"We can now distinguish between efficient and inefficient regions of a nanostructured photovoltaic film," lead researcher David Ginger told optics.org. "The technique we have developed measures photo-induced charge buildup in polymer solar cells with a sufficient spatial resolution of less than 100 nanometers."

According to the research, carried out by David Coffey and David Ginger at the University of Washington, the technique could help developers better understand how today's organic solar cell devices are working at a microscopic level.

"We found that the rate of photo-induced charging under the tip was a good indicator of the photovoltaic efficiency of the material in that region," said Ginger. "By directly imaging photo-induced charging, we hope this technique will help facilitate more rational device design."

EFM, a version of atomic force microscopy, measures long-range Coulombic forces between a sample and the conductive tip of an atomic force microscope. As the cantilever tip scans across the surface of the sample, changes in the electric field between the tip and the sample result in changes in the measured cantilever motion.

This makes it possible to detect very weak electric fields from very few photoinduced charges, and with a spatial resolution of below 100 nm. In the particular type of EFM employed by the team, the cantilever is set oscillating at its resonant frequency. Any electric-field gradient between the tip and sample causes a change in the effective spring constant of the system, which results in a change in the resonant frequency of the cantilever.

"The change in resonance frequency is the signal that we detect. Our innovation was to measure this 'frequency shift' as it changed with time while the photovoltaic polymer film was being illuminated from underneath. This allowed us to measure how fast the photogenerated charge was building up," said Ginger.