The photovoltaic effect of ferroelectric crystals in solar cells can be increased by a factor of 1,000 if three different materials are arranged in a grid. Researchers at Martin Luther University (MLU) Halle-Wittenberg (Germany) have shown this in a study. To do this, they created crystalline layers of barium titanate, strontium titanate and calcium titanate, which they placed alternately on top of each other.
Currently, most solar cells are based on silicon, but their efficiency is limited. For some years, research has therefore been conducted into new materials, such as ferroelectrics, like barium titanate, a mixed oxide of barium and titanium. "Ferroelectric means that the material has spatially separated positive and negative charges," explains physicist Dr Akash Bhatnagar from the Centre for Innovation Competence SiLi-nano at MLU. "The charge separation leads to an asymmetric structure that enables electricity generation under light." Unlike silicon, ferroelectric crystals do not require a pn junction for the photovoltaic effect, i.e. no positively and negatively doped layers, which makes the production of solar modules much easier.
However, pure barium titanate absorbs only little sunlight and consequently generates a comparatively low luminous flux. Recent research has shown, however, that the combination of different materials in extremely thin layers significantly increases the yield of solar energy. "The important thing here is that a ferroelectric material alternates with a paraelectric material. Although the latter does not have separate charges, it can become ferroelectric under certain conditions, such as low temperature or slight modifications to the chemical structure," explains Bhatnagar.
Bhatnagar's research group has now discovered that the photovoltaic effect is significantly enhanced again when the ferroelectric layer alternates not just with one, but with two different paraelectric layers. For this purpose, the scientists have embedded the barium titanate between strontium and calcium titanate. To do this, the crystals are vaporised with a high-power laser and redeposited on carrier