Quantitative elemental analysis of photovoltaic Cu(In,Ga)Se2 thin films using MCs+ clusters

In the process of optimizing solar cells a quantitative and depth-resolved elemental analysis of photovoltaic thin films is strongly required. Regarding Cu(In,Ga)Se₂ (CIGS) thin film solar cells, depth dependent stoichometric changes of Ga and In are of great interest because the In/Ga ratio has a large effect on solar cell efficiencies. In this paper we investigate the elemental composition of CIGS 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. Quantification of the data is obtained by comparison to X-ray Photoelectron Spectroscopy (XPS) depth profiles. The detection of MCs⁺-clusters is used for semiquantitative elemental analysis of CIGS thin films. 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. Additional high resolution 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.