Investigation of detection limits of ZnSe and Cu2SnSe3 secondary phases in Cu2ZnSnSe4

Abstract

Quaternary Cu2ZnSnSe4 (CZTSe) is a promising semiconductor material for absorber layer in thin lm solar cells due to direct band gap around 1eV and high absorption coe cient (> 104cm1) (7). The highest conversion e - ciency of CZTSe solar cells is above 11% (8). Nevertheless, a low open circuit voltage with respect to the band gap is a common phenomenon in CZTSe photovoltaic devices. A plausible reason for this is a reduction in the e ective band gap due to inhomogeneities in structure, phase, or composition. To gain a detailed knowledge of the in uence of phase inhomogeneities on the performance of solar cells, the understanding of detection limits of conventionally used characterization methods is essential. The aim of this work is to study the sensitivity limits of X-ray di raction and Raman spectroscopy to the presence of two very common secondary phases for Cu2ZnSnSe4{ZnSe and Cu2SnSe3. Polycrystalline powder of two CZTSe samples (slightly Zn-rich) and one Cu2SnSe3 sample have been grown using the solid state reaction method in evacuated silica tubes. Additionally, an industrially produced powder of ZnSe has been used to produce a number of mixtures of corresponding CZTSe with 1%, 2%, 3%, 5%, 10% and 20% of ZnSe or Cu2SnSe3 respectively. The structural characterization of the starting materials as well as of mixtures was carried out by powder X-ray di raction (PXRD) and subsequent Rietveld analysis of the di raction data using the FullProf suite (11). Rietveld re nement of di raction data of the mixtures was performed, paying a special attention to the in uence of amounts of ZnSe and Cu2SnSe3 on the di raction patterns of the mixtures. The amounts of secondary phases determined by Rietveld re nement have been compared with the initial data, determining in this way the detection limits of PXRD for these secondary phases. To study the crystal structure of the synthesized mixtures at the micrometer scale Raman spectroscopy has been employed. In these measurements a 632:8nm laser line was employed and it was found to be e cient for both ZnSe and Cu2SnSe3 phase detection. By performing Raman line scan measurements we evaluated characteristic Raman mode intensities corresponding to the di erent phases and thus are able to estimate the mixture composition.