We synthesized a novel low-band-gap, conjugated polymer, poly[4,7-bis(3',3'diheptyl-3,4-propylenedioxythienyl)-2,1,3-benzothiadia zole] [poly(heptyl(4)-PTBT)], consisting of alternating electron-rich, diheptyl-substituted propylene dioxythiophene and electron-deficient 2,1,3-benzothiadiazole units, and its photovoltaic properties were investigated. A thin film of poly(heptyl(4)-PTBT) exhibited an optical band gap of 1.55 eV A bulk-heterojunction solar cell with indium tin oxide/poly(3,4-ethylenedioxythiophene)/ poly(heptyl(4)-PTBT): methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (1:4)/LiF/Al was fabricated with poly(heptyl(4)-PTBT) as an electron donor and PCBM as an electron acceptor and showed an open-circuit voltage, short-circuit current density, and power conversion efficiency of 0.37 V 3.15 mA/cm(2), and 0.35% under air mass 1.5 (AM1.5G) illumination (100 mW/cm(2)), respectively. A solid-state, dye-sensitized solar cell with a SnO2:F/TiO2/N3 dye/poly(heptyl(4)-PTBT)/Pt device was fabricated with poly(heptyl(4)-PTBT) as a hole-transport material. This device exhibited a high power conversion efficiency of 3.1%, which is the highest power conversion efficiency value with hole-transport materials in dye-sensitized solar cells to date. (c) 2006 Wiley Periodicals, Inc.