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Title: Effect of polymer and metal oxide properties on hybrid light emitting diodes
Author: Costa Dantas Faria, Jorge
ISNI:       0000 0004 5994 3708
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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Hybrid organic/inorganic light-emitting diodes (HyLEDs) combine thin, metal oxide layers together with light-emitting organic semiconductors to create devices that are more resilient to ambient conditions than standard devices. In this thesis, HyLED performance is measured as a function of the individual organic and oxide layer properties with the aims of addressing several perceived gaps within reported literature and to consequently optimise future device design. Reported herein are the results of the insertion of a vertical zinc oxide nanorod array as a non-planar, bottom-cathode electron injection layer. Using facile solution-processing methods, a well-aligned and uniform array was deposited into which the light emitting polymer poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) was then melt-processed to create a massive, 3D interfacial contact area for electron injection. This study recorded, for the first time, efficiency and luminance values for a vertical nanorod-based LED with maximum figures of 1.66 cd/A and 8602 cd/m2 showing their potential for display and lighting applications. The successful demonstration of these nanorod HyLEDs was due to the insertion of a poly[(9,9-dioctylfluore-nyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine)] (TFB) at the F8BT/anode interface to block electrons and reduce the probability of interfacial exciton dissociation. Using post-deposition annealing and solution processing, a robust method of casting TFB onto F8BT without the need for lift-off steps is presented. In planar devices, the insertion of TFB increases the maximum recorded efficiencies from 0.024 to 1.0 cd/A and its role as an optical emission tuning layer was also demonstrated; by simply varying the layer thickness the F8BT electro-luminescence was tuned from green emission to orange. Finally, the molecular weight of F8BT was seen to significantly influence the performance of HyLEDs, with six separate batches ranging from 36 to 300 kg/mol providing a systematic device study. Importantly, these batches were extracted from a single commercial source using gel permeation chromatography to negate the possible influence of different synthesis routes and chain ends. The thesis is organised into six chapters. Chapter 1 focuses on the principles behind device design and operation with an overview of the current state of HyLED literature and a discussion of the principle materials studied. Chapter 2 provides a general description of the experimental procedures and techniques used throughout. Chapters 3 to 5 each focus on a specific experimental investigation and are largely self-contained with each presenting - along with a discussion of the results - a literature review, a list of aims and an experimental section particular to the study conducted. Finally, Chapter 6 provides general concluding remarks, expanding on the ideas of further work discussed within Chapters 3-5.
Supervisor: McLachlan, Martyn ; Campbell, Alasdair Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral