Photovoltaic Semiconductors by Room Temperature E–ALD Procedure: a XAS Study of CuxZnySz Thin Films

In the present study, we report the results of a microanalytical and X-ray Absorption Spectroscopy study carried out on different films belonging to the Cu-Zn-S ternary system, related to the kesterites, Cu2ZnSn(S,Se)4, in the light of their potential application to solar cell production. In recent years, in fact, kesterites attracted a worldwide interest because they encompass reduced production costs and low environmental risks. The thin films analysed in the present study were realised through the E-ALD (Electrochemical Atomic Layer Deposition) technique. Cu-Zn-S films were prepared alternating underpotential depositions of the elements onto a Ag (111) substrate. The obtained thin films were thus characterised by Scanning Electron Microscopy (SEM), X-ray Absorption Spectroscopy (XAS) spectroscopy and DRS. The results of the morphological and spectroscopic characterisation of the films reveal a wide degree of heterogeneity, involving the morphological organisation of the films, as well as their phase composition. On the basis of the experimental results, E-ALD is proposed to operate a progressive and conventional coverage of the Ag (111) surface through a nanometric polycrystalline film consisting of oriented microcrystals. The insertion of a unit of ZnS in the deposition sequence apparently induces the formation of nanowhiskers on the film surface. From a structural point of view, Cu and Zn follow completely different paths in the crystallisation of the film. The XAS data, in fact, point to a poorly ordered low coordinated structure for Cu as in the mineral chalcocite. Conversely, Zn forms a crystalline structure in excellent agreement with the sphalerite model compound. From an applicative point of view, the three films exhibit band gap values variable within the series and progressively approaching the requirements for solar energy conversion, thus strongly candidating E-ALD technique as a versatile tool to synthesise thin films of sulfide photovoltaics at room temperature conditions.