Researchers revealed in a study that the photovoltaic effect of ferroelectric crystals can be increased by a factor of 1,000 if different materials are periodically arranged.
Commercially available solar cells are mostly silicon-based and their efficiency is limited due to the material’s properties. Researchers are trying to identify new materials for capturing solar energy more efficiently. Some of those materials are ferroelectrics like barium titanate, a mixed oxide made of barium and titanium. Ferroelectric materials exhibit ferroelectricity, which is the ability of the material to have a spontaneous electric polarization. The polarization can be reversed by applying an external electric field.
Ferroelectric materials have separated positive and negative charges. This charge separation leads to an asymmetric structure that enables electricity to be generated from light. These materials do not require PN junction to create a photoelectric effect which makes them much easier to manufacture.
However, pure barium titanate does not absorb much sunlight, resulting in relatively low current. Researchers at Martin Luther University Halle-Wittenberg (MLU) have indicated that combining extremely thin layers of different materials significantly increases the solar energy yield. They achieved high energy efficiency by creating crystalline layers of barium titanate, strontium titanate and calcium titanate which they alternately placed on top of one another. Their findings were published in the journal Science Advances.
“The important thing here is that a ferroelectric material is alternated with a paraelectric material. Although the latter does not have separated charges, it can become ferroelectric under certain conditions, for example at low temperatures or when its chemical structure is slightly modified,” explains physicist Dr. Akash Bhatnagar from MLU’s Centre for Innovation Competence SiLi-nano.
The research group discovered that the photovoltaic effect increases if the ferroelectric layer alternates not only with one, but with two different paraelectric layers. Yeseul Yun, a Ph.D. student at MLU and first author of the study, explains: “We embedded the barium titanate between strontium titanate and calcium titanate. This was achieved by vaporizing the crystals with a high-power laser and redepositing them on carrier substrates. This produced a material made of 500 layers that is about 200 nanometers thick.”
According to the researchers, as compared to pure barium titanate of a similar thickness, the current flow was up to 1,000 times stronger. The researchers are confident that the potential demonstrated by the new concept can be used for practical applications in solar panels.
