CZTS — solar cells from abundant elements

Sometimes solar cells contain rare or expensive elements, for example indium for CIGS and tellurium for CdTe cells. There are therefore concerns about the cost and long-term sustainability in the use of these materials and the potential for the corresponding solar cell technologies in mass production. CZTS is an abbreviation of Cu2ZnSn(S,Se)4, a material composed of only abundant and non-toxic elements, which therefore has the advantage of a lower cost and reliable access to material. They are also more environmentally friendly since it is easier to access the materials.

We started working on CZTS as an alternative material to CIGS in 2010 and the group now consists of around 10 people. The project includes development of two vacuum-based deposition techniques; sputtering and annealing of precursor films, which are both used for manufacturing the solar cells. These are showed in the picture below. In the reactive sputtering process, sputtering takes place in Ar/H2S-plasma from metals or compounds that are then deposited on the substrate. By controlling the substrate temperature, pressure and power, we can deposit films with different degree of crystallinity, metal composition and sulfur content. For the annealing of CZTS a custom-made oven was built in the beginning of the project. This is based on a quartz-tube where the samples are moved between a cold and a hot zone with a controlled atmosphere. In the process we use today we have a box of graphite with a sulfur atmosphere to avoid unwanted reactions at the surface and back contact at the high temperatures. 

Due to big research fundings from the Swedish Foundation for Strategic Research (SSF) and Wallenberg Foundations, we have recently been able to extend our laboratory with a new sputter and oven. The sputter is a CMS-18 from Lesker. It has six sources, which gives the opportunity to test more material combinations. The oven from DSE Tech is custom-made with separate control of pressure, temperature and influx of sulfur and selenium. This facilitates the possibility to control the sensitive annealing. We also use materials characterization, device fabrication and opto-electronic characterization.

CZTS is a new technology which have not been developed for very long, and therefore the efficiency of the solar cells are still lower than for example CIGS and CdTe. Our research aims to improve the solar cells’ performance by optimizing the manufacturing process and the materials themselves. For example, we study band gap variations, i.e. the energy levels in the materials, and how they affect the solar cell’s properties. 

Contact: Charlotte Platzer-Björkman

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