Cu2ZnSnS4

Cu2ZnSnS4 (CZTS) is a quaternary copper-based chalcogenide semiconductor composed of copper, zinc, tin, and sulfur. It adopts a kesterite-type crystal structure and is studied as an earth-abundant, scalable alternative for thin-film photovoltaic absorbers, particularly when selenium is incorporated to form CZTSSe. It is related to the copper-indium-gallium-sulfide (CIGS) family of solar absorbers.

In its ideal form, CZTS crystallizes in a tetragonal kesterite structure (space group I-4). Real films often exhibit cation disorder among Cu, Zn, and Sn sites, producing antisite defects that influence the bandgap, open-circuit voltage, and recombination. Secondary phases such as Cu2S or ZnS can form if stoichiometry deviates from the nominal composition, complicating device performance.

CZTS is a direct bandgap semiconductor with a bandgap around 1.5 eV for sulfur-rich films, suitable for single-junction photovoltaics. Replacing sulfur with selenium yields CZTSSe with tunable bandgaps roughly in the 1.0 to 1.5 eV range, enabling versatility for absorber design. The material exhibits strong optical absorption, allowing the use of relatively thin absorber layers in solar cells.

Thin films of CZTS are produced by vacuum-based methods (co-evaporation, sputtering) or solution-based processes (electrodeposition, chemical bath deposition) followed by sulfurization or annealing. Key challenges include phase segregation, copper–zinc disorder, and defect-related recombination, as well as achieving high phase purity and stoichiometric control to minimize Voc deficits.

CZTS/CZTSSe devices have demonstrated laboratory efficiencies approaching 12–13%, but the technology has yet to surpass leading thin-film technologies in commercial production. The material remains an area of active research due to its earth-abundant constituents and potential for scalable, lower-toxicity photovoltaic absorbers.