NOVELTY - A pi -electron conjugated copolymer composition comprises pi -electron conjugated random copolymer having a number average molecular weight of 30000-10000000 and pi -electron conjugated block copolymer containing polymer block (A) and polymer block (B) containing mutually different thieno(3,4-b)thiophene-containing monomeric units (I). The mass ratio of pi -electron conjugated random copolymer with respect to pi -electron conjugated block copolymer is 8:2-2:8.    USE - pi -Electron conjugated copolymer composition is used in organic semiconductor composition for forming layer of photoelectric conversion element (all claimed) used in organic thin film solar cell and optical sensor.    ADVANTAGE - The pi -electron conjugated copolymer composition has excellent electron-donating property and self-organization ability due to the presence of block copolymer having high crystallinity and cohesiveness, and provides organic semiconductor composition having excellent solubility with respect to solvent and controlled morphology due to the formation of ideal phase separation structure and photoelectric conversion element with excellent photoelectric transfer characteristics and high photoelectric conversion efficiency.    DETAILED DESCRIPTION - A pi -electron conjugated copolymer composition comprises pi -electron conjugated random copolymer having a number average molecular weight of 30000-10000000 and pi -electron conjugated block copolymer containing polymer block (A) and polymer block (B) containing mutually different thieno(3,4-b)thiophene-containing monomeric units of formula (I). The mass ratio of pi -electron conjugated random copolymer with respect to pi -electron conjugated block copolymer is 8:2-2:8.    R1,R2=optionally substituted 1-18C alkyl, alkoxy, aryl, heteroaryl, alkylcarbonyl, or alkyloxycarbonyl;and    X1=H or halo.    An INDEPENDENT CLAIM is included for organic semiconductor composition, which contains the pi -electron conjugated copolymer composition and an electron-accepting material.