Multijunction photovoltaic cell

A tandem solar cell ( also: stack solar cell, multi-junction solar cell, English: multi -junction solar cell) consisting of two or more solar cells with different materials that are monolithically stacked. The purpose of this arrangement is to increase the efficiency of the entire assembly. The solar cells are thereby optimized for a particular wavelength range. Together by a broader spectrum of sunlight is absorbed than with a single cell leading to a higher efficiency. When applied to organic solar cells is meant by the term, combinations of various organic materials, also with different absorption behavior. By thin-film technologies, the concept can be implemented in a single solar cell.

Advantage of a tandem solar cell

The problem with conventional solar cells is the low conversion of the incident light spectrum of the sun. The part of the solar spectrum, the energy of which is smaller than that of the band gap of the semiconductor material used, may not be absorbed and, therefore, is converted into electricity. However the light is absorbed is greater than the band gap of an energy, the excess energy is converted into heat ( Thermalisation ). Optimal transformation is carried out for radiation with energy equal to that of the band gap. A tandem solar cell is now combined solar cells of several materials which have optimal for each individual spectral band gaps.

Materials

For tandem solar cells, both of the indirect semiconductor silicon as well as combinations of the most direct III- V semiconductors can be used. The aim is to coordinate the materials to each other so that the light of the correct wavelength lands in the solar cell layer designed for this purpose. The design can be further ensured that the photons of the sun light can be held by the reflection in the respective layers ( photon recycling).

• Gallium arsenide - Germanium: While GaAs has a high band gap and therefore absorbs only high-energy radiation, it is permeable to some of the lower-energy radiation. This is then absorbed from the germanium layer.
• Gallium indium arsenide - gallium indium phosphide: Are better suited to their specific absorption properties of the different spectral regions of sunlight and thus achieve a higher efficiency. These monolithic solar cells reached an efficiency of 31% in 2001, which increased to 2009 to 41.1 %.
• Silicon - silicon: This combination of layers of amorphous ( a-Si) and microcrystalline ( mu.C - Si) are used currently most. The top cell ( a-Si) absorbs light mainly in the range 400-600 nm, the bottom cell ( mu.C -Si) in the range of 500-1100 nm But tandem solar cells with two amorphous layers (a -Si a-Si) can be produced.

Research and production

November 2012 was the world record efficiency for multi -junction solar cells 44%. The first decade of the 21st century shows a separation of research and production of solar cell technology. Supported by the promotion of renewable energies the U.S. Obama administration has been researched for solar cell technology at Rice University and patents pending. The company Stion was supported by the U.S. National Renewable Energy Laboratory (NREL ) with research funds in the amount of one million U.S. dollars. The company Natcore developed on the basis of research results and production mediates these processes to solar cell manufacturers. According to Natcore are using this technology tandem solar cells with an efficiency of " over 30 % " economically. In March 2011, a technology transfer with the Chinese solar cell manufacturer TLNZ Solar Technology in Hunan was decided by Natcore. The contents of this Agreement affects tandem solar cells and industrial production of black silicon. In Germany in 2005 by Siemens " 10,326,547 (A1 ) DE " was registered for tandem solar cells the patent. Since 2007 is worked out by scientists at " solar cells of the third generation" of research funding by the Thuringian Ministry of Science. In Switzerland, Oerlikon Solar designs of technologies to " Micromorph " tandem solar cells.

Applications

Because of the expensive and complex manufacturing processes tandem solar cells previously only in extraterrestrial applications (eg satellites) were used. With concentrator systems can concentrate the sunlight, so that only a small portion is still needed to semiconductor material. This also effective photovoltaic systems from solar panels on Earth can be constructed. They require a mechanical tracking, to be focused by the combined lens sunlight on the individual solar cells.

By making use of new plant technologies, the production of tandem solar cells based on silicon but also for ordinary terrestrial applications became attractive. There are manufacturers in Germany, Japan and the United States.