Organic-inorganic hybrid metal halide perovskite materials are widely used in optoelectronic functional devices such as solar cells, light-emitting diodes, phosphors, photodetectors, and lasers due to their unique semiconductor properties. Their crystal dimensions, spatial structures, optoelectronic properties, and photophysical processes, etc., can be regulated by factors such as crystal structure design, composition regulation, temperature, and pressure.

Recently, supported by the National Natural Science Foundation of China (22075022 and 21703008), Cui Binbin, associate researcher of the Institute of Advanced Research Institute of Multidisciplinary Sciences of Beijing Institute of Technology, Professor Chen Qi of the School of Materials Science & Engineering, Professor Omar F. Mohammed of King Abdullah University of Science and Technology (KAUST), Kingdom of Saudi Arabia, and researcher Jin Shengye, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, published the latest research progress of the team on the luminescence and mechanism of 2D perovskite crystals in the international top journal "ACS Energy Letters" in the field of materials and chemistry (impact factor 23.101) with the title of Exciton Self-trapping for White Emission in 100-oriented Two-dimensional Perovskites via Halogen Substitution. Han Ying, a doctoral student at the Institute of Advanced Research Institute of Multidisciplinary Sciences of Beijing Institute of Technology, and Dr. Yin Jun, a postdoctoral fellow at King Abdullah University of Science and Technology co corresponding authors of the work.

Figure 1 Free excited state emission (left) and self-trapped broadband emission (right) of two-dimensional perovskite based on 100 orientation

In recent years, low dimensional organic-inorganic hybrid metal halide perovskite has opened up a new field of single component white light phosphors. The research shows that its broadband white light emission is generated by the return of self trapped excitons (STEs) to the ground state, and is subject to the lattice distortion affecting the electron phonon coupling strength of the material and the interaction between inorganic skeleton and organic cations. Previous reports have found that halogen substitution can modulate the STE-broadband white light emission of low-dimensional perovskite crystals, but the underlying mechanism remains unclear. In order to explore the effect of different halogens on the self-trapped luminescence mechanism of 2D perovskite STEs, the team synthesized two 100-oriented two-dimensional 2D light-emitting diodes using protonated benzimidazole as the organic cation and Br- and Cl- as the halogen anions, respectively. It is worth noting that although the two have similar crystal structures and the crystals are almost undistorted in the inorganic octahedral framework, they exhibit completely different luminescence characteristics and mechanisms (Figure 1).

The BM-Br crystal has a weak electron-phonon coupling effect, which reflects the "Huang-Rhys" factor S="13 of the electron-phonon coupling strength, and produces deep blue narrow-band photoluminescence at room temperature; The BM-Cl crystal exhibits STE broadband white light emission due to strong electron-phonon coupling at room temperature, with S = 181, which is the highest value among all bulk semiconductor materials at present (Fig. 2). Therefore, this work reveals the intrinsic regulation mechanism of halogen on 2D perovskite STE emission: in addition to changing the lattice deformation potential and the potential of self-trapping STE, halogen substitution enhances the electron-phonon coupling in BM-Cl crystals by up to tenfold, thereby affecting the kinetics of STE, which provides a general and feasible new idea for regulating the electron-phonon coupling strength in low-dimensional perovskites.

Fig.2 Crystal structures and optical characteristic spectra of BM-Cl and BM-Br (photoluminescence PL, color gamut, Huang-Rhys factor calculation and transient thermal exciton relaxation)

Finally, the team applied two 2D perovskite single crystal materials to electroluminescent diodes (LEDs) and photoluminescent devices, respectively. The LED device based on the BM-Br light-emitting layer emits bright deep blue light, and its CIE color coordinate is (0.159, 0.097), which corresponds well to the standard coordinates of the dark blue index of the display; The application of BM-Cl phosphors in photoluminescence devices can emit bright broadband white light. In addition, the BM-Cl phosphor also exhibits a negative thermal quenching (NTQ) range and reaches a maximum intensity around 300 K at room temperature, with weaker thermal quenching before 450 K. Both the deep blue LED based on the BM-Br light-emitting layer and the white phosphor based on BM-Cl show excellent stability (Fig. 3), and have great application potential in the field of display lighting.

Fig.3 Luminescent properties and stability of light-emitting devices based on BM-Br and BM-Cl

Paper information:

Exciton Self-trapping for White Emission in 100-oriented Two-dimensional Perovskites via Halogen Substitution, ACS Energy Letters , 2021, DOI: 10.1021/acsenergylett.1c02572.

Link to the paper: https://doi.org/10.1021/acsenergylett.1c02572

About the author:

Cui Binbin, a member of the Jiusan Society, obtained his doctorate from the Institute of chemistry of the Chinese Academy of Sciences in 2016 and visited Nanyang Technology University in Singapore from September 2015 to April 2016. He is currently a pre-appointed assistant professor (special associate researcher) and doctoral supervisor of the Advanced Research Institute of Multidisciplinary Sciences, Beijing Institute of Technology. His main research direction is organic-inorganic hybrid photoelectric functional materials. A series of important research progress has been made in "efficient and stable organic-inorganic hybrid metal halogen perovskite photovoltaic devices" and "structure and photoelectric function regulation of low dimensional organic-inorganic hybrid perovskite crystal materials", such as energy & environ Sci., 2020, 13, 4344-4352； Adv. Sci., 2021, 8, 2004805; Nat. Commun., 2019, October, 5190-5198, etc. In recent years, he has published more than 30 research papers in Energy & Environ Sci., Nat. Commun., J. Am. Chem. Soc., Angew. Chem. Int. Ed., Adv. SCI and other journals. He presided over the National Natural Science Foundation of China General and Youth Fund projects.