The development of new-type thin-film solar cell technology is groundbreaking, it opens up the possibility of introducing clean energy into modern families most cordially, and will bring a new way of life.
Today, people are particularly optimistic about the development potential of these new-type thin-film solar cells. Unfortunately, such solar cells usually present low photoelectric conversion efficiency and poor stability, leading to an improvement of efficiency at the expense of the environment. Therefore, it is urgent to explore environment-friendly technologies to promote the conversion efficiency and the stability of PV devices.
After years of research, scientists have found that some materials with a certain asymmetric structure (the so-called multiferroics) represent not only ferroelectric and ferromagnetic ordering simultaneously, but many interesting coupling, widening greatly their application.
In the 1970s, scientists found inadvertently a new important physical phenomenon-ferroelectric PV effect. It is essentially different from the traditional p-n junction, thus it is also called the anomalous or bulk photovoltaic effect. Recently, with the continuous warming of energy research, researchers have introduced multiferroics with appropriate bandgap structure into solar cells as the light absorption layer of devices, forming a new multiferroic solar PV device.
At present, scientists have put forward many theories about the physical mechanism of the ferroelectric PV effect. Whereas, it is most accepted that this effect is closely related to the polarization characteristics of materials. In 2015, Canadian scientists Riad Nechache and Federico Rosei reported the tuning of the bandgap and the ferroelectric PV effect of multiferroic Bi2FeCrO6 thin films. It is testified that the regulation of ferroelectric polarization on PV devices is indeed feasible.
It is known that there is also a certain relationship between magnetic field and light. but whether the PV response can also be adjusted by magnetization for multiferroics? And what is the mechanism?
In a new paper published in Light Science & Application, a team of scientists, led 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 developed an inexpensive new-type all-inorganic oxide solar cell and studied the regulation of PV response of the devices.
Based on self-made black silicon, they designed a multiferroic heterojunction solar cell by introduced non-toxic bismuth layered perovskite heterojunction as the light absorber and graphene as the anode which presents a high photoconversion efficiency of up to 3.90%. Their solar cell still maintains 90% of the initial efficiency after 30 days of stability tracking test.
More interestingly, they studied the regulation mechanisms of applied polarizing and magnetizing fields on the photovoltaic response of the device from the built-in field-driven carrier separation and the above-bandgap change, which is of great significance to improve the efficiency of traditional ferroelectric oxide solar cells. Therefore, their study shows people the great potential of multiferroic materials in the field of new photovoltaic devices and the possibility of exceeding the Shockley-Queisser limit. The reported method and technique will 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 solar cell is centered around multiferroic bismuth layered perovskite heterojunction, and black silicon acted as a light-harvesting and backscattering layer. The multiferroicity of the absorber makes it show ferroelectricity, ferromagnetism, and a variety of couplings simultaneously, thus providing a basis for the corresponding regulation of PV responses. Therefore, they try to regulate the device's performance by applying an electric field and magnetic field. The results show that both ferroelectric polarization and magnetization can effectively tune the photovoltaic response. These scientists summarize 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. 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."
"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 added.
"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 scientists forecast.