Cd-free kesterite solar cells: State-of-the-art and perspectives

In the scenario of the new emerging photovoltaics, kesterites play a lead role in the thin-film solar cell technologies. This class of compounds, mainly represented by the pure-sulfide form Cu2ZnSnS4 (CZTS) and the sulfo-selenide form Cu2ZnSn(S,Se)4 (CZTSSe), shows unique characteristics and stands as a promising p-type absorber material thanks to its high absorption coefficient, high cost-effectiveness and low toxicity. However, CdS is commonly used as the n-type partner (buffer layer) in kesterite solar cells but, beyond its toxicity, it has a nonoptimal band alignment with kesterites and exhibits parasitic absorption at low wavelengths due to its bandgap. Several efforts have been made in the last decade, to replace CdS with a suitable, Cd-free, both environmentally and economically sustainable buffer layer, and many times with successful results allowing not only to equal, but also to overcome in few cases the performances of the corresponding CdS-based reference devices. Zn1-xSnxO for instance leads to higher efficiencies than CdS when coupled with pure-sulfide CZTS, while Zn(O,S) seems to couple better with CZTSSe. TiO2 has been also considered as suitable buffer layer for kesterites and, in the last few years, several works have been reported both theoretical and experimental, showing very promising results. In this review we summarize the efforts and the improvements recorded by the scientific community working on this topic in the last ten years, with the aim to supply a landmark useful to design future experiments in a more efficient way and to push forward the related research activities, in order to fully overcome CdS limitations and to promote thin-film kesterite devices to higher performances.