Scientists at Martin Luther University Halle-Wittenberg (MLU) have discovered that 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.
In their study, they created crystalline layers of barium titanate, strontium titanate, and calcium titanate which they alternately placed on top of one another. This breakthrough finding could significantly increase the efficiency of solar cells and was published in the journal "Science Advances".
Currently, most solar cells are silicon-based, and their efficiency is limited. Therefore, researchers have been exploring new materials such as ferroelectrics like barium titanate, a mixed oxide made of barium and titanium. Unlike silicon, ferroelectric crystals do not require a pn junction to create the photovoltaic effect. This means that they do not require positively and negatively doped layers, making it easier to produce solar panels.
However, pure barium titanate does not absorb much sunlight, resulting in a relatively low photocurrent. The latest research has shown that combining extremely thin layers of different materials significantly increases the solar energy yield.
The researchers discovered that the photovoltaic effect is greatly enhanced if the ferroelectric layer alternates not only with one, but with two different paraelectric layers. This was achieved by embedding barium titanate between strontium titanate and calcium titanate using a high-power laser to vaporize and redeposit the crystals on carrier substrates, producing a material made of 500 layers that is about 200 nanometres thick.
When conducting the photoelectric measurements, the new material was irradiated with laser light. The results were surprising, as compared to pure barium titanate of similar thickness, the current flow was up to 1,000 times stronger, 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, allowing electrons to flow much more easily due to the excitation by the light photons. This effect is very robust and remained nearly constant over a six-month period.
Further research is necessary to determine exactly what causes this outstanding photoelectric effect. However, the potential demonstrated by this 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, and the crystals are significantly more durable and do not require special packaging.