Wan-Chin Yu

Optimization of dye adsorption time and film thickness for efficient ZnO dye-sensitized solar cells with high at-rest stability

Photoelectrodes for dye-sensitized solar cells were fabricated using commercially available zinc oxide (ZnO) nanoparticles and sensitized with the dye N719. This study systematically investigates the effects of two fabrication factors: the ZnO film thickness and the dye adsorption time. Results show that these two fabrication factors must be optimized simultaneously to obtain efficient ZnO/N719-based cells. Different film thicknesses require different dye adsorption times for optimal cell performance. This is because a prolonged dye adsorption time leads to a significant deterioration in cell performance. This is contrary to what is normally observed for titanium dioxide-based cells. The highest overall power conversion efficiency obtained in this study was 5.61%, which was achieved by 26-μm-thick photoelectrodes sensitized in a dye solution for 2 h. In addition, the best-performing cell demonstrated remarkable at-rest stability despite the use of a liquid electrolyte. Approximately 70% of the initial efficiency remained after more than 1 year of room-temperature storage in the dark. To better understand how dye adsorption time affects electron transport properties, this study also investigated cells based on 26-μm-thick films using electrochemical impedance spectroscopy (EIS). The EIS results show good agreement with the measured device performance parameters.

Enhancing performance of ZnO dye-sensitized solar cells by incorporation of multiwalled carbon nanotubes

A low-temperature, direct blending procedure was used to prepare composite films consisting of zinc oxide [ZnO] nanoparticles and multiwalled carbon nanotubes [MWNTs]. The mesoporous ZnO/MWNT films were fabricated into the working electrodes of dye-sensitized solar cells [DSSCs]. The pristine MWNTs were modified by an air oxidation or a mixed acid oxidation treatment before use. The mixed acid treatment resulted in the disentanglement of MWNTs and facilitated the dispersion of MWNTs in the ZnO matrix. The effects of surface property and loading of MWNTs on DSSC performance were investigated. The performance of DSSCs was found to depend greatly on the type and the amount of MWNTs incorporated. At a loading of 0.01 wt%, the acid-treated MWNTs were able to increase the power conversion efficiency of fabricated cells from 2.11% (without MWNTs) to 2.70%.

Flower-shaped ZnO nanocrystallite aggregates synthesized through a template-free aqueous solution method for dye-sensitized solar cells

Hierarchically structured flower-shaped aggregates composed of ZnO nanocrystals were synthesized through a template-free aqueous solution method. The synthesized nanocrystallite aggregates were demonstrated to be promising photoanode materials for dye-sensitized solar cells (DSSCs). Compared with commercially available ZnO nanoparticles (ZnONPs), the flower-like aggregates (ZnONFs), each having an overall dimension of 400–600 nm, exhibited similar dye loading but higher light-scattering ability, which led to a substantial increase in the light-harvesting efficiency of resulting cells. The unique morphology of ZnONFs also boosted the absorbed photon-to-electric current generation efficiency. Consequently, DSSCs constructed from ZnONFs showed significantly improved photocurrent and achieved an overall conversion efficiency of 4.42%, which was 47% higher than that attained by ZnONP-based cells.

Influence of high-frequency vibration on the morphological instability in the directional crystallization of binary melts

The influence of various types of vibration on the morphological instability of the directional crystallization front in binary melts is investigated numerically under microgravity and terrestrial conditions. The vibration frequency is assumed to be high and the amplitude to be small and an averaged approach is used. It is shown that high-frequency rotational vibration generates an intense mean flow localized in the neighborhood of the crystallization front and the direction of this flow is opposite to the direction of gravity-convection flow. Under terrestrial conditions the interaction between vibration flow and gravity convection leads to the gravitational vortex being pushed away from the crystallization front. Under both terrestrial and microgravity conditions rotational vibration has a strong stabilizing action on the morphological instability and prevents the formation of an axial hollow.

Electrochemical Deposition of ZnO Porous Nanoplate Network for Dye-Sensitized Solar Cells

Mesoporous ZnO films composed of interconnected porous nanoplates were prepared by an electrochemical deposition-pyrolytic conversion approach and constructed into the photoanodes of dyesensitized solar cells (DSSCs). Precursor nanoplates grown on conducting glass substrates were transformed into ZnO porous nanoplates by calcination at 400 °C for 1 h. Correlations between the ZnO film thickness and the electrochemical deposition time were determined in order to prepare ZnO films of various thicknesses and to study the effect of the film thickness on the photovoltaic performance of DSSCs. The optimal film thickness was determined to be approximately 27 μm, and the best performing cell reached an energy conversion efficiency of 2.91%. The results show that the ZnO porous nanoplate network so prepared is suitable for DSSC applications.

Enhanced Performance of Dye-sensitized Solar Cells Aided by Olive-shaped ZnO Nanocrystallite Aggregates as the Light-scattering Layer

Olive-shaped ZnO nanocrystallite aggregates were synthesized for dye-sensitized solar cells (DSSCs). The submicron-sized hierarchical nanostructure is composed of highly crystalline ZnO nanoparticles about 20 nm in diameter and has an overall dimension of approximately 150 × 300 nm. An economical and environment-friendly aqueous solution method was developed to synthesize the olive-like aggregate. This template-free self-assembly method involved the mixing of zinc nitrate and sodium hydroxide aqueous solutions at a low temperature (80 °C) and aging the mixture for a particular length of time. We employed a low-temperature (150 °C for 1 h) thermal treatment process for the fabrication of bilayer photoelectrode, with commercial ZnO nanoparticles (~ 20 nm) as the underlayer and submicron-sized structures as the light-scattering overlayer. The N719-sensitized DSSCs containing the aggregate overlayer reached a power conversion efficiency of 4.4 %, 33 % higher than that attained by DSSCs incorporating large solid particles (200–500 nm) as the scattering layer. The enhanced overall conversion efficiency of aggregate-based cells was correlated with a prominent increase in the short-circuit current density. Optical and dye-loading investigations show that this improvement can be attributed to the dual functionality of the olive-shaped nanocrystallite aggregates, which have excellent light-scattering ability to enhance photon capture while providing a large surface area for sufficient dye adsorption.

Dye-sensitized solar cells based on electrodeposited zinc oxide films

Dye-sensitized solar cells (DSSCs) based on porous nanocrystalline ZnO films were fabricated, and the effect of film thickness on power conversion efficiency was studied. The ZnO films, consisting of nanosheet networks, were prepared by pyrolysis of nanostructured precursors electrodeposited on conducting glass substrates. The film thickness of ZnO had a great impact on the performance of the resulting DSSCs. Peak conversion efficiency of 2.91 % was obtained with a film thickness of 27 μm.