Integration of TiO2 in Oxide/Metal/Oxide-Electrodes for Increased Color Rendering of CIGS Solar Cells

Kurzfassung

Cu(In,Ga)Se2 (CIGS) solar cells are high-efficient low-cost thin-film solar cells. The world record cell reaches 23.4 %, which is comparable to crystalline silicon-based solar cells. They are especially suitable for building integrated photovoltaics. However, the standard modules are black and therefore not attractive for all buildings. One way to colour CIGS modules is to exchange the thick aluminum-doped zinc oxide (Al:ZnO) front electrode of a CIGS solar cell by an oxide/metal/oxide layer stack (OMO). By varying the layer thickness of the top most oxide layer the color appearance can be changed. A challenge is that the solar cells are encapsulated with a glass layer for weather resistance, which leads to a decrease of the color purity. The aim of this thesis is to increase the color purity of the encapsulated layer stack by exchanging the top most OMO layer of Al:ZnO, which is the coloring layer, by a titanium dioxide (TiO2) layer. The TiO2 layer has a higher refractive index n of up to 2.77 compared to 1.9 for Al:ZnO. The color of the layer stack is dependent on the reflected spectrum. It is expected that the reflection of the cell stack increases with an increasing refractive index, which should lead to an increased color purity. In this thesis an electron beam evaporation process for a high refractive index TiO2 layer was developed. The TiO2 layer was then integrated into the OMO layer stack, called TiO2OMO. Four TiO2OMO layer stacks of different colors were deposited first on glass and then on CIGS solar cells. They were optically characterized and compared to a reference OMO layer stack. Further, an optical model was used to calculate more possibly achievable colors. The thesis is divided into the theoretical background, followed by an introduction to the used methods, the presentation of the experimental results, the discussion of the results and the final conclusion as well as an outlook