Quantitative analysis of graded Cu(In1−x,Gax)Se2 thin films by AES, ICP-OES, and EPMA

The overall composition and the compositional profile of the quaternary semiconductor Cu(In_1−x,Ga_x)Se₂ (CIGS) have strong effects on the performance of photovoltaic devices based on it. Recent work that has yielded ∼20% efficient solar cells based on CIGS has forced extra attention on quantitative analysis of the absorber layers. In this paper we present details of the procedures used to generate detailed compositional profiles of graded Cu(In_1−x,Ga_x)Se₂ thin films by Auger electron spectroscopy (AES) that when integrated, agree quantitatively with inductively-coupled plasma optical emission spectrometry (ICP-OES) data on the same films. The effects of sample rotation during sputter depth profiling on the quantification results are described. Details of the procedures used for the ICP-OES and wavelength-dispersed electron probe microanalysis (EPMA) analyses are also presented. Finally, we show why X-ray microanalysis techniques alone should not be used to argue that specific windows of copper and gallium concentrations can yield high performance devices.