In kesterite CZTSSe solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered or double-layered absorber. So far, only segregated pieces of research exist to characterize this double layer, its formation dynamics and its effect on the performance of devices. Here, we review the existing research on double-layered kesterites and evaluate the different mechanisms proposed. Using a cosputtering-based approach, we show that the two layers can differ significantly in morphology, composition and optoelectronic properties, and complement the results with a statistical dataset of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 nm to 300 nm, we show that the double layer segregation is alleviated. In turn, we see a progressive improvement in the device performance for lower thickness, which alone would be inconsistent with the known case of ultrathin CIGS solar cells. By comparing the results with CZTS grown on monocrystalline Si substrates, without a native Na supply, we show that the alkali metal supply does not determine the double layer formation, but merely reduces the threshold for its occurrence. Instead, we propose that the main formation mechanism is the early migration of Cu to the surface during annealing and formation of Cu2-xS phases, in a self-regulating process akin to the Kirkendall effect. We comment on the generality of the mechanism proposed, comparing our results to other synthesis routes, including our own in-house results from solution processing and pulsed laser deposition of sulfide and oxide-based targets. Although the double layer occurrence largely depends on the kesterite synthesis route, the common factors determining the double layer occurrence appear to be the presence of metallic Cu and/or a chalcogen deficiency in the precursor matrix.
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