Tandem solar cell

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A tandem solar cell (also: stacking solar cell , multiple solar cell ; English: multi-junction solar cell ) is composed of two or more solar cells made of different materials one above the other are laminated. A distinction is made between mechanically stacked tandem solar cells, in which the materials are separated from one another, and monolithic solar cells, in which all solar cells are built on the same substrate. The uppermost part of the solar cell facing the light absorbs light with a short wavelength (equivalent to high energy) and lets long-wave light through. The second solar cell arranged underneath, in turn, absorbs part of the spectrum up to a threshold wavelength, which in semiconductors is determined by the so-called band gap energy. In principle, any number of partial solar cells can be arranged one above the other. The purpose of this arrangement is to increase the efficiency of the conversion of sunlight into electricity compared to single solar cells. This is achieved on the one hand by the fact that short-wave (high-energy) light generates a higher voltage in the uppermost partial solar cells. On the other hand, the absorption in the longer-wave (lower-energy) spectral range can be expanded by sub-solar cells arranged below. In a tandem solar cell, the absorption range is expanded compared to a single solar cell, and the efficiency of the conversion in the short-wave spectral range is increased.

The highest levels of efficiency are achieved today with tandem solar cells made from III-V semiconductor compounds. When applied to organic solar cells , the term also includes combinations of different organic materials with likewise different absorption behavior. New concepts rely on the combination of a Si sub-cell with partial solar cells made of III-V compound semiconductors or perovskites .

Advantage of a tandem solar cell

Sectional view through a tandem solar cell (a) and frequency ranges of the absorption of the individual layers (b)

Conventional single solar cells can not make optimal use of the incident light spectrum from the sun. That part of the solar spectrum whose energy is smaller than that of the band gap of the semiconductor material used cannot be absorbed and converted into electricity. If, on the other hand, light is absorbed with an energy greater than the band gap, the excess energy is converted into heat (thermalization). Optimal conversion occurs for radiation whose energy corresponds to that of the band gap. A tandem solar cell now combines solar cells made of several materials, which each have optimal band gaps for individual spectral ranges.

materials

Tandem solar cells can contain indirect semiconductors such as silicon or germanium , as well as combinations of direct III-V semiconductors , perovskites , amorphous or microscrystalline absorber layers. The aim is to absorb broadband sunlight in a cascade of partial solar cells with decreasing band gap energy. The upper solar cells are transparent for the light that is to be absorbed in the solar cells below. The partial solar cells are connected in series in a monolithic arrangement via tunnel diodes and therefore usually have two external contacts, identical to a conventional single solar cell . In the case of tandem solar cells mechanically stacked on top of one another, however, one finds configurations with separate contacts for each partial solar cell. A suitable construction can also ensure that the photons of sunlight are held in the respective layers by reflection (photon recycling).

  • Gallium indium phosphide - gallium indium arsenide - germanium : The most widespread tandem solar cell consists of three partial solar cells made of GaInP, GaInAs and Ge, one on top of the other. Such solar cells are used, for example, to power satellites , but also in concentrator PV power plants. These monolithic solar cells achieved an efficiency of 31% in 2001, which was increased to 41.1% by 2009.
  • Gallium indium phosphide-gallium arsenide-gallium indium arsenide phosphide-gallium indium arsenide: Quadruple solar cells made from these III-V semiconductors (abbreviated as GaInP, GaAs, GaInAsP, GaInAs) achieve 46.1% efficiency at 312 times the concentration of sunlight.
  • Silicon-silicon: At the moment, combinations of layers of amorphous (a-Si) and microcrystalline (µc-Si) silicon are mostly used here. The top cell (a-Si) absorbs the light mainly in the range 400 to 600  nm , the bottom cell (µc-Si) in the range 500 to 1100 nm. However, tandem solar cells with two amorphous layers (a-Si – a- Si) produced.
  • Gallium indium phosphide-gallium arsenide-silicon: The latest developments allow III-V solar cells to be combined with silicon solar cells. Here efficiencies of 30.2% below the AM1.5g solar spectrum were achieved without concentration.

Research and production

In November 2012, the world record for efficiency for multi-junction solar cells was 44%. The first decade of the 21st century shows a separation between research and production in solar cell technology. Supported by the promotion of renewable energies of the US Obama administration has been to solar cell technology at the Rice University research and patents pending. Stion was supported by the US National Renewable Energy Laboratory ( NREL ) with research funding of one million US dollars. The company Natcore developed production processes on the basis of the research results and passed these processes on to solar cell manufacturers. According to Natcore, tandem solar cells with an efficiency of “over 30%” can be produced economically with this technology. In March 2011 Natcore decided to transfer technology with the Chinese solar cell manufacturer TLNZ Solar Technology in Hunan . The content of this agreement concerns tandem solar cells and the industrial production of black silicon . In Germany, Siemens applied for the patent "DE 10326547 (A1)" for tandem solar cells in 2005. Since 2007 scientists have been working on "third generation solar cells" with research funds from the Thuringian Ministry of Science. In Switzerland, Oerlikon Solar is developing technologies for “micromorph” tandem solar cells. In 2015, the Helmholtz Center Berlin and the Eidgenössische Materialprüfungs- und Forschungsanstalt succeeded in manufacturing tandem solar cells with a top cell made of perovskite and a silicon bottom cell.

Applications

Because of the expensive and complex manufacturing processes, tandem solar cells were previously only used in extraterrestrial applications (e.g. satellites ). With concentrator systems, sunlight can be bundled so that only a small amount of semiconductor material is required. This means that effective photovoltaic systems can also be constructed from solar modules on earth. These require mechanical tracking to that with lenses concentrated sunlight on the individual solar cells focused to make.

The use of new system technologies made the production of tandem solar cells based on silicon attractive for common terrestrial applications. There are manufacturers in Germany, Japan and the USA.

Web links

credentials

  1. Fraunhofer ISE researchers take second place in the FEE Innovation Prize Energy - 41.1% record efficiency for solar cells again awarded  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. - Press release from the Fraunhofer Institute for Solar Energy Systems on March 2, 2010.@1@ 2Template: Toter Link / www.ise.fraunhofer.de  
  2. NREL Solar Cell Sets World Efficiency Record at 40.8 Percent ( Memento of the original from September 7, 2015 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (English) - National Renewable Energy Laboratory , on August 13, 2008. @1@ 2Template: Webachiv / IABot / www.nrel.gov
  3. World record: 41.1% efficiency for multi-junction solar cells at Fraunhofer ISE  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. - Fraunhofer Institute for Solar Energy Systems , January 14, 2009.@1@ 2Template: Toter Link / www.ise.fraunhofer.de  
  4. Frank Dimroth, Thomas ND Tibbits, Markus Niemeyer, Felix Predan, Paul Beutel, Christian Karcher, Eduard Oliva, Gerald Siefer, David Lackner, Peter Fuß-Kailuweit, Andreas W. Bett, Rainer Krause, Charlotte Drazek, Eric Guiot, Jocelyne Wasselin , Aurélie Tauzin, Thomas Signamarcheix: Four-Junction Wafer Bonded Concentrator Solar Cells . Ed .: IEEE Journal of Photovoltaics. Vol. 6, No. 1 , 2016, p. 343 .
  5. Hanna Brummack : Optimization of drift-determined solar cells made of amorphous and nanocrystalline silicon. Institute for Physical Electronics at the University of Stuttgart , Stuttgart, 2000.
  6. 30.2 percent - a new record for silicon-based multi-junction solar cells. Fraunhofer ISE, November 9, 2016, accessed on November 15, 2016 .
  7. Solar Cell Efficiency Tables onlinelibrary.wiley.com (English; accessed on February 6, 2013.)
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  9. Rice University patent applications European Patent Office (accessed May 14, 2011.)
  10. Solarglobalnet "Tandem solar cell increases efficiency" www.solar-global.net (accessed on May 14, 2011.)
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  12. Natcore Solar Archived Copy ( Memento of the original from March 22, 2011 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (English; accessed May 14, 2011.) @1@ 2Template: Webachiv / IABot / www.natcoresolar.com
  13. Natcore Technology - Sale to TLNZ Solar Technology ( Memento of the original from November 29, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. at Cleantechaktien.com (accessed May 14, 2011) @1@ 2Template: Webachiv / IABot / www.cleantechaktien.com
  14. Siemens patent tandem solar cell patent application European Patent Office (accessed on May 14, 2011.)
  15. Innovations Report third generation solar cells (viewed on May 14, 2011.)
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  17. Albrecht et al .: Energy Environ. Sci. 2015 http://pubs.rsc.org/en/Content/ArticleLanding/2016/EE/c5ee02965a#!divAbstract