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Title: Role of thin organic interlayers inserted at the electrode interfaces for efficient polymer LEDs
Author: Bailey, Jim
ISNI:       0000 0004 6061 4530
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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This thesis discusses role of thin organic interlayers at the electrode interfaces of polymer light-emitting diodes (LEDs) in increasing their efficiencies. The effect of varying the anode-side IL material, its thickness, and its p-doping level on poly(dioctylfluorene-alt-benzothiadiazole) (F8BT) PLEDs is examined. Then the impact of varying the IL material is explored in Lumation Green 1300 PLEDs to determine whether a relation exists between the role of the IL and the light emitting layer's properties. It is found that excitons are formed in F8BT adjacent to the interface with the IL and are thus exceptionally sensitive to the energetics at that interface, with wide energy gaps helping to reduce luminance quenching significantly. The general effects of ionic cathode-side ILs on PLED efficiencies and response times are investigated. The novel materials used contain imidazolium cationic groups. The impact of varying the ionic IL main conjugated backbone on PLED characteristics is studied, since the conjugated backbone is primarily responsible for the optoelectronic properties of semiconducting polymers. Efficiencies of ~15 lm W-1 at 3 V are achieved. The light emitting layer is varied from F8BT to Super Yellow to probe the influence of the active layer's properties on the role of the IL in determining device characteristics. The exact combination of active layer and IL strongly influence device performance. The influence of the mobile anion and the immobile cationic group of cathode-side ILs on PLED performance is investigated. Our investigation focused on ionic ILs with a F8BT conjugated backbone. Ionic ILs with various mobile counter ions are created to study the role of the mobile ion on charge injection. Increased size of anion leads to poorer device performances. Experiments were carried out to understand what factors determine the response times and efficiencies of PLEDs with ionic cathode-side ILs. Response times of 4.24 μs are recorded and Scanning Kelvin Probe Microscopy (SKPM) experiments on thin films of the ionic polymers are conducted to shed light on the movement of ions within the IL during PLED operation. Good energy level matching with the cathode metal is found for the ionic cathode-side ILs, allowing for good electron injection properties.
Supervisor: Kim, Ji-Seon ; Bradley, Donal Sponsor: Engineering and Physical Sciences Research Council ; Erasmus Mundus Program ; World Class University Project in Korea (WCU)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral