Dye-sensitized solar cells based on ZnO nanotetrapods

Abstract

In this paper, we reviewed recent systematic studies of using ZnO nanotetrapods for photoanodes of dye-sensitized solar cells (DSSCs) in our group. First, the efficiency of power conversion was obtained by more than 3.27% by changes of conditions of dye loading and film thickness of ZnO nanotetrapod. Short-circuit photocurrent densities (J_sc) increased with the film thickness, J_sc would not be saturation even the film thickness was greater than 35 μm. The photoanode architecture had been charactered by good crystallinity, network forming ability, and limited electron-hopping interjunctions. Next, DSSCs with high efficiency was devised by infiltrating SnO₂ nanoparticles into the ZnO nanotetrapods photoanodes. Due to material advantages of both constituents described as above, the composite photoanodes exhibited extremely large roughness factors (RFs), good charge collection, and tunable light scattering properties. By varying the composition of the composite photoanodes, we had achieved an efficiency of 6.31% by striking a balance between high efficiency of charge collection for SnO₂ nanoparticles rich films and high light scattering ability for ZnO nanotetrapods rich films. An ultrathin layer of ZnO was found to form spontaneously on the SnO₂ nanoparticles, which primarily was responsible for enhancing open-circuit photovoltage (V_oc). We also identified that recombination in SnO₂/ZnO composite films was mainly determined by ZnO shell condition on SnO₂, whereas electron transport was greatly influenced by the morphologies and sizes of ZnO crystalline additives. Finally, we applied the composite photoanodes of SnO₂ nanoparticles/ZnO nanotetrapods to flexible DSSCs by low temperature technique of “acetic acid gelation-mechanical press-ammonia activation.” The efficiency has been achieved by 4.91% on ITO-coated polyethylenenaphtalate substrate. The formation of a thin ZnO shell on SnO₂ nanoparticles, after ammonia activation, was also found to be critical to boosting V_oc and to improving inter–particles contacts. Mechanical press, apart from enhancing film durability, also significantly improved charge collection. ZnO nanotetrapods had been demonstrated to be a better additive than ZnO particles for the improvement of charge collection in SnO₂/ZnO composite photoanodes regardless of whether they were calcined.

Keywords

dye-sensitized solar cell (DSSC) / metal oxides / nanostructure / ZnO nanotetrapod / photoanode / flexible solar cell

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Acknowledgements

This work was supported by the Research Grants Council of Hong Kong under the General Research Funds (No. 604809). We wish to thank all those who contributed to the work reviewed here.

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