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Let us think about the future clean energy systems. For large-scale electric power generation by solar cells, efficient use of high purity silicon is the key. We are trying to improve the efficiency several-ten times by making highly-crystalline films in one minute instead of bulk substrates. Transparent electrodes are important for both solar cells (yielding electricity from light) and display & lighting devices (generating lights from electricity). We try to replace rare-metal-based oxide semiconductors with carbon nanotubes or graphene. Nanotube-silicon hybrids are promising to realize Li ion batteries of larger capacities for (hybrid) electric vehicles. In this way, nanotechnology can bring innovations widely even with abundant carbon and silicon elements, and contributes to sustainable technological society.
But nanomaterials can never be made in macro-scale if we artificially manipulate atoms/molecules one-by-one. Self-organization, i.e. spontaneous formation of materials from numerous atoms/molecules, is the key. We are trying to understand the processes of chemical reactions of atoms/molecules, formation of nanostructures, and evolution of higher-order structures fundamentally. Based on the fundamental understandings with flexible thinking and idea, we are proposing and developing novel processes for nanomaterials and their devices.
Carbon nanotubes (CNTs) are a unique 1D nanomaterial as thin as ~nm and as long as ~mm.
They have good electrical conductivity, high tensile strength, thermal and chemical stability similarly with inorganic materials, and have small mass, flexibility, and compatibility with printing process similarly with organic materials.
Thus, many applications have been proposed for them. On the other hand, as their price (higher than gold) shows, their fabrication process is still under development and their practical applications are very much limited. Chemistry & engineering should lead the innovations for their production and manufacturing. We have developed rapid growth process of millimeter-long single-wall CNTs and are trying to realize their practical production. Please click here for details. We are developing mass-production processes of CNTs by utilizing three-dimensional space of reactors, and direct fabrication of various devices by growing CNTs on device substrates.
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Rapid SWCNT growth Larger Movie. Continuous production by fluidized bed: Movie |
Graphene is a unique 2D nanomaterial of a single atomic layer having excellent conducting, transparent, and mechanical properties.
But their practical production method has not been developed yet.
We are developing new processes including direct synthesis of graphene films on substrates for electronic device applications, and low-cost production of its transparent conductive films for solar cells and touch panels.
Such thin films can also be fabricated easily by dispersion and printing of CNTs.
We are developing a loss-free fabrication process of thin films of CNTs from their powder. Solar cells are one of the important application target of these studies. Although various types have been extensively researched, bulk single- or poly-crystalline Si type still accounts for >90% of all solar cells. To make solar cells cheaper and install them at larger scale, we are developing a rapid vapor deposition process of highly crystalline Si films at >10 times larger area than the conventional Si substrates in 1 minute. We also work on a simple cell fabrication process coating the Si film with CNTs. And we started technology assessment of their environmental, energy, and economy aspects.
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1-min-epitaxy and lift-off of Si films for solar cells |
Energy devices must be produced at low cost and installed at large scale in order to contribute to the sustainable energy and environmental systems.
We are developing next-generation, high energy and power density rechargeable batteries via rapid, high-yield fabrication process using low-cost sources. Self-supporting, sponge-like films of single-wall and few-wall CNTs can be easily obtained by dispersion and filtration. For example, 0.1 mm-thick paper of our few-wall CNTs by fluidized-bed CVD is as light as 0.3 mg/cm3 and 3 mg/cm2, has a 80-90% porosity and 100 S/cm electrical conductivity. Differently from the conventional electrodes fabricated by coating active materials on heavy current collectors of Cu or Al foils with binder and conductive fillers, we are developing light-weight, high-capacity electrodes by capturing various active materials in the CNT papers. Especially, Si has a huge theoretical capacity 10-times as large as the current graphite anode, however is suffering from the degradation due to the volumetric change during charge-discharge cycles. Vapor deposition is a common method in basic research in preparing thin films slowly and carefully under ultrahigh vacuum. But it enables rapid, low-cost fabrication of aluminum thin films for gas barriers in snack packaging in industry. We elevate the vapor pressure of the evaporation source by heating it to a temperature much higher than the melting point, deposit the vapor on substrates at lower temperature, and realize several micrometer-thick porous films quickly in 1 minute. We are applying this method also for various metals to realize high-performance battery electrodes via simple, low-cost processes.
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1-min deposition of porous Si films for Li ion batteries |