The control and removal of secondary phases is one of the major challenges for the development of Cu 2 ZnSn(S,Se) 4 (CZTSSe)-based solar cells. Although etching processes have been developed for Cu(S,Se), Zn(S,Se), and CuSn(S,Se) secondary phases, so far very little attention has been given to the role of Sn(S,Se). In this paper, we report a chemical route using a yellow (NH 4 ) 2 S solution to effectively remove Sn(S,Se). We found that Sn(S,Se) can form on the surface either because of stoichiometric deviation or by condensation. After etching, the efficiency of devices typically increases between 20 and 65% relative to the before etch efficiencies. We achieved a maximum 5.9% efficiency in Se-rich CZTSSe-based devices. It is confirmed that this feature is related not only to the removal of Sn(S,Se) but also to the unexpected passivation of the surface. We propose a phenomenological model for this passivation, which may open new perspectives for the development of CZTSSe-based solar cells.

Zn‐rich Cu2ZnSnSe4 (CZTSe) films were prepared by a two‐step process consisting in the DC‐magnetron sputtering deposition of a metallic stack precursor followed by a reactive anneal under a Se + Sn containing atmosphere. Precursor composition and annealing temperature were varied in order to analyze their effects on the morphological, structural, and optoelectronic properties of the films and solar cell devices. Raman scattering measurements show the presence of ZnSe as the main secondary phase in the films, as well as the presence of SnSe at the back absorber region of the films processed with lower Zn‐excess values and annealing temperatures. The ZnSe phase is found to accumulate more towards the surface of the absorber in samples with lower Zn‐excess and lower temperature annealing, while increasing Zn‐excess and annealing temperature promote its aggregation towards the back absorber region of the devices. These measurements indicate a strong dependence of these process variables in secondary phase formation and accumulation. In a preliminary optimization of both the composition and reactive annealing process, a solar cell with 4.8% efficiency has been fabricated, and potential mechanisms limiting device efficiency in these devices are discussed. Copyright © 2014 John Wiley & Sons, Ltd.

This work reports a process based on the use of an ultrathin (10 nm) ZnO intermediate layer for the improvement of the absorber/back contact interface region in Cu 2 ZnSnSe 4 (CZTSe) kesterite solar cells.Raman microprobe measurements performed directly on the substrate surface after mechanical removal of the absorber layer indicate the occurrence of a decomposition reaction of Cu 2 ZnSnSe 4 in contact with the Mo substrate. This leads to a significant degradation of the quality of the absorber/back contact interface, with the formation of a high density of voids. The presence of an intermediate ZnO layer on the Mo coated substrates inhibits the decomposition reaction, because it prevents interaction between the CZTSe and Mo layers during the annealing process. This leads to a significant improvement in the interface morphology as observed by detailed cross-section scanning electron microscopy. It also correlates with the observed increase of the device conversion efficiency from 2.5% up to 6.0%. The improvement in the optoelectronic characteristics of the cells could be related to a significant decrease of the device series resistance due to the formation of a smoother interface with low density of voids, resulting from the effective inhibition of the CZTSe decomposition reaction at the Mo back contact layer.

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