Microstructure and Surface Chemistry of Nanoporous 'Black Silicon' for Photovoltaics

We report our detailed investigation of the microstructure and surface chemistry of nanoporous black Si layers using transmission electron microscopy techniques. We find that the one-step nanoparticle-catalyzed liquid etch creates deep conical nanovoids. The cones provide the density-graded surface that suppresses reflection. The surface of the as-etched nanoporous black Si is an amorphous Si suboxide (SiO_x) produced by the strongly oxidizing nanocatalyzed etch. The oxygen concentration decreases monotonically away from the nanovoid surface. This suboxide tends to be thinner near the cone tip than nearer to the wafer surface. The c-Si/suboxide interface is rough at the nanometer scale. Diffraction contrast reveals a high density of point defects in the c-Si near the c-Si/suboxide interface. These features account for the poor blue response of as-etched black Si solar cells. The passivation treatment is seen to convert the Si suboxide quite completely into SiO₂ with a smooth c-Si/SiO₂ interface. These changes are essential to achieve our 16.8%-efficient solar cells.