Jung-Yu Li

Highly efficient green organic light emitting diode with a novel solution processable iridium complex emitter

We demonstrate a high-efficiency green organic light-emitting diode (OLED) with a solution-processed emissive layer composed of a novel green light emitting iridium complex, bis [5-methyl-8-trifluoromethyl-5H-benzo(c) (1,5)naphthyridin-6-one]iridium(pyrazinecarboxylate). By coupling with a proper host, the green device shows at 1000 cd m−2 an external quantum efficiency of 23.8%, current efficiency of 95.6 cd A−1, and efficacy of 60.8 lm W−1, the highest among all reported OLEDs with a solution-processed emissive layer. The high efficiency may be attributed to the host possessing a zero electron injection barrier, resulting in a more balanced carrier-injection.

High efficiency yellow organic light-emitting diodes with a solution-processed molecular host-based emissive layer

Highly efficient yellow organic light-emitting diodes (OLEDs) with a solution-process feasible emissive layer were fabricated by simply using molecular hosts doped with an iridium-complex based yellow emitter. The best yellow OLED device studied here showed for example, at 100 cd m−2, a power efficiency of 32 lm W−1, a 113% improvement compared with the prior record of 15 lm W−1 based on the same emitter with a polymeric host. The marked efficiency improvement may be attributed to the device being composed of an electron-injection-barrier free architecture, a device structure that led the excitons to generate preferably on the host to enable the efficiency-effective host-to-guest energy transfer to occur and the employed molecular host that exhibited a good host-to-guest energy transfer. The efficiencies were further improved to 53, 39 and 14 lm W−1 at 100, 1000 and 10 000 cd m−2, respectively, with the use of a micro-lens. This study also demonstrates the possibility of achieving relatively high device efficiency for wet-processed OLED devices via balancing the injection of carriers with commercially available OLED materials and limited designs in device structure.

The use of a polarity matching and high-energy exciton generating host in fabricating efficient purplish-blue OLEDs from a sky-blue emitter

We reveal in this communication a new finding regarding the use of a sky-blue emitter to generate purplish-blue emission from organic light emitting diodes (OLEDs) with a polarity matching and high-energy exciton generating host. The resulting device exhibits CIExy coordinates of (0.155, 0.063) and a 3.5% external quantum efficiency, with a 1.2 lm W−1 power efficiency at 100 cd m−2 as a sky-blue emitter, 1-((9,9-diethyl-9H-fluoren-2-yl)ethynyl)pyrene with CIExy of (0.190, 0.241) is doped into a host of 4,4′-bis(9-carbazolyl)-biphenyl, for example. The resulting purplish-blue emission enables a greater than 100% color saturation. The extraordinarily marked blue-shift may result from a low doping concentration to prevent bathochromic shift due to emitter segregation, a polarity matching host to further disperse the emitter, and an efficient host and guest energy level pairing that enables excitons to be generated on the host to trigger short wavelength emission. Furthermore, the host is capable of generating excitons with higher energy to facilitate the triggering of emission with a shorter wavelength. The high efficiency may be attributed to the low doping concentration preventing efficiency roll-off caused by concentration-quenching, the excitons generated on the host facilitating the occurrence of the effective host-to-guest energy transfer, and the employed host possessing an effective host-to-guest energy transfer effect. Notably, the new approach also works for other light-blue emitters in yielding a highly desirable deep-blue light, provided their molecular structure is free of steric hindrance.

A lighting mechanism for flat electron emission lamp

We propose a lighting mechanism for generating uniform planar light. The device integrates electron beams induced by gas discharge with cathodoluminescence at the anode, where the spectra of the emitted light depend entirely on the phosphor materials coated on the anode. Consequently, ultraviolet is not required and the usage of mercury can be avoided. In addition, the features of double-side lighting, transparency, and gray-scale images indicate that the flat electron emission lamp might become potential candidate for the next generation green lighting source.

Discharge and photo-luminance properties of a parallel plates electron emission lighting device

The gas discharge and photo-luminance properties of a planar lighting source featuring highly uniform light emission and mercury-free design were studied. The current density-voltage characteristics and the associated gas discharge of the devices operating with the values of the ratio of electric field to gas pressure (E/p) between 4.3 kV/Torr-cm and 35.7 kV/Torr-cm indicate that the width of the cathode fall extends over the entire gap between the two electrodes and the device is mostly in the obstructed discharge regime. The optical emission analysis confirmed the electron collision-induced gas emissions and strong effect of gas pressure on the phosphor emission when operated at constant current density, both are indicative of the primary roles played by the electron energy.

Significant color space blue-shift of green OLED emitter with sustaining lifetime and substantial efficiency enhancement

In this study, we demonstrate that by embedding a plasmonic coupling metal layer beneath the active layer of an organic light-emitting diode, the resultant device is capable of inducing significant blue shifts in CIE color space coordinates of emitting light from the green emitting material without compromising the lifetime of the parent material. The implemented device consists of multilayers of organic emitting materials sandwiched by two thin metal layers to form a metal-dielectric-metal (MDM) cavity-like structure. The original green emission with CIE coordinates of (0.22, 0.56) was significantly color space blue-shifted to CIE coordinates of (0.10, 0.53). The MDM device exhibits an efficiency of 62 cd/A at a luminance of 1000 cd/m2, which represents a two-fold enhancement of current efficiency. Moreover, the spectral peak intensity is 4.3 times higher than that in a conventional device, which is much higher than that expected for an optical microcavity model, suggesting that the Purcell effect result...

Secondary electron emission characteristics of oxide electrodes in flat electron emission lamp

The present study concerns with the secondary electron emission coefficient, γ, of the cathode materials used in the newly developed flat electron emission lamp (FEEL) devices, which essentially integrates the concept of using cathode for fluorescent lamp and anode for cathode ray tube (CRT) to obtain uniform planar lighting. Three different cathode materials, namely fluorine-doped tin oxide (FTO), aluminum oxide coated FTO (Al2O3/FTO) and magnesium oxide coated FTO (MgO/FTO) were prepared to investigate how the variations of γ and working gases influence the performance of FEEL devices, especially in lowering the breakdown voltage and pressure of the working gases. The results indicate that the MgO/FTO bilayer cathode exhibited a relatively larger effective secondary electron emission coefficient, resulting in significant reduction of breakdown voltage to about 3kV and allowing the device to be operated at the lower pressure to generate the higher lighting efficiency.

Microstructure- and optic-related characteristics of magnesium oxide thin film

Plasma display panel is one of the most potential flat devices for large-size displays. Magnesium oxide thin films have been used as a protective layer to improve discharge characteristics and the lifetime of panel. So magnesium oxide thin film is worthy to be studied. Magnesium oxide thin films were prepared by e-beam gun evaporation at a substrate temperature of 200°C. Heat annealing treatment and ion assisted deposition effect on thin films were investigated. The relation between these characteristics of films has been studied. The characteristics of the deposited films, including optical properties, crystalline structure, and microstructure were discussed.