A team of researchers, including Vasilii Balanov, Jani Peräntie, Jaakko Palosaari, Suhas Yadav, and led by Yang Bai, associate professor at the University of Oulu Finland has made an advancement in the field of multifunctional energy harvesting.
Their latest study, published in the journal of Advanced Electronic Materials, advances in understanding the photovoltaic effect in ferroelectric crystals. The article reports the team’s recent research results regarding improving the electric output of the bulk photovoltaic effect (BPVE) via manipulation of ferroelectric domains in oxide perovskite crystals.
“In ordinary solar cells, the mechanism of harvesting the solar energy and then converting them into green electricity is based on the formation of p-n junctions of semiconductors. While the p-n junction has been invented for more than a century, widely used in the silicon industry nowadays, the BPVE is a more recently discovered physical phenomenon from the 1960s-1970s. The BPVE does not rely on p-n junctions to work under solar energy. It forms its own ‘‘self-junction’’ and, theoretically, it may break the physical limit of the Shockley-Queisser limit that prevents single p-n junction-based solar cells from being more efficient,” Yang Bai said.
Using BPVE in practice is challenging, as the output power of BPVE-based cells is still negligible compared to those of p-n junction-based photovoltaic cells. In the study, Bai’s team showed that by creating a stacked domain structure, a 35 % improvement on the output power of BPVE-based cells can be achieved. A domain is a submicron-sized region containing spontaneous polarizations orienting in the same direction, which can be switched by applying an external electric field.
The improvement of electric output from Bai’s BPVE device is achieved by applying an AC poling electric field, under which the microstructure (domains) inside the crystals will be better aligned compared to the situation under the conventionally used DC field. After removing the electric field, the domains remain at this better aligned state. The better aligned domains help to reduce recombination of electric charge carriers, and thus the energy conversion efficiency increases. The results of the work pave a way towards developing more efficient BPVE cells that can help to unlock multifunctionality in future photonic, computing, sensing and energy harvesting devices.
“The first concrete applications will be in small-scale sensing and computing devices, where in addition to the electric signals, we can input light of different wavelengths as an extra degree of freedom for operation. For example, we have previously proven the use of BPVE in filterless colour sensor. Other examples include components for neuromorphic computing and multi-source energy harvesters for IoT (internet of things) devices,” Yang Bai said.
Despite the breakthrough, there is still a lot of research work ahead.
“While we are advancing in the working mechanism inside the materials, the challenge still lies in the band gap of the materials, where we ideally need a material that simultaneously has a narrow band gap (to maximise visible light absorption) and a large spontaneous polarization (to maximize the open-circuit voltage). We have limited options for such materials. Most available materials nowadays only possess either a narrow band gap or a large spontaneous polarization, not both. In the near future, we will attempt to expand the material options,” Yang Bai noted.
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