Ching-Chiun Wang

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.

Characterization of porous silicate for ultra-low k dielectric application

Thermal stability of a porous low-k film is a critical issue for application consideration in the back-end-of-line. In this study, thermal stability of the porous silicate has been investigated by changing the thermal processing temperatures. Experimental results have shown that the dielectric constant of the porous silicate still remains below 2.0 after thermal processing at 500 °C for 1 h. A series of material analysis techniques and electrical characteristics have been used to verify the physical and chemical characteristics of the porous silicate film.

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 roll-up character

Highly efficient organic light-emitting diodes (OLEDs) are strongly demanded for both display and illumination applications. High efficiency would also help prolong the device lifespan. However, many OLEDs encounter significant roll-off problems, leading to undesired low device efficiency at high luminance, which is unfavorable to their commercial realization for lighting. Hence, OLED devices with mild or even little roll-off are highly expected. We have, nevertheless, observed some OLEDs that exhibit roll-up phenomenon, i.e., that their external quantum efficiency (EQE) or current efficiency increases as the applied voltage or brightness is increased. By taking such advantage of device architecture design, OLEDs with an approaching or even above the theoretical limit EQE are obtained at high luminance. In this report, we present how this works for a yellow OLED that exhibits a record-high power efficiency among reported fluorescent yellow OLEDs, a very-low color temperature OLED with a record-breaking efficacy based on the same color-temperature, and a green OLED. Notably, the yellow OLED exhibits an EQE that increases from 5.4 to 6.2% and current efficiency from 16.4 to 18.7  cd/A as the luminance increases from 1000 to 10,000  cd/m2. Plausible mechanisms regarding why roll-up occurs in these devices are discussed.

Very low color-temperature Organic Light-Emitting Diodes for lighting at night

Light sources with low color temperature (CT) are essential for their markedly less suppression effect on the secretion of melatonin, and high power efficiency is crucial for energy-saving. To provide visual comfort, the light source should also have a reasonably high color rendering index (CRI). In this report, we demonstrate the design and fabrication of low CT and high efficiency organic light-emitting diodes. The best resultant device exhibits a CT of 1,880 K, much lower than that of incandescent bulbs (2,000–2,500 K) and even as low as that of candles, (1,800–2,000 K), a beyond theoretical limit external quantum efficiency 22.7 %, and 36.0 lm/W at 100 cd/m2. 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.

OLED Fabrication by Using a Novel Planar Evaporation Technique

Organic light-emitting diode fabrication is suffering from extremely high material wasting during deposition especially using a typical point or even line source. Moreover, the need of depositing a high number of emitters and host(s) with a precise composition control in a single layer makes traditional vapor codeposition systems nearly impossible, unless otherwise with a very low yield. To improve, we have developed a novel thin-film deposition system with a planar source loadable with any premetered solvent-mixed organic compounds, plausibly with no component number limitation. We hence demonstrate experimentally, along with a Monte Carlo simulation, in the report the feasibility of using the technique to deposit on a large area-size substrate various organic materials with a relatively high material utilization rate coupling with high film uniformity. Specifically, nonuniformity of less than ±5% and material utilization rate of greater than 70% have been obtained for the studied films.

Candle light-style OLED: a plausibly human-friendly safe night light

Candles emit sensationally-warm light with a very-low color-temperature, comparatively most suitable for use at night. In response to the need for such a human-friendly night light, we demonstrate the employment of a high number of candle light complementary organic emitters to generate mimic candle light based on organic light emitting diode (OLED). One resultant candle light-style OLED shows a very-high color rendering index, with an efficacy at least 300 times that of candles or twice that of an incandescent bulb. The device can be fabricated, for example, by using four candle light complementary emitters, namely: red, yellow, green, and sky-blue phosphorescent dyes, vacuum-deposited into two emission layers, separated by a nano-layer of carrier modulation material to maximize both the desirable very-high color rendering index and energy efficiency, while keeping the blue emission very low and red emission high to obtain the desirable low color temperature. With different layer structures, the OLEDs can also show color tunable between that of candle light and dusk-hue. Importantly, a romantic sensation giving and supposedly physiologically-friendly candle light-style emission can hence be driven by electricity in lieu of the hydrocarbon-burning and greenhouse gas releasing candles that were invented 5,000 years ago.

Highly efficient low color temperature organic LED using blend carrier modulation layer

Color temperature (CT) of light has great effect on human physiology and psychology, and low CT light, minimizing melatonin suppression and decreasing the risk of breast, colorectal, and prostate cancer. We demonstrates the incorporation of a blend carrier modulation interlayer (CML) between emissive layers to improve the device performance of low CT organic light emitting diodes, which exhibits 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 result shows a CT much lower than that of incandescent bulbs, which is 2500 K with 15 lmW-1 efficiency, and even as low as that of candles, which is 2000 K with 0.1 lmW-1. The high efficiency of the proposed device may be attributed to its CML, which helps effectively distribute the entering carriers into the available recombination zones.