Almost every week, science publishes advances in the efficiency of solar cells. Now, researchers at Martin Luther University (MLU) Halle-Wittenberg (Germany) have found that the energy generation of ferroelectric crystals in solar cells can be increased by a factor of a thousand if three different materials are arranged in a grid capable of generating electricity.
To achieve this increase in electrical energy production, the researchers created crystalline layers of barium titanate, strontium titanate, and calcium titanate, which they placed alternately on top of one another, separating the positive and negative charges in the same photovoltaic device. This arrangement could greatly increase the efficiency of solar panels.
Most solar cells are currently silicon-based, but 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. Unlike silicon, ferroelectric crystals do not require a so-called PN junction to create the photovoltaic effect, i.e., no positively and negatively doped layers, which makes the production of solar modules much easier.
However, pure barium titanate, a ferroelectric crystal, does not absorb much sunlight and consequently generates a comparatively low photocurrent. By experimenting with the different combinations of the materials, the researchers found 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,” explains physicist Dr. Akash Bhatnagar from MLU’s Centre for Innovation Competence SiLi-nano.
Bhatnagar and his team 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.
In comparison with pure barium titanate, the new photoelectric material irradiated with laser light had a current flow 1,000 times stronger and more efficient, even with the reduction in the proportion of the base element of the mixture 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,” explains Bhatnagar. The measurements also showed that this effect is very robust: it remained nearly constant over a six-month period.
Further research must now be done to find out exactly what causes the outstanding photoelectric effect. Bhatnagar is confident that the potential demonstrated by the new concept can be used for practical applications in solar panels.