Xiaoyan Gan

Solution-derived 40 µm vertically aligned ZnO nanowire arrays as photoelectrodes in dye-sensitized solar cells

Well-aligned ZnO nanowire arrays with a long length of more than 40 microm were prepared successfully by using the polyethylenimine (PEI)-assisted preheating hydrothermal method (PAPHT). Several important synthetic parameters such as PEI content, growth time, preheating time and zinc salt concentration were found to determine the growth of ultralong ZnO nanowire arrays, including length, diameter, density and alignment degree. The photoluminescence (PL) spectrum of as-grown ultralong ZnO nanowire arrays revealed a UV emission and a yellow emission, which was attributed to the absorbed hydroxyl group based on the peak shift after annealing in various atmospheres. The performance of dye-sensitized solar cells (DSSCs) increased with increasing length of ZnO nanowire arrays, which was mainly ascribed to the aggrandized photocurrent and reduced recombination loss according to electrochemical impedance spectroscopy (EIS). A maximum efficiency of 1.3% for a cell with a short-circuit current density (J(sc)) = 4.26 mA cm(2), open-circuit voltage (V(oc)) = 0.69 V and (fill factor) FF = 0.42 was achieved with a length of 40 microm.

Toward Hierarchical TiO2 Nanotube Arrays for Efficient Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) have been considered as one of the low cost alternatives for conventional silicon solar cells since the fi rst application of transparent TiO 2 nanocrystalline fi lms in 1990s. [ 1 ] Light-harvesting ability and internal carrier collection effi ciency via the sensitized photoanode constitute the key factors that determine the performances of DSCs. Effi ciencies exceeding 11% have already been achieved in laboratories with the sensitizers of N719 and C101, [ 2 ] whereas the overall performances are still lower than the predicted values due to poor utilization of near infrared photons and recombination losses in the random network of TiO 2 nanocrystallines. [ 3 ]

A facile route to aligned TiO2 nanotube arrays on transparent conducting oxide substrates for dye-sensitized solar cells

TiO2 nanotube arrays (NTAs) on transparent conducting oxides (TCO) have attracted great attention due to the potential application in dye-sensitized solar cells (DSCs). Here, we introduce the template-assisted process for direct fabrication of aligned TiO2 NTAs on TCO substrates, involving layer-by-layer adsorption and reaction (LBL-AR) assembled TiO2 coating on ZnO nanorods (NR). Key factors of the fabrication process on the microstructures of TiO2 NTAs are analyzed, and the geometry effects of TiO2 NTAs on the performance and electron transport properties of DSCs are investigated by using electrochemical impedance spectra (EIS). An efficiency of 4.25% (under AM1.5 irradiation, 100 mW cm−2) is obtained from N719 sensitized 20 μm thick TiO2 NTAs with a wall thickness of 20 nm, with Jsc = 8.2 mA cm−2, Voc = 0.81 V and FF = 63%.

Coaxial multi-shelled TiO2 nanotube arrays for dye sensitized solar cells

The performance of one-dimensional (1D) TiO2 nanotube based dye-sensitized solar cells (DSCs) was limited by the insufficient surface area of TiO2 nanotubes. To solve this issue, coaxial multiple-shelled TiO2 nanotube arrays were successfully synthesized on the transparent conductive oxide (TCO) substrates by using improved ZnO nanorod template assisted layer by layer absorption and reaction (LbL-AR) technique. To fabricate tube-in-tube nanostructures, LbL-AR TiO2 coatings were successively deposited on the exterior walls of the ZnO nanowires and the sacrificial sol–gel ZnO spacers, which were removed together by selective etching to form the hollow tubal structures. The performance of dye-sensitized solar cells (DSCs) increases with increasing the shell number of multi-shelled TiO2 nanotube photoanodes, attributed to the increase of the surface area, which was confirmed by N2 adsorption-desorption isotherms and the dye-loading capacities. A maximum efficiency of 6.2% was achieved for a quintuple shelled TiO2 nanotube photoanode with a short-circuit current density (Jsc) = 15 mA cm2, open-circuit voltage (Voc) = 0.73 V and fill factor (FF) = 0.57.

Aerogel based SiO2-TiO2 hybrid photoanodes for enhanced light harvesting in dye-sensitized solar cells

It is intriguing and challenging to utilize aerogels with a very high surface area to enhance the light-harvesting capacity of a photoanode. Here we demonstrate highly efficient aerogel-modified photoanodes by modifying both the aerogels microstructure and the photoanodes functions. A low-cost sol–gel and ambient-drying route is developed to fabricate high-quality SiO2 and SiO2–TiO2 hybrid aerogels with tunable surface area (500–1170 m2 g−1) and packing density (0.03–0.27 g cm−3). Two series of hybrid photoanodes integrating SiO2–TiO2 aerogel microparticles and TiO2 nanocrystallites are designed, to overcome the intrinsic weakness of the mesoporous structure in the electron-transporting property. The aerogel-modified photoanodes yield significantly higher photocurrent densities than the TiO2 nanocrystallite photoanode due to their increased dye-loading capacity and the enhanced visible light scattering effect. A distinctive interaction enhancement of the cell efficiency between the dye-loading and the light-scattering effect induced by the aerogel microparticles is observed. At optimal conditions (aerogel: 0.202 g cm−3 packing density, 15 wt% incorporation), the primitive aerogel-modified photoanode exhibits a conversion efficiency of 7.57%, much higher than the pure TiO2 counterpart (3.67%). When optimized with TiCl4 treatment and a higher film thickness, it shows the highest efficiency of 9.41%, 16% higher than the TiCl4-treated TiO2 photoanode modified with a conventional scattering layer. The work affords a simple, powerful and versatile route towards highly efficient hybrid photoanodes utilizing mesoporous materials, and also shows great promise for extension to other applications such as supercapacitors or photocatalysis.

TiO2 nanorod arrays functionalized with In2S3 shell layer by a low-cost route for solar energy conversion.

We report the fabrication and characterization of a TiO(2)-In(2)S(3) core-shell nanorod array structure for application of semiconductor-sensitized solar cells. Hydrothermally synthesized TiO(2) nanorod arrays on FTO glass substrates are functionalized with a uniform In(2)S(3) shell layer by using the successive ion layer adsorption and reaction (SILAR) method. This low-cost technique promotes a uniform deposition of In(2)S(3) nanoshells on the surface of TiO(2) nanorods, thus forming an intact interface between the In(2)S(3) shell and TiO(2) core. Results show that the thickness of In(2)S(3) shell layers as well as the visible light absorption threshold can be effectively controlled by varying the coating cycles during the SILAR process. The best reproducible performance of the sandwich solar cell using the TiO(2)-In(2)S(3) core-shell nanorod arrays as photoelectrodes was obtained after 30 SILAR cycles, exhibiting a short-circuit current (I(sc)) of 2.40 mA cm(-2), an open-circuit voltage (V(oc)) of 0.56 V, a fill factor (ff) of 0.40 and a conversion efficiency (η) of 0.54%, respectively. These results demonstrate a feasible and controllable route towards In(2)S(3) coating on a highly structured substrate and a proof of concept that such TiO(2)-In(2)S(3) core-shell architectures are novel and promising photoelectrodes in nanostructured solar cells.

Branched double-shelled TiO2 nanotube networks on transparent conducting oxide substrates for dye sensitized solar cells

A novel three-dimensional branched double-shelled TiO2 nanotube network (3DNTNs) structure was constructed on transparent conducting oxide substrates (TCO) by using a ZnO-nanorod array template-assisted method. Various morphological features, such as branch length and population density, could be easily tailored by simply modifying the growth time. The performances of dye-sensitized solar cells (DSCs) fabricated with TiO2 3DNTNs are higher than those assembled with 1-dimensional nanotube arrays. The hollow branches filling in the spaces between the double-shelled stem nanotubes increase the amount of dye-loading, resulting in an enhanced light-harvesting ability.

Nanocrystalline/nanoporous ZnO spheres, hexapods and disks transformed from zinc fluorohydroxide, their self-assembly and patterned growth

The synthesis of ZnO nanostructures via the transformation strategy from zinc-based compounds has long been hampered by the difficulties in obtaining high quality nanostructures. Herein we report a simple and effective method toward nanocrystalline/nanoporous ZnO nanostructures with well-defined shapes transformed from zinc fluorohydroxide Zn(OH)F deposited on Si substrate. The traditional reaction route of zinc chloride-hexamethylenetetramine system toward ZnO was deviated by the use of fluorion (F−) additive through a simple solvothermal process in H2O–ethanol component solvent, resulting in the formation of Zn(OH)F crystals. Three kinds of micron shape (sphere, hexapod and hexagonal disk) and two kinds of microstructure (nanocrystalline and nanoporous) were demonstrated, merely by varying the concentration of F− ions and the sintering temperature. While the concentration of F− ions directly determined the phase and morphology of the as-deposited nanostructures, the ultra smooth nature of the Si substrate and the component solvent affected significantly the nucleation, the self-assembly behavior and the crystallinity of each Zn(OH)F prism. The formation mechanism of Zn(OH)F hexapod was discussed from two points, the Si-controlled nucleation on already-formed particles, and the [F−]-dependent crystallization process. Furthermore, the self-assembly of multiple Zn(OH)F/ZnO disks and the selective growth of Zn(OH)F hexapods was demonstrated by reducing the growth rate and by inducing the nucleation on the patterned gold layer, respectively.

Measurement of the valence band-offset in a PbSe/ZnO heterojunction by x-ray photoelectron spectroscopy

A heterojunction of PbSe/ZnO has been grown by molecular beam epitaxy. X-ray photoelectron spectroscopy was used to directly measure the valence-band offset (VBO) of the heterojunction. The VBO, ΔEV, was determined as 2.51 ± 0.05 eV using the Pb 4p3/2 and Zn 2p3/2 core levels as a reference. The conduction-band offset, ΔEC, was, therefore, determined to be 0.59 ± 0.05 eV based on the above ΔEV value. This analysis indicates that the PbSe/ZnO heterojunction forms a type I (Straddling Gap) heterostructure.