Alkylsulfanyl-bithiophene-alt-Fluorene:p-conjugated polymers for organic solar cells

We describe the synthesis of alternating bithiophene–fluorene copolymers (P1–P3) with different regiochemistry of the bithienyl unit and different alkylsulfanyl chain lengths. The structural, electrochemical and photophysical properties of these polymers were investigated by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), NMR, UV/Vis and photoluminescence (PL) spectroscopy and cyclic voltammetry (CV), and the polymers were used to assemble organic solar cells (OSCs), in combi-nation with the fullerene derivative methyl [6,6]-phenyl-C61- butyrate (PCBM). P3, with a head-to-head bithienyl unit, shows a broader absorption and a lower band gap with respect to P1 and P2, which have tail-to-tail bithienyl units. The PL intensity of P1–P3 is dramatically quenched in the presence of PCBM, demonstrating that an efficient charge transfer between donor and acceptor occurs. The best OSC device was obtained with P