TECHNOLOGYSOLAR CELLS
Tuning the triple-junction
Extracting the very best performance from a triple junction photovoltaic demands optimisation of the absorption edge of every sub-cell. Incorporating quantum wells into the cells can realise this, while allowing the device to be tailored for the spectral conditions where it will be deployed, say QuantaSol’s Keith Barnham, Alison Dobbin, Matt Lumb and Tom Tibbits.
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the global economy, deployments continue to rise at an exponential rate in many countries, according to a recent report from the International Energy Agency’s Photovoltaic Power Systems Programme. Germany continues to lead the way in photovoltaic deployment thanks in part to the success of its feed-in tariff program, and in 2009 it installed nearly 4 GW. Now its recipe for success is being adopted by several other countries, including the UK, through the introduction of their own market stimulation packages.
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High concentration systems are automated to track the sun and keep the sunlight focused on the small, high efficiency cells. The first systems have been deployed in locations with large amounts of direct solar insolation
Image courtesy of Daido Steel
Maintaining this rapid growth in cell deployment hinges on further cost reductions, which is possible with the introduction of third generation cells. Those that are tipped to make the first major contribution are the triple-junction cells based on GaInP, GaAs and germanium. This type of device is already the cell of choice to power satellites, and when it is used on earth it can realise efficiencies in excess of 40 percent – roughly three times that of second-generation, thin-film cells made from materials such as cadmium telluride and copper indium gallium selenide.
The key to realizing low costs compared to second- generation cells is to use the triple-junction devices
hotovoltaic installations are going up and up. Despite the sharp downturn in
in concentrator systems that employ cheap plastic mirrors and lenses to focus sunlight by factors of around 500 onto the expensive III-V cells.
Like any new technology, the initial cost of these concentrating photovoltaic (CPV) systems is high. This partly accounts for their initial deployment in the very sunniest climes, where it is most cost- effective. Here engineers are honing the technology that is used to track the path of the sun across the sky and ensure that the sunlight is optimally focused on the cells. As time goes on, increased sales of this technology will drive down costs at both the system and cell level. This will increase the appeal of this technology, which can generate up to three times the electricity from the same system area as one based on second- generation cells.
Quantasol, a spin-out from Imperial College London was founded in 2007 and is well placed to supply high-efficiency cells to the growing number of CPV module and system manufacturers. One of our key strengths is our exclusive, patent- protected, quantum-well (QW) technology that boosts the efficiency of triple-junction cells by around 3 percent in absolute terms (see Figure 1). This means that we can regularly realise 40 percent median efficiency across our production wafers, making the performance of our cells comparable to those made by the market leaders.
What’s more, our novel cells promise to set a new benchmark for efficiency that cannot be matched with conventional triple-junction cells, a factor that has helped us run a successful sampling campaign with customers through the US, Europe and Asia. This has swelled our order book, which stands in excess of 1MW of solar cells for delivery through the first half of 2011. We have been pioneering the commercial development of III-V
www.solar-pv-management.com Issue III 2011
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