NOVELTY - Light emitting thin film comprises a polymer compound (A) comprising fused thiophene- or fused furan-units (I)-(V) and having a polystyrene reduced number average molecular weight of 103-108.    USE - (A) is useful in a light emitting thin film, a conductive thin film or organic semiconductor thin film, which is useful in a polymer LED. (A) is useful in an ink composition. The polymer LED is useful in flat light source, and segment display apparatus or dot-matrix display apparatus and as back light of a liquid crystal display (all claimed), where the display devices are useful as a display of a computer, TV, portable terminal, portable telephone, car navigation, and view finder of a video camera. The polymer LED is useful for a light source of curved surface for lighting, and in flat-panel display of dot matrix. (A) is useful in charge transporting material. (A) is useful as coloring matter for lasers, material for organic solar cell, an organic transistor and a luminescent thin-film material which emits fluorescence or phosphorescence.    ADVANTAGE - (A) has excellent electronic transportation ability. (A) provides high performance polymer LED, which can be driven at a low-voltage and efficiently.    DETAILED DESCRIPTION - Light emitting thin film comprises a polymer compound (A) comprising fused thiophene- or fused furan-units of formula (I)-(V) and having a polystyrene reduced number average molecular weight of 103-108.    X = S or O;    R1-R10 = amino, silyl or carboxyl (all optionally substituted), alkyl, alkoxy, alkylthio, aryl, aryloxy, arylthio, arylalkyl, arylalkoxy, arylalkylthio, arylalkenyl, arylalknyl, halo, acyl, acyloxy, imino, amide, imide or monovalent heterocyclic group;    a, b = 0-3;    c-f = 0-5;    g-j = 0-7, where a+b, c+d, e+f, g+h and i+j = 1 or more.    INDEPENDENT CLAIMS are included for:    (1) a conductive thin film comprising (A);    (2) an organic semiconductor thin film comprising (A);    (3) a polymer light emitting device (LED) comprising at least one organic layer which is a thin film; and    (4) an ink composition comprising at least one polymer (A) and a solvent for processing a thin film.