Tiny Crystals Could Oust Traditional Solar Absorbers

Traditional absorbers in thin film-based solar cells may have some competition in a new technique that incorporates nontoxic and relatively common elements.

The new approach, developed by a team from the University of Nantes, uses sulfide materials that contain copper, tin and zinc (known as kesterites) as solar cell absorbers. This could result in technology that is more sustainable than rare, precious metals.

A scanning electron microscopy image of kesterite single crystals. Courtesy of A. Lafond, Institut of Materials Jean Rouxel, Nantes, France.

In the study, the researchers prepared a powdered precursor using a ceramic synthesis at a high temperature from the corresponding elements Cu, Zn, Sn and S. The resulting product went through a quenching process in which it was heated to strengthen it before being put into ice water to lock in the chemical structure present at the elevated temperature.

Tiny single crystals were then picked out of the powder. The product’s purity was tested using X-ray diffraction and energy-dispersive X-ray spectroscopy analyses. The researchers carried out high-performance resonant diffraction, allowing adjustment of the radiation wavelength.

The researchers found that the strengthening process generates a disordered structure that can be distinguished from the kesterite structure, despite the otherwise similar X-ray scattering pattern that would be generated by the copper and zinc ions in the ordered form.

It is now possible to carry out resonant diffraction of a single crystal of the semiconductor Cu2ZnSnS4 (CZTS). Traditionally, kesterites have resisted conventional X-ray diffraction, as the copper and zinc ions have been indistinguishable.

Also, creating a thin absorber film from CZTS in a solar panel can be done at an elevated temperature, the researchers say.

“The next step in this research is to determine the relationship between the synthesis conditions — quenching or slow cooling — and the actual Cu/Zn distribution in the kesterite structure,” said one of the researchers, Alain Lafond of the University of Nantes.

The research is published in Acta Crystallographica Section B (doi: 10.1107/S2052520614003138).