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We focus on the determination of the internal luminescence quantum efficiency of a green-emitting organic light-emitting diode (OLED). By considering different geometrical configurations of OLED thin-film stacks, we elucidate the role of the internal luminescence quantum efficiency of the emitter in the thin-film microcavity. Combining optical simulations with experimental results, a comprehensive efficiency analysis is performed. Here the electroluminescence of a set of OLEDs is characterized. Additionally, the devices are characterized using time-resolved photoluminescence measurements. The experimental data are analyzed using optical simulations. This analysis leads to a quantification of internal luminescence quantum efficiency and allows conclusions about competing mechanisms resulting in nonradiative recombination of charge carriers.

Organic light-emitting diodes (OLEDs) are efficient large-area light sources facing their market entry. Still, the development of stable and more efficient blue emitters and the enhancement of light outcoupling remain challenges for further device improvements. Here, we review the working principles of OLEDs and highlight ongoing efforts to improve their efficiency, in particular by coupling out more light.

A device containing a microcavity organic light-emitting diode (OLED) and a magnetooptically active bismuth iron garnet (BIG) Bi3Fe5O12 waveguide combines a planar source for polarized light generation with the material exhibiting the highest known Faraday rotation at room temperature. To build such a device an optimization of garnets and OLEDs has to be done. For a good functionality of the device it is essential to maximize the light coupled from the OLED into the waveguide and to seperate s- and p-polarized emitted light. To optimize the OLED emisson numerical simulations have been performed where material and thickness of the metal anode, as well as the thickness of the hole and the electron conducting layers were varied. The stacks with the best separation of s- and p-polarized light and the highest coupling into the waveguide were determined, fabricated, and characterized regarding their electrical and optical properties. OLEDs have to be deposited on plane surfaces for exhibiting low leakage currents and thus being functional. In order to get plane and crack free BIG surfaces the garnet growth and surface formation were examined on various gadolinium gallium garnet (GGG) Gd3 Ga5 O12 and buffered non-garnet substrates. GGG substrates with different cuts and lattice constants were characterized as well as yttrium iron garnet (YIG) Y3Fe5O12 and GGG buffered sapphire, silicon, and fused silica substrates. The garnet had to be structured to fabricate a planar waveguide. Therefore laser structuring and plasma etching techniques were utilized. The structured garnets were characterized regarding the wall roughness. The optical constants of YIG and BIG were determined from films deposited on silicon using ellipsometric measurements. The combination of microcavity OLED and garnet waveguide resulted in an integrated magnetooptical modulator whose functionality has been proven by applying an external magnetic field and measuring the rotation of the polarized light.

In spite of huge progress in improving the internal quantum efficiency of organic light-emitting diodes (OLEDs), these devices still suffer from poor light out-coupling. Loss mechanisms are for example waveguiding in the organic layers and the substrate as well as the excitation of surface plasmons at metallic electrodes. Their relative strength and the mutual dependence on the OLED structure have been studied both experimentally and by numerical simulation. Here, we consider the impact of the radiative quantum efficiency of the emitter material on predictions of light extraction from OLEDs. Competing processes resulting in non-radiative recombination of charge carriers usually reduce the emitter quantum efficiency in a real device. We show that optical simulation leads to erroneous conclusions when neglecting these competing processes. Furthermore, we demonstrate a method, which allows determining both the radiative quantum efficiency and the charge recombination factor via simulation based analysis of experimental data. This analysis of device efficiency is applied on a set of red-emitting electrophosphorescent devices.

The efficiency of organic light-emitting diodes is limited as only a fraction of the consumed electrical power is converted into light that is finally extracted to air. Especially, the radiative quantum efficiency of the guest-host system is of interest and should be close to unity to achieve highly efficient devices. We show that the red phosphorescent emitter Ir(MDQ)2(acac) doped in an α-NPD matrix exhibits a profound non-isotropic dipole orientation. Ignoring this feature leads to a significant overestimation of the emitter efficiency. Furthermore, we demonstrate the huge potential for efficiency enhancement of mainly parallel emitter orientation in phosphorescent organic light-emitting diodes.

Small molecule organic light-emitting diodes (SM-OLEDs) are efficient large area light sources facing their market entry. However, a low light outcoupling efficiency of typically 20% still strongly limits device performance. Here, we highlight the potential of employing dye-doped emission layers with emitting molecules having horizontally oriented transition dipole moments. The effect of molecular orientation is explained by studying optical simulations that distinguish between horizontal and vertical dipole orientation. In addition, an experimental method that enables straightforward determination of dipole orientation in guest–host systems is presented and used for the analysis of two materials that are very similar except for their orientation. By measuring the external electroluminescence quantum efficiency of SM-OLEDs based on these materials, evidence is found that a mainly horizontal dipole orientation enhances light outcoupling by around 45%. Furthermore, the effect of orientation in SM-OLEDs offers many additional benefits concerning stack design and has fundamental implications for material choice.

20 EPN 42/4 Organic light-emitting diodes (OLEDs) are efficient large-area light sources facing their market entry. Still, the development of stable and more efficient blue emitters and the enhancement of light outcoupling remain challenges for further device improvements. Here, we review the working principles of OLEDs and highlight ongoing efforts to improve their efficiency. Wolfgang Brutting, Jorg Frischeisen, Bert J. Scholz, and Tobias D. Schmidt DOI: 10.1051/epn/2011402 Institute of Physics, University of Augsburg, 86135 Augsburg, Germany Email: wolfgang.bruetting@physik.uni-augsburg.de More light

The orientation distribution of the emissive sites in a phosphorescent Organic LED has been measured utilizing two independent optical methods. In contradiction to common expectations we find a clearly non-isotropic, predominantly parallel emitter orientation in the well-known triplet emitting guest–host system of Ir(MDQ) 2 (acac) blended in an α-NPD matrix. This result emphasizes the necessity of more sophisticated assumptions on active emitter properties in quantitative optical OLED analysis, and demonstrates a highly promising approach for OLED efficiency optimization.

The orientation of the transition dipole moments of fluorescent organic molecules doped into a matrix material is determined by photoluminescence measurements of the angular dependent emission spectra and by comparison with simulations. The analysis of two small molecular materials doped into a 4,4′-bis(N-carbazole)-biphenyl matrix is demonstrated, yielding a horizontal orientation of 91% for 4,4′-bis[4-(diphenylamino)styryl]biphenyl and a completely random orientation in case of tris-(8-hydroxyquinoline)aluminum (Alq3). This expeditious technique does not require detailed information about the optical properties of the dopant, making this method particularly suitable for characterizing newly developed materials for organic light-emitting diodes with enhanced light-outcoupling efficiency.

The internal quantum efficiency of organic light-emitting diodes (OLEDs) can reach values close to 100% if phosphorescent emitters to harvest triplet excitons are used, however, the fraction of light that is actually leaving the device is considerably less. In this work we use numerical simulations to optimize light outcoupling from different OLED stacks. First, we change the distance of the emission zone to the cathode, which minimizes the excitation of surface plasmons. Then the influence of different dipole orientation of the emitter material on the light outcoupling is studied. Finally, a metal-free, transparent OLED stack reported by Meyer et al.,1 where no plasmons can be excited, is investigated for improved outcoupling efficiency.

A novel surface plasmon resonance (SPR) sensor based on an integrated planar and polychromatic light source is presented. The sensor comprises an organic light emitting diode (OLED) and a metallic sensing layer located on opposite sides of a glass prism. We successfully fabricated and tested prototype sensors based on this approach by the use of different prism geometries and OLEDs with blue, green and red emission color. We investigated the angular and wavelength dependent SPR dispersion relation for sensing layers consisting of silver and gold in contact with air. Further on we demonstrated the sensor function by real time monitoring of temperature changes inside an adjacent water reservoir as well as by recording the dissolving process of sodium chloride in water. The presented technique offers the advantage that there is no necessity to couple light from external bulky sources such as lasers or halogen lamps into the sensing device which makes it particularly interesting for miniaturization.

The internal quantum efficiency of organic light-emitting diodes (OLEDs) can reach values close to 100% if phosphorescent emitters to harvest triplet excitons are used, however the fraction of light that is actually leaving the device is considerably less. Loss mechanisms are for example waveguiding in the organic layers and the substrate as well as the excitation of surface plasmon polaritons at metallic electrodes. In this work we use numerical simulations to identify and quantify different loss mechanisms. Changing various simulation parameters, for example layer thicknesses, enables us to study their influence on the fraction of light leaving the OLED. With these simulations we therefore can enhance the light output of the OLED stack. We present simulations of bottom-emitting OLEDs based on the green emitter tris-(8-hydroxyquinoline) aluminum (Alq3) with transparent indium tin oxide anode and a metallic cathode, as well as microcavity devices with two metallic electrodes. The results of the simulations are compared with experimental data on the angular dependent emission spectra and published efficiency data.

The internal quantum efficiency of organic light-emitting diodes (OLEDs) can reach values close to 100% if phosphorescent emitters to harvest triplet excitons are used; however, the fraction of light that is actually leaving the device is considerably less. Loss mechanisms are, for example, waveguiding in the organic layers and the substrate as well as the excitation of surface plasmon polaritons at metallic electrodes. Additionally, absorption in the organic layers and the electrodes can play a role. In this work we use numerical simulations to identify and quantify different loss mechanisms. Changing simulation parameters, for example, the distance of the emitter material to the cathode or thicknesses of the various layers, enables us to study their influence on the fraction of light leaving the OLED. An important parameter in these simulations and for the actual device is the radiative quantum efficiency q, which is defined as the efficiency of radiative exciton decay in an unbounded space filled by th...

We present a method to achieve a consistent, comprehensive efficiency analysis of fluorescent organic light-emitting diodes (OLEDs) showing non-isotropic emitter orientation and triplet-to-singlet up-conversion. Combining photoluminescence lifetime and external quantum efficiency measurements on OLEDs with varying cavity length allows for an independent determination of the radiative emitter efficiency under optical as well as electrical excitation. The difference clearly shows a significant enhancement of the singlet exciton fraction to more than 25% under electrical operation. Furthermore, the presented method does not require detailed information about the emitting system and is generally applicable for a comprehensive efficiency analysis of bottom-emitting OLEDs.

The efficiency of organic light-emitting diodes (OLEDs) is still limited by poor light outcoupling. In particular, the excitation of surface plasmon polaritons (SPPs) at metal-organic interfaces represents a major loss channel. By combining optical simulations and experiments on sim-plified luminescent thin-film structures we elaborate the conditions for the extraction of SPPs via coupling to high-index media. As a proof-of-concept, we demonstrate the possibility to extract light from wave-guided modes and surface plasmons in a top-emitting white OLED by a high-index prism.

Organic light-emitting diodes (OLEDs) usually exhibit a low light-outcoupling efficiency of only 20%. Typically, more than 30% of the available power is lost to surface plasmons (SPs). Consequently, the overall efficiency could be strongly enhanced by recovering SP losses. Therefore, three suitable techniques for extracting SPs-index coupling, prism coupling, and grating coupling-are discussed from a theoretical point of view and investigated experimentally in simplified OLED-like structures. The basic physical processes are clarified by systematic variations of the involved layer thicknesses and by excited state lifetime measurements. In addition, the analysis of the results is supported by optical simulations based on a dipole model. Finally, the advantages and disadvantages of each method, their potential efficiency for recovering SP losses, as well as the applicability in OLEDs are compared.

Organic light-emitting diodes (OLEDs) usually exhibit a low light outcoupling efficiency because a large fraction of power is lost to surface plasmons (SPs) and waveguide modes. In this paper it is demonstrated that periodic grating structures with almost µm-scale can be used to extract SPs as well as waveguide modes and therefore enhance the outcoupling efficiency in light-emitting thin film structures. The gratings are fabricated by nanoimprint lithography using a commercially available diffraction grating as a mold which is pressed into a polymer resist. The outcoupling of SPs and waveguide modes is detected in fluorescent organic films adjacent to a thin metal layer in angular dependent photoluminescence measurements. Scattering up to 5th-order is observed and the extracted modes are identified by comparison to the SP and waveguide dispersion obtained from optical simulations. In order to demonstrate the low-cost, high quality and large area applicability of grating structures in optoelectronic devices, we also present SP extraction using a grating structure fabricated by a common DVD stamp.