Researchers at the Technion and Ben-Gurion University discovered the reason * As part of the research published in Nature Materials, an innovative method was developed to measure the conversion efficiency of photons into mobile electrical charges (electrons and holes) in semiconductors. With this method they discovered an unknown factor that limits the conversion efficiency of photons to electric current in iron oxide (hematite)
Researchers from the Technion and Ben-Gurion University present new findings concerning the conversion of solar energy into electricity and hydrogen fuel. As part of the research published in Nature Materials, an innovative method was developed to measure the conversion efficiency of photons into mobile electric charges (electrons and holes) in semiconductors. With this method, they discovered an unknown factor that limits the conversion efficiency of photons to electric current in iron oxide (hematite). The discovered factor is expected to affect the efficiency of other materials, in particular in materials with correlated electrons (electron correlated materials), in which there is a strong interaction between the electrons and the atoms in the crystal. The developed method is expected to yield new knowledge about the interaction between light and matter in these materials and help in the development of new materials for solar cells with unique characteristics.
background:
The advantages of solar energy - energy that comes from the sun - have been known for years, and the attempt to harness it for human needs has led to many developments, including photovoltaic solar cells. These devices absorb the light particles (photons) in a semiconductor material, and these give their energy to mobile electric charges known as electrons and holes and give them an electrical voltage that allows them to do work (free energy). This work is expressed as electrical energy in photovoltaic cells, and in principle such cells may satisfy all our energy needs.
The main difficulty in switching to sustainable solar energy stems from the changing availability of sunlight throughout the hours of the day and the dependence of this light on clouds and haze. The electricity output of a photovoltaic cell is derived from the intensity of the light hitting it, therefore it varies from hour to hour, from season to season, and on many days of the year it is limited by clouds and haze. In order to be able to rely on solar energy as a central energy source that can satisfy the requirements for electricity, heat, fuel and other needs at all hours of the day and in all seasons of the year, it is necessary to convert it into storable energy (storage) and convert it back into electricity, heat and fuel according to the requirement. Thus, for example, it is possible to store the electricity produced in photovoltaic solar cells in rechargeable batteries, and convert it back to electricity when needed. Storage in batteries increases the cost of solar electricity and is suitable for use within a day at most. Long-term storage, for example from season to season, requires other technological solutions, as do other energy needs, besides electricity production, such as domestic and industrial heating, transportation fuel, and more.
Such a solution can be implemented using photo-electrochemical cells that work in a similar way to photovoltaic cells, but instead of electricity they produce hydrogen by splitting water molecules into their two components - oxygen and hydrogen. These are stored for future use, either to produce electricity or to drive hydrogen-powered electric vehicles using a fuel cell that replaces the battery array in electric vehicles such as Tesla and others. Hydrogen created in this way is called "green" hydrogen and is an ideal substitute for hydrocarbon fuel since its production and use are not accompanied by the emission of greenhouse gases or anything else except distilled water.
Photoelectrochemical cells for converting solar energy into "green" hydrogen employ many research and development groups in academia, but these technologies have not yet matured for application. This is mainly due to the limitations of the materials that can be used in them. Similar to the photovoltaic cells, the heart of the cell here is also made of a semi-conducting material that absorbs the photons and converts their energy (radiation energy) into work, but here the work is expressed in chemical energy stored in the chemical bonds of the hydrogen and oxygen molecules created by the splitting of the water molecules. For this purpose, the semiconductor is immersed in an aqueous electrolyte with corrosive properties that do not allow the use of ordinary semiconductors such as the silicon used in photovoltaic cells. These and other limitations require other semiconductors with unique properties that are not normally found.
The research findings and their importance:
For over a decade, Prof. Avner Rothschild and his research group at the Technion have been researching a mineral called hematite, which is a type of iron oxide with a chemical composition similar to rust, and which has the spectrum of properties needed for a photoelectrochemical cell to split water into hydrogen and oxygen. This research has led to scientific and technological breakthroughs published in leading scientific journals from the Nature group and others. Despite the improvements achieved in the properties of the material and the structure of the photoelectrochemical cell, the conversion efficiency of photons to electric current in hematite-based devices reaches less than half of the theoretical limit for this material. By comparison, silicon photovoltaic cells reach close to 100% of silicon's theoretical limit.
After years of research in which the researchers turned every stone on the way to improve the properties of the material and the structure of the cell, they came to the conclusion that much of the hidden is over the visible and there must be a unique disappearing factor for hematite that prevents reaching the theoretical limit known to this material. In their latest study, just published in the scientific journal Nature Materials, they uncover and discover this missing factor, and propose a new method for its characterization in hematite and other materials. Unlike silicon and other semiconductors used in solar cells and other optoelectronic devices, where the photons absorbed in the material produce mobile electric charges (electrons and holes) that can move freely and generate an electric current, a significant portion of the photons that reach the hematite are absorbed in a different way through local electronic transitions in which the electron moves from an energetic state (called an orbital) one after the other in the same atom or in the chemical bond between two neighboring atoms. Since such an electron remains located at a specific site in the crystal, it does not have the ability to move (mobility) and therefore cannot contribute to the creation of an electric current. Therefore, the photons absorbed in this way are "wasted" and do not contribute to the creation of an electric current (in a photovoltaic cell) or hydrogen (in a photo-electrochemical cell).
Due to unique chemical and physical properties expressed, among other things, in a strong interaction between the electrons and atoms in the crystal, a significant part (about half) of the photons absorbed by hematite create local electronic transitions of this type. This property was characterized for the first time in the present study as a dependence on the wavelength of the photons striking the material, in hematite and some other semiconducting oxides where the phenomenon was found to be marginal compared to hematite. However, the researchers point out that this is an unknown phenomenon and therefore most likely it also exists in other semiconductors. The very act of revealing the phenomenon and charting a path to its characterization constitutes a scientific breakthrough in the study of the interaction between light and matter in materials with correlated electron materials, and it has practical importance in the study of new materials for solar cells with unique characteristics, such as the photoelectrochemical cells for splitting water into hydrogen and oxygen, the subject of the current research .
The article published in the prestigious journal Nature Materials was led by researchers from the research group of Prof. Avner Rothschild from the Faculty of Materials Science and Engineering at the Technion, including Dr. Daniel Groh (senior scientist in the Department of Materials Engineering at Ben-Gurion University of the Negev), Dr. David Ellis and doctoral student Yifat Pickner from the Grand Technion Energy Program (GTEP), in collaboration with researchers from the Solar Fuels Research Institute led by Prof. Roel van de Krol at the Helmholtz-Zentrum Berlin. The study was supported by the Research Center on Photocatalysts and Photoelectrodes for Hydrogen Production in the National Science Foundation's (ISF) Petroleum Substitutes for Transportation Program, the Grand Batech Technion Energy Center (GTEP) and the Russell Berry Research Institute in Nanotechnology (RBNI).
For an article in Nature Materials
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