Fluorenone core donor-acceptor-donor pi-conjugated molecules end-capped with dendritic oligo(thiophene)s: synthesis, liquid crystalline behaviour, and photovoltaic applications

We have synthesized a new series of donor-acceptor-donor (D-A-D) pi-conjugated molecules, consisting of fluorenone core end-capped with dendritic oligo(thiophene)s of increasing generation (abbreviated as FG0, FG1, and FG2). In view of the application of these new organic semiconductors in photovoltaic devices, we have explored their spectroscopic, redox, and structural properties. The thermal behaviour of the new organic semiconductors was investigated by differential scanning calorimetry and polarized-light optical microscopy. Liquid crystalline behaviour has been found in the case of FG1, corresponding to a smectic ordering with a triclinic symmetry (Sm-obl) upon heating, as confirmed by variable temperature small-angle X-ray diffraction studies. In order to evaluate their photovoltaic performances, devices with an active area of 0.28 cm(2) were fabricated. Under AM1.5 simulated sunlight (100 mW cm(-2)) conditions, a device containing FG1/[70]PCBM blends showed a power conversion efficiency of ca. 0.8%.