Holl on Hybrids

Since his 1970s Pamphlet Architecture series on American architecture, Steven Holl has exhibited a unique ability to pinpoint intersections of culture and building. To gauge exactly how this current of thought might be running through Holl's present explorations, which range from small-scale domestic homespun projects to substantial schemes in China, guest-editor Everardo Jefferson interviews Holl at his New York base. Copyright © 2005 John Wiley & Sons, Ltd.     

Color stable and low driving voltage white organic light-emitting diodes with low efficiency roll-off achieved by selective hole transport buffer layers

White organic light-emitting diodes (WOLEDs) showing high color stability, low operating voltage, high efficiency and low efficiency roll-off by adopting different hole transport buffer layers which also behaves as electron/exciton blocking layers (EBL) have been developed. The characteristics of WOLEDs based on blue–green and orange phosphors could be easily manipulated by hole transport buffer layer, which tailors charge carrier transportation and energy transfer. Our WOLEDs show low operating voltages, 100 cd/m² at 3.2 V, 1000 cd/m² at 3.7 V and 10000 cd/m² at 4.8 V, respectively, and achieve a current efficiency of 35.0 cd/A, a power efficiency of 29.0 lm/W at a brightness of 1000 cd/m², and a low efficiency roll-off 8.7% calculated from the maximum efficiency value to that of 5000 cd/m².     

Low driving voltage white organic light-emitting diodes with high efficiency and low efficiency roll-off

Single emission layer white organic light-emitting diodes (WOLEDs) showing high color stability, low turn-on voltage, high efficiency and low efficiency roll-off by incorporating iridium(III) bis[(4,6-difluo-rophenyl)-pyridinato-N,C2] (FIrpic) and bis(2-phenylbenzothiazolato) (acetylacetonate)iridium(III) (Ir(BT)₂(acac)) phosphors dyes have been demonstrated. Our WOLEDs without any out-coupling schemes as well as n-doping strategies show low operating voltages, low turn-on voltage (defined for voltage to obtain a luminance of 1 cd/m²) of 2.35 V, 79.2 cd/m² at 2.6 V, 940.5 cd/m² at 3.0 V and 10 300 cd/m² at 4.0 V, respectively, and achieve a current efficiency of 40.5 cd/A, a power efficiency of 42.6 lm/W at a practical brightness of 1000 cd/m², and a low efficiency roll-off 14.7% calculated from the maximum efficiency value to that of 5000 cd/m². Such improved properties are attributed to phosphors assisted carriers transport for achieving charge carrier balance in the single light-emitting layer (EML). Meanwhile the host–guest energy transfer and direct exciton formation process are two parallel pathways serve to channel the overall excitons to dopants, greatly reduced the unfavorable energy losses.     

Mesostructured Silica Chemically Doped with RuII as a Superior Optical Oxygen Sensor

Novel oxygen sensors consisting of a [Ru(bpy)₂phen]²⁺ (bpy: 2,2′‐bipyridyl, phen: phenathroline) portion covalently grafted to a mesostructured silica‐based network are prepared in situ via a sol–gel approach with the help of cetyltrimethylammoniumbromide (CTAB) surfactant. 1,10‐Phenanthroline covalently grafted to 3‐(triethoxysilyl)propyl isocyanate is used as not only the sol–gel precursor but also as the second ligand of the Ru(bpy)₂Cl₂ · 2H₂O complex to prepare the sol–gel‐derived mesostructured silicates for an oxygen sensor. For comparison purposes, the oxygen sensors in which [Ru(bpy)₂phen]Cl₂ is conventionally physically incorporated into the matrix are also prepared. Elemental analysis, NMR, Fourier transform IR, UV‐vis electronic absorption, luminescence‐intensity quenching Stern–Volmer plots, and excited‐state decay analysis are used to characterize the obtained oxygen sensors. These obtained bulk xerogels and spin‐coated thin films show that the homogeneity and the sensitivity of the covalently grafted samples are superior to those of the physically incorporated ones, and the highest sensitivity is obtained in the mesostructured bulk xerogel. This improvement in oxygen sensitivity is attributed to the increased diffusivity of oxygen in the uniform and nearly parallel porous structure of the Mobil Catalytic Material 41 mesostructured matrix, the enhanced homogeneity results from the covalently grafted propyl group in –Si–(CH₂)₃– that acts as the fundamental spacer which prevents interaction between the attached Ru^II complex and the silica matrix, and optimal dispersion in the mesopores during the sol–gel polycondensation. Furthermore, the greatly minimized leaching effect of the sensing molecules could be observed in the covalently grafted system. The covalent grafting strategy presented in this paper provides superior optical oxygen sensors with homogeneous distribution, improved sensitivity, and simplified calibration plots.     

Improving device efficiencies of solid-state white light-emitting electrochemical cells by adjusting the emissive-layer thickness

Exciton quenching in the recombination zone close to electrochemically doped regions would be one of the bottlenecks for improving device efficiencies of solid-state white light-emitting electrochemical cells (LECs). To further enhance device efficiencies of white LECs for practical applications, we adjust the emissive-layer thickness to reduce exciton quenching. In white LECs with properly thickened emissive-layer thickness, the recombination zone can be situated near the center of the emissive layer, rendering mitigated exciton quenching and thus enhanced device efficiencies. High external quantum efficiencies and power efficiencies of optimized devices reach ca. 11% and 20 lm/W, respectively, which are among the highest reported for white LECs. These results confirm that tailoring the thickness of the emissive layer to avoid exciton quenching would be a feasible approach to improve device efficiencies of white LECs.