Research Center for Photovoltaics

In order to cut the power generation cost of photovoltaic systems, it is essential to improve the efficiency and to reduce the production cost of photovoltaic modules. At the Advanced Processing Team, we develop and investigate the fabrication process of silicon-based materials and devices, aiming for realizing high-efficiency and liw-cost solar cells.

Silicon-based solar cells can be classified into two groups: crystalline silicon (c-Si) and thin-film silicon technologies. In both types of solar cells, hydrogenated amorphous silicon (a-Si:H) and its alloy materials are extensively used as a junctuion / surface passivation layer as well as a light-absorbing layer. Aiming at high efficiency a-Si*H/c-Si heterojunction and thin-film silicon solar cells, we develop deposition processes of a-Si:H to improve the film quality while preventing damages on underlying and interface layers during the film growth. On the other hand, light management technique is one of the most important research topics in silicon-based solar cells, as the light absorption in silicon is relatively weak. We develop various light trapping structures based on sub-micron-sized patterning to maximize the light-absorption in a silicon layer of limited thickness. Fuethermore, the developments of next-generation silicon materials are being implemented towards significant coat reduction and higher efficiency.

For the growth of high quality a-Si:H, we investigate and control the detrimental deposition species (such as reactive radicals and high-energy ions) generated in plasma-enhanced chemical vapor deposition (PECVD), through various in-situ diagnostic techniques and material characterization.  As a result, we have developed a novel deposition process (so-called triode PECVD) and demonstrated a stabilized conversion efficiencies of 10.2% (the world highest independently-confirmed efficiency at March 2016) in a single-junction a-Si:H solar cell.
　For light management, randomly textured substrates are commonly used to enhance light absorption in the cells.  We have developed a periodically-textured substrate with a honeycomb-shaped patterning, which enables us to provide efficient light trapping and to grow high-quality microcrystalline silicon layers at the same time.  Accordingly a high current density of 32.9 mA/cm2 with an absorber thickness of only 4 μm, and a high conversion efficiency of 11.8% (the world highest independently-confirmed efficiency at March 2016) have been achieved in single-junction microcrystalline silicon solar cells. 
　These R&D efforts have been integrated into multijunction devices, leading to stabilized efficiencies of 12.7% and 13.6% for double- and triple-junction solar cells, respectively (the world highest independently-confirmed efficiency for respective device configurations at March 2016).  Furthermore, our technologies including thin-film silicon deposition, process monitoring and high-efficiency device fabrication are applied to a-Si:H/c-Si heterojunction solar cells, particularly for improving the surface passivation and light trapping with cost-effective thinner device designs.
　As a next-generation technology, we investigate the liquid-phase crystallization process using a high-density and line-shaped laser scanning system to realize efficient ultra-thin crystalline silicon solar cell.  Processing technology of Si nanocrystals and the potential of Si nanocrystals for solar cell application are investigated.

  Figure.1　Outline of research areas and subjects in Advanced Processing Team.

  Figure.2　Schematic illustration of real-time diagnostic tools equipped in a PECVD system for developing high-quality and low-damage film deposition processes.

  Figure.3　Left: Surface profile of the size-variable honeycomb-textures patterned on c-Si surface.  Right: Appearance of the thin-film silicon solar cells formed on a honeycomb-textured substrate (5 cm in diameter).

 ⇒  recent publication list