The photoelectric effect of ferroelectric crystals can be increased by a factor of 1000 if three different materials are periodically arranged in a network. This was revealed in a study conducted by researchers at Martin Luther University Halle-Wittenberg (MLU). They achieved this by creating crystal layers of barium titanate, strontium titanate, and calcium titanate that were alternately layered on top of each other. Their findings, which can significantly increase the efficiency of solar cells, have been published in the journal science progress.
Most solar cells currently rely on silicon; However, its efficiency is limited. This has prompted researchers to examine new materials, such as ferroelectric materials such as barium titanate, a mixed oxide made of barium and titanium. Physicist Dr. Akash Bhatnagar of MLU’s Center for Innovative SiLi-nano Efficiency explains: “Viroelectric energy means that a 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 photoelectric effect, in other words, there are no positive and negative doped layers. This makes the production of solar panels much easier.
However, pure barium titanate does not absorb much sunlight and therefore generates a relatively low photocurrent. The latest research shows that combining ultra-thin layers of different materials significantly increases the solar energy yield. “The important thing here is that a ferroelectric substance alternates with a quasi-electric substance. Although the latter does not have separate charges, it can become ferroelectric under certain conditions, for example at lower temperatures or when its chemical structure is modified Slightly” Bhatnagar.
The Bhatnagar research group discovered that the photovoltaic effect is greatly improved if the ferroelectric layer is exchanged with not just one, but two different quasi-ferroelectric layers. Yeol Yoon, Ph.D. The MLU student and first author of the study explains: “We combined barium titanate between strontium titanate and calcium titanate. This was achieved by evaporating the crystals with a high-powered laser and repositioning them on carrier substrates. This produces a 500-layer material that is about 200 nanometers thick.” .
When performing the photovoltaic 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 about two-thirds. “It appears that the interaction between the lattice layers leads to a much higher permittivity – in other words, the electrons are able to flow more easily due to the excitation of light photons,” explains Akash Bhatnagar. Measurements also showed this effect to be very strong: it remained roughly constant over six months.
More research now needs to be done to find out exactly what causes the remarkable photoelectric effect. Bhatnagar is confident that the potential shown by the new concept can be used for practical applications in solar panels. “The layer structure shows higher yields in all temperature ranges compared to pure ferroelectrics. The crystals are also significantly more durable and do not require special encapsulation.”
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Yeseul Yun et al, Enhanced and strongly tunable photovoltaic effect in semi-electric photovoltaic superlattices science progress (2021). DOI: 10.1126 / sciadv.abe4206
Presented by Martin Luther University Halle-Wittenberg
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