Hone-Zern Chen

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.

Grain Growth and Electrical Properties in ZnO Varistors with Various Valence States of Additions

Grain growth and electrical properties in ZnO system with various valence states of manganese and cobalt were studied. The results were discussed by means of defects produced by the additions. The grain growth was analyzed from the kinetic grain growth equation: Gn=Dtexp (-E/RT). In this work, the grain growth kinetic exponent n was 6 and activation energy was 230±21 kJ/mol. The grain size increased with the valence states of manganese and cobalt. The compositions with lower trap density have higher nonlinear coefficient, higher voltage ratio and lower leakage current. The varistor properties were improved by increasing the sintering temperature and time because of the more obvious tunneling effect.

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.

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.

Magnetic behavior in the La0.7Pb0.3Mn1−xCoxO3 perovskite compounds

Compositions with the rhombohedral structure have been successfully synthesized in the La 0.7 Pb 0.3 Mn 1-x Co x O 3 system. DC magnetization and AC susceptibility measurements were carried out for the entire series. The Curie temperature T c decreases with increasing x from 306 K (x = 0.0) down to 60 K (x = 0.9). The antiferromagnetic state was found near the Co-rich side (x = 1.0). Co doping tends to destroy the double exchange in Mn 4+ -O-Mn 3+ and broadens the coexistence region of the cluster-glass and ferromagnetic states.

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%.

Thickness-dependent leakage current of (polyvinylidene fluoride/lead titanate) pyroelectric detectors

The novel pyroelectric IR detectors have been fabricated using the Polyvinylidene Fluoride (PVDF)/Lead Titanate (PT) pyroelectric bilayer thin films, which were deposited onto Pt(111)/SiO2/Si(100) substrates by a sol-gel process. The ceramic/polymer structure was constructed of the randomly oriented polycrystalline PT film (approximately 1 microm) heated at 700 degrees C for 1 h and the beta-phase PVDF film crystallized at 65 degrees C for 2 h. The effects of PVDF thin film thickness (100 approximately 580 nm) on the pyroelectric response of IR detectors were studied. The results show that the depositions of PVDF thin films onto the PT films will cause the leakage current (J) of the detectors decrease from 6.37 x 10(-7) A/cm2 to 3.86 x 10(-7) A/cm2. The specific detectivity (D*) measured at 100 Hz decreased from 2.72 x 10(7) cm x Hz(1/2)/W for detector without PVDF to 1.71 x 10(7) cm x Hz(1/2)/W for detector with PVDF thickness of 580 nm. By optimizing the ratio of the specific detectivity (D*) to leakage current, D*/J, the detector with PVDF thickness of 295 nm exhibits the best performance.

LiTaO3 Thin Films Prepared by a Diol-Based Sol-Gel Process and Crystallized by Conventional and RTA Processes

The LiTaO3 thin films were prepared by a diol-based sol-gel method and crystallized by conventional and RTA processes. Based on the results, the c-axis orientation of the films crystallized by the RTA process presented better crystallization than those crystallized in a conventional furnace. The remanent polarization (P r ) and the coercive field (Ec) of the films crystallized in a conventional furnace were about 2.4 μ C/cm2 and 87 kV/cm. On the contrary, for LiTaO 3 films crystallized by the RTA method, the remanent polarization (P r ) and the coercive field (Ec) were about 10.7 μC/cm2 and 45 kV/cm, respectively.