Researchers at Martin Luther University Halle-Wittenberg achieved an increase in the photovoltaic effect of ferroelectric crystals by a factor of 1,000 by creating crystalline layers of barium titanate, strontium titanate and calcium titanate which they alternately placed on top of one another.
Their findings, which were published in the journal Science Advances, could significantly increase the efficiency of solar cells.
In their paper, the researchers explained that most solar cells are currently silicon-based, which means that their efficiency is limited. This is what prompted them to examine the properties of barium titanate, a mixed oxide made of barium and titanium.
“Ferroelectric means that the material has spatially separated positive and negative charges,” Akash Bhatnagar, co-author of the study, said in a media statement. “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 solar panels.
Bhatnagar explained that pure barium titanate does not absorb much sunlight and consequently generates a comparatively low photocurrent. However, the latest research has shown that combining extremely thin layers of different materials significantly increases the solar energy yield.
“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,” the physicist said.
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.
Thus, they 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 are about 200 nanometres thick.
When conducting the photoelectric measurements, the new material was irradiated with laser light. The result was unexpected: compared to pure barium titanate of a similar thickness, the current flow was up to 1,000 times stronger – and this even though the proportion of barium titanate as the main photoelectric component was reduced by almost two thirds.
“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,” Bhatnagar said. The measurements also showed that this effect is very robust: it remained nearly constant over a six-month period.
The scientists also said that further research must now be done to find out exactly what causes the outstanding photoelectric effect. Yet, he is confident that the potential demonstrated by the new concept can be used for practical applications in solar panels.
“The layer structure shows a higher yield in all temperature ranges than pure ferroelectrics. The crystals are also significantly more durable and do not require special packaging.”
