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Photoluminescence and Electroluminescence Imaging Workstation

Photo of a silver rectangular cabinet sitting on top of a small three-drawer desk. A computer tower sits under the table. The cabinet is open on the front. The interior walls of the cabinet are black.  A metal frame in the cabinet holds the illumination source on the top and a sample stage on the bottom.

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


Images showing areas with varying shades of gray.

PL image of multicrystalline silicon wafer (left) and (right) finished solar cell.



Two square photos. Left one shows various shades of gray. Right one is black background with numerous scattered small white dots.

(Left) EL image of a multicrystalline Si solar cell; (right) Reverse-bias EL image of a multicrystalline Si solar cell showing junction breakdown at shunts and defects.

This page provides details on the photoluminescence and electroluminescence imaging workstation in the Stand-Alone Measurements and Characterization bays of the Process Development and Integration Laboratory. This capability is currently operational.

Photoluminescence (PL) is the measure of radiative recombination when a sample is illuminated to excite excess carriers. As light generates excess carriers, their concentrations build up to values that depend on defects, impurities, and other recombination mechanisms in that region. Photoluminescence intensity is proportional to the carrier concentration: so, in general, bright areas indicate higher minority-carrier lifetime regions, whereas dark areas indicate higher defect concentration. PL is a contactless technique, which allows it to be applied between many processing steps within the solar cell processing. However, it does require optical filtering to eliminate the reflected illumination wavelength from the PL emissions.

Electroluminescence (EL) is similar to photoluminescence except that excess carriers are injected across the junction of a solar cell using an applied forward bias instead of using light to generate carriers. The carriers recombine, and electroluminescence shows regions of high and low defect concentrations. Electroluminescence is also sensitive to resistance and shunts when driving current through the solar cell. So this technique can also show breaks in metallization, higher resistance regions, shunts, and other device features. Reverse-biased electroluminescence can also be used to identify locations in a photovoltaic device where junction breakdown can occur, such as shunts, impurity decorated dislocations, and other crystallographic defects. EL is a contacted technique that requires a completed device.

Both photoluminescence and electroluminescence can identify severe shunts due to the high degree of non-radiative recombination that occurs within a shunt.

Applications:

  • PL imaging (proportional to minority-carrier lifetime) at any point of solar cell processing (bare wafer, wafer with oxide or nitride, patterned finished cell). Lifetime—which is the time constant for carrier recombination—correlates with cell efficiency
  • PL imaging with resistive load for series resistance imaging, where voltages and currents can be applied
  • EL imaging on finished cells—comparable to diffusion length and open-circuit voltage (VOC)
  • EL imaging with varying voltage and current can characterize diode ideality factor (n)
  • EL imaging with applied reverse bias for shunt/short defect characterization

Special features:

  • The system uses a Si charge-coupled device (CCD) camera with 1024 x 1024 pixels and is thermoelectrically cooled to -70°C. The detector is back illuminated and deep depleted to enhance sensitivity in the near-infrared range of about 1100-nm wavelength.
  • Entire sample in field of view, or use a zoom lens for about 10-µm spatial resolution
  • Exposure times can be varied from about 10 ms to seconds and minutes. PL exposure times on passivated samples or finished cells are typically 1 s or a few seconds. PL exposure times on bare, unpassivated wafers are longer, up to minutes, depending on surface recombination velocity. EL exposure times are typically 1 s or a few seconds for currents near 1-sun operating condition.
  • Illumination is from a laser diode with 810-nm wavelength and up to 60 W of power to spread out over a 6" x 6" area to achieve at least 1-sun intensity
  • Can image a wide variety of photovoltaic materials including single-crystalline silicon (Si) wafers, multicrystalline Si wafers, copper indium gallium diselenide thin films, and cadmium telluride thin films.
  • Can transfer a sample between the table-top (stand-alone) enclosure and the vacuum chamber located on the Integrated Measurements and Characterization cluster tool.

Contact Steve Johnston for more details.