Machine Learning Identifies High-Performing Solar Materials


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Even after this first pass, the researchers still had approximately 3,000 dye candidates to consider. To further refine the selection, the scientists screened for dyes that contained carboxylic acid components that could be used as chemical ​“glues,” or anchors, to attach the dyes to titanium dioxide supports. Then, the researchers used Theta to conduct electronic structure calculations on the remaining candidates to determine the molecular dipole moment — or degree of polarity — of each individual dye.

“We really want these molecules to be sufficiently polar so that their electronic charge is high across the molecule,” Cole said. ​“This allows the light-excited electron to traverse the length of the dye, go through the chemical glue, and into the titanium dioxide semiconductor to start the electric circuit.”

After having thus narrowed the search to approximately 300 dyes, the researchers used their computational setup to examine their optical absorption spectra to generate a batch of roughly 30 dyes that would be candidates for experimental verification. Before actually synthesizing the dyes, however, Cole and her colleagues performed computationally intensive density functional theory (DFT) calculations on Theta to assess how each of them were likely to perform in an experimental setting.

The final stage of the study involved experimentally validating a collection of the five most promising dye candidates from these predictions, which required a worldwide collaboration. As each of the different dyes had been initially synthesized in different laboratories throughout the world for some other purpose, Cole reached out to the original dye developers, each of whom sent back a new sample dye for her team to investigate.

“It was really a tremendous bit of teamwork to get so many people from around the world to contribute to this research,” Cole said.

In looking at the dyes experimentally at Argonne’s Center for Nanoscale Materials, another DOE Office of Science User Facility, and at the University of Cambridge and the Rutherford Appleton Laboratory, Cole and her colleagues discovered that some of them, once embedded into a photovoltaic device, achieved power conversion efficiencies roughly equal to that of the industrial standard organometallic dye.

“This was a particularly encouraging result because we had made our lives harder by restricting ourselves to organic molecules for environmental reasons, and yet we found that these organic dyes performed as well as some of the best known organometallics,” Cole said.

A paper based on the study, ​“Design-to-device approach affords panchromatic co-sensitized solar cells,” appeared as the cover article in the February 1 issue of Advanced Energy Materials. Other Argonne authors of the paper included Liliana Stan and Álvaro Vázquez-Mayagoitia. Authors from the University of Cambridge, Rutherford Appleton Laboratory (UK), Indian Institute of Technology-Roorkee, Tianjin University of Technology (China), Hong Kong Baptist University, University of Zaragoza (Spain), and University of Naples (Italy) also contributed.

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