Sun-Zen Chen

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.

Effect of fabrication parameters on three-dimensional nanostructures of bulk heterojunctions imaged by high-resolution scanning ToF-SIMS.

Solution processable fullerene and copolymer bulk heterojunctions are widely used as the active layers of solar cells. In this work, scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to examine the distribution of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly(3-hexylthiophene) (rrP3HT) that forms the bulk heterojunction. The planar phase separation of P3HT:PCBM is observed by ToF-SIMS imaging. The depth profile of the fragment distribution that reflects the molecular distribution is achieved by low energy Cs(+) ion sputtering. The depth profile clearly shows a vertical phase separation of P3HT:PCBM before annealing, and hence, the inverted device architecture is beneficial. After annealing, the phase segregation is suppressed, and the device efficiency is dramatically enhanced with a normal device structure. The 3D image is obtained by stacking the 2D ToF-SIMS images acquired at different sputtering times, and 50 nm features are clearly differentiated. The whole imaging process requires less than 2 h, making it both rapid and versatile.

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.

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.

Direct evidence for stress-induced (001) anisotropy of rapid-annealed FePt thin films

Roles of rapid thermal annealing (RTA) on the evolution of crystallographic anisotropy of single-layered FePt films have been characterized. We observed a huge biaxial tensile stress of 2.18 GPa induced with increasing heating rate from 0.5 to 40 K/s. The result is a transition of orientation from (111) to perfect (001) texture. The later then degrades at heating rates ≥80 K/s due to morphological variation. The advantages of RTA are to induce tensile stress by densification reaction within a very short time and to simultaneously impede thickness-dependent dynamic stress relaxation.

Effect of Fabrication Parameters on Three-Dimensional Nanostructures and Device Efficiency of Polymer Light-Emitting Diodes

By using 10 kV C(60)(+) and 200 V Ar(+) ion co-sputtering, a crater was created on the light-emitting layer of phosphorescent polymer light-emitting diodes, which consisted of a poly(9-vinyl carbazole) (PVK) host doped with a 24 wt % iridium(III)bis[(4,6-difluorophenyl)pyridinato-N,C(2)] (FIrpic) guest. A force modulation microscope (FMM) was used to analyze the nanostructure at the flat slope near the edge of the crater. The three-dimensional distribution of PVK and FIrpic was determined based on the difference in their mechanical properties from FMM. It was found that significant phase separation occurred when the luminance layer was spin coated at 30 degrees C, and the phase-separated nanostructure provides a route for electron transportation using the guest-enriched phase. This does not generate excitons on the host, which would produce photons less effectively. On the other hand, a more homogeneous distribution of molecules was observed when the layer was spin coated at 60 degrees C. As a result, a 30% enhancement in device performance was observed.

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.

A universal, easy-to-apply light-quality index based on natural light spectrum resemblance

Light-quality is extremely crucial for any light source to be used for illumination. However, a proper light-quality index is still missing although numerous electricity-driven lighting measures have been introduced since past 150 yr. We present in this communication a universal and easy-to-apply index for quantifying the quality of any given lighting source, which is based on direct comparison of its lumen spectrum with the natural light counterpart having the same color temperature. A general principle for creating high quality pseudo-natural light is accordingly derived. By using organic light-emitting diode technology, for example, daylight-style emission with a 96% natural light resemblance is obtained as a high number of organic emitters with diffused colors spanning throughout the entire visible range are employed. The same principle can be extended to other lighting technology such as light-emitting diode to generate natural light-style emission.

Magnetic properties of sintered permanent magnets R-Fe-Cu-B (R=Prand Nd)

Radially anisotropic Pr/sub 15/Fe/sub 77.5/Cu/sub 1.5/B/sub 6/ permanent magnets have been prepared from hydrogen decrepitated (HD) powders by conventional powder metallurgy processes. The best magnetic properties obtained along the radial directions are as follows: (BH)/sub max/=10.5 MG-Oe, B/sub r/=7.5 kG, and iH/sub c/=8.5 kOe. Since Pr/sub 2/(Fe,Cu)/sub 14/BH/sub x/ exhibits planar anisotropy, the tetragonal c-axis of the HD powders should lie in the plane perpendicular to the field direction during alignment compaction. This style of radial alignment is not changed in the subsequent dehydrogenation and sintering processes. The easy direction of magnetization of the magnet can be changed by alloy design, switching from radial directions to the field direction with the addition of 33 wt.% or more of Nd-Fe-B. Further addition of Nd-Fe-B monotonically increases both B/sub r/ and (BH)/sub max/. For the Nd-Fe-Cu-B system, a small concentration of 1.5 at.% Cu dramatically deteriorates the magnetic properties of the sintered magnet due to the extensive precipitation of free iron. >