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-ENVIRONMENT & ENERGY & ELECTRONICS-
-Thin Film Lab.-

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Thin Film Laboratory provides research and education concerning the formation of oxide semiconductors and photovoltaic devices (solar cells) based on chemical thermodynamics and solid state physics. The members in our laboratory are striving for efficient utilization of finite resources and energies, and creation of novel renewable energy sources.


The number of researches about direct formation of oxide semiconductor thinfilms utilizing electrolysis in aqueous solution are getting increased year by year. Deposition methods from aqueous solution including electrochemical deposition have many advantages such as safety, low environmental road, requirement of simple and inexpensive experimental apparatus, and easiness to control film thickness and surface morphology.


Chemical states of metal ions can be predicted by potential-pH diagrams contrived by Marcel Pourbaix. The diagrams show the most stable chemical species in the aqueous solution at a certain electrode potential and a solution pH. Therefore, they helps us to design appropriate experimental conditions such as solution compositions and electrode potentials. For example, the principles of cathodic ZnO deposition by hydroxide generation through reductions of nitric ions or dissolved oxygen molecules, and anodic Ag2O deposition by acid generation through oxidation of water expected from their potential-pH diagrams are shown above.


Electrodeposition of ZnO utilizing arrayed polystylene particles on electrodes as templates can realize the fabrication of various nano-structured materials such as nano-cauliflower arrays and ZnO nano-wall arrays.


We are aiming to apply these oxide semiconductors to new light-absorbing materials for solar cells.


 

Moreover, heteroepitaxial growth due to lattice matching to substrate materials gives oxide semiconductor films with high crystallinity and orientation. By adjusting formation conditions, very uniform and smooth ZnO layer with few defects and ZnO nano-pillars grown vertically against the substrate surface can be obtained. These thin films show not only visible luminescence due to defects but also ultraviolet luminescence based on the recombination of exitons under ambient temperature. This indicates that the method is able to be one of the formation methods of high-quality ZnO.

 


A solar cell composed only of metal oxides can be fabricated by stacking n-type ZnO layer and p-type Cu2O layer. Researches on ZnO/Cu2O-type photovoltaics are obtaining more attention recently. The conversion efficiency of this type of solar cells, which was below 0.3% in 2005, are increasing to reach at 2% in 2007, and more improvement is expected. We are making effort for fabricating oxide solar cells with combinations of other oxide materials.


We are also studying the formation of Zn(O,S) layers by chemical bath deposition (CBD) as buffer layers of Cu(In,Ga)Se2 (CIGS) solar cells, which are increasingly commercialized as the next generation photovoltaics. CdS thinfilm, which is commonly used as the buffer layer of CIGS solar cell, contains highly toxic material, i.e. Cd. Therefore, it is important to use less toxic Zn(O,S) in place of CdS for constructing environmentally-friendly solar cells. We are precisely investigating the effects of local structures of Zn(O,S) depending on formation condition on its solar-cell characteristics utilizing SPring-8, the third-generation synchrotron radiation facility.

 

 


Thank you for

NEDO (New Energy and Industrial Technology Development Organization) Research and Development on Innovative Solar Cells –Research and Development on Thinfilm Multijunction Solar Cells with Highly Ordered Structures (Wide-Bandgap Oxides) (2008-2014)

NEDO (New Energy and Industrial Technology Development Organization) Development of Next-generation High-performance Technology for Photovoltaic Power Generation System (Development of High-Efficiency Flexible CIGS Solar Cell Modules) (2010-2015)

JST-CREST (Japan Science and Technology Agency - Core Research for Evolutional Science and Technology ) Creative research for clean energy generation using solar energy (Bandgap Science for Organic Solar Cells) (2009-2014)

JSPS-KAKENHI (Japan Society for the Promotion of Science - Grants-in-Aid for Scientific Research) Preparation of High-Efficiency Oxide Solar Cells by Electrochemical Heteroepitaxial Growth (2010-2012)

JSPS-SAKURA (Japan-France Integrated Action Program) Electrochemical Construction of High Quality and Nano-Structured Oxide Photovoltaic Devices (2011-2012) ( Collaborated with Centre National de la Recherche Scietifique (CNRS), Ecole Nationale Superieure de Chimie de Paris(ENSCP))


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