Unlike petroleum and coal, the sun’s rays are free for conversion into electricity. Photovoltaic solar cells convert sunlight into electricity, but at night and on cloudy days, they are not productive. 

To be used when the sun is not shining, energy must be stored to ensure a stable power supply. One approach for doing so is to charge rechargeable batteries during the day, using solar power, and to discharge them to the grid during the night. This requires large-scale battery storage that increases the cost of solar power and is only effective for short-term storage. Long-term seasonal storage demands other solutions.

Another approach, which is the subject of studies by Prof. Avner Rothschild from the Technion-Israel Institute of Technology in Haifa and his research group, is to use photoelectrochemical cells to convert sunlight not into electricity, but into hydrogen fuel produced by splitting water molecules into hydrogen and oxygen. 

The stored hydrogen can be used later for producing electricity or can be put to other uses such as heating, fuelling fuel-cell electric vehicles, and various industrial processes such as steel making, petrochemical refining and ammonia production. 

The vital component of both photovoltaic and photoelectrochemical solar cells is a semiconductor photoabsorber – a material capable of absorbing photons and generating free-charge carriers (electrons and holes) that contribute to the photocurrent. But where commercial solar cells use silicon for that purpose, photoelectrochemical cells must depend on other materials that display greater compatibility to the conditions in which the cell must operate, such as stability in aqueous electrolytes. A promising material for that purpose is hematite, an abundant form of iron oxide whose chemical composition is similar to that of rust. 

Until now, however, hematite has frustrated scientists – despite half a decade of research, scientists have been able to obtain from it less than half of the solar energy conversion efficiency that theory predicts. Rothschild’s group now shows in a paper in Nature Materials under the title “Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin-film metal oxide photoanodes for solar water splitting” why this is the case. They present a new way to assess the actual efficiency limit that might be obtained from hematite and other semiconductors.

The group postulated that the efficiency loss in hematite is not caused solely from charge carrier recombination, a well-known effect that can be mitigated by nanostructuring and light trapping techniques but occurs also due to internal light–matter interaction effects that cannot be mitigated by these approaches. According to their theory, a portion of the electrons excited by absorbed photons are excited into electronic states that cannot move freely within the material. The absorbed photons that give rise to these localized electronic transitions are thus “wasted” without contributing to the photocurrent. 

Using an ultrathin hematite film, the group was able to measure the effect in correlation to wavelength, extracting the so-called wavelength dependent photogeneration yield spectrum. In collaboration with the research group of Prof. Roel van de Krol from the Institute for Solar Fuels in Helmholtz-Zentrum Berlin, they measured a similar spectral response of photogenerated charge carriers by another, microwave-based technique. Obtaining similar results by the two different methods serves as a verification of the method and demonstrates that the photogeneration yield is an overlooked, yet fundamental limitation responsible for the underperformance of hematite photoelectrodes for solar energy conversion and storage.

The group’s novel method makes possible the characterization of other materials in the same way they characterised hematite, providing information on the limitations of different materials and giving access to information about light-matter interaction in correlated electron materials with non-trivial opto-electronic properties. This will open the way to more efficient construction of photoelectrochemical cells, giving access to renewable energy and green hydrogen fuel.