The development of white organic light-emitting diodes (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized. Nevertheless, the overall device power efficiencies are still considerably below the 60-70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist, details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W(-1) (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates, and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W(-1) at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W(-1) if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron-photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W(-1). This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.

Flexible pressure sensors are essential parts of an electronic skin to allow future biomedical prostheses and robots to naturally interact with humans and the environment. Mobile biomonitoring in long-term medical diagnostics is another attractive application for these sensors. Here we report the fabrication of flexible pressure-sensitive organic thin film transistors with a maximum sensitivity of 8.4 kPa À 1 , a fast response time of o10 ms, high stability over 415,000 cycles and a low power consumption of o1 mW. The combination of a microstructured polydimethylsiloxane dielectric and the high-mobility semiconducting polyisoindigobithiophene-siloxane in a monolithic transistor design enabled us to operate the devices in the subthreshold regime, where the capacitance change upon compression of the dielectric is strongly amplified. We demonstrate that our sensors can be used for non-invasive, high fidelity, continuous radial artery pulse wave monitoring, which may lead to the use of flexible pressure sensors in mobile health monitoring and remote diagnostics in cardiovascular medicine.

White organic light‐emitting diodes (OLEDs) are highly efficient large‐area light sources that may play an important role in solving the global energy crisis, while also opening novel design possibilities in general lighting applications. Usually, highly efficient white OLEDs are designed by combining three phosphorescent emitters for the colors blue, green, and red. However, this procedure is not ideal as it is difficult to find sufficiently stable blue phosphorescent emitters. Here, a novel approach to meet the demanding power efficiency and device stability requirements is discussed: a triplet harvesting concept for hybrid white OLED, which combines a blue fluorophor with red and green phosphors and is capable of reaching an internal quantum efficiency of 100% if a suitable blue emitter with high‐lying triplet transition is used is introduced. Additionally, this concept paves the way towards an extremely simple white OLED design, using only a single emitter layer.

White organic LEDs are seen as one of the next generation light-sources, with their potential to reach internal efficiencies of unity and their unique appearance as large-area and ultrathin devices. However, to replace existing lighting technologies, they have to be at least on par with the state-of-the-art. In terms of efficiency, the fluorescent tube with 60-70 lumen per Watt (lm W-1) in a fixture is the current benchmark. In the scientific literature, so far only values of 44 lm W-1 have been published for white OLEDs.Here, we present results (Reineke et al., Nature 459, 234 (2009)) of white OLEDs with 90 lm W-1 at an illumination relevant brightness of 1,000 candela per square meter (cd m-2). Extracting all light from the glass substrate using a 3D light extraction system, we even obtain 124 lm W-1. In order to achieve such high efficacy values, we reduced the energetic losses prior to photon emission that include ohmic and thermal relaxation losses, leading to very low operating voltages. This is accomplished by the use of doped transport layers and a novel, very energy efficient emission layer concept. Equally important, we addressed the optics of the OLED architecture, because about 80% of the generated light remains trapped in conventional devices. Therefore, we used high refractive index substrates to couple out more light and placed the emission to the second field antinode to avoid plasmonic losses. Our devices are also characterized by an outstandingly high efficiency at high brightness, reaching 74 lm W-1 at 5,000 cd m-2.

A novel concept for white organic light emitting diodes (OLEDs) enabling the utilization of all electrically generated excitons for light generation and achieving a high luminance at very low voltage is introduced. The key feature is a fluorescent blue emitter with high triplet energy, rendering it possible to harvest its triplet excitons by letting them diffuse to an orange phosphorescent iridium complex. This results in a total internal quantum efficiency of almost unity. The high potential of this concept is demonstrated in that a total external power efficiency of 57.6 lm W -1 at a brightness of 100 Cd m -2 (20.3 % external quantum efficiency) and 37.5 lm W -1 (16.1 %) at 1000 Cd m -2 were achieved. This is the most efficient white OLED reported in scientific literature so far. Lighting uses a significant part of the world's energy resources, with a large share still consumed by inefficient incandescent lamps. White organic light emitting diodes (OLEDs) show promise as major role-players in future ambient lighting [1][2][3] due to their favorable properties such as homogenous large-area emission, good color rendering, and potential realization on flexible substrates. Although white OLED efficiencies superior to those of incandescent lamps have been reported, it remains a challenge to reach or surpass the efficiency of fluorescent tubes. A key requirement for such high efficiencies is that all electrically generated excitons, i.e., both singlet and triplet excitons are employed for emission. While suitable molecules have been found for the green and red, stable and efficient blue phosphorescent emitters are still a challenge. [4] Recently, OLED concepts for white light generation, which combine fluorescent blue emitters with phosphorescent green and red ones, have shown an improved operational stability. However, up until now, devices comprising fluorescent blue do not utilize all the generated excitons for light emission, thus giving rise to rather low efficiencies at an illuminationrelevant brightness. [5][6][7] A new concept consisting in the transfer of singlet excitons generated on a host to a fluorescent blue emitter as well as triplets to phosphorescent guests [5] has improved exciton harvesting. Nevertheless, this concept suffers from the disadvantage of the triplet excitons, which are either directly generated on or transferred by Dexter transfer to the blue emitter, being lost. This is a result of the excitons' inability to leave the blue emitter because of its low-lying triplet state. Here, we report on a novel concept for highly efficient white OLEDs. We demonstrate the possibility of harvesting triplet excitons on the fluorescent blue emitter since an emitter molecule with very high triplet energy is used. This permits the transfer of the triplet excitons to the lower-energy colors of the white spectrum. In particular, we demonstrate the achievement of an internal quantum efficiency of almost unity, which may render it possible to completely dispense blue phosphorescent emitters in future high-eff...