Pyrene- benzo[1,2,5]thiadiazole based conjugated polymers for application in BHJ solar cells

Ethylhexyloxy-functionalised pyrene (PEH) was prepared and copolymerised with both dithienyl-benzo[c]-[1], [2], [5]thiadiazole and dibithiophenyl-benzo[c]-[1], [2], [5]thiadiazole via a Stille coupling polymerisation method to yield PPEH-DTBT-8 and PPEH-DT2BT-8, respectively. A comparative study was conducted to assess the impact of substituting thiophene for bithiophene repeat units upon the resulting properties of the conjugated polymers. PPEH-DT2BT-8 which has bithiophene spacers between pyrene and benzothiadiazole repeat units, exhibited a narrower optical and electrochemical band gap relative to PPEH-DTBT-8; a consequence of the incorporating bithiophene spacer units which promote intramolecular charge transfer between the electron donating and electron accepting moieties. Both PPEH-DTBT-8 and PPEH-DT2BT-8 showed deep HOMO levels of -5.54 and -5.50 eV, respectively. The polymers possess good thermal stabilities with degradation temperatures in excess of 310 °C. The photovoltaic performance of the two polymers was studied by fabricating bulk heterojunction (BHJ) photovoltaic devices using PC70BM as the acceptor. PPEH-DTBT-8 and PPEH-DT2BT-8 demonstrated efficiencies of 0.33 and 1.83%, respectively. The higher efficiency of PPEH-DT2BT-8 can be attributed to vastly improved FF and Jsc values.