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  • Tiny Crystals Could Oust Traditional Solar Absorbers

Photonics.com
Apr 2014
NANTES, France, April 4, 2014 — 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.


The ordered kesterite structure. Courtesy of University of Nantes.


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).


GLOSSARY
diffraction
As a wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form various diffraction patterns.  
ion
An atom that has gained or lost one or more electrons and, as a result, carries a negative or positive charge.
quenching
The inhibition or elimination of one process by another process. The stimulated emission of a laser oscillator can be quenched by a pulse of radiation of the same frequency traversing the oscillator in a different direction. This pulse induces the excited ions to emit radiation in a direction apart from the oscillating mode, and hence the oscillation is decreased.
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