The photovoltaic effect of ferroelectric crystals can be multiplied by 1000 if three different materials are periodically arranged in an array. This was revealed in a study conducted by researchers at Martin Luther University in Halle-Wittenberg (MLU). They achieved this by creating crystalline layers of barium titanate, strontium titanate and calcium titanate which they alternately overlapped. Their findings, which could dramatically increase the efficiency of solar cells, have been published in the journal Scientists progress.
Most solar cells are currently silicon-based; however, their effectiveness is limited. This prompted researchers to look at new materials, such as ferroelectrics like barium titanate, a mixed oxide made up of barium and titanium. “Ferroelectric means that the material spatially separated the positive and negative charges,” says physicist Dr Akash Bhatnagar of MLU SiLi-nano’s Innovation Competence Center. “The separation of charges leads to an asymmetric structure which allows electricity to be generated from light.” Unlike silicon, ferroelectric crystals do not require a so-called pn junction to create the photovoltaic effect, that is to say no positively and negatively doped layers. This makes the production of solar panels much easier.
However, pure barium titanate does not absorb much sunlight and therefore generates a relatively low photocurrent. The latest research has shown that the combination of extremely thin layers of different materials dramatically increases solar energy yield. “The important thing here is that a ferroelectric material is alternated with a paraelectric material. Although the latter does not have separate charges, it can become ferroelectric under certain conditions, for example at low temperatures or when its chemical structure is slightly modified, ”explains Bhatnagar.
Bhatnagar’s research group found that the photovoltaic effect is greatly enhanced if the ferroelectric layer alternates not only with one, but with two different paraelectric layers. Yeseul Yun, PhD student at MLU and first author of the study, explains: “We integrated 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 composed of 500 layers approximately 200 nanometers thick. “
During the photoelectric measurements, the new material was irradiated with laser light. The result surprised even the research group: compared to pure barium titanate of a similar thickness, the current flow was up to 1000 times stronger – and this despite the fact that the proportion of barium titanate as main photoelectric component has been reduced by almost two-thirds. “The interaction between the layers of the network seems to lead to a much higher permittivity – in other words, electrons can flow much more easily due to the excitation by light photons,” says Akash Bhatnagar. The measurements also showed that this effect is very robust: it has remained almost constant over a period of six months.
More research now needs to be done to find out exactly what causes the exceptional photoelectric effect. Bhatnagar is convinced that the potential demonstrated by the new concept can be used for practical applications in solar panels. “The layered structure shows higher efficiency in all temperature ranges than pure ferroelectrics. The crystals are also significantly more durable and do not require special packaging.”
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