An acetylene polymer is formed on single-walled carbon nanotubes (SWNTs) using in situ polymerization. The acetylene polymers/SWNTs composite is hydrophilic even water-soluble, and has a structure of donor/acceptor dyad. In measurements of photocurrents-voltage curves, the composite film exhibits a power conversion efficiency of 1.50 x 10(-2%) under illumination (I = 80 mW/cm2, air mass 1.5 condition).