Experimental study of Cu2ZnSnS4 thin films for solar cells
Cu2ZnSnS4 (CZTS) is a semiconductor with a direct band gap of about 1,5 eV and anabsorption coefficient of 10^4 cm^-1, and is for this reason a potential thin film solarcell material. Demonstrated efficiencies of up to 6,8% as well as use of cheap andabundant elements make CZTS a promising alternative to current solar cells.The aim of this study was to fabricate and characterize CZTS films and to evaluatetheir performance in complete solar cells. For the fabrication of CZTS we applied atwo-step process consisting of co-sputtering of the metal or metal-sulphurprecursors, and subsequent sulphurization by heating at 520°C in sulphur atmosphereusing sealed quartz ampoules.The work included a systematic comparison of the influence of composition on qualityand efficiency of CZTS solar cells. For this purpose films with various metallic ratioswere produced. The results show that the composition has a major impact on theefficiency of the solar cells in these experiments. Especially zinc-rich, copper-poor andtin-rich films proved to be suitable for good cells. The worst results were received forzinc-poor films. An increase in efficiency with zinc content has been reportedpreviously and was confirmed in this study. This can be explained by segregation ofdifferent secondary phases for off-stochiometric compositions. According to thephase diagram, zinc-poor films segregate mainly copper sulfide and copper tin sulfidecompounds which are conductive and therefore detrimental for the solar cell. Zincsulfide, that is supposed to be present in the other regions of the phase diagramexamined in this study, could be comparatively harmless as this secondary phase isonly isolating and by this ’just’ reduces the active area. This is less disadvantageousthan the shunting that can be caused by copper sulfides. Contrary to the efficiencyresults, metal composition had no major impact on the morphology.A comparison of the composition before and after the sulphurization revealed thatmetal precursors showed higher tin losses than sulphur containing precursors. Apossible explanations for this was given.Another central point of this work was the examination of the influence of sulphur inthe precursor. Less need of additional sulphur in the film might lead to better materialquality. This is based on the assumption that the film is subjected to less diffusion ofthe elements and so to less dramatic changes within the film, which might result infewer voids and defects. However, our experiments could find only a weak trend thatsulphur in the precursor increases the performance of the solar cells; concerningmorphology it was observed that more compact films with smaller grains developfrom metal-sulphur-precursors.The best efficiency measured within this work was 3,2%.
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