Showing papers by "Sebastian Reineke published in 2009"

White organic light-emitting diodes with fluorescent tube efficiency

Abstract: 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.

Triplet Harvesting in Hybrid White Organic Light‐Emitting Diodes

Abstract: 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 Light-Emitting Diodes with Fluorescent Tube Efficiency

Abstract: 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.

Highly phosphorescent organic mixed films: The effect of aggregation on triplet-triplet annihilation

Abstract: The efficiency roll-off at high brightness levels is a key factor limiting the application of organic light emitting diodes. We investigate triplet-triplet annihilation in an archetype phosphorescent host-guest system. We show that the currently used host-guest systems are not at the physical limit set by intrinsic annihilation, but have an increased roll-off due to aggregate formation. The existence of these aggregates is directly proven by transmission electron microscopy.

Direct observation of host–guest triplet–triplet annihilation in phosphorescent solid mixed films

Abstract: Triplet–triplet annihilation (TTA) between excited states of host and guest species in organic mixed films, comprising a phosphorescent heavy-metal complex, is usually only accessed indirectly, as one is only able to monitor the radiative guest phosphorescence. We compare two different host materials, 4,4′,4″-tris(N -carbazolyl)-triphenylamine (TCTA) and 4,4′-N,N ′-dicarbazole-biphenyl (CBP) in combination with the green emitter fac -tris(2-phenylpyridine) iridium which differ in their triplet state energy. At excitation levels in the saturation range of guest emission, we observe increasing non-linearities in photoluminescence transients with increasing excitation power for the CBP host material, which can directly be attributed to TTA of host and guest excited states. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Measurement and simulation of exciton decay times in organic light-emitting devices with different layer structures.

Abstract: The decay time of an exciton depends on the coupling between the dipole oscillator and the optical environment in which it is placed. For an organic light-emitting device this environment is determined by the thin-film layer structure. The radiative decay competes with nonradiative decay channels and in this way influences the luminescent efficiency and the external quantum efficiency of the device. We describe a method to estimate the dependency of the exciton decay time and the luminescent efficiency on the thin-film stack and validate the results experimentally.

Controlling Excitons: Concepts for Phosphorescent Organic LEDs at High Brightness

Abstract: This work focusses on the high brightness performance of phosphorescent organic light-emitting diodes (OLEDs). The use of phosphorescent emitter molecules in OLEDs is essential to realize internal electron-photon conversion efficiencies of 100 %. However, due to their molecular nature, the excited triplet states have orders of magnitude longer time constants compared to their fluorescent counterparts which, in turn, strongly increases the probability of bimolecular annihilation. As a consequence, the efficiencies of phosphorescent OLEDs decline at high brightness – an effect known as efficiency roll-off, for which it has been shown to be dominated by triplet-triplet annihilation (TTA). In this work, TTA of the archetype phosphorescent emitter Ir(ppy)3 is investigated in time-resolved photoluminescence experiments. For the widely used mixed system CBP:Ir(ppy)3, host-guest TTA – an additional unwanted TTA channel – is experimentally observed at high excitation levels. By using matrix materials with higher triplet energies, this effect is efficiently suppressed, however further studies show that the efficiency roll-off of Ir(ppy)3 is much more pronounced than predicted by a model based on Förster-type energy transfer, which marks the intrinsic limit for TTA. These results suggest that the emitter molecules show a strong tendency to form aggregates in the mixed film as the origin for enhanced TTA. Transmission electron microscopy images of Ir(ppy)3 doped mixed films give direct proof of emitter aggregates. Based on these results, two concepts are developed that improve the high brightness performance of OLEDs. In a first approach, thin intrinsic matrix interlayers are incorporated in the emission layer leading to a one-dimensional exciton confinement that suppresses exciton migration and, consequently, TTA. The second concept reduces the efficiency roll-off by using an emitter molecule with slightly different chemical structure, i.e. Ir(ppy)2(acac). Compared to Ir(ppy)3, this emitter has a much smaller ground state dipole moment, suggesting that the improved performance is a result of weaker aggregation in the mixed film. The knowledge gained in the investigation of triplet-triplet annihilation is further used to develop a novel emission layer design for white organic LEDs. It comprises three phosphorescent emitters for blue, green, and red emission embedded in a multilayer architecture. The key feature of this concept is the matrix material used for the blue emitter FIrpic: Its triplet energy is in resonance with the FIrpic excited state energy which enables low operating voltages and high power efficiencies by reducing thermal relaxation. In order to further increase the device efficiency, the OLED architecture is optically optimized using high refractive index substrates and thick electron transport layers. These devices reach efficiencies which are on par with fluorescent tubes – the current efficiency benchmark for light sources.

Zeit für eine neue Lichtquelle

Abstract: Weise organische Leuchtdioden (OLED) fur Beleuchtungszwecke werden seit etwa 15 Jahren intensiv erforscht. Damit sich diese Technologie als wirkliche Alternative etablieren kann, gilt es seither, die Effizienz und Lebensdauer drastisch zu steigern. Kurzlich ist es unserer Gruppe an der TU Dresden gelungen, mit weisen OLED die Leistungseffizienz von Leuchtstoffrohren zu ubertreffen.

Modeling exciton decay time and radiative efficiency in organic light emitting devices

Abstract: The exciton decay time depends on the optical environment in which the decay takes place. In the case of an organic light-emitting device this environment is determined by the thin film layer structure. The radiative and non-radiative decay channels compete and in this way influence the radiative efficiency and the external quantum efficiency of the device. This paper shows an experimentally verified model to estimate the dependency of the exciton decay time and the radiative efficiency on the thin film stack.