Traditional solar cell architectures require expensive, high-quality crystalline materials due to their long carrier diffusion lengths. However, 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].

Silicon nano and microwires 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 and Au, Cu, and Ni catalysts. Transmission electron microscopy (TEM) indicates that wires are single crystalline and grow along the <111> direction. While Au is traditionally employed as a catalyst for VLS wire growth, our studies show that Cu and Ni can also catalyze the growth of vertically aligned wires. In particular, Ni is not a deep-level trap in silicon and is significantly cheaper than Au, an important consideration for large-area applications such as photovoltaics.

Vertically aligned, densely packed, large-area arrays of silicon nano and microwires are fabricated by either self-assembling or templating metal catalysts on Si(111) substrates. Arrays with wire diameters between 50 and 250 nm are created via drop casting and self-assembly of Au colloid. Highly ordered microwire arrays are fabricated by photolithographic patterning and etching of a thermally grown oxide, evaporation of catalyst metal, and resist lift-off. The oxide serves as a surface diffusion barrier for metal catalyst atoms, preventing droplet ripening and maintaining pattern fidelity during the growth process. Arrays with nearly 100% vertically aligned wires, exceeding 75 μm in length can be achieved over areas of at least 1 cm². Achievement of large-area arrays is an important step toward realizing a functioning device.

Final device fabrication requires properly doping wires to obtain radial pn junctions, passivating surfaces to reduce recombination rates, conformally depositing an optically transparent material for structural support, and deposition of front and back electrical contacts. Formation of radial pn junctions can be achieved through a combination of in situ doping during wire growth and ex situ diffusion doping. The optical and electrical properties of arrays and individual wires passivated by thermal oxidation and organic functionalization are investigated with photoluminescence (PL) and near-field scanning optical microscopy (NSOM). Conformal filling of regions between these high-aspect ratio wires can be accomplished with spin-on-glasses, CVD deposited films, as well as polymeric materials.

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