The Role of Oxide Thin Layer in Inverted Structure Polymer Solar Cells
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Abstract
The role of wide band gap oxide thin layer in inverted structure polymer solar cells was investigated by employing oxide films of TiO<sub>2</sub> and Nb<sub>2</sub>O<sub>5</sub>approximately 10 nm in thickness deposited onto FTO substrates. The experimental results 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<sub>2</sub> buffer layer leads to power conversion efficiency as high as 2.8%. As for Nb<sub>2</sub>O<sub>5</sub>, 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<sub>2</sub>. These experimental results suggest a tunneling mechanism for the electrons to transport from the PCBM to collecting electrode over the oxide film, instead of a diffusion through the oxide film arising from either energy or concentration difference of the photogenerated electrons.
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