Laser Chemical Processing (LCP) of Poly-Silicon Thin Film on Glass Substrates

Abstract

Laser chemical processing (LCP), based on the patented LaserMicroJet technology by Synova® S.A, was introduced by Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE), as a novel approach for selective doping for high efficiency (> 20%) silicon wafer solar cells. The technique consists of coupling a laser beam into a highly pressurised thin liquid jet. Total reflection inside the liquid jet enables laser light to be wave-guided towards the sample of interest. If the liquid contains a dopant source, selective doping is possible via the laser-induced, physical and chemical interactions of the substrate and doping medium. To date, this process was primarily investigated for silicon wafer solar cells. In this work, we report on a novel application of LCP for n-type doping of poly-silicon thin films on glass substrates. By using phosphoric acid as the doping medium, we have successfully realised n-type doping of poly-silicon thin films through LCP. Proof-of-principle experimental results are promising in terms of sheet resistance (< 5 kΩ/□) and active dopant concentration of 5x1018 to 1x1019 cm-3 at a doping depth of less than 250nm as measured by electrochemical capacitance-voltage (ECV) profiling. The obtained sheet resistance and doping concentration levels of LCP doped areas opens a new frontier for LCP processing. In the future, the LCP technique will be applied to fabricate back surface fields (BSF) for poly-silicon thin film solar cells.