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Final Report Summary - R2M-SI (Roll to Module processed Crystalline Silicon Thin-Films for higher than 20% efficient modules)

The aim of this project was to lift off c-Si-thin-films in a cost effective manner, and to process them to integrated interconnected solar cells. One main goal was therefore to provide material for solar cells with an efficiency potential of >20%. Recent scientific publications as well as founding of startup companies aiming to use comparable material show the general interest in the topic in the PV community.
As handling issues stopped quick progress so far, the main goals of the R2M-Si project were to enable the use of lift-off films in a nearly handling-free approach. Thereby limitations arising from handling issues can be avoided. For this purpose Fraunhofer (ISE), University of Konstanz (UKON), Interuniversity Microelectronics Centre (IMEC), Stiftelsen for industriell og teknisk forskning (SINTEF), Ioffe Physical-Technical Institute of the Russian Academy of Sciences (IOFFE), Rena Sondermaschinen GmbH (RENA), and the company S’Tile joined forces.
The expected impact of this project was not only the ability to provide high-quality, large-area silicon foils, but also a tool beyond state of the art, which is able to produce silicon foils with high throughput and very low costs. Along with a tailored solar cell and module concept this should lead to potential conversion efficiencies of 20% and higher.
Within the R2M-Si project we worked successfully on both the theoretical understanding and the actual process development of porosifying and lifting off layers from flat wafers as well as from the circumference of a silicon rod. Studies on grain orientation dependencies of the porosification along with tool development towards a continuous formation of a separation layer were accompanied by in depth understanding and thus optimisation of the reorganisation and subsequent epitaxial growth process and the introduction of a special gluing process.
As results of this work a circumference of a 150 mm ingot was etched in a quasi-continuous mode and subsequently successfully detached. A comparable but smaller piece of porous Si has also been successfully detached, glued both to a standard multi crystalline wafer as well as a sintered Si wafer and after gluing reorganised and epitaxially thickened.
The consortium was also able to show epitaxially thickened attached and non-attached reference foils with diffusion lengths > 150 µm and corresponding cell efficiencies of up to 15.2%. This process has been transferred to a CVD reactor able to work in a continuous mode (ConCVD), resulting in epitaxial layers showing etch pit densities as low as 4e4 cm-2 and diffusion lengths of up to 70 µm. The latter translates in solar cell results of open circuit voltage up to 640 mV, fill factors up to 79% and respective efficiencies of up to 13.8%. Additionally an in-situ p-n junction epitaxy process grown in just one pass through the ConCVD has also been developed within the project.
Further down the line in the value chain also a new integrated solar cell and module concept has been proposed and also shown. Accompanied by thoroughly conducted simulations using 1, 2 and 3 D simulations a device structure theoretically capable of more than 18% has been identified. By preparing actual mini modules following the new concept design open circuit voltages of 3,05 V with 5 cell mini module has been shown.
All these steps represent a big step towards the initial goal of providing material for solar cells with an efficiency potential of >20% at low costs.

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