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This paper reports the large area quantum dots light emitting diodes (QLEDs) that are composed of CdSe/ZnS core/shell quantum dots (QDs) as the light emitting layer (EML), ZnO nanoparticles (NPs) as the electron transport/injection layer (ETL) and organic polymers as the hole transport
layer (HTL). By varying the thickness of ETL and EML, the perfo...
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... The ZnO NPs exhibit n-type properties, attributed to interstitial Zn atoms and the oxygen vacancies [25,26]. These characteristics make them widely used as the ETL in QLEDs, offering advantages such as high mobility and compatible interface energy levels [27,28]. However, ZnO NPs often contain a significant concentration of oxygen vacancies, leading to nonradiative recombination with QDs [29][30][31]. ...
Many quantum dot light-emitting diodes (QLEDs) utilize ZnO nanoparticles (NPs) as an electron injection layer (EIL). However, the use of the ZnO NP EIL material often results in a charge imbalance within the quantum dot (QD) emitting layer (EML) and exciton quenching at the interface of the QD EML and ZnO NP EIL. To overcome these challenges, we introduced an arginine (Arg) interlayer (IL) onto the ZnO NP EIL. The Arg IL elevated the work function of ZnO NPs, thereby suppressing electron injection into the QD, leading to an improved charge balance within the QDs. Additionally, the inherent insulating nature of the Arg IL prevented direct contact between QDs and ZnO NPs, reducing exciton quenching and consequently improving device efficiency. An inverted QLED (IQLED) utilizing a 20 nm-thick Arg IL on the ZnO NP EIL exhibited a 2.22-fold increase in current efficiency and a 2.28-fold increase in external quantum efficiency (EQE) compared to an IQLED without an IL. Likewise, the IQLED with a 20 nm-thick Arg IL on the ZnO NP EIL demonstrated a 1.34-fold improvement in current efficiency and a 1.36-fold increase in EQE compared to the IQLED with a 5 nm-thick polyethylenimine IL on ZnO NPs.
... The ZnO nanoparticles (NPs) exhibit n-type properties, attributed to interstitial Zn atoms and the oxygen vacancies [19,20]. These characteristics make them widely used as the ETL in QLEDs, offering advantages such as high mobility and compatible interface energy levels [21,22]. However, ZnO NPs often contain a significant concentration of oxygen vacancies, leading to non-radiative recombination with QDs [23][24][25]. ...
Many quantum dot light-emitting diodes (QLEDs) utilize ZnO nanoparticles (NPs) as an electron injection layer (EIL). However, the use of ZnO NP ETL material often results in charge imbalance within the quantum dot (QD) emitting layer (EML) and exciton quenching at the interface of QD EML and ZnO NP EIL. To overcome these challenges, we introduced an Arginine (Arg) interlayer (IL) onto the ZnO NP EIL. The Arg IL elevated the work function of ZnO NPs, thereby suppressing electron injection into the QD, leading to improved charge balance within the QDs, Additionally, the inherent insulating nature of the Arg IL prevented direct contact between QDs and ZnO NPs, reducing exciton quenching and consequently improving device efficiency. The inverted QLED (IQLED) utilizing a 20-nm-thick Arg IL on ZnO NP EIL exhibited a 2.22-fold increase in current efficiency and a 2.28-fold increase in EQE and compared to an IQLED without an IL. Likewise, the IQLED with a 20-nm-thick Arg IL on ZnO NP EIL demonstrated a 1.34-fold improvement in current efficiency and a 1.36-fold increase in EQE compared to IQLED with a 5-nm-thick PEI IL on ZnO NPs.
... ZnO nanoparticles (NPs) is widely employed as the electron transport layer (ETL) in QLEDs due to their advantages, including high mobility, transparency, and suitable interface energy level [20]. However, ZnO NPs often exhibits a significant concentration of oxygen vacancies, leading to nonradiative recombination at the QD-ZnO interface [17,21]. ...
Zn0.9Mg0.1O nanoparticle (NP) were employed as electron transport layers (ETLs) with varying thicknesses to investigate their influence on the efficiency of the top-emission quantum dot light-emitting diodes (TE-QLEDs) fabricated inside the bank. An increase in the thickness of the Zn0.9Mg0.1O NP ETL led to a decrease in the concentration of oxygen vacancies, reducing the conductivity of the Zn0.9Mg0.1O and resulting in lower current density in the TE-QLEDs. The decrease in conductivity of Zn0.9Mg0.1O NP ETL was confirmed through electron-only device (EOD) characterization. Furthermore, it was noted that when the thickness of Zn0.9Mg0.1O NP ETL was 30 nm, the concentration of hydroxyl species reached its minimum. By minimizing the presence of hydroxyl species, exciton quenching at the quantum dot (QD) and Zn0.9Mg0.1O NP ETL interface was minimized, enhancing charge balance within the QD, significantly improving the efficiency of QLED. We successfully demonstrated that TE-QLED with a 30 nm-thick Zn0.9Mg0.1O NP ETL exhibits outstanding performance, achieving a maximum current efficiency of 91.92 cd/A and a maximum external quantum efficiency of 21.66%. These results suggest that Zn0.9Mg0.1O NP ETL, when tailored to an appropriate thickness, can serve as an ETL for TE-QLEDs, effectively suppressing exciton quenching and enhancing the charge balance in the TE-QLEDs.
... Moyen et al. have shown that the defect density also decreases with decreasing ZnO nanoparticle size. Therefore, reducing nanoparticles' size not only prevents exciton quenching in the charge-transport layer and emission-layer interface but also improves the charge balance in QLEDs [16,17]. ...
... Thus, the wide energy bandgap of the ZnO nanoparticle layer not only improved the electron injection from the cathode into the emission layer but also prevented the leakage of holes to the ZnO nanoparticle layer. This restricted the excitation-recombination region and prevented exciton quenching in the poly-TPD/QD interface, hence potentially improving the charge recombination efficiency [13,16,17,[28][29][30][31]. ...
Quantum dots (QDs) have attracted a lot of attention over the past decades due to their sharp emission spectrum and color, which can be tuned by changing just the particle size and chromophoric stability. All these advantages of QDs make quantum dot light-emitting diodes (QLEDs) promising candidates for display and light-source applications. This paper demonstrates a large-emitting-area QLED fabricated by a full-solution process. This QLED is composed of indium tin oxide (ITO) as the anode, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) as the hole injection layer (HIL), and poly(N,N′-bis-4-butylphenyl-N,N′-bisphenyl)benzidine (poly-TPD) as the hole-transport layer (HTL). The light-emitting layer (EML) is composed of green CdSe/ZnS quantum dots. By applying the ZnO nanoparticles as the electron-injection/transport layer, QLED devices are prepared under a full-solution process. The large-emitting-area QLED exhibits a low turn-on voltage of around 2~3 V, and the International Commission on Illumination (CIE) 1931 coordinate value of the emission spectrum was (0.31, 0.66). The large emitting area and the unique QLED structure of the device make it possible to apply these features to inkjet printing quantum dot light sources and quantum dot display applications.
... By controlling those materials and engineering the interface of each layer, a notable improvement in QD-LED performance has been achieved by adopting metal oxide nanoparticles (NPs) such as TiO 2 or ZnO to make an efficient ETL [3][4][5][6][7][8][9]. Specifically, solution synthesized ZnO NPs with wide bandgap and high mobility showed excellent QD-LED performance exhibited by highly conductive electron transport and /or efficient hole blocking [10][11][12][13][14]. However, most reported literature using ZnO NPs as the ETL of QD-LEDs has not independently investigate the influence of intrinsic defects, such as interstitial Zn (Zn i ) and oxygen vacancies (V O ), on the performance of ZnO NPs because they have either been combined with various extrinsic doping materials such as Al, Li, and Ga [15][16][17] or had a modifier layer such as PEIE and PMMA inserted [18][19][20]. ...
ZnO nanoparticles (NPs) of 4-5 nm, widely adopted as an electron transport layer (ETL) in quantum dot light emitting diodes (QD-LEDs), were synthesized using the solution-precipitation process. It is notable that synthesized ZnO NPs are highly degenerate intrinsic semiconductors and their donor concentration can be increased up to N D = 6.9 × 1021 cm-3 by annealing at 140 °C in air. An optical bandgap increase of as large as 0.16-0.33 eV by degeneracy is explained well by the Burstein-Moss shift. In order to investigate the influence of intrinsic defects of ZnO NP ETLs on the performance of QD-LED devices without a combined annealing temperature between ZnO NP ETLs and the emissive QD layer, pre-annealed ZnO NPs at 60 °C, 90 °C, 140 °C, and 180 °C were spin-coated on the annealed QD layer without further post-annealing. As the annealing temperature increases from 60 °C to 180 °C, the defect density related to oxygen vacancy (V O) in ZnO NPs is reduced from 34.4% to 17.8%, whereas the defect density of interstitial Zn (Zni) is increased. Increased Zni reduces the width (W) of the depletion region from 0.21 to 0.12 nm and lowers the Schottky barrier (ФB) between ZnO NPs and the Al electrode from 1.19 to 0.98 eV. We reveal for the first time that carrier conduction between ZnO NP ETLs and the Al electrode is largely affected by the concentration of Zni above the conduction band minimum, and effectively described by space charge limited current and trap charge limited current models.
... [17][18][19] However, the ZnO QDs and other QDs materials are potential semiconductor materials in electronic devices, such as lasers, OLEDs, and photovoltaic devices, the usual shell materials or doping materials may reduce surface defects and meanwhile bring adverse effect on the electronic and luminescent properties of QDs. [20][21][22][23][24] Another possible way is to keep them at relatively low temperature in closed cavity to avoid contact with the atmosphere, such as in glass capsules, for long service time in device. Further methods of solving the problem of stability of QDs need to be developed to utilize the QDs with high efficient quantum size effect. ...
The ZnO quantum dots (QDs) were synthesized with improved chemical solution method. The size of the ZnO QDs is exceedingly uniform with a diameter of approximately 4.8 nm, which are homogeneously dispersed in ethanol. The optical absorption edge shifts from 370 nm of bulk material to 359 nm of QD materials due to the quantum size effect, while the photoluminescence peak shifts from 375 nm to 387 nm with the increase of the density of ZnO QDs. The stability of ZnO QDs was studied with different dispersion degrees at 0 °C and at room temperature of 25 °C. The agglomeration mechanisms and their relationship with the emission spectra were uncovered for the first time. With the ageing of ZnO QDs, the agglomeration is aggravated and the surface defects increase, which leads to the defect emission.
A ZnMgO and ZnO double-layered structure was prepared to create a stepwise interfacial electronic structure to improve the electron-injection and electron-transport behaviors in quantum-dot light-emitting diodes (QLEDs). The current density of the electron-only device (EOD) with ZnMgO/ZnO was higher than that of the EOD with only ZnMgO. The detailed QLED interfacial electronic structure was measured using X-ray and ultraviolet photoelectron spectroscopy. A stepwise interfacial electronic structure for electron injection and electron transport was observed connecting the aluminum cathode to the ZnMgO conduction band minimum (CBM) via the ZnO CBM. The QLEDs with the ZnMgO/ZnO double electron transport layer showed an improved performance, with a maximum luminance and current efficiency of 90,892 cd m⁻² and 19.2 cd A⁻¹, respectively. Moreover, the turn-on voltage of the device was significantly reduced to 2.6 V due to the stepwise interfacial electronic structure between the aluminum cathode and ZnMgO CBM. This research provides a useful method for developing highly efficient and low turn-on voltage QLEDs using a ZnMgO/ZnO double ETL for next-generation display.
Ink-jet printing is a promising deposition technology, which is capable of large-area fabrication and mask-free patterning. For ink-jet-printed quantum dot (QD) light-emitting diodes (LEDs), the QDs are commonly dissolved in a mixture of solvent and thickener ink system. However, the hole transport layer could be eroded by this QD ink, leading to a rough surface morphology and resulting in the leakage of carriers and low device performance. This phenomenon was first and directly observed by using an atomic force microscope and a cross-sectional scanning electron microscope. We, therefore, redesigned the annealing process of the hole transport layer to achieve an optimized smooth surface with a reduced number of defects for ink-jet-printed QD LEDs (QLEDs). Optimized morphology brings back a maximum luminance of over 30,000 cd/m2 and an external quantum efficiency of 7.52% for the ink-jet-printed red QLEDs using CdSe QDs, which are comparable to those of the spin-coated device. Moreover, the operation lifetime of the ink-jet-printed device is also enhanced by the restored surface morphology. An enhanced T50 lifetime of the ink-jet-printed device at 1000 cd/m2 is improved from 26 to 127 h, which converted to a long T50 lifetime of 8013 h, when operated at 100 cd/m2.
Metal halide perovskite nanocrystals (NCs) have sparked considerable attentions in the area of light-emitting diodes (LED) by virtue of their remarkable color purity and spectral tunability (400-700 nm). However, the optoelectronic performance of LED devices based on perovskite NCs is severely limited by the problem of charge injection since (i) the charge injection layers exhibit significant differences in energy levels and charge mobility, (ii) one thick NCs emitting layer can obstruct the electrons transmission and one thin NCs emitting layer can change the charge recombination region, inducing series of troubles in fabrication and application of LED devices. Herein, a series of LED devices based on cesium lead bromide (CsPbBr3) green-emitting perovskite NCs are reported by adopting Poly-N-vinylcarbazole (PVK)/Poly[9,9-dioctylfluoreneco-N-[4-(3-methylpropyl)]diphenylamine] (TFB) and Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) as the hole transport layer (HTL) materials. With the increase of applied voltage, the CsPbBr3 NCs LED device with PTAA as HTL displays better transport characteristics, such as a lower turn-on voltage and a higher luminance, than that of the device with TFB and PVK as HTL. By optimizing the spin-coating craft parameter of the CsPbBr3 NCs layer, an efficient green-emitting CsPbBr3 NCs LED device with maximum luminance of 4531 cd m⁻², current efficiency of 14.4 cd A⁻¹, power efficiency of 14.1 lm W⁻¹ and maximum external quantum efficiency of 4.28% was obtained. These results provide an approach for the practical applications of CsPbBr3 NCs electroluminescence LED devices.
Efficiency and operational durability are crucial characteristics for the high-performance quantum-dots light-emitting diodes (QLEDs). In order to improve the efficiency and the stability of the QLEDs, we have inserted an inorganic Al 2 O 3 interfacial modification layer (IML) next to the electron transport layer (ETL) using atomic layer deposition (ALD). We conducted a comparative analysis of two different positions of Al 2 O 3 IML inside the device, inserting IML before ETL and after ETL, to find an optimized structure for the efficient QLEDs. As a result, when Al 2 O 3 IML was located after ETL, reduction of oxygen vacancy in the ZnO layer was occurred due to the reaction of O 2 plasma reactant that used during the ALD process. It can have the effects of decreasing the exciton quenching phenomenon and balancing the charge injection. Therefore, improved device performances were observed, and the maximum luminance was 56 108 cd m ⁻² when 1 nm Al 2 O 3 IML was deposited after ZnO ETL. These results suggest Al 2 O 3 IML deposition would provide a useful way to improve the performance of QLEDs.
