TiO2 Nanowires/ZnO Nanosheets Hierarchically Structure on Ti Foil as Flexible Photoanodes for Dye-Sensitized Solar Cells
Authors: Gao Rui , Cui Yixiu , Liu Xiaojiang , Wang Liduo , Cao Guozhong 
Published: February 2020

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.

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