Changchun Yinghua Institute reveals the exciton behavior of quasi-two-dimensional perovskite

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[ Instrument Network Instrument Development ] Qin Chuanjiang, a researcher at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and an international research team led by Professor Anda Qianboya of Kyushu University in Japan revealed the mechanism that led to the low luminous efficiency of a class of quasi-two-dimensional perovskites, and proposed a solution. The solution developed a high-efficiency green light-emitting diode based on this type of material. Related results were published online November 12 in Nature Photonics (2019).
Organic-inorganic hybrid perovskites are widely used in the field of optoelectronics research because of their low cost, easy processing, and excellent optoelectronic properties. Light-emitting diodes based on such materials also have great potential as next-generation lighting and display components. Among them, three-dimensional perovskite is formed by alternating organic and inorganic components in three-dimensional space. Two-dimensional perovskite is a layered structure formed by alternating two components, while quasi-two-dimensional perovskite is two types. The mixed structure of perovskites, that is, three-dimensional perovskites of different sizes are wrapped by a large-sized organic shell. Due to the existence of a naturally occurring quantum well structure in the quasi-two-dimensional perovskite, it has a larger exciton binding energy than the conventional three-dimensional perovskite, which is more favorable for luminescence. Although some quasi-two-dimensional perovskite light-emitting diodes have achieved higher electro-optic conversion efficiencies, the reason for the low efficiency of some green-light devices when using different organic components is still unknown. In this research work, the researchers answered this question through a large number of relevant experimental data obtained through international cooperation. Qin Chuanjiang, the first author and co-director of the paper, said: "At present, most researchers believe that such perovskites exhibit more characteristics of traditional inorganic semiconductors. However, we have proved that quasi-two-dimensional perovskites have many properties of organic semiconductors. Therefore, it is necessary to consider exciton behavior with different energies."
Unlike typical inorganic semiconductors, organic semiconductors first form exciton states and then relax luminescence during electroluminescence. Due to the spin characteristics of the electrons, excitons of two different properties, singlet and triplet, will be formed. Although the regulation of singlet and triplet excitons is the basis for the design and development of high-efficiency organic light-emitting diodes, it has not been considered in the study of perovskite light-emitting diodes. In this study, the researchers compared two types of perovskite luminescent materials with similar crystal properties but different organic components, and found that the triplet excitons in one of the perovskite materials disappeared. Through analysis, organic components with low triplet energy levels are used in this type of perovskite. The reason for the poor luminescence performance is that the triplet excitons are transferred to the lower energy organic part, resulting in non-radiative energy loss. When an organic component having a high triplet level is used, the triplet excitons remain in the perovskite luminescent body, thereby obtaining high luminous efficiency. In addition, the researchers further found that in certain quasi-two-dimensional perovskites, dark-state triplet excitons can also be upconverted to radiant singlet excitons, enabling the realization of all excitons in quasi-two-dimensional perovskite devices. Utilization is possible.
Based on the above findings, the research team prepared a quasi-two-dimensional perovskite light-emitting diode capable of efficiently capturing triplet excitons by selecting appropriate organic components, and obtained 12.4% electro-optical conversion efficiency. “We not only explained the experimental phenomena observed before, but some new findings also provide guidance for the development of high-efficiency perovskite optoelectronic devices such as light-emitting diodes, lasers and solar cells,” said Anda Qianbo, who led the research.
The above research work was supported by the start-up funds for the overseas introduction of talents from Changchun Yinghua Institute.

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