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).
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