Figure: Examples of complex epitaxial nanowire structures. (a) GaP nanotrees; (b) axial InAs/InP double-barrier heterostructure; (c) GaN nanowires grown on GaN; radial GaAs/GaInP core-shell heterostructures (d) from the side, (e) from the top.

Nanowires, which do not exist in nature, are defined as structures with diameters in the range of tens of nanometers and lengths in the order of nanometers to micrometers. In practice, nanowires are usually symmetric in the two smaller dimensions with a round or polyhedral cross-section. The most controlled way to fabricate these nanowires is via metal particle assisted epitaxial growth. As the growth front of the wire is defined by the interface between the top plane of the wire and the bottom of the Au particle, the growing wire lifts the particle. The diameter of the wire is mainly determined by the diameter of the Au particle.

Today, Sol routinely grows nanowires several micrometers in length and with only 5–200 nm in diameter. Since the end of the 1990s, the nanowire research field has significantly expanded worldwide.  To date nanowires have been grown in most III–V materials combinations and with several epitaxial techniques, e.g. MOVPE (metal organic vapor phase epitaxy), CBE (chemical beam epitaxy), and MBE (molecular beam epitaxy). Even growth methods without metal seed particles have been developed, using selective-area growth (SAG).

Rapidly growing attention is being given to solar-generated electricity as an attractive, carbon-neutral renewable energy source. However, state-of-the-art (SOA) photovoltaic conversion efficiencies remain below 41% even for the most sophisticated and expensive multi-junction concentrator cells and typically at or well below 20% for more common Si PV cells offered in the market. As a result,  cost / kWh remains high in comparison to fossil fuel generation costs.

Semiconductor nanowires are very promising for making a solar cell that is both  inexpensive and efficient enough to finally compete on a cost basis with other energy production methods. Nanowire-based solar cells are being researched by several groups around the world, aiming at improving the collection of electrical current, the light absorption in the material, or the antireflective properties of the nanowire structure, as compared to the same material in planar form. The nanowire materials investigated include silicon, III-V and II-VI semiconductors and are produced by a variety of methods. Most nanowire approaches require that the wires grow from a substrate in a reactor, and such batch processing methods are today by  their nature slow and costly.

However, Sol Voltaics is developing a novel production method for nanowire solar cells which combines the low cost and substrate versatility of thin films with the high conversion efficiency of III-V materials. The Company's approach is twofold.  First, Sol Voltaics develops technology (called Aerotaxy) for initially producing self-assembling, free-flying nanowires in the gas phase, in a continuous and scalable flow process. Each wire is a small solar cell (diode)  that can be applied directly from the gas phase, or portably stored in a liquid or as a dry powder for later use by a customer. Secondly, the Company also develops process technology for aligning the nanowire diodes self-assembled in the gas phase and then vertically depositing them on any substrate, to form a dense array of identical solar cells, which are contacted in parallel to form a solar panel of any size.

The nanowires can be made in a variety of materials, which absorb different parts of the solar spectrum, to make solar cells tailored to the specific needs of a customer. In the future, more advanced structures are possible in each nanowire, such as core-shell, or even multi-junction cells. Multi-junction nanowire cells pave the way for efficiency improvements beyond that of single-junction and could lead to conversion efficiencies of 40%, 50% and even 60%. Sol Voltaics has already demonstrated multi-junction nanowires using MOCVD and intends, over time, to develop means to be able to port it to its evolving Aerotaxy technology.