Yung-Cheng Jou

Using light-emitting dyes as a co-host to markedly improve efficiency roll-off in phosphorescent yellow organic light emitting diodes

We discovered in this study the feasibility of using regular light-emitting dyes as an effective co-host, rather than a sensitizer, to markedly improve the efficiency of phosphorescent organic light emitting diodes. At 10000 cd m−2, for example, the efficacy of a yellow emitter containing device was increased from 11.7 lm W−1 to 15.4 lm W−1, an increment of 32%, as a sky-blue phosphorescent dye, bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic), was blended into a host of 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA). The efficacy at 1000 cd m−2 was 26.7 lm W−1, the highest among all reported yellow OLEDs with a solution-processed emissive layer. The marked efficiency improvement may be attributed to the co-host having an electron trapping character, enabling excitons to generate on itself instead of on the guest, creating an additional efficiency-effective energy transfer route, and having a very efficient co-host to guest energy transfer. The most effective co-host may vary with the variation of the host employed, depending on the energy level pairing of the co-host and host.

High efficiency low color-temperature organic light-emitting diodes with a blend interlayer

Low color temperature (CT) lighting sources are crucial for their low suppression of melatonin secretion, and high power efficiency is essential for energy-saving. This study demonstrates the incorporation of a blend interlayer between emissive layers to improve the device performance of low CT organic light emitting diodes. The resulting devices exhibit a CT much lower than that of incandescent bulbs, which is ∼2500 K with a ∼15 lm W−1 efficiency, and even as low as that of candles, which is ∼2000 K with ∼0.1 lm W−1. The best device fabricated shows an external quantum efficiency of 22.7% and 36 lm W−1 (54 cd A−1) with 1880 K at 100 cd m−2, or 20.8% and 29 lm W−1 (50 cd A−1) with 1940 K at 1000 cd m−2. The high efficiency of the proposed device may be attributed to its interlayer, which helps effectively distribute the entering carriers into the available recombination zones.

Deep-blue emitting pyrene–benzimidazole conjugates for solution processed organic light-emitting diodes

New pyrene–benzimidazole conjugates containing different π-linkers such as phenyl, thiophene and triarylamine were synthesized and characterized by photophysical, electrochemical, thermal and electroluminescence studies. Triarylamine-containing dyes displayed red-shifted absorption spectra and positive solvatochromism in emission spectra due to the pronounced intramolecular charge transfer (ICT) from the triarylamine donor to pyrene acceptor in the excited state. All derivatives were used as emitting dopants in multilayered organic light-emitting diodes exhibiting deep blue electroluminescence. The solution processed device fabricated by utilizing 1-phenyl-2-(pyren-1-yl)-1H-benzo[d]imidazole as an emitter displayed promising deep blue emission characteristics with a maximum luminance of 714 cd m−2, external quantum efficiency of 1.5%, CIE coordinates of (0.16, 0.05) at 100 cd m−2 and 100% color saturation.

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.

High efficiency yellow organic light emitting diodes with a balanced carrier injection co-host structure

We demonstrate herein the design and fabrication of a highly efficient yellow organic light-emitting diode (OLED) with a balanced carrier injection device architecture having a zero electron-injection-barrier host blended with a hole-injection aiding co-host. The resultant yellow OLED showed, at 1000 cd m−2 for example, an efficacy of 59 lm W−1, current efficiency of 71 cd A−1 and external quantum efficiency (EQE) of 23%, with values of 42 lm W−1, 47 cd A−1 and 15% EQE without a co-host. The co-host effect that resulted in very balanced carrier injection was also valid for other yellow OLED devices and their efficiency improvement was also very marked. With the use of a micro-lens, the device efficiency is further improved to 79 lm W−1, 96 cd A−1 and 30% EQE.

An Energy Efficient and High Color Rendering Index Candle Light-style Organic Light Emitting Diode for Illumination

We demonstrate in this communication the feasibility of using organic light-emitting diode (OLED) technology to fabricate a environmental-friendly, energy efficient and high quality candle light-style OLED for illumination devices. Resulting device with yellowish orange emission, CIE coordinates (0.52, 0.43), with a color temperature of 2,000 K and color rendering index 93, closely matching to (0.52, 0.42) and 1,914 K of a white candle studied. The emissive spectrum of candle light OLED shows an 81 % similarity with that of the white candle. The candle light-style OLED exhibits a color rendering index of 93 and power efficiency of 19 lm/W, which is nearly 20 times more efficient than the conventional candles.

Disruptive characteristics and lifetime issues of OLEDs

Abstract: Nowadays, organic light-emitting diodes (OLEDs) have potential to disrupt the conventional and mainstream solid state lights (SSLs) and flat panel displays (FPDs) due to their numerous superlative characteristics like sunlight style color-temperature tunability, energy-saving, environmental-friendly, physiologically- and psychologically-friendly, and a very high color rendering index (CRI), etc. However, the commercial viability of OLEDs would strongly depend on their extrinsic and intrinsic lifetime because OLED efficacy and lifetime greatly suffer from exposure to atmospheric oxidants (moisture and oxygen). The effect of oxidants can be minimized by the use of abundant emerging encapsulation techniques while the intrinsic lifetime can be improved by a balanced device structure and high glass transition temperature based materials, etc.