// Bottom device development for (flexible) tandem solar cells based on industry-compatible CIGS compound semiconductor thin films

The aim of the project is to modify the Cu(In,Ga)Se2 (CIGS) cell with a view to its subsequent use as a so-called bottom cell in a tandem. The goal is to achieve an efficiency of 10% (module 9%) under filtered lighting with an effective band gap of 1 eV. In the ZSW subproject, preparation is carried out exclusively on commercially available soda-lime glass substrates with a thickness of 3 mm. To achieve the goal, the absorber is optimized in terms of band gap and its progression, roughness, and alkali supply, as are the buffer used and the front electrode. For the buffer layer, the aim is to use materials that give the bottom cell improved stability against thermal stress that would occur when the top cell is applied. In addition, its function should not be impaired by interface-related metastability effects due to the lack of blue light in the filtered light spectrum. The ZSW is focusing on magnetron-sputtered Zn(O,S) and GaOx layers. With regard to the front electrode, TCO layers with high charge carrier mobility and the associated reduced absorption are being used and further optimized. H- and Zn-doped indium oxide (IOH, IZO) are of particular relevance for the tandem system. All activities are accompanied by comprehensive material analysis and electro-optical investigations.