Potential of CZTSe Solar Cells Fabricated by an Alloy-Based Processing Strategy

Abstract

In this manuscript, we give an overview of the main insights into our growth procedure for kesterite solar cells and show the possibilities that are provided by this approach. The importance of using Cu–Sn alloy instead of elemental Sn and Cu in the precursor is shown. We discuss how the alloy approach stabilises the composition and helps guide the process along a preferred reaction pathway. A summary of our previously reported findings in the context of our latest results on kesterite solar cells prepared from Cu–Sn alloyed precursors is drawn. The positive impact of an alloy precursor configuration on the formation pathway, process control, and process resilience is demonstrated. Furthermore, a new optimisation strategy for kesterite, based on the reported pathway, is discussed, including a smooth phase transition from Cu-rich to Cu-poor kesterite. Finally, we demonstrate results on buffer optimisation and the application of a promising hybrid buffer configuration of CdS/Zn(O,S), which can reduce the optical losses in the solar cell structure.

The effect of excess selenium on the opto-electronic properties of Cu2ZnSnSe4 prepared from Cu–Sn alloy precursors

“…Hence, the kesterite formation reaction takes place with the suppressed formation of SnSe 2Àx (which is prone to evaporation), and the Sn loss is directly prevented, which we have described in detail elsewhere; 7,8 however, at extremely low selenium amounts, the overall selenization reaction slows down due to insufficient vapor pressure of selenium; this leaves more time for the metallic precursor stack to undergo the re-alloying reactions that can ultimately lead to the Cu-Sn alloy disintegration and formation of elemental Sn. 7,8 In this case, the SnSe 2Àx phases can start to form and escape from the absorber layer; this leads to a net loss in the Sn amount at the end of the annealing process. However, the rst reason can only explain the higher Cu/Sn ratio in the absorber selenized with selenium at a very low amount (i.e.…”

“…This elemental Sn acts as an additional source of the SnSe 2Àx vapor during the hightemperature stage of the annealing process. 7,8 As the effect of the Se amount on this mechanism of additional Sn incorporation was not previously investigated, we performed two annealing experiments in which we interrupted the selenization process at 400 C and performed a Raman analysis on the initial elemental Sn-wire used in the susceptor. Fig.…”

“…The addition of Sn into the susceptor can be used as a tool to tune the material composition and properties, opening up a new optimization parameter. 8 It is nally essential to note that the changes occurring in the absorber with chalcogen are not only important during the fabrication of the kesterite lm, but can also be signicant for the post-treatment steps; at posttreatment temperatures above 220 C, selenium depletion can start to occur in the absorbers, and also, the possible changes in the predominant defect clusters, band gap, and composition have to be taken into account. The approach and ndings presented in this study appear to be of a general nature for CZTSe processing and can therefore also be used as a template for optimizing the selenization processes of other congurations.…”

“…Moreover, one of the approaches proposed to prevent this Sn loss is the use of precursors in which Sn is mainly embedded in the Cu-Sn alloy conguration, which in combination with a properly designed selenization procedure can suppress the formation of volatile Sn(S,Se) 2Àx ; this would ultimately help to maintain a constant Sn amount throughout the annealing process. 7,8 An appropriate conguration of alloy precursors during kesterite processing is also known to improve the thin lm homogeneity; 9,10 moreover, using this alloy structure, reproducible and resilient kesterite absorber processing with device efficiencies above 11% can be achieved. 11,12 In this study, we report the optimization of the selenium amount during the annealing process for precursor stacks with Cu-Sn alloy layers.…”

Vapor-Phase Incorporation of Ge in CZTSe Absorbers for Improved Stability of High-Efficiency Kesterite Solar Cells

“…In brief, the Ge is incorporated into the already-formed kesterite-absorber layer via the vapor phase during the high-temperature phase of the selenization process. During this step the composition of the layer is usually shifted from Cu-rich to Cu-poor [11,12]. Since the main kesterite phase formation route is unaltered, the process offers high reproducibility in terms of layer morphology and targeted final-layer composition.…”