Macroporous Si as an absorber for thin-film solar cells

We separate 15 μm to 32 μm thick macroporous Si films from n-type crystalline Si wafers by electrochemical etching under back-side illumination. These layers are surface passivated and have openings in the oxide, for local doping and contacting, over 1% of the cell area. We measure effective lifetimes within a range of 2 to 3 μs. With an analytical model for the effective lifetime in macroporous Si we find an average surface recombination velocity of 75 cm/s, dominantly caused by the contact openings. The measured optical absorption of a 26 μm thick layer allows for a maximum short circuit current density of 37 mA/cm². The potential efficiency with the current surface passivation quality is 17%. The mate- rial loss for separating the macroporous absorber from the wafer is 5 μm.