Figure: Examples of complex epitaxial nanowire structures grown by Sol. (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, on-grid solar generation plants are uncommon and relatively small (10 MW or below), and cost / kWh remains high in comparison to fossil fuel generation costs.

Sol will extend photovoltaic technology beyond today’s medium-efficiency / medium-cost silicon based cells and high-efficiency / high-cost III-V cells that are made from multiple planar layers and III–V heterostructure materials, lattice matched grown on expensive and small size III–V or Ge substrates. While the commercially available silicon cells are simply integrated in modules without any optics, the latter approach is in general used for higher-end applications (such as space or military) in combination with optical concentrators and tracking systems.  The optical concentrators focus sunlight by a factor of 400-1000 times and reduce the required cell size of these expensive III–V devices.  The geometries of the concentration optics require cells of different areas, and in some cases require mounting on materials for cooling.

 Sol Voltaics has developed technologies to fabricate perfect one-dimensional crystalline semiconductor structures (nanowires), and complex structures composed of nanowire-trees , with direct control of morphology and chemical composition, including atomically-sharp heterostructures.  Such nanowires hold great promise for solar electrical conversion because:

1. they are produced to a large extent with self-assembly techniques, providing complex structures with relatively simple processing
2. the limited radial extent of the nanowires allows heteroepitaxial combination of materials with virtually no regard to lattice matching or thermal expansion
3. a high absorption coefficient can be achieved in the nanowires
4. the need for large area coherent growth is avoided because nanowire growth is inherently more tolerant to inhomogeneities over a large area wafer than current state of the art, thus relaxing processing tolerances and thereby increasing manufacturing yields and reducing manufacturing cost.

Together, these advantages will permit simple growth of complex networks of optically active heterostructured materials with monolithically integrated electronic devices on inexpensive mono and poly-crystalline Si substrates.

The Company’s goal is to develop and commercialise highly efficient multiple heterostructure III–V or III–nitride nanowire based multi-junction PV cells fabricated on readily available low-cost and large area silicon wafers using enhanced, lower cost epitaxial growth processes that are readily scalable to mass production. We anticipate developing and offering nanowire-based concentrator PV solar cells to the market with efficiencies equal to or higher than state-of-the art planar multi-junction cells while at the same time dramatically reducing cost.

When achieved, the initial market to be addressed will, in cooperation with selected concentrator module manufacturers, be the on-grid concentrator solar plant market at expected generating costs / kWh approaching fossil fuel generation. Later markets to be addressed will include roof-top commercial scale CPV systems.