Design and synthesis of novel photoactive block copolymers towards use in efficient organic photovoltaic cells

Abstract

The opposing hydrophilic properties of certain materials in organic photovoltaic cells (OPVs) produce interfaces which are prone to delamination and charge loss. As amphiphilic block copolymers possess both hydrophilic and hydrophobic properties, they could be used to efficiently bind these materials. The opposing nature of the components of an amphiphilic block copolymer means their synthesis can sometimes be difficult due to the lack of common solvents which is the case for the conjugated polymers used in OPV cells. To address this problem, the group have previously bonded a thermolabile hydrophobic group to a hydrophilic monomer and created a hydrophobic block copolymer which became amphiphilic when heated. As heating polymers can provoke unwanted events in solar cells, using ultraviolet light to provoke the hydrophilic transition would be preferable. The aim of this project is to create an analogous hydrophobic block copolymer which becomes amphiphilic under UV-light. Firstly, 4-vinylbenzenzoic acid (VBA) was functionalised with a photolabile group (2-nitrobenzyl (NB)) to create the novel 2’nitrobenzyl 4-vinylbenzoate (NBVB) using two different techniques (nucleophilic addition/elimination and Steglich esterification) as the first yielded a meagre amount of product. To polymerise this monomer, a reversible addition-fragmentation chain transfer (RAFT) polymerisation system composed of 2-cyanomethyl-S-dodecyltrithiocarbonate and 1,1′-azobis(cyclohexanecarbonitrile) was chosen and tested on styrene which yielded a polystyrene of impressive quality. NBVB did not polymerise in a controlled manner supposedly due to suppression effects incurred by the NB group. Instead, VBA was polymerised and subsequently functionalised with NB to yield the novel poly(2’nitrobenzyl 4-vinylbenzoate) (PNBVB). The method developed herein for the RAFT polymerisation of VBA yields polymers with lower dispersity and better control over chain length than any other reported method. A polystyrene-block-poly(2’nitrobenzyl 4-vinylbenzoate) (PS-b-PNBVB) copolymer was synthesised using a PS macroRAFT. The resulting block copolymer possessed the desired molecular weight and satisfactory dispersity. NBVB, PNBVB and PS-b-PNBVB were then irradiated and their photocleavable abilities were demonstrated.