Significant progress has been achieved recently in developing dye-sensitized solar cells (DSSCs) [1] for low-cost solarpower devices using typical thick (∼ 12 lm) films of TiO 2 nanoparticles. [1][2][3] However, in TiO 2 nanoparticle-based DSSCs, the photoconverison efficiency is often limited by the disordered electrode morphology, which gives rise to interfacial interferences for electron transport. [4][5][6][7] To overcome this limitation, wide bandgap semiconducting oxides comprising 1D and 2D nanostructures have been proposed as promising solutions. [8][9][10][11][12][13][14] These include oriented single-crystalline ZnO nanowires, [8] quasi-ordered arrays of TiO 2 nanotubes, [15][16][17][18] core/shell nanostructures, [19][20][21][22] and 2D hollow structures assembled by colloidal templates.[23]In conjunction with these efforts, growing attention has also been paid to the importance of thin-film devices for use in DSSCs.[24] The key challenge here has been to enhance the surface area of thin-film electrodes. A TiO 2 electrode with a high surface area is necessary to effectively adsorb the dye and achieve a high photocurrent.[1] So far, however, there have been only a few reports on processing strategies designed to provide markedly enhanced surface activities and photocurrent efficiency for thin-film photoelectrodes (≤ 1-2 lm).[15] In particular, there are very few studies on TiO 2 electrodes prepared by physical vapor deposition (PVD), i.e., sputtering, which is a conventional method used in preparing thin films. [25,26] The sputtered film shows a dense columnar microstructure that provides efficient electron paths, a large internal surface area, and mitigates recombination processes. [25,27] Even though sputtered films give rise to a faster electron diffusion coefficient, the amount of dye they adsorbs is still small compared to nanoparticle-based TiO 2 electrodes because the sputtered films typically exhibit densely packed or pore-free morphologies. [25,26] In order to satisfy the requirements for fast electron transport and high surface area in thin-film photoelectrodes, we have combined colloidal templates and rf-sputtering to deposit quasi-ordered hollow TiO 2 hemispheres [28] on conducting glass substrates. This fabrication route produced monolayer-dispersed colloidal templates, providing flexible dimensional control over such features as colloidal diameter (hemisphere size) and shell thickness. Enhanced photoconversion efficiency was obtained due to the predominant role of the hollow structure in promoting electron transport [29] as well as a large surface area for enhanced dye loading. Moreover, the macroporous structure with hollow hemispheres allowed even viscous electrolytes to easily penetrate up to the glass substrate. In this work, the suitability of ordered hollow TiO 2 hemisphere films for highefficiency photoelectrodes in DSSCs was examined further. The scheme in Figure 1 illustrates the procedure used to fabricate the DSSCs used in this study. Detailed processing procedures are described...

Dye-sensitized solar cells (DSCs) are more spotlighted than conventional photovoltaic devices due to their relatively low cost, easy fabrication and high efficiency. However, there are limitations to increase the conversion efficiency of DSCs. The limiting factors are the quantity of dye adsorption and charge recombination between TiO 2 electrode and electrolyte. Coating other materials such as high energy band gap insulators or semiconductors on the TiO 2 electrode enhances dye adsorption and reduces charge recombination. We fabricated DSCs based on bare TiO 2 nanotube arrays and 0.02 and 0.04 M MgO coated TiO 2 nanotube arrays. MgO layer increased the photovoltage and photocurrent. The overall conversion efficiency of DSCs using 0.02 M MgO coated TiO 2 nanotubes was 1.61%. MgO formed insulating layers between TiO 2 nanotube array electrode and electrolyte. Charge recombination was inhibited at the interfaces of TiO 2 nanotube array electrode and electrolyte by MgO insulating layers. MgO coating also improved dye adsorption because iso-electric point (IEP) of MgO was larger than TiO 2 . When the IEP of coating material is larger than TiO 2 , the chemical attraction between the electrode surface and Ru-based dye molecule is increased.

For a working electrode in dye-sensitized solar cells (DSSCs), an anatase TiO 2 electrode with a hollow hemisphere structure (with diameters of 0.4, 1.5, and 5.0 μm) was fabricated using colloidal templating and RF-sputtering techniques. The experimental results of a short-circuit current density of 1.96 mA/cm 2 , an opencircuit voltage of 0.73 V, and a power conversion efficiency (AM 1.5) of 0.67% were obtained with a shell thickness of 300 nm, a hemisphere diameter of 0.4 μm, and a 100 nm thick blocking layer. The microstructural features, i.e. hemisphere diameter and shell thickness, played an important role in promoting the charge-collection ability.

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