Shiang-Hau Peng

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.

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.

Highly efficient color-temperature tunable organic light-emitting diodes

Color temperature (CT) is crucial to human physiology. A simple organic light-emitting diode with CT tunable between 1700 and 5200 K and a high current efficiency of 23.7 cd A−1 at 1000 cd m−2 is demonstrated. The wide CT span may be attributed to the use of a hole-modulation material with high triplet energy and high electron mobility, and the high efficiency to the use of emitters with high quantum yield. The efficiency is further improved to 31.3 cd A−1 by using double hole-modulation layers, which can more effectively prevent excessive hole-injection, leading to a better carrier-injection balance.

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.

Organic light-emitting diodes with direct contact-printed red, green, blue, and white light-emitting layers

We demonstrate the feasibility of using direct contact-printing in the fabrication of monochromatic and polychromatic organic light-emitting diodes (OLEDs). Bright devices with red, green, blue, and white contact-printed light-emitting layers with a respective maximum luminance of 29 000, 29 000, 4000, and 18 000 cd/m2 were obtained with sound film integrity by blending a polymeric host into a molecular host. For the red OLED as example, the maximum luminance was decreased from 29 000 to 5000 cd/m2 as only the polymeric host was used, or decreased to 7000 cd/m2 as only the molecular host was used. The markedly improved device performance achieved in the devices with blended hosts may be attributed to the employed polymeric host that contributed a good film-forming character, and the molecular host that contributed a good electroluminescence character.

Plant Growth Absorption Spectrum Mimicking Light Sources

Plant factories have attracted increasing attention because they can produce fresh fruits and vegetables free from pesticides in all weather. However, the emission spectra from current light sources significantly mismatch the spectra absorbed by plants. We demonstrate a concept of using multiple broad-band as well as narrow-band solid-state lighting technologies to design plant-growth light sources. Take an organic light-emitting diode (OLED), for example; the resulting light source shows an 84% resemblance with the photosynthetic action spectrum as a twin-peak blue dye and a diffused mono-peak red dye are employed. This OLED can also show a greater than 90% resemblance as an additional deeper red emitter is added. For a typical LED, the resemblance can be improved to 91% if two additional blue and red LEDs are incorporated. The approach may facilitate either an ideal use of the energy applied for plant growth and/or the design of better light sources for growing different plants.

Spirally Configured (cis-Stilbene) Trimers: Steady-State and Time-Resolved Photophysical Studies and Organic Light-Emitting Diode Applications

This article reports for the time-resolved photophysical studies of spirally configured ( cis-stilbene) trimers and their spin-coated organic light-emitting diode (OLED) device performances. Transient absorption profiles of spirally configured, ter-( cis-stilbene) were studied by pulse radiolysis. The emission profiles after charge recombination of their incipient radical ions in benzene provides insights into the emission mechanism and efficiency in OLED devices. Blue-, sky blue-, and green-emitting OLED devices for a maximum external quantum efficiency are 4.32%, 4.70%, and 2.77%, respectively, by solution process.