Thin 60 μm-Thick Crystalline Silicon Solar Cell on Ceramic Substrate by Al-Bonding

The fabrication of a highly efficient thin silicon wafer solar cell by bonding a 60 μm-thick monocrystalline silicon wafer to a cost-effective SiC ceramic substrate in the early stage of the cell fabrication sequence was demonstrated. In the process sequence, the rear side of a thin textured silicon wafer and the top side of the ceramic carrier were coated with aluminium. A phosphorous dopant was applied on top side of the wafer by a spin-on technique. For bonding the silicon wafer to the carrier and for the formation of a pn-junction a single annealing step in a rapid thermal processing (RTP)-furnace was applied. This sintering step simultaneously generates: i) a reliable adhesion to the ceramic carrier, ii) a low ohmic metallic base contact on the rear side, and iii) an n-type emitter with a sheet resistance of 50-60 ohm/sq. In this work we report on the successful realization of an effective aluminium back surface field (BSF) formed in the same process step. From a fit of the internal quantum efficiency (IQE)-curve the effective diffusion length is determined to be L_eff>200 μm. Compared with a cell bonded by an Ag-containing metal paste we see a clear increase in the spectral response in the long wavelength region between 800 nm and 1050 nm indicating the formation of a BSF. The best 60mum-thick solar cell with 4 cm² cell area produced with this technique on a 4" wafer shows a conversion efficiency of 15.0%