Tobias D. Schmidt

Evidence for non-isotropic emitter orientation in a red phosphorescent organic light-emitting diode and its implications for determining the emitter’s radiative quantum efficiency

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.

Charge accumulation at organic semiconductor interfaces due to a permanent dipole moment and its orientational order in bilayer devices

Charge accumulation at the organic heterointerfaces in multilayer organic light-emitting diodes (OLEDs) is an important process for understanding their device operation, efficiency, and degradation properties. Charge accumulation behavior has typically been analyzed in terms of the energy barrier and difference of the charge carrier mobility across heterointerfaces. In this study, we demonstrate that permanent dipole moments and their orientational order also play a significant role in the charge behavior at organic semiconductor interfaces. The charge accumulation properties of bilayer devices composed of polar or nonpolar molecules deposited on a 4,4’-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl layer between the anode and cathode were examined by displacement current measurement and impedance spectroscopy. In addition, Kelvin probe measurements for the corresponding bilayer structures excluding the cathode were performed to analyze the relationship between the potential profile and charge accumulation pr...

High-efficiency fluorescent organic light-emitting diodes enabled by triplet-triplet annihilation and horizontal emitter orientation

A green organic light-emitting diode with the fluorescent emitter Coumarin 545T shows an external quantum efficiency ( ηEQE) of 6.9%, clearly exceeding the classical limit of 5% for fluorescent emitters. The analysis of the angular dependent photoluminescence spectrum of the emission layer reveals that 86% of the transition dipole moments are horizontally oriented. Furthermore, transient electroluminescence measurements demonstrate the presence of a delayed emission originating from triplet-triplet annihilation. A simulation based efficiency analysis reveals quantitatively the origin for the high ηEQE: a radiative exciton fraction higher than 25% and a light-outcoupling efficiency of nearly 30%.

Comprehensive efficiency analysis of organic light-emitting devices

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.

Strategies for light extraction from surface plasmons in organic light-emitting diodes

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.

Analyzing degradation effects of organic light-emitting diodes via transient optical and electrical measurements

Although the long-term stability of organic light-emitting diodes (OLEDs) under electrical operation made significant progress in recent years, the fundamental underlying mechanisms of the efficiency decrease during operation are not well understood. Hence, we present a comprehensive degradation study of an OLED structure comprising the well-known green phosphorescent emitter Ir(ppy)3. We use transient methods to analyze both electrical and optical changes during an accelerated aging protocol. Combining the results of displacement current measurements with time-resolved investigation of the excited states lifetimes of the emitter allows for a correlation of electrical (e.g., increase of the driving voltage due to trap formation) and optical (e.g., decrease of light-output) changes induced by degradation. Therewith, it is possible to identify two mechanisms resulting in the drop of the luminance: a decrease of the radiative quantum efficiency of the emitting system due to triplet-polaron-quenching at trapped charge carriers and a modified charge carrier injection and transport, as well as trap-assisted non-radiative recombination resulting in a deterioration of the charge carrier balance of the device.

Efficiency Analysis of Organic Light-Emitting Diodes Based on Optical Simulations

Organic light-emitting diodes (OLEDs) are promising new large-area light sources on their way to commercialization. However, there is still much room for improvement in terms of device efficiency and long-term stability under electrical operation. In this paper, we review the current issue of efficiency analysis based on optical simulations of state-of-the-art OLED stacks. In detail, we present a method to determine the radiative quantum efficiency of the emitter, figure out the crucial points for nonisotropic emitter orientation, and discuss the application of the developed method to analyze degradation effects during electrical operation.

Evidence for different origins of the magnetic field effect on current and electroluminescence in organic light-emitting diodes

An external magnetic field can change the current through an organic light-emitting diode and the luminance it emits. Existing models predict that both phenomena have the same behaviour and, therefore, a common origin; however, there are indications that they are not completely linked. As a direct proof, we measured the magnetic field effect in multilayer organic light-emitting diodes using Alq3 as emission layer. After successively adding blocking layers, we found a decrease of the magnetic field effect on the current, whereas the effect on the luminance remained at the same level. Thus, both effects can be separated from each other.

Degradation induced decrease of the radiative quantum efficiency in organic light-emitting diodes

The efficiency decrease during electrical operation of organic light-emitting diodes is a crucial issue for both applied and fundamental research. In order to investigate degradation processes, we have performed an efficiency analysis for phosphorescent state-of-the-art devices in the pristine state and after an accelerated aging process at high current density resulting in a luminance drop to less than 60% of the initial value. This loss in efficiency can be explained by a decrease of the radiative quantum efficiency of the light-emitting guest/host system from 70% to 40%, while other factors determining the efficiency are not affected.

Extracting the emitter orientation in organic light-emitting diodes from external quantum efficiency measurements

Emitter orientation will play a major role in future applications of organic light-emitting diodes due to its strong impact on the efficiency of the devices. Up to now, determining the orientation of transition dipole moments required elaborate angular-dependent measurements of the light emission pattern. In this paper, we present a simplified and straightforward method to extract the emitter orientation from external quantum efficiency measurements. We demonstrate the validity of the method on three different dye-doped emitting systems.