Scientists have discovered a method to power the “engine” of sustainable fuel production by slightly tweaking the materials. Researchers led by the University of Cambridge are developing low-cost, light-harvesting semiconductors that power devices that 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 they do not approach the properties of silicon, which dominates the semiconductor market.

But researchers found that by growing copper oxide crystals in a specific direction so that electrical charges travel diagonally between the crystals, the charges travel much faster and farther, greatly improving performance. Tests of a copper oxide photocathode based on this fabrication technique showed a 70% improvement over current state-of-the-art copper oxide photocathodes and also demonstrated significantly improved stability.

Researchers say their results have been published in the journal NatureIt shows how low-cost materials can be designed to transition from fossil fuels to clean, sustainable fuels that can be stored and used within existing energy infrastructure.

Copper oxide problems and potential

Copper(I) oxide, or copper oxide, has been touted for years as a cheap alternative to silicon because it is efficient enough to capture sunlight and convert it into electrical charge. However, much of this charge tends to be lost, limiting the performance of the material.

One of the authors, Dr. D. from Cambridge’s Department of Chemical Engineering and Biotechnology. “Like other oxide semiconductors, copper oxide has its own problems,” said Linfeng Pan. “One of these problems is the mismatch between how deeply light is absorbed and how far charges travel inside the material, so most of the oxide beneath the top layer of the material is effectively dead space.”

“For most solar cell materials, it is the defects on the surface of the material that cause performance to degrade, but with these oxide materials it is the opposite: the surface is mostly good, but some volumes cause losses,” said Professor Sam Stranks. , the person who manages the study. “This means that the way the crystals grow is vital to their performance.”

To improve copper oxides to the point where they can seriously compete with known photovoltaic materials, they must be optimized to efficiently generate and transport electrical charges consisting of an electron and a positively charged electron “hole” in sunlight. beats them

Impact and future directions

One potential approach to optimization is monocrystalline thin films, which are very thin slices of material with a highly ordered crystal structure, frequently used in electronics. However, the production of these films is often a complex and long process.

Using thin-film deposition techniques, the researchers succeeded in growing 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 in a specific direction. 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 electrons travel an order of magnitude further when they move diagonally from the inside of the cube toward the body, rather than from the face or edge of the cube,” Pan said. “The further the electrons travel, the better the performance.”

“Something about the cross direction in these materials is magic,” Stranks said. “We need to do more work to fully understand why and optimize this further, but so far it has resulted in a huge increase in performance.” Tests of copper oxide photocathodes 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 orientation makes films much more stable, although factors other than volumetric properties may also 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 path,” 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 how they grow, how they form, and ultimately how they work.”