Detailed information on energy research

Projektinfo 02/2014

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 com-bined 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 de-ployed 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 in-creasing 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 materi-als, components and solar cells to the module and system construction. To ensure that the overall system can work highly efficiently, the high-performance cells, op-tics, 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.

Energy from a thousand sunsMulti-junction solar cells with concentrator systems utilise a wide solar spectrum

This research project has been funded by the

Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)

Multi-junction solar cells achieve top performancesThe 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 sev-eral 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 expen-sive semiconductor materials. Depending on the concen-tration 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 An-dreas 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 con-siderably 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 pas-sively removed using a suitable heat sink or by means of active cooling.

Semiconductor stack makes optimum use of the solar spectrumA typical monolithic triple-junction solar cell consists of approximately 30 individual monocrystalline layers, which are grown in a single process on a Ge substrate. In triple-junction solar cells, three sub-cells convert different are-as of the light spectrum into electricity. GaInP is used as the material for the top sub-cell and GaInAs is used for the middle sub-cell. The Ge substrate is activated during the deposition process and serves as the third sub-cell. The three cells are arranged vertically and are internally connected via tunnel diodes and other functional semi-conductor layers. For the layered crystal growth of these 5-8-µm thin-film solar cells (size corresponds to fine dust), the metal organic vapour phase epitaxy (MOVPE) method is deployed. The deposition takes place on a Ge wafer with a diameter of 100 mm. 885 cells with a 3 x 3 mm2 format can be produced from this. In III-V multi-junction solar cells, several diodes (pn junc-tions) with sensitivities in the blue to infrared spectral range are stacked on top of each other. In order to convert sunlight into electricity even more effi-ciently and cheaply in future, it is intended that the next generation of these highly efficient cells will combine four or five sub-cells as quadruple- or quintuple-junction cells. Since 1995, III-V multi-junction solar cells have been used to provide power for satellites. Researchers from AZUR SPACE and Fraunhofer ISE are currently developing manufacturing processes for the next-generation cells. They are further improving these multi-junction cells that are specially tailored to the conditions in space. Their goal is to enable them to generate even more electrical

output per unit area, to be thinner and lighter, and to better withstand highly charged particles in orbit.

Lenses and mirrors bundle the lightAn optical system focuses the sunlight as efficiently, precisely and homogene-ously as possible onto the tiny solar cells. Under concentrated light, the effi-ciency of the energy conversion increases even more.High-concentration systems can be constructed either as point-focus systems with lenses and, if required, additional secondary optics, or as central receiver systems with mirror optics similar to large-scale solar thermal plants. Point-focus modules, such as the FLATCON® or CONCENTRIX™ systems, usually use Fresnel lenses as the primary optics and work with concentration factors of 350 – 500. The efficiency can be further increased using additional secondary optics (by <1.3 % absolute with FLATCON®). Passive cooling of the metal heat sink is sufficient (Fig. 4).With receiver systems, the radiation is focussed by up to a 1,000 times on a tightly packed concentrator module, which needs to be actively cooled.

Module assembly requires maximum precision The CPV systems must be able to sustainably withstand high thermal, mechani-

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Anti-reflective coating

n-dot. buffer and barrier layer Active Ge substrate, p-doped

p-GaInAs base n-GaInAs emitter n+-AlGaInP/AlInAs barrier layer p++-AlGaAs p+-AlGaInP barrier layer p-GaInP base

n-GaInP emitter n+-AlInP window layer

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Si: 19 % (1 x) 35 % (1 x), 43 % (500 x)GaAs triple-junction solar cell:

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Fig. 2 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.

Fig. 1 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.

cal and electrical loads, including energy densities in excess of 100 W/cm2, high temperatures, temperature fluctuations and wind loads. During the as-sembly of the modules in automated production lines, the cells, optical con-centrators and the heat sink units are precisely positioned and permanently fixed. For a plant with a capacity of 1 MW, more than 2 million individual ele-ments have to be assembled.

Precision tracking of the sunWhilst for low-concentration photovoltaics (concentration factor under 50), single-axis tracking with a mechanical accuracy in the degree range is often sufficient, higher concentrating systems require dual-axis tracking. The demands on the accuracy and stability depend on the concentration factor and optics deployed. With precise control, mechanical accuracies better than 0.1 degree can be achieved with 500-fold concentrating systems.The tracking systems work economically and the power for the drive is, on average, well below 100 W for system sizes in the kW range.

Power plants for sunny locationsRegions such as the southern Mediterranean, which has high direct solar ra-diation (> 1.800 kWh/m2/p.a.), are particularly suitable for constructing CPV

3BINE-Projektinfo 02/2014

power plants. The yield of the plants depends on the precise production, light distribution, tracker control and inverters. Compared with other PV systems, CPV systems generate a greater yield over a longer period during the course of the day, which is due to the higher output of the PV cells and the tracking (Fig. 4).Larger CPV power plants achieve system efficiencies of over 25 %. More than 160 MW were already installed in the sunbelt by 2013. The technology is particularly strong in the USA. There, power plants already operate with out-puts of more than 30 MW. A 44-MW power plant deploy-ing Soitec’s CONCENTRIX™ technology is currently being put into operation in South Africa. At such locations, concentrated photovoltaic technology achieves lower electricity prices than conventional silicon modules, which is demonstrated by economic feasibility calculations and the performance values of existing plants.

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Researchers are optimising the cells

In the joint project entitled “Economic production ca-pability for III-V concentrator solar cells” (WiFerKon), engineers and scientists from AZUR SPACE and Fraun-hofer ISE have focussed on the most important manu-facturing aspects of multi-junction solar cells, ranging from the structure production to proving the reliability. During this project, they have managed to develop a 40 %-class CPV cell with cost-optimised production and successfully introduce it to the market. In addition, the most important developments were made for a new, next-generation metamorphic triple-junction solar cell with a 42 %-class cell efficiency. The deposition rates during crystal growth (the so-called epitaxy) were also increased. In addition, the processes used with the pre-viously deployed wafers with a diameter of 100 mm have been transferred to the 150-mm format. This has enabled the energy and cost efficiency to be substan-tially increased in the production of CPV cells. The cells are already being manufactured in the produc-tion facility that commenced operation during the course of the project.

Another joint project, entitled “Development of inno-vative and cost-effective technologies for high-effi-ciency solar cells for next-generation concentrator modules” (InKoTeK), builds on the WiFerKon project. The aim of the project is to reduce the manufacturing cost per watt at the module level by 40 % by 2015. Based on the current production at AZUR SPACE, the intention is to optimise the entire value chain, rang-ing from the substrate, epitaxy and cell production to the assembly. The researchers are developing new production processes and are modifying the solar cell structure for efficiencies greater than 41 %. The project objectives have already been clearly exceeded during the course of the project. A new solar cell with an effi-ciency of 42 % has already been transferred to produc-tion. In view of the progress made with the production process for CPV, it is also clear that the potential for higher efficiencies and lower production costs is not yet exhausted.

Fig. 4 In concentrated PV solar power plants, lenses or mirrors focus the sunlight on small III-V multi-junction solar cells. Left a plant with CONCENTRIX™ modules. Right a plant with parabolic mirrors. The receiver is cooled and the heat collected is utilised.

Fresnel lens; solar cell

A B50 mm

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Aspheric total internal reflection facets

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Fig. 3 Examples of optical concentrators: Top left: Total reflection lens with secondary lens directly on the cell, below left: Fresnel lens; right: Dome-shaped Fresnel lens with beam homogeniser/light collector.

Fig. 2 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.

Fig. 1 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.

BINE Projektinfo 01/20104 BINE-Projektinfo 02/2014

One collector for electricity and heat Two technological paths for generating electricity and heat in sunny regions are proven and available: solar thermal and photovoltaic power plants enable the high solar irradiance to be utilised efficiently. Both use mirrors and optics for their concentrator systems.An interesting “hybrid form” of these technologies is provided by combining PV electricity and heat generation in so-called CPVT (concentrated photovoltaic and thermal) systems. These can achieve an overall efficiency of up to 70 % if the thermal energy can be used as process heat, for solar cooling or for thermal seawater desalination. Researchers at Fraunhofer ISE are investigating how the optics and receivers for such CPVT systems can be optimally designed. In the KoMGen project, they are developing a new concentrator technology where the concentration factor is increased by mirror optics. This deflects the light onto solar cells in a densely packed, central receiver unit that is mechanically separated from the optical system. This actively cooled central receiver enables the resulting heat energy to be decoupled and used.

Short energy payback timeThe researchers expect GaAs-based, multi-junction solar cells to be able to achieve efficiencies of up to 50 % by 2020. CPV is a highly promising technology for low-cost, sustainable electricity production, which promises further reductions in electricity generation costs for regions with high direct solar irradiance. It is remarkable how quickly CPV systems generate the energy that was used for their production: for example, the energy payback time for a FLATCON® system is less than 8 months.

Project participants >> Economic production capability for III-V concentrator solar cells (WiFerKon):

Fraunhofer-Institute for Solar Energy Systems ISE, Dr Andreas Bett, Freiburg, Germany, www.ise.fraunhofer.de AZUR SPACE Solar Power GmbH, Heilbronn, Germany, Dr Gerhard Strobl, Dr Victor Khorenko, www.azurspace.com

>> Development of the next generation of FLATCON concentrator modules: Soitec Solar GmbH, Freiburg, Germany, Dr Andreas Gombert, www.soitec.com

>> Development of innovative and cost-effective technologies for high-efficiency solar cells for nextgeneration concentrator modules (InKoTeK) joint project:

AZUR SPACE Solar Power GmbH, Heilbronn, Dr Gerhard Strobl, Dr Victor Khorenko, www.azurspace.com Photonic Sense GmbH, Eisenach, Dr Christian Hell, www.photonic-sense.com Philipps-Universität Marburg, Prof. Dr Kerstin Volz, www.physik.uni-marburg.de Sempa Systems GmbH, Dresden, Dr Jörg Koch , www.sempa.de CS Clean Systems AG, Dr Ismaning, Dr Andreas Feller, www.cscleansystems.com Dausinger + Giesen GmbH, Stuttgart, Dr Steffen Sommer, www.dausinger-giesen.de

Links and literature (in German) >> www.cpvconsortium.org >> www.innovationsallianz-photovoltaik.de

More from BINE Information Service>> Innovations in photovoltaics. BINE-Themeninfo brochure II/2011>> Solar thermal power plants. BINE-Themeninfo brochure II/2013>> This Projektinfo brochure is available as an online document at www.bine.info under

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ImprintProject organisation Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)11055 Berlin Germany

Project Management Jülich Forschungszentrum Jülich GmbHDr Hermann Bastek52425 Jülich Germany

Project number 0325125, 0327567, 0327567A, 0327662, 0325379A-G

ISSN0937 - 8367

PublisherFIZ Karlsruhe · Leibniz Institute for Information Infrastructure Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany

AuthorGerhard Hirn

Copyright Cover image, Fig. 2, Fig. 4 left: AZUR SPACE Solar Power GmbHFig. 1, Fig. 3: Fraunhofer ISEFig. 4 right: Zenitsolar

Text and illustrations from this publication can only be used if permission has been granted by the BINE editorial team. We would be delighted to hear from you.

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