Ming-Cheng Kao

Structure and photovoltaic properties of ZnO nanowire for dye-sensitized solar cells

Aligned ZnO nanowires with different lengths (1 to approximately 4 μm) have been deposited on indium titanium oxide-coated glass substrates by using the solution phase deposition method for application as a work electrode in dye-sensitized solar cells (DSSC). From the results, the increases in length of zinc oxide (ZnO) nanowires can increase adsorption of the N3 dye through ZnO nanowires to improve the short-circuit photocurrent (Jsc) and open-circuit voltage (Voc), respectively. However, the Jsc and Voc values of DSSC with ZnO nanowires length of 4.0 μm (4.8 mA/cm2 and 0.58 V) are smaller than those of DSSC with ZnO nanowires length of 3.0 μm (5.6 mA/cm2 and 0.62 V). It could be due to the increased length of ZnO nanowires also resulted in a decrease in the transmittance of ZnO nanowires thus reducing the incident light intensity on the N3 dye. Optimum power conversion efficiency (η) of 1.49% was obtained in a DSSC with the ZnO nanowires length of 3 μm.

Properties of LiTaO3 Thin Films Derived by a Diol-Based Sol–Gel Process

Lithium tantalite (LiTaO3) thin films have been successfully deposited on Pt(111)/SiO2/Si(100) substrates by means of sol–gel spin-coating technology. The XRD results show that the LiTaO3 thin films heated at temperatures higher than 700°C exhibit randomly oriented polycrystalline. The relative dielectric constant (er) increases from 35 to 300 at temperature ranging from 30 to 610°C, then decreases as the temperature increases above 610°C. The dielectric loss factor (tan δ) is as low as 0.006 at temperatures ranging from 30 to 770°C. For the first time, the ferroelectric properties of LiTaO3 thin films (~0.5 µm) deposited on Pt(111)/SiO2/Si(100) substrates have been investigated. The sample shows a phase transition at 610°C, a coercive field (Ec) of 22.3 kV/cm, a remnant polarization (Pr) of 2.4 µC/cm2 and a maximum pyroelectric coefficient (γ) of 4.0×10-8 C/cm2K at 62°C. The high figures of merit (FV and Fm of 2.1×10-10 Ccm/J and 2.4 ×10-8 Ccm/J, respectively) of LiTaO3 films reveal that a good infrared response of this LiTaO3 thin film can be utilized in its application to detectors.

Influence of Y-doped induced defects on the optical and magnetic properties of ZnO nanorod arrays prepared by low-temperature hydrothermal process

One-dimensional pure zinc oxide (ZnO) and Y-doped ZnO nanorod arrays have been successfully fabricated on the silicon substrate for comparison by a simple hydrothermal process at the low temperature of 90°C. The Y-doped nanorods exhibit the same c-axis-oriented wurtzite hexagonal structure as pure ZnO nanorods. Based on the results of photoluminescence, an enhancement of defect-induced green-yellow visible emission is observed for the Y-doped ZnO nanorods. The decrease of E2(H) mode intensity and increase of E1(LO) mode intensity examined by the Raman spectrum also indicate the increase of defects for the Y-doped ZnO nanorods. As compared to pure ZnO nanorods, Y-doped ZnO nanorods show a remarked increase of saturation magnetization. The combination of visible photoluminescence and ferromagnetism measurement results indicates the increase of oxygen defects due to the Y doping which plays a crucial role in the optical and magnetic performances of the ZnO nanorods.

Effects of Membrane Thickness on the Pyroelectric Properties of LiTaO3 Thin Film IR Detectors

High-performance pyroelectric infrared (IR) detectors have been fabricated using lithium tantalite (LiTaO3) thin films deposited on Pt(111)/Ti/SiO2/Si(100) substrates by the diol-based sol–gel method. The thermal isolation of detecting elements was achieved by anisotropic wet etching of the back of the silicon substrate. In order to reduce the thermal mass and thermal time constant of the detector, the sensing element was fabricated on a thin membrane. The effects of membrane thickness on the response of pyroelectric IR detectors were studied by changing the membrane thickness (20–350 µm). As the membrane thickness decreased, the voltage responsivity (Rv) increased from 4300 up to 8398 V/W, and the specific detectivity (D*) also increased from 1.2×108 to 2.7×108 cmHz1/2/W. Experimental results reveal that the thermally isolated detectors with membrane thickness of 20 µm exhibit excellent sensitivity.

Characterization of Calcium-Modified Lead Titanate Thin Films Derived from a Diol-Based Sol-Gel Process

Polycrystalline thin films of Ca-modified lead titanate (PCT) were deposited on Pt/SiO2/Si substrates using a diol-based sol–gel process. Calcium acetylacetonate hydrate was adopted as a starting material instead of the calcium acetate or calcium nitrate tetrahydrate used conventionally. By changing the Ca content (5–35 mol%) and heating temperature (500–800°C), the influences of various processing parameters on the characteristics of thin films were studied. With the increase of the Ca content, the relative dielectric constant (er) of PCT thin film increased from 43 to 70 at the heating temperature of 700°C. It was found that the coercive field (Ec) and the remanent polarization (Pr) decreased, but the pyroelectric coefficient (γ) increased with an increase of Ca content. The results reveal that PCT thin film with a Ca content of 25 mol% exhibits the largest figures of merit for the voltage responsivity and the specific detectivity at a heating temperature of 700°C.

Pyroelectric Properties of Sol–Gel Derived (Pb,Ca)TiO3 Thin Film Detectors

Calcium-modified lead titanate (PCT) thin film pyroelectric infrared (IR) detectors were fabricated on Pt(111)/SiO2/Si(100) substrates using a diol-based sol–gel process. The randomly oriented PCT thin film, which has not undergone the poling treatment, exhibits a relatively small dielectric constant and a large pyroelectric coefficient. The pyroelectric properties of PCT thin film point detectors with various Ca contents on modulation frequency are measured and compared. The optimized composition of PCT thin film IR detectors was found to be the PCT(25), which exhibits a large voltage responsivity of 4533 (V/W) at 20 Hz and a specific detectivity (D*) of 6.45×107 cmHz1/2/W at 100 Hz. The results show that Pb1-xCaxTiO3 thin film with x = 0.25 [PCT(25)] was most suitable for application to the pyroelectric thin film IR detector.

Properties of Sol–Gel Derived (Polyvinylidene Fluoride/Lead Titanate) Pyroelectric Bilayer Thin Films

Ferroelectric polymer/ceramic bilayer thin films composed of polar β-phase polyvi-nylidene fluoride (PVDF) and 1-µm-thick polycrystalline lead titanate (PbTiO3) thin films were prepared, using a sol–gel method. The influence of various processing parameters on the characteristics of PVDF/PbTiO3 thin films was studied by varying the thickness of PVDF thin films (100–580 nm). The results show that the surface of the obtained PVDF film was smooth with less lower porosity above the PbTiO3 film, enabling a significant decrease in the leakage current of bilayer thin films. Although the coercive field (Ec), remanent polarization (Pr) and pyroelectric constant (p) of bilayer thin films decreased with the increase of PVDF thin film thickness, the deposition of PVDF thin films onto the PbTiO3 films caused the leakage current (J) measured at 40 kV/cm to decrease from 1.57 ×10-6 A/cm2 to 3.86 ×10-7 A/cm2 as well as the relative dielectric constant (er) from 63 to 20. The figures of merit (FV and Fm) due to relative low-dielectric constant (er) revealed that good infrared responses of bilayer thin films can be utilized in their application to detectors.

Structural Dependence of Photoluminescence and Room-Temperature Ferromagnetism in Lightly Cu-Doped ZnO Nanorods

Well-defined 1-D ZnO and ZnO:Cu semiconductor nanostructures have been fabricated by a low temperature solution process. Cu (0.073 nm) is chosen as a dopant due to the similar ionic radius with Zn (0.074 nm). The radius of ZnO:Cu nanorods observed by FE-SEM is lager than that of pure ZnO which indicates the growth rate of the nanorods can be obviously enhanced by the light doping of Cu. The XRD patterns of both compositions with single diffraction peak (002) show the same wurtzite hexagonal structure. Photoluminescence spectra show a red-shift of the UV emission peak position and a decrease of the luminescence intensity in green-yellow region. From the room temperature hysteresis loop, ferromagnetism is observed and the saturation magnetization decreases with the increase of the Cu concentration for ZnO:Cu nanorods.

Au-Mediated Surface Plasmon Enhanced Ultraviolet Response of p-Si/n-ZnO Nanorods Photodetectors

Surface plasmon resonance mediated by Gold (Au) nanoparticles (NPs) was employed to enhance the ultraviolet (UV) response of p-Si/n-ZnO nanorod (NR) heterojunction photodetectors (HPDs). In the ZnO NRs with Au NP covering, the photoluminescence spectrum shows that the band-to-band emission of ZnO (384 nm) is increased by a magnitude of 3, and the deep-level emissions (450-700 nm) are drastically decreased as compared with HPDs without coverings of Au NPs. Such a result increases UV-to-visible rejection ratio from 214 to 6180 and it attributes that the defect-level emissions of ZnO induce surface plasmon resonance in Au NPs and then enhances the emissions, exciting a large quantity of electrons crossing over the Au NP/ZnO interface barrier. Consequently, these electrons are transferred to the ZnO conduction band from the Au NPs, causing a photocurrent gain and the quantum efficiency is as high as 1157% at 2 V reverse bias.

Improved Energy Conversion Efficiency of TiO2 Thin Films Modified with Ta2O5 in Dye-Sensitized Solar Cells

Tantalum-doped TiO2 thin films [(TiO2)1-x(Ta2O5)x, x=0–0.8%] were prepared on fluorine-doped tin oxide (FTO)-coated substrates by sol–gel technology for uses in dye-sensitized solar cells (DSSCs). The effects of Ta content on the growth and properties of the TiO2 thin films were investigated. The crystallization and microstructures of the thin films were examined by X-ray diffraction, scanning electron microscopy, and Brunauer–Emmett–Teller analyses. The performance of DSSCs based on Ta-doped TiO2 thin films was also studied. From the obtained results, the increases in Jsc and Voc may be due to the increased electron concentration of TiO2 thin film and the flat-band potential of the TiO2 shifted by tantalum doping, respectively. The optimum properties of DSSCs of Voc=0.68 V, Jsc=7.84 mA/cm2, FF=45.1%, and η=2.4% were obtained using the Ta-doped TiO2 thin film with x=0.5%.