Developing a colloidal quantum-dot light-emitting device (QDLED) with an enhancement on efficiency and reliability by inhibiting HTL degradation.

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Operation lifetime of a blue OLED exhibiting triplet-triplet annihilation increased by 89.3% with the incorporation of 10% assistant host in the emitting layer, which increased diffusion length of singlet exciton, reduced the quenching of triplet exciton, and blocked the triplet pair diffusion. Author Keywordsorganic light-emitting diode, lifetime, TTA, OLED Stability and Degradation Mechanisms Objective and BackgroundOrganic light-emitting diode (OLED) is a promising lighting and display technology [1]. Contrary to red and green OLEDs based on phosphorescent emitter which exhibit high efficiency and long lifetime, fluorescent emitter with triplet-triplet annihilation (TTA) characteristics is typically used for commercial blue OLED, and it results in lower efficiency and shorter lifetime [1]. Emission of TTA-OLED from singlets and triplets can be separated from turnoff dynamics of transient electroluminescence (TrEL) measurement due to the delayed emission of triplets [1]. To improve the lifetime of an OLED, a common strategy is to widen the emission zone. By mixing the hole-and electron-transporting materials to form a bipolar emitting layer (EML), recombination zone is extended and hence increases the operation lifetime [2]. Besides, emission zone can be also achieved by increasing the exciton diffusion length.In this paper, we co-evaporated an assistant host (AH) with volume concentration of 10% into the EML of a TTA-blue OLED, it was found that operation lifetime increased by 89.3% (measurement condition: T95@ 2000 cd/m2, which meant the required time for OLED luminance decreased to 95% of its initial value (2000 cd/m2) with constant current driving). This AH and the TTA host exhibited hole-and electron-transporting characteristic, respectively. However, we did not observe the recombination zone broadening. On the other hand, singlet and triplet exciton energy of AH were higher than that of TTA host. And we did observe the diffusion length of singlet exciton increased by 24.3% due to the dilution effect [3]. After electrical aging, triplet exciton experienced less quenching with the incorporation of AH. Besides, it also decreased the quenching of intermediate triplet pair. ExperimentsIn our blue OLED, TTA material was used as the host of the EML, with the incorporation of 4% blue dopant material. Concentration of AH was 0% and 10% in the EML for the two devices used in this paper, respectively. EML was sandwiched by hole-and electron-transporting layer (HTL and ETL). Indium-tin oxide (ITO) and Al was used as the anode and cathode, respectively. 20-nm thin film consisting of the host material of the EML with and without AH was deposited on the glass substrate and 50-nm C60 upon glass substrate, respectively, for obtaining the diffusion length. C60 here was used as a stronger exciton quencher. By comparing the photoluminescence (PL) intensity with and without quencher (C60), diffusion length can be obtained. Steady-state electroluminescence (EL) and spectrum was obtained by spectrometer (Minolta CS1000)...

We have demonstrated that degradation of the hole-transporting layer material in the blue quantum-dot light-emitting diode (QLED) is the main reason accounts for the efficiency loss operated under constant current driving, and the operation lifetime can be elongated by 85% under pulse-mode driving compared to DC case.Keywords QLED, blue emission, lifetime. Figure 3. (a) PL spectra of QLED excited at 340 nm. PL spectra of QLED excited at 420 nm with different HTL thickness: (b) 0 nm, (c) 7 nm, (d) 14 nm, and (e) 28 nm.Figure 4. Enhancement factor for the QLED drive by pulsemode with 10 kHz and different duty cycle.

In this study, we report on the application of a dielectric/ultra-thin metal/dielectric (DMD) multilayer consisting of ytterbium (Yb)-doped molybdenum oxide (MoO3)/silver (Ag)/MoO3 stacked as the transparent cathode in top-emitting green quantum dot light-emitting diodes (QLED). By optimizing the Yb doping ratio, we have highly improved the electron injection ability from 0.01 to 0.35. In addition, the dielectric/ultra-thin metal/dielectric (DMD) cathode also shows a low sheet resistance of only 12.2 Ω/sq, which is superior to the resistance of the commercially-available indium tin oxide (ITO) electrode (~15 Ω/sq). The DMD multilayer exhibits a maximum transmittance of 75% and an average transmittance of 70% over the visible range of 400–700 nm. The optimized DMD-based G-QLED has a smaller current leakage at low driving voltage. The optimized DMD-based G-QLED enhances the current density than that of G-QLED with indium zinc oxide (IZO) as a cathode. The fabricated DMD-based G-QLED shows a low turn-on voltage of 2.2 V, a high current efficiency of 38 cd/A, and external quantum efficiency of 9.8. These findings support the fabricated DMD multilayer as a promising cathode for transparent top-emitting diodes.

In this work, we have demonstrated a top‐emission green quantum dot light‐emitting diode (QLED) with dielectric‐metal‐dielectric (DMD) structure as transparent cathode, which consisted of ytterbium (Yb)‐doped molybdenum oxide (MoO3)/Yb:Ag/MoO3. The DMD‐QLED exhibited a reduced operational voltage and higher efficiency by 1.43‐folds enhancement in external quantum efficiency relative to reference QLED due to the improvement of electron injection and transport as well as less exciton quenching.

We have demonstrated a high efficiency blue quantum‐dot light‐emitting diode (QLED) with positive aging treatment. While device was statically stored 602 hrs, the external quantum efficiency (EQE) of QLED increased from 4.8% to 7.8%, which resulted from the enhancement in electron injection at quantum dot (QD)/ electron transporting layer (ETL) interface. It was confirmed by the measurement of the depth profile of X‐ray photoelectron spectroscopy (XPS). It indicated that cathode material, aluminum (Al), diffused into ETL material, zinc oxide (ZnO), leading to aluminium zinc oxide (AZO) alloy, Al doped ZnO.

In this work, green top emission quantum dot light-emitting diodes with different structure, that are normal and inverted structure, were demonstrated. The device characterization of these two device are compared. The QLED with inverted structure exhibits better device performance than normal one. Current efficiency is improved from 26.9 to 42.5 cd/A, as well as the external quantum efficiency is improved from 5.32 to 9.42 %. Operational lifetime of inverted structure is longer and more reliable than normal structure by ~10-times.