Photovoltaic cells, with a conducting polymer/fullerene (C 60 ) interpenetrating heterojunction structure fabricated by spin-coating a conducting polymer onto a C 60 thin film, have been investigated and demonstrated a high efficiency as solar cells based on organic materials. The photovoltaic properties of the solar cells with a structure of indium-tin-oxide (ITO)/C 60 /poly(3-hexylthiophene) (PAT6)/Au have been improved by the insertion of a molybdenum trioxide (VI) (MoO 3 ) layer as a cathode buffer layer. In the solar cells with the structure of ITO/C 60 /PAT6/MoO 3 /Au, the energy conversion efficiency has been improved to 1.15% under AM1.5 (100 mW/cm 2 ) illumination.