Recently, the University of Cambridge in the UK, the Federal Institute of Technology in Lausanne in Switzerland and Nankai University have teamed up to make new progress in the field of photocatalytic water splitting to produce hydrogen.
For the first time, a room-temperature liquid-phase epitaxial growth method has been developed, which can be used for low-cost, high-quality, large-scale preparation of cuprous oxide (Cu2O) single crystal films.
For the first time, researchers have discovered the anisotropy of cuprous oxide phase carrier transport. Among them, the carrier mobility along the [111] direction is 1 order of magnitude higher than in the other directions, and the diffusion distance is more than 1 order of magnitude longer than the average diffusion distance.
In terms of photoelectric water splitting performance at critical potentials, it is 70% higher than the current state-of-the-art flat plate cuprous oxide equipment.
These results significantly improve the performance record of cuprous oxide photoelectrodes, bringing them closer to large-scale applications. A series of optoelectronic property parameters obtained for the first time in this study also provide important and precise guidance for the design of cuprous oxide-based optoelectronic devices.
Photocatalytic hydrogen production: providing solutions for renewable and clean energy
In recent years, human activities have led to drastic changes in the environment, and the sustainable development and application of energy has received unprecedented attention. Although nuclear fusion technology is expected to be high, it is still far from being able to land in terms of time.
Clean energy technologies such as solar fuels and solar cells will be in huge demand in the coming decades or even hundreds of years, and they are intermittent, unstable, and have low energy density.
As a renewable energy technology, photocatalytic hydrogen production provides a solution to directly convert solar energy into hydrogen energy and store it in fuel.
However, this is a huge challenge, and the large-scale adoption of this technology must be achieved with high efficiency, low manufacturing costs, and high stability.
The most efficient solar fuel devices available today are all made of III-V semiconductor light-absorbing materials. However, it should not be ignored that there are limitations in the application of this material, including extremely high price, complex preparation process, and extremely high requirements.
At the 2024 Materials Research Society (MRS) Spring Meeting in Seattle, USA, at the end of April, project leaders using the material for solar water splitting to produce hydrogen reported that the cost of producing target hydrogen is still too far away.
At present, it is generally believed that cuprous oxide material is the best choice for the light cathode, and the electrodeposition method used is an industrially recognized preparation method with low production cost. In addition, this method has the advantages of low equipment requirements and simple preparation conditions.
At present, the photoelectric performance of cuprous oxide photoelectrode has been comparable to that of photoelectrode based on mature photovoltaic materials, and if further promoted, it is very likely to become the best photoelectric water splitting material.
The anisotropy of carrier transport is discovered
Cuprous oxide is one of the ideal materials for photocatalytic hydrogen production electrodes, but it cannot be ignored that oxides generally have the bottleneck problem of short carrier transport distance. Moreover, the problem is "innate", and it is difficult to solve the root cause even by changing the synthesis method to control the doping concentration or optimize the crystal.
Therefore, in order to have a comprehensive understanding of cuprous oxide and find a solution, it is necessary to conduct a basic study of the material and optoelectronic properties of cuprous oxide.
At the level of basic research, the simpler the material system, the more likely it is to precisely control the variables and thus obtain more reliable results. As a result, the team's first thought was a single crystal thin film, which has a clear, ordered crystal structure and is less affected by various types of defects.
Drawing on a thin film release process, they innovatively developed a liquid phase epitaxial growth method at room temperature, resulting in a very valuable platform for single crystal cuprous oxide thin film materials.
Pan Linfeng, first author of the paper and a postdoctoral researcher at the University of Cambridge, said: "Due to the unique nature of epitaxial growth, cuprous oxide single crystal thin films of any crystal orientation can be obtained by selecting the crystal orientation of the substrate. ”
Figure丨Cuprous oxide film with anisotropic photoelectrochemical properties and mobility (source: Nature)
Combined with the advanced femtosecond laser transient reflectance spectroscopy technology of the Cavendish Laboratory at the University of Cambridge, the researchers accurately quantified the propagation distance of charge carriers in each crystal-oriented film in the bulk phase of cuprous oxide thin films for the first time, thus discovering the anisotropy of carrier transport.
In general, the temporal resolution of transient spectroscopy is from picosecond to nanosecond, and the most important carrier dynamics parameters in oxide materials are not observed at all.
Due to the limitations of the layered structure of the device, the team designed and customized advanced spectroscopy instruments. In subsequent data interpretation, the team also provided Hilbert variation, which uses transient absorption to analyze the data.
"This study shows that spectroscopy technology with high time resolution and spatial resolution plays an important role in the research of optoelectronic devices. In addition, it is expected to promote scholars in the field of solar fuels to increase their attention to spectroscopy technology. Pan Linfeng said.
Thanks to these carrier dynamics, researchers have been able to achieve a new understanding of the data based on precise quantitative, and have achieved a breakthrough in performance in the regulation of the crystal structure of semiconductor thin films.
Figure丨Polycrystalline cuprous oxide photocathode with optimal orientation (source: Nature)
It is worth noting that, as described at the end of the paper, the optoelectronic property parameters tested for the first time in this study are of great value to various cuprous oxide optoelectronic devices. Among them, the research strategy of optoelectronic anisotropy is also applicable to various optoelectronic devices such as photovoltaics, detectors, light-emitting diodes and so on.
Therefore, this research provides a unique material platform for basic research, device construction, and exploration and application in the field of catalysis, including a complete set of templates for material preparation, material characterization, optoelectronic property testing, and property utilization.
At the same time, it provides an important strategy to solve the problem of short charge carrier transmission distance in oxide semiconductors.
The research group "strong combination" will do the research to the extreme
In this study, many of the test data were "first-in-the-first", and the researchers wanted to do everything they could to make the test standardized and the data reliable.
As a result, after the first single crystal cuprous oxide thin films, they spent a lot of time optimizing the parameters to improve repeatability. This has proven to be time-consuming and labor-intensive, but very valuable.
The research was carried out by four research groups, among which the Department of Materials and the Department of Electrical Engineering at the University of Cambridge focused on the characterization of crystallography, while the researchers from the Department of Physics at the University of Cambridge focused on the optoelectronic properties and carrier dynamics of single crystal thin films.
The research group at EPFL focuses on the development of material preparation methods, material fabrication and photoelectrode device construction, and the main contribution of the researchers at Nankai University is to provide the indispensable test characterization.
In addition, both the characterization of material crystallographic parameters and the spectroscopic characterization of carrier dynamics are applied for the first time on this unique material platform.
In fact, the various teams have been working on this material for many years, and the researchers who characterize it are experts in their fields, but many questions and difficulties still arise when it comes to testing and data analysis.
To this end, they adopt communication methods suitable for different time scales, such as instant messaging office software, email, online meetings, synchronous office, etc., to maintain the efficiency and quality of communication.
"We maintain a very open and honest attitude, and we are responsible for the work to the extreme, which greatly improves the efficiency of problem solving and provides a strong guarantee for the final high-quality results. Pan Linfeng said.
Picture丨Pan Linfeng (Source: Pan Linfeng)
近日，相关论文以《在氧化亚铜光电极中，沿 [111] 取向表现出高载流子迁移率》（High carrier mobility along the [111] orientation in Cu2O photoelectrodes）为题发表在 Nature 上[1]。
Linfeng Pan and Linjie Dai, postdoctoral researchers at the University of Cambridge, are the co-first authors.
Samuel M. of the University of Cambridge Prof. Samuel D. Stranks, Prof. Michael Grätzel and Prof. Anders Hagfeldt of the Ecole Polytechnique Fédérale de Lausanne and Prof. Luo Jingshan of Nankai University are the co-corresponding authors.
Figure丨Related papers (source: Nature)
Gather the "last piece of the puzzle" for a comprehensive study of cuprolic oxide photocathode
Linfeng Pan received his master's degree from East China University of Science and Technology, and his master's supervisor is Professor Yang Huagui.
He then received his Ph.D. from the Ecole Polytechnique Fédérale de Lausanne in Switzerland, where he worked on oxide solar fuels under the supervision of Prof. Michael Grätzel and Prof. Anders Hagfeldt, the "fathers of dye-sensitized solar cells".
Beginning in 2020, he studied at the University of Cambridge, Samuel Brown. Prof. Samuel D. Stranks' research group is engaged in postdoctoral research, focusing on anisotropic electrons and photophysics of semiconductors, especially oxide materials.
Up to now, he has published 26 papers in academic journals and received more than 3,000 citations. During his Ph.D., he achieved fruitful results at the electron extraction end and hole transport end of the photoelectrode.
At the electron extraction end, an efficient coaxial p-n heterojunction was constructed on cuprous oxide nanowires by atomic layer deposition (ALD), which greatly improved the efficiency of photon absorption, charge separation and extraction. At the same time, the world's highest photocurrent density and photovoltage were obtained at that time.
On top of this, they also built and demonstrated an unbiased independent solar water splitting system with all oxides, creating a conversion efficiency of 3% from solar to hydrogen, which was the highest of its kind at the time[2].
At the hole transport end, Pan Linfeng and his collaborators used cuprous thiocyanate layer (CuSCN) with hole transport selectivity. Due to the existence of the tail state energy level, the hole transport process becomes very smooth. This led to a significant increase in the fill factor of the PV performance curve, resulting in a record conversion efficiency of 4.5% from solar to hydrogen [3].
Picture丨Samuel M. Prof. Samuel D. Stranks' group (Source: Linfeng Pan)
This research focusing on the light absorption of cuprous oxide is the "last piece of the puzzle" to form a comprehensive study of cuprous oxide photocathode.
"When I presented the results at the MRS Spring Meeting, many of my peers were very interested in the preparation of monocrystalline thin films, and I explained the key points in detail. Pan Linfeng said.
It should be understood that in this study, although the stability of cupmer oxide photoelectrodes performed well among all photoelectrodes, there is still a long way to go from the requirements of practical application.
Therefore, the researchers plan to combine spectroscopic techniques with high spatial resolution to conduct in-situ observation and analysis to try to explore the instability factors of cuprous oxide photoelectrodes and optimize them in a targeted manner.
Talking about the future prospects of this technology, Pan Linfeng said that he hopes to apply the spectroscopic technology learned in the Cavendish laboratory to research in the field of solar fuels in the future. While promoting the large-scale application of this technology, it provides effective solutions for the country's "dual carbon" strategic goals.
Resources:
1.Pan, L., Dai, L., Burton, O.J. et al. High carrier mobility along the [111] orientation in Cu2O photoelectrodes. Nature 628, 765–770 (2024). https://doi.org/10.1038/s41586-024-07273-8
2.Pan, L., Liu, Y., Yao, L. et al. Cu2O photocathodes with band-tail states assisted hole transport for standalone solar water splitting. Nature Communications 11, 318 (2020). https://doi.org/10.1038/s41467-019-13987-5
3.Pan, L., Liu, Y., Yao, L. et al. Cu2O photocathodes with band-tail states assisted hole transport for standalone solar water splitting. Nature Communications 11, 318 (2020). https://doi.org/10.1038/s41467-019-13987-5