Martin-Luther-Universit├Ąt Halle-Wittenberg found the photovoltaic effect of ferroelectric crystals can be increased by a factor of 1,000 if three different materials are arranged periodically in a lattice. Researchers achieved this by creating crystalline layers of barium titanate, strontium titanate and calcium titanate which they alternately placed on top of one another. These findings could significantly increase the efficiency of solar cells.

Ferroelectric paraelectric superlattice solar cell design segment graphic. Image Credit: Uni Halle / Yeseul Yun – Martin-Luther-Universit├Ąt Halle-Wittenberg. Click image for the largest view.

The findings, which could significantly increase the efficiency of solar cells, have been published in the journal Science Advances.

Most solar cells are currently silicon based; however, their efficiency is limited. This has prompted researchers to examine new materials, such as ferroelectrics like barium titanate, a mixed oxide made of barium and titanium.

Physicist Dr Akash Bhatnagar from MLU’s Center for Innovation Competence SiLi-nano explained, “Ferroelectric means that the material has spatially separated positive and negative charges. The charge separation leads to an asymmetric structure that enables electricity to be generated from light.”

Unlike silicon, ferroelectric crystals do not require a so-called pn junction to create the photovoltaic effect, in other words, no positively and negatively doped layers. This makes it much easier to produce the solar panels.

However, pure barium titanate does not absorb much sunlight and consequently generates a comparatively low photocurrent. The latest research has shown that combining extremely thin layers of different materials significantly increases the solar energy yield.

Bhatnagar noted, “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.”

Bhatnagar’s research group discovered that the photovoltaic effect is greatly enhanced if the ferroelectric layer alternates not only with one, but with two different paraelectric layers.

Yeseul Yun, a PhD student at MLU and first author of the study, explained, “We embedded the barium titanate between strontium titanate and calcium titanate. This was achieved by vaporizing the crystals with a high-power laser and re-depositing them on carrier substrates. This produced a material made of 500 layers that is about 200 nanometers thick.”

When conducting 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 1,000 times stronger – and this despite the fact that the proportion of barium titanate as the main photoelectric component was reduced by almost two thirds.

Bhatnagar explained, “The interaction between the lattice layers appears to lead to a much higher permittivity – in other words, the electrons are able to flow much more easily due to the excitation by the light photons.” The measurements also showed that this effect is very robust: it remained nearly constant over a six-month period.

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Now we know not everyone is chasing Perovskite research. Ferroelectric crystals have been out there, known of, but not notably researched. This team has just made ferroelectric crystals notable, indeed. A 1,000 fold increase is newsworthy. And yet 1,000 times a little bit might not be much. But ferroelectric is back in the game and in fundamental research more targets begets more results, some of which are going to matter a lot.


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