Low-bandgap conjugated copolymers based on a donor-acceptor structure have been synthesised via palladium-complex catalysed direct arylation polymerisation. Initially, we report the optimisation of the synthesis of poly(cyclopentadithiophene-alt-benzothiadiazole) (PCPDTBT) formed between cyclopentadithiophene and dibromobenzothiadiazole units. The polymerisation condition has been optimised, which affords high-molecular-weight polymers of up to M(n) = 70 k using N-methylpyrrolidone as a solvent. The polymers are used to fabricate organic photovoltaic devices and the best performing PCPDTBT device exhibits a moderate improvement over devices fabricated using the related polymer via Suzuki coupling. Similar polymerisation conditions have also been applied for other monomer units.

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We study the performance of PCPDTBT : C71-PCBM organic photovoltaics (OPVs) for three hole transporting layers (HTL); PEDOT : PSS, nickel oxide (NiO) and molybdenum trioxide (MoO3). We show that devices fabricated with nickel oxide HTL demonstrate the highest power conversion efficiency and theoretical data using a transfer matrix model confirms that this is as a result of increased absorption in the active layer as well as a result of improved series resistance and improved matching of energy levels. Device degradation is studied and lifetime is highest for the NiO and MoO3 based devices, proving that OPVs with this material system are less sensitive to the environmental effects of water, oxygen and irradiance.

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