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.---From ElectronicsWeekly.com 30 June ,2009