Colloidally synthesized cesium lead halide (CsPbX3; X = Cl, Br, and I) perovskite nanocrystals (PNCs) often suffer from poor ambient and environmental stability conditions, limiting their practical applications. The commonly used surfactant oleylamine is converted to the oleylammonium cation, which pulls out the halide anion from the PNC surface, thus disrupting the structural integrity and stability of the nanocrystal. We developed a simple, completely amine-free colloidal synthesis with a hot injection method under open-atmospheric conditions and introduced bromooctane as a bromine precursor to overcome the above issues. These as-synthesized amine-free PNCs show a photoluminescence quantum yield (PLQY) of around 60%, and the size of PNCs is ∼25 nm. Moreover, these amine-free PNCs were highly stable in colloidal solutions and thin films for more than 5 months under ambient conditions, with 66% of their initial PLQY. In addition, these PNCs show exceptional stability under different environmental conditions, with 44% of the initial PL even after water treatment under 6 h and 28% of the initial PL under ethanol treatment of 120 min. Furthermore, they exhibit excellent photostability for 96 h and retain 36% of their initial PL under ceaseless UV light irradiation at 365 nm (8 W/cm2). In addition, these PNCs have good stability upon heat treatment and maintain 34% of the initial PL upon heating up to 90 °C. Finally, we fabricated a green-emitting down-conversion light-emitting diode (LED) using these amine-free PNCs. Therefore, we envision that these amine-free CsPbBr3 PNCs are perhaps ideal candidates for perovskite-based display applications.
In recent years, colloidal cesium lead halide (CsPbX3) perovskite nanocrystals (PNCs) have attracted significant attention from researchers due to their unique optical properties and potential use in optoelectronic applications. In colloidal synthesis, oleic acid and oleylamine are commonly used as surface-capping ligands. Although oleylamine plays a crucial role in maintaining the colloidal stability and surface passivation of PNCs, its dynamic equilibrium with oleic acid leads to the formation of labile oleylammonium, which pulls halides from the surface of PNCs and thus degrades the crystals. In this Perspective, we summarize the various approaches for eliminating the amines to make high-quality, photostable, and amine-free CsPbX3 PNCs. In addition, we look over the prospects of these PNCs regarding stability in different environmental conditions, photoluminescence properties, and optoelectronic device performance. This perspective will give a broad overview of amine-free PNCs starting from their synthesis, challenges, and optoelectronic properties to their future prospects.
The most commonly used surface capping ligands, like oleic acid and oleylamine, passivate the surface of perovskite nanocrystals (PNCs) to enhance their stability and optical properties. However, due to their inherent insulating nature, charge transport across the surface of the PNCs is hindered, limiting their application in devices. In this study, we have post-treatment CsPbBr 3 PNCs with short chain ligands benzoic acid (BA) and ascorbic acid (AA) and observed that both acid-treated PNCs show enhanced stability and optical properties. Still, BA-treated PNCs show the highest charge transport rate due to their conjugating nature. The photoelectrochemical measurements also show the most efficient electron flow across the surface of the PNC with BA-treated PNCs. A longer carrier lifetime and fast charge transfer make BA-treated PNCs ideal candidates for application in real-life devices.
As-synthesized, green-emitting graded-alloy core/shell “giant” quantum dots have outstanding properties that can be utilized for different optoelectronic applications. But often, defects arise upon nonuniform growth of the shell. These surface defects affect the photoluminescence quantum yield (PLQY), lifetime, and stability of the green-emitting graded-alloy core/shell “giant” quantum dots. So, to overcome these surface traps, the graded-alloy core/shell g-QDs were passivated with z-type ligands (ZnCl2, CdCl2, and MgCl2). The change in the PLQY, lifetime, and photoluminescence emission confirmed that the z-type ligands are passivating the trap states of g-QDs by bonding with an unsaturated chalcogenide atom on the surface, and this was confirmed by X-ray photoelectron spectroscopy. All three cases (treated with ZnCl2, CdCl2, and MgCl2) showed outstanding enhancement in the photoluminescence quantum yield. Fascinatingly, the ZnCl2-treated g-QDs showed maximum enhancement in the PLQY from 62% to unity. Furthermore, the photostability test was performed under continuous UV-light irradiation for 24 h, which clearly showed superior photostability. Moreover, the temperature-dependent stability of untreated and treated g-QDs was studied from 10 to 90 °C. Furthermore, the green-emitting down-converted LED was fabricated by utilizing ZnCl2-treated g-QDs, which show great potential for display application.
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