To reveal the inner structures and interactions of these BHJ "highways,"
"Solar cells are like pancakes, flat with a big surface area," Eisaman said. "Sunlight typically hits the solar cell from the top side and passes through its thin layers. This is called normal incidence."
Previously scientists would observe the photoactive layer by shining a laser through the top of the solar cell, similar to sunshine. But probing solar cells with normal incidence is an incomplete method--light shone from above will tend to have higher intensity at the top of the photoactive layer, decreasing as it gets absorbed through the material and limiting the resolution. The new method that Eisaman and his team developed sends light horizontally through the photovoltaic rather than just from the top.
"Guided optical modes allow for better control of the position of the light," Eisaman said. "The light propagates within the plane of the pancake, providing more precise information."
Fullerene and polymer materials do not uniformly mix throughout the BHJ photoactive layer. Instead, the materials tend to "phase segregate," with one side being polymer-rich and the other side being fullerene-rich. This phase segregation affects both the propagation of light and the passage of electrons and holes through the layer. Using their high-resolution picture of the BHJ photoactive layer, the scientists then mapped out how electrons travel through the solar cell.
"The electrons and holes are like two different makes of cars that travel on two different types of highways," said
The new method enabled Eisaman and his team to selectively excite regions within the BHJ photoactive layer so that they could measure, with unprecedented accuracy and simplicity, the distance the electrons travel.
"With the normal incidence method, you're creating a lot of cars scattered somewhere between Exit 35 and 50," Eisaman said. "But with our guided-mode technique, we're able to effectively create cars at exactly Exit 60. So we can observe how many of them traveled safely from that exit to the end of the highway, clearly drawing the path and revealing the pot-holes, road-blocks and accidents."
Added Dissanayake, "This technique gives you a fundamental understanding of how composition within a solar cell affects charge extraction and the efficiency of a device. It gives people guidelines on how to formulate high efficiency solar cells--not limited to organic, but also other types of nanomaterial-based photovoltaics."
The researchers used instruments at the
"The complimentary capabilities of the new optoelectronic techniques being developed in our lab and the world-class fabrication and materials characterization facilities at CFN make Brookhaven a perfect place to do this work," Eisaman said.
"This technique is core to our strategy for building new and unique capabilities for photovoltaic device characterization," said
In addition to Eisaman and Dissanayake,
The National Synchrotron Light Source (NSLS) provides intense beams of infrared, ultraviolet, and x-ray light for basic and applied research in physics, chemistry, medicine, geophysics, and environmental and materials sciences. Supported by the
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