TiO2 Nanowires/ZnO Nanosheets Hierarchically Structure on Ti Foil as Flexible Photoanodes for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have attracted much attention of researchers since it was reported first in 1991¹. The highest power conversion efficiency of DSSCs has reached over 12% until now². The grain boundaries of nanocrystals in the TiO₂ nanoparticle photoanode usually used could cause slow electron mobility. To overcome the slow electron transport of nanocrystalline TiO₂, researches focused on the ordered one-dimensional (1D) structures. These 1D structures provided direct pathways for injected electrons and then improved electron transport³.

However, DSSCs based on 1-D structured photoanode have not obtained expected extremely high power conversion due to the much smaller surface area, which resulted in smaller dye-loading compared with nanoparticles. To further improve the photovoltaic performance of DSSCs, constructing hierarchical morphologies based on 1-D structures and nanoparticles or nanosheets turns out to be an effective way⁴. The hierarchical structure could enhance the dye-loading and maintain the fast electron transport at the same time.⁵

In this work, a novel TiO₂ Nanowires/ZnO Nanosheets (TiO₂ NW/ZnO NS) hierarchical structure has been fabricated on Ti foil substrate through a two-step process of hydrothermal and homogenous precipitation, which is much easier and lower-power consumption than commonly two-step hydrothermal method⁶.

As shown in Fig. 1 A and B, the obtained TiO₂ NWs are well ordered 1-D structure, forming TiO₂ NWs arrays on Ti foil substrate. The length of TiO₂ NWs is about 12 μm, and the diameter of TiO₂ NWs is about 100 nm. After the homogenous precipitation process, a TiO₂ NW/ZnO NS hierarchical structure has been fabricated. The diameter of NWs increased clearly with ZnO NS attaching, enhancing to about 400 nm, which is about 4 times of TiO₂NWs, which could increase the surface area and dye-loading significantly.

As shown in Fig. 1 E, DSSCs device based on such structured flexible photoanode showed a significantly improvement of power conversion efficiency compared to that based on TiO₂ NWs, which increased from 0.80% to 1.41% under 100 mW cm^-2 illumination, enhancing 76.2% relatively. The improvement of conversion efficiency is mainly due to the much larger surface area and resulting increased dye-loading amount and photocurrent of TiO₂ NW/ZnO NS hierarchical structure.

References

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