HIGH QUALITY EPITAXIAL FOILS, OBTAINED BY A POROUS SILICON BASED LAYER TRANSFER PROCESS, FOR INTEGRATION IN BACK CONTACTED SOLAR CELLS

摘要

The creation of foils by lifting off a thin, high quality layer of a silicon substrate is one of thepromising substitutes for wafer sawing to create substrates thinner than 100 μm. The porous silicon-based layertransfer process is a well known method to obtain high quality foils. Despite a number of convincing lab-based solarcell show-cases, there is no breakthrough of this technology at (semi)-industrial level, because of the poor yield ofprocessing free standing foils. This paper presents a method to fabricate back contacted solar cells based on epitaxialfoils avoiding processes on free-standing foils. First, a porous silicon layer is electrochemically etched, acting as aweak sacrificial layer to detach the foil that is epitaxially grown on top of the porous silicon layer. An effectivelifetime around 100 μs is obtained for those epitaxial foils, indicating sufficiently good quality for solar cellfabrication. Front-side processing is done while the epitaxial foil is still attached to its parent substrate. Crackinitiation is obtained by laser scribing the foil. A UV nanosecond pulse laser is used to induce the weakening byscribing trenches. Afterwards, the front-side of the foil is permanently bonded to a quartz carrier by siliconeadhesives and detached from its parent substrate. This lift-off process is studied in detail. A yield of around 80% isobtained for samples that went through the process chain up to the detachment step. The rear side of the solar cell isprocessed while bonded to glass. So far, the rear-side processing sequence was performed on Float-zone referencewafers as a proof of concept and still needs to be applied on epitaxial foils.