Shih-Ming Shen

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.

High-efficiency blue organic light-emitting diodes using a 3,5-di(9H-carbazol-9-yl)tetraphenylsilane host via a solution-process

We present a solution-processed blue organic light-emitting diode (OLED) with markedly high current efficiency of 41.2 cd A−1 at 100 cd m−2 and 31.1 cd A−1 at 1000 cd m−2. The high efficiency was partly attributed to the use of a molecular host, 3,5-di(9H-carbazol-9-yl)tetraphenylsilane, which possesses a wide triplet band gap, high carrier mobility, ambipolar transport property and high glass transition temperature. Besides the intrinsically good physical properties, the solution-process also played an important role in fabricating the high-efficiency device, since it could make the molecular distribution of host and guest homogeneous in the emissive layer. Moreover, the device efficiency at higher brightness could be markedly enhanced by using an electron-blocking layer. As the microlens was introduced on the glass substrate to enhance the light outcoupling, the resultant device efficiency of the blue OLED further increased to 50.1 cd A−1 at 100 cd m−2 and 37.3 cd A−1 at 1000 cd m−2.

Highly efficient blue organic light-emitting diode with an oligomeric host having high triplet-energy and high electron mobility

We report a high-efficiency blue organic light-emitting diode (OLED) with a solution-processed emissive layer composed of an oligomeric host of poly[3-(carbazol-9-ylmethyl)-3-methyloxetane] (PCMO) that possesses high triplet-energy and high electron mobility. The device exhibited a current efficiency of 40.4 cd A−1 with an external quantum efficiency (EQE) of 21.6% and power efficiency of 28.2 lm W−1 at 230 cd m−2 or 24.7 cd A−1, 10.3%, and 15.5 lm W−1 at 1 000 cd m−2. The high efficiency may be attributed to the host possessing a high electron mobility and lower electron injection barrier, resulting in a more balanced carrier-injection. Moreover, the high electron-mobility favors the transport of electrons, resulting in a more balanced carrier-injection in the emissive layer. The device efficiency has been further enhanced to 42.6 cd A−1 (22.9%, 29.7 lm W−1) at 124 cd m−2 or 28.8 cd A−1 (15.4%, 17.8 lm W−1) at 1 000 cd m−2 by pre-heating the emissive solution at an elevated temperature before spin-coating.

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, 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.

Extraordinarily high efficiency improvement for OLEDs with high surface-charge polymeric nanodots.

The efficiency of highly efficient blue, green, red, and white organic light-emitting diodes (OLEDs) has been substantially advanced through the use of high surface-charge nanodots embedded in a nonemissive layer. For example, the blue OLEDs markedly high initial power efficiency of 18.0 lm W(-1) at 100 cd m(-2) was doubled to 35.8 lm W(-1) when an amino-functionalized polymeric nanodot was employed. At high luminance, such as 1000 cd m(-2) used for illumination applications, the efficiency was improved from 12.4 to 21.2 lm W(-1), showing a significant enhancement of 71%. The incorporated highly charged nanodots are capable of effectively modulating the transportation of holes via a blocking or trapping mechanism, preventing excessive holes from entering the emissive layer and the resulting carrier-injection imbalance. Furthermore, in the presence of a high-repelling or dragging field arising from the highly charged nanodots, only those holes with sufficient energy are able to overcome the included barriers, causing them to penetrate deeper into the emissive layer. This penetration leads to carrier recombination over a wider region and results in a brighter emission and, therefore, higher efficiency.

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 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.

Nearly non-roll-off high efficiency fluorescent yellow organic light-emitting diodes

This paper demonstrates the employment of a multi-emissive-layer device architecture with smooth cascading energy levels to improve markedly the efficiency, roll-off, and color-stability of a fluorescent yellow organic light-emitting diode. As a tri-emissive layer structure was used in lieu of neat film, for example, the resultant external quantum efficiency was increased from 2.2 and 1.8% to 8.2 and 8.6% between 100 and 1000 cd m−2, while current efficiency increased from 5.8 and 4.6 cd A−1 to 26.5 and 27.2 cd A−1 and efficacy from 4 and 2.4 lm W−1 to 20.6 and 18.3 lm W−1. Moreover, the color stability of the tri-emissive layer structure has become of even higher quality than the neat film counterpart has. The reason why the multi-emissive-layer device exhibits both high efficacy and high color-stability may be attributed to the stepwise energy levels that enable a significant reduction in injection barriers, a wider recombination zone, and a more effective carrier confinement. Additionally, the paired host and guest energy-levels in two of the emissive layers allow excitons to generate on the host to facilitate the occurrence of host-to-guest energy transfer and, thus, high device efficiency. The little roll-off may be due to the different paired host and guest energy-levels in the third emissive layer allowing excitons to generate predominantly on the guest at low voltage, but with increasing excitons generating on the host as the voltage increases, fully utilizing all the possible recombination sites.

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.