Copper indium gallium selenide solar cells
Since the distances of the photocarriers between generation and collection are shorter due to the small film thickness, the requirements on the absorber material are lower than for crystalline silicon solar cells. Therefore, thin film solar cells made of polycrystalline CIGS are produced, which reduces the required energy expenditure and the cost compared to the production of monocrystalline silicon. Due to the small layer thicknesses also lightweight and even flexible solar modules for applications in the field of photovoltaics can be produced with appropriate substrate choice. In addition, modules can be directly produced in a production line - without having to use individual solar cells, which are then interconnected.
The efficiency of modules is currently at 11-14 %. Manz AG presented in September 2012, a CIGS solar module with an efficiency of 14.6 % on total module surface and 15.9 % on Aperture before, which was produced on a mass production line. In small laboratory cells, higher efficiencies can be achieved. With 20.1% ( 0.5 cm ²) at the Centre for Solar Energy and Hydrogen Research Baden- Württemberg ( STW ) reaches the worldwide highest value for thin-film solar cells in April 2010. Previously, for 16 years the highest efficiency, the U.S. research institute NREL achieved. This value was increased to 20.3% in July 2010. This value was of a CIGS solar cell on plastic wrap to 20.4 % and in October 2013, increased from STW to 20.8%, higher than the efficiency of crystalline solar cells in January 2013. Due to the low layer thickness, the resources are conserved and in appropriate quantities there is to be a cost-effective manufacturing than in the thick-film technology. Production technology also allows the manufacture of semi-transparent modules. Limitations in the mass production of CIGS modules there could be, as the raw material indium is relatively scarce and in other technological products based on semiconductors (eg flat panel ) is employed. However, only very small amounts of indium for solar cell production are needed, so that mass production will exacerbate the scarcity of indium not clear.
The graph shows a schematic cross section of a Cu (In, Ga) Se2 solar cell with the corresponding layer thicknesses. Although flexible substrates offer advantages, is as yet mostly used glass as a substrate. The substrate is coated with Molybdenum (Mo), which serves as the back contact. The pn junction is a heterojunction, that is, the p- and n- doped layers are made of different semiconductors.
The eponymous semiconductor Cu (In, Ga) Se2 is also referred to as the absorber, since a large part of the incident light is absorbed. He is lightly p - doped by intrinsic defects of the material. As a n- doped layer of zinc oxide ( ZnO) with aluminum (Al) heavily doped. This also aluminum zinc oxide ( AZO) said layer forming a transparent conductive oxide layer. Due to the relatively high band gap of zinc oxide ( Eg, ZnO = 3.2 eV ), this layer is transparent to visible light. Therefore, it is also referred to as a window. Between windows and absorbers buffer layers are made of cadmium sulfide ( CdS) and undoped ZnO. The research is due to toxicity of cadmium sulfide and the hope of power gains with alternative buffer materials ( In2S3, Zn (O, S), ( Zn, Mg) O, and others). The asymmetric doping of the layers results in an asymmetric space charge zone, the deeper into the absorber extends as in the ZnO.
While the Mo and ZnO layers are produced by sputter deposition in a chemical bath and CdS deposited (English chemical bath deposition, CBD) is, there are several variations to produce the absorber. The most widely used at the same time thermal evaporation of the elements or the deposition of the metals ( Cu, In, Ga) by electroplating, sputter deposition, or other method, followed by heating in a selenium atmosphere.
There are factories for the production of copper -indium- diselenide modules and Others at the company Manz CIGS Technology GmbH ( part of Manz AG, formerly Würth Solar) in Schwäbisch Hall, in the Avancis GmbH in Torgau and in the CIS Solar GmbH in Hamburg. A pilot production of solar modules based on copper indium disulfide is at the company Soltecture in Berlin. From a pilot production of solar modules based on copper indium gallium diselenide based in Uppsala / Sweden the company Solibro has emerged as a 100 percent subsidiary of Q -Cells AG. Solibro as well as the company Miasole (U.S.) have now been taken over by the sixth largest utility in China, Hanergy. More companies in the U.S. and Japan have production or pilot lines, including a 900 MW production line of the company Solar Frontier, which are already produced with RoHS compliant buffer layers on CBD -Zn (S, O, OH ) basis.
Flexible CIGS solar cells
In addition to the deposition on glass substrates is working on the market introduction of flexible CIGS solar cells and modules, including at the company Global Solar in Tucson / Arizona and Berlin. As substrates in addition to the metal foils and high temperature polymers are used such as polyimide. In polyimide film in 2011 efficiencies of 18.7 % and 2013 of 20.4 % was achieved in the laboratory at EMPA in Switzerland. In order to transfer this technology into mass production different companies have built pilot plants and production reached in 2009, efficiencies of up to 13.4%.