Synthesis of conjugated D-A polymers bearing bi(dithienogermole) as a new donor component and their applications to polymer solar cells and transistorsElectronic supplementary information (ESI) available: Experimental procedure for monomer preparation, 1H NMR spectra of pDTG2-BT and pDTG2-PT, XRD patterns and AFM images of the polymer films optimized geometry, and LUCO and HOCO profiles of model polymer pDTG2-PT0, and response curves of TFTs with pDTG2-BT and pDTG2-PT]. See DOI: 10.1039/c4ra1674

New donor-acceptor π-conjugated polymers composed of alternating bi(dithienogermole) (DTG2) and benzo-(BT) or pyridinothiadiazole (PT) units ( pDTG2-BT or pDTG2-PT ) were prepared and their optical, electrochemical, photovoltaic, and semiconducting properties were investigated. pDTG2-BT showed two absorption maxima at 738 and 686 nm, respectively. Of the two, one at longer wavelength likely due to the aggregated polymer segments was relatively intensified, when the spectrum was measured in film. In contrast, pDTG2-PT showed one broad band at 686 nm, which shifted to longer wavelength at 748 nm in film. These results indicate stronger interchain interaction for pDTG2-BT . Cyclic voltammograms of the polymer films showed pseudo-reversible profiles and the anodic peak of pDTG2-BT appeared at lower potential than that of pDTG2-PT , indicating the higher-lying HOMO (Highest Occupied Molecular Orbital) for pDTG2-BT . The polymer electronic states were also examined by the quantum chemical calculations on the polymer models. Of those polymers, pDTG2-BT exhibited the higher photovoltaic performance. The power conversion efficiency of the cell (ITO/PEDOT:PSS/ pDTG2-BT :PC 71 BM/LiF/Al) reached 4.30% with V oc = 0.53 V, J sc = 12.68 mA cm −2 , and FF = 0.64, presumably reflecting the stronger interchain interaction, although pDTG2-PT afforded higher V oc , arising from its lower-lying HOMO. pDTG2-BT also exhibited higher performance as a thin film transistor material than pDTG2-PT . The performance of the cell with pDTG2-BT was also higher than that based on a similar alternating dithienogermole (DTG)-BT polymer, reported previously. Donor-acceptor π-conjugated polymers with alternating bi(dithienogermole) and benzo- or pyridinothiadiazole units were prepared and their potential applications to bulk heterojunction-type polymer solar cells and thin film transistors were explored.