Solar Energy Research

(1) Improvement of Efficiencies of Organic Solar Cells: Development of Materials and Novel Design of Device Structure
(Institute of Advanced Energy) Takashi Sagawa, Susumu Yoshikawa

Research Target in FY2008
　Polymer solar cells are a promising new type photovoltaic conversion device with the advantages of lightweight, large-area, fl exible and low cost roll-to-roll production by using the convenient well-developed solution-based thin film deposition technology. For the sake of highly effi cient photocurrent conversion effi ciencies 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 and designed novel device structures in FY2008.

Research Plan and Achievement
　We fi rstly made guidelines for development of novel donors, acceptors, and electrodes for organic photovoltaic devices and tried to optimize and evaluate the fabrication process of single cell with commercially available materials. Followings are main research achievements in the year of 2008.



(2) Design of the Artificial Photosynthetic Enzyme Driven by Solar Energy
(Institute of Advanced Energy) Takashi Morii

Purpose and results in FY2008
　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 aimed at the construction of photoelectric transducers consist of light-harvesting antenna and charge transporter in this year.



(3) Electrode Materials for Lithium-Ion Battery with High Energy Density and High Power Density
(Graduate School of Energy Science) Mitsuhiro Hibino, Takeshi Yao

　Lithium-ion batteries have already gained a large market share in the field of small-scale batteries and are becoming increasingly important as an essential energy storage medium, in particular for utilization of solar energy and other “new energies”, and further for any types of electric vehicles including hybrid and plug-in hybrid ones. Our group has targeted to develop electrode materials for lithium-ion batteries in terms of syntheses of new electrode materials and investigations of their basic electrochemical properties. In our recent studies, we succeeded to fabricate a composite material of ferric oxide and carbon that was applicable to cathodes for rapidly chargeable and dischargeable lithium ion batteries. In this study, we develop new electrode materials and examine them from a standpoint of the practical application in which lithium-ion batteries are incorporated in photovoltaic power generation system in the form of power storage device. To this, this year we have intended to establish the fabrication method of a practical electrode sheet in the 2032-type cell using our newly developed materials. Also we have continued to conduct a basic study on new materials in terms of synthesis, characterization, and evaluation of electrochemical property.



(4) Preparation and Evaluation of Semiconductor Thin Films by Spray Layer-by-Layer Method
(Institute of Advanced Energy) Yoshikazu Suzuki

Targets and outcomes in FY2008
　For dye-sensitized solar cells and quantum dot-sensitized solar cells, thin film processing on conducting glass substrate for wide-area, low cost and low environmental impact have been required. Homogeneity and good mechanical properties of thin films are also important for such a thin film. Furthermore, low (ambient) temperature process is favored to apply the plastic conducting films. In FY2008, we have studied on the preparation and evaluation of semiconductor thin fi lms by spray layer-bylayer (LbL) method. Titanate nanowire thin film was successfully coated on glass substrate by using spray LbL method with controlling the thickness of tens of nanometers. (J. Ceram. Soc. Jpn., 117, 381 (2009)).



(5) Development of Low-cost Production Method for Solar-grade Silicon
(Graduate School of Energy Science) Rika Hagiwara, Toshiyuki Nohira

Purpose and results in FY2008
　Crystalline silicon solar cells currently hold more than 80% of the total solar cell production. Since they have high conversion effi ciency, 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 FY2008, we especially concentrated on the electrolytic reduction of SiO2 in molten CaCl2. The plans of FY2008 were to conduct a detailed purity analysis on the produced silicon and to device experimental setup which prevents contamination. Two types of SiO2 contacting electrodes were devised, in which only Si is in contact with SiO2 (Fig. 4-6). By using these electrodes, SiO2 plates and tubes were successfully reduced to silicon in molten CaCl2 at 1123 K. Metal impurities in the produced Si were signifi cantly decreased compared with our previous study owing to the adoption of the new electrodes and a quartz vessel. It was also found that the remaining CaCl2 and some metal impurities in the produced Si can be removed effectively by using NH4OH/H2O2/H2O, HCl/H2O2/H2O and HF/H2O2/H2O as cleaning solutions. Most of the impurity elements were below our target levels which were calculated from the acceptable impurity levels for SOG-Si and the segregation coeffi cients for the impurity elements.



(6) Nanoprocessing with Femtosecond Laser Pulses for the Development of Effi cient Solar Cells
(Institute of Advanced Energy) Kenzo Miyazaki, Godai Miyaji, Kazumichi Yoshii

Objective and plan in FY2008
　The purpose of this research is to develop a new technology of nanoprocessing with femtosecond (fs) laser pulses, for the purposes of achieving a high effi ciency of thin-fi lm solar cells. The study in 2008 was concerned with the following objectives: (1) One was to understand the physical mechanism of periodic nanostrcuture formation on thin fi lm targets that we observed for the fi rst time with fs laser pulses. It has been found in our previous experiment that the nanoscale ablation is initiated with the generation of near fi eld (local fi eld) around small surface roughness. We intended to make clear its subsequent interaction process for periodic nanostructure formation to develop a theoretical model. The experiment was designed to observe the most initial stage of nanostructuring on diamond-like carbon (DLC) fi lm with linearly and circularly polarized femtosecond (fs) laser pulses. Based on the results, we tried to control the nanostructure; (2) The other was to demonstrate the validity of a quantum mechanical theory for high-order harmonic generation from coherently rotating molecules, which we have recently developed. Especially, we have shown that HHG from aligned molecules is able to provide us with a new method for accurate measurement of degree of molecular alignment and rotational temperature.



(7) Evaluation of Interfaces for Solar Energy Conversion
(Institute of Advanced Energy) Tetsuo Sakka, Kazuhiro Fukami, Yukio H. Ogata

Target
　Efficiency of solar energy conversion by a solid surface depends on microstructure and chemical component of the surface. Evaluation of the interface in situ in the fabrication process or under actual use is important for monitoring the processes and controlling the process parameters. In the present work we aim at the establishment of micro-LIBS (laser induced breakdown spectroscopy) for in situ elemental mapping of photo-electrodes. In this fi scal year, we tried to clarify the reason of the pulse-to-pulse fl uctuation of the spectral intensity, especially focusing on the instability observed in the analysis of an alloy in water.



(8) Frequency-conversion of Mid-infrared Laser Pulses
(Institute of Advanced Energy) Chengpu Liu, Takashi Nakajima

Objective in FY2008
　To synthesize new effi cient materials for solar cells it is often useful to utilize a mid-infrared laser with short pulse duration for the analysis, since the mid-infrared “photon” energy corresponds to the “phonon” energy of the materials. It is, however, desired that the laser is available at more than a single wavelength: If perfectly synchronized laser pulses are simultaneously available at different wavelengths such as the mid-infrared, nearinfrared, and perhaps to the visible region, the usefulness of a polychromatic light source is out of doubt.
　One way to realize a polychromatic light source is to use frequency-conversion techniques such as second harmonic generation (SHG), third harmonic generation (THG), and fourth harmonic generation (FHG) processes, etc. If we initially have mid-infrared laser pulses at the wavelength of ～ 12 μm, even the FHG is at 3 μm and still in the infrared region. To produce shorter wavelength pulses, we have to utilize high-order harmonic generation (HHG), which is possible if the initial mid-infrared pulses have suffi cient pulse energies.
　In this work we investigate the interaction of alkali-metal (potassium) atoms with mid-infrared laser pulses, the purpose of which is to clarify at what mid-infrared laser intensity the high-order harmonic photons are produced



(9) Development of energy materials by use of MIR-FEL
(Institute of Advanced Energy) Taro Sonobe, Toshiteru Kii, Kai Masuda, Hideaki Ohgaki

　Our research group aims at developing a novel evaluation method for solar cell materials by use of Our research group aims at developing a novel evaluation method for solar cell materials by use of 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 effi ciency solar cell by use of microwave heating. Particularly, we will study the energy states of mid and sub-band, and life time and band-edges through investigating a photoluminescence (PL) by UV-visible laser synchronizing a FEL laser, in order to understand the correlation factors of surface stats and mid/sub-band structures of semiconducting materials for solar cells.
　For the above purpose, we successfully developed the microwave material processing to control surface states of wide-gap semiconducting materials such as TiO2 and ZnO in cooperation with Research Institute for Sustainable Humanosphere (RISH), and applied a patent in this year. 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 12μm was observed for fi rst 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 13.2μm has been achieved for the fi rst time in May 2008. Now we are developing the FEL beamline for chemical and renewable energy research by using MIR-FEL (5-20μm). At same time, we installed a photoluminescence (PL) measurement system with He-Cd laser (325nm/ 442nm), and have started to measure a PL spectra for TiO2 and ZnO. In next year, we are going to develop the in-situ PL measurement system during FEL irradiation, and establish novel optical measurement methods of semiconducting materials as well as solar cells to develop a high effi ciency solar cell.