C. T. Yip

Dye-sensitized solar cells using ZnO tetrapods

One dimensional (1D) ZnO nanostructures are of interest for applications in dye-sensitized solar cells (DSSCs) since they exhibit significantly improved electron transport compared to that in porous films. However, 1D nanostructures also have a significantly lower surface area than the porous films. Thus, the achieved solar cell efficiencies are typically much lower in spite of the improved charge transport. In this work, we investigated DSSCs based on ZnO tetrapods to achieve an increased surface area compared to that of 1D nanostructures. The cell performance as a function of the tetrapod film thickness and the dye used was studied. To further increase the surface area, mixed morphologies (tetrapods with nanoparticles) were also investigated. Under optimal conditions, an AM 1.5 power conversion efficiency of ∼1.2% was achieved.

Core‐shell ZnO solar cell application

In this study, the ZnO nanorod arrays coated with different oxide shells (TiOx, MgO, SnO2, and Al2O3) were fabricated. We compared the effect of various kinds of oxide layer coating (TiOx, MgO, SnO2, and Al2O3) on the performance of ZnO based solar cells. The thickness of the oxide shell layer is determined by transmission electron microscopy. Different kinds of oxide layers had a significant effect on the photovoltaic performance. The effect of different shells on the properties and performance of ZnO based core/shell nanorod array photoanode is discussed.

Surface modification of TiO 2 and ZnO nanosurfaces and applications

Nanomaterials are of great interest for a variety of practical applications. To achieve improved performance of nanomaterial-based devices it is essential to understand and modify their surface properties. Here we present an investigation of properties of nanosurfaces of ZnO and TiO2, and discuss their applications in photovoltaics and antibacterial coatings.

Effect of fabrication processes on bulk heterojunctions (BHJ) photovoltaic device performance

Organic photovoltaic (OPV) devices have attracted much interest in recent decades because they have a great potential for low cost solar cells. Among different kinds of organic solar cells, conjugated polymer/fullerene bulk heterojunction (BHJ) solar cells have exhibited improvements in the power conversion efficiency (PCE) in recent years. The performance of BHJ solar cells is highly dependent on different fabrication processes. To address this issue, we focus on the dependence of different photovoltaic parameters on the fabrication methods. BHJ solar cells fabricated using platinum metallopolyyne (P1) with a low band gap of 1.85 eV as an electron donor and phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor have been studied. The fabrication parameters, such as ratios of P1 to PCBM, solvents used, thickness of the active layers and top contact materials, have been systematically investigated. Blend ratio and solvent used had most significant influence on photovoltaic performance with several times higher efficiency of the best condition compared to the worst condition. They affected all photovoltaic parameters [open circuit voltage (Voc), short circuit current density (Jsc) and the fill factor (FF)]. Top contact materials affected the Voc and the FF, while thickness of the active layer mainly affected the Jsc and FF. The influence of different fabrication conditions on photovoltaic performance has been discussed.

Efficiency enhancement for ZnO tetrapod dye-sensitized solar cells by TiO 2 coating and ammonium treatment

Dye sensitized solar cells (DSSCs) have aroused much interest in recent years. However, the efficiency still needs improvement. In order to enhance the performance of the cells, it is important to increase the active surface area for dye adsorption so as to increase the light absorbance. In this work, zinc oxide (ZnO) tetrapods, which have four legs extending from a common core, were used to fabricate the photoanode due to its large surface area. Also, it is important to reduce the recombination rate and increase the carrier transport rate so that a higher efficiency can be achieved. Therefore, the effect on the performance of cells after coating a titanium oxide (TiO2) film and ammonium treatment on the ZnO nanostructures were investigated, too. The former helped suppress the recombination rate while the latter helped improve the carrier concentration and charge transport. Moreover, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy were performed on the as-grown and ammonia annealed samples. Under optimal conditions, AM 1.5 power conversion efficiency of 1.02% was achieved.

Hydrothermal growth of periodic ZnO nanorod arrays using polystyrene sphere templates

Zinc Oxide (ZnO) photonic crystals (PCs) are of great interest in recent years due to its potential applications in optoelectronic devices. However, most of the growth temperatures in fabricating ZnO nanostructures in periodic structures reported up to date are very high (>500°C), which is not favorable for low cost manufacture. In this work, a method involving low growth temperature (90°C), hydrothermal synthesis, was used to fabricate ZnO nanostructures. Self-assembly of polystyrene (PS) spheres was used to fabricate periodic arrays. PS spheres were self-assembled on the silicon (Si) or fluorine-doped tin oxide (FTO) substrate with ZnO seed layer derived from zinc acetate. After crosslinking the PS sphere layer, hydrothermal growth was performed. The influence of growth conditions (concentration, addition of PEI, etc.) on fabricated ZnO nanorod array has been studied. Then, PS spheres were removed by annealing in air, and fabricated ZnO nanorod arrays were studied by scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. Periodicity of the array was strongly substrate dependent (better on Si than on FTO). Light emitting diodes were also fabricated in order to demonstrate the potential use of this ZnO periodic array.

Titania nanotube array based photovoltaic cells

It has been shown that dye sensitized solar cells (DSSCs) based on porous titanium dioxide (titania) layers have efficiencies exceeding 10%. Although porous structure has the advantage of large surface area for light harvesting, electron transport through the random nanoparticle network forming a porous film results in electron mobilities which are two orders of magnitude lower compared to the single crystal materials. Therefore, considerable efforts have been made to fabricate DSSC based on one dimensional nanostructures, such as nanowires or nanotubes. Titania nanotube arrays are typically made by anodization of titanium, followed by annealing to improve crystallinity. In this work, we investigated the influence of annealing temperature and annealing atmosphere on the crystal structure, the electron transport, and the solar cell performance of titania nanotube arrays. The titania nanotube arrays were prepared from electrochemically anodized titanium foils and their morphology and crystal structure were characterized by scanning electron microscopy and transmission electron microscopy. The crystal phases and the compositions of nanotube arrays were further investigated by X-ray diffraction for different annealing temperatures and X-ray photoelectron spectroscopy for different annealing atmospheres. For optimal annealing conditions, the short circuit current density of 4.27 mA/cm2 and power conversion efficiency of 1.30% could be achieved under AM 1.5 simulated solar irradiation for 2 μm long nanotubes.