(543a) Aligned Self-Assembly of Quantum Confined CsPbBr? Perovskite Nanocrystals in for Application in Enhanced External Quantum Efficiency Light Emitting Diodes and Displays

With the world's population on the rise and the escalating impacts of climate change, there is a pressing need to improve the efficiency of all devices to minimize our ever-growing energy usage. The pathway to high-efficiency optoelectronics is through light management: by directing the light where we want it to go, we minimize loss pathways and increase efficiency (Figure 1). For light-emitting diodes (LEDs) and displays (Figure 1 a), the light should be projected out the front face of the device into forward propagating modes. However, a significant amount of light is lost into total internal reflection modes, and thus the light is trapped inside the LED. Even for materials that have near unity internal quantum efficiency (nearly 100 % efficiency of creating a photon from electrical excitation), the overall efficiency of the device drops to 15-20% because of this light trapping [1].

The external quantum efficiency (EQE) on any optoelectronic device depends on the average orientation or angle of electronic transitions between the emissive and ground state (transition dipole moment (TDM) angle) of a material [2]. In most bulk materials, these light emissions are isotropic (angle 35°) because the electronic transitions are all in random directions. However, the light emission of nanocrystals and nanomaterials can be highly anisotropic depending on the size and the assembly of the nanocrystals in the medium. Perovskite nanocrystals have gained a lot of interest over the last few years because of their exceptional optical properties and ease of fabrication [2]. However, perovskites are highly surface sensitive: interactions between the perovskite nanocrystals and their substrate can modify the transition dipole moment beyond what is expected of the given shape, size, and alignment. Past work has shown that perovskite nanocrystals will have higher transition dipole moments than expected (upwards of 45-55°) because of these surface effects which are detrimental to LED and display efficiencies [3].

Here, we fabricated large-scale, highly confined perovskite materials via the self-assembly of individual nanocrystals to try to reduce these surface effects and achieve the transition dipole moments (0°) to maximize the EQE of LEDs and displays. We dispersed as-synthesized various CsPbBr₃ nanocrystal shapes in hexane, heptane, and octane and observed how they assembled into much larger fused nanocrystals that can maintain their confinement. For example, we synthesized CsPbBr₃ 2-D nanoplates (15-20 nm length) and achieved large, but still confined nanowires with lengths around 1 um through solvent evaporation and temperature control (Figure 1b). The final shape and aspect ratio of these fused nanocrystals depended largely on the capillary forces between the solvent and the perovskite and the polarity of the solvents. For example, octane evaporates slower than hexanes and heptane’s which lead to the formation of extremely anisotropic ally-shaped (i.e. long and skinny) particles as seen in Figure 1 f, g. Using back focal plane imaging, we determined transition dipole angles (TDM) of 22.5-25° for nanoplates and 23-25° for nanowires, indicating lower angles compared to isotropic cubes (40-45°), thus demonstrating enhanced alignment and in-plane electronic transitions in the lower dimension structures. By reducing this TDM to 15-20° (by 51.1%) through these quantum confined nanocrystals, the EQE of an LED can be enhanced to 30-35% (increase by 15%) which would significantly reduce the power requirements of these devices.