Probing microscopy technique produces green solar cells

Ashley N. Paddock, ashley.paddock@photonics.com

A new microscopy technique will help engineers peer more closely inside plastic cells to predict a new way to make them more energy-efficient.

Most plastic solar cells today are made from a blend of semiconducting polymers and other carbon-rich molecules to form bulk heterojunctions, said Guangyong Li, assistant professor of electrical and computer engineering at the University of Pittsburgh. Although the material costs very little and is usable, it does not assist with energy efficiency – although it could – he said.

Current plastic solar cells have achieved an energy efficiency rate of 8.6 percent. Li said that if he can produce solar cells with a 10 percent or higher efficiency rate, they would have a broad impact on the energy market. “For large-scale commercialization, the power conversion efficiency of organic solar cells needs to be further improved such that electricity can be generated at a comparable cost to that from silicon-based solar cells.”

These organic solar cells are made from bulk heterojunctions formed from blending donor and acceptor. Courtesy of Guang-yong Li, University of Pittsburgh.

But the cells’ complex makeup has impeded further improvements. “The nanoscale morphology of the bulk heterojunctions is the critical link from material properties and processing conditions to the overall performance of organic solar cells,” Li explained. “So far, the only feasible way to optimize the device performance is through trial-and-error experiments, which are not only expensive, but also inaccurate.”

Using traditional force microscopy does not provide good enough resolution, so the researchers could not properly study the domains they needed to examine, Li said. Instead, Li used Kelvin probe force microscopy (KPFM), a method that studies the surface potential of cells. Although KPFM is not a new idea, he plans to use it in a different way.

He plans to develop an instrument to help detect the domains formed from different materials. It could help him determine the conditions that plastic solar cells should have for better energy efficiency.

“By developing an instrumental tool to characterize the nanoscale morphology, we expect to unveil the ‘blackbox’; thus, to find … a simple and direct way to optimize the device for best performance,” Li said.

Next, he plans to develop multiscale simulation on organic solar cells based on the correct understanding of the nanoscale morphology of the bulk heterojunctions. Such simulation will help scientists find the optimal materials, design and processing conditions that could lead to the best performance for the device.

The research was funded by the National Science Foundation.