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Energy from a thousand suns

Projektinfo – Detailed information on energy research read short description Our four-page `Projektinfo` articles will keep you informed of the latest results in the field of energy research. With our well laid-out, critical and easy-to-understand presentation, you will find out what the actual benefits of these projects are. Find out today about tomorrow`s topics and projects also available as a subscription. subscribe projektinfos

New multi-junction solar cell reaches an efficiency of over 44 per cent with focusing optics at an up to thousand-fold concentration of sunlight.

© AZUR SPACE Solar Power GmbH



Concentrated photovoltaic technology Projektinfo 02/2014



With multi-junction solar cells, the sub-cells supplement one another, each specialising on a partial area of the solar spectrum. Since they therefore use a significantly greater proportion of the solar spectrum, multi-junction cells achieve a greater efficiency than silicon cells.

© Fraunhofer ISE



Comparison of a silicon solar cell (left) and a solar cell from III/V semiconductors (right). The rainbow strip shows the spectral range used by the cells. The triple-junction cell is composed of up to 30 layers.

© AZUR SPACE Solar Power GmbH



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Energy from a thousand suns Multi-junction solar cells were originally developed as energy sources for space flight applications. Their output is high but their manufacture is complex. With their perfectly matched layers, the cells utilise a much wider spectrum of sunlight than silicon cells. For terrestrial use, the multi-junction solar cells are combined with concentrator systems. Like a magnifying glass, these focus a hundred- to thousand-fold concentrated beam of light onto the pinhead-sized solar cells. For this purpose, a solar tracker is used to enable the entire system to track the precise position of the sun. A new record solar cell has achieved an efficiency of more than 44 per cent.

Concentrated photovoltaic (CPV) technology is particularly suitable for generating electricity in areas with high direct solar irradiance, where it achieves double the efficiency of conventional modules based on silicon cells. In particular, CPV is deployed in large power plant parks with up to 100 megawatt capacities. Research establishments such as the Fraunhofer Institute for Solar Energy Systems ISE and solar power companies such as AZUR SPACE or Soitec are working together on increasing the efficiency of concentrator solar cells and reducing the manufacturing costs. They are developing new production processes and modifying the structure of the solar cells. The activities cover all production stages, ranging from the materials, components and solar cells to the module and system construction. To ensure that the overall system can work highly efficiently, the high-performance cells, optics, tracker and control system must be optimally matched with one another. This close interrelationship between the system components needs to be taken account of in the development, system improvements and industrial production.

Multi-junction solar cells achieve top performances The core of CPV power plants are the multi-junction solar cells. They differ fundamentally in terms of their structure and manufacturing process from the widely used silicon solar cells. Multi-junction cells consist of more than just one semiconductor material. Instead they consist of several cells made of different semiconductor materials from group III and V of the periodic table, which are stacked on top of one another. Gallium indium phosphide (GaInP), gallium indium arsenide (GaInAs), and germanium (Ge) are used for example. Each of these semiconductors uses a different wavelength range of sunlight to generate electricity (Fig. 1). The interaction of these sub-solar cells creates a high efficiency. The researchers are combining the expensive-to-manufacture cells with low-cost optical concentration. “Deploying the cost-effective focussing optics enables sparing use of the comparatively expensive semiconductor materials. Depending on the concentration factor, you only require five hundredth to one thousandth of the semiconductor material and this also increases the efficiency of the solar cell,” explains Dr Andreas Bett, Deputy Head of Fraunhofer ISE. These solar cells currently achieve an average efficiency of 40 % in production and up to 44.7 % in the laboratory. The CPV modules produced have an efficiency of 32 % and the CPV system’s AC efficiency amounts up to 28 %. The GaAs-based multi-junction solar cells withstand considerably higher temperatures than silicon-based cells. To ensure that they do not heat up too much owing to the extremely intense radiation, the thermal energy is passively removed using a suitable heat sink or by means of active cooling.