The Role of Oxide Thin Layer in Inverted Structure Polymer Solar Cells

Abstract

The role of wide band gap oxide thin layer in inverted structure polymer solar cells was investigated by employing ox-ide films of TiO 2 and Nb 2 O 5 approximately 10 nm in thickness deposited onto FTO substrates. The experimental re-sults demonstrated that the thin oxide layer serving to separate the electron collecting electrode and the photoactive film of a blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) was necessary to promote the formation of continuous uniform PCBM film to block holes in P3HT from being recombined with electrons in collecting electrode. A use of TiO 2 buffer layer leads to power conversion efficiency as high as 2.8%. As for Nb 2 O 5 , in spite the fact that its conduction band is higher than the LUMO level of PCBM polymer acting as electron transport material, a power conversion of 2.7%, which was only slightly different from the 2.8% achieved for the cell employing TiO 2 . These experimental results suggest a tunneling mechanism for the electrons to transport from the PCBM to col-lecting electrode over the oxide film, instead of a diffusion through the oxide film arising from either energy or concen-tration difference of the photogenerated electrons.