International audienceCu2ZnSnS4 (CZTS) is an interesting material for sustainable photovoltaics, but efficiencies are limited by the low open-circuit voltage. A possible cause of this is disorder among the Cu and Zn cations, a phenomenon which is difficult to detect by standard techniques. We show that this issue can be overcome using near-resonant Raman scattering, which lets us estimate a critical temperature of 533 +/- 10K for the transition between ordered and disordered CZTS. These findings have deep significance for the synthesis of high-quality material, and pave the way for quantitative investigation of the impact of disorder on the performance of CZTS-based solar cells. (C) 2014 AIP Publishing LLC

The material Cu2ZnSn(S,Se)4 (CZTS) offers a promising indium-free alternative to Cu(In,Ga)Se2 for the absorber layer in thin-film solar cells. It is known that the highest solar energy conversion efficiencies are reached for Cu-poor, Zn-rich CZTS compositions and that too much disorder at the Cu and Zn sites can have a negative impact on the device performance. In this article, we investigate the structures of [VCu + ZnCu] A-type and [2ZnCu + ZnSn] B-type defect complexes and their impact on the long-range Cu/Zn disorder. To that end, we use (119)Sn, (65)Cu, and (67)Zn solid-state NMR and Raman spectroscopy to characterize powdered CZTS samples. For both A- and B-type substitutions, our NMR investigations demonstrate the clustering of the complexes. Moreover, we show that (A+B)-type compounds should be considered as A-type and B-type compounds, since no interaction seems to exist between [VCu + ZnCu] and [2ZnCu + ZnSn] defect complexes. In addition, it is worth noting that [2ZnCu + ZnSn] complexes have only a minor impact on the level of disorder at the Cu and Zn sites. In contrast, [VCu + ZnCu] complexes seem to restrain the random distribution of Cu at the Zn site and of Zn at the Cu site; i.e., the long-range Cu/Zn disorder. Raman characterization of the CZTS samples was also conducted. The Q = I287/I303 and the newly introduced Q' = I338/(I366 + I374) ratios determined from Raman spectra collected at 785 nm turn out to be very sensitive to the level of Cu/Zn disorder. Moreover, they can be used to differentiate the nature of the substitution in slow-cooled materials.