesting candidates for thin film solar cells on large scale because they contain only abundant, cheap and non-toxic raw materials. Their viability has been shown recently by an efficiency close to 10%. Current solar cell structures are based on the structures developed for chalcopyrite solar cells (CIGS). We are working on an industrially relevant co-evaporation process, which has so far achieved efficiencies close to 6%. We foresee several ways to improve this efficiency and to further the basic knowledge about these materials. Deliberately grading the bandgap within the absorber can suppress recombination at front and back surfaces. This can be achieved either by varying the metallic composition or by varying the Se/S ratio across the absorber. The Mo back contact has already been shown to be problematic because during absorber preparation a MoSe2 layer forms, which creates a barrier for charge carrier transport. We will investigate different metals for back contacts, guided by their tendency to form selenides and by their work function. This will at the same time allow conclusions on the band structure of the kesterites. It was shown that CIGS solar cells profit from Na diffusing from the glass substrate. The influence of Na on the efficiency, morphology and electronic properties of kesterites will be investigated in this project.The objective is an efficiency beyond the current 10% level and an improved understanding of the structural and electronic properties of kesterites.