Optimizing sulfurisation conditions in the fabrication of Cu 2 ZnSnS 4 absorber layers from electroplated precursors
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1210-Q06-03
Optimizing sulfurisation conditions in the fabrication of Cu2ZnSnS4 absorber layers from electroplated precursors Jonathan J. Scragga,*, Daniel Wolversonb, Guillaume Zoppic and Laurence M. Petera a
University of Bath, Department of Chemistry (bPhysics), BA2 7AY, UK c Northumbria photovoltaics applications centre, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK. *corresponding author: [email protected] ABSTRACT Using direct photoelectrochemical measurement of the photocurrent obtained from Cu2ZnSnS4 (CZTS) absorber layers made by a two-stage electroplating-sulfurisation process, the influence of processing conditions (temperature, time, and pressure) on material quality was investigated with a view to understanding the long sulfurisation times usually found in the literature. The improvement in photocurrent due to KCN etching was also studied, and seems to be due both to removal of surface phases and also slower etching of the bulk material. The optimum sulfurisation time was found to be around 50 minutes, despite evidence that sulfur incorporation and phase formation are complete within 5 minutes. Slow grain growth was suggested as a rate-limiting factor, and a rate constant was derived based on a simple model. INTRODUCTION The cost reduction potential of the thin-film photovoltaic materials Cu(In,Ga)(S,Se)2 and CdTe compared to silicon is one of the key factors motivating continued research into them. However, in the context of large-scale deployment up to TW levels, it has been suggested that the rarity of Tellurium and Indium will limit their applicability. Cu2ZnSnS4 (CZTS) has suitable properties for a thin-film absorber, but contains only naturally abundant, non-toxic components [1]. If the fabrication of efficient devices based on CZTS can be achieved, then it promises both significant cost reductions and the potential for sustainable production. The best device to date was made using a two-stage process of precursor production by RF co-sputtering followed by thermal sulfurisation, giving an efficiency of 6.7 % [1]. There are many examples of similar two-stage processes, the key step in all cases being the sulfurisation. As detailed in Table I, sulfurisation conditions in the literature vary greatly, for example times from 10 minutes up to 3 hours are reported. Pressure [mbar] Temperature [°C] Time [mins] Source of S Ref [1] 0.56 525 10 S [3] 1×10-4 300-500 60 S [1] Not given 580 180 H2S [4] Not given 550 180 S this work 1-800 500-600 0-200 S Table I Sulfurisation conditions used for several two-stage processes in the literature.
If the two-stage process is to be industrially viable, the sulfurisation time in particular must be minimized, however no detailed study of the effect of sulfurisation time – or indeed sulfurisation parameters in general – has yet been published. The use of a rapid thermal processing system with heating/cooling rates up to 20°Cs-1 allows us to perform such a study
with excellent time resolution. Stacked metallic precursors were produced using an electropl
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