The influence of hydrogen in the incorporation of Zn during the growth of Cu2ZnSnS4 thin films

Cu2ZnSnS4 (CZTS) is a p-type semiconductor, candidate to replace Cu(In,Ga)Se-2 as absorber layer in thin film solar cells. The best solar cells based on CZTS present efficiencies up to 6.8%. These results were improved when metallic Zn was replaced by ZnS, which may imply a different chemical path for the formation of CZTS. In this study it is compared with the diffusion of Zn on Cu2SnS3 by introducing metallic Zn or ZnS. For this CZTS films were grown by sulphurization of Cu2SnS3, some with a Zn layer and others with a ZnS layer. The influence of H-2 during the annealing process is also studied and for this some sulphurizations were done in the presence of a partial atmosphere of H-2. The SEM micrographs of the samples show a columnar growth structure of the films with different degrees of compactness. The compactness is improved in the samples where a ZnS layer was present in the precursor and the sulphurization was done in the presence of H-2. EDS chemical profiling revealed regular zinc distribution for the samples with metallic Zn whilst the ones with ZnS exhibited a Zn-rich surface. X-ray diffraction (XRD) indicated the presence of CZTS and Cu2-x. phases in all samples. These results were confirmed by Raman scattering. It was concluded that the sulphurization of Cu2SnS3 films with the use of ZnS layers under H-2 atmosphere produces better quality CZTS thin films, since it promotes Zn diffusion and avoids Zn losses by evaporation. (C) 2011 Elsevier B.V. All rights reserved.