Bert J. Scholz

Extraction of surface plasmons in organic light-emitting diodes via high-index coupling

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 simplified 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.

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.

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.

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.

Comprehensive efficiency analysis of organic light-emitting diodes featuring emitter orientation and triplet-to-singlet up-conversion

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.

Non-isotropic emitter orientation and its implications for efficiency analysis of organic light-emitting diodes

The efficiency of organic light-emitting diodes (OLEDs) is limited since only a small fraction of the consumed electrical power is converted into visible light that is finally extracted to air. Most of the efficiency loss is caused by suboptimal radiative quantum efficiency (RQE) of the emitting guest-host system and by dissipating a huge part of the radiated energy to optical modes such as surface plasmons or waveguided modes, which cannot easily be extracted by common outcoupling structures. In order to increase the external quantum efficiency (EQE) of OLEDs new approaches are needed. Recent studies show that the EQE can be enhanced considerably by horizontally oriented emitters, a feature that is well known for fluorescent emitters and has lately been demonstrated in phosphorescent state-of-the-art OLEDs. By means of optical simulations we investigated the influence of non-isotropic emitter orientation on the effective RQE and the outcoupling factor. We show that in order to achieve a consistent efficiency analysis it is indispensable to account for possible deviations from isotropy. Ignoring these orientation effects leads to significant misinterpretation of the RQE and other factors, which determine the external quantum efficiency of a device. Furthermore, we demonstrate the huge potential for efficiency enhancement of mainly parallel dipole emitter orientation in both fluorescent and phosphorescent OLEDs.

Influence of molecular orientation on the coupling of surface plasmons to excitons in semitransparent inverted organic solar cells

We investigate the coupling between surface plasmons and excitons for different donor materials in semitransparent organic solar cells. Surface plasmons can be excited at the interface between the semitransparent anode and the surrounding dielectric medium in Kretschmann configuration, if the resonance condition for wavelength and angle is fulfilled. In solar cells with nearly upright standing diindenoperylene donor molecules in close proximity to the metal, this can lead to an enhancement in photo-current. By contrast, for cells with dibenzo-tetraphenyl-periflanthen as donor, the lying orientation of molecules is unfavorable for coupling to surface plasmons. In this case, the excitation of surface plasmons acts like a parasitic absorption and reduces the photo-current.