Synthesis of conjugated D–A polymers bearing bi(dithienogermole) as a new donor component and their applications to polymer solar cells and transistors

Abstract

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:PC₇₁BM/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.