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The design, synthesis and characterisation of thirteen new semiconducting polymers for use in organic photovoltaic (OPV) and field effect transistor (OFET) devices are reported. The rational design of each polymer is discussed and their structures related to their varying chemical and physical properties, which are further used to rationalise the specific device performances. Various structural modifications are investigated with a focus on the electron-deficient bis-lactam structures diketopyrrolopyrrole (DPP) and isoindigo, that are flanked by thieno[3,2-b]thiophene donor groups. Alkyl chain optimisation of thieno[3,2-b]thiophene diketopyrrolopyrrole (DPPTT) based co-polymers was thoroughly examined and it was found that increased alkyl chain size affords improved solubility and a wider range of accessible co-monomer units. Exploiting this improved solubility, the new DPPTT-T polymer was fractionated using recycling gel permeation chromatography (GPC). This gave fractions with increased molecular weights and narrowed mass distributions resulting in OPV power conversion efficiency (PCE) ehancements of greater than 50 %. Continuing with DPPTT-T alkyl chains, a new OPV structural design consideration is introduced in which the alkyl chain branching position is systematically moved further from the polymer backbone. This resulted in higher molecular weight polymers with stronger π - π interactions and significantly enhanced device performances due to increased intermolecular interactions, with PCEs in excess of 8 %. Using the new higher performing branched alkyl chains the role of differing chalcogenophene co-monomers OPV devices was also investigated and was found that increased heteroatomic size, from thiophene to selenophene to tellurophene, resulted in narrowed optical band gaps and increased heteroatom – heteroatom interchain interactions. When these differences are taken into consideration, thiophene is shown to be the highest performing chalcogenophene comonomer of the series. Moving to isoindigo, a new thieno[3,2-b]thiophene flanked structure (iITT) was designed and synthesised for the first time. The resultant narrow band gap co-polymers were shown to be excellent candidate materials for ambipolar OFET applications. Through a comparative literature and computational study, the new iITT unit is shown to be one of the highest performing units within this family of polymer structures.