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Photothermal Deflection Spectroscopy Tool

This page provides details on the photothermal deflection spectroscopy (PDS) tool in the silicon area of the Process Development and Integration Laboratory. This capability should be operational within 2009. It supports the U.S. Department of Energy (DOE) Technology Roadmaps for crystalline and amorphous silicon, copper indium gallium diselenide, cadmium telluride, organic photovoltaics, transparent conducting oxides, and reliability. Access the Technology Roadmaps on the DOE Solar Energy Web site.

Photo of top of metal optical bench with various optical components such as a circular mirror held by four metal "fingers" and the end of an aperture device.  Several cables snake across the top of the table.

Photothermal deflection spectroscopy unit that is one of the Stand-alone Measurements and Characterization tools in the Process Development and Integration Laboratory.

This sophisticated tool uses automated photothermal deflection spectroscopy (PDS) to measure small amounts of light absorbance, which detects defect levels in silicon film photovoltaic (PV) materials. Using an inert, temperature-sensitive liquid, this tool can measure film absorbance with an accuracy of 10-5 over wavelengths from 300 nm (ultraviolet) to 3000 nm (infrared)—one of just a few PV systems with such sensitivity. By attaining accurate absorbance measurements at wavelengths well into the infrared spectrum, the PDS tool is ideal for amorphous silicon, nanocrystalline silicon, and other novel silicon film PV materials.


  • Measuring defect levels and tail states in any PV technology, although the technique is most applicable to amorphous/nanocrystalline silicon
  • Recording photon absorption as a function of photon energy ()

Special features:

  • Completely custom system design, including optical table and air cushion to minimize noise
  • Transparent substrate with deposited layer <1 mm thick, or a layer deposited on a custom substrate made of infrared quartz or sapphire
  • Absorbance values can be measured down to 10-5
  • Spot size is 0.5 mm x 2 mm
  • Class 3a intensity-stabilized He-Ne laser measures 100 μm from the sample surface
  • Sample is submerged in FluorinertTM, a non-reactive alternative to carbon tetrachloride developed by 3M
  • Position-sensitive detector (lock-in amplifier) is two photodiodes, sensitive to beam-position changes of <10 nm (angular shift of about 10-6 degrees), with readings recorded by computer
  • Motorized 3-axis motion for iterative sample alignment (x-y and rotational), with two cameras for monitoring sample position
  • Absorbance measured over the light spectrum from ultraviolet to infrared (300 to 3000 nm)
  • Measurements performed in air at atmospheric pressure
  • At least four filters help to eliminate high-order diffraction from grating monochromator
  • Custom software with sample positioning, data acquisition, and image processing developed by the National Renewable Energy Laboratory

Contact Pauls Stradins for more details.