Fig: Structural characterization of superlattices. (A) Cross-sectional STEM acquired from sample SBC222. (B) High-resolution STEM from a part of the scanned region. The schematic depicts the arrangement of unit cells. RSM acquired around (103) reflection in (C) BTO, (D) SBC555, (E) SBC252, and (F) SBC222. Star and yellow arrows indicate the STO substrate and satellite peaks from SL, respectively.
Most solar cells are currently silicon based; however, 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. Ferroelectric means that the material has spatially separated positive and negative charges. 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 the solar panels.
However, pure barium titanate does not absorb much sunlight and consequently generates a comparatively low photocurrent. 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.
When conducting the photoelectric measurements, the new material was irradiated with laser light. The result surprised even the research group: compared to pure barium titanate of a 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. 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. 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. The crystals are also significantly more durable and do not require special packaging.
Yeseul Yun et al, Strongly enhanced and tunable photovoltaic effect in ferroelectric-paraelectric superlattices, Science Advances (2021). DOI: 10.1126/sciadv.abe4206