In recent years, metal halide perovskite heterojunction has received widespread attention in solar cells, display and photoelectric detection. However, because ionic crystals such as perovskites are extremely sensitive to the preparation conditions, changes in the interface properties of heterojunctions during synthesis are often accompanied by changes in the crystal phases, size and crystal orientation of each component. These complications make it particularly difficult to study the properties of the heterojunction interface and to precisely regulate its properties.
In view of this problem, the team of Academician Huang Wei, Professor Huang Xiao and Professor Wang Lin of Nanjing University of Technology has developed a universal method for preparing 3D/2D perovskite epitaxial heterojunction by directional self-assembly at room temperature, which is expected to broaden the material range of perovskite epitaxial heterojunction and achieve more regulation of composition, interface and orientation-related properties. At the same time, the method of spontaneous orientation assembly at room temperature has important scientific significance in the field of crystal growth, and provides theoretical guidance for the epitaxial growth of other organic-inorganic hybrid material systems based on weak interactions such as Van Der Waals.
On July 25, 2022, the research was published in the international academic journal Nature Materials under the title “Room temperature epitaxial welding of 3D and 2D perovskites”. Zhu Zhaohua, a master’s degree graduate of Nanjing University of Technology, Zhu Chao, a researcher at Southeast University, Yang Lei, a master’s degree graduate from Nanjing University of Technology, Chen Qian, a doctoral student at Northwestern Polytechnical University, and Zhang Linghai, a researcher at Nanjing University of Technology, are the co-first authors.
Point 1: Epitaxial self-assembly of 3D/2D perovskite heterojunction
Taking the CsPbBr3/PEA2PbBr4 heterojunction as an example, the team of researchers showed how the orientation of crystal assembly can be regulated by changing the perovskite surface ligand. When the surfaces of the two perovskites are the same PEA molecule, the CsPbBr3 nanocrystals are epitaxially assembled on the surface of the POA2PbBr4 nanosheets, and the thickness of the interfacial layer is consistent with the organic layer in the POA2PbBr4 crystal, indicating the order of the interface molecules.
Figure 1: Preparation of CsPbBr3/PEA2PbBr4 heterojunctions by directional assembly and random assembly.
Figure 2: Interface characterization of the CsPbBr3/PEA2PbBr4 heterojunction.
Key point 2: Formation mechanism and universality of 3D/2D perovskite heterojunction
This room temperature epitaxial assembly method is suitable for a variety of 3D (CsPbBr3, CsPbCl3, Cs2AgBiBr6, etc.) and 2D perovskites (PEA2PbBr4, BA2PbBr4, NMA2PbBr4, etc.). In addition to the directional interactions such as van der Waals and dipole moments between crystals, the interaction between the two layers of molecules at the heterojunction interface is affected by the layer spacing and relative angle, which is the main driving force for inducing the orientation assembly of perovskite crystals.
Figure 3: Schematic diagram of various 3D and 2D perovskites.
Figure 4: Characterization of a variety of 3D/2D perovskite epitaxial heterojunctions.
Point 3: The epitaxial interface enhances energy and charge transfer between perovskite crystals
Compared with the random assembly interface formed by the traditional method, the epitaxial heterojunction interface has a stronger interface coupling effect, which enhances the energy and charge transfer between perovskite crystals. In addition, this spontaneous epitaxial assembly process induced the surface lattice distortion and band gap change of 2D perovskite, so that the material exhibited multi-exciton luminescence in an ultra-wide wavelength range at low temperatures.
Figure 5: Optical properties of the CsPbBr3/PEA2PbBr4 heterojunction.
Figure 6: Stress analysis and band calculation at the CsPbBr3/PEA2PbBr4 heterojunction interface.
(Source: Science Network)
Related thesis information: https://doi.org/10.1038/s41563-022-01311-4