In the future, climate-neutral hydrogen will play an important role as a fuel and raw material. Hydrogen is produced by electrolysis of water, using an indirect method where an external energy source (solar panel or wind turbine) supplies the electrolysis cell with voltage, or using a direct method: a photoelectrochemical cell where the photoelectrode itself supplies the electricity. energy for electrolysis (PEC cell). This direct method has some advantages, but is not yet competitive.
Currently, this is mainly due to the lack of good photoelectrodes. Metal oxides are considered suitable in principle; they are inexpensive, non-toxic, stable in aqueous solution and often also have catalytic properties that facilitate the desired chemical reaction. Sunlight releases charge carriers in metal oxides, thus creating an electrical voltage.
But compared to doped semiconductors like silicon, these charge carriers are not very mobile, they are slow, or immediately return to the lattice and localize. This is due to different mechanisms at different time and length scales that are still poorly understood.
In the femtosecond laser laboratory of HZB, the team led by Dr. Dennis Friedrich and Dr. Hannes Hempel is now investigating in detail for the first time what limits the conductivity of metal oxides. “We want to know how strongly the charge carriers are localized and how this reduces their mobility at different times,” said Markus Schleuning, first author of the study, who made his doctorate in this subject. The work is published in the journal Advanced Functional Materials.
“First, we developed a new method to determine the diffusion lengths. The simple equation can also be used in other types of materials such as halide perovskites or silicon,” explains Hempel.
“Then we found out that it doesn’t work for certain materials, and precisely when charge carriers are present,” Friedrich added.
In the femtosecond laboratory, all samples are investigated with the same terahertz method (OPTP) and microwave spectroscopy (TRMC), both measurement methods initially provide information on the movement and lifetime of the charge carriers-but in different time scales.
The results can be very different, indicating that the carriers are localized in the current. From ultrafast processes on the order of 100 femtoseconds to slower processes lasting 100 microseconds, the team was able to determine the dynamics of charge carriers in materials. By way of comparison, extrapolated to our human perception of time, this corresponds to changes in the time span of one second to 31 years.
Physicists used this combination of methods to analyze 10 metal oxide compounds, including Fe2or3CuFeO2α-SnWO4BaSnO3 and CuBi2or4. For all materials, the movements are small compared to conventional semiconductors. A heat treatment, annealing, improves the mobility of BaSnO3.
The best performer is the well-known bismuth vanadate (BiVO4), which shows little carrier localization at the length scales studied. The study shows how metal oxide compounds can be identified to identify and develop the best materials for photoelectrodes.
Markus Schleuning et al, Carrier Localization in Nanometer-Scale Transport Limits in Metal Oxide Photoabsorbers, Advanced Functional Materials (2023). DOI: 10.1002/adfm.202300065
Provided by the Helmholtz Association of German Research Centers
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