Changsheng Shi

Solution-processable small molecules as efficient universal bipolar host for blue, green and red phosphorescent inverted OLEDs

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode are of great interest for large-size active-matrix displays due to their easy integration with n-type thin film transistors (TFTs) based on low-cost and highly-uniform amorphous silicon and oxide. In this work, a solution-processable electron transporting material 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13) is employed to blend with a solution-processable hole transporting material 4,4′,4′′-tri(9-carbazoyl)triphenylamine (TCTA) to be used as a universal bipolar co-host for blue, green and red phosphors, and for the first time, phosphorescent IOLEDs are fabricated by solution-processing small molecules. High efficiency and reduced efficiency roll-off are achieved in the solution-processed IOLEDs, which mainly contribute to the high quality of the solution-processed small molecule films as well as the balanced charge injection in the co-host system. Importantly, the solution process is advantageous over vacuum evaporation to deposit multi-component small molecule films, and can be expected to reduce manufacturing costs. Our results demonstrate a promising approach to fabricate low-cost and high-performance IOLEDs for n-type TFT-based displays.

A Small‐Molecule Zwitterionic Electrolyte without a π‐Delocalized Unit as a Charge‐Injection Layer for High‐Performance PLEDs

Small-molecule zwitterionic materials were found to be more efficient as charge-injection materials in an organic electronic device than a previously described polymer (see structures). Furthermore, the superior device performance observed for 1 indicates that it is not necessary to focus only on π-delocalized systems and that solid ionic liquids may be promising alternative candidates for charge-injection materials.

Room-Temperature Sol-Gel Derived Molybdenum Oxide Thin Films for Efficient and Stable Solution-Processed Organic Light-Emitting Diodes

Molybdenum oxide (MoO3) thin films were prepared by sol-gel methods at room temperature from the precursors of MoO3 powder mixing into NH3 or H2O2 solution and then directly treated by UV-ozone instead of widely used high-temperature annealing. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) characteristics demonstrated that the room-temperature sol-gel derived MoO3 thin films exhibited excellent uniformity, unchanged chemical structure, and high work function. For the first time, the novel solution-processed MoO3 thin films were successfully applied as the hole injection layers (HILs) for solution-processed organic light-emitting diodes (OLEDs). The efficiencies of the resulting OLEDs were comparable or even higher than that of the device using PEDOT:PSS as the HIL. More importantly, the lifetimes of the solution-processed OLEDs are improved by nearly 2 orders of magnitude. This study should provide a potential approach to develop low-cost, high-performance, and long-lifetime OLEDs for practical applications.

Solution-processed single-emitting-layer white organic light-emitting diodes based on small molecules with efficiency/CRI/color-stability trade-off.

By doping blue, green and red dyes into a bipolar host system, a simple single-EML white organic light-emitting diode (WOLED) with efficiency and color-stability trade-off was achieved by solution process. The resulting WOLED shows high efficiency (i.e., 36.5 cd/A and 15.7% at 1141 cd/m(2)), reduced efficiency roll-off (i.e., critical current density jc is as high as 140 mA/cm(2)) and, especially, extremely stable electroluminescence spectra with a slight CIE coordinate variation of (0.404 ± 0.004, 0.436 ± 0.001). The superior performance of the WOLED is attributed to the effective suppression of exciton quenching and charge trapping in the bipolar EML.

Tuning the coercivity of Fe-filled carbon-nanotube arrays by changing the shape anisotropy of the encapsulated Fe nanoparticles

To tune the coercivity of Fe-filled carbon-nanotube (CNT) arrays, the shape anisotropy of encapsulated Fe nanoparticles (Fe-NPs) was investigated. Four Fe-filled CNT-array samples with different Fe-NP aspect ratios were prepared by catalytic pyrolysis of acetylene using ferrocene as catalyst. The coercivity of the Fe-filled CNT arrays increased from ∼300 to ∼800 Oe at room temperature when the mean aspect ratio of the encapsulated Fe-NPs changed from 1.6 to 6.0. This clear dependence of the coercivity of the Fe-filled CNT arrays on the aspect ratio of the Fe-NPs might be interpreted in terms of the Stoner–Wohlfarth model. This result indicates that changing the shape anisotropy of the encapsulated Fe-NPs is an effective method to tune the coercivity of the Fe-filled CNTs.

Efficient electron injection layer based on thermo-cleavable materials for inverted bottom-emission polymer light emitting diodes

The electron injection material PF-EP was found to be thermo-cleavable. Its solubility in chlorobenzene can be adjusted by thermal treatment at different temperatures. By analyzing the results of TGA, FT-IR, and 13C solid-state NMR, we interpret that the solubility transition is caused by partial acidification, crosslinking through a hydrogen-bonded network and coordination of O, P and Li. Based on this thermo-cleavable approach, efficient fully solution-processed IBPLEDs were successfully fabricated. The maximum current efficiency of the device with 2.5% Li2CO3 doped PF-EP as the EIL reaches nearly 1.7 times of that of a conventional device. We attribute the high performance to the good electron injection and hole blocking abilities of PF-EP and Li2CO3.

Highly efficient inverted organic light-emitting diodes using composite organic heterojunctions as electrode-independent injectors

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode have attracted increasing attention for display applications because of their easy integration with the n-type transistors based on low-cost and highly-uniform amorphous silicon (a-Si), and transparent amorphous oxide semiconductors (TAOSs). Up to date, indium tin oxide (ITO) has been widely used as the transparent electrode, but the dogged issue of using ITO as the cathode in IOLEDs is the large energy barrier for electron injection due to its high work function. In this work, a kind of organic charge generation layer (CGL), comprising of a p-type semiconductor/bulk heterojunction (BHJ)/n-type semiconductor (p/BHJ/n), is introduced on the ITO cathode to fabricate high performance red, green and blue IOLEDs. It is found that the utilization of the composite organic heterojunction CGL as an electron injector greatly enhanced the electron injection, thus significantly improving the electroluminescence efficiency of the resulting IOLEDs. More importantly, their performances are independent of the work function of the used cathode. It is experimentally demonstrated that the electrons injected into the emitting layer are from the generated charges in composite organic heterojunction CGLs, which are completely different in the injection manner from electrodes in conventional OLEDs. It is believed that our studies provide a promising method to fabricate high performance IOLEDs regardless of the choice of electrodes, which will benefit to integrate IOLEDs on the n-type TFTs.

Managing excitons for high performance hybrid white organic light-emitting diodes by using a simple planar heterojunction interlayer

High performance hybrid white organic light-emitting diodes (WOLEDs) were fabricated by inserting a planar heterojunction interlayer between the fluorescent and phosphorescent emitting layers (EMLs ...

Highly efficient charge generation and electron injection of m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction on ITO cathode for high efficiency inverted white organic light-emitting diodes

The charge generation and electron injection characteristics of m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction made of 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) p-type organic semiconductor and 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile (HAT-CN) n-type semiconductor were well studied. It was found that m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction showed better charge generation ability than m-MTDATA/HAT-CN organic heterojuntion, and realized highly efficient electron injection when using it as charge generator on indium tin oxide (ITO) cathode. The investigations of capacitance-frequency and current density-voltage characteristics of the electron-only devices based on m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction demonstrated that the amounts of the injected electrons were dependent on the properties of the used n-doping electron transporting layer (n-ETL). Therefore, by optimization, high efficiency inverted white organic light-emitting diodes...

High performance hybrid tandem white organic light-emitting diodes by using a novel intermediate connector

We propose a novel intermediate connector for fluorescence/phosphorescence hybrid tandem white organic light emitting diodes (WOLEDs). The core of our concept is to insert a thin layer of calcium (Ca) between lithium 8-hydroxyquinolinolate (Liq) and 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) to construct the intermediate connector. The resultant hybrid tandem WOLEDs exhibit very high device performance. For the color-complementary devices, the maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) reach 106.3 cd A−1, 51.4 lm W−1 and 39.6%, respectively, without any out-coupling techniques. At the luminance intensity of 1000 cd m−2, the efficiencies can still be maintained at 102.8 cd A−1, 46.9 lm W−1 and 38.8%. We also obtain a color rendering index (CRI) as high as 93 in a three-primary-color device. Moreover, both the complementary and three-primary-color devices possess excellent stability for the luminance increased from 1000 cd m−2 to 10 000 cd m−2.