The new-type thin-film solar cell technology is a revolutionary development that will pave the way toward introducing clean energy into modern homes and changing the way of life.
a. Micromorphology of both the full device stack and the individual layers. b. Schematic of energy band alignment in a magnetic field. c and d Schematic of energy band alignment in different polarization directions. Image Credit: by Kaixin Guo, Xu Wang, Rongfen Zhang, Zhao Fu, Liangyu Zhang, Guobin Ma, and Chaoyong Deng
Nowadays, people are mostly optimistic concerning the development prospect of these new-type thin-film solar cells. Regrettably, such solar cells commonly display poor stability and low photoelectric conversion efficiency, leading to an upgrade in efficiency at the cost of the environment.
Thus, it is crucial to discover eco-friendly technologies to boost the stability and the conversion efficiency of PV devices.
After prolonged years of study, researchers have discovered that certain materials with a specific asymmetric structure (the so-called multiferroics) represent not only ferroelectric and ferromagnetic ordering at the same time, but also many remarkable coupling, broadening their application significantly.
In the 1970s, researchers unintentionally discovered a new critical physical occurrence — ferroelectric PV effect. It is fundamentally diverse from the traditional p-n junction, thus it is also referred to as the anomalous or bulk photovoltaic effect.
In recent times, with the nonstop warming of energy research, scientists have incorporated multiferroics with suitable bandgap structures into solar cells as the light absorption layer of devices, creating a new multiferroic solar PV device.
Now, researchers have come up with a number of theories about the physical mechanism of the ferroelectric PV effect. However, it is mostly recognized that this effect is closely associated with the polarization features of materials.
In 2015, Canadian researchers Federico Rosei and Riad Nechache reported the alteration of the bandgap and the ferroelectric PV effect of multiferroic Bi2FeCrO6 thin films. It is established that the regulation of ferroelectric polarization on PV devices is indeed achievable.
It is proven that there is also some relationship between light and magnetic field. It is yet to be seen whether the PV response can also be modified by magnetization for multiferroics. And what will the mechanism be?
In a new study reported in Light Science & Application, a research team, headed by Professor Chaoyong Deng from Key Laboratory of Electronic Composites of Guizhou Province, College of Big Data and Information Engineering, Guizhou University, China, and co-workers have created a low-cost new-type all-inorganic oxide solar cell and examined the regulation of PV response of the devices.
Based on self-made black silicon, they engineered a multiferroic heterojunction solar cell by incorporating non-toxic bismuth layered perovskite heterojunction as the light absorber and graphene as the anode which offers a high photoconversion efficiency of nearly 3.90%. Their solar cell still sustains 90% of the preliminary efficiency after 30 days of the stability tracking test.
More remarkably, they examined the regulation mechanisms of applied polarizing and magnetizing fields on the photovoltaic response of the device from the incorporated field-driven carrier separation and the above-bandgap change, which is of huge importance to enhance the efficiency of conventional ferroelectric oxide solar cells.
Thus, their research exposes people to the significant potential of multiferroic materials in the field of new photovoltaic devices and the likelihood of surpassing the Shockley-Queisser limit. The reported method and system will offer vital references for the application of multiferroic materials and the design for future high-performance photovoltaic devices, without causing environmental pollution.
The solar cell is placed around multiferroic bismuth layered perovskite heterojunction while the black silicon served as a light-harvesting and backscattering layer. The multiferroicity of the absorber makes it display ferromagnetism, ferroelectricity and a range of couplings concurrently, thus offering a foundation for the corresponding regulation of PV responses.
As a result, they try to order the performance of the device by applying a magnetic field and an electric field. The results reveal that both magnetization and ferroelectric polarization can effectively tweak the photovoltaic reaction. These researchers review the regulation mechanism of their solar cells.
Except for the well-known Rashba effect, we attribute this regulation mainly to the built-in field-driven carrier separation and the corresponding bandgap alignments. The different directions of the built-in depolarization field caused by ferroelectric polarization and the built-in field determined by the piezoelectric effect promote (or hinder) the directional movement of photogenerated electrons and holes.
Dr. Kaixin Guo, Study Researcher, Guizhou University
“Meanwhile, The accumulation of positive (negative) surface charges at the head (or tail) side of the polarization vector makes the energy levels of the active layer down (or up), resulting in a decrease (or increase) of the barrier height, which becomes large enough for positive poling to reverse the original band bending of the device structure,” Dr. Kaixin Guo continued.
The energy levels of the transition metal atoms in the system will be polarized and split due to the Zeeman effect, which makes these impurity levels drifted away from the bandgap center, reducing the recombination rate of recombination centers to minority-carriers, prolonging the lifetime of minority-carriers and thus improving the efficiency of solar cells.
Dr. Kaixin Guo, Study Researcher, Guizhou University
The researchers state, “the presented technique is simple and easy to popularize, which can be used to improve the performance of photovoltaic devices controllably, and the high stability and the environmental friendliness make it expected to cut a striking figure in the field of new-type solar cells.”
“This breakthrough could provide valuable references for the application of multiferroic materials and the design for future high-performance photovoltaic devices, without causing pollution to the environment,” the researchers estimate.
Journal Reference:
Guo, K., et al. (2021) Multiferroic oxide BFCNT/BFCO heterojunction black silicon photovoltaic devices. Light: Science & Application. doi.org/10.1038/s41377-021-00644-0.
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