Cyclopentadithiophene–benzothiadiazole copolymers with permutations of repeating unit length and ratios; synthesis, optical and electrochemical properties and photovoltaic characteristics

Abstract

Conjugated copolymers with varying ratios and lengths of cyclopentadithiophene (CPDT) to benzothiadiazole (BT) repeating units, –[(CPDT)_x–(BT)_y]_n–, have been synthesized by palladium complex catalysed Suzuki coupling polymerisation, direct arylation polymerisation or oxidative polymerisation using iron(III) chloride. The different permutations of the co-polymers allow for tuning of the optical and electrical properties. To our knowledge, this is one of the first studies of its kind for conjugated polymers for use in OPVs. The optical band gaps were measured between 1.7 and 2.0 eV in film, in which the polymer with ratios of CPDT : BT (x : y = 1 : 1) units showed the lowest bandgap followed by x : y = 1 : 2, 2 : 1, 2 : 2 and 3 : 3. Hole mobility and solar cell performance of these polymers with PC₆₁BM as the electron acceptor were measured. A relatively small variation in hole mobility was observed between polymers. However, the best reported power conversion efficiency (PCE) was measured at 2.5% when processed in the absence of any additives using the polymer with ratios of CPDT : BT (x : y = 2 : 2), which was followed by the ratios of x : y = 2 : 1, 1 : 1, 1 : 2 and 3 : 3, indicating that variation of the ratios and length of donor and acceptor units affect OPV performance and better performance is achievable by careful consideration of the donor-to-acceptor ratio.