Paul-Anton Will

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

This work was funded by the European Social Fund and the free state of Saxony through the OrganoMechanics project. Support from the excellence cluster “cfaed” is gratefully acknowledged.

High-performance organic light-emitting diodes comprising ultrastable glass layers

Organic light-emitting diodes with ultrastable glass emission layers show increased efficiency and device stability. Organic light-emitting diodes (OLEDs) are one of the key solid-state light sources for various applications including small and large displays, automotive lighting, solid-state lighting, and signage. For any given commercial application, OLEDs need to perform at their best, which is judged by their device efficiency and operational stability. We present OLEDs that comprise functional layers fabricated as ultrastable glasses, which represent the thermodynamically most favorable and, thus, stable molecular conformation achievable nowadays in disordered solids. For both external quantum efficiencies and LT70 lifetimes, OLEDs with four different phosphorescent emitters show >15% enhancements over their respective reference devices. The only difference to the latter is the growth condition used for ultrastable glass layers that is optimal at about 85% of the materials’ glass transition temperature. These improvements are achieved through neither material refinements nor device architecture optimization, suggesting a general applicability of this concept to maximize the OLED performance, no matter which specific materials are used.

Light trapping for flexible organic photovoltaics

Here we investigate light trapping substrates and electrodes for enhancing the performance of organic photovoltaics (OPVs). Their power conversion efficiency (PCE) can be improved by a factor of 1.16 using laser patterned PET substrates and by a factor of 1.13 using commercial, structured display films. Furthermore, we prepare light trapping electrodes using as flexible conductive polymer with embedded TiO2 nanoparticles, improving the PCE by a factor of 1.08 as compared to a neat polymer electrode. However, nano-imprinted conductive polymer electrodes does not provide light trapping effect due to the small size (50 nm) of the structures. Moreover flexible OPV devices, integrating the above light trapping elements, show non-degraded performance after bending tests.

Bragg Scattering of Non-radiative Modes in Red Top-emitting Organic Light Emitting Diodes with Variation of Cavity Length

The spectral radiant intensity and external quantum efficiency of red top-emitting organic light emitting diodes with integrated one dimensional periodic gratings is studied in dependence of the cavity length with comparison to optical simulations.

Improved light outcoupuling of organic light-emitting diodes by combined optimization of thin film layers and external textures

We present improvements in light outcoupling for the example of red, bottom-emitting, ITO free OLEDs. As an optimization tool we use experimentally verified coupled modelling approach, where we simulate a complete OLED device, including thin-film coherent stacks as well as thick microtextured incoherent layers (substrate). We calibrate the combined model on a fabricated small sample OLED. The research of lateral limitations and limited integrating sphere opening effects show that small area effects can lead to large deviations in outcoupling efficiency with respect to the large area devices commonly used in lighting applications. On the large area devices, we focus on the optimization of the thinfilm stack cavity in the OLED by tuning the thicknesses of thin layers. We show the importance of including the complete device in the optimization process, including the thin-film stack and the thick substrate with the outcoupling textures. We show that an OLED with an optimized planar cavity and applied external positive shaped dome texture can reach up to 50.5 % light extraction efficiency according to simulations.

Quantitative analysis of charge transport in intrinsic and doped organic semiconductors combining steady-state and frequency-domain data

Single-carrier devices are an excellent model system to study charge injection and charge transport properties of (doped) transport layers and to draw conclusions about organic electronics devices in which they are used. By combining steady-state and impedance measurements at varying temperatures of hole-only devices with different intrinsic layer thicknesses, we are able to determine all relevant material parameters, such as the charge mobility and the injection barrier. Furthermore, the correlation and sensitivity analyses reveal that the proposed approach to study these devices is especially well suited to extract the effective doping density, a parameter which cannot be easily determined otherwise. The effective doping density is crucial in organic light-emitting diodes (OLEDs) for realizing efficient injection, charge balance, and lateral conductivity in display or lighting applications. With the fitted drift-diffusion device model, we are further able to explain the extraordinary two-plateau capacitance–frequency curve of these hole-only devices, which originates from charges that flow into the intrinsic layer at zero applied offset voltage. We demonstrate that the observation of this behaviour is a direct indication for ideal charge injection properties and the observed capacitance–frequency feature is linked to the charge carrier mobility in the intrinsic layer. The extracted material parameters may directly be used to simulate and optimize full OLED devices employing the investigated hole-injection and -transport materials.Single-carrier devices are an excellent model system to study charge injection and charge transport properties of (doped) transport layers and to draw conclusions about organic electronics devices in which they are used. By combining steady-state and impedance measurements at varying temperatures of hole-only devices with different intrinsic layer thicknesses, we are able to determine all relevant material parameters, such as the charge mobility and the injection barrier. Furthermore, the correlation and sensitivity analyses reveal that the proposed approach to study these devices is especially well suited to extract the effective doping density, a parameter which cannot be easily determined otherwise. The effective doping density is crucial in organic light-emitting diodes (OLEDs) for realizing efficient injection, charge balance, and lateral conductivity in display or lighting applications. With the fitted drift-diffusion device model, we are further able to explain the extraordinary two-plateau capacit...

Thermally Activated Delayed Fluorescence in a Y3N@C80 Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies

Abstract The endohedral fullerene Y3N@C80 exhibits luminescence with reasonable quantum yield and extraordinary long lifetime. By variable‐temperature steady‐state and time‐resolved luminescence spectroscopy, it is demonstrated that above 60 K the Y3N@C80 exhibits thermally activated delayed fluorescence with maximum emission at 120 K and a negligible prompt fluorescence. Below 60 K, a phosphorescence with a lifetime of 192±1 ms is observed. Spin distribution and dynamics in the triplet excited state is investigated with X‐ and W‐band EPR and ENDOR spectroscopies and DFT computations. Finally, electroluminescence of the Y3N@C80/PFO film is demonstrated opening the possibility for red‐emitting fullerene‐based organic light‐emitting diodes (OLEDs).

Thermally-activated delayed fluorescence in Y3N@C80 endohedral fullerene: time resolved luminescence and electron paramagnetic resonance studies

Endohedral fullerene Y3N@C80 exhibits luminescence with reasonable quantum yield and extraordinary long lifetime. By variable-temperature steady-state and time-resolved luminescence spectroscopy, we demonstrate that above 60 K the Y3N@C80 exhibits thermally-activated delayed fluorescence with maximum emission at 120 K and a negligible prompt fluorescence. Below 60 K, a phosphorescence with a lifetime of 192 ms is observed. Spin distribution and dynamics in the triplet excited state is investigated with X- and W-band EPR and ENDOR spectroscopies and DFT computations. Finally, electroluminescence of the Y3N@C80/PFO film is demonstrated opening the possibility for red-emitting fullerene-based organic light-emitting diodes (OLEDs).