Quantum dot (QD) light-emitting diodes (LEDs) are a promising candidate for high-efficiency, color-saturated displays. This work reports on the size effect of sol−gel synthesized ZnO nanoparticles (NPs) in which sizes of 2.9, 4.0, and 5.5 nm, were used as an electron transfer layer in QLEDs. The size of the NPs was estimated by transmission electron microscopy (TEM) and its effect on QLED performance was investigated by photoluminescence decay lifetime and electron mobility of ZnO NPs. It was found that as the size of the NP decreased from 5.5 to 2.9 nm, the conductivity increased, whereby the electron mobility was enhanced from 7.2 × 10 −4 cm 2 /V•s to 4.8 × 10 −3 cm 2 /V•s and electron decay lifetime increased from 5.11 to 6.68 ns. A comparison of NP size effects shows that the best performance is achieved with the 2.9 nm sized ZnO, which yields a turn on voltage of 3.3 V, a maximum current efficiency of 12.5 cd/A, power efficiency of 4.69 lm/W and external quantum efficiencies (EQE) of 4.2%. This is most likely due to the higher electron mobility in the smaller ZnO NPs, which facilitates electron transfer from the NPs to QDs, along with the slow exciton dissociation in the QD layer as a result of more favorable energy level alignment at the interface of smaller ZnO NPs and the adjacent emissive layer.
Surface plasmon-enhanced electroluminescence (EL) has been demonstrated by incorporating gold (Au) nanoparticles (NPs) in quantum dot light-emitting diode (QLED). Time-resolved photoluminescence (TRPL) spectroscopy reveals that the EL enhancement is ascribed to the near-field enhancement through an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Au NPs. It is found that the size of Au NPs and the distance between the Au NPs and the emissive layer have significant effects on the performance of QLED. The enhancement can be maximized as the SP resonance wavelength of Au NPs matches well with the PL emission wavelength of the QD film and the distance between Au NPs and the emissive layer maintains 15 nm. The photoluminance (PL) and EL intensity can be enhanced by 4.4 and 1.7 folds with the incorporation of Au NPs. The maximum current efficiency of 4.56 cd/A can be achieved for the resulting QLEDs by incorprating Au NPs with an enhancement factor of 2.0. In addition, the enhancement ratio of 2.2 can be achieved for the lifetime of resulting QLED.
A novel size-controllable germanium quantum dot (Ge QD) is synthesized and decorated onto reduced graphene oxide (RGO) fragments to overcome the low infrared (IR) photoresponses (∼0.1 A/W)13,14 of pristine graphene. With the integration of flexible substrate, monolayer graphene (MLG) electrode and n-type zinc oxide (ZnO), a high-performance QD-decorated-RGO/ZnO heterostructure infrared photodetector is reported in this study. The Ge QD-decorated-RGO hybrid photosensitive composite improves the responsivity (∼9.7 A/W, 1400 nm) in IR waveband without sacrificing the response speed (∼40 μs rise time and 90 μs recovery time). In addition, the effective barrier formed between graphene and ZnO interface restricts the dark current (∼1.4 nA, -3 V) to guarantee the relatively excellent rectifying behavior and high on/off ratio (∼10(3)) for this IR photodetector. With these superior inherent properties and micron-sized sensing active area, this photodetector manifests great potential in the future application of graphene-based IR photodetector.
High performance, mechanically flexible quantum dot light emitting diodes (QLEDs) based on ZnO nanoparticles used as an electron transfer layer (ETL) are reported.
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