Realizing 22.3% EQE and 7-Fold Lifetime Enhancement in QLEDs via Blending Polymer TFB and Cross-Linkable Small Molecules for a Solvent-Resistant Hole Transport Layer.

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4'-(N-(4-butylphenyl)))] (TFB) has been widely used as a hole transport layer (HTL) material in cadmium-based quantum dot light-emitting diodes (QLEDs) because of its high hole mobility. However, as the highest occupied molecular orbital (HOMO) energy level of TFB is -5.4 eV, the hole injection from TFB to the quantum dot (QD) layer is higher than 1.5 eV. Such a high oxidation potential at the QD/HTL interface may seriously degrade the device lifetime. In addition, TFB is not resistant to most solvents, which limits its application in inkjet-printed QLED display. In this study, the blended HTL consisting of TFB and cross-linkable small molecular 4,4'-bis(3-vinyl-9H-carbazol-9-yl)1,1'-biphenyl (CBP-V) was introduced into red QLEDs because of the deep HOMO energy level of CBP-V (-6.2 eV). Compared with the TFB-only devices, the external quantum efficiency (EQE) of devices with the blended HTL improved from 15.9 to 22.3% without the increase of turn-on voltage for spin-coating-fabricated devices. Furthermore, the blended HTL prolonged the T90 and T70 lifetime from 5.4 and 31.1 to 39.4 and 148.9 h, respectively. These enhancements in lifetime are attributed to the low hole-injection barrier at the HTL/QD interface and high thermal stability of the blended HTL after cross-linking. Moreover, the cross-linked blended HTL showed excellent solvent resistance after cross-linking, and the EQE of the inkjet-printed red QLEDs reached 16.9%.

Boosting the efficiency of inverted quantum dot light-emitting diodes by balancing charge densities and suppressing exciton quenching through band alignment.

We report an inverted and multilayer quantum dot light emitting diode (QLED) which boosts high efficiency by tuning the energy band alignment between charge transport and light emitting layers. The electron transport layer (ETL) was ZnO nanoparticles (NPs) with an optimized doping concentration of cesium azide (CsN3) to effectively reduce electron flow and balance charge injection. This is by virtue of a 0.27 eV upshift of the ETL's conduction band edge, which inhibits the quenching of excitons and preserves the superior emissive properties of the quantum dots due to the insulating characteristics of CsN3. The demonstrated QLED exhibits a peak current efficiency, power efficiency and external quantum efficiency of up to 13.5 cd A-1, 10.6 lm W-1 and 13.4% for the red QLED, and correspondingly 43.1 cd A-1, 33.6 lm W-1 and 9.1% for green, and 4.1 cd A-1, 2.0 lm W-1 and 6.6% for the blue counterparts. Compared with QLEDs without optimization, the performance of these modified devices shows drastic improvement by 95.6%, 39.4% and 36.7%, respectively. This novel device architecture with heterogeneous energy levels reported here offers a new design strategy for next-generation high efficiency QLED displays and solid-state lighting technologies.

Hybrid Printing Metal-mesh Transparent Conductive Films with Lower Energy Photonically Sintered Copper/tin Ink.

With the help of photonic sintering using intensive pulse light (IPL), copper has started to replace silver as a printable conductive material for printing electrodes in electronic circuits. However, to sinter copper ink, high energy IPL has to be used, which often causes electrode destruction, due to unreleased stress concentration and massive heat generated. In this study, a Cu/Sn hybrid ink has been developed by mixing Cu and Sn particles. The hybrid ink requires lower sintering energy than normal copper ink and has been successfully employed in a hybrid printing process to make metal-mesh transparent conductive films (TCFs). The sintering energy of Cu/Sn hybrid films with the mass ratio of 2:1 and 1:1 (Cu:Sn) were decreased by 21% compared to sintering pure Cu film, which is attributed to the lower melting point of Sn for hybrid ink. Detailed study showed that the Sn particles were effectively fused among Cu particles and formed conducting path between them. The hybrid printed Cu/Sn metal-mesh TCF with line width of 3.5 µm, high transmittance of 84% and low sheet resistance of 14 Ω/□ have been achieved with less defects and better quality than printed pure copper metal-mesh TCFs.

Embedded Ag/Ni Metal-Mesh with Low Surface Roughness As Transparent Conductive Electrode for Optoelectronic Applications.

Metal-mesh is one of the contenders to replace indium tin oxide (ITO) as transparent conductive electrodes (TCEs) for optoelectronic applications. However, considerable surface roughness accompanying metal-mesh type of transparent electrodes has been the root cause of electrical short-circuiting for optoelectronic devices, such as organic light-emitting diode (OLED) and organic photovoltaic (OPV). In this work, a novel approach to making metal-mesh TCE has been proposed that is based on hybrid printing of silver (Ag) nanoparticle ink and electroplating of nickel (Ni). By polishing back the electroplated Ni, an extremely smooth surface was achieved. The fabricated Ag/Ni metal-mesh TCE has a surface roughness of 0.17 nm, a low sheet resistance of 2.1 Ω/□, and a high transmittance of 88.6%. The figure of merit is 1450, which is 30 times better than ITO. In addition, the Ag/Ni metal-mesh TCE shows outstanding mechanical flexibility and environmental stability at high temperature and humidity. Using the polished Ag/Ni metal-mesh TCE, a flexible quantum dot light-emitting diode (QLED) was fabricated with an efficiency of 10.4 cd/A and 3.2 lm/W at 1000 cd/m2.

Pyridine-Based Electron-Transport Materials with High Solubility, Excellent Film-Forming Ability, and Wettability for Inkjet-Printed OLEDs.

Film morphology has predominant influence on the performance of multilayered organic light-emitting diodes (OLEDs), whereas there is little reported literature from the angle of the molecular level to investigate the impact on film-forming ability and device performance. In this work, four isomeric cross-linkable electron-transport materials constructed with pyridine, 1,2,4-triazole, and vinylbenzyl ether groups were developed for inkjet-printed OLEDs. Their lowest unoccupied molecular orbital (∼3.20 eV) and highest occupied molecular orbital (∼6.50 eV) levels are similar, which are mainly determined by the 1,2,4-triazole groups. The triplet energies of these compounds can be tuned from 2.51 to 2.82 eV by different coupling modes with the core of pyridine, where the 2,6-pyridine-based compound has the highest value of 2.82 eV. Film formation and solubility of the compounds were investigated. It was found that the 2,6-pyridine-based compound outperformed the 2,4-pyridine, 2,5-pyridine, and 3,5-pyridine-based compounds. The spin-coated blue OLEDs based on the four compounds have achieved over 14.0% external quantum efficiencies (EQEs) at the luminance of 100 cd m-2, and a maximum EQE of 12.1% was obtained for the inkjet-printed device with 2,6-pyridine-based compound.

Inkjet-Printed Quantum Dot Light-Emitting Diodes with an Air-Stable Hole Transport Material.

High-efficiency quantum dot light-emitting diodes (QLEDs) were fabricated using inkjet printing with a novel cross-linkable hole transport material N,N'-(9,9'-spirobi[fluorene]-2,7-diylbis[4,1-phenylene])bis(N-phenyl-4'-vinyl-[1,1'-biphenyl]-4-amine) (SDTF). The cross-linked SDTF film has excellent solvent resistance, high thermal stability, and the highest occupied molecular orbital (HOMO) level of -5.54 eV. The inkjet-printed SDTF film is very smooth and uniform, with roughness as low as 0.37 nm, which is comparable with that of the spin-coated film (0.28 nm). The SDTF films stayed stable without any pinhole or grain even after 2 months in air. All-solution-processed QLEDs were fabricated; the maximum external quantum efficiency of 5.54% was achieved with the inkjet-printed SDTF in air, which is comparable to that of the spin-coated SDTF in a glove box (5.33%). Electrical stabilities of both spin-coated and inkjet-printed SDTF at the device level were also investigated and both showed a similar lifetime. The study demonstrated that SDTF is very promising as a printable hole transport material for making QLEDs using inkjet printing.

Highly Air-Stable Electron-Transport Material for Ink-Jet-Printed OLEDs.

A novel cross-linkable electron-transport material has been designed and synthesized for use in the fabrication of solution-processed OLEDs. The material exhibits a low LUMO level of -3.51 eV, a high electron mobility of 1.5×10-5  cm2 V-1 s-1 , and excellent stability. An average 9.3 % shrinkage in film thickness was observed for the film after thermal curing. A maximum external quantum efficiency (EQE) of 15.6 % (35.0 cd A-1 ) was achieved for blue-phosphorescent OLEDs by spin-coating and 13.8 % (31.0 cd A-1 ) for an ink-jet-printed device, both of which are better than the EQE of a control device prepared by vacuum-deposition (see figure).