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In Situ Optical Diagnostic for Monitoring or Control of Sodium Diffusion in Photovoltaics Manufacturing

National Renewable Energy Laboratory

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PDF Document PublicationUS Patent 9,136,184 (1,521 KB)

Technology Marketing Summary

To measure the thickness and density of Cu(InxGa1-x)Se2 (CIGS) layers, manufacturers use methods such as X-ray fluorescence (XRF) and transmission electron microscopy (TEM). XRF, while the standard diagnostic tool for molybdenum films in CIGS manufacturing, is not sensitive to film density and is only sensitive to the number of atoms within the sample’s volume. Due to this, CIGS manufacturers must assume a constant density and infer film thickness during production. TEM, conversely, quantifies the density of very small samples removed from the production line. However, this method may take days to determine the sample’s density. This delay in determining the density of molybdenum films and the assumptions of constant density and film thickness do not account for the variations in the density of molybdenum films that can occur during the production process, which, as researchers at NREL have discovered, can significantly impact the diffusion of sodium (Na) from the soda lime glass (SLG) substrate of CIGS films and adversely affect the device’s efficiency. Therefore, there is still a need for a method to determine the density of molybdenum films in-situ, in-line, and in real time. 


Researchers at NREL have developed an in-situ, in-line, and real time diagnostic method to measure the density of molybdenum films. This method comprises directing a beam of polarized light through the substrate to the molybdenum layer and monitoring the change in the polarization state of the light upon reflection to determine its dielectric function and, in turn, an accurate measurement of the molybdenum film’s density. The resulting density measurements can then be used as a feedback control on the deposition process and as a feedforward control for CIGS processing to adjust to changes in Na diffusion caused by variations in the molybdenum layer’s density. 

  • In situ, in-line, and real-time
  • Applicable to control deposition process
  • Monitors effect of Na diffusion
Applications and Industries
  • Photovoltaics
  • Density measurement
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated

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To: Bill Hadley<>