MECHANICAL WAFER ENGINEERING FOR SEMITRANSPARENT POLYCRYSTALLINE SILICON SOLAR-CELLS

A concept for the realization of semitransparent bifacially active hig hly efficient and light weight crystalline silicon solar cells is pres ented. The concept is based on the preparation of perpendicular V-groo ves in silicon blanks by mechanical abrasion using a dicing saw and be veled blades. Holes of variable diameter are formed automatically in t he processing step, which provide a connection between the passivated phosphorus doped front and back side emitters. A maximum bulk-emitter distance of approximately 30 mum has been realized in 200 mum thick st ructures which should result in highly efficient solar cell devices ev en in small grain low quality polycrystalline material. The partial tr ansparency of the presented solar cell structure opens the way for new applications (crystalline Si photovoltaic windows, etc.). The feasibi lity of the mechanical grooving process has been demonstrated on Wacke r SILSO cast silicon. Double-side V-grooved structures (distance betwe en grooves 90 and 140 mum, bevel angle 35-degrees-) with hole diameter s in the range 10-70 mum, corresponding to a transmittance of up to 30 % in the visible, have been prepared with excellent uniformity and mec hanical stability over a large area (5 x 5 cm2) . An average total ref lectance in the range 500-1000 nm of R(av) = 0.9% has been measured on a structure with a geometrical hole fraction of 1.7% after growth of a 1170 angstrom thick layer of thermal oxide. This SILSO structure had an effective silicon thickness of 120 mum, whereas the absorptance sp ectrum near the band edge was similar to a 5.5 mm thick nongrooved sil icon wafer, indicating the excellent light trapping obtained.