QUANTITATIVE ANALYSIS OF MATRIX ELEMENTS AND SODIUM IN PHOTOVOLTAIC Cu(In, Ga)Se_2 THIN FTMS BY THE USE OF TIME-OF-FLIGHT SECONDARY ION MASS SPECTROMETRY

摘要

The efficiency of CIGS solar cells is strongly related to a quantitative control of the elemental composition of photovoltaic thin film systems. In the case of Cu(In,Ga)Se_2 thin film solar cells variations of the Ga and In stoichiometry within the ab-sorber layers are one important key parameter. Additionally, the spatial distribution of Na is of great interest because local enrichments of Na within the grain structure have a large effect on solar cell efficiencies. In the present study, we investigate the elemental composition of Cu(In,Ga)Se_2 thin film solar cells based on secondary ion intensities in Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiling, providing high sensitivities and high spatial resolution. For semi quantitative elemental analysis of Cu(In,Ga)Se_2 thin films the detection of MCs~+-Clusters is used and compared to depth profiles acquired with X-ray photoelectron spectroscopy (XPS). Correlation plots of the intensities of GaCs~+ and InCs~+ indicate that there is no relevant matrix effect for In and Ga due to changes in stoichiometry in the layer. Additionally, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) measurements show a strong correlation between the ratio of the bulk concentrations of Ga and In and the ratio of integrated ToF-SIMS intensities of GaCs~+ and InCs~+ therefore supporting the quantitative interpretation of MCs~+ data. Furthermore, the total concentration of Na is estimated within the complete Cu(ln.Ga)Se_2 layer stack using ICP-MS. Of particular interest is the depth dependent distribution of Na, especially the amount of Na located at layer interfaces. The depth dependent Na concentration is investigated while considering beam induced migration effects and possible interface agglomerations.