Search for novel photovoltaic absorber materials: Kesterite and Briartite

Photovoltaics is one of the most easily implementable renewable energy source. Highly efficient thin-film solar cells use chalcogenides such as Cu(In,Ga)Se₂ or halide perovskites like (Cs,FA,Ma)Pb(I,Br)₃ as absorbers. Since the availability of indium as well as the limited stability of halide perovskites is an object of concern regarding large scale production of solar cells, the search for novel absorber materials with long term stability is an ongoing challenge.

We studied the chalcogenides Kesterite (Cu₂ZnSnS₄) and Briartite (Cu₂ZnGeS₄) as promising candidates for photovoltaic applications. These semiconductors show a high absorption coefficient of solar radiation and long term stability. Kesterite-based thin film solar cells already reach power conversion efficiencies > 12% [1].

To understand the materials’ optoelectronic properties, deep insights into the structural properties are crucial. Both compounds crystallize in the tetragonal Kesterite structure, which is characterized by a network of corner sharing tetrahedra. Additionally Briartite shows an orthorhombic modification (Wurtz-Kesterite structure). The symmetry reduction is caused by a distortion of the coordination tetrahedra.

We investigated the structural and optoelectronic properties of Kesterite and Briartite using powder samples synthesized by solid state reaction. The formation of different Briartite modifications depends heavily on the sulfur pressure, tailored during the synthesis. The challenge in structural analysis is the differentiation of the electronic similar elements Cu, Zn and Ge. We solved this problem by applying neutron diffraction as well as multiple energy anomalous diffraction.

The presentation will give an overview of structure-property relations of these promising photovoltaic absorber materials.

[1] M.Kauk.Kuusik, J.Mater.Chem. A11 (2023) 23640