Silicon nano and microwire arrays have the potential to enable low-cost, high-efficiency solar cells via efficient radial minority carrier collection in materials with low minority carrier diffusion lengths. This can be achieved by creating an optically thick array of wires aligned normal to a substrate where each wire has a radial, solid state pn junction, enabling light absorption along the entire wire length but minority carrier collection radially along the wire minimum dimension [1].

Densely packed, large-area, vertically aligned silicon nanowires for photovoltaic applications have been grown by a vapor-liquid-solid (VLS) chemical vapor deposition (CVD) process at atmospheric pressure on Si(111) substrates with SiCl₄/H₂. Au, Cu, and Ni all catalyze silicon wire growth near 1000^oC. The percentage of wires oriented vertically with respect to the substrate significantly improves with increasing growth temperature. Transmission electron microscopy (TEM) indicates that wires are single crystalline and grow along the <111> direction.

Near-field scanning optical microscopy (NSOM) measurements show that surface recombination dominates bulk recombination in VLS grown wires up to several microns in diameter. Therefore, highly ordered arrays of order micron diameter wires are also being investigated. Microwire arrays are fabricated by photolithographic patterning and etching of a thermally grown oxide, evaporation of catalyst metal, resist lift-off, and VLS wire growth. The oxide serves as a surface diffusion barrier for metal catalyst atoms during the growth process, preventing droplet ripening and maintaining pattern fidelity. Arrays with nearly 100% vertically aligned wires, exceeding 75 μm in length can be achieved over areas of at least 1 cm². Efforts towards fabrication and characterization of a completed photovoltaic device are ongoing.

[1] B. M. Kayes, H. A. Atwater, and N. S. Lewis, J. Appl. Phys., 97, 114302 (2005).