Synthesis and Characterisation of Polymers for Application in Organic Electronics

Abstract

Organic Photovoltaics (OPVs) have attracted great interest in recent years due to their potential for offering low-cost solar energy conversion. However, problems, such as low efficiency and short lifetimes limit their application. Mechanical and electrical instability of OPVs is a result of complex degradation processes such as delamination. As this process occurs at the weakest interface in OPV devices, located between poly(3,4-ethylenedioxythiophene):poly(p-styrene sulfonate) (PEDOT:PSS) and the photoactive layer, the adhesion could be improved by introduction of an interlayer between the aforementioned layers. To this end, new amphiphilic interfacial copolymers were designed. The aim was to obtain a block copolymer consisting of a hydrophilic block being in contact with the PEDOT:PSS layer and a hydrophobic block interacting with the photoactive layer. Since, roll-to-roll OPV production requires deposition of materials from solution, a post-deposition method of thermal transformation of the hydrophobic copolymer into amphiphilic material was developed. Firstly, a protocol for the synthesis of thermally modifiable R-alkyl p-styrene sulfonates was optimised using Reversible Addition-Fragmentation chain Transfer (RAFT) polymerisation. Subsequently, thermal behaviour of the products comprising various alkyl protecting groups was investigated. As a result, the isobutyl group was identified as the most thermally labile, but storage-stable, protecting group, and therefore, was used for synthesis of the thermally modifiable poly(isobutyl p-styrene sulfonate) (PiBSS) block. In the next step, the best conditions for preparation of the poly(dodecyl p-styrene sulfonate) (PDDSS) hydrophobic block were optimised. Finally, a library of new interfacial PiBSS-b-PDDSS copolymers with varied lengths of both blocks was synthesised. Thermal behaviour of the copolymers was tested in the bulk and in films, with the thin film morphologies studied further by Grazing Incidence Small- and Wide-angle X-ray Scattering. Preliminary electrical studies on devices with the incorporated interlayer were performed showing the potential for further use of these copolymers in OPV. In the industrial component of this thesis (Chapter 4), a series of poly(ethylene oxide)-poly(p-styrene sulfonate) (PEO-PSS) block copolymers was synthesised for application as counter ion templates in EDOT polymerisation. Significant differences in PEDOT dispersion quality were observed whist using PEO-b-PSS, PSS-b-PEO-b-PSS and PEO-PSS multiblock copolymers, showing that the latter gave the most stable dispersions. Overall, this work presents the preparation of new copolymers particularly attractive to roll-to-roll industrial printing of organic electronics for their long-term stability.