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Photocurrents resolved from CT-AFM of a polycrsytalline CdTe solar cell during illumination

A new tomographic AFM imaging technique developed at the University of Connecticut (UConn; US) and the U.S. Department of Energy's Brookhaven National Laboratory (BNL) reveals that microstructural, planar defects — previously believed to be detrimental — actually improve conductivity in cadmium telluride solar cells by acting as pathways for hole conduction, physically and energetically separated from electron conduction pathways at grain boundaries.

AFM stands for atomic force microscopy.

“Such a physical separation would increase the diffusion length and lifetime of carriers and, thus, could have a significant impact on short-circuit current and open-circuit voltage,” says Bryan Huey, associate professor and head of HueyAFM Labs at UConn, confirming that this newfound knowledge could have profound impact on the design of the next generation of more efficient solar cells. His team is hoping efforts will be made in engineering the density, orientation and interconnectivity of planar defects to confirm these observations and consequentially maximize solar cell efficiencies.

Tomographic AFM imaging enabled breakthrough

Elaborating on the role this new tomographic AFM imaging technique has played in enabling them to make this research breakthrough, Huey explains that while standard AFM imaging techniques are able to measure quantities on the surface or a cross section of material, tomography offers the ability to image throughout the depth of the solar cell. “In CdTe, this allowed our team to directly observe the crucial role that planar defects serve in these devices,” the materials scientist says. “Previous results with such systems, from our group and others, hinted at such microstructural features, but we could only confirm that the features were planar once we acquired volumetric data.”

Cadmium telluride as advantageous alternative to silicon

Another reason why this research advance could be important is that it helps scientists to better understand cadmium telluride as promising solar material, which is being floated as alternative to silicon that could even offer a number of advantages. As a direct band gap material, CdTe, for one, has a higher absorbance and less radiative recombination than silicon, according to the expert. “From an economic standpoint, CdTe has the smallest carbon footprint, lowest water use and shortest energy payback time of all photovoltaic technologies,” Huey says, further pointing out that CdTe does not suffer from the same price volatility as silicon, as it is not widely used in other semiconductor industries. “Furthermore,” the expert adds, “at about 5% market share, with the primary manufacturer, First Solar, based here in the United States, growth in this technology presents the potential for local economic opportunity.”

Next steps for the researchers

”We are eager to work more closely with solar cell manufacturers to guide and test new microstructures,” Huey sats. “We are also looking forward to applying these concepts to related photovoltaics — such as CIGS, multilayer, etc. — as well as with other classes of functional materials.”

The work is detailed in the paper “Charge transport in CdTe solar cells revealed by conductive tomographic atomic force microscopy,” published in Nature Energy.

Written by Sandra Henderson, Research Editor, Novus Light Technologies Today

Labels: Tomographic AFM imaging,atomic-force microscopy,University of Connecticut,Brookhaven National Laboratory,planar defects in solar cells,CdTe Solar Cells,Bryan Huey,HueyAFM Labs

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