Measurement of the sodium concentration in CIGS solar cells via laser induced breakdown spectroscopy

The control of sodium in CIGS solar cells is critical to achieve high efficiency devices, but to date composition measurement techniques either cannot detect the sub-one atomic percent levels (for example, x-ray fluorescence (XRF)) or are expensive, time consuming, and destructive (examples include SIMS, XPS/ESCA). We employed an inexpensive, fast, and minimally destructive method to measure the concentration of sodium in CIGS solar cells fabricated at the Hawaii Natural Energy Institute (HNEI). Laser induced breakdown spectroscopy (LIBS) was used to determine the relative concentration of sodium. Two different analysis methods of the LIBS data were explored: the first assumes local thermal equilibrium (LTE) of the plasma and is calibration-free while the second employs comparison of relative peak heights after calibration to determine the concentration. Analysis is presented for solar cells produced on thin titanium foils where sodium fluoride is included in the deposition process to incorporate sodium into the CIGS layer.