Researchers at the US Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley have uncovered the secret of ferroelectric materials producing high-voltage electricity under light.
Ferroelectric material refers to a type of material with ferroelectric effect, which is a branch of pyroelectric material. Ferroelectric materials and their application research has become one of the most popular research topics in the field of condensed matter physics and solid electronics. Scientists have learned that the atomic structure of ferroelectric materials can make them spontaneously polarize, but it is still unclear how the photoelectric process occurs in ferroelectric materials. If this photovoltaic mechanism can be understood and applied to solar cells, it will effectively improve the efficiency of solar cells.
The ferroelectric material used by the researchers is bismuth ferrite film (BFO). This specially made film has unusual characteristics, with different electric domains arranged regularly and regularly within a distance of hundreds of microns. The electric domains are strip-shaped, each electric domain is 50 nanometers to 300 nanometers wide, the domain wall is 2 nanometers, and the polarity of adjacent electric domains is opposite. In this way, researchers can clearly know the precise position of the built-in electric field and its electric field strength, which is convenient for research on the micro scale, and at the same time avoids the errors caused by the impurity atoms surrounding and the polycrystalline material.
When the researchers irradiated the bismuth ferrite film with light, they obtained a voltage much higher than the band gap voltage of the material itself, indicating that photons can release electrons and form holes in the domain wall, so that even if there is no semiconductor PN The structure can also form a current perpendicular to the domain wall. Through various experiments, the researchers determined that the domain wall plays a very important role in increasing the voltage. Based on this, they developed a model that allows domains with opposite polarities to create excess charges and transfer them to adjacent domains. This situation is a bit like the process of transferring a bucket. As excess charge is continuously injected into the zigzag adjacent electric domains, the voltage can be significantly increased step by step.
On both sides of the domain wall, due to the opposite electrical properties, an electric field can be formed to separate the carrier. On one side of the domain wall, electrons accumulate and holes repel each other; on the other side, holes accumulate and electrons repel each other. The reason why solar cells lose efficiency is that electrons and holes will quickly combine, but this situation will not occur on the bismuth ferrite film because the polarity of adjacent domains is reversed. According to the principle of homosexual repulsion and heterosexual attraction, electrons and holes will move in opposite directions, and since the number of electrons far exceeds the number of holes, excess electrons will overflow to adjacent domains.
Bismuth ferrite film itself is not a very good solar cell material, because it only reacts to blue and near ultraviolet rays, and it can not produce high enough current while generating high voltage. But the researchers are convinced that in any ferroelectric material with a zigzag structure, a similar process will occur.
Researchers are currently investigating and studying other better alternative materials. They believe that if this technology is applied to solar cells, it will cause the solar cells to generate higher currents, and can greatly improve the efficiency of solar cells, and is expected to produce powerful solar cells.
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