Pyrene–benzothiadiazole-based copolymers for application in photovoltaic devices

The preparation and characterisation of four narrow band gap pyrene-benzothiadiazole based alternating copolymers is presented. An investigation of the impact of attaching different solubilising groups to the pyrene repeat units on the optical, electrochemical and thermal properties of the resulting materials was undertaken along with studies on the aggregation of polymer chains in the solid state. Unsurprisingly, polymers which had the smaller 2-ethylhexyl chains attached to the pyrene units (PPEH-DTBT and PPEH-DTffBT) displayed lower molecular weights relative to polymers with larger 2-hexyldecyl substituents (PPHD-DTBT and PPHD-DTffBT). Despite this, the 2-ethylhexyl substituted polymers displayed narrower optical band gaps relative to their analogous 2-hexyldecyl substituted polymers. Of all polymers synthesised, PPEH-DTBT displayed the lowest optical band gap (1.76 eV) in the series. All polymers display degradation temperatures in excess of 300°C. Polymers with smaller alkyl chains on the pyrene units display shallower HOMO levels which could be due to increased intramolecular charge transfer between the donor and acceptor units. Preliminary investigations on bulk heterojunction solar cells with a device structure ITO/PEDOT:PSS/Polymer:PC70BM/Ca/Al were undertaken. Polymer/PC70BM blend ratios of 1/3 were used in these studies and have indicated that PPEH-DTBT displayed the highest efficiency with a PCE of 1.86 %.