Organic laser diodes: modelling and simulation

This thesis analyzes the impact of various loss processes on the threshold current density of organic semiconductor laser diodes by numerical simulation. Design concepts based on organic double-heterostructures are evaluated and design rules are derived which can be used in order to reduce the impact of loss processes and in order to improve the device performance

Immense magnetic response of exciplex light emission due to correlated spin-charge dynamics

As carriers slowly move through a disordered energy landscape in organic semiconductors, tiny spatial variations in spin dynamics relieve spin blocking at transport bottlenecks or in the electron-hole recombination process that produces light. Large room-temperature magnetic-field effects (MFE) ensue in the conductivity and luminescence. Sources of variable spin dynamics generate much larger MFE if their spatial structure is correlated on the nanoscale with the energetic sites governing conductivity or luminescence such as in co-evaporated organic blends within which the electron resides on one molecule and the hole on the other (an exciplex). Here we show that exciplex recombination in blends exhibiting thermally-activated delayed fluorescence (TADF) produces MFE in excess of 60% at room temperature. In addition, effects greater than 4000% can be achieved by tuning the device's current-voltage response curve by device conditioning. These immense MFEs are both the largest reported values for their device type at room temperature. Our theory traces this MFE and its unusual temperature dependence to changes in spin mixing between triplet exciplexes and light-emitting singlet exciplexes. In contrast, spin mixing of excitons is energetically suppressed, and thus spin mixing produces comparatively weaker MFE in materials emitting light from excitons by affecting the precursor pairs. Demonstration of immense MFE in common organic blends provides a flexible and inexpensive pathway towards magnetic functionality and field sensitivity in current organic devices without patterning the constituent materials on the nanoscale. Magnetic fields increase the power efficiency of unconditioned devices by 30% at room temperature, also showing that magnetic fields may increase the efficiency of the TADF process.Comment: 12 pages, PRX in pres

Organic laser diodes: modelling and simulation

This thesis analyzes the impact of various loss processes on the threshold current density of organic semiconductor laser diodes by numerical simulation. Design concepts based on organic double-heterostructures are evaluated and design rules are derived which can be used in order to reduce the impact of loss processes and in order to improve the device performance

Magnetoresistance in organic light-emitting diode structures under illumination

Copyright 2007 by the American Physical Society. Article is available at

Coupled opto-electronic simulation of organic bulk-heterojunction solar cells: parameter extraction and sensitivity analysis

A general problem arising in computer simulations is the number of material and device parameters, which have to be determined by dedicated experiments and simulation-based parameter extraction. In this study we analyze measurements of the short-circuit current dependence on the active layer thickness and current-voltage curves in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based solar cells. We have identified a set of parameter values including dissociation parameters that describe the experimental data. The overall agreement of our model with experiment is good, however a discrepancy in the thickness dependence of the current-voltage curve questions the influence of the electric field in the dissociation process. In addition transient simulations are analyzed which show that a measurement of the turn-off photocurrent can be useful for estimating charge carrier mobilities.Comment: 10 pages, 12 figures, 2 tables, Accepted for publication in Journal of Applied Physic

Investigation of Magnetic Field Dependent Electroluminescence and Charge Injection in Organic Light Emitting Diodes

After 20 years of development, conjugated polymers have been extensively applied in organic light emitting diodes (OLED), solar cells, transistors, and chemical or bio-sensors. Recently it is discovered that magnetic field can tune the electroluminescence intensity and conductivity in OLEDs, leading to the development of organic magneto-optoelectronics. However, the underlying mechanisms are still unclear. In this dissertation, we investigated a wide range of conjugated polymers and low molecular weight molecules and proposed that the magnetic field effect on electroluminescence and magnetoresistance arise from the magnetic field enhanced polaron pair dissociation and reduced triplet-charge reaction. The final magnetic field effects are determined by the sum of the two contributions. The magnetic field effect on polaron pair dissociation can be tuned by varying the spin-orbital coupling of the organic semiconductor. Stronger spin-orbital coupling leads to the reduction of magnetic field effect on both electroluminescence and magnetoresistance. Phosphorescent dye doping can also tune the magnetic field effects through energy transfer process and intermolecular interaction. Triplet-charge reaction can be largely controllable by manipulating the bipolar injection. It has found that unbalanced bipolar injection enhance the triplet-charge injection, leading to more positive magnetoresistance and more negative magnetic field effect on electroluminescence. Balanced bipolar injection reduces triplet charge reaction, resulting in more negative magnetoresistance and more positive magnetic filed effect on electroluminescence. The triplet-charge reaction can also be morphologically tuned. In poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) based OLEDs, low energy crystalline domains can be induced in PFO amorphous matrix by either high boiling point solvent or annealing treatments. The low energy domains can both spatially confine both excitons and charges to enhance the triplet-charge reaction. Consequently the enhanced triplet-charge reaction reduces the magnitude of magnetic field effects Our study successfully built a bridge between the magnetic field effects and the spin dependent excitonic processes in OLEDs. Scientifically, the excitonic processes, e.g. intersystem crossing, triplet-charge reaction, can be investigated by simply measuring the magnetic responses. Technically, this tunable magnetic field effects have the potential to be used to in new generation smart screens, magnetic sensors