ZnO Nanoparticles for Quantum-Dot-Based Light-Emitting Diodes

Abstract

Zinc oxide (ZnO) nanoparticles (NPs) are widely used as electron-transport layers in quantum dots (QDs) light-emitting diodes (QLEDs). In this work, we show that the size of the NPs can be tuned with the sol–gel synthesis temperature while keeping a constant mass yield. As the NP size decreases, the surface defect density reduces and the band gap broadens. In return, it prevents exciton quenching at the ZnO NPs/emitting QDs (core–shell CdSe@ZnS) interface. Moreover, as the conductivity of the ZnO NP films decreases, the electron–hole balance in QLEDs improves. When the synthesis temperature decreases from 60 to 0 °C, the diameter of the NPs shrinks from 5.4 to 2.8 nm. The optical band gap broadens from 3.44 to 3.66 eV and the energy of the minimum of the conduction band increases from −3.81 to −3.64 eV below the Fermi level. Consequently, the radiative decay rate of a CdSe@ZnS QDs layer coated on ZnO NP films increases from 11.62 to 13.75 ns. The smaller NPs exhibit a faceted surface with a lower density of defects. Under UV illumination, the intensity of the band to band emission from the ZnO NPs increases while the emission from defects decreases as the NPs diameter becomes smaller. The conductivity of the ZnO film decreases by more than 1 order of magnitude, and the current efficiency of QLEDs increases from 35.8 to 50.8 cd/A.

Influence of back donation effects on the structure of ZnO nanoclusters

“…ZnO semiconducting nanoparticles (NP) are currently a very widespread topic of research, they are characterized by their large band gap and high exciton binding energy 6 while also being relatively inexpensive 7 . They are also exceptionally versatile and have an incredibly wide range of applications including the development of lasers 8 and light emitting diodes (LEDs) 9,10 . They can also serve as an electron transport layers (ETL) in solar cells particularly in perovskite 11 and organic cells 12 and they have also been used in the manufacturing of core-shell NPs with fine-tuned 13 and multifunctional properties 14 .…”

Synthesis of UV/blue light-emitting aluminum hydroxide with oxygen vacancy and their application to electrically driven light-emitting diodes

“…Furthermore, the smaller the size of ZnO, the higher the CBM, which makes it easier to inject electrons into the emissive layer owing to the quantum confinement effect of ZnO. 8,24 Fig. S4(b) and (c)† show the UPS spectrum and Tauc plot of the synthesized ZnO nanoparticles.…”

“…Additionally, luminescent Al(OH) 3− x nanoparticles we made were presented for the electrically driven active LEDs structure, which quantum dot-based LEDs research has been well known. 8,10,22–24 The Al(OH) 3− x based LEDs were efficiently fabricated by the solution processing method. The architecture of the active LEDs with Al(OH) 3− x emissive layers (EML) was ITO (cathode)/ZnO nanoparticle films (40 nm)/Al(OH) 3− x active layers (∼2 monolayers)/2,2′-bis(4-(carbazol-9-yl)phenyl)-biphenyl (BCBP, 50 nm)/MoO 3 (10 nm)/Al (100 nm).…”

An efficient organic and inorganic hybrid interlayer for high performance inverted red cadmium-free quantum dot light-emitting diodes

“…Because of the low barrier to electron injection and high electron mobility, ZnO led to excess electron injection resulted in a severe charge unbalance issue in the QD EML. [14][15][16] Not only charge unbalance, but there is an exciton quenching of the QD EML, which is induced by the adjacent ZnO layer due to spontaneous charge transfer. 24,32,33 To solve such issues, it needs to modulate electron injection to attain proper charge balance within the QD EML and also needs to reduce spontaneous charge transfer to suppress exciton quenching of the QD.…”