Abstract
The role of wide band gap oxide thin layer in inverted structure polymer solar cells was investigated by employing oxide films of TiO2 and Nb2O5approximately 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 TiO2 buffer layer leads to power conversion efficiency as high as 2.8%. As for Nb2O5, 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 TiO2. 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.
Highlights
Photovoltaic cells based on conjugated polymer and fullerene bulk heterojuction composites have attracted much attention for renewable energy due to their promising properties such as low production cost, their lightness, light weight, mechanical flexibility and the possibility of fabricating them on large area [1,2,3,4,5]
The role of wide band gap oxide thin layer in inverted structure polymer solar cells was investigated by employing oxide films of TiO2 and Nb2O5 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
Summary
Photovoltaic cells based on conjugated polymer and fullerene bulk heterojuction composites have attracted much attention for renewable energy due to their promising properties such as low production cost, their lightness, light weight, mechanical flexibility and the possibility of fabricating them on large area [1,2,3,4,5]. In order to overcome these problems, the inverted device structures is an alterative solution to improve the durability, because it uses a more air-stable high work function electrode such (Ag, Au) as back contact to collect holes while using an inorganic semiconductor for buffer layer to collect electrons. In this work we report the fabrication of inverted polymer solar cells and investigate the role of thin dense metal oxide films in the inverted polymer solar cells using dense film of TiO2 and Nb2O5. These two oxides were purposely selected in view of the bottom of conduction band of TiO2 lower than the LUMO level of PCBM and that of Nb2O5 higher than the LUMO level of PCBM. It was found that such a very thin metal oxide films between the electron collecting electrode and active layers was necessary to promote the formation of continuous uniform PCBM film and block the holes in P3HT from being recombined with the electrons in collecting electrode
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