CIGS-based thin film solar cells

CIGS-solceller

World record! Recently, research from our collaboration with the company Evolar (now First ETC) joined the list of world records [link to NREL and Progress in Photovoltaic record tables]. The world record is published in an article in Nature Energy.

CIGS is an acronym for Cu(In,Ga)Se2. This semiconductor material is a highly efficient light absorber. The material is self-doped and high quality CIGS materials are therefore always p-type. To use CIGS in solar cells, it needs to be combined with several other materials to make a p-n junction, to contact the solar cell and to passivate the surfaces. On top of a substrate, a back contact is coated. The CIGS layer is coated on the back contact by co-evaporation. A buffer layer is used to get an efficient PN transition and on top a transparent conductor is coated as a window layer.

Aiming for high efficiency

The research seeks to maximise the efficiency of the solar cells by using different materials, different combinations of materials and changing the manufacturing process. Our projects have different focuses, which can be described in terms of how the solar cells will be used. One area is tandem solar cells where CIGS can be part of a top cell, which should mainly absorb visible light, or as a bottom cell where the material should instead have high absorption in the infrared range. We are also working on solar cells that can be illuminated from two directions, both from the front (directly from the sun) or from the back (reflected light). To achieve high efficiency, high quality is required both in the absorbing layers and high quality in the contact layers, where the key words are long lifetime of charge carriers and high mobility.

In the transitions between different layers, the materials are required to
have the ability to conduct current, but at the same time have a low probability of recombination of the charge carriers. We are experimenting with various additives, such as silver (replacing a small amount of copper in the CIGS layer), as well as heavier alkali metals (Cs and Rb), which are added through a post-processing step to achieve low recombination.

Our CIGS research is supported by advanced characterisation methods, such as
transmission electron microscopy and various types of synchrotron-based methods. We also have a theoretical part in our research with DFT methods and connection to nano-structured methods.