page_title: Copper indium gallium selenide solar cells
section_title: Electrodeposition followed by selenization
hierarchical_section_title: Copper indium gallium selenide solar cells / Production / Film production / Electrodeposition followed by selenization
caption_attribution_description: English: CIGS electroplating setup
context_page_description: A copper indium gallium selenide solar cell is a thin-film solar cell used to convert sunlight into electric power. It is manufactured by depositing a thin layer of copper, indium, gallium and selenium on glass or plastic backing, along with electrodes on the front and back to collect current. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials.
  CIGS is one of three mainstream thin-film photovoltaic technologies, the other two being cadmium telluride and amorphous silicon. Like these materials, CIGS layers are thin enough to be flexible, allowing them to be deposited on flexible substrates. However, as all of these technologies normally use high-temperature deposition techniques, the best performance normally comes from cells deposited on glass, even though advances in low-temperature deposition of CIGS cells have erased much of this performance difference. CIGS outperforms polysilicon at the cell level, however its module efficiency is still lower, due to a less mature upscaling.
context_section_description: Precursors can be deposited by electrodeposition. Two methodologies exist: deposition of elemental layered structures and simultaneous deposition of all elements (including Se). Both methods require thermal treatment in a Se atmosphere to make device quality films. Because electrodeposition requires conductive electrodes, metal foils are a logical substrate. Electrodeposition of elemental layers is similar to the sputtering of elemental layers. 
  Simultaneous deposition employs a working electrode (cathode), a counter electrode (anode), and a reference electrode as in Figure 4. A metal foil substrate is used as the working electrode in industrial processes. An inert material provides the counter electrode, and the reference electrode measures and controls the potential. The reference electrode allows the process to be performed potentiostatically, allowing control of the substrate's potential.
  Simultaneous electrodeposition must overcome the fact that the elements' standard reduction potentials are not equal, causing preferential deposition of a single element. This problem is commonly alleviated by adding countering ions into solution for each ion to be deposited (Cu²⁺, Se⁴⁺, In³⁺, and Ga³⁺), thus changing that ion's reduction potential. Further, the Cu-Se system has a complicated behavior and the film's composition depends on the Se⁴⁺/Cu²⁺ ion flux ratio which can vary over the film surface. This requires the precursor concentrations and deposition potential to be optimized. Even with optimization, reproducibility is low over large areas due to composition variations and potential drops along the substrate.
  The resulting films have small grains, are Cu-rich, and generally contain Cu2−xSeₓ phases along with impurities from the solution. Annealing is required to improve crystallinity. For efficiencies higher than 7%, a stoichiometry correction are required. The correction was originally done via high temperature physical vapor deposition, which is not practical in industry.
  Solopower is currently producing cells with >13.7% conversion efficiency as per NREL.
caption: Figure 4: CIGS electrodeposition apparatus
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