Katsuyasu Kawano

Application of rare-earth complexes for photovoltaic precursors

Abstract By applying rare-earth complexes as photovoltaic precursors, the conversion efficiency of solar cells were improved. Rare-earth complexes absorb light at shorter wavelengths and subsequently emit light at longer wavelengths. The high-energy region of the solar spectrum is shifted to longer wavelengths, hence the cell-output power is higher because the emitted light matches with a higher sensitivity region of the basic Si solar cell. For a CaF 2 :Eu single crystal placed on top of an a-Si solar cell experiments showed a maximum relative conversion efficiency of 1.5 Eu ions in CaF 2 SrF 2 mixed crystals, whose emission is even more red-shifted compared with CaF 2 crystals, showed slight improvements. The ion implantation of rare-earth ions into CaF 2 evaluated potential application for antireflection (AR) coatings. Futhermore, porous glass doped with Eu-chelete complexes have been investigated.

High-Conversion-Efficiency Solar Cell Using Fluorescence of Rare-Earth Ions.

An experiment was performed to obtain a high-conversion-efficiency solar cell by shifting UV-region sunlight to the visible region, in which the optical absorption and emission properties of rare-earth Eu activator ions were exploited. A thin disc of CaF2:Eu single crystal was used as an alternative to the antireflection (AR) film in a solar cell. The effectiveness of rare-earth ions was examined. Their relative efficiencies were found to be higher at Eu concentrations from 0.001 to 2 mol%, with a maximum value of 1.5 at 0.05 mol%, than those of nondoped crystal when subjected to low illuminance by a Xe-lamp-based solar simulator. For the purpose of putting the crystal to practical use as an AR film, rare-earth ions were implanted into CaF2 crystal at a dose of 1014–1015 ions/cm2 and an energy of up to 140 keV.

Organic dye-doped thin films for wavelength conversion and their effects on the photovoltaic characteristics of CdS/CdTe solar cell

For the improvement of the conversion efficiency of CdS/CdTe solar cells by applying the wavelength conversion properties, the spectroscopic studies of the fluorescent laser dye Rhodamine 6G (Rh6G)-doped into polyvinylbutyral (PVB) and polymethylmethacrylate (PMMA) were carried out. It was found that all the photoluminescence excitation (PLE) spectra below 540 nm were effectively converted to photoluminescence (PL) spectra in the wavelength region above 550 nm where the solar cell possessed a high spectral quantum efficiency. The external quantum efficiencies of PVB:Rh6G and PMMA:Rh6G films were estimated to be 0.9±0.05 and 0.4±0.1, respectively. When the PVB:Rh6G film was placed on the top of the CdS/CdTe solar cell, both short-circuit current and conversion efficiency increased by 11% compared with the cell with no film.

Laser silicon ablation studied by time-of-flight mass spectrometry : generation of Si ions and neutrals and their translational properties

Laser pulses of Nd:YAG (532 and 266 nm) were irradiated on a Si target to investigate the ablation process. Monoatomic ions and neutrals were simultaneously ablated, with ions having higher most probable kinetic energy (MPKE) and broader velocity distribution than neutrals. The two beams showed close ablation fluence threshold, but different exponential relationships of the intensity of the ablated species vs laser fluence, with the 266 nm beam having a higher increasing rate than that of the 532 nm beam. The 266 nm beam also resulted in higher kinetic energy and a higher fraction of ions in the ejected particle stream. The time-of-flight (TOF) results fitted the shifted Maxwell-Boltzmann distribution well. The laser Si ablation mechanism was discussed.

Transient Response of Charge Collection by Single Ion Strike in 4H-SiC MESFETs

The radiation damage of 4H-SiC Metal Semiconductor Field Effect Transistors (MESFETs) due to gamma rays was studied. The threshold voltage and Schottky property of gate contact varied only slightly after absorbed dose of 10.4 MGy(SiC). In addition, the transient response of charge collection was studied by using transient ion beam induced current (TIBIC) system. It was found that the collected charge was several orders of magnitudes higher than the charge induced in SiC by direct ionization. The most likely explanation of the enhanced charge collection was the bipolar and the channel modulation effects.

Transient Currents Generated by Heavy Ions With Hundreds of MeV

Single Event Transient (SET) current is one of the most interesting issues for understanding and predicting Single Event Effects (SEEs) in real space environments. Since space radiation consists of high energy heavy ions, estimation of the energy dependence of SET currents is required. In order to estimate this effect, we have developed the Transient Ion Beam Induced Current (TIBIC) system combined with collimated high energy ion beams accelerated by AVF Cyclotron. We have demonstrated the function of TIBIC developed here. Using the newly developed TIBIC system and the Technology Computer Aided Design (TCAD) simulator, the influence of ion energy on SET currents is estimated

Impact of Auger Recombination on Charge Collection of a 6H-SiC Diode by Heavy Ions

Charge collection efficiency (CCE) generated in a 6H-SiC p+n diode by impact of heavy ions was evaluated by the transient ion beam induced current (TIBIC) technique. Numerical analysis by using technology computer aided design (TCAD) concludes that the Auger recombination process reduces CCE. Comparing experimentally measured CCEs with calculated ones revealed that the ambipolar Auger coefficient of about 3 times 10-29 cm6/s.

Electron spin resonance study of laser-annealed (Zn,Mn)O ceramics

Investigations have been made on the effects of pulse laser annealing on the ceramics (Zn,Mn)O by means of electron spin resonance (ESR) measurements. A remarkable change in the ESR spectrum was observed after annealing by a Nd:YAG pulse laser (λ=1.064 μm, 5–7 J/cm2). It suggests that segregated Mn2+ precipitates at grain boundaries in ZnO are diffused into crystal grains by preferential heat conduction through the boundaries. The scanning electron microscope observation verified enough melting features within the ceramics.

ERR of Off-Center Eu2+ Ions with Resolved Structures in RbCl

In RbCl: Eu 2+ crystals, there are found three groups of EPR spectra with an intensity ratio of 2: 1: 1, which are attributed to off-center displacements of Eu 2+ ions along [111] axis from normal lattice sites. The spectra of the off-center Eu 2+ ions can be described by combining the spin Hamiltonian in an orthorhombic field and the perturbation terms depending on the magnetic field as follows; \(\mathscr{H}{=}g\beta\textbf{\itshape HS}+\sum_{l{=}1}^{3}\sum_{m{=}0}^{l}\text{B\)_2l^2m O _2l^2m\(}+\textbf{\itshape AIS}+u\beta[S_{x}^{3}H_{x}+S_{y}^{3}H_{y}+S_{z}^{3}H_{z}-\frac{1}{5}(\textbf{\itshape SH})[3S(S+1)-1]]+U[S_{x}^{3}I_{x}+S_{y}^{3}I_{y}+S_{z}^{3}I_{z}-\frac{1}{5}(\textbf{\itshape SI})[3S(S+1)-1]]\), where g =1.995, b 2 0 =294.6, b 2 2 =-139.8, A 151 =-31.3, u =-0.2∼0.9, U =0∼-0.05 (10 -4 cm -1 ) at room temperature. The displacement is estimated from the angles between the principal axes and [110] directions to be 0.082 a ( a : anion to cation distance), and the induced dipole moment to be 2.57 ...

Studies on Thermal Migration of Eu Ion Doped into TiO2 Nanoparticles

The location of the Eu ion in the TiO2 lattice was investigated as a function of sintering temperature using X-ray photoelectron spectroscopy (XPS), to clarify the role of the Eu ion in TiO2 nanoparticles synthesized by the simple sol–gel method. The crystalline structure as well as the average crystallite size were evaluated using X-ray diffraction (XRD), and the direct observation of nanoparticles was carried out using atomic force microscopy (AFM). A model was proposed in which, in the as-synthesized gel state, the Eu ion occupies the core part of a nanoparticle, and with increasing sintering temperature, it migrates toward the particle shell to finally reach the surface at high sintering temperatures above 900 °C, at which sufficient thermal energy is provided to allow the Eu ion to migrate to the surface of the nanoparticles, resulting in the formation of Eu2Ti2O7 with a pyrochlore structure and an abrupt decrease in the photoluminescence (PL) intensity of the Eu ion.