Light Propagation Made Visible in Solar Cells

Near-field optical microscopy has provided a glimpse inside the workings of thin-films solar cells, potentially offering a route to optimizing them and other nanophotonic devices.

The technique uses a glass fiber tip to measure the amount of light captured in a thin-film solar cell through quantum mechanical tunneling. A team at the Jülich Research Center applied the technique to thin-film silicon solar cells that feature periodic nanostructures designed to more efficiently absorb the IR portion of the solar spectrum.

Using near-field optical microscopy, the researchers were able to measure the amount of light that had actually been captured in a thin-film solar cell. Courtesy of the Jülich Research Center.

They observed the best light absorption in cases where the period of the electric field intensity of the waveguide mode matched the periodicity of their nanophotonic 2-D grating.

Thin-film solar cells are easier to manufacture and require less material than conventional crystalline silicon cells, but they are not as efficient. Energy conversion takes place in a micron-thick layer, meaning light of longer wavelengths is absorbed poorly. Nanoscale waveguides — photonic crystals, nanowires, or plasmonic gratings — allow these layers to absorb a wider swath of incident sunlight.

Until recently, light trapping within periodically nanostructured solar cells could only be analyzed using indirect and macroscopic methods, the researchers said.

Near-field optical microscopy enables local analysis of light coupling to waveguide modes in a number of optoelectronic devices for light trapping and emission, they said.

The research was published in Nano Letters (doi: 10.1021/nl503249n).

For more information, visit www.fz-juelich.de.