Improving the open-circuit voltage of alkylthio-substituted photovoltaic polymers via post-oxidation

Molecular designing of photovoltaic polymers based on benzodithiophene (BDT) building blocks for high power conversion efficiency (PCE) in polymer solar cells (PSCs) arouse much attention in the past few years. To meliorate the energy levels of photovoltaic polymers featuring alkylthio substituted BDT units, a novel post-polymerization oxidation method was proposed applied in converting sulfur atom into sulfonyl group on side chains of the pristine polymer PBT-S. After treating with tiny amount of meta-chloroperoxybenzoic acid (m-CPBA) and hydrogen peroxide (H2O2), two batches of the target polymers, namely, PBT-SO2-M and PBT-SO2-H were prepared for the first time, respectively. The photochemical and electrochemical results indicate that both the HOMO levels are distinctly dropped with almost no influence on band gaps by introducing strong electron-withdrawing sulfonyl groups on side chains of BDT. Accordingly, the photovoltaic results reveal that the V-oc of devices based on PBT-SO2-M and PBT-SO2-H are 0.81, 0.71 V which are 0.17 and 0.07 V higher than that of pristine polymer PBT-S, respectively. Moreover, the J(sc) and PCE of PBT-SO2-H devices are comparable with those of the devices based on PBT-S. Overall, this work suggests that the molecular energy levels of D-A copolymers can be effectively tuned by a post-oxidation method. (C) 2015 Elsevier B.V. All rights reserved.