Triisopropylsilylacetylene-functionalised anthracene-alt-benzothiadiazole copolymers for application in bulk heterojunction solar cells

Abstract

Three triisopropylsilylacetylene-functionalised anthracene (TIPSAnt) based polymers were synthesised by copolymerising TIPSAnt with either dithienyl-5,6-difluoro-benzo[c]-[1,2,5]thiadiazole, dithienyl-benzo[c]-[1,2,5]thiadiazole or dibithiophenyl-benzo[c]-[1,2,5]thiadiazole to yield PTATffBT, PTATBT-8 and PTAT2BT-8, respectively. PTAT2BT-8 demonstrated a reduced optical and electrochemical band gap, relative to PTATffBT and PTATBT-8. The HOMO level of PTAT2BT-8 (−5.32 eV) is significantly shallower compared to its counterparts. This can be attributed to increased intramolecular charge transfer along the polymer backbone; a consequence of the incorporation of additional thiophene spacer units. The photovoltaic properties of the polymers were investigated by fabricating bulk heterojunction (BHJ) polymer solar cells using PC₇₀BM as the electron acceptor. PTATffBT displayed limited solubility in common organic solvents and could not be used for the fabrication of photovoltaic cells. Optimised photovoltaic devices fabricated from PTATBT-8 and PTAT2BT-8 demonstrated power conversion efficiencies of 2.36% and 3.15%, respectively. PTAT2BT-8 provided better efficiencies chiefly as a result of better J_sc and FF values.