Na incorporation in Mo and CuInSe2 from production processes

Results of characterization of thin films of Mo deposited by DC magnetron sputtering on soda-lime glass (Mo/SLG) and CuInSe₂ (CIS) on Mo/SLG are presented. The primary objective of the work was to clarify the factors determining the concentration of Na in commercial-grade CIS. Mo films were deposited by three laboratories manufacturing CIS thin film solar cells. Analysis was by secondary ion mass spectrometry, scanning electron microscopy and X-ray diffraction. Changes in Mo deposition parameters in general affected the Na level but there was no obvious link to any single Mo deposition parameter. Oxygen content directly affected the Na level. The Na behavior was not obviously connected to film preferred orientation. Selenization of the Mo layers was also examined. Elemental Se vapor was found to produce significantly less selenization than H₂Se. The amount of selenization was also strongly dependent upon Mo deposition conditions, although a specific source of the change in reaction rate was not found. Na distributions in the CIS deposited on the Mo were not limited by the diffusivity of the Na. The Na concentration in the CIS was increased by annealing the Mo films both with and without intentionally added Na. The Na level in the CIS appears to be set more by the CIS deposition process than by the Na concentration in the Mo so long as the Mo contains sufficient Na to saturate the available sites in the CIS.