Ming-Hsuan Wu

Sunlight-style color-temperature tunable organic light-emitting diode

We demonstrate a man-made lighting device of organic light-emitting diode (OLED) capable of yielding a sunlight-style illumination with various daylight chromaticities, whose color temperature ranges between 2300 and 8200 K, fully covering those of the entire daylight at different times and regions. The OLED employs a device architecture capable of simultaneously generating all the emissions required to form a series of daylight chromaticities. The wide color-temperature span may be attributed to that the recombination core therein can easily be shifted along the different emissive zones simply by varying the applied voltage via the use of a thin carrier-modulating layer.

Color-stable, efficient fluorescent pure-white organic light-emitting diodes with device architecture preventing excessive exciton formation on guest

Color-stable, high-efficiency fluorescent pure-white organic light-emitting diodes were fabricated using an electroluminescence-efficient blue host 2-(N,N-diphenylamino)-6-[4-(N,N-diphenylamino)styryl]naphthalene and yellow 5,6,11,12–tetra-phenylnaphthacene in a single emissive layer. The resultant power efficiency, at 100cd∕m2, for example, was 9.5lm∕W, and its emission changed from (0.321, 0.357) to (0.315, 0.344) for brightness increasing from 100to10000cd∕m2. The high color stability may be attributed to the device structure enabling the generation of excitons on host so that lesser excitons would form on guest, preventing exciton-quenching-caused blueshift.

Highly efficient orange-red phosphorescent organic light-emitting diode using 2,7-bis(carbazo-9-yl)-9,9-ditolyfluorene as the host

We demonstrate an efficient orange-red organic light-emitting diode using a host, 2,7-bis(carbazo-9-yl)-9,9-ditolyfluorene, doped with tris(2-phenylquinoline) iridium(III). The device exhibits a high current efficiency of 44.8 cd/A at 1000 cd/m2. This may be attributed to the adoption of the host, which favors the injection of holes, as well as the emissive-layer architecture enabling excitons to form on host and hence favoring efficient energy-transfer from host to guest. Moreover, an electron-confining layer is used to modulate excessive holes to be injected into emissive layer and confine the electrons, which would in turn balance the injection of both carriers and improve efficiency.

High-efficiency flexible white organic light-emitting diodes

A high-efficiency flexible white organic light-emitting diode (OLED) was fabricated using effective device structure on high glass-transition plastic substrate with a thin silicon dioxide pre-coat and highly conductive indium tin oxide (ITO) deposited by radio frequency magnetron sputtering at elevated temperature. Sputtering ITO at 200 °C on a 150 A SiO2-modified polyethersulfone yielded a high power efficiency of 6.5 lm/W at 800 cd m−2 and a maximum external quantum efficiency of 3.2% with a pure-white emission of (0.321, 0.339). Besides device structure, the power efficiency of the flexible OLEDs depends strongly on the conductivity of ITO, which in turn depends on its surface topology, which can be most effectively improved by adding a SiO2 pre-coat of optimal thickness.

High-efficiency, very-high color rendering white organic light-emitting diode with a high triplet interlayer

This study reports the fabrication of a highly efficient, very high color-rendering index (CRI) white organic light-emitting diode (OLED) using five organic dyes doped into two different phosphorescent and fluorescent emissive layers separated by a high triplet-energy interlayer. The resulting white OLED achieves a 93 CRI with a power efficiency of 23.3 lm W−1 at 100 cd m−2, or 14.3 lm W−1 at 1000 cd m−2. This high CRI is attributable to the five dyes employed in this design, which together emit a relatively wide spectrum that nearly covers the entire range of visible light. At the proper thickness, the interlayer enables the device to balance the distribution of carriers in the two emissive zones and achieve a maximum power efficiency while maintaining high CRI.

High efficiency deep-blue organic light-emitting diode with a blue dye in low-polarity host

A high efficiency deep-blue organic light-emitting diode was fabricated using a blue light-emitting dye, 2,7-bis{2[phenyl(m-tolyl)amino]-9,9-dimethyl-fluorene-7-yl}-9,9-dimethyl—fluorene, doped in a low-polarity host, 4,4′-bis(9-carbazolyl)-biphenyl. The resulting device showed a highly saturated blue light with Commission Internationale de l’Eclairage coordinates of (0.14, 0.08) and a record-high external quantum efficiency of 5.1% at 100cd∕m2. The highly saturated blue emission may be attributed to effective solid-state solvation of the employed host, being able to effectively separate the blue dopants due to similar host-guest polarity. The high efficiency may result from efficient energy transfer from the host to dopant and a proper device architecture, leading excitons to favorably form on the host instead of on the dopant.

Highly efficient orange-red organic light-emitting diode using double emissive layers with stepwise energy-level architecture

This study demonstrates a highly efficient orange-red organic light-emitting diode with a double emissive layer architecture that exhibits a record-breaking power efficiency of 47 lm W−1 at 100 cd m−2, or 32 lm W−1 at 1 000 cd m−2. Two factors may contribute to the high efficiency. First, the device architecture has a stepwise energy-level, leading to a significantly reduced energy-barrier for both hole and electron to transport, revealed by the relatively low driving voltage. Second, employing a hole-transporting host and an electron-transporting host leads to a nearly perfect balanced injection of hole and electron to the emissive zones, indicated by the obtained external quantum efficiency that reaches the 20% theoretical limit.

Sunlight-style organic light-emitting diodes

We demonstrate organic light-emitting diodes (OLEDs) capable of yielding sunlight-style illumination with various daylight chromaticities, whose color temperature (CT) covers those of the daylight at different times and regions. Besides having the disruptive characters like being plane-type and luminaire-ready, flexible, printable, thin, and light, the emissive spectrum of OLEDs closely resembles that of sunlight. The degree of sunlight spectrum resemblance is 63%, for example, for one high color rendering OLED, while 2% for a sodium lamp, 16% for a fluorescent tube, 49% for a light-emitting diode, and 72% for an incandescent bulb. By incorporating one hole-modulating layer (HML), a fluorescent sunlight-style OLED yields CT between 2300 and 7900 K, while the CT span can be further expanded between 2400 and 18000 K by using double HMLs. For a phosphorescent sunlight-style OLED, the CT spans between 1900 and 3100 K with a power efficiency of 17.6 lm/W at 1000 cd/m2.

Some approaches for fabricating high-efficiency OLEDs

High-efficiency is strongly desired for organic light-emitting diodes (OLEDs) to be fully realized as the future display and lighting technology. To replace current illumination tools, such as incandescent bulbs and fluorescent tubes, for examples, OLEDs with much higher efficiency are demanded. We will present herein some approaches for fabricating high-efficiency OLEDs of blue and white emission. Besides employing highly efficient electroluminescent guests and thin device architecture, low injection barriers to carriers, high carrier-transporting character, effective carrier/exciton confinement, balanced carrier-injection, exciton generation on host, effective host-to-guest energy-transfer and improved light-coupling efficiency are essential. Amongst, the incorporation of nano-dots in emissive- and non-emissive-layers can markedly improve the device efficiency. The enhancement is especially marked as small polymeric nano-dots are incorporated into the non-emissive layers. Since the incorporation is not in the emissive layer, the efficiency improvement mechanism works for both fluorescent and phosphorescent devices. Importantly, the efficiency improvement is also a strong function of the surface charge density of the nano-dots. Regardless positively or negatively charged, the improvement becomes more pronounced as the charge density increases. Results regarding some lately achieved extraordinarily highly-efficient OLEDs containing nano-dots with high surface charge will be presented.

P‐248L: Late‐News Student Poster: High‐Efficiency Deep‐Blue Organic Light Emitting Diode with a Novel Blue Dye in Low‐Polarity Host

A high-efficiency deep-blue organic light emitting diode was fabricated by using a novel blue light-emitting dye 2,7-Bis{2[phenyl(m-tolyl)amino]-9,9-dimethyl-fluroene-7-yl}-9,9-dimethyl- fluroene doped in 4,4′-bis(9-carbazolyl)-biphenyl in a low-polarity host. The resultant device showed a highly-saturate blue light with CIE coordinates of (0.14, 0.06) with a record-high external quantum efficiency of 4.6 % at 100 cd/m2, favoring the use for wide-color-gamut and low-energy-consumption OLED display.