Research Targets in FY2009

Polymer solar cells are a promising new type photovoltaic conversion device with the advantages of lightweight, large-area, flexible and low cost roll-to-roll production by using the convenient well-developed solution-based thin film deposition technology. For the sake of highly efficient photocurrent conversion efficiencies of organic thin film solar cells in terms to reduction of carbon dioxide emissions, we intended to develop some materials for such organic thin film solar cells designed and evaluated novel device structures in FY2008-2009.

Research Plans and Achievements

We developed donors such as porphyrin, polythiophenes, and so on and acceptors of fullerenes, which are consisting of active layer. While design and evaluation of electron transporting layer with TiO2 and/or ZnO have been explored and the construction and examination of single-cells were performed in the FY 2009.

1）Molecular design of donors and acceptors for active layer

Porphyrin lipid, which is able to form fibrous aggregates, as donor and fullerene C60 lipid as acceptor were designed and focused. It was found that the emission of porphyrin was remarkably quenched by the addition of fullerene especially in the case of both lipid systems. Therefore, enhancement of the efficiency of the charge separation was confirmed by using the molecular assembling system in addition to the improvement of the light harvesting property through the increase of the absorption.

2）Development of materials for electron transporting layer

We prepared ZnO nanorod arrays and TiO2 nanotube arrays as the electrodes for hybrid type solar cells. Especially, hybrid organic-inorganic solar cells have been prepared using poly(3-hexylthiophene) and (6,6)-phenyl C61 butylic acid methyl ester as the bulk heterojunction onto ZnO followed by the further coating of PEDOT:PSS as a hole transporting layer. It was confirmed that the rectification property of the device was effectively improved and attained a power conversion efficiency of 3.2%.

3）Construction and examination of single cells

We tried to fabricate and evaluate the single cells with commercially available compounds and feedback the results for next plan after FY 2009. Introduction of TiOx layer into the polymer solar cell based on poly (3-hexylthiophene) and (6, 6)-phenyl C61- butyric acid methyl ester revealed the improvement of the homogeneousness of the film and the interface, confirmed by the laser-beam induced current technique. Carrier mobility and the lifetime were measured by charge extraction by linearly increasing voltage (CELIV) method. It was found that the lifetime of the charge increased twice as compared to the no TiOx layer as shown in Fig. 4-1.

Research Targets in FY2009

Toward sustainable society, chemical conversion of solar energy as artificial photosynthesis is potentially promising for efficient utilization of renewable energy sources in addition to the well-established thermal and electrical utilization of solar energy. Before the development of the photo-driven oxidase, which was designed by mimicking the material conversion process in photosynthesis, we designed and constructed photoelectric transducers consist of light-harvesting antenna and charge transporter in FY2009.

Research Plans and Achievements

Since double stranded DNA forms highly organized self-assembly and hole migration process through DNA have been studied extensively, DNA scaffold would be appropriate for hole transporter. Because of the utility of ruthenium(II) (Ru(II)) complex as oxidant to probe DNA charge transfer by the excitation of visible light, Ru(II) complex was an attractive photosensitizer. Therefore, we designed DNA-modified films containing Ru(II) complex as a photoelectric transducer. Ru(II) complex tethered complementary DNA was constructed and immobilized on a Au surface (Figure 1a). A stable cathodic photocurrent was immediately observed under the photoirradiation of the modified gold electrode at 436 nm, whereas the photocurrent was instantly disappeared without the photoirradiation (Figure 1b). Furthermore, as the potential on the gold electrode became decreasing under 0 V, the cathodic photocurrent dramatically increased (Figure 1c). Thus, the photocurrent generation was controlled by a positive charge transport, i.e., hole transport between the gold electrode and the DNA. The photocurrent process is mainly divided into four processes, that is, charge injection, charge recombination, charge conduction along the DNA, and charge hopping to the Au electrode. In summary, we developed the photoelectric transducer consist of Ru(II) complex and DNA scaffold. The DNA-modified films tethering Ru(II) complex showed cathodic photocurrent under visible light irradiation due to photoinitiated hole transport through DNA duplexes.

Fig. 4-2. (a) Schematic representation of cathodic photocurrent generation along the DNA duplex, which is immobilized on a gold electrode. (b) Photocurrent responses of DNA films at a potential of −0.2 V upon illumination (436 nm). (c) Photocurrent versus applied potential curves for the DNA-modified gold electrode with Ru(II) complex (red) or without Ru(II) complex (black). The bias on the electrode was changed from −0.2 to 0.2 V versus SCE.

Research Targets in FY2009

For effective use of new energy and various electric vehicle systems such as HEV, P-HEV and pure EV, there is a growing need for electric energy storage with high power density as well as high energy density. Iron oxide is one of the most promising materials as an electrode of lithium-ion batteries due to its low toxicity and low cost. Our strategy for preparation of rapidly dischargeable and chargeable electrode materials using iron oxide is as follows: A number of lithium ions have to move from the anode to cathode (during discharging), and from the cathode to the anode (during charging) in short time for such rapid discharge and charge. Then diffusion length of lithium ion in iron oxide particles is limited to short distance. Adequately small particles permit lithium to reach all parts of particles even if such a short diffusion length. For preparation of iron oxide in the small particle, aqueous solution method, which can be, in general, conducted at low cost, is appropriate. On the other hand, since the rapid discharge and charge means fast electrochemical reactions, high electronic conductivity is necessary. It is effectively achieved by combination of iron oxide particles with conducting additives such as graphitic carbon materials.

Research Plans and Achievements

We attempted to prepare a composite including carbon material. From the above viewpoints, in order to obtain an iron oxide/carbon composite with favorably contacting condition between particles, carbon material as conducting additive is introduced during the synthetic stage of iron oxide small particles by aqueous solution method. In this study, we adopted γ-Fe2O3 for the iron oxide component, and acetylene black (AB) or ketjen black (KB) for the carbon component in the composite. The AB and the KB are representative materials for conducting additives to electrodes. These composites (γ-Fe2O3/AB andγ-Fe2O3/KB) are examined as cathodes of lithium-ion batteries and they exhibit high coulombic efficiency and high cycle performance. Furthermore the γ-Fe2O3/KB composite is found to allow rapid discharge and charge. The specific capacity of the γ-Fe2O3/KB composite was 80 mA h g–1 at a current density of 4 A g–1: the capacity 80 mA h g–1 could be discharged in 1.2 minutes. At the same time, the composites exhibited high retention rate of specific capacity; the ratio of discharge capacity of the 50th cycle to that of the 5th cycle was 97.8% forγ-Fe2O3/KB composite. These results indicate that the γ-Fe2O3/KB composite is a promising cathode material of rapidly discharging and charging lithium-ion batteries.

Research Targets in FY2009

Crystalline silicon solar cells currently hold more than 80% of the total solar cell production. Since they have high conversion efficiency, high reliability and low environmental impact, they are expected to be mass-produced and widely used all over the world in the future. However, the cost is rather high for conventional production methods of solar-grade silicon, which is the most important challenge for the silicon solar cell industry. Thus, the purpose of this project is to develop a new and low-cost production method of solar-grade silicon. We focus on the electrochemical processing in molten salts for this purpose. In PY2009, we especially concentrated on the electrolytic reduction of SiO2 in molten CaCl2. The plans of PY2009 were to develop a new method of utilizing SiO2 powder as feedstock and to achieve the target levels of purity.

Research Plans and Achievements

The SiO2 powder was pressed into a donut-shaped pellet, which was then attached to a silicon rod (Fig.4-3--a). This SiO2 pellet was successfully reduced to silicon in molten CaCl2 at 1123 K (Fig.4-3--b). The produced silicon was analyzed by GD-MS. It was confirmed that most of the impurity elements were below our target levels which were calculated from the acceptable impurity levels for SOG-Si and the segregation coefficients for the impurity elements. The elements, which have not been cleared the target levels, are only boron and carbon.

Fig. 4-3. Photographs of the SiO2 contacting electrodes; A donut-shaped pellet made from SiO2 powder is fixed to a polycrystalline Si rod. (a) Before the electrolysis. (b) After the electrolysis in molten CaCl2 at 1123 K.

Research Targets in FY2009

We are studying new nanoprocessing technologies using femtosecond (fs) laser pulses, for the purposes of the development of efficient thin-film solar cells. The studies are concerned with (1) the experimental demonstration of our physical model for the nanostructure formation on solid surfaces with femtosecond (fs) laser pulses, and (2) the development of a new method to reconstruct the angle-dependent distribution of high-order harmonic generation from single molecules aligned with fs laser pulses.

Research Plans and Achievements

• We did fs-laser ablation experiments of semiconductor materials, Si, InP, GaN, GaAs, and InAs, and the formation of two types of periodic structures were observed on the target surface. When the targets were irradiated in water at low fluence, the observed size of a larger structure was about half of laser wavelength λ, and the other was　λ/5 –λ/4. The nanostructure size is in good agreement with that calculated with our model.
• 2) Applying the high-order harmonic generation (HHG) induced by ultrafast response of aligned molecules, we have succeeded to develop a new versatile method to accurately measure molecular rotational temperature with high temporal and spatial resolutions in pulsed supersonic N2, O2, and CO2 beams. We have proposed and demonstrated a new way to reconstruct angle-dependent harmonic yield from a single molecule. It is shown that the harmonic distribution is strongly dependent on the highest occupied molecular orbital of molecules.

Research Targets in FY2009

Efficiency of solar energy conversion by semiconductors depends on their microstructures as well as chemical components of the surfaces. In the present program we aim at the development of highly-functional novel microscopic structures of interfaces, and the evaluation of interfaces in situ in the fabrication processes to control the process parameters. In the present academic year we aim at the establishment of laser-ablation-based atomic emission spectroscopy for in situ elemental mapping of solid surfaces in liquid, and clarification of the relation between the irradiation damage on the target surface and the spatial resolution of the elemental analysis.

Research Plans and Achievements

We investigate the damage on a target surface after the irradiation of the target in water by tightly focused laser pulses. The size and shape of the irradiation damage are investigated at various irradiation conditions, and the spatial resolution of the LIBS measurement is discussed. In order to investigate the effects of sample surface structure upon the measurement, thin metallic films on a glass plate as well as a bare metal plate were investigated. Furthermore, the application to in situ elemental compound monitoring during the electrodeposition of composite semiconductors will be planned.
A comparatively deep pore at the center of the irradiation damage is found when the ablation laser is tightly focused onto the surface. The pore diameter was ~10 µm although the laser spot was 1.6 µm. By assuming that the emission of atoms is limited to this region, the spatial resolution of 10 µm should be obtained for in situ surface elemental analysis in water. On the other hand the metal thin film on a glass plate gives the pore or the thin-film removal of 60 µm. This means that the spatial resolution of the measurement is sensitive to the surface structure.

Research Targets in FY2009

　Knowing the fact that the available wavelength range from a single free-electron laser facility is rather limited, how to make the available wavelength range broader is an urgent issue. Our goal in this GCOE project is to develop an efficient frequency-conversion technique particularly suitable for the mid-infrared free-electron laser (KU-FEL) in our institute. This year we plan to complete the basic design for the second and fourth harmonic generations and investigate the transverse-mode dependence of pulse propagation in a medium.

Research Plans and Achievements

　To design the frequency-conversion scheme, we must take into account the facts that the pulse duration is rather short (0.5-1ps) and also the pulse energy is rather moderate (1μJ/pulse), since they determine the maximum length of the nonlinear crystal and conversion efficiency, respectively. Moreover, we cannot focus the beam too much to improve the conversion efficiency, because all kinds of nonlinear crystals for the mid-infrared do not have so much resistance against the energy. After examining several different nonlinear crystals such as AgGaSe2, AgGaS2, ZGP, and GaSe, etc., we have found that a AgGaSe2 crystal with a length of 3-6 mm is most suitable for the SHG if the incident wavelength is 8-14μm. Similarly, AgGaSe2 or ZGP with a length of 6 mm is most suitable for the FHG. The expected conversion efficiency for the SHG with a 3 mm (6 mm) crystal is 15 % (48 %) at the peak intensity of 100 MW/cm2, and 50 % for the FHG with a 3 mm crystal. The problem, however, is that we do not know the damage threshold of the nonlinear crystal. The only known data from the literature is that the AgGaSe2 crystal’s damage threshold is 25 MW/cm2 for a 1ns pulse.
　It is next year’s subject to experimentally examine the damage threshold for our specific case (with a sub-ps pulse). If it turns out that the intensity can be no more than 25 MW/cm2 in our case, we intend to use a few nonlinear crystals with a group delay compensation to improve the conversion efficiency.
　As for the transverse-mode dependence of the incident beam on the pulse propagation, we have theoretically compared the Gaussian and Bessel incident beams in terms of the propagation in a rare gas, and found that the Bessel incident beam has much more stability during the propagation. We have successfully identified that the energy pooled in the peripheral region of the Bessel pulse serves as an energy reservoir to provide the inner part of the beam with energy for stable propagation.

Research Targets, Plans and Achievements in FY2009

　Our research group aims at developing a novel evaluation method for solar cell materials by use of a Mid-Infrared Free Electron Lasers (KU-FEL), as well as investigating a new material processing to control the energy bandgap structure of wide-bandgap semiconducting materials for high efficiency solar cell by use of microwave heating. Particularly, we will study the selective excitation of lattice vibration (phonon) of metal oxides using KU-FEL with short pulse, high energy, and tunable wave length, while paying attention to the direct observation through Raman scattering, temperature dependency of electric resistivity, as well as changes in electronic states through Photoluminescence at low temperature.
　For the above purpose, we successfully developed the microwave material processing to introduce the lattice deficiency in wide-bandgap semiconducting materials such as TiO2 and ZnO in cooperation with Research Institute for Sustainable Humanosphere (RISH). In addition, a mid-infrared free electron laser (MIR-FEL) facility (KU-FEL: Kyoto University Free Electron Laser) has been constructed for energy science in Institute of Advanced Energy (IAE), Kyoto University. Lasing at 12μm was observed for first time at IAE in March 2008. A beam loading compensation method with an RF amplitude control in the thermionic RF gun was used to qualify the electron beam. A developed feedforward RF phase control was applied to stabilize the RF phase shifts. As a result FEL gain saturation at 13.2 µm has been achieved for the first time in May 2008. Now we have developed the FEL beamline for chemical and renewable energy research by using MIR-FEL (5-20 μm). At same time, we are installing a cryostat system for measurement of photoluminescence (PL) with He-Cd laser (325nm/ 442nm) at low temperature, and have started to measure PL spectra for TiO2 and ZnO. In next year, we are going to start the in-situ PL measurement during FEL irradiation, and investigate the correlation between lattice deficiency and electronic state, then establish an novel optical measurement methods of semiconducting materials as well as solar cells to develop a high efficiency solar cell.

pagetop