Layer model makes solar cells more potent
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 substrates. The material produced in this way consists of 500 layers and is about 200 nanometres thick.
For the photoelectric measurements, the new material was irradiated with laser light. The result surprised even the research group: compared to pure barium titanate of 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. “Apparently, the interaction of the lattice layers leads to a much higher permittivity – that is, to the fact that the electrons can flow off much more easily due to the excitation by the light photons”, explains Akash Bhatnagar. On top of that, the measurements showed that this effect is very robust: it was almost constant over a period of six months.
Further research must now show exactly which causes are responsible for the outstanding photoelectric effect. Bhatnagar is confident that the demonstrated potential of the new concept can be used for practical applications in solar modules. The layer structure shows a higher yield than a pure ferroelectric in all temperature ranges. In addition, the crystals used are significantly more durable and do not require special packaging.
The results of the study were published in the journal Science Advances. The study was supported by the German Federal Ministry of Education and Research (BMBF), the German Research Foundation and with funding from the European Regional Development Fund (ERDF).