Enhancing performance of inverted quantum-dot light-emitting diodes based on a solution-processed hole transport layer via ligand treatment

The performance of inverted quantum-dot light-emitting diodes(QLEDs)based on solution-processed hole transport layers(HTLs)has been limited by the solvent-induced damage to the quantum dot(QD)layer during the spin-coating of the HTL.The lack of compatibility between the HTL’s solvent and the QD layer results in an uneven surface,which negatively impacts the overall device performance.In this work,we develop a novel method to solve this problem by modifying the QD film with 1,8-diaminooctane to improve the resistance of the QD layer for the HTL’s solvent.The uniform QD layer leads the inverted red QLED device to achieve a low turn-on voltage of 1.8 V,a high maximum luminance of 105500 cd/m2,and a remarkable maximum external quantum efficiency of 13.34%.This approach releases the considerable potential of HTL materials selection and offers a promising avenue for the development of high-performance inverted QLEDs.

Effect and mechanism of encapsulation on aging characteristics of quantum-dot light-emitting diodes

The aging characteristics,e.g.,the evolution of efficiency and luminance of quantum-dot light-emitting diodes(QLEDs)are greatly affected by the encapsulation.When encapsulated with ultraviolet curable resin,the efficiency is increased over time,a known phenomenon termed as positive aging which remains one of the unsolved mysteries.By developing a physical model and an analytical model,we identify that the efficiency improvement is mainly attributed to the suppression of hole leakage current that is resulted from the passivation of ZnMgO defects.When further encapsulated with desiccant,the positive aging effect vanishes.Tofully take the advantage of positive aging,the desiccant is incorporated after the positive aging process is completed.With the new encapsulation method,the QLED exhibits a high external quantum efficiency of 20.19%and a half lifetime of 1,267 h at an initial luminance of 2,800 cd·m^(-2),which are improved by 1.4 and 6.0 folds,respectively,making it one of the best performing devices.Our work provides an in-depth and systematic understanding of the mechanism of positive aging and offers a practical encapsulation way for realizing efficient and stable QLEDs.

Developing near-infrared quantum-dot light-emitting diodes to mimic synaptic plasticity

The quantum-dot light-emitting diodes(QLEDs)that emit near-infrared(NIR)light may be important optoelectronic synaptic devices for the realization of artificial neural networks with complete optoelectronic integration.To improve the performance of NIR QLEDs,we take advantage of their low-energy light emission to explore the use of poly(3-hexylthiophene)(P3 HT)as the hole transport layer(HTL).P3 HT has one of the highest hole mobilities among organic semiconductors and essentially does not absorb NIR light.The usage of P3 HT as the HTL indeed significantly mitigates the imbalance of carrier injection in NIR QLEDs.With the additional incorporation of an interlayer of poly[9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-flourene]-alt-2,7-(9,9-dioctylfluorene)],P3 HT obviously improves the performance of NIR QLEDs.As electroluminescent synaptic devices,these NIR QLEDs exhibit important synaptic functionalities such as short-and long-term plasticity,and may be employed for image recognition.

The influence of H_(2)O and O_(2) on the optoelectronic properties of inverted quantum-dot light-emitting diodes

The influence of H_(2)O and O_(2) on the performances of Mg-doped zinc oxide (ZnMgO) and ZnMgO-based inverted quantum-dot light-emitting diodes (QLEDs) are studied. With the involvement of H_(2)O from ambience, ZnMgO exhibits a high conductivity, whereas the resultant QLEDs show a low efficiency. The efficiency of QLEDs can be enhanced by annealing ZnMgO in H_(2)O-free glovebox;however, the uniformity and the current of the devices are degraded due to the presence of O_(2), which adsorbs on the surface of ZnMgO and captures the free electrons of ZnMgO. By exposing the devices with ultraviolet (UV) irradiation, the adsorbed O_(2) can be released, consequently leading to the increase of driving current. Our work discloses the influence of the annealing ambience on the conductivity of ZnMgO, and reveals the interaction of H_(2)O/O_(2) and UV with the ZnMgO and its effect on the performance of the resultant inverted QLEDs, which could help the community to better understand the mechanisms of ZnMgO-based QLEDs.

Quantum-dot and organic hybrid light-emitting diodes employing a blue common layer for simple fabrication of full-color displays

Colloidal quantum-dot(QD)light-emitting diodes(QLEDs)have been in the forefront of future display devices due to their outstanding optoelectronic properties.However,a complicated solution-process for patterning the red,green,and blue QDs deteriorates the QLED performance and limits the resolution of full-color displays.Herein,we report a novel concept of QD–organic hybrid light-emitting diodes by introducing an organic blue common layer(BCL)which is deposited through a common mask over the entire sub-pixels.Benefitted from the optimized device structure,red and green QLEDs retained their color coordinates despite the presence of the BCL.Furthermore,adopting the BCL improved the external quantum efficiency of green and red QLEDs by 38.4%and 11.7%,respectively,due to the Förster resonance energy transfer from the BCL to the adjacent QD layers.With the BCL structure,we could simply demonstrate a full-color QD-organic hybrid device in a single substrate.We believe that this device architecture is practically applicable for easier fabrication of solution-processed,highresolution,and full-color displays with reduced process steps.

Quantum-dot light-emitting diodes with Fermi-level pinning at the hole-injection/hole-transporting interfaces

Quantum-dot light-emitting diodes(QLEDs)are multilayer electroluminescent devices promising for next-generation display and solid-state-lighting technologies.In the state-of-the-art QLEDs,hole-injection layers(HILs)with high work functions are generally used to achieve efficient hole injection.In these devices,Fermi-level pinning,a phenomenon often observed in heterojunctions involving organic semiconductors,can take place in the hole-injection/hole-transporting interfaces.However,an in-depth understanding of the impacts of Fermi-level pinning at the hole-injection/hole-transporting interfaces on the operation and performance of QLEDs is still lacking.Here,we develop a set of NiOx HILs with controlled work functions of 5.2–5.9 eV to investigate QLEDs with Fermi-level pinning at the hole-injection/hole-transporting interfaces.The results show that despite that Fermi-level pinning induces identical apparent hole-injection barriers,the red QLEDs using HILs with higher work functions show improved efficiency roll-off and better operational stability.Remarkably,the devices using the NiOx HILs with a work function of 5.9 eV demonstrate a peak external quantum efficiency of~18.0%and a long T95 operational lifetime of 8,800 h at 1,000 cd·m^(−2),representing the best-performing QLEDs with inorganic HILs.Our work provides a key design principle for future developments of the hole-injection/hole-transporting interfaces of QLEDs.

Enhanced efficiency of top-emission InP-based green quantum dot light-emitting diodes with optimized angular distribution

High resolution and wide color gamut are two key requirements for novel display technologies. Owing to the distinguishing advantages over conventional displays, such as intrinsic wide color gamut and the possibility to achieve high resolution, quantum dot light-emitting diodes (QLED) have drawn considerable attention in recent years. On the other hand, indium phosphide quantum dots (InP QDs) have shown a great potential as a replacement for cadmium selenide (CdSe) QDs in display applications due to the inherent toxicity of cadmium-based QDs. In this study, we investigate a top-emission InP-based green QLED with optimized angular distribution. By adjusting the electrical and optical architecture, the device exhibits improved properties with a maximum current efficiency of 30.1 cd/A and a narrowed full width at half maxima (FWHM)of 31 nm, which are the best results ever reported to our knowledge.

ZnSeTe blue top-emitting QLEDs with color saturation near Rec.2020 standards and efficiency over 18.16%

ZnSeTe blue Cd-free quantum dot(QD)has emerged as a promising emitter for display applications due to its nontoxicity,tunable wavelength,and high efficiency.However,ZnSeTe-based quantum-dot light-emitting diodes(QLEDs)usually exhibit unsaturated emissions with broad spectra.Herein,a top-emitting structure,equipped with a transparent indium-zinc-oxide(IZO)top electrode and an IZO phase tuning layer(PTL),is developed to modulate the emission spectra and the efficiency of the devices.Saturated blue emissions with color coordinates beyond Recommendation ITU-R BT.709(Rec.709)and near Rec.2020 standards are achieved.Moreover,benefiting from the improved outcoupling efficiency and the enhanced charge balance,the top-emitting QLED demonstrates a high external quantum efficiency of 15.14%,which is further improved to 18.16%by capping the devices with SiO2 nanospheres.Simulation analysis reveals that the surface plasmon polariton(SPP)losses are effectively reduced by applying a 100 nm PTL,leading to an outcoupling efficiency of 41.2%at a wavelength of 478 nm.Due to the simultaneously enhanced color saturation and efficiency,a high chroma efficiency(current efficiency/y coordinate in Commission Internationale de l'Eclairage chart)of 123 is obtained.The developed top-emitting architecture could enable the realization of efficient and saturated QLEDs for wide color gamut high-definition display applications.