Preparation and Characterisation of Indacenodithiophene Based Donor Acceptor Conjugated Polymers for Organic Photovoltaic Applications

Abstract

The growing demand for clean and renewable energy has intensified research into advanced solar technologies. Organic photovoltaics (OPVs) have emerged as promising alternatives to conventional inorganic devices owing to their low fabrication cost, light weight, flexibility, and solution processability. Within bulk heterojunction (BHJ) OPVs, donor-acceptor (D-A) conjugated polymers are crucial in governing light absorption, exciton dissociation, and charge transport. The rational design of such polymers with tuneable band gaps and controlled solid-state organisation is therefore key to improving device performance. This thesis presents the synthesis and systematic investigation of twelve indacenodithiophene (IDT)-based conjugated polymers prepared via Stille cross-coupling polymerisation. The IDT donor was coupled with three families of electron deficient acceptors, thienopyrroledione (TPD), benzothiadiazole (BT), and benzothiadiazole imide (BTI), while side chain engineering and the introduction of thiophene or selenophene linkers were employed to modify structural and electronic properties. All polymers displayed good solubility and high molecular weight suitable for optoelectronic analysis. Optical and electrochemical studies revealed tunable band gaps (~1.7–2.1 eV) and stable energy levels, with BTI-based polymers exhibiting the lowest LUMO levels and narrowest electrochemical gaps. Incorporation of selenophene linkers led to red-shifted absorption and enhanced backbone planarity compared to thiophene analogues. Thermogravimetric analysis confirmed high stability across the series, with all decomposition temperatures exceeding 250 °C; notably, the BTI-based polymer P9 exhibited a Td of 459 °C. Powder X-ray diffraction indicated semi-crystalline morphologies with pronounced π–π stacking in the selenophene- and BTI-based systems. Overall, this study establishes clear structure-property relationships among TPD-, BT-, and BTI-containing polymers, offering molecular design guidelines for next-generation IDT-based D-A polymers optimised for organic solar cells and related optoelectronic applications.