See also: UK firm QuantaSol breaks solar cell world record
See also: Electronics Weekly focus on solar cells
QuantaSol's record-breaking solar cell is GaAs-based with multiple quantum wells to broaden its absorption spectrum, and is made in the UK.
The Surrey-based spin-out from Imperial College revealed the cell earlier this week, claiming that the single-junction cell was 28.3% efficient illuminated by 500x concentrated sun light.
The quantum wells to lower the junction's effective bandgap, allowing it to absorb wavelengths too long to interact with unmodified GaAs.
The quantum wells are made from layers of InGaAs, a material not lattice-matched to GaAs substrates.
"We need fifty quantum wells for a solar cell, which would normally just form dislocations and relax, so we work with a compensating barrier," CTO Keith Barnham told Electronics Weekly.
InGaAs has a bigger lattice constant than GaAs, so the firm is interspersing barrier layers of GaAsP which has a smaller lattice constant than GaAs.
The result is stresses between each layer, but a balanced overall structure which matches the substrate.
"We can grow up to 65 quantum wells without dislocations, and we think we can go higher," said Barnham, who added that QuantaSol has grown similar stacks on germanium substrates.
At high sunlight concentration, quantum-well radiative recombination is a significant loss mechanism - photons form when conduction band electrons at the bottom of wells join valence band holes at the top.
"Fortunately, a feature of our particular design means these photons will only be absorbed by the substrate," said Barnham - and the company forms a Bragg reflector on the substrate to reflects these back into the junction where some are recycled.
The start-up intends to become a significant fabless solar cell maker.
"We use all UK foundries," CEO Kevin Arthur told Electronics Weekly with out revealing which ones. "Our objective is to ramp-up to high-volume manufacture."
The cells are aimed at makers of concentrator photovoltaic (CPV) assemblies for solar farms.
These use lenses or mirrors to concentrate sunlight - traditionally by 500 times - onto small - usually 10x10mm - solar cells: typically yielding 14W from a 1cm2 cell.
Proponents of CPV say that the cost of focusing optics, steering system, heat sinking and exotic semiconductor materials in a CPV system is still less per kWh than conventional flat panel photovoltaic systems.
The argument is that, although flat plate systems use silicon which is far cheaper than GaAs, they need at least 500 times more semiconductor area to catch the same amount of sunlight.
And as silicon solar cells are only 12-15% efficient, 1cm2 of GaAs in a CPV system should actually be compared with 1,000cm[super2] of flat plate silicon.
"The sweet-spot for CPV is installations from a few hundred kilowatts to 100MW," said Arthur.
Flat panel makers would argue with these costing assumptions, but the CPV brigade is upping the anti. "The main players are starting to increase the concentration ratio, aiming for 1,000x," said Arthur.
According to QuantaSol, there are already 35-50 CPV system suppliers, supplying mainly California, but also Europe; and the first Asian CPV supplies are appearing.
QuantaSol claims that it is unique amongst CPV cell suppliers because it can modify its quantum wells to engineer the absorption spectrum, whereas competitors can only change semiconductor.
"One of the great things about quantum wells is that they allow you to adjust the bandgap," said CTO Barnham. "We can manufacture cells specifically for different locations in the world."
The firm is looking to engineer junctions specifically for multi-junction cells.
Its aims are 32.6% from a tandem cell consisting of an InGaP top junction epitaxially grown on a GaAs junction, with wells in both junctions to broaden the absorption spectra; and in the last quarter of this year to have a 38% efficient triple junction cell.