Monocrystalline silicon film process is ten times faster for solar cells
This could improve the raw material yield to nearly 100% and drastically reduce manufacturing costs while maintaining the power generation efficiency of monocrystalline silicon solar cells, which are used in most high efficient solar cells. Raw material costs account for about 40% of the current tandem modules, and by making them flexible and lighter with a thin film process, they can be used in more places with lower installation costs.
Monocrystalline Si solar cells using a lift-off process have challenges with the formation of a high-quality thin film Si at the wafer level, as well as achieving a porous structure that can easily be lifted and peeled off. Improving the growth rate and the yield of the raw material and being being able to use the substrate after lift-off without any waste are two other challenges.
The joint research team f Professor Manabu Ihara and Assistant Professor Kei Hasegawa of the Tokyo Tech, and Professor Suguru Noda of Waseda University developed a high-quality thin film monocrystalline silicon with a thickness of about 10 μm and a reduced crystal defect density down to the silicon wafer level.
This used a double-layer nano-order porous silicon generated on the surface of a monocrystalline wafer using an electrochemical technique. The surface was then smoothed to a roughness of 0.2 to 0.3 nm via a unique zone heating recrystallization method (ZHR method), and this substrate was used for high-speed growth to obtain a moonocrystalline thin film with high crystal quality.
The grown film can easily be peeled off using the double-layer porous Si layer, and the substrate can be reused or used as an evaporation source for thin film growth, which greatly reduces material loss.
When the surface roughness of the underlying substrate is reduced by changing the ZHR method conditions, the defect density of the thin film crystal that was grown decreased, and the team eventually succeeded in reducing it to the Si wafer level of about 1/10th. This quantitatively shows that a surface roughness in the range of only 0.1-0.2 nm, just a few atoms thick, has an important impact on the formation of crystal defects, which is also of interest as a crystal growth mechanism.
The film formation rate and the conversion rate of the silicon source to the thin film are bottlenecks in the production of thin-film monocrystalline Si. With traditional chemical vapour deposition (CVD) at 1414ºC, which is mainly used for epitaxy, the maximum film forming rate is a few μm/h and the yield is about 10%. Instead the researchers at the Noda Laboratory of Waseda University vaporised the silicon at temperatures over 2000ºC in a rapid evaporation method (RVD) capable of depositing Si at 10 μm/min.
It was found that the ZHR technology developed this time can resolves technical problems and drastically reduce the manufacturing cost of the lift-off process.
The team is now measuring the carrier lifetime of the thin film, which is directly connected to the performances of solar cells, and using the lift off thin films for low cost bottom cells in tandem type solar cells with an efficiency of over 30%.
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