Quantitative analysis of CuIn1−xGaxSe2 thin films with fluctuation of operational parameters using laser-induced breakdown spectroscopy

To enable a quantitative analysis of CuIn1−xGaxSe2 (CIGS) thin solar cell films using laser-induced breakdown spectroscopy (LIBS), a new method based on the internal standard principle is proposed to identify the spectral line pairs unaffected by the fluctuations of measurement conditions. Due to nonlinear calibration curves resulting from the high concentration of constituent elements of CIGS, the standard deviation of measured line intensity ratios divided by the slope of calibration curves was introduced as a new figure of merit to determine the precision of LIBS analysis along with the linearity of data points in the correlation plots. From the analyses of 39 Cu, 10 In, and 5 Ga emission lines, Cu 324.754 nm–In 325.608 nm, Cu 219.975 nm–Ga 287.424 nm, and In 303.935 nm–Ga 287.424 nm line pairs were identified as the spectral lines of which intensity ratios were least influenced by the fluctuations of spot size, the position of collection optics, and/or laser energy. The selected line pairs, except the Cu 219.975 nm line, had similar upper level energy and consisted of strong resonance lines. It was found that the use of these strong lines has advantages of better reproducibility and precision for the analysis of elements with high concentration for which the calibration curve may become nonlinear due to self-absorption.