Direct linear polarization electroluminescence from perovskite nanosheet superlattices

• Linearly polarized light is crucial in many application scenarios, including optical measurement analysis, biological imaging, counterfeit detection, liquid crystal displays, and emerging three-dimensional display technologies. Current technology converts ordinary light into linearly polarized light by using special linearly polarizing filters. Common linear polarizing filters are composed of highly oriented long-chain molecules or metal meshes with photonic structures. This conversion process causes the brightness to be at least halved, resulting in significant energy loss and the need for additional optics. Therefore, it is particularly important to develop light sources that directly emit linearly polarized light.

  
  

  To address this problem, research teams including Professor Robert Hoye (correspondence) and Dr. Ye Junzhi (first author) from the University of Oxford, Professor Akshay Rao (correspondence) and Dr. Dai Linjie (correspondence) from the University of Cambridge developed a CsPbI 3 perovskite colloidal nanosheet. Superlattices serve as the light-emitting layer of light-emitting diodes (LEDs) for direct emission of linearly polarized light. The research team used solvents with different vapor pressures to achieve self-assembly and orientation control of nanosheets in the film (the nanosheets can lie facing up or stand facing the sides), thereby arranging their transition dipole moments. Benefiting from the highly uniform arrangement of the nanosheets and their strong quantum and dielectric confinement, this nanosheet film exhibits remarkable exciton fine structure splitting, enabling an optical structure without the need for any additional optical structures. Highly polarized (74.4%) pure red LED. This work demonstrates the potential of perovskite nanosheets as linearly polarized light sources, laying the foundation for the development of next-generation three-dimensional display technology and linearly polarized light applications such as optical communications.

  
  

  According to literature reports, perovskite colloidal quantum dots can emit light with a certain degree of linear polarization, especially quantum dots with anisotropic shapes such as nanowires and nanorods. The degree of linear polarization of a single asymmetric quantum dot can exceed 70%. However, when these quantum dots are made into films or devices, the arrangement of the quantum dots in the film is often disordered due to factors such as uneven size, which significantly reduces the luminescence polarization performance of the device. In order to solve this problem, the research team first successfully prepared nanosheets with highly consistent sizes by adjusting the synthesis temperature and the ratio of precursors. The longitudinal thickness of these nanosheets was only 2.5 nanometers. By selecting different types of solvents to adjust the evaporation rate, the research team achieved an orderly arrangement of nanosheets on the film into a superlattice structure (as shown in Figure 1). Through structural characterization techniques such as grazing incidence wide-angle X-ray diffraction and optical characterization methods such as angular transformation momentum space Fourier microscopy, the research team confirmed the formation of oriented superlattice and the orderly arrangement of transition dipole moments (as shown in the figure) 2).

  
  

  Linearly polarized luminescence in perovskite quantum dots originates from the fine structure splitting of excitons in the electronic energy bands. The research team first confirmed through static fluorescence spectroscopy that the main luminescence in the ultrathin nanosheets was excitonic luminescence, and used a lead iodide precursor solution to passivate surface defects of the nanosheets. Through ultrafast transient absorption spectroscopy and fluorescence spectroscopy analysis, it is proved that the non-radiative recombination process of defect recombination and exciton-exciton annihilation in passivated nanosheets is reduced, thereby improving the luminous efficiency (as shown in Figure 3) . The passivated material exhibits higher device efficiency, and the standing-aligned nanosheet films and devices have higher polarized luminescence than the lying nanosheets (as shown in Figure 4). In contrast, the luminescence of weakly confined nanocrystals (CsPbI 3 ) and bulk films (FAPbI 3 ) has no obvious linear polarization. Through low-temperature fluorescence measurements, the research team found that nanosheets have higher exciton fine splitting than weakly confined nanocrystals and bulk films, due to the larger excitons generated by their strong quantum confinement and dielectric confinement effects. Binding energy (shown in Figure 5).

  
  

  Finally, the research team summarized the three key points for realizing such a highly polarized light-emitting device (1) forming a superlattice with highly uniformly arranged nanosheets, (2) achieving high photoluminescence quantum yields (PLQYs), and the luminescence mechanism is mainly affected by the band Radiative recombination control of edge excitons, and (3) preservation of large exciton fine structure splitting due to strong quantum and dielectric confinement at the film level. This work opens a new research frontier, providing more efficient linearly polarized LEDs for applications such as display technology and optical communications.

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• Ye, J., Ren, A., Dai, L. et al. Direct linearly polarized electroluminescence from perovskite nanoplatelet superlattices. Nat. Photon. (2024).

  https://doi.org/10.1038/s41566-024-01398-y