Despite the rapid development of light-emitting diodes (LEDs) based on emerging perovskite nanocrystals (PENCs), it is still challenging to achieve integrated high-efficiency and high-brightness devices due to the insulating long-chain ligands used by PENCs.
Here, researchers from UESTC and other units developed highly luminescent and stable formamidine lead bromide PENCs, capped with rationally designed 2-naphthalenesulfonic acid (NSA) short aromatic ligands, for use in LEDs. Compared with the commonly used oleic acid ligands, the NSA molecules not only maintained the surface properties of PENS during the purification process, but also significantly improved the electrical properties of the assembled emissive layer, ensuring efficient charge injection/transport in the device. The electroluminescence of the resulting best LED shows a high brightness of 67115 cd cm-2 and a peak external quantum efficiency of 19.2 percent .
The related paper was published in the journal ACS Energy Letters with the title "High-Brightness Perovskite Light-Emitting Diodes Based on FAPbBr3 Nanocrystals with Rationally Designed Aromatic Ligands".
Metal halide perovskite nanocrystals (PENCs) have recently emerged as candidates for low-cost, high-performance light-emitting diodes (LEDs). High-quality PENCs with near-unit luminescence quantum yield (PLQY) can be obtained under mild synthesis conditions due to their unique high defect-tolerance property. Furthermore, the light emission of PENCs exhibits wide tunability and narrow emission linewidth across the entire visible spectrum, resulting in an ultra-wide color gamut covering about 100 percent . This makes it the best candidate for high-resolution display applications.
In the past few years, with extensive research on the material synthesis and surface passivation of PENCs, the external quantum efficiency (EQE) of related LEDs has rapidly increased from about 0.1 percent to high values of more than 20 percent , which is comparable to commercial luminescence technologies. Despite impressive progress in realizing PENS-based high-efficiency LEDs, most state-of-the-art devices have little to no system-integrated high EQE and high brightness. This is mainly limited by poor charge injection/transport in the device, which originates from the insulating properties of long-chain organic ligands commonly used in PENCs.
To tune the surface properties of PENCs, in addition to the commonly used long-chain oleylamine and oleic acid (OA), a series of alternative ligands have been explored and many useful post-synthesis ligand management strategies have been developed. However, due to the highly dynamic ligand binding and ionic crystal structure of perovskites, the surface properties and structural integrity of PENCs are very sensitive to the surrounding chemical environment during colloidal synthesis and post-processing, which makes the realization of rational ligand engineering of PENCs extremely high. challenging. Therefore, appropriate surface ligands that can maintain the high luminous efficiency and excellent colloidal stability of PENCs, while ensuring the effective electrical properties of the resulting emissive layer, are crucial for the future performance improvement of related LEDs.
In this work, the authors report a high{{0}}brightness, high-efficiency LED based on pure green-emitting lead formamidine bromide (FAPbBr3), revealing efficient ligand exchange and/or PENS compensation during facile purification using NSA-containing solvents , thereby improving the stability of the colloidal solution and high PLQYs. In addition, the assembled emissive layers of NSA PECs show good charge transport properties, which may originate from the strong coupling of PECs induced by NSA molecules. By embedding an optimized NSA FAPbBr3 emitter into a complete LED, the device exhibits a green electroluminescence (EL) peak at 532 nm with a half-height width (fwhm) of approximately 21 nm, corresponding to the International Commission on Illumination (CIE) coordinates (0.19,0.77), the green primary color.
Fig.1 Material synthesis and characterization
Figure 2 Ligand binding properties of PENCs
Figure 3 Single-carrier device characteristics
Figure 4 LEDs based on FAPbBr3 perovskite nanocrystals