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

Abstract

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.