New strategies for in-situ controllable solid films of full-substrate perovskite quantum dots

On December 22, 2022, Beijing time, a scientific research team led by researcher Yuan Mingjian and academician Chen Jun of Nankai University and the team of Professor Edward H. Sargent of the University of Toronto published an article in Nature entitled “Synthesis-on-substrate of Quantum Dot Solids”.

In this study, aiming at the challenge of difficult preparation of high-performance conductive semiconductor quantum dot solid films, through the rational design of organic ligand structure, and the controllable adjustment of the structural spatial dimension and electronic band information of perovskite materials, a new strategy (Synthesis-on-Substrate, SoS) for high-quality perovskite quantum dot materials on the substrate surface was developed, and finally realized the efficient construction of multi-material, cross-size perovskite three-color electroluminescent devices.

The corresponding authors of the study are Yuan Mingjian, Academician Chen Jun of Nankai University and Professor Edward H. Sargent of the University of Toronto. Dr. Jiang Yuanzhi, Sun Changchang, Xu Jian and Li Saisai are co-first authors of this paper.

Perovskite materials have excellent optical and electrical properties, which have been widely studied and made great progress in the field of electroluminescence. However, the external quantum efficiency of blue-emitting perovskite LEDs is still low, which greatly hinders the commercial progress of perovskite materials in the field of full-gamut display. At present, CsPbBr3 perovskite quantum dots with strong quantum confinement are the most promising candidates for achieving efficient and stable blue light emission perovskite light-emitting diodes. However, the synthesis of ultra-small-sized CsPbBr3 perovskite quantum dot monodisperse solution based on traditional methods presents great challenges. At the same time, the surface ligands of the obtained ultra-small size perovskite quantum dots are easily shed during post-processing, which makes it difficult for the quantum dot solution to be converted into a high-quality perovskite quantum dot solid film, resulting in the construction of high-efficiency blue-emitting perovskite light-emitting diodes.

Recently, a team led by Yuan Mingjian, a researcher and academician Chen Jun from the School of Chemistry of Nankai University and the Key Laboratory of Advanced Energy Materials Chemistry of the Ministry of Education, and the team of Professor Edward H. Sargent of the University of Toronto researched, aiming at the challenge that high-performance conductive semiconductor quantum dot solid films are difficult to prepare, through structural screening and design of organic ligands, a new strategy for the in-situ synthesis of ultra-small size blue-emitting perovskite quantum dot solid films with high optical and electrical properties on the substrate surface was proposed. A pure blue-emitting perovskite light-emitting diode with an external quantum efficiency of up to 17.9% is realized, and it is proved that the strategy is universally applicable to a variety of substrates and various types of perovskite materials.

Figure 1: Solid film strategy for all-substrate in situ synthesis of perovskite quantum dots

In previous work, the research team found that the chemical structure of organic amine cationic ligands has a decisive influence on the macroscopic (such as zero, one, two, three-dimensional) and microscopic properties (such as band structure, electron transport and operation behavior) of perovskite materials, which provides a solution for breaking through the traditional colloidal quantum dot synthesis process, developing a new controllable preparation method for high-quality conductive quantum dot solid films, and realizing high-performance pure blue-emitting perovskite luminescent materials. Therefore, the research team took the phenethylamine cation (PEA+) ligand with good conductivity as a model, and rationally designed its head end (alkyl ammonium on the benzene ring) and tail end (alkyl ammonium parasite), and proposed a new ligand for α-methyl-4-bromobenzylammonium cation (Br-MBA+) suitable for one-step base in situ synthesis strategy. Through theoretical calculations and experimental results, the research team found that the introduction of additional methyl groups into the head end of the ligand to form a large head end steric hindrance can effectively inhibit the layered perovskite phase, thereby ensuring the formation of cubic phase CsPbBr3 perovskite. By further introducing halogen substituents at the tail end of the ligand, the binding energy of the ligand on the surface of CsPbBr3 perovskite can be increased, and the restricted growth of CsPbBr3 perovskite crystals can be realized, and finally an ultra-small CsPbBr3 perovskite quantum dot solid film with strong quantum confinement can be obtained.

The research team then controlled the CsPbBr3 perovskite quantum dot size with high precision by simply changing the ligand concentration in the perovskite precursor, achieving continuous adjustment of the band gap from 2.37 eV to 2.68 eV. At the same time, the research team further proposed α, α-dimethyl-4-bromobenzylammonium cation cation (Br-DMA+) ligand for CsPbI3 perovskite quantum dot system, realizing the controllable preparation of solid films of red emission perovskite quantum dots.

Figure 2: Exploration of the film-forming mechanism of perovskite quantum dot solid films

The research team used in situ synchrotron radiation GIWAXS to explore the formation mechanism of CsPbBr3 perovskite quantum dots: during the crystallization process, the film showed a gradual phase transition from PbBr2-2•DMSO intermediate phase to cubic phase CsPbBr3. The research team believes that the strong binding ability of ligands to perovskite surfaces is a prerequisite for the formation of ultra-small-size perovskite quantum dots with strong quantum confinement, and the introduction of metastable mesophase and its induced controllable crystallization process may be the key factor in the formation of perovskite quantum dots with uniform size distribution.

The research team then characterized the obtained perovskite quantum dot solid film with transient fluorescence, photoinduced external quantum efficiency, conductivity, etc. The results show that the obtained perovskite film has low defect density, moderate surface ligand density, and good optical and electrical properties. At the same time, perovskite quantum dot solid films also exhibit ideal thermal stability.

Figure 3: Perovskite LED related device performance

The research team assembled the obtained high-quality conductive perovskite quantum dot solid film into light-emitting diodes and explored their device properties. The experimental results show that the electroluminescence wavelength of pure blue-emitting perovskite light-emitting diode device constructed from ultra-small perovskite quantum dots is 480 nm, and its external quantum efficiency reaches 17.9%, and shows good spectral stability. At the same time, the research team also further extended the strategy to a variety of perovskite materials, and achieved close proximity to Rec. The controllable construction of red light, green light and deep blue emission perovskite light-emitting diodes of the 2100 display color standard provides a new research scheme for the research of perovskite luminescent materials and display devices and other related fields.

The above research work was supported by the Key Research and Development Program of the Ministry of Science and Technology, the Innovation Group of the National Natural Science Foundation of China, and the China Postdoctoral Science Foundation. (Source: Science Network)

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