Lateral phase separation in Cu-In-Ga precursor and Cu(In,Ga)Se2 absorber thin films

•Lateral phase separation and dewetting of Mo during precursor heating is observed.•Dewetting is strongly decreased by applying faster heating rates.•NaF deposited on top/underneath the precursor de-/increases the dewetting of Mo.•Reduced dewetting during heating enables delayed Se supply during selenization.

Sequential Cu(In,Ga)Se2 fabrication with a thermally activated reaction of Cu-In-Ga metal precursor layers in chalcogen atmosphere is an industrially attractive route for preparation of Cu(In,Ga)Se2 absorber based thin film solar cells. Recent results show that controlling the selenium supply during rapid thermal processing has a huge impact on absorber growth. Especially a two stage process applying a first annealing step with or without Se at temperatures up to 400 °C was shown to have a positive effect on the elemental in-depth distribution. However, during this annealing, lateral phase separation, dewetting and coarsening may occur in the metal phase, leading to lateral non-uniformity of the absorber. In this study we show how the dewetting can be strongly decreased by adjusting the precursor architecture, applying faster heating rates and NaF addition on top of a precursor. In contrast, NaF deposited underneath the precursor increases the dewetting effect. Further we show that lateral phase separation during annealing increases with temperature and leads to phase domain sizes of several micrometers at 580 °C.

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