Photoluminescence Spectroscopy Uncovers Photovoltaic Properties

Fluorescence spectroscopy is the key to new research into photovoltaic materials with the objective of developing more efficient, flexible and less costly solar cells.

A team from Texas A&M University-Central Texas uses photoluminescence to gauge the quality of solar cells, materials that convert light to electricity. The luminescence of a solar cell can indicate the quality of the solar cell crystal. Semiconductors, which are the basis of solar cells, luminesce at a very specific wavelength.

Generally, the better the luminescence of the materials, the better the efficiency of the solar cells, so researchers measure the luminescence of samples to gauge the potential semiconductor properties.

The research team is trying to develop new materials. Besides silicon, they do some work with mostly thin film photovoltaic materials like cadmium telluride, copper, indium, gallium and selenide.

The team looks at whether a cell is luminescing along with the uniformity of the luminescence. The individual materials should all have the same wavelengths of light and the same distribution.

Sometimes scientists see different spots that have dissimilar wavelengths of luminescence. That tells them the manufacturing process has introduced some level of variability in the photovoltaic materials.

The work encompasses both basic and applied research to want to understand the materials and how to process them so it can be manufactured effectively.

The team is part of a center with the long-term goal to help establish photovoltaic electricity as a major source of energy in the world.

The Texas A&M University group uses a HORIBA MicOS (a fully integrated microscope spectrometer) to do a broad array using photoluminescence across the entire sample. Its aim is to aid in the development of new materials.

The folks at Texas A&M are trying to find those poorer performing parts of the sample to understand it better, for basic research and applied applications. They are trying to understand how to get rid of the variations that are less efficient. Patterning, the deposit of poorly or non-photovoltaic materials, is something they want to eliminate. They do this through photoluminescence by engineering out the poor fluorescing components.

They will get a sample from a collaborator and take a rough scan of it with the MicOS, and do a broad array using photoluminescence across the entire sample. As a result, they have mapped the sample and might see some areas of interest.

If the team identifies one spot on a solar cell that’s more interesting, it can put it in the scanning electron microscope with the HORIBA CLUE (Cathodoluminescence Universal Extension), and zoom in on that area. Then the researchers can do micro-mapping in areas that are interesting to them.

The researchers can do micro-mapping this way in areas that are show unusual photoluminescence. Or they may be trying to understand how to get rid of the variations that are less efficient, like the way the photovoltaic materials were deposited. It lets them look at different ways to eliminate that patterning.

Researchers are trying to identify materials that can be much less expensive than silicon, the historical standard for photovoltaics. They also look for properties of substances that can do what silicon can’t do, like be lightweight, bend, and better integrate with our everyday lives. The paint mentioned earlier can be made on a solar cell on any low-cost material, like plastics and paper.  In labs around the world, novel materials include those made from semiconducting nanoparticles (quantum dots), perovskite crystals, semiconducting polymers, and even biological materials and materials made from plant-based extracts.  All of these are being investigated for better semiconducting properties. 

The key is to understand the efficiencies of the materials. Once achieved, the technology will become more commercial.

The hitch is this - the ability of these photovoltaic materials to convert light into electricity. These paints haven’t reached the efficiency of silicon – yet.

Silicon dominates the market, accounting for about 90 percent of solar cells. The remaining 10 percent is cadmium telluride. But silicon has severe limitations. It’s a poor absorber of light, requiring thick layers to absorb the energy. Researchers like those at Texas A&M are looking for more flexible less expensive alternatives.