Routes for efficiency enhancement in fluorescent TADF exciplex host OLEDs gained from an electro‐optical device model

Fluorescence-based organic light-emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) have increasingly attracted attention in research and industry. One method to implement TADF is based on an emitter layer composed of an exciplex host and a fluorescent dopant. Even though the experimental realization of this concept has demonstrated promising external quantum efficiencies, the full potential of this approach has not yet been assessed. To this end, a comprehensive electro-optical device model accounting for the full exciton dynamics including triplet harvesting and exciton quenching is presented. The model parameters are fitted to multiple output characteristics of an OLED comprising a TADF exciplex host with a fluorescent emitter, showing an external quantum efficiency of >10%. With the model at hand, an emission zone analysis and a parameter study are performed, and possible routes for further efficiency enhancement are presented

Pinpointing the origin of the increased driving voltage during prolonged operation in a phosphorescent OLED based on an exciplex host

We report on the origin of the reduced power efficiency in a red phosphorescent OLED with an exciplex host after prolonged operation. The power efficiency is reduced solely by an increased driving voltage while the radiant flux remains constant. An electrical model describing the driving voltage increase is, thus, sufficient to explain the reduced power efficiency. The electrical model of the fresh OLED and at different stages of degradation was devised from four different measurement methods. Using multiple measurement methods to determine the model parameters results in a rather unique set of model parameters, despite the large number of model parameters (38) as revealed by a correlation analysis. The increase in driving voltage could be reproduced by modifying only 7 out of the 38 model parameters. A sensitivity analysis identified the parameters with the largest effect (66%) on the driving voltage increase to be the trap density and the mobility of the employed hole transporting layer. This work highlights the benefit of using multiple measurement methods to derive reliable model parameters and the use of a sensitivity analysis to pinpoint the origin of the investigated property

Finite element modeling for analysis of electroluminescence and infrared images of thin-film solar cells

Sheet resistance losses and local defects are challenges faced in solar module fabrication and upscaling processes. Commonly used investigation tools are non-invasive optical and thermal imaging techniques, such as electroluminescence, photoluminescence as well as illuminated and dark infrared imaging. Here, we investigate the potential of computationally efficient finite element simulation of solar cells and modules by considering planar electrodes coupled by a local current–voltage coupling law. Sheet resistances are determined by fitting current simulation results of an OPV solar cell to electroluminescence imaging data. Moreover, a thermal model is introduced that accounts for Joule heating due to an electrothermal coupling. A direct comparison of simulated temperature maps to measured infrared images is therefore possible. The electrothermal model is successfully validated by comparing measured and simulated temperature profiles across four interconnected organic solar cells of a mini-module. Furthermore, the influence of shunts on the thermal behavior of OPV modules is investigated by comparing electrothermal simulation results to dark lock-In IR thermography images

Characterization and Modelling of the Emission Zone and Exciton Dynamics in Doped Organic Light-Emitting Diodes

Only recently organic light-emitting diode (OLED) technology has successfully managed the transition from research labs into the consumer market, taking a 60% share of the global mobile display market in 2018. The latest discovery of thermally activated delayed fluorescence attracted a lot of attention in research and industry due to the potential to fabricate fluorescence-based OLEDs with high efficiencies comparable to the currently used phosphorescence-based OLEDs, but with the advantage of possibly cheaper and more sustainable emitter materials (no Ir-, Pt-complexes). For achieving high efficiencies in OLEDs, a substantial number of layers and interfaces of the multilayer stack have to be optimized. A particularly important role is assigned to the emission layer within which light is generated by charge recombination and subsequent energy transfer and radiative decay of excitons. The understanding of charge recombination and exciton dynamics and the determination of the position of light generation are essential for the fabrication of modern OLEDs and are the goal of this thesis. Therefore two different OLED types, phosphorescence-based OLEDs and state-of-the-art TADF exciplex host OLEDs incorporating a fluorescent emitter, are studied by electro-optical characterization and device modelling. In a first step the emission zones are determined and analyzed by angle-dependent steady-state measurements at different biases and optical simulations. In both OLED types split emission zones are obtained with densities of emissive excitons that decay way from both emission layer interfaces toward the center. For the phosphorescence-based OLEDs an additional bias-dependence of the split emission zone is observed, meaning that at low bias the main emission is located at the cathode side and shifts to the anode side for increasing bias. In a second step, with transient EL decay measurements and electro-optical simulations the split emission zones are correlated to an EL peak appearing after OLED turn-off. To study the influence of the emission zone and the exciton dynamics on the OLED efficiency an electro-optical device model is established to reproduce the experimentally obtained measurement data. As the model includes charge carrier dynamics, light outcoupling and time- and position-dependent exciton processes, such as the formation, diffusion, transfer, decay and quenching, the physical mechanisms in the OLEDs are elucidated. For the phosphorescence-based OLED a surprising current efficiency increase of up to 60% for increasing bias as well as a subsequent decrease is explained with the shift of the emission zone and its influence on exciton quenching and light outcoupling. Similarly, for the TADF exciplex host OLEDs a model parameter study illustrates promising EQE enhancement routes, which could lead to EQEs as high as 42%. This thesis emphasizes the need of accurate knowledge of the emission zone and its bias-dependence due to its potentially strong influence on the OLED efficiency and its importance for the optimization of the OLED layer stack. In addition, this thesis shows that full electro-optical device modelling (including electrons, excitons and photons) combined with advanced electro-optical characterization techniques is crucial for elucidating the physical mechanisms in state-of-the-art OLEDs as well as for the prediction of promising routes for future efficiency enhancements

Influence of the bias-dependent emission zone on exciton quenching and OLED efficiency

We present an electro-optical model of a three-layer phosphorescent OLED which accurately describes the measured current efficiency and transient electroluminescence decay for different biases. Central findings are a bias-dependent emission zone, which influences light outcoupling as well as exciton quenching, and the presence of strong triplet-polaron quenching even at low bias. The measured current efficiency initially increases up to 9 V before it decreases, where the increase is found to be caused by reduced triplet-polaron quenching with holes, while the decrease is caused by a reduced light outcoupling and increased triplet-triplet annihilation. The numerical model allows identifying the individual contributions of the exciton continuity equation and explains the electroluminescence decay, which deviates significantly from a mono-exponential decay due to the dominating influence of exciton generation and quenching after the external bias is removed

Analysis of the bias-dependent split emission zone in phosphorescent OLEDs

From s-polarized, angle-dependent measurements of the electroluminescence spectra in a three-layer phosphorescent organic light-emitting diode, we calculate the exciton distribution inside the 35 nm thick emission layer. The shape of the exciton profile changes with the applied bias due to differing field dependencies of the electron and hole mobilities. A split emission zone with high exciton densities at both sides of the emission layer is obtained, which is explained by the presence of energy barriers and similar electron and hole mobilities. A peak in the transient electroluminescence signal after turn-off and the application of a reverse bias is identified as a signature of a split emission zone

Time-Dependent p–i–n Structure and Emission Zone in Sandwich-Type Light-Emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) are one of the simplest electroluminescent devices and consist of a single emissive organic/salt layer sandwiched between two electrodes. The unique attribute of an operated LEC is the development of a p-doped/intrinsic/n-doped (p–i–n) structure in the active layer in which the doped regions allow for facile transport of electronic charges to the intrinsic region, where charge recombination and light emission occur. However, due to the complex and simultaneous motion of ionic and electronic charges, the examination of the p–i–n structure and the zone where the light is emitted (EZ) is challenging during operation. By analyzing incident photon-to-current conversion efficiency and angular emission measurements with optical simulations, and correlating the results with capacitance measurements, we are able to obtain a clear picture of the p–i–n situation and the EZ within the active layer of a sandwich-type LEC during operation. It is found that the p-doped zone grows to the center of the active layer while the EZ stays closer to the metal electrode and is unchanged over time. Furthermore, optical simulations reveal that the determined EZ limits the external quantum efficiency of the LEC by outcoupling efficiencies of less than 10%

High‐Pressure Studies of Correlated Electron Systems

Tuning the electronic properties of transition-metal and rare-earth compounds by virtue of changes of the crystallographiclattice constants offers controlled access to new forms of order. We review the development oftungsten carbide and moissanite Bridgman cells conceived for studies of the electrical resistivity up to 10GPa,as well as bespoke diamond anvil cells developed for neutron depolarization studies up to 20GPa. For the diamondanvil cells, the applied pressure changes as a function of temperature in quantitative agreement withthe thermal expansion of the pressure cell. A set-up based on focussing neutron guides for measurements of thedepolarization of a neutron beam by samples in a diamond anvil cell is described. Resistivity measurementsand neutron depolarization provide evidence of ferromagnetic order in SrRuO$_3$ up to 14GPa close to a putativequantum phase transition. Combining hydrostatic, uniaxial, and quasi-hydrostatic pressure, the emergenceof incipient superconductivity in CrB$_2$ is observed. The temperature dependence of the electrical resistivity inCeCuAl$_3$ is consistent with emergent Kondo correlations and an enhanced coupling of magneto-elastic excitationswith the conduction electrons at low and intermediate temperatures, respectively