NOVELTY - Substituted oligothiophene compounds (I) are new.    USE - (I) are useful in photovoltaic devices, dye sensitized solar cells (all claimed), photodetectors, organic solar cells, bulk heterojunction organic solar cells and bilayer organic solar cells.    ADVANTAGE - (I) are capable of charge transportation and exhibit improved absorption of visible light than current oligothiophene-based materials.    DETAILED DESCRIPTION - Substituted oligothiophene compounds of formula (I) are new.    Ar1 = aromatic group and heteroaromatic group (both optionally substituted); either    R1, R2 = 1-20C alkyl, 3-8C cycloalkyl, aromatic group or heteroaromatic group (all optionally substituted); or    NR1R2 = optionally saturated ring (optionally substituted, optionally containing heteroatoms including O, N or S, and optionally further fused to at least one other ring);    L = linker including 2C alkenylene or 2C alkynylene (both optionally substituted) or direct bond;    T1 = substituted thiophene moiety of formula (a1), substituted thieno(3,2-b)thiophene moiety of formula (a2), substituted dithieno(3,2-b;2',3'-d)thiophene moiety of formula (a3) or substituted 4H-dithieno(3,2-b;2',3'-d)pyrrole moiety of formula (a4); either    R3, R4 and R9 = 1-10C alkyl, 3-8C cycloalkyl or 1-10C alkoxy (all optionally substituted) or H; or    pair of R3, R4 and R9 = optionally saturated ring (optionally substituted, optionally containing at least one heteroatom including O, N or S, and optionally further fused to at least one other ring);    R5 = 1-8C alkyl, 3-8C cycloalkyl or aromatic group (all optionally substituted) or H;    R6 = 1-8C alkyl, 1-8C perfluorinated alkyl, 3-8C cycloalkyl, aromatic group or heteroaromatic group (all optionally substituted);    R7 = 1-30C alkyl, 2-12C alkenyl, 2-8C alkynyl (at least one C of alkyl, alkenyl and alkynyl is optionally replaced with at least one O, S, NR8, carbonyl or thiocarbonyl), 3-8C cycloalkyl, 3-12C alkoxy, aromatic group or heteroaromatic group (all optionally substituted);    R8 = H or R6; and    n = 1-10.    INDEPENDENT CLAIMS are also included for:    (1) preparation of (I);    (2) a photovoltaic device comprising a first electrode, a second electrode, and an active material in electrical contact with the first and second electrodes, where the device generates an electrical potential upon the absorption of photons, and the active material comprises (I) and a second material which is a charge accepting material; and    (3) a dye sensitized solar cell comprising an anode (2), a cathode (4), a charge accepting material on one electrode, (I) in contact with the charge accepting material, and a charge transport material in contact with (I) and the other electrode.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic illustration of a photovoltaic device in the form of a bilayer photovoltaic cell.    Bilayer organic solar cell (1)    Anode (2)    Transparent thin film support (3)    Cathode (4)    Electron acceptor material (6)