Former Empa doctoral students Shih-Chi Yang and Amogh Kinikar received the Empa Research Awards 2023 for their outstanding scientific work. Both researchers came across their discoveries partly by chance – but also had the presence of mind and perseverance to follow up on their lucky punch and to recognize its significance.

An unexpected reaction

As a doctoral student in Empa's nanotech@surfaces laboratory, Amogh Kinikar originally wanted to synthesize certain types of graphene nanoribbons. These flat carbon structures, only one atom thick and a few atoms wide, possess unique and interesting electromagnetic properties. To produce them, Kinikar wanted to make starting molecules – hydrocarbons with bromine reaction centers – react with each other on a gold surface.

Instead of linking to each other to form long ribbons, however, the molecules paired up to form dimers. Nor did the reaction take place at the expected reaction center. Instead, the molecules reacted at so-called isopropyl groups, which consist only of carbon and hydrogen and normally hardly react with anything.

Using scanning tunneling microscopy, Kinikar identified the newly formed structure in the middle of the dimer as a benzene ring, or more technically a phenylene ring. What the trained physicist did not know at the time was that the synthesis of benzene rings is considered extremely difficult in organic chemistry. The few reaction mechanisms known to produce such rings all require highly reactive starting molecules. "A chemist would never have dared to claim that this was a benzene ring," laughs Kinikar.

However, he was proved right: He had discovered a completely new synthesis pathway for benzene rings. "The fact that a compound that was demonstrably difficult to synthesize was created from a combination of unreactive groups on an unreactive metal surface was indeed a surprise," says the researcher. The chemists in his team initially refused to believe him.

But over time, Kinikar and his colleagues succeeded in understanding the mechanism behind the unexpected reaction. Through a collaboration between chemists, physicists and computational scientists, they were able to prove that benzene rings do indeed form in the process. The team published their results in the journal Nature Synthesis. First author Kinikar has now been awarded the Empa Research Award for this paper. After completing his doctorate in 2022, Kinikar remained at Empa as a postdoc at the nanotech@surfaces lab.

Side project leads to record efficiency

Shih-Chi Yang also originally planned to focus on a different project during his doctorate. He worked in Empa's Thin Films and Photovoltaics laboratory on so-called CIGS solar cells. CIGS stands for copper indium gallium diselenide. These thin-film cells are material-saving and lightweight and, unlike conventional silicon solar cells, can be produced on flexible substrates.

In addition, it is theoretically possible to produce bifacial CIGS solar cells. The rear side can capture reflected and diffuse light and thus further increase the overall efficiency of the cell. "Unfortunately, in practice, there was a major hurdle," says Yang. "CIGS cells are normally produced at temperatures of more than 400 to 500 degrees Celsius. At these temperatures, gallium oxide forms at the back interface, which severely impairs performance."

As luck would have it, Yang had refined the manufacturing process for monofacial CIGS cells in a previous project, allowing it to take place at a lower temperature with the help of silver. The project was aimed at reducing the substrate temperature while maintaining good CIGS absorber quality – but Yang also thought of its potential for producing bifacial cells. He began a side project, in which he attempted to use the new process to make bifacial CIGS cells and tackle the long-standing challenges with their production.

The initial results were so convincing that the side project quickly turned into the main project of Yang's doctoral thesis. With his new approach, he achieved a record efficiency for bifacial CIGS solar cells. Together with his colleagues, he published the results in the journal Nature Energy – and now received the Empa Research Award for this publication.

Bifacial CIGS cells are not ready to hit the market yet. "For industrial applications, the production processes still need to be improved," says Yang. "But we were able to show: This technology works, and it's worth exploring even further." Yang, however, is leaving future research to his former colleagues at Empa. He has been working as a Machine Learning data scientist in industry since April 2023. "The analytical skills and critical thinking that I was able to develop during my doctorate at Empa are serving me very well," he says.

Empa Research Awards 2023: When serendipity meets excellence