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    Amphiphilic Block Copolymers for Morphological Control in Bulk Heterojunction Solar Cells
    Ahluwalia, Gagandeep ( 2021)
    Organic solar cell (OSC) is a type of solar cell that utilise a solution-processable organic semiconductor material as a photoactive component. The use of less toxic, flexible organic semiconductors in solar cells leads to a formation of a lightweight device with a short energy payback time. Along with this, it is easy to fabricate organic solar cells on a large scale by roll-to-roll printing, which is the driving force for industrialisation. The recent efforts on organic materials such as small molecules or polymers have pushed the OSCs towards a milestone efficiency of more than 18 %. However, achieving the desired control over the morphology by blending of the donor (electron-rich) and acceptor (electron-deficient) materials and reproducibility for industrial scope has remained a challenge, based on the fact that often deposition techniques used in the laboratory are different to those in continuous operation, i.e. spin coating compared to slot die deposition. Moreover, the use of chlorinated solvents for processing restricts OSCs large-scale manufacturing scope. In order to increase the commercial applicability of OSCs, a bi-continuous interpenetrating network of the donor and acceptor block is required, where donor and acceptor materials should have 10-20 nm domain sizes with continuous interfaces to promote the charge generation/separation process. Also, the photoactive component should show solubility in non-chlorinated or industrial relevant solvents. Fully conjugated block copolymers, where a conjugated donor and acceptor block are covalently linked with each other, have an ability to attain a defined morphology by manipulating the Flory–Huggins segment–segment interaction parameter. The BCP materials could self-assemble into a thermodynamically favoured morphology and offers continuous interface, control over domain size, long term stability, and reproducibility. However, the BCP’s have failed to deliver a high performing OSC so far. The primary reason is the difficulty of achieving a clean phase separation in BCP’s due to the high inter-block interactions parameter between donor and acceptor block, which enhances the charge recombination over charge separation process. Moreover, developing a fully conjugated block copolymer with a specific molecular weight, block ratio, and high purity is lacking behind due to the unavailability of the synthetic approach. The synthesis of BCP via a well-known step-growth polymerisation usually ends-up containing a mixture of the desired BCP along with a significant number of polymer contaminants, which impacts device performance. Furthermore, the BCP’s reported in the literature usually require chlorinated solvents for the processing, which limits its industrial scope. For industrial adoption of OSCs, we have developed a strategy to control the synthesis of an amphiphilic di-block copolymer containing high performing push-pull donor and acceptor blocks to achieve a clean phase separation and solubility in non-chlorinated solvents. In order to obtain control synthesis of a block copolymer, firstly, we have developed a strategy to control the molecular weight/ end-group functionality of homopolymers. The strategy involves designing and synthesising of asymmetric functionalised push-pull monomers that undergo a Suzuki or a Stille pseudo-living polymerisation. Herein, studies on four different homopolymers, i.e., p(BDT-BT), p(BDT1-BT), p(IID-TT), p(NDI-TT), were performed and control over the p(BDT-BT), p(BDT1-BT) polymer via Suzuki catalyst transfer polymerisation and p(NDI-TT) via Stille catalyst transfer polymerisation was achieved. Two fully conjugated amphiphilic di-block copolymers with a specific molecular weight and block ratio (1:1) were synthesised via a stepwise or one-pot procedure. In a stepwise method, TfO-p(NDI-TT) was initially synthesised using a pseudo-living Stille polymerisation with a single triflate (OTf) end group and specific molecular weight. Subsequently, TfO-p(NDI-TT) was used as a macro-initiator to grow an amphiphilic di-block copolymer (BCP1) via a grafting-into approach, where the donor polymer block was grown using a pseudo-living Suzuki polymerisation. Moreover, the one-pot synthesis of an amphiphilic di-block copolymer (BCP2) with the sequential addition of donor and acceptor monomer was performed utilising similar optimised conditions developed for stepwise block copolymer synthesis. Furthermore, the preliminary morphological behaviour of BCP1 and BCP2 was investigated using X-ray scattering techniques. At last, a device containing a BCP1 as an active layer achieved an efficiency of 3.2 %, whereas BCP2 showing a maximum efficiency of 2.7 % was reported. This work has demonstrated a potential route of utilising asymmetrically functionalised push-pull monomers to achieve control over di-block copolymer synthesis containing high performing donor and acceptor polymer.