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Title: Novel organic semiconductors for light-emitting diodes
Author: Rapidis, Alexandros Georgios
ISNI:       0000 0004 8507 5254
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2019
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The intriguing properties of organic semiconductors in various optoelectronic applications, ranging from molecular wires for integrated circuits, to organic photovoltaics (OPV), organic transistors (OFET) and organic light-emitting diodes (OLEDs) have motivated the work presented in this thesis. More precisely, this thesis focuses on narrow energy gap materials, suitable for OLEDs emitting in the red and near-infrared (NIR) regions. Conjugated materials emitting at low energies face great challenges to have efficient light emission. This thesis proposes two very interesting and novel strategies to cope with the limiting factors of efficient light-emission from conjugated compounds. Firstly, a series of porphyrin oligomers is presented. With emission ranging from red to pure NIR, three architectures of these oligomers were studied. Zinc oligomers at various length that allowed for fine tuning of the emission, five hexamers with different coordinating metals at the centre of each unit, allowing for phosphorescence emission, and a pentamer, with single acetylene instead of butadiyne bonds connecting porphyrin units, resulting in a shorter oligomer with an extended pi-conjugation and a bathochromic shift of the emission. Two limitations can be identified for efficient NIR emission, the so-called 'energy-gap law' and aggregation quenching. Both limitations are addressed, resulting in unprecedented external quantum efficiencies when the oligomers are used in OLEDs and remarkable devices lifetimes, considering the non-optimised diodes. Secondly, three diketopyrrolopyrrole-based copolymers are presented and their photophysical properties discussed. These polymers represent a different strategy to prevent aggregation; by engineering covalent bonds, the conjugated core is sheathed within its cyclic sidechains. The strategy is proven highly successful and the three polymers are compared with their unprotected counterparts, where emission is severely quenched. The advantages of the encapsulation are more pronounced when the polymers are incorporated in OLEDs, where the encapsulated ones achieved up to 16 times higher external quantum efficiencies compared to the unprotected ones.
Supervisor: Cacialli, F. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available