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Madrid team sets dual-junction solar record

Researchers working on lattice-matched III-V solar cells at Madrid's Polytechnic University set a new world-best efficiency mark of 32.6 per cent at over 1000-sun concentration.

Researchers in Spain, a country now at the forefront of solar energy development, have set a new efficiency record for dual-junction solar cells.

The team, led by Carlos Algora at the Universidad Politécnica de Madrid, has made a lattice-matched III-V device that converts 32.6 per cent of received sunlight into electricity at 1026-sun concentration.

The result represents a significant improvement on the previous best of 31.0 per cent, set eight years ago by the Fraunhofer Institute in Freiburg, Germany.

In fact, the same Fraunhofer group was closely involved in setting the latest milestone, having independently measured the cell efficiency at its calibration laboratory.

Speaking to compoundsemiconductor.net, Algora highlighted two key aspects of the Madrid group s cell that set it apart from similar designs that are based on III-V materials.

Most developers of these high-efficiency cells use lattice-mismatched structures, Algora said. While this allows far more freedom in selecting material combinations that absorb in the desired wavelength range, it does have drawbacks relating to defect control and material quality.

Using lattice-matched materials instead means that defects are far less of an issue, something that ought to lead to good yields when the cells are manufactured in high volumes.

Based on a GaAs/GaInP epitaxial structure and a GaAs substrate, the Madrid approach also reduces series resistance in the cells, which Algora believes leads to the second key advantage of the lattice-matched design.

This relates to the trade-off between cell efficiency and the degree of focus used. In lattice-mismatched designs, efficiency tends to peak at around 200-300 suns. However, cells that are optimized to more intense focusing, equivalent to 500 or even 1000 suns, should provide more energy per dollar.

"We are aiming for optimum performance at about 1000 suns," said Algora. "So the structure, including the tunnel junction and the front metal grid, are specifically designed to reach the maximum efficiency [at this concentration]." Significantly, the Madrid cell maintains a very high efficiency of 31.1 per cent, even at nearly 3000-sun concentration.

Algora believes that the existing dual-junction design could be pushed up to efficiencies of almost 35 per cent, by improving the current-matching between the top and bottom cells.

The Madrid group is also working to incorporate a third junction, based on germanium, to improve efficiency. "We should be able to grow a similar dual junction cell on a germanium substrate, with a third junction created in the substrate by dopant diffusion."

According to the group's modeling studies, such a structure would result in a 41 per cent efficiency cell at 1000-sun concentration.

Although the elevated efficiencies of triple-junction cells have received much more attention, Algora says that the dual-junction approach could have an impact in commercial CPV systems:

"The efficiency difference between triple- and dual-junction cells can be considerably reduced when operated under real conditions," he observed.

"In such conditions, the variation of the spectrum throughout the day and the year could result in average electricity production [that is] only slightly higher for triple-junction cells."

"Then, open issues such as the long-term reliability of both types of cells could become crucial."

Currently partnering with both domestic and foreign solar energy companies, the Madrid team also holds a patent protecting a method for producing both dual- and triple-junction cells. Algora describes this method as "the LED-like approach", so-called because it details the most efficient and cheapest way to make such cells.

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