Relative surface composition (in at%) of the three investigated samples (Cu, Zn, Sn, S, and Se only), determined from XPS intensities and using the corresponding photoionization cross sections, inelastic mean free paths, and analyzer transmission values.

	Cu	Zn	Sn	S	Se
Reference	16 ± 2	21 ± 2	16 ± 2	0	47 ± 5
One spike	8 ± 1	30 ± 3	10 ± 1	27 ± 3	24 ± 2
Three spikes	7 ± 1	28 ± 3	7 ± 1	38 ± 4	20 ± 2