Sputterheizung CdS-Kammer (Foto: Gabi Haindl)

Cu(In,Ga)Se2 thin film solar cells

Cu(In,Ga)(S,Se)2 (CIGS) thin film solar cells are investigated in the department of surface science within the “GRACIS“ (gradients in Cu(In,Ga)(S,Se)2 thin film solar cells) project. We focus on the interface between the absorber (CIGS) and the buffer layer (CdS or ZnS). The measurements are done with X-ray photoelectron spectroscopy (XPS) at the Darmstädter Integrated System for solar cell research (DAISY-Sol).

The buildup of a CIGS solar cell is a structure of different layers in the substrate configuration. A soda lime glass or a flexible foil made of metal or polymer is used as a substrate for the following layers. As a back contact metallic Molybdenum is used. The absorber (CIGS) and a buffer layer (CdS, ZnS, In2S3…) are deposited by different methods on top of the back contact. The front contact consist of a transparent conductive oxide (TCO) like ZnO. To increase the conductivity, a Ni/Al contact in grid finger geometry is deposited on top of the TCO. In the last production step the solar cell is encapsulated in glass to protect the cell from environmental effects.

The schematic band devolution of a solar cell gives information about properties like the generation and the separation of charge carriers. The devolution of the valence band is determined by XPS analysis. The conduction band is calculated by the band gaps of the different materials and the determined valence band offset. The charge carrier generation takes pΔlace in the absorber material (here CIS) and is caused by the solar radiation. The generated electrons are moved to the conduction band. Because of the band bending, the electrons are transported to the front contact. The holes in the valence band are transported to the back contact. To enable an effective charge transport in the conduction and valence band, the different barriers (ΔEVB and ΔECB) at the interfaces between the materials have to be as low as possible.

Lab scale CIGS thin film solar cells have an efficiency of 20,3 % (status: july 2010, sample from ZSW). Commercially available modules have an efficiency of 10 to 13 % and are sold by Würth Solar and Solibro in Germany:

The research partners of the GRACIS project are: