The photo of pure inorganic manganese bismuth perovskite single crystal under visible light (upper) and ultraviolet light (lower) shows orange light emission under ultraviolet light irradiation. Photo courtesy of Wei Junhua
The perovskites currently used in the photovoltaic field are widely favored by researchers due to their excellent photoelectric properties. However, as a lead-based material, perovskite also has its shortcomings. For example, it has the toxicity of lead, and its humidity and temperature stability are not as good as pure inorganic materials in application.
Is it possible to design a pure lead-free inorganic material, which has the superior photoelectric properties of perovskite without the toxicity of lead, and its stability is better than similar organic materials?
Recently, a group of professor Kuang Daibin from the School of Chemistry of Sun Yat-sen University published a paper in the sister journal of Cell, saying that the group has constructed an all-inorganic bimetallic Cs4MnBi2Cl12 perovskite single crystal. Sensitization achieves 25.7% luminescence quantum efficiency.
Is it permissible to mix manganese skeleton with perovskite?
The superior photoelectric performance of perovskite is achieved by the orbital hybridization between B-site cation and halogen anion. If you want to replace toxic lead, it is feasible to try a B-site metal element similar to lead but with lower toxicity.
"Our research group has made many attempts in the research of non-lead materials. The pure indium-based perovskite reported earlier has a large Stokes shift, which can reduce self-absorption and thereby obtain higher luminescence quantum efficiency." Kuang Daibin told "China "Science News", "On the one hand, I want to try the bimetallic perovskite that no one has done before, on the other hand, I want to make pure inorganic materials with high stability."
Bivalent manganese ion is an excellent luminescence center with large Stokes shift and high luminescence quantum efficiency. By adjusting its coordination number and crystal field environment, the luminescence spectrum can be adjusted. Usually, the four-coordinate is green light emission. The coordination is yellow to red light emission.
Previously, in related applications, manganese was mainly used as a dopant for lead-based perovskites. Such systems have made great progress, proving that the photogenerated excitons in the perovskite host can be effectively transferred to Mn2+ doping The agent produces strong orange light emission. However, the amount of doping has a great influence on the luminescence performance. The uncertain distribution of Mn2+ as an impurity in the perovskite crystal structure and the presence of various organic ligands make the in-depth study and comparison of electronic interactions in such systems difficult.
Can you try manganese as the skeleton metal in the perovskite structure?
Wei Junhua, the first author of the article and a doctoral student of the School of Chemistry of Sun Yat-sen University, told reporters that although Mn2+ is a good luminescence center, manganese has both parity forbidden and spin-forbidden transitions in an octahedron with a symmetric center. It is difficult to form photogenerated excitons, so manganese-based halide materials generally have low extinction coefficients and low luminous efficiency.
So, how can we enhance the luminous efficiency of Mn2+ and the photoelectric performance of manganese-based perovskite?
Photoelectric synergetic effect of manganese and bismuth under three-layer structure
One strategy is to introduce photosensitizers. Kuang Daibin's team found that the combination of manganese and bismuth has not been tried.
"In the beginning, we mainly synthesized materials by one-pot method in hydrochloric acid at room temperature, and found that the luminous efficiency of manganese bismuth perovskite crystallites was low, only 7.8%." Wei Junhua said, "It is generally believed that single crystals have a higher Crystallinity and less defect density, therefore, we grow manganese bismuth perovskite single crystal by hydrothermal method, so that the luminous efficiency of the material is increased to 25.7%."
Through single crystal X-ray diffraction analysis of the crystal structure, they found that the manganese bismuth perovskite formed the next [BiCl6]3--[MnCl6]4--[BiCl6]3-trilayer octahedral structure.
Why is the luminous efficiency of manganese improved under this structure?
It turns out that [BiCl6]3-octahedron has strong absorption in the ultraviolet region, and bismuth ions can effectively sensitize manganese ions, thereby improving the luminous efficiency of the manganese center.
"Our theoretical calculations also show that the atomic orbitals of manganese and bismuth are well-hybridized, which is conducive to efficient energy transfer between bismuth and manganese." Wei Junhua told the China Science Journal, "In addition, large reports in the literature The luminous intensity of most metal halide perovskite materials will quench with increasing temperature, and we have found that the luminous intensity of manganese bismuth perovskite materials gradually increases with increasing temperature, showing a thermal activation trend, so at room temperature Can get better luminous performance, the operability will be stronger in practical applications. And the single crystal thermal decomposition temperature is as high as 480 ℃, after 90 days in the air (relative humidity 60%), there is no change in the physical phase, showing Better structural stability."
"Constructing dissimilar metals perovskite through manganese alloying will be very meaningful. Different metals in dissimilar metal compounds may have a synergistic effect on their photoelectric properties." Kuang Daibin said.
Medical X-ray display skills
Considering the high extinction coefficient of bismuth element to X-rays, the team of Kuang Daibin applied this newly synthesized material to the field of medical X-ray imaging.
With the assistance of Zhongshan University Hospital, the imaging film made of manganese, bismuth and perovskite materials can show the internal structure of the object through the plastic shell wrapped under the medical X-ray irradiation.
"Medical applications require that perovskite materials are very sensitive to X-rays to achieve high imaging resolution. At present, high-end medical X-ray imaging materials are mainly imported from abroad, and sometimes they encounter foreign export restrictions. Even if they can be purchased, they are often priced. Very expensive." Kuang Daibin said.
In addition, they mixed the manganese bismuth perovskite single crystal with commercial blue and green phosphors to obtain white phosphors, and then assembled the resulting white phosphors with ultraviolet chips to obtain white LEDs, which also showed in the field of lighting. Great potential.
Kuang Daibin said: "The metal halide perovskite material has a high defect tolerance and is inexpensive to prepare. It can be prepared by a solution method at a low temperature, which is preferred for new luminescent materials."
This study initially revealed the application potential of manganese bismuth perovskite materials in medical X-ray imaging and illumination display. Kuang Daibin believes that the next step is to further prepare single crystal materials with low defect density, higher luminous efficiency, and larger crystal size through the synthesis process and the adjustment of the material composition, in-depth exploration of the material's luminescence mechanism, and exploration of large-scale preparation technology Etc., to lay a solid foundation for future commercial applications. (â– Chi Han, a trainee reporter of this newspaper)
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