• ZnO hierarchical structures for efficient quasi-solid dye-sensitized solar cells
Dye-sensitized solar cells (DSCs) have been considered as a promising alternative to conventional silicon-based solar cells owing to their low cost and relatively high efficiency. In DSCs, mesoporous TiO2 is usually employed as the photoanode material. In recent years, ZnO nanostructures have been extensively investigated as a promising photoanode material for DSCs because they possess a similar band-gap structure, excellent electrical properties and flexibly controlled morphologies. However, the photoanodes simply made from ZnO nanoparticles (NPs) usually suffer from deficient light-scattering even with adequate dye-loading being offered.
ZnO hierarchically structured photoanodes have been known to be more efficient for DSCs since dye-loading and light-scattering can be optimized simultaneously. However, most of the ZnO hierarchical structures so far reported were constructed by simply packing the primary nano-sized particles together.12,13 In this case, the photogenerated electrons still need to hop across massive boundaries before being collected, leading to a serious charge recombination and a low electron collection efficiency.
We dreveloped a direct precipitation method for mass production of ZnO microflowers (MFs, see Fig,1) containing hierarchical structures. The ZnO MFs are constructed by interlaced single crystalline and porous nanosheets which are ideal photoanode material for dye-sensitized solar cells (DSCs) because the MFs can largely improve the energy harvesting performance and the efficiency of DSCs. Compared with other forms of nano-sized structures, the novel hierarchical structures show obvious advantages in DSC application because of their large surface area for dye-loading, good light scattering efficiency and excellent electrical transport property. The quasi-solid state DSCs fabricated with the MF hierarchical structures (Fig.2) exhibited an efficiency of 4.12%, much higher than that of ZnO nanoparticle-based DSCs, indicating a great potential for the development of highly-efficient quasi-solid DSCs.
Fig.1 (a) Hierarchical ZnO spheres; (b) Interlaced ZnO nanoribbons formed 3D network; (c) Schematic diagram of the DSCs configuration using ZnO hierarchical ZnO nanostructures.
Fig. 2 (a) and (b) Cross-sectional SEM images of the NP-based and MF-based photoanodes respectively; (c) schematic diagrams of the NP-based and MF-based photoanode structures; (d) J–V curves of the quasi-solid DSCs fabricated using NP- and MF-based photoanodes
See details: - Chun Cheng, Yantao Shi, Chao Zhu, Wei Li, Lin Wang, Kwok Kwong Fung and Ning Wang, Phys. Chem. Chem. Phys. 13 (2011)10431.
• Optimizing nanosheet-based ZnO hierarchical structure through ultrasonic-assisted precipitation for remarkable photovoltaic enhancement in quasi-solid dye-sensitized solar cells (Efficiency = 6.19%)
We introduce ultrasonic-assisted precipitation for fabricating novel nanosheet-based ZnO hierarchical flowers (HFs) in aqueous solution. With the powerful ultrasound irradiation, these nanosheets on the HFs are not only interlaced and monocrystalline, but also axially oriented, porous and ultrathin. Furthermore, broad channels enclosed by adjacent nanosheets can deeply extend into the inner parts of the HFs. Structural improvements reveal that the specific area of the novel HFs as well as their performances on light-capturing and electron transport have been largely improved compared with those prepared through direct precipitation. Remarkably, when assembled into quasi-solid DSCs, ZnO HF photoanodes show a high conversion efficiency up to 6.19% (under AM 1.5, 100 mW cm2 illumination), the highest record of quasi-solid ZnO-based DSCs up to now.
Please see details:
• Yantao Shi, Chao Zhu, Lin Wang, Wei Li, Chun Cheng, Kin Ming Ho, Kwok Kwong Fung and Ning Wang*, “Optimizing nanosheet-based ZnO hierarchical structure through ultrasonic-assisted precipitation for remarkable photovoltaic enhancement in quasi-solid dye-sensitized solar cells”, JOURNAL OF MATERIALS CHEMISTRY Volume: 22 Issue: 26 Pages: 13097-13103 .
• Chao Zhu, Yantao Shi, Chun Cheng, Lin Wang, Kwok Kwong Fung, and Ning Wang, "Correlation between theMorphology and Performance Enhancement of ZnO Hierarchical Flower Photoanodes in Quasi-Solid Dye-Sensitized Solar Cells", Journal of Nanomaterials,Volume 2012, Article ID 212653, doi:10.1155/2012/212653.
• Yantao Shi, Chao Zhu, Lin Wang, Wei
Li, Kwok Kwong Fung and Ning Wang, “Asymmetric ZnO Panel-like
Hierarchical Architectures with Highly Interconnected Pathways for Free
Electron Transport and Photovoltaic Improvements”, Chemistry – A
European Journal, 19(1), (2013) 282-7. doi: 10.1002/chem.201202527.
• Shi, YT, Zhu, C, Wang, L, Zhao, CY , Li, W, Fung, KK, Ma,
TL, Hagfeldt, A, Wang, N, “Ultrarapid Sonochemical Synthesis of ZnO
Hierarchical Structures: From Fundamental Research to High Efficiencies up to
6.42% for Quasi-Solid Dye-Sensitized Solar Cells”, CHEMISTRY OF MATERIALS 25(2013)1000-1012.