A new series of donor-π-acceptor organic K-dyes based on thieno[2,3,a]carbazole moiety as a new electron donor, trithiophene as a π-linker and cyanoacrylic acid as an electron acceptor were designed and synthesized for achieving high performances in dye-sensitized solar cells (DSCs). The substituent effect and the molecular planarity for photophysical properties and DSC performances of K-dyes have been investigated, and the highest power conversion efficiency of 7.4% has been achieved. EIS and IMVS were employed to study the effect of molecular structure on charge transfer process and electron life time.