In this work, the exciton diffusion model coupled with a drift-diffusion solver is used to simulate three bilayer TTF-OLEDs devices, which triplet tank layer (TTL) is DMPPP (Device A), DCPPP (Device B), and PPC (Device C), respectively. The simulation results are matched to the experimental data, and the efficiency and loss mechanisms are studied. The main reason for IQE loss is triplet-polaron quenching (TPQ) and the ability of triplet diffusion. The experimental result of the device has a recorded 15.5% efficiency in TTF OLEDs, which are benefitted due to the high diffusion coefficient and fewer electrons accumulated in the converting layer to avoid the TPQ processes. This is due to the LUMO of PPC being matched to the second layer to avoid carrier accumulation at the interface. Device A has a good diffusion ability and low TPQ coefficients but suffers from electron accumulation at the interfaces. The worst case (B) has a low diffusion coefficient with a high TPQ coefficient, which has a weak triplet density in the NPAN layer to induce the TTF processes. Besides the bilayer studies, the single-layer structures are also studied to extract some key parameters for bilayer studies. It is interesting to find that material with high TPQ coefficients can quench the triplets to stop the triplet-singlet annihilation, which will have a higher efficiency in the single-layer material. However, it plays the opposite role in bilayer structures because triplets are quenched before they reach the NAPA layers.
Organic light-emitting diode (OLED) has gain numerous attentions since an efficient OLED was firstly demonstrated by Tang and VanSlyke in 1987. Thanks to lots of efforts paid on their progress including materials and device architecture in past three decades, red and green OLEDs have great success in efficiency and lifetime. However, the development of high efficiency deep-blue counterparts with Commission Internationale de L'Eclairage (CIE) coordinate of y<0.1 currently remains in demand in the market of full-color display applications. For example, to realize the BT.2020 color space standard, the standard blue emission must have CIE coordinates of (0.131,0.046), and that is extremely challenging, especially in material development.
Here, a new compound consisting of phenyls groups to connect a benzene core was successfully synthesized. The newly obtained compound exhibited a super wide bandgap of 3.5 eV and a deep-blue emission of approximately 397 nm as well as a photoluminescence quantum yield (PLQY) by 68% in thin film. Consequently, a non-doped OLED using the pristine new compound as emitting layer showed a peak efficiency of 4.9% in external quantum efficiency (EQE) and deep-blue emission with CIE coordinates of (0.16, 0.04). Note that the OLED configuration was bottom emission, which meant the such deep-blue emission resulted from the material itself, rather than microcavity effect. Grazing incidence wide-angle X-ray scattering (GIWAXS) of new compound displayed an order parameter (SGIWAXS) of 0.44, indicating molecules primarily aligned horizontally to the substrate, which contributed to the high efficiency.
Although phosphorescent and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLED) exhibit high efficiency and long lifetime for red and green emission, blue OLED is still a bottleneck. In mass production, triplet-triplet emission (TTA) OLED is the main stream for reasonable lifetime together with limited efficiency. To improve the efficiency of blue TTA-OLED, a bilayer emitting layer (EML) was employed. Compared to single-EML device, external quantum efficiency of the bilayer OLED increased from 9.4% to 13.0%, which mainly resulted from the increase of delayed emission from 15.0% to 37% with enhanced TTA process.
Recently, Quantum-dot light-emitting diode (QLED) has attracted much attention due to narrow electroluminescence (EL) spectra, low driving voltage, tunable emission colors and simple fabrication. In conventional QLED structure, inorganic zinc oxide (ZnO) nanoparticles was usually used as electron transporting layer material by spin-coating. However, defects in solution-processed ZnO film may quench quantum dot (QD) emission and increase the driving voltage. In this study, we fabricated ZnO by sputtering process with the inverted structure. Compared to the QLED with solution-processed ZnO as the ETL, driving voltage of the device with sputtered-ZnO as the ETL significantly decreased from 7.04 V to 2.95 at current density of 20 mA/cm2, while the current efficiency remained at 11.46 and 11.70 cd/A at current density of 80 mA/cm2.
In this study, we report a bistriazoles derivative to be the wide bandgap host for blue emitters. In particular, this bistriazoles derivative possesses a wide bandgap of 4.0 eV and bipolar carrier transportation behavior. In addition, its bipolar behavior is classified to the rare case of electron-favorable bipolar carrier transportation, which was identified by the time-of-flight measurement and unipolar device. The electron mobility is little faster than hole one, which benefits the adjustment of carrier balance in the emitting layer. Employing to be the host of blue phosphorescent OLED, the device exhibits a high-efficiency performance with a 30.2%EQE.
Lots of attentions owing to its superior properties such as narrow electroluminescence (EL) spectra, tunable emission colors, high luminance, and simple fabrication process. Typically, in a QLED, quantum dots (QD) layer is sandwiched by organic materials as hole transporting layer (HTL) and inorganic zinc oxide (ZnO) nanoparticles as electron transporting layer (ETL), respectively. Because the electron mobility of ZnO is typical higher than the hole mobility of organic material, it results in carrier unbalance and reduces the efficiency. Hence, it is important to improve the hole transporting ability to achieve charge balance condition for higher efficiency. In this study, we have fabricated green QLEDs with two different HTL materials. By using HTL with high mobility and suitable energy level, voltage decreased from 11.1 V to 5.8 V at 10 mA/cm2, together with enhancement of current efficiency from 21.8 cd/A to 58.1 cd/A, and external quantum efficiency from 5.94% to 16.0%, corresponding to 2.6-times improvement.
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