**4. Conclusion**

*Nanostructures*

D149-sensitized ZnO DSCs and could also be rationalized as a negative shift in edge of ZnO conduction band caused by D205 dye adsorption. The alkyl chain of the terminal rhodanine moiety of D205 has been extended from ethyl to octyl, which

electrons injection into the photoelectrodes [53]. The dyes with the hydrophobic alkyl chains principally not only could form a barrier layer on the sensitizer dye to protect the dye layer against water intrusion from the electrolyte but also rearrange

to the electron recombination resistance which means a superior circuit in D205 sensitized ZnO DSCs than that of D149-sensitized ZnO DSC. Like **Figure 12**, some interior parameters of the devices can be further derived by well-fitting the impedance data based on the modified equivalent circuit of DSCs as shown in **Figure 20**. The detail parameters are listed in **Table 3**. The electron loss rate *keff* in the D205-sensitized ZnO DSCs is smaller than the D149-sensitized ones, which causes the prolonged electron lifetime *τeff* in the D205-sensitized ZnO DSCs. The larger charge-transfer resistance *Rk* value for D205-sensitized ZnO DSCs indicates the less interfacial recom-

As shown in **Figure 19b**, the obvious middle semicircle of the Nyquist plots belongs

Moreover, the effective electron diffusion coefficient *Deff* is also enhanced with utilization of D205 sensitizer. It is reasonable that the photocurrent density may be directly affected by changes in the electron recombination rate. The amphiphilic D205 may help the formation of a self-assembled dye monolayer that prevents photoelectrons from being resorbed by triiodide ions in the electrolyte, resulting in a higher *Voc* and *Jsc* [54].

<sup>−</sup> and efficient

−

of electrolyte.

cause the effective suppression of electron recombination between I3

bination occurring between the injected electrons of ZnO and the I3

*The equivalent circuit model of ZnO DSCs composed with hierarchical nanoparticles [50].*

*FF* **η (%)**

*keff* **(s<sup>−</sup><sup>1</sup> )**

D149 10.94 0.641 0.71 4.95 57.85 17.29 12.55 2.47 2.14 × 10<sup>−</sup><sup>3</sup> D205 12.17 0.653 0.67 5.34 47.12 21.22 14.43 1.98 2.51 × 10<sup>−</sup><sup>3</sup>

*Performances and electron transport properties of the D149- and D205-sensitized DSCs (27-μm-thick ZnO* 

**τ***eff* **(ms)**

*Rk* **(Ω)**

*R<sup>w</sup>* **(Ω)**

*Deff* **(cm2 /s)**

dyes that are more perpendicular to the ZnO surface.

**56**

**Table 3.**

**Figure 20.**

**ZnO DSCs**

*Jsc* **(mA/ cm2 )**

*Voc* **(V)**

*photoelectrode) determined by J-V characteristics and EIS analysis [50].*

Low-dimensional NPs, QDs, and NWs have attracted considerable attention owing to their interesting physical and chemical properties. ZnO NWs can shed the light to conduct electronic, optoelectronic, electrochemical, and electromechanical devices with nanoscale dimensions because of the excellent electrical transport and photonic interconnection due to their crystallinity. ZnO QDs and NPs are of great interest because of the three-dimensional confinement of carrier, and phonon leads not only continuous tuning of the optoelectronic properties but also improvement in device performance. As a wide-bandgap semiconductor, ZnO has been reported as an alternative for DSCs because ZnO offers a large direct bandgap which is similar to TiO2 and even higher electron mobility. ZnO also can be tailored to various nanostructures that provides a promising means for improving the performance of the photoelectrode in DSCs. It is of great urgency to effectively design and control the process window that enables the seamless integration not only the mass production but reproducibility of ZnO nanostructures for the near future.
