Blue-Emitting Core/Crown Nanoplatelets of CdSe/CdS for LED Application

Colloidal inorganic nanocrystals are an attractive class of nanomaterials for both fundamental research and technical applications thanks to their size- and shape-dependent properties, but also because of their excellent chemical processability.¹ In particular, colloidal CdSe nanoplatelets (NPLs) are fascinating nanocrystals to explore the potential of two-dimensional (2D) materials for next generation optoelectronic devices.² The strong quantum confinement in the vertical direction leads to peculiar optical properties that depend on their thickness,³ such as, narrow emission, large exciton binding energy, relatively small dielectric constant, giant oscillator strength, large linear/nonlinear absorption cross sections and short radiative fluorescence lifetime.⁴ In addition, the large volume of NPLs makes Auger recombination less efficient.⁵ All of these advantages make NPLs promising for application in Light-Emitting Diodes (LEDs).<br/>However, application of CdSe NPLs in the ultraviolet/ blue region remains an open challenge due to charge trapping leading to limited photoluminescence quantum yield (PLQY) and sub-bandgap emission in core-only NPLs.⁶<br/>Furthermore, for LEDs based on II-VI semiconductor quantum dots (QDs), the efficiency and stability of red and green QD based devices is close to standards for commercial applications, while blue QLEDs (Quantum dots-LEDs) lag behind in terms of device efficiency and stability.⁷<br/>The specific problems of blue QLEDs are as follows: surface defects and lattice mismatch of blue QDs, carrier injection imbalances, high absorption coefficient of charge injection layers, fluorescence quenching caused by exciton dissociation induced by an electric field, instability of the organic transport layer, and the loss of light caused by plane-laminated structures.⁸<br/>We recently focused on the synthesis of 3,5 ML CdSe/CdS core/crown blue emitting nanoplatelets developed in 2023.⁹ After a significant improvement in core-only synthesis¹⁰ with a precise control over the aspect ratio (length/width), we raised the PLQY of core/crown NPLs up to 60% with complete elimination of trap state band and undesirable second phases. More specifically, our crowning procedure increases the quantum efficiency of 4 times with respect to the core-only using much less material than a core/shell structure.<br/>Using CdSe/CdS core/crown NPLs emitting in the pure blue region of electromagnetic spectrum (2.7 eV = Eg, λ = 460 nm), we then obtained the highest EQE (2.8%) existing for blue core/crown structured CdSe-based material in Light-Emitting Diodes fabrication.<br/>We take advantage of some important strategies to improve EQE in our proposed LED. In contrast to the majority of works present in the literature, we avoid using ZnO film as ETL due to its absorption in the same region of NPLs emission and in order to reduce self-absorption in NPLs, we use a very low thickness of NPLs as emissive layer. The electroluminescence measurement shows a 460 nm peak with a turn-on voltage of 3.3 V.<br/>Our results show an effective way for the PLQY improvement of core-crown CdSe/CdS NPLs and good performances in hybrid organic-inorganic Light Emitting Diodes application.<br/><br/>1. Chem. Rev. 2010, 110, vol. 1, 389–458.<br/>2. Energy Mater. Adv. 2022, ID 857943.<br/>3. Nature Mater 10, 2011, 936–941.<br/>4. Light Sci Appl 10, 2021, 112.<br/>5. Physical Review Letters, 2003, vol. 91, no. 22, 227401.<br/>6. ACS Appl. Nano Mater. 2022, vol. 5, 1367−1376.<br/>7. Adv. Optical Mater. 2023, 2203152.<br/>8. J. Mater. Chem. C, 2020, 8, 10160.<br/>9. Nano Lett. 2023, 23, 8, 3224–3230.<br/>10. Chem. Mater. 2020, 32, 21, 9260–9267.