Scientists have put together a couple more pieces of the puzzle to explain how energy levels align in a critical group of advanced materials, which could have implications for developing sustainable technologies such as dye-sensitized solar cells and organic LEDs. It’s been known for years that transition metal oxides, also known as superconductors, make excellent electrical contacts in organic-based devices, but the reason why hasn’t been known until now. “What we found was shocking,” said Mark T. Greiner, a materials science and engineering doctoral candidate at the University of Toronto. “We wanted to understand what is so special about oxides that makes them so useful in organic electronic devices. We soon realized that the property we were aiming to understand was the same property that makes oxides valuable catalysts: their ability to transfer charge to molecules.” Teasing out the mysteries of transition metal oxides could help progress solar cells. Courtesy of Zu Lab, University of Toronto. To better understand this property, Greiner and Zheng-Hong Lu, professor and Canada Research Chair in organic optoelectronics, examined as many materials as possible. They tested a large number of metal oxides, spanning a wide range of properties – from insulators to p- and n-type semiconductors to metallic conductors. What they discovered was a surprise. All of the oxides followed the same trend, regardless of their electronic properties, Greiner said. He explained that this trend could be represented by a single simple equation. “The findings imply that charge transfer between metal oxides and molecules depends primarily on the oxide’s electron chemical potential and the molecule’s ionization energy,” Greiner said. Providing a piece of the puzzle to further understand energy-level alignment, Greiner is hopeful that their findings will drive further developments in the field and that the discovery will extend into other fields of research as well, including catalysis. The engineers are now at work to refine their model. With their results indicating that the charge-injection barrier plateaus at a minimum value, the team is designing experiments to investigate the physical causes behind this minimum injection barrier, and will explore the limitations of the model by testing it on non-oxide materials. The research appeared online in Nature Materials (doi: 10.1038/nmat3159).