Guogang Xue

Fabrication of hierarchically assembled microspheres consisting of nanoporous ZnO nanosheets for high-efficiency dye-sensitized solar cells

Porous nanosheet-assembled ZnO microspheres (PNMSs) were synthesized via one-pot hydrothermal treatment followed by calcination and were used as photoanodes for dye-sensitized solar cells (DSSCs). An overall light conversion efficiency of up to 5.16% has been achieved with this unitary material and structure. Superior to the referenced porous nanosheet-assembled ZnO microflowers (PNMFs) and porous dispersed ZnO nanosheets (PDNs), as well as precedent nanostructures, the present PNMS fully accommodates all indispensable characteristics for a high-performance DSSC through: (1) the hierarchical ZnO microsphere generates a prominent aggregation-induced light scattering center, which is in favor of enhancing the light absorption and the light propagation; (2) the single crystal nature of the nanosheet enhances the transport of injected electrons along the nanosheet, providing a direct electron pathway throughout the film and lowering the hole–electron recombination; (3) the thin thickness and porous character of the nanosheets results in a large surface area for dye adsorption; (4) intersectional contact with one another of the nanosheets of neighboring spheres increases the transport channel of the injected electrons through the adjacent spheres, avoiding the high resistance existing in the nanoparticle-based sphere aggregates due to the relatively small contact area among the aggregates; (5) these spherical assemblies form large external pores in the photoelectrode film, thus providing a “main trunk” for the quick electrolyte diffusion throughout the ZnO layer in the film. Simultaneously, the nanopores embedded in the nanosheet act as “branch lines” for more efficient electrolyte diffusion into the interstices of the densely packing nanosheets in the sphere.

Vertically building Zn2SnO4 nanowire arrays on stainless steel mesh toward fabrication of large-area, flexible dye-sensitized solar cells

Zn(2)SnO(4) nanowire arrays were for the first time grown onto a stainless steel mesh (SSM) in a binary ethylenediamine (En)/water solvent system using a solvothermal route. The morphology evolution following this reaction was carefully followed to understand the formation mechanism. The SSM-supported Zn(2)SnO(4) nanowire was utilized as a photoanode for fabrication of large-area (10 cm × 5 cm size as a typical sample), flexible dye-sensitized solar cells (DSSCs). The synthesized Zn(2)SnO(4) nanowires exhibit great bendability and flexibility, proving potential advantage over other metal oxide nanowires such as TiO(2), ZnO, and SnO(2) for application in flexible solar cells. Relative to the analogous Zn(2)SnO(4) nanoparticle-based flexible DSSCs, the nanowire geometry proves to enhance solar energy conversion efficiency through enhancement of electron transport. The bendable nature of the DSSCs without obvious degradation of efficiency and facile scale up gives the as-made flexible solar cell device potential for practical application.

Interfacial modification of photoelectrode in ZnO-based dye -sensitized solar cells and its efficiency improvement mechanism

A ZnO compact layer prepared by a sol–gel method was introduced into a photoelectrode at the interface between fluorine-doped tin oxide (FTO) substrate and a mesoporous ZnO layer in ZnO-based dye-sensitized solar cells (DSSCs). The ZnO compact layer was characterized by field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and UV-Vis spectroscopy. The compact layer increased the photoelectric conversion efficiency of ZnO-based DSSCs by 20%. An electrochemical impedance spectroscopy (EIS) study demonstrated that the compact layer strikingly reduced the interfacial resistance in the device by enhancing the conductive contact between nanocrystalline ZnO and FTO substrate. The photocurrent density–voltage characteristics in the dark suggests that the compact ZnO layer also plays the role of a blocking layer suppressing the charge recombination, which is illustrated by the suppression of dark current density. The two effects effectively elevate the short circuit current density (JSC) and open circuit voltage (VOC), and finally improve the overall conversion efficiency of ZnO-based DSSCs.

Understanding of the chopping frequency effect on IPCE measurements for dye-sensitized solar cells: from the viewpoint of electron transport and extinction spectrum

The incident monochromatic photon to electron conversion efficiency (IPCE) is an essential characterization method for the photoelectrical performance of solar cells. An IPCE measurement apparatus involving alternating current (ac) and direct current (dc) methods was set up. A chopping frequency effect on IPCE measurements was found for dye-sensitized solar cells (DSSCs), that is, with the increase in chopping frequency, the IPCE spectrum decreased significantly, and the different bands of the IPCE spectrum declined to different degrees. The chopping frequency effect was studied in detail by measuring the short-circuit current waveform, the extinction spectrum of the dye-coated TiO2 photoelectrode film and electrochemical impedance spectroscopy. The mechanism of the chopping frequency effect was investigated from the electron transport and extinction spectrum. The electron transport properties of the TiO2 photoelectrode film determined the slow response of DSSCs. From the extinction spectrum, the transport distance of electrons in the TiO2 film varied under the illumination of different monochromatic light. For DSSCs, the ac method was remarkably influenced by the trap states of electrons and the optical penetration depth, while the dc method was a steady-state measurement avoiding the impact of these two factors. Thus, the dc method is more suitable than the ac method for IPCE measurements of DSSCs.