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  Veloso Ferreira, Rafael: Investigation of the detection limits of ZnSe and Cu₂SnSe₃ secondary phases in Cu₂ZnSnSe₄ by XANES. , Coimbra, Universidade de Coimbra, thesis, 2017

Abstract:
The quaternary Cu2ZnSnSe4 (CZTSe) is a promising semiconductor material for absorber layers in thin film solar cells due to its direct band gap around 1 eV and high absorption coefficient larger than 104 cm-1 [6]. The highest conversion efficiency of CZTSe solar cells achieved so far is above 11.6% [7]. However, one troubling and common phenomenon in CZTSe-based photovoltaic devices is a low open-circuit voltage with respect to the band gap energy. A plausible reason for this could be a reduction in the effective band gap due to inhomogeneities in structure, phase, or composition in the absorber layer. To gain a detailed knowledge on the influence of phase inhomogeneities on the performance of solar cells, the understanding of detection limits of investigation methods is essential. The sensitivity limits of the conventionally used methods such as X-ray diffraction and Raman spectroscopy were studied recently [10]. This work aims to understand the real sensitivity levels of XANES to the presence of Cu2SnSe3 and ZnSe in Cu2ZnSnSe4, two very common secondary phases for this compound. Additionally, the effects of subjecting highly off-stoichiometric CZTSe samples containing many secondary phases to a second thermal annealing process were investigated, focusing on changes in their phase contents and the compositions of the CZTSe phases. To achieve the main purpose of this work, two sets of polycrystalline powder samples were used. So as to simulate secondary phase-containing CZTSe absorbers, single-phase CZTSe powder was mixed with determined amounts of secondary phase powder – single-phase Cu2SnSe3 or ZnSe – in order to obtain a calibration series of CZTSe mixtures containing 1%, 2%, 3%, 5%, 10% and 20% of secondary phase. The two mixture sets were prepared into pellets and measured by transmission mode XANES at the Se and Cu K-edges. Data from these measurements were analysed with the ATHENA program [21] using a linear combination fitting (LCF) method. The phase characterization of the re-annealed CZTSe samples was carried out by powder X-ray diffraction, while the compositional analysis was performed by wavelength-dispersive X-ray spectroscopy. Results obtained have shown that XANES is capable of quantifying the ZnSe secondary phase at all concentrations within a small error bar. With this technique, it was also possible to detect the Cu2SnSe3 phase down to 5% concentration. Investigation on the influence of the CZTSe standard’s stoichiometry and the addition of various secondary phase standards to the LCF analysis had largely inconclusive results, particularly with the ZnSe mixtures, but revealed the necessity for further investigation on these topics. Results from the techniques applied to the re-annealed samples showed that the secondary phase contents had remained identical in most samples and that the amount of distinct CZTSe phases did not decrease in any of them, although their composition had changed with respect to those previously present. In the end, it was determined that the annealing had no major benefits in increasing the homogeneity of the samples.