Kinetic Monte-Carlo modelling of charge and exciton dynamics in phosphorescent organic light-emitting diodes

Stephen Sanderson studied the operation of phosphorescent organic light-emitting diodes using kinetic Monte-Carlo simulation techniques combined with molecular dynamics modelling for accurate representations of the molecular structure. His research has contributed to an improved understanding of the underlying physical processes in these devices

The Investigation of Phosphorescent Dopants and Novel Blue Fluorescent Polymer Hosts for PLED Devices

This thesis has focused on using experimental and simulation based techniques in an attempt to understand the interactions between polymer hosts and phosphorescent dopants in Organic Light Emitting Devices (OLEDs). The viability of the SEmiconducting Thin Film Optics Simulation (SETFOS) software as a modelling tool has been established using the well documented material poly(3,4-ethylenedioxythiophene) (PEDOT). Parameters including resistivity and work function were extracted using SETFOS and the trends observed compared favourably to the commercially provided values, despite some limitations. SETFOS was then used, along with steady state and transient electroluminescence characterisation, to investigate the effects of both phosphorescent dopant colour and concentration on device performance and extract important device parameters, such as the density of states and carrier mobilities. Different device behaviours were observed depending upon the dopant colour and concentration, highlighting the importance of both to device performance. SETFOS was again found to be able to produce quantitative values for a number of device parameters, but several more limitations within the models were identified, which makes further analysis and investigation necessary. Having gained an understanding of host and dopant interactions in OLED devices, the information gathered was used in the characterization of novel high triplet host polymers for OLED applications. Seven polyfluorene based copolymers were investigated in devices with a range of different coloured phosphorescent dopants and charge transport molecules. Unfortunately, they were found to be unsuitable for use as host materials in OLEDs, acting instead as charge traps. These polymers, along with four others, were alternatively assessed on their ability to perform as deep blue, or violet, fluorescent materials in undoped Polymer LED (PLED) devices. These devices were found to have some of the highest device characteristics currently detailed in the literature, and represent a variety of new ways of achieving efficient deep blue emission using PLED devices

Photon Generation and Dissipation in Organic Light-Emitting Diodes

By using phosphorescent and thermally activated delayed fluorescence emitters, the internal quantum efficiency of organic light-emitting diodes (OLEDs) can now reach 100%. However, a major fraction of generated photons is trapped inside the device, because of the intrinsic multi-layer device structure and the mismatch of refractive indices. This thesis comprises different approaches for the efficiency enhancement of planar OLEDs. In particular, outcoupling strategies to extract trapped photons to obtain highly efficient OLEDs are investigated

Modellierung von Ladungs- und Exzitondynamik in amorphen organischen Halbleitern = Modeling of charge and exciton dynamics in amorphous organic semiconductors

Organische Halbleiter finden Anwendungen in vielen Technolgien, wie organischen Leuchtdioden, organischen Solarzellen oder organischen Transistoren. Die Effizienz der Bauteile wird hierbei maßgeblich von den elektronischen Transporteigenschaften der zugrundeliegenden organischen Halbleiter bestimmt, deren oft amorphe Struktur zu Ladungsperkolationseffekten auf einer Größenordnung von 100nm führen kann. In dieser Arbeit werden Methoden vorgestellt, die es ermöglichen mesoskopischen Ladungstransport in amorphen organischen Halbleitern, auf der Basis zugrundeliegender quantenchemischer Rechnungen, effizient zu simulieren. Dabei werden Methoden zur effizienten Auswertung der Coulomb-Wechselwirkung zwischen Ladungsträgern in kinetischen Monte-Carlo Verfahren (KMC), sowie Algorithmen zur Beschleuningung von Vielteilchen-KMC-Verfahren vorgestellt. Die Methoden werden gegen den prototypischen organischen Halbleiter alpha-NPD validiert. Es wird gezeigt, dass Ladungstransport in gemischten Emissions-Transport Materialien, wie sie in organischen Leuchtdioden Einsatz finden, durch Hüpftransport zwischen Emittermolekülen stattfindet, indem räumliche Distanzen zwischen End- und Anfangszustand durch virtuelle Zustände auf Transportmolekülen überbrückt werden. Desweitern wird gezeigt, dass Ladungsträgerinjektion an metallisch-organischen Grenzflächen bei hohen elektrischen Feldern durch virtuelle Übergangszustände auf organischen Molekülen in der Nähe der Grenzfläche um mehrere Größenordnungen verstärkt wird

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