Experimental Evaluation of Cache-Related Preemption Delay Aware Timing Analysis

In the presence of caches, preemptive scheduling may incur a significant overhead referred to as cache-related preemption delay (CRPD). CRPD is caused by preempting tasks evicting cached memory blocks of preempted tasks, which have to be reloaded when the preempted tasks resume their execution. In this paper we experimentally evaluate state-of-the-art techniques to account for the CRPD during timing analysis. We find that purely synthetically-generated task sets may yield misleading conclusions regarding the relative precision of different CRPD analysis techniques and the impact of CRPD on schedulability in general. Based on task characterizations obtained by static worst-case execution time (WCET) analysis, we shed new light on the state of the art

Controlling Excitons: Concepts for Phosphorescent Organic LEDs at High Brightness

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 investi- gated 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 differ- ent 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.Diese Arbeit richtet ihren Fokus auf die Untersuchung der Leistungsfähigkeit von phosphoreszenten, Licht-emittierenden organischen Dioden (OLEDs) im Bereich hoher Betriebshelligkeiten. Phosphoreszente Emittermoleku ̈le werden in OLEDs eingesetzt, um interne Elektron-Photon Konversionseffizienzen von 100% zu erreichen. Begründet in ihrer chemischen Struktur, weisen die angeregten Triplett-Zustände dieser Emitter um Größenordnungen längere Zeitkonstanten als die Emission fluo- reszenter Materialien auf, sodass die Wahrscheinlichkeit bimolekularer Auslöschung stark ansteigt. Dies resultiert in einem deutlichen Effizienzrückgang phosphoreszenter OLEDs bei großen Leuchtdichten. Dieser als Roll-off bekannter Effekt wird bei hohen Anregungsdichten hauptsächlich durch Triplett-Triplett Annihilation (TTA) bestimmt. In der Arbeit wird TTA an einem Modellmolekül, dem phosphoreszenten Emit- ter Ir(ppy)3, in zeitaufgelösten Photolumineszenz Experimenten untersucht. Für das bekannte Emittersystem CBP:Ir(ppy)3 wird bei hohen Anregungsdichten Host-Guest TTA beobachtet, was einen zusätzlichen, ungewünschten TTA Kanal darstellt. Dieser Effekt wird durch das Verwenden von Matrix Materialien mit höherer Triplett Energie vermieden, jedoch zeigt sich in weiteren Untersuchungen, dass der Roll-off deutlich stärker ist als von einem auf Förster Energieübertrag basierendem Modell vorhergesagt, welches selbst ein intrinsisches Limit für TTA in phosphoreszenten Systemen beschreibt. Die Diskrepanz zwischen experimenteller Beobachtung und Modellvorhersage wird durch eine starke Tendenz des Emitters, Aggregate zu bilden, erklärt, was TTA deutlich verstärkt. Diese Aggregate werden mit Hilfe von Transmissionselektronenmikroskopie an Ir(ppy)3-dotierten Mischsystemen direkt nachgewiesen. Basierend auf diesen Resultaten werden zwei Konzepte entwickelt, um die Effizienz phosphoreszenter Systeme bei hohen Helligkeiten zu verbessern. Im ersten Ansatz werden dünne intrinsische Schichten des Matrixmaterials in die Emissionsschicht eingebaut, was die Exzitonenbewegung in einer Raumrichtung und damit auch TTA stark unterdrückt. Das zweite Konzept reduziert den Effizienz Roll-off durch die Verwendung eines phosphoreszenten Emitters Ir(ppy)2(acac) mit einer leicht abgeänderten Molekularstruktur. Im Vergleich mit Ir(ppy)3 weist dieser ein deutlich kleineres Dipolmoment im molekularen Grundzustand auf, wodurch die Aggregation vermindert wird. Aufbauend auf den Ergebnissen der TTA wird ein neuartiges Emissionsschicht-Design für weißes Licht entwickelt. In diesem Konzept werden drei phosphoreszente Materialien für blaue, grüne und rote Farbe in eine Vielschicht-Architektur eingebracht. Das Hauptmerkmal der Emissionsschicht ist die Wahl des Matrix-Materials für dem blauen Emitter FIrpic: Seine Triplett Energie liegt resonant zu dem FIrpic Triplett Zustand, wodurch niedrige Betriebsspannungen und hohe Leistungseffizienzen ermöglicht werden, da die thermische Relaxierung reduziert wird. Um die Ef- fizienz dieser weißen OLEDs weiter zu erhöhen, wird die entwickelte OLED Architektur zusätzlich durch die Verwendung von hochbrechenden Substraten und dicken Elektronen-Transportschichten optisch optimiert. Bei beleuchtungsrelevanten Helligkeiten erreichen diese OLEDs das Effizienzniveau von Leuchtstoffröhren – letztere stellen heute den Effizienz-Maßstab dar

Cache-Related Preemption Delay Computation for Set-Associative Caches - Pitfalls and Solutions

In preemptive real-time systems, scheduling analyses need - in addition to the worst-case execution time - the context-switch cost. In case of preemption, the preempted and the preempting task may interfere on the cache memory. These interferences lead to additional reloads in the preempted task. The delay due to these reloads is referred to as the cache-related preemption delay (CRPD). The CRPD constitutes a large part of the context-switch cost. In this article, we focus on the computation of upper bounds on the CRPD based on the concepts of useful cache blocks (UCBs) and evicting cache blocks (ECBs). We explain how these concepts can be used to bound the CRPD in case of direct-mapped caches. Then we consider set-associative caches with LRU, FIFO, and PLRU replacement. We show potential pitfalls when using UCBs and ECBs to bound the CRPD in case of LRU and demonstrate that neither UCBs nor ECBs can be used to bound the CRPD in case of FIFO and PLRU. Finally, we sketch a new approach to circumvent these limitations by using the concept of relative competitiveness

Response-time analysis for fixed-priority systems with a write-back cache

This paper introduces analyses of write-back caches integrated into response-time analysis for fixed-priority preemptive and non-preemptive scheduling. For each scheduling paradigm, we derive four different approaches to computing the additional costs incurred due to write backs. We show the dominance relationships between these different approaches and note how they can be combined to form a single state-of-the-art approach in each case. The evaluation explores the relative performance of the different methods using a set of benchmarks, as well as making comparisons with no cache and a write-through cache. We also explore the effect of write buffers used to hide the latency of write-through caches. We show that depending upon the depth of the buffer used and the policies employed, such buffers can result in domino effects. Our evaluation shows that even ignoring domino effects, a substantial write buffer is needed to match the guaranteed performance of write-back caches

Highly efficient, dual state emission from an organic semiconductor

We report highly efficient, simultaneous fluorescence and phosphorescence (74% yield) at room temperature from a single molecule ensemble of (BzP)PB dispersed into a polymer host. The slow phosphorescence (208 ms lifetime) is very efficient (50%) at room temperature and only possible because the non-radiative rate for the triplet state is extremely low. The ability of an organic molecule to function as an efficient dual state emitter at room temperature is unusual and opens new fields of applications including the use as broadband down-conversion emitters, optical sensors and attenuators, exciton probes, and spin-independent intermediates for F\"orster resonant energy transfer

Film the Film: A new method to measure oxygen diffusion in polymer films using light.

Organic materials such as polymer films surround us in many everyday applications ranging from food packaging and smartphone displays to medical purposes. One of their main usage scenarios is the thin, lightweight, and easy processable encapsulation to protect a particular target from molecular oxygen. Hence, the oxygen diffusion properties in these polymer films represent a key parameter. This work demonstrates a new method to determine and model the oxygen distribution in thin polymer films using light. It provides a significant advantage over many common methods since no vacuum machinery is needed. The working principle is based on the phosphorescent emission of an organic dopant which is quenched in the vicinity of molecular oxygen at room temperature. The model system used in this study consists of a polystyrene layer, which is doped with PtOEP (Platin(II)-2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin) and covered with a barrier layer of Ex-cevalTM. The oxygen concentration in the doped polystyrene film can be locally depleted under excitation with 365 nm UV light. To determine the oxygen diffusion coefficient, a concentration gradient is created and the time evolution of the luminescent pattern is recorded with a CCD-camera. The recorded data is reconverted to oxygen concentration equivalents and the impact of photoconsumption during the recording process is eliminated, before fitting it with a diffusion simulation. The result reveals a significant dependency on the processing conditions of the film, i.e. D = (1.49 ± 0.08) × 10−7 cm^2/s for unannealed and D = (0.71 ± 0.09) × 10−7 cm^2/s for an-nealed samples. The values lie well in the range reported in the literature.:Problem Goal Conception The physics behind the phenomenon Results Mor

Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses

We demonstrate enhanced light extraction for monochrome top-emitting organic light-emitting diodes (OLEDs). The enhancement by a factor of 1.2 compared to a reference sample is caused by the use of a hole transport layer (HTL) material possessing a low refractive index (1.52). The low refractive index reduces the in-plane wave vector of the surface plasmon polariton (SPP) excited at the interface between the bottom opaque metallic electrode (anode) and the HTL. The shift of the SPP dispersion relation decreases the power dissipated into lost evanescent excitations and thus increases the outcoupling efficiency, although the SPP remains constant in intensity. The proposed method is suitable for emitter materials owning isotropic orientation of the transition dipole moments as well as anisotropic, preferentially horizontal orientation, resulting in comparable enhancement factors. Furthermore, for sufficiently low refractive indices of the HTL material, the SPP can be modeled as a propagating plane wave within other organic materials in the optical microcavity. Thus, by applying further extraction methods, such as micro lenses or Bragg gratings, it would become feasible to obtain even higher enhancements of the light extraction.Comment: 11 pages, 6 figures, will be submitted to PR