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Scientists have discovered a method of supercharging the "engine" of sustainable fuel production - by slightly changing the materials. Researchers led by the University of Cambridge are developing low-cost light-harvesting semiconductors that power devices to convert water into pure hydrogen fuel using only energy from the sun. These semiconductor materials, known as copper oxides, are cheap, common and non-toxic, but do not come close to silicon, which dominates the semiconductor market, in terms of performance.
However, the researchers found that by growing the copper oxide crystals in a specific orientation so that electrical charges traveled across the crystals diagonally, the charges traveled much faster and farther, greatly improving performance. Tests of a copper oxide photocathode based on this fabrication technique showed a 70% improvement over existing state-of-the-art copper oxide photocathodes and also showed significantly improved stability.
The researchers say that their results published in the journal Nature, show how low-cost materials can be engineered to transition from fossil fuels to clean, sustainable fuels that can be stored and used in existing energy infrastructure.
Problems and potential of copper oxide
Copper(I) oxide, or copper oxide, has been touted for years as a cheap potential replacement for silicon because it is efficient enough to capture sunlight and convert it into an electrical charge. However, much of this charge tends to be lost, limiting the performance of the material.
"Like other oxide semiconductors, copper oxide has its own intrinsic problems," said one of the authors, Dr Linfeng Pan from Cambridge's Department of Chemical Engineering and Biotechnology. "One of those problems is the mismatch between how deeply the light is absorbed and how far the charges travel inside the material, so most of the oxide under the top layer of the material is effectively dead space."
"For most solar cell materials, it's the defects on the surface of the material that cause performance degradation, but with these oxide materials it's the other way around: the surface is mostly good, but some bulk causes losses," said Professor Sam Stranks, who led the study. "This means that the way the crystals are grown is vital to their performance."
In order to develop copper oxides to the point where they can compete reliably with known photovoltaic materials, they need to be optimized so that they can efficiently generate and move electrical charges – consisting of an electron and a positively charged electron “hole” – under sunlight. beats them
Impact and future directions
One potential approach to optimization is monocrystalline thin films – very thin slices of material with a highly ordered crystalline structure, often used in electronics. However, making these films is usually a complex and lengthy process.
Using thin film deposition techniques, the researchers were able to grow high-quality copper oxide films at ambient pressure and room temperature. By precisely controlling the growth and flow rate in the chamber, they were able to "move" the crystals into a specific orientation. Then, using high temporal resolution spectroscopic techniques, they were able to observe how the orientation of the crystals affects how efficiently electrical charges move through the material.
"These crystals are basically cubes, and we found that when the electrons move through the cube diagonally to the body, rather than along the face or edge of the cube, they move an order of magnitude further," Pan said. "The further the electrons travel, the better the performance."
"Something about that diagonal direction in these materials is the magic," Stranks said. "We need to do further work to fully understand why and optimize it further, but so far it has resulted in a huge jump in performance." Tests of a copper oxide photocathode made using this technology have shown an increase in efficiency of more than 70% compared to existing modern electrodeposited oxide photocathodes.
"In addition to improved performance, we found that the orientation makes the films much more stable, but factors other than bulk properties may be involved," Pan said.
Much more research and development is still needed, but this and related families of materials could play a vital role in the energy transition, the researchers say.
"There's still a long way to go, but we're on an exciting trajectory," Stranks said. "There's a lot of interesting science to be learned from these materials, and I'm interested in connecting the physics of these materials to their growth, how they form, and ultimately how they work."
